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239 Cards in this Set
- Front
- Back
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1. What is the purpose of a telescope?
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Capture as many photons as possible from a given region of the sky and concentrate them onto a focused beam for analysis
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2. What is refraction?
What is a refracting telescope? |
The bending of a beam of light as it passes from one transparent medium (air) into another (glass)
Uses a lens to gather and concentrate a beam of light |
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3. What is a lens?
How does a refracting lens work? |
A series of prims
Light ray refracts when it passes from air into glass and then is refracted a second time when it passes from glass into air *Changes direction of light ray by an amount that depends on the angle between the prism's faces |
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4. How does the deflection angle change?
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When the angle between the faces is large, the deflection is large
When the angle is small so is the deflection |
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5. What is the focus?
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A single point that all refracted light rays arriving parallel to the lens's axis pass through
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6. What is the focal length?
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The distance between the primary mirror and the focus
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7. What is a reflecting telescope?
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A telescope that uses a curve mirror instead of lens to focus the incoming light
Light striking a polished surface is reflected back, leaving the mirror at the same angle at which is arrived |
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8. What is the prime focus?
Draw a reflecting mirror. |
The focus of the primary mirror
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9. How are images by a mirror formed?
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Light coming from different points on a distant object are focused to slightly different locations
Result is image formed around the prime focus that is inverted (upside down) and actually small |
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10. Describe the two basic types of telescopes?
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1. Refractor (lens)
-image formed at focus and magnified by eyepiece 2. Reflector (mirrors) -image formed at prime focus and use eye piece to magnify |
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11. What are the four types of reflecting telescopes?
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1. Prime focus
*uses one mirror vs rest which use a secondary mirror 2. Newtonian focus -light is intercepted before it reaches prime focus and is deflected by 90° usu to eyepiece at side 3. Cassegrain focus -secondary mirror intercepts and reflects light back down through small hole at center of primary mirror -Cassegrain focus is point behind primary mirror where light converges 4. Nasmyth/coude focus -complex and starlight is reflected by several mirrors |
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12. Which type of telescope, reflecting or refracting, are dominant today?
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Reflecting telescopes
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13. Why are most telescopes reflectors and not refractors?
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1. Light traveling through lens is refracted differently depending on wavelength (chromatic aberration)
2. Some light traveling through lens is absorbed (esp problem with IR and UV radiation) 3. Large lenses can be heavy and only supported at edge (lens deforms under own weight) 4. Lens need 2 high-precision surfaces (mirror needs one so is cheaper) |
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14. What is chromatic abberation?
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Amount of refraction depends on wavelength so different colors of light focus at different distances
Bend blue light (shorter wavelength) more than red light so focused slightly closer to lens than red component Image has a colored "halo" |
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15. Why build large telescopes?
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1. More collecting area provides morel light-gathering power (see fainter detail)
2. If not limited by atmosphere, resolution is proportional to wavelength and inversely proportional to telescope size (bigger telescope has better resolving power) |
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16. What does resolving power mean?
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They can distinguish objects that are closer together
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17. What is the collecting area?
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Total area capable of gathering radiation
*large reflecting mirror, more light it collects, easier to study and measure radiative properities |
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18. How is observed brightness related to telescope size?
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Number of photons collected from object per unit time is proportional to square of radius of mirror
So, 5-m telescope produces an image 25 times as bright as a 1-m telescope Also produces an image 25 times faster than the 1-m telescope So to collect same amount of light, observe w/ a small telescope for a long time or a large one for shorter time |
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19. What are the Keck telescopes?
What is an advantage to building it this way |
36 1.8-m mirrors combines for equivalent collecting area of a single 10-m reflector
Has a Cassegrain focus and and Nasmyth focus with Coude room Makes it cheaper to build and faster since they are smaller mirrors |
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20. What is the largest single mirror as compared to the segmented design of Keck?
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8.3-m Subaru telescope
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21. What is resolution?
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Ability of any device to form distinct, separate images of objects lying close together in the filed of view
The finer the resolution, better we distinguish objects, more detail |
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22. How does diffraction impact telescope resolution?
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It's an intrinsic property that limits telescope resolution depending on wavelength and size
Diffractions introduces a "fuzziness" or loss of resolution |
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23. Why do large telescopes produce less defraction than small ones?
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Amount of diffraction is proportional to wavelength and inversely proportional to diameter of telescope
Angular resolution (arcsec) = 0.25 x wavelength(um)/mirror diameter(m) *1 um = 10^-6m |
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24. How do resolution numbers work?
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A bigger number, poorer resolution
Lower arcsec means a better image |
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25. What are charge-coupled devices (CCDs)?
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Used in modern telescopes (replaced photographic equipment) and their output goes directly to a computer
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26. What are pixels?
How do they work to form 2-D image? |
Tiny picture elements on a CCD
Light strikes pixel, electric charge builds up, charge is monitored and 2-D image formed *Amount of charge is directly proportional to number of photons striking each pixel |
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27. What two advantages do CCDs have over photographic equipment?
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1. More efficient (show fainter objects and record same level of detail faster)
2. Digital format representation |
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28. What is the field of view?
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Distance from center to where the image becomes unacceptable defines the field of view
Large reflectors are good are good at forming images of narrow fields of views |
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29. What is photometry?
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Measurement of brightness
The brightness we measure from an image is really an average over the entire exposure |
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30. What is a photometer?
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It measures the total amount of light received in all or part of the field of view
Used when highly accurate and rapid measurements of light intensity are required *"Throw away" spatial detail and no image is produced |
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31. Do large telescopes achieve the diffraction limit?
Why or why not? |
Rarely
Atmospheric blurring due to atmospheric turbulence (light rays strike telescope at slightly different locations due to turbulence) |
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32. What is seeing?
What is the seeing disk? |
Seeing is used to describe the effects of atmospheric turbulence
The circle over which a star's light is spread |
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33. How does atmospheric turbulence vary with wavelength?
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Has less effect on light of longer wavelength - ground based astronomers generally "see" better in IR
BUT, atmosphere is wholly or partially opaque to IR so best position in on top of mountain |
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34. What are the solutions to atmospheric blurring or turbulence?
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1. Put telescopes on mountain tops, esp in desert to reduce light pollution
2. Put telescopes in space 3. Adaptive optics |
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35. What is adaptive optics?
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Track atmospheric changes with laser and adjust mirrors in real time
Analyze image while light is still being collected and adjust telescope from moment to moment Control environmental and mechanical fluctuations |
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36. With adaptive optics in place what becomes the main agent limiting a telescope's resolution?
In what part of electromagnetic spectrum is adaptive optics most effective and why? |
Earth's atmosphere
The infrared Atmospheric distortions are smaller and b/c longer IR wavelengths impose less stringent requirements on precise shape of the mirror |
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37. How big is the Hubble Space Telescope?
What range can it observe in? |
2.4-m diameter main mirror
Observe visible, infrared, and ultraviolet radiation (100nm to 2200nm) *Don't deal with blurring of light |
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38. What are radio telescopes?
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1. Similar to optical reflecting telescopes (collecting area that reflects light to focus where there is a detector)
2. Observe at Prime Focus 3. Build at ground base b/c atmosphere is no hindrance to long wavelength radiation |
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39. How do radio telescopes differ from optical telescopes?
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Optical instruments can detect all visible wavelengths simultaneously
Radio detectors normally register only a narrow band of wavelengths at any one time Poorer angular resolution than counterpart (enormous size only partly offsets this effect) |
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40. How are radio telescopes built?
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1. Cheaply
2. Large (need to be b/c cosmic radio sources are extremely rare) 3. Less sensitive to imperfections (due to longer wavelength) 4. Don't need to be as smooth at optical counterparts that collect shorter visible radiation |
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41. What is one disadvantage of large radio telescopes like the one the Arecibo telescope?
What is a general disadvantage of radio telescopes? |
Cannot be pointed very well to follow cosmic objects across the sky
Longer wavelengths means poor angular resolution |
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52. What are some advantages or value of radio astronomy?
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1. Can observe 24 hours a day
2. Clouds, rain, and snow don't interfere 3. Observation at entirely different frequency so get totally different information |
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53. How do we get new info?
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1. Many of the strongest radio sources emit little or no visible info
2. Visible light can be absorbed by interstellar dust, radio waves are not 3. Many parts of universe cannot be seen at all by optical means |
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54. What is interferometry?
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Technique to overcome poor angular resolution of radio telescopes
Combine information from several widely spread radio telescopes as if they came from a single dish Combined instruments make up interferometer |
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55. What is the resolution of an interferometer?
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Resolution will be that of dish whose diameter equals the largest separation between dishes
The larger the distance separating the telescopes (longer the baseline) the better is the attainable resolution |
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56. What does interferometry involve?
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Combine signals from two receivers
The amount of interference depends on the direction of the signal (constructive vs destructive interference) |
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57. How good is interferometry?
Can it be done with any other forms of radiation? |
Can get radio images whose resolution is close to optical
Can also be done with visible light but much more difficult due to shorter wavelengths |
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58. What is good about infrared observations?
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Can image where visible radiation is blocked by dust
Some of the most useful IR observing is done from ground on mountain tops in desert |
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59. Where is the atmosphere opaque?
So where must UV astronomy be done? |
Partially opaque to radiation below 400nm and totally to 300nm and below
Must be done in space as the atmosphere absorbs almost all UV ray |
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60. What is special about x-ray and gamma rays?
What can we do with x-rays? |
They will not reflect off mirrors so need new techniques
They will reflect at a very shallow angle and can therefore be refocused (have serious of nested cylindrical mirrors) |
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61. What about gamma-rays?
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They cannot be focused at all so images are coarse
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62. What are some general consideration with radio wave?
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1. Penetrate dusty regions of interstellar space
2. Atmosphere largely transparent to them 3. Detected at daytime and nightime 4. High resolution at long wavelength requires very large telescopes and interferometers |
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63. What are some general consideration with IR waves?
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1. Penetrate dusty regions
2. Atmosphere only partially transparent to IR so mountain tops and space |
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64. What are some general considerations with visible light?
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1. Atmosphere transparent to it
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65. What are some considerations with UV waves?
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1. Atmosphere opaque so observations made from space
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66. What are some general considerations with x-rays?
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1. Atmosphere opaque so observe from space
2. Need special mirror configurations |
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67. What are some general considerations with gamma-rays?
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1. Opaque atmosphere
2. No image formed |
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68. How do we measure sun's rotation?
What is special about it? |
Measure timing of sun spots and other surface features
Rotates in about a month but not as a solid body Spins differentially - faster at the equator and longer at the poles |
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69. How do we calculate the Sun's luminosity?
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Luminosity is the total amount of energy it radiates per second
Calculated from the fraction of that energy that reaches Earth |
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70. What is the solar "constant"?
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Amount of the Sun's energy reaching Earth (1400 W/m2)
About 50% to 70% of incoming energy from Sun reaches the Earth (rest is intercepted by atmosphere or reflected away by clouds) |
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71. On a sunny, clear day how much solar energy does a sunbather with surface area of 0.5m2 receive?
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0.5m2 X 0.70 (70%) x 1400 W/m2
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72. What is the Sun's total luminosity?
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4 x 10^26 W
This is the total rate at which energy leaves the Sun's surface |
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73. How do we determine the solar interior?
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Use mathematical models consistent with observation and physical principles, provide information about the Sun's interior
Theoretical models generally begin by assuming the Sun is in a state of hydrostatic equilibrium |
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74. What is hydrostatic equilibrium?
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The inward gravitational force must be balanced by outward pressure
Otherwise the Sun would expand/explode or contract/implode |
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75. What kind of internal pressure must the Sun have?
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Very high internal pressure to equalize strong gravitational pull due to Sun's large mass
High pressure in turn requires high central temperature |
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76. What is helioseismology?
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Doppler shifts of solar spectral lines indicate a complex pattern of vibrations
These vibrations are the result of internal pressure waves that reflect off photosphere and repeatedly cross solar interior Helioseismology is the study of solar surface patterns |
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77. What is the standard solar model?
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1. Core: 15 mil K, dense, energy by nuclear fusion
2. Radiation Zone -7 mil K, energy transported by electromagnetic radiation, transparent 3. Convection zone: -2 mil K, energy carried by convection, opaque 4. Photosphere: -5800K, electromagnetic radiation can escape, part we see 5. Chromosphere: -4500K, cool lower atmosphere 6. Transition zone: -8000K, rapid increase in temp 7. Corona: -3 mil K, hot/dense upper atmopshere 8. Solar wind |
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78. Does the sun have a surface?
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Not a solid surface
The "surface" is the photosphere (what we see) Photosphere is thin which is why we see a well-defined sharp edge (it is opaque so we can't see through it) |
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79. Describe radiative energy transport.
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Atoms collide less frequently and violently than in core so electrons can bind to parent nuclei
By outer edge of radiation zone all photons produced in core have been absorbed *no physical movement of material with radiation |
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80. Describe convective energy transport.
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Hot solar gas moves outward while cooler gas above it sinks creating a characteristic pattern of convection cells
Escaping energy reaches surface by convection |
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81. How does the visible top layer of the convection zone appear?
What do granules form? |
It is granulated with areas of upwelling material surrounded by areas of sinking material
Granules are regions of bright and dark gas Each granule forms the topmost part of a solar convection cell |
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82. How are the solar convection cells arranged?
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Sizes of convection cells become progressively larger at greater depths
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83. How does temperature and density change from the core to outer parts?
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Very dense at core and then density drops rather sharply at first and then decreases more slowly near photosphere
Temperature also decreases with increasing radius in solar interior, but not as rapidly as the density |
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84. What does spectral analysis of the Sun's spectrum tell us?
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Can tell us what elements are present in the chromosphere and photosphere
Some 67 elements have been identified |
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85. Why do spectral lines arise again?
What element makes up most of the Sun? What is next? |
Electrons in atoms or ions make transitions between states of well-defined energies emitting or absorbing photons of specific energies
Hydrogen (91.2%) and the Helium |
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86. Why are spectral lines seen?
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Because the Sun is more opaque close to atomic transitions, so we only see cooler, outer layers at those wavelength
Photons with energies well away from any atomic transition can escape from relatively deep in the photosphere |
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87. Where is the chromosphere?
Can we see it? When can we? |
Cooler and above photosphere
Difficult to see directly as photosphere is too bright When Moon covers photosphere and not chromosphere during eclipse (reddish hue is visible) |
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88. When can the solar corona been seen?
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During an eclipse if both photosphere and chromosphere are blocked - can see ghostly corona
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89. What is the temperature of the corona like?
What is the corona's energy source? |
Much hotter BUT much less dense than layers below it
Heated by interaction with the Sun's magnetic field (magnetic disturbances in solar photosphere) |
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90. What are sunspots?
What is their different coloration due to? |
Dark areas on the sun that often occur in groups
Appear dark b/c they are slightly cooler than their surroundings (4500K vs 5800K of photosphere) If removed them from sun, they would glow brightly like any other hot object |
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91. How was solar magnetism discovered?
How is the magnetic field in a sunspot? |
Observations of broadening or splitting of spectral lines by a magnetic field
1000 times greater than the field in undisturbed photosphere region Sunspots are linked in pairs by connecting magnetic field lines |
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92. Why may sunspots be cooler?
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Abnormally strong fields tend to block or redirect convective flow of hot gays
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93. What is the polarity of magnetic field lines at sunspots?
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Emerge from interior at S
Dive below photosphere at N *All the sunspots pairs in the same solar hemisphere have the same magnetic configuration |
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94. How do sunspots originate?
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When magnetic field lines are distorted by Sun's differential rotation
To limit twisting, field lines burst out of surface |
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95. What is the sunspot cycle?
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Sunspots are not steady, change size and shape and come and go
The sunspot cycle is a 11-year sunspot cycle during which sunspot numbers rise, fall, and then rise again *individual spots do not move but new spots appear closer to equator as older ones at higher latitude fade *start with solar minimum and reach solar max 4 yrs in |
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96. What is this 11-year cycle a part of?
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A 22-year cycle because the spots (and the Sun's magnetic poles) switch polarities between the northern and southern hemisphere every 11 years
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97. What is the Maunder minimum?
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Few, if any, spots from 1640-1710
Was this solar inactivity just a cycle too long? |
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98. What are active regions?
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Sites of energetic events
Areas around sunspots are active and large eruptions may occur in the photosphere *Most frequent and violent around the time of solar max |
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99. What are prominences?
What causes them? |
Loops or sheets of glowing gas ejected from active regions on the solar surface
Magnetic instabilities in the strong fields found in and near sunspots groups may cause prominence |
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100. What are quiescent prominences?
What are active prominences? |
Persist for days or even weeks hovering high over photosphere
Come and go more erratically |
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101. What are flares?
What type of radiation is associated with them? |
Large explosions on Sun's surface emitting a similar amount of energy to a prominence BUT in seconds or minutes rather than days or weeks
X-ray and UV emissions are intense in compact hearts of flares (100 mil K) |
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What are coronal mass ejections?
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Giant magnetic "bubbles" (confined by magnetic fields)of ionized gas that separate from solar atmosphere and escape into space
Associated (but not always) with flares and prominences |
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103. How can coronal mass ejections affect Earth?
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If there fields are properly orient, they can mere with Earth's magnetic field (reconnection)
Potential cause widespread communication and power disruptions |
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104. What are some examples of solar-terrestrial relations?
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1. Near start of last 8 22-yr solar cycles, there have been droughts in N. America
2. Link between solar activity and increased atmospheric circulation 3. Maunder min corresponds with colder years of Little Ice Age 4. Sun's luminosity is greatest when many dark spots cover surface 5. Correlation between solar activity and geomagnetic disturbances on Earth |
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105. What does the coronal gas radiate at?
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High frequencies primarily in the X-ray range
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106. How do solar winds escape?
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Through coronal holes which can be seen in X-ray images
These structures are deficient in matter (density is lower than tenuous corona) |
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107. Why are coronal holes lacking in matter?
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as is able to stream freely into space at high speeds
In coronal holes, solar magnetic field lines extend from surface far out into interplanetary space Because of the "open" field structure flares and other magnetic activity tend to be suppressed there |
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108. How does the solar corona change?
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It varies with sunspots
At sunspot min = regular corona that uniformly surrounds sun At sunspot max = larger and more irregular in appearance and extends farther from solar surface |
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109. What do astronomers believes heats the corona primarily?
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Solar surface activity
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110. How efficiently does the Sun generate energy?
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Every kg of the sun produces 0.2 milliwatts of energy
This is not much BUT it continues through the 10 billion year lifetime Total lifetime energy output is 3 x 10^13 J/Kg |
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111. What provides the source of energy for the Sun?
Describe it. |
Nuclear fusion: combining of light nuclei into heavier ones
nucleus 1 + nucleus 2 yields nucleus 3 + energy *total mass decreases so that energy is produce (energy comes from mass) |
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112. Is iron good to use for fission or fusion?
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No, won't yield energy because most stable
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113. How is mass converted into energy?
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E = mc2
Even small amount of mass is the equivalent of a large amount of energy b/c speed of light is soo larger *Law of conservation of mass and energy (mass + energy remains constant) |
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114. What does nuclear fusion entail?
How can this be done? |
Like-charge nuclei get close enough to each other to fuse
Need to get them close enough so strong nuclear force binds them This can only happen if temperature is extremely high (over 10 mil K or electromagnetic repulsion is too strong) |
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115. Describe the first step in the proton-proton chain.
What is a positron? What is a neutrino? |
proton + proton yields deuteron (H with extra neutron) + neutrino + positron
Positively charged electron (antimatter counterpart to electron - they annihilate one another producing gamma-ray photons) Low mass, no electrical charge, move at almost speed of light, interact with hardly anything, interaction with matter is governed by weak nuclear force |
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116. What is the second step of the proton-proton chain?
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Isotope of He formed
2H + 1H yields 3He + energy |
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117. What is the third step in the proton-proton chain?
What is the net reaction? What does the energy released ultimately become? |
Two He-3 isotopes forms a He-4 and two protons and energy
4(H1) -> (He4) + 2 neutrinos + energy (form of gamma rays) Energy released ultimately becomes sunlight that warms our planet Neutrinos escape without interacting |
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118. Describe the gravitational force?
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Weakest of natural forces
All particles interact through gravity b/c all have mass |
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119. Describe the electromagnetic force.
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Only affects charged particles
For subatomic particles it is much stronger than gravity Can repel (like charges) or attract (opposite charges) Above microscopic level, most object are close to neutrally charge |
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120. Describe the weak nuclear force.
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Can affect any subatomic particles regardless of charge
Governs emission of radiation from some radioactive atoms |
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121. What is the electroweak force?
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Electromagnetic and weak force are two aspects of this basic force
At low temperatures though like on Earth or even in stars, the two forces have distinct properties |
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122. Describe the strong nuclear force.
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Affects protons and neutrons (not electrons)
Strongest of all forces Binds atomic nuclei and subatomic particles together Governs generation of energy in Sun and all other stars Operates only at very close range |
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123. Rank the strengths of the forces.
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Strong nuclear force: 1
Electromagnetic: <0.01x as strong Weak nuclear: 3x10^-7x as strong Gravity: 6x10^-39x as strong |
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124. For how many years can the Sun continue fusion (has enough H in core to keep fusion going)?
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Go for about another 5 billion years
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125. Can we observe solar neutrinos?
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They are emitted directly from the core and the Sun and escape
They interact w/ virtually nothing |
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126. What would neutrino observation allow us to do?
Is neutrino observation likely? |
Give us a direct picture of what is happening in the core
They are not real likely to interact with Earth-based detectors Only way to spot them is th have a huge detector volume and be able to observe single interaction events |
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127. How is observation neutrinos?
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1. Typical solar neutrino detectors, the resolution is poor
2. There has always been a deficit in the type (#) of neutrinos produced in fusion reactions w/in the Sun (neutrino problem) |
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128. What does recent research neutrinos reveal?
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Sun is emitting about as many neutrinos as standard solar model predicts BUT the neutrinos change into other types of neutrinos between the Sun and Earth causing the apparent deficit
This change indicates that neutrinos have some mass |
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129. What is parallax?
How do we measure stellar distance? |
Apparent shift of a foreground object relative to some distant background as the observer's point of view changes
Use parallax *As distance to object increases, parallax becomes smaller and harder to measure |
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130. How can we measure parallax from Earth?
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Observe star at different times of the year
Have to extend baseline to the diameter of Earth's orbit around the Sun (2AU) Need this enormous baseline for stellar parallaxes to be measurable |
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131. What is a parsec?
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The distance at which parallax (for 1 AU baseline) is 1 arc second
1 pc = 3.3 light years Best parallax measurements are accurate down to 200 pc or 0.0005" |
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132. How are parallax and distance related?
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Inversely proportional
distance(pc) = 1/parallax(") |
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133. What are our nearest neighbors?
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1. Closest star to Earth is Proxima Centauri (display largest known stellar parallax) and is 4 ly away
2. Bernard Star at 6 ly away and has largest proper motion of any star 3. 30 or so stars withing 4 pc (13 ly) of Earth |
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134. How were the stars named?
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1. Brightest stars were known to and named by ancients and many have kept Arabic names (Rigel, Betelgeuse)
2. In 1604, stars within a constellation were ranked in order of brightness and labeled with Greek letters (Alpha Centauri) 3. Early 18th century, stars were numbered from west to east in a constellation (19 Ori) 4. Different naming schemes as more stars were discovered 5. Label stars by celestial coordinates |
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135. What are the two components of stellar motion?
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1. Radial velocity, along the line of sight, can measured using Doppler effect
2. Transverse velocity, perpendicular to line of sight, determined by monitoring position of star in sky (proper motion) |
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136. What is proper motion?
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Annual movement of a star across the sky, as seen from Earth and corrected for parallax
Expressed in arc sec per year |
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137. What is luminosity?
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Absolute brightness
A measure of the total power radiated by a star Units are energy per sec Does not depend on location or motion |
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138. What is apparent brightness?
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How bright a star appears when viewed from Earth
It depends on the absolute brightness BUT also on the distance of the star Measure of energy flux (energy per unit area per unit time) |
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139. Is luminosity a constant?
Is apparent brightness a constant? |
Yes, amount of radiation leaving a star per unit of time is constant
It is inversely proportional to the square of the distance from the star and directly proportional to luminosity |
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140. Give the proportionality for apparent brightness (energy flux)
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Energy flux ∝ luminosity/ distance2
Doubling distance from star makes it appear 4 times dimmer Tripling distance makes it appear 9 times dimmer |
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141. If two star appear equally bright do they have the same luminosity?
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No, one might a closer dimmer star and the other a farther, brighter star
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142. What is the magnitude scale?
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Used instead of measuring apparent brightness
Ranking in terms of apparent brightness Large magnitude means fainter star So "first magnitude" would mean bright in astronomy |
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143. What type of scale is the magnitude scale?
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Logarithmic scale
Change of 5 magnitudes corresponds to a change of a factor of 100 in apparent brightness Change in 1 magnitude corresponds to a factor of about 2.5 in apparent brightness |
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144. How have we changed Hipparchus' magnitude scale?
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1. Change in magnitude of 5 corresponds to exactly a factor of 100 in apparent brightness
2. Called apparent magnitudes 3. Not limited to whole numbers 4. Can have ranges outside 1 and 6 |
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145. What is apparent magnitude?
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Tells us how bright an object appears from Earth
Depends on both luminosity and distance |
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146. What is absolute magnitude?
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Measures how bright an object would appear if it were exactly 10 pc away
It measures luminosity, rather than brightness |
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147. A reduction of 5 in magnitude corresponds to an increase by how much in luminosity?
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A factor of 100
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148. When a star farther than 10 pc away is moved to a point 10 pc how does apparent brightness and magnitude change?
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Apparent brightness increases
Apparent magnitude decreases |
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149. If a star at a distance of 100 pc were moved to a standard 10 pc distance how would stuff all change?
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Distance would decrease by a factor of 10
Apparent brightness would increase by factor of 10^2 or 100 Apparent magnitude would decrease by 5 |
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150. How can astronomers measure a stars temperature?
What colors correspond with cooler stars? hotter stars? |
Determine surface temp by measuring apparent brightness at several frequencies and match observations to black body curve
Red stars are cooler Blue stars are hotter |
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151. What is the radiation from stars?
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Primarily black body radiation
Since black body curve is not symmetric, observations at two wavelengths are enough to define temperature |
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152. Describe approximate surface temp, color, and color index (b flux/v flux).
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1. blue-violet, 30,000K, 1.3
2. blue, 20,000K, 1.2 3. white, 10,000K, 1 4. yellow-white, 7000K, 0.72 5. yellow, 6000k, 0.55 6. orange, 4000k, 0.33 7. red, 3000k, 0.21 |
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153. What is photometry?
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Type of non-spectral-line analysis using a standard set of filters
Estimate temp by measuring and comparing amount of light received through different colored filters |
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154. How many categories of stellar spectra are there?
What do they correspond to? What are the categories from highest to lowest? |
Seven
Correspond to different temps O B A F G K M (Oh Be A Fine Girl, Kiss Me) |
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155. Describe the stellar spectral classes.
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1. O ; 30,000K; ionized He strong, multiply ionized heavy elements (O,N), H faint
2. B; 20,000K; neutral He moderate, singly ionized heavy elements, H moderate 3. A; 10,000; neutral He very faint, singly ionized heavy elements (Ca), H strong 4. F; 7000K; singly ionized heavy elements, neutral metals, H moderate |
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156. Describe the stellar spectral classes.
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5. G; 6000K; singly ionized heavy elements, neutral metals, H relatively faint
6. K; 4000K; singly ionized heavy elements, neutral metals strong, H faint 7. M; 3000K; neutral atoms strong, molecules moderate, H very faint |
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157. What is speckle interferometry?
When can it be used? |
Many short-exposure images of a star, each too brief for Earth's turbulent atmosphere to smear it out into a disk are combined to make a high resolution map of a star's surface
A few very large, very close stars can be imaged directly by using this (i.e. Betelgeuse) |
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158. How is size determined for the vase majority of stars that cannot be imaged directly?
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Calculated knowing luminosity and temperature
Radius-luminosity-temperature relationship L ∝ r^2 x T^4 SO in solar units: r = L^1/2 X T^4 or r = square root of L/T^4 |
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159. How do Dwarf stars radii compare to the Sun?
What about Giant stars? What about supergiant stars? |
Less than or equal to Sun
Radii 10-100 times the Sun's Radii greater than 100 times the Sun's |
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160. What is the Hertzsprung-Russell (H-R) diagram?
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Diagram that plots stellar luminosity (or absolute magnitude) against surface temperature (or color)
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161. What is the main sequence?
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Well-defined band stretching diagonally from the top left (high temp, high luminosity) to the bottom right (low temp, low luminosity)
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162. How does temperature range in main sequence stars?
How does luminosity range? |
Temp ranges from 3000K (M class) to over 30,000K (O class)
Luminosity range is very large covering some 8 orders of magnitude |
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163. What do the dashed lines on a H-R diagram indicate?
Along a constant-radius line, how are temperature and luminosity related? |
Where stars of the same radius lie
Luminosity ∝ T^4 |
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164. Where is the white dwarf region?
What are stars like here? |
Bottom left hand corner
Hot but not very luminous and quite small |
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165. Where are the blue giants?
What are stars like here? |
Top left corner
Large, hot, and bright star (blue in color) *The very large are called blue supergiants |
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166. Where are red giants?
What are stars like here? Are red giants near the Sun? |
Upper right corner
Cooler but very luminous (bright) No red giants w/in 5 pc of the Sun BUT many of the brightest stars seen are red giants |
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167. Why do the brightest stars in the sky appear bright?
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Because their luminosities are enormous, its not their proximity
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168. Where are the red dwarfs?
What are they like? How common are they? |
Lower right corner
Small, cool, and faint Red dwarfs are the most common type of star in the sky (80% of all stars) |
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169. On an H-R diagram of 20,000 nearby stars what is apparent?
What percent of stars make up each region? |
Main sequence is clear as is the red giant region
*bright, blue main sequence stars are rare 90% of stars lie on main sequence, 9% are red giants, 1% are white dwarfs |
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170. If temperature is the same, how do size of stars compare if one is at top left of H-R diagram and other is at bottom left of H-R diagram?
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The star near the top is bigger
L ∝ r^2 -as luminosity increases, so does radius |
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171. How can we yield distance?
Up to what distance can we use parallax to measure? |
Use measurement of apparent brightness combined w/ knowledge of luminosity
Can only measure parallax for stars up to 200 pc away To get distance for other stars, need to use less direct means |
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172. What is spectroscopic "parallax"?
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Technique that can be used for greater distance
Uses spectroscopy to find distance |
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173. Describe spectroscopic parallax in a brief?
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1. Measure star's apparent magnitude and spectral class
2. Use spectral class to estimate luminosity (H-R diagram) 3. Apply inverse-square law to find distance from luminosity and observed brightness |
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174. What is the equation?
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Apparent brightness ∝ (luminosity/distance^2)
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175. How do we measure apparent magnitude?
What is spectral class? |
It's just a measurement of observed brightness in magnitude units
(fainter = larger magnitude) O/B/A/F/G/K/M - more detail classifications can indicate whether star is on main sequence, giant, etc. |
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176. How do we use spectral info to distinguish between giants, super giants, and main sequence stars?
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Line width is sensitive to density in stellar photosphere
Density is in turn correlated with luminosity (class star categorized into is known as luminosity class) |
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177. How do the line widths vary?
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Spectral lines are broader for main sequence sequence stars than giants
This is because star is denser (so denser equal wider line) |
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178. What are the stellar luminosity classes?
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Ia. Bright super giants
Ib. Super giants II. Bright giants III. Giants IV. Sub-giants V. Main sequence stars |
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179. What things define a specific star?
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Its....
1. Spectral class 2. Luminosity class 3. Temperature 4. Luminosity |
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180. How do we use spectral class to estimate luminosity?
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Use H-R diagram
If we're looking as an AV star we know... 1. Its main sequence 2. T ~ 10,000K 3. L ~ 100 times as luminous as Sun |
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181. Suppose star in question has apparent brightness 100 time (5 magnitudes) fainter than another A V star that's 100 pc, how much more distant is this star?
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10 times more distant
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182. How are various stellar distances measure?
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1. Up to ~1Au use radar ranging
2. Up to ~200 pc us stellar parallaz 3. Up to ~10,000 pc use spectroscopic parallax |
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183. What is a binary-star system?
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Consist of two stars in orbit about a common center of mass, held together by the mutual gravitational attraction
Most stars are in binary pairs |
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184. What can we use binary pairs to do?
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Determine stellar masses
Measurement of their orbital motion allows determination of the masses of the star Remember force of gravity depends on mass |
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185. What do we use to do this?
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Newton's modification to Kepler's Third Law
(P/P earth)^2 = (a/a earth)^3/(M total/M sun) *P is orbital period *a is semi-major axis |
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186. What are visual binaries?
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They have widely separated members that are bright enough to be observed and monitored separately
We can see the orbit on the sky Get P from orbital period and a from separation on the sky |
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187. How do we estimate sizes from angles again?
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diameter = distance x (angular diameter/57.3 degrees)
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188. What are spectroscopic binaries?
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They are the most common and they are too distant to be resolved in separate stars
Can be detected because their Doppler shifts change with time (red shift when away, blue shift when towards) |
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189. How do we get P and a in spectroscopic binaries?
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Measure period directly
Infer a from orbital speed v = (2∏a)/P *lot of planets found this way |
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190. What is an eclipsing binary?
What do we study in eclipsing binaries? |
Orbital plane of the pair of stars is almost edge-on to our line of sight
The light curve or the variations in the light from binary system to get info about orbit, mass, and radius |
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191. In a binary star system, which star moves more?
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The lighter one moves more
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192. Why is mass important?
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Largely controls a stars properties and density
*Main determinant of where a star will be on the main sequence |
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193. Where are low mass stars?
How abundant are they? |
Low-mass stars are cool, faint, and at bottom of main sequence
Lower-mass stars are more abundant |
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194. Where are massive stars?
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They are hot and bright and lie at top of main sequence
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195. How does radius correlate with mass?
How does luminosity increase with mass? |
Radius tends to go up with mass on main sequence
Strongly related in that L is proportional to M^4 |
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196. How is mass related to stellar lifetime?
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Amount of fuel star has for fusion should be proportional to its mass
Rate at which is consumes fuel must be proportional to luminosity stellar lifetime ∝ mass/luminosity |
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197. What is the mass-luminosity relationship?
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lifetime ∝ 1/(stellar mass)^3
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198. What three conclusions then can we draw about stellar lifetime and stellar mass?
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1. Most massive stars have shortest lifetimes (O and B stars we observe must be young) - have lots of fuel BUT burn it rapidly
2. Small red dwarfs burn fuel extremely slow and can have lifetime of trillion of years or more 3. Sun is in between two extremes and has total lifetime of 10 billion years (half way through life) |
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199. Why are massive stars rare?
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1. Only created rarely
2. Die out quickly |
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200. Two stars have same mass but A is more luminous than B. Which star has shorter stellar lifetime?
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A does because more luminous star burns through fuel quicker
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201. What is the interstellar medium (ISM)?
How much of the galaxy does it make up? What does it provide? |
"The stuff in between stars"
Makes up significant fraction of mass of our galaxy (15%) Provides raw material for star formation |
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202. What are the two component of the ISM?
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1. Gas
-atoms and small molecules, mostly H and He 2. Dust -large molecules or conglomerates of molecules (1000 to millions of atoms) |
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203. How much of the ISM is gas?
What is the gas composition similar to? What is the density of the gas like? |
99% of ISM (by mass) is gas
Similar composition to the Sun (>90% H, 8% He) Low density, 1 atom/cm^3 is typical This is 0.1 solar masses in a 1 pc radius sphere |
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204. How are young stars?
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They put out a lot of UV which excites electrons in H atoms
Young stars have warm gas and emission lines are produced |
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205. What does "nebula" mean?
What are emission nebula? |
Generic term for any fuzzy object in the sky
Many clouds of gas lit up by young stars that recently formed in them , seen in emission line *They are birth place for stars |
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206. What are molecular clouds?
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This is in the densest region where it is cold enough (10K) so that atoms combine readily into molecules
These 100-10^6 solar-mass clouds are where stars form |
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207. How do molecular clouds appear in visible light?
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Appear black
Can't see through them |
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208. What is the Horsehead Nebula?
What temperature is molecular gas? |
A dust cloud seen in front of an emission nebula
Cold |
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209. What are the dark regions in the Milky Way?
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Regions of dust (and gas) clouds that block lights from beyond stars
Dust particularly absorbs blue/UV light or it bounces off it |
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210. When are dust grains brightest?
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Dust glows in the infrared
Dust grains are typically heath to 100K so brightest at about 30 um (well into infrared) *Sunset appears red because of dust |
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211. What is interstellar dust made up?
What is it similar to? How much of ISM does it make up? |
Conglomerates of molecules
Similar to soot or smoke 1% of ISM and is distributed throughout (not just in dense regions) |
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212. What does dust to light that passes through it?
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Absorbs or scatters light that passes through obscuring our view
*Only does this if wavelength is less than grain size (do this more to blue and UV radiation) |
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213. So what is the net effect of this scattering of light?
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Red (or IR) is less affect by dust than blue (or UV) light
Blue light is more scattered than red light White light source will appear redder than before (like the Sun at sunset) |
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214. What causes the color of a red/pink nebula?
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Photons emitted by electrons changing energy levels in atoms
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215. How is star formation?
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It is ongoing
There are star-forming regions in our galaxy as well as in other Occurs in dense molecular clouds |
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216. What is the overview of star formation?
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It happens when part of a molecular cloud begins to contract under its own gravitational force
As it collapses, the center becomes hotter and hotter until nuclear fusion begins in the core In general, T increases, central density increases, and diameter decreases |
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217. Why does the ISM collapse everywhere?
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In stable objects (like Sun) pressure and gravity are perfectly balanced in hydrostatic equilibrium
For part of a gas cloud to collapse, gravity must be stronger than the pressure |
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218. How is gravity when there are a few atoms?
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Gravitational force is nowhere strong enough to overcome the random thermal motion/pressure
Thus, only dense regions (above some mass threshold) will collapse under gravity to form stars |
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219. Briefly describe the stages of prestellar evolution of solar type star.
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1. Central and surface temp of 10K and its just an interstellar cloud
2. Central temp 100K, cloud fragments 3. Central temp 10,000K 4. Protostar, central temp 1 mil K 5. Protostar, central temp 5 mil K 6. Star, central temp 10 mil K 7. Main-sequence star, central temp 15 mil K |
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220. Describe stage 1 where the cloud collapse begins.
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Interstellar cloud starts to contract
Perhaps triggered by shock or pressure wave from nearby star As it contracts, a cloud fragments into smaller pieces |
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211. What happens during stage two?
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Individual fragments collapse
Once the density is high enough, there is no further fragmentation A presolar cloud concentrates 1-2 solar masses in a sphere 0.01 pc radius |
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212. How is the cloud during stage two?
What happens to the radiation |
Transparent
Almost all energy released via gravitational collapse just gets carried off by radiation |
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213. What happens to the core during stage 3?
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Innermost core of cloud becomes opaque and begins to to heat up
Because it is opaque it can no longer radiate efficiently and starts heating up to 10,000K+ |
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214. What happens significantly at stage 4?
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A protostar is born
Large, opaque object with a well-defined photosphere at the core of the cloud Can now plot it on the H-R diagram |
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215. By the time a protostar is born what else has begun?
Is protostar in equilibrium? |
Planetary formation
No, so all heating comes from gravitational collapse (not fusion) |
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216. What occurs during stage 5 in terms of size?
What about luminosity? |
Protostar grows - gains mass and gets denser and hotter at its core
It also shrinks in size, releasing energy from gravitational collapse Luminosity decreases even though temperature rise b/c it is becoming more compact |
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217. What is the big thing that happens at stage 6?
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A star is born!
Core reaches temp of ~10 mil K Nuclear fusion |
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218. What happens at stage 7?
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It reaches main sequence where it remains unchanged for long time
It takes time for star to stop contracting and settle into hydrostatic equilibrium |
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219. What about formation of stars bigger and smaller than our Sun?
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The shape of their path on H-R diagram is similar but they end up on different places on the Main Sequence
Main difference is the time it takes them to form as a star *More massive stars form quicker whereas low-mass stars can take up to a billion years to form |
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220. Do stars move along the main sequence?
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No they do not
Once they reach it they are in equilibrium and do not move until their fuel begins to run out |
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221. What are failed stars?
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Cloud fragments that are too small for fusion to begin so they gradually cool off and become dark
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222. How much mass must a protostar have to become a star?
What about failed stars? |
0.08 mass of the Sun (80 times the mass of Jupiter) so its dense and hot enough for fusion to begin
"failed stars" are about 12 Jupiter masses |
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223. What is a failed star called?
Can we see them? |
Brown dwarf
It is luminous when first formed Difficult to observe directly as they are very dim |
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224. Where do clouds collapse more easily?
What does this cause? |
Along a cloud's axis of rotation
The gas around a protostar to settle into a disk |
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225. What happens to most of the material falling into a collapsed cloud?
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It is ejected in jets
Jets are matter being expelled from around protostar |
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226. What happens to jets with time?
What do they become? |
Jets from protostars are believed to widen over time
Become powerful stellar winds capable of clearing out an area roughly the size of the solar system |
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227. Where do the jets come out of when clouds collapse to form disks of materials?
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Coming out of the poles
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228. What is the role of magnetic field in the collapsing of clouds of dust and gas?
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Magnetism helps support intersellar clouds against collapse (like gas pressure)
When clouds do collapse, the magnetic field gets more concentrated and stronger, this may help form jets |
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229. How can we see inside star-forming regions?
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Look for infrared radiation or radiation of longer wavelength
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330. Have we discovered any protostellar disks?
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Yes, many in the past 10 years
There are two protostars in the Orion nebula at around stage 5 in development |
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331. When will a cloud start collapsing?
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Only in dense regions so if you increase density of a cloud it might start to form stars
When looking as just a few atoms, the gravitational force is nowhere near strong enough to overcome pressure |
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332. What can cause gas to start compressing?
Where do these come from? |
Shock waves (high temps and density behind the wave)
1. From nearby star formation, this can trigger collapse process in an interstellar cloud 2. Death of a nearby Sun-like star 3. Death of a massive star (supernova) 4. Galaxy collisions (most violent) *Density waves in galactic spiral arms |
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333. Why are star clusters important?
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Allow us to study effect of mass on stellar evolution in clusters stars are approximately of the same age and composition
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334. What is an open cluster?
|
Can contain thousands of stars that are formed together (younger)
Pleiades is an example -contains many blues stars so is younger |
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335. What are globular clusters?
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These clusters can contain millions of stars in a small volume (older)
Very compact Omega Centauri is an example -contains only red stars so is older |
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336. How are globular clusters on the H-R diagram?
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Absence of massive Main Sequence stars (due to old age, these stars have already used up all their fuel and moved off main sequence)
Heavily populated Red Giant region |
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337. How can the presence of massive, short-lived O and B stars affect star clusters?
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They can blow away dust and gas before smaller stars have time to form
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338. Is there an upper limit to stellar mass?
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More massive stars may not be stable enough to reach the main sequence
Blow self apart like Eta Carinae's did 150 years ago (>100 times mass of Sun) |
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339. What are hints that a problem might involve proportionalities?
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How much/by how many times will X increase
Request answers in solar units or in solar luminosity |
