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371 Cards in this Set
- Front
- Back
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How do lenses change the direction of light? |
By refraction |
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What shape are converging lenses? |
• Convex • Thicker in the middle |
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In which direction do converging lenses bend rays of light? |
Together |
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What is the horizontal axis through the centre of a lens called? |
The principle axis |
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What is a vertical axis through the centre of a lens called? |
The lens axis |
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What are axial rays? |
Rays travelling parallel to the principle axis |
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Where will axial rays passing through the lens converge? |
On a point called the principle focus. |
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What are non-axial rays? |
Rays that aren't travelling parallel to the principle axis |
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Where will non-axial rays converge after passing through a convex lens? |
On the focal plane |
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What is the focal plane? |
The plane perpendicular to the principal axis which contains the principle focus. |
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What is the focal length of the lens? |
Focal length is the perpendicular distance between the lens axis and the focal plane. |
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What is the symbol for focal length? |
f |
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What does a ray diagram show? |
How light rays travel from an object through a lens. |
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How do you draw a ray diagram? |
1. Set up your axis, object and lens position 2. Draw two rays: - One parallel to the principle axis passing through the principle focus - One passing straight through the centre of the lens which doesn't get refracted |
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Where is the image formed? |
Where the rays cross |
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What can a ray diagram tell you about the image? |
It's size, orientation and whether it is real or virtual. |
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If the image is closer to the lens then the object, what size will the image be? |
Diminished |
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If the object is closer to the lens axis then the image, what size will the image be? |
Magnified |
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If the image is the same distance from the lens axis as the object what size will the image be? |
The same size |
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What is the definition of a real image? |
When light rays have traveled through space and can be captured on a screen |
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What is the definition of a virtual image? |
When the light rays appear to have come from a different point in space, so the image can't be projected onto a screen. |
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What kind of image is formed when the object is further than the focal length away from a lens? |
A real, inverted image |
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What kind of image is formed when the object is closer than the focal length from a lens? |
A virtual, upright image |
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What is the symbol for distance between an object and the lens axis? |
u |
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What does v represent? |
The distance between the image and the lens axis |
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When is v positive? |
When the image is real |
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Why would v be negative? |
If the image was virtual |
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What is the lens equation? |
Where: • f is the focal length • u is the distance between the object and the lens axis • v is the distance between the image and the lens axis |
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What is an astronomical refracting telescope consist of? |
Two converging lenses |
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What does the objective lens do? |
Converges the rays from an object to form a real image inside the telescope. |
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What does the eye lens do? |
Acts as a magnifying glass on the real image by forming a magnified virtual image which can be observed. |
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What is the benefit of assuming that the light from objects in space is coming from infinity? |
The rays will be parallel to each other. |
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Where is the real image formed when using an astronomical refracting telescope? |
On the focal plane of the objective lens. |
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What does a telescope being in normal adjustment imply? |
The principle focus of the objective lens is in the same position as the principle focus of the eye lens. |
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Where does the object appear to be when the telescope is in normal adjustment? |
At infinity |
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What is the symbol for the focal length of the objective lens? |
fₒ |
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What is the symbol for the focal length of the eye lens? |
fₑ |
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What is the length of the telescope equal to? |
fₒ+fₑ Focal length of the objective lens added to the focal length of the eye lens. |
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What does a ray diagram for an Astronomical Refracting Telescope look like? |
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How do you draw a ray diagram for an astronomical refracting telescope? |
1. Draw a straight, non-axial ray passing through the centre of the objective lens which ends at the eye lens axis 2. Draw two parallel, non-axial rays on either side of the first ray which end at the objective lens axis 3. Draw a straight line from where these rays meet the objective lens axis, so that these rays all cross at the same point on the focal plane and reach the eye lens axis 4. Draw a dashed construction line through the point the rays cross and the centre of the eye lens, continue the three rays drawn so the are refracted at the eye lens axis parallel to the construction line. 5. Extending these rays backwards using dashed lines shows the virtual image being formed at infinity. |
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What is the symbol for magnification of a telescope? |
M |
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What is the angular magnification of a telescope? |
The angle subtended by the image at the eye divided by the angle subtended by the object at the unaided eye. |
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Which two formulas can be used to find angular magnification? |
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What do reflecting telescopes use to reflect and focus light? |
Mirrors |
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What kind of mirror is used to form a real object in a reflecting telescope? |
A parabolic concave primary mirror |
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How does a parabolic concave primary mirror form a real image? |
By converging parallel axial rays from an object, at its focal point. |
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What kind of lens is used to magnify this image? |
An eye lens |
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Where is the focal point of the primary mirror? |
In front of the mirror |
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Why is the Cassegrain arrangement used? |
So that the observer doesn't block incoming light |
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What does the Cassegrain arrangement use? |
A secondary convex mirror |
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What does a ray diagram for an Cassegrain reflecting telescope look like? |
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What are CCDs used as? |
Sensitive light detectors |
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What do CCDs do? |
Capture images digitally |
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What are CCDs composed from? |
Postage stamp sized silicon chips divided into a grid of millions of identical picture elements (pixels). |
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Why are CCDs made from silicone? |
Silicone is a semiconductor with not many free electrons. |
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What happens when light is shined on a pixel? |
1. The silicone releases a number of electrons proportional to the light's intensity. 2. The electron is captured by a potential well underneath the pixel. |
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What happens when electrons build up in the potential wells? |
A pattern identical to the image formed on the CCD chip is created |
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What happens when the chip has been exposed to light for the desired amount of time? |
The charge is processed and converted into a digital signal which can then be sent to computers world wide. |
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What are photons? |
Discrete packets of electromagnetic radiation energy. |
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Why do photons release electrons in the silicone of the CCD? |
Because of the photoelectric effect |
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What is quantum efficiency? |
The percentage of incident photons which cause an electron to be liberated. |
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What is the average quantum efficiency of a CCD? |
70%-80% |
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What is the average quantum efficiency of the eye? |
About 1% |
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What is the resolving power of a telescope? |
A measure of how much detail you can see |
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What is the definition of resolving power? |
The resolving power of an instrument is the smallest angular separation at which it can distinguish two points. |
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What is resolution limited by? |
Diffraction |
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What happens when a beam of light passes through a circular aperture? |
A diffraction pattern of bright maxima and dark minima is formed |
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What is the central circle of the diffraction pattern called? |
The Airy disc |
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What is the minimum point at which two light sources can be distinguished? |
If the centre of the Airy disc from one light source is equal to or further then the first minima of the other source. |
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What is the Rayleigh Criterion? |
Where: • θ is the minimum angle that can be resolved (radians) • λ is the wavelength (metres) • D is the diameter of the aperture (metres) |
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What is θ measured in? |
Radians |
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What is λ measured in? |
Metres |
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What is D measured in? |
Metres |
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What is D for telescopes? |
The diameter of the objective lens or objective mirror |
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What is needed to see more detail? |
Larger lenses or mirrors. |
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What is chromatic aberration? |
Where glass refracts different coloured lights by different amounts meaning each colour focuses in a slightly different position causing a blurry image. |
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What colour light is refracted best by glass? |
Blue light |
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What does chromatic aberration look like? |
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What are four disadvantages to refracting telescopes? |
1. Chromatic aberration. 2. Bubbles and impurities can scatter light meaning faint objects aren't seen. It is difficult and expensive to build good quality, large, lenses. 3. Large lenses are heavy and can only be supported at the edges, meaning the shape can become distorted. 4. For a large magnification, the objective lens needs to have a long focal length. This means that the telescope would need to be very long leading to very large, expensive buildings being needed to house them. |
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What are the benefits of reflecting telescopes? |
1. Large mirrors of good quality are cheaper and easier to build then large lenses 2. They can be supported from underneath so they don't distort as much as lenses 3. They don't suffer from chromatic aberration |
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What is spherical aberration? |
If the shape of the mirror isn't perfect, parallel rays reflecting off different parts of the mirror won't converge onto the same point. |
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What does spherical aberration look like? |
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How can the secondary mirror in a Cassegrain telescope cause problems? |
• The mirror and supports will block incoming light • Some light will defract around the secondary mirror |
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What will the problems caused by the secondary mirror in a Cassegrain telescope lead to? |
A decrease in image clarity |
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What is the main feature of a radio telescope? |
A parabolic dish |
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What does the parabolic dish act in the same way as? |
The objective mirror of an optical reflecting telescope. |
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What can be used instead of a polished mirror? |
Wire mesh |
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Why can wire mesh be used? |
Long wavelength radio waves are unaffected by gaps |
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What happens to EM radiation when it reaches the dish? |
It is reflected and focused by the dish on an antenna which acts as a detector at the focal point. |
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What does a preamplifier do? |
Amplifies weak radio signals without distorting the signal. |
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What happens to the signal after it leaves the preamplifier? |
1. A second amplifier amplifies the signal further 2. It is then passed through a tuner 3. It is sent to a computer which produces a false-colour image of the detected radio signals |
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What is the purpose of passing the signal through a tuner? |
The tuner filters out any unwanted wavelengths |
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What do the different colours represent on a false-colour image? |
Different wavelengths or intensities |
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Why are most radio telescopes manoeuvrable? |
The telescope can track the source of the waves as the Earth rotates |
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How much longer then wavelengths of light are the wavelengths of radio waves? |
About a million times longer |
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What gives the resolving power of a telescope? |
The Rayleigh criterion |
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What is the resolving power of a single radio telescope worse than? |
The unaided eye |
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How do astronomers get around the issue of resolving power? |
By linking lots of radio telescopes together, combining the data to form a single computer image. |
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Are radio telescopes easier to make then optical telescopes? |
Yes |
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Why is making the parabolic dish out of wire mesh better than using optical reflectors? |
Mesh is cheaper and easier to use |
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Why does it not matter if there are imperfections in the shape of the dish? |
Longer wavelengths won't be affected as much by imperfections |
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What are infrared (I-R) and ultraviolet (U-V) telescopes similar to? |
Optical reflecting telescopes |
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How are I-R, U-V and optical reflecting telescopes alike? |
• All use a parabolic mirror set-up to focus radiation onto a detector • All use CCDs (or special photographic paper) as radiation detectors |
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Do the mirrors in I-R telescopes need to be more perfectly shaped then the mirrors in optical telescopes, or less? |
Less |
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Do the mirrors in U-V telescopes need to be more perfectly shaped then the mirrors in optical telescopes, or less?
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More |
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What is a common problem with infrared telescopes? |
They produced their own infrared radiation due to their temperature. |
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How do you reduce the effect of I-R telescopes producing infrared radiation? |
Lower the temperature |
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What can be used to lower the temperature of an I-R telescope? |
• Liquid helium • Refrigeration units |
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What do X-ray telescopes have a different structure to other telescopes? |
X-Rays don't reflect straight off surfaces |
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What usually happens to an X-ray when it strikes a surface? |
It is absorbed by the material or passes straight through |
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When do X-rays reflect off mirrors? |
When they just graze the surface |
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How can X-rays be brought to focus on a detector? |
By having a series of nested mirrors which gradually alter the direction of the X-Rays. |
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What is an X-ray telescope called? |
A grazing telescope |
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What can be used to detect X-rays? |
• Modified Geiger counters • Fine wire mesh • Highly sensitive CCD cameras |
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Why is the atmosphere a problem when positioning telescopes? |
It only lets through certain wavelengths of electromagnetic radiation |
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Which wavelengths of electromagnetic radiation can pass through our atmosphere? |
• Visible light • Radio waves • Some U-V and I-R |
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Why can optical and radio telescopes be used from the surface of the Earth? |
Because the atmosphere is transparent to these wavelegths |
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What absorbs infrared radiation in the atmopsphere? |
Water vapour |
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Where is the best place to put an I-R telescope? |
• High, dry places • Such as Mauna Kea volcano in Hawaii |
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How can telescopes be sent to higher altitudes? |
By strapping them to weather balloons or aeroplanes |
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What is the ideal location for U-V, X-ray and I-R telescopes? |
Above the atmosphere, in orbit around the Earth |
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What is the resolving power of a telescope limited by? |
1. The Rayleigh criterion 2. The quality of the detector |
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Why does the Rayleigh criterion limit the resolving power of a telescope? |
• Resolving power depends on wavelength of radiation and the diameter of the objective mirror or dish. • For the same size of dish, a U-V telescope has better resolving power than a radio telescope, as the radiation it detects has a shorter wavelength. |
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Why does the quality of the detector limit the resolving power of a telescope? |
• The resolving power of the telescope is limited by the resolving power of the detector. |
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What affects the resolving power of a CCD detector? |
How many pixels there are |
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What affects the resolving power of a wire mesh X-ray detector? |
How fine the wire mesh is |
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What is the collecting power of a telescope proportional to? |
Collecting area |
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What is the collecting area of a radio, optical, U-V or I-R telescope? |
The area of the objective mirror or dish |
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What is the collecting area of an X-ray telescope? |
The size of the opening through which X-rays can enter the telescope |
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Which telescope generally has a lower collecting power then other telescope? |
X-ray telescopes |
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What is the advantage to a telescope having a bigger dish or mirror? |
• Collects more energy from an object in a given time • Creates a more intense image • The telescope can observe fainter objects |
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What is collecting power? |
Energy collected per second |
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What is collecting power proportional to? |
dish diameter² |
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What is parallax? |
A meausure of how much a nearby object appears to move in relation to a fixed background, due to the observers motion |
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How can the distance to nearby stars be calculated? |
By observing how they move relative to stars which are so distant they appear not to move at all |
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How is the distance to nearby stars calculated? |
By comparing the position of the nearby star in relation to the background stars at different parts of the Earth's orbit |
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What is parallax measured in terms of? |
The angle of parallax |
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How do you measure the angle of parallax? |
1. Observe the position of the star at either end of the Earth's orbit (6 months apart) 2. The angle of parallax is half the angle that the star moves in relation to the background stars. |
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What does a greater parallax angle imply? |
The object is closer to the Earth |
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What does a diagram for angle of parallax look like? |
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What do you need to calculate the distance to the nearby star? |
• The angle of parallax • The radius of the Earth's orbit |
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What formula can you use to work out the distance to the star when θ is small? |
Where: • d is the distance to the star • r is the radius of the Earth's orbit • θ is the angle of parallax (radians) |
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What is the angle of parallax measured in? |
Radians |
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How do you convert the angle in degrees into the angle in radians? |
angle in degrees x 2π/360 |
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What is the unit of distance for parallax? |
Parsec (pc) |
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What is a parsec? |
The distance of an object from Earth if its angle of parallax is equal to 1 arcsecond = (1/3600)° |
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When is a star exactly one parsec away? |
If the angle of parallax, θ = 1 arcsecond |
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What is one arcsecond equal to? |
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What is 1 pc in metres? |
3.08 x10^16 m |
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What is 1 Mpc (megaparsec) in parsecs? |
1x10^6 pc |
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What is 1 astronomical unit (AU) defined as? |
The average distance between the Earth and the Sun |
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What is the value of 1 AU? |
1.50 x10¹¹ m |
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What do all electromagnetic waves travel at in a vacuum? |
The speed of light |
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What is a light year (ly)? |
The distance that electromagnetic waves travel through a vacuum in one year |
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What is 1 light year in metres? |
9.46x10^15 m |
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What is 1 pc in light years? |
3.26 ly |
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Why do astronomers search for distant galaxies? |
So they can see what the universe was like billions of years ago |
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Not including parallax, what are other ways of measuring distance? |
1. Standard Candles 2. Redshift 3. Quasars |
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Why is important to have several different ways of measuring distance? |
1. Measuring the same distance using several methods makes the measurement more reliable 2. Some methods only work for certain distances |
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What is luminosity? |
The total amount of energy emitted in the form of electromagnetic radiation each second |
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What is luminosity also known as? |
The power output of a star |
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What is luminosity measured in? |
Watts (W) |
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What is the Sun's luminosity? |
4x10^26 W |
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What is the intensity (I) of an object? |
• The power received from it per unit area of the Earth. • It is the effective brightness of an object |
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What is apparent magnitude? |
A measure of brightness of an object viewed from Earth |
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What is the apparent magnitude of a star based on? |
• It's power output • It's distance from the Earth |
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What is the difference in intensity of a magnitude 1 star compared to a magnitude 6 star? |
A magnitude 1 star has an intensity 100 times greater than a magnitude 6 star. |
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What can be used to calculate the brightness ratio between two stars? |
Where: • I2 is intensity of star 2 • I1 is the brightness of star 1 • m1 is the apparent magnitude of star one • m2 is the apparent magnitude of star 2 |
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Do the brightest objects in the sky have a negative or positive apparent magnitude? |
Negative |
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What is the apparent magnitude of the Sun? |
-26.7 |
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What is the apparent magnitude of the dimmest objects observed by the Hubble Space Telescope? |
Around 30 |
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Which star is said to have an apparent magnitude of 0? |
Vega |
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What is the definition of absolute magnitude (M) of an object? |
What an object's apparent magnitude would be if it were 10 parsecs away from the Earth. |
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What is the absolute magnitude of a star based on? |
Luminosity |
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What formula gives the relationship between M and m? |
Where: • m is the apparent magnitude • M is the absolute magnitude • d is the distance from Earth (pc) |
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What is d measured in? |
Parsecs |
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What are standard candles? |
Objects that you can calculate the brightness of directly, without knowing their distance. |
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What is a good example of a standard candle? |
A type 1a supernovae |
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How can you use a type 1a supernova to work out distance? |
1. All type 1a supernovae have the same peak absolute magnitude 2. You can work out the apparent magnitude from Earth 3. The magnitude formula can be rearranged to find d by substituting in the values for m and M |
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What is a black body? |
An object with a pure black surface which emits radiation strongly and absorbs all light incident on it. |
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What is the definition of a black body? |
A black body is a body that absorbs all electromagnetic radiation of all wavelengths and can emit all wavelengths of electromagnetic radiation. |
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Why do blackbodies emit a continuous spectrum of electromagnetic radiation? |
They emit all wavelengths of electromagnetic radiation |
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What is black body radiation? |
A continuous spectrum of electromagnetic radiation |
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What are black body curves? |
A graph of radiation power output against wavelength |
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What do black body curves vary with? |
Temperature |
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What do all black body curves have in common? |
Their shape |
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What can black body radiation curves be used for? |
Making estimations about an objects temperature and other properties |
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How does a higher surface temperature for a black body star affect the maximum wavelength? |
The higher the temperature, the shorter the wavelength. |
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What is Wien's displacement law? |
λₐₓT=0.0029 m K Where: • λₐₓ is the peak wavelength (m) • T is temperature • 0.0029 is the Wien constant (metres kelvin) |
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What is λₐₓ measured in? |
metres (m) |
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What is T measured in? |
Kelvin (K) |
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What is the Wien constant measured in? |
Metres Kelvin (m K) |
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What are the properties of a hot black body? |
• Emits more radiation of shorter wavelengths • Has a higher total power output (assuming surface area is the same) |
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Why might a hotter star not appear to be as bright as a cooler star? |
Because it is mostly emitting radiation that isn't in the visible part of the electromagnetic spectrum. |
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How can you estimate a source's black-body temperature? |
By using Wien's displacement law to estimate the surface temperature of the source using the peak wavelength. |
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What is the power output of a star related to? |
Surface temperature and surface area of a star |
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What is the power output of a star proportional to? |
The fourth power of the star's temperature |
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What is the luminosity of a star directly proportional to? |
The stars surface area |
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What is Stefan's law? |
P=σAT^4 Where: • P is the power output of the star in (W) • σ is the Stefan constant • T is the surface temperature (K) • A is the surface area (m²) |
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What is the Stefan constant equal to? |
5.67x10^-8 Wm^-2K^-4 |
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What is intensity? |
The power of radiation per square metre |
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What is the inverse square law? |
Where: • I is intensity (Wm-²) • P is the power output of the star (W) • d is the distance from the star (m) |
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What is intensity measured in? |
Watts per meter squared (Wm-²) |
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What do you need to assume to use the inverse square law? |
• The star is spherical • It gives out an even amount of power in every direction |
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What is the only way in which electrons exist in atoms? |
In certain well-defined energy levels |
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What is the ground state? |
The lowest energy level an electron can be in |
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What value represents the ground state? |
n=1 |
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When is an atoms said to be excited? |
When one or more of it's electrons is at an energy level higher than ground state. |
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How do electrons move up energy levels? |
By absorbing photons |
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Why can the energy of each photon absorbed only take a certain value? |
Because photons are discrete energy packets with a set value |
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What is the energy of a photon given by? |
E=hf Where: • h is plank's costant • f is frequency |
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What is produced by splitting the light from a star using a prism or diffraction grating? |
A spectrum |
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Why do you get absorption lines in a spectrum? |
Absorption lines are formed when radiation from a star passes through a cooler gas such as those found in the atmosphere |
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What happens when electrons de-excite? |
A wavelength of light equal to the energy of the photon absorbed is emitted and radiated in all directions |
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What does wavelengths of light being emitted in all directions cause? |
• A reduction in the intensity of the light hitting Earth. • Dark lines in the spectrum corresponding to the absorbed wavelengths |
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What is the intensity of the absorption line? |
How dark it is |
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What does a more intense absorption line at a particular wavelength imply? |
That more radiation of that wavelength has been absorbed |
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What are wavelengths corresponding to the visible part of hydrogen's line absorption spectrum cause by? |
The electrons in atomic hydrogen moving between the first excitation level and higher energy levels. |
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What is value given to the first excitation level? |
n=2 |
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What does electrons moving in atomic hydrogen between n=2 and higher energy levels lead to? |
The Balmer series |
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When do you see hydrogen Balmer absorption lines? |
In stellar spectra where light emitted by the star has been absorbed by hydrogen atoms in the atmosphere as light passes through it. |
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What condition needs to be met for a hydrogen absorption line to occur in the visible part of a stars spectrum? |
Electrons in the hydrogen atom already need to be in the n=2 state |
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When are electrons naturally in the n=2 state? |
At high temperatures |
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What happens when electrons collide in the n=2 state? |
They gain extra energy |
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What happens if the temperature is too high? |
The majority of electrons will reach the n=3 level so there will be less Balmer transitions. |
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What does the intensity of the Balmer lines depend upon? |
The temperature of the star |
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What is the range in temperature on a Hertzsprung Russell diagram? |
50 000K - 3000K |
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What are spectral classes? |
Groups which stars are classified into according to temperature and relative strength of certain absorption lines |
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How many main spectral classes are there? |
Seven |
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What are the seven main spectral classes in order of decreasing temperature? |
O, B, A, F, G, K, M |
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Which stars are the hottest? |
O class stars |
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What colour do O class stars appear to be? |
Blue |
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Which class of stars are coolest? |
M class stars |
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What colour do M class stars appear to be? |
Red |
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What class of star is the Sun? |
G class |
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What is the temperature of stars in the O spectral class? |
25 000K - 50 000K |
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Which absorption lines are found in the O spectral class? |
Strongest: • Helium-plus ion (He^+) absorptions • Helium atom (He) absorptions Weak: • Hydrogen Balmer (H) lines |
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What colour do B class stars appear to be? |
Blue |
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What colour do A class stars appear to be? |
Blue-White |
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What colour do F class stars appear to be? |
White |
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What colour do G class stars appear to be? |
Yellow-White |
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What colour do K class stars appear to be? |
Orange |
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What is the temperature of stars in the B spectral class? |
11 000K - 25 000K |
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What is the temperature of stars in the A spectral class? |
7500K - 11 000K |
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What is the temperature of stars in the F spectral class? |
6000K - 7500K |
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What is the temperature of stars in the G spectral class? |
5000K - 6000K |
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What is the temperature of stars in the K spectral class? |
3500K - 5000K |
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What is the temperature of stars in the M spectral class? |
< 3500K |
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Which absorption lines are found in the B spectral class? |
• Helium atom (He) absorptions • Hydrogen (H) absorptions |
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Which absorption lines are found in the A spectral class? |
• Strong hydrogen Balmer (H) lines • Metal ion absorptions |
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Which absorption lines are found in the F spectral class? |
• Strong metal ion absorptions |
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Which absorption lines are found in the G spectral class? |
• Metal ion absorptions • Metal atom absoptions |
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Which absorption lines are found in the K spectral class? |
• Neutral metal ion spectral lines |
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Which absorption lines are found in the M spectral class? |
• Neutral atom spectral lines •Molecular band absorptions from compounds like titanium oxide (TiO) |
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What is titanium oxide present in M class stars? |
Because these stars are cool enough for molecules to form. |
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What is the Hertzsprung-Russell diagram? |
A graph of absolute magnitude against temperature/spectral class. |
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What does the Hertzsprung-Russell diagram look like? |
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Starting from the top, what is the scale for absolute magnitude on the Hertzsprung-Russell diagram? |
-10, -5, 0, 5, 10 ,15 |
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Where can main sequence stars be found on the Hertzsprung-Russell diagram? |
In the central, long diagonal band. |
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Where can red giants and red supergiants be foundon the Hertzsprung-Russell diagram? |
In the top right corner of the Hertzsprung-Russell diagram. |
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Where can white dwarves be found on the Hertzsprung-Russell diagram? |
In the bottom left corner of the Hertzsprung-Russell diagram. |
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What is the absolute magnitude of the sun? |
5 |
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What kind of star is the sun? |
A main-sequence star |
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What are main-sequence stars? |
Stars that are undergoing their longest, most stable phase where they are fusing hydrogen into helium. |
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Why can we presume red giants have a huge surface area? |
Stephan's Law, they have: • High luminosity and • Low surface temperature |
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What are red giants? |
Stars that have moved off the main sequence, and fusions reactions other then hydrogen to helium are also happening in them. |
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Why can we presume white dwarfs are very small?
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Stephan's Law, they have:
• Low luminosity and • High surface temperature |
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What are white dwarf stars? |
Stars that are at the end of their lives, where all of their fusion reactions have stopped and they are slowly cooling down. |
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How do all stars start? |
• As clouds of dust and gas (leftover from supernovae). • Denser clumps of the cloud slowly contract under gravity |
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What happens when clumps of cloud get dense enough? |
• The cloud fragments into regions called protostars • They continue to contract and heat up. |
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What happens when the temperature at the centre of a protostar reaches a few million degrees? |
Hydrogen nuclei start to fuse together and form helium. |
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What does the production of helium do? |
• Releases enormous amounts of energy • Creates enough radiation pressure to stop gravitational collapse |
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When has a star reached the main sequence? |
When it is producing helium |
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What happens once a star has reached the main sequence stage? |
•It will remain realitivly unchanged until it runs out of hydrogen to fuse into helium |
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What is core hydrogen burning? |
When the pressure produced from hydrogen fusion in a stars core balances the gravitational force trying to compress the star. |
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What happens when the hydrogen in the core runs out? |
1. Nuclear fusion stops 2. Outward pressure stops 3. The core contracts and heats up under the weight of the star 4. Outer layers expand and cool 5. The star becomes a red giant |
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What is shell hydrogen burning? |
When the material surrounding the core still contains lots of hydrogen which then has it's temperature raised by the contracting core causing the hydrogen to fuse. |
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What is core helium burning? |
The core continues to contract after shell hydrogen burning ntracts until it gets hot and dense enough for helium to fuse into carbon and oxygen |
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What does core helium burning do? |
Releases huge amounts of energy, which pushes the outer layers of the star further outwards. |
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What happens when the helium runs out? |
The carbon-oxygen core contracts again and heats a shell around it so that helium can fuse in this region |
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What is shell helium burning? |
When helium fuses in a shell around a carbon-oxygen core |
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What happens in low mass stars which won't go through shell burning? |
The carbon-oxygen core won't get hot enough for further fusion so it continues to contract under it's own weight. |
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What happens once the core has shrunk to Earth size? |
•Electrons exert electron degeneracy pressure to stop further collapse • The helium becomes increasingly unstable as the core contracts. |
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What happens when the star pulsates and ejects it's outer layers into space as a planetary nebula? |
A hot dense core is left behind |
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What is the name of this hot dense solid core? |
A white dwarf |
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What happens to white dwarfs? |
They cool down and fade away |
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How will a sun like star move around the H-R diagram as it evolves? |
1. Start in the main sequence 2. Become a red giant when it runs out of hydrogen. 3. When helium fusion is complete, ejects its outer layers to became a white dwarf |
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Do high mass stars have a longer or shorter life then lower mass stars? |
Shorter |
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Why do high mass stars have a shorter life? |
Because they burn their fuel more quickly and don't spend as long as main sequence stars |
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What happens to a high mass star during the core burning to shell burning stages? |
They will fuse more then helium, building up layers in an onion like structure. |
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What is the last element that can be fused? |
Iron |
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What happens when the core of a high mass star runs out of fuel? |
It starts to contract and forms a white dwarf core. |
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What happens if the mass of the core is more than 1.4 times the mass of the sun? |
Electron degeneracy won't stop the core contracting |
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How does a supernova occur? |
1. The core of the star continues to contract. 2. Outer layers of the star fall in and rebound off the core causing huge shockwaves. 3. The shockwaves cause the star to cataclysmically explode. |
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What happens to the core of a high mass star after a supernova? |
It becomes either a neutron star or a black hole |
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What happens to the absolute magnitude of a star exploding in a supernova? |
It experiences a brief and rapid increase, temporary outshining the rest of the galaxy before fading. |
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How are neutron stars formed? |
As the core of the star contracts, electrons in the core material get compressed onto atomic nuclei and combine with protons to form neutrons and neutrinos. If the stars core is between 1.4 and 3 solar masses, the star can't contract any further so the core collapses to become a neutron star made of neutrons. |
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What value for solar mass must a star's core have if it to become a neutron star? |
Between 1.4 and 3 solar masses |
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What is the approximate density of a neutron star? |
Incredibly dense, 4x10^17 kgm-³ |
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What is the size of a neutron star? |
Very small, diameter of about 20km |
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How fast can neutron stars rotate? |
Very fast, up to 600 times per second |
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What are pulsars? |
Two rotating beams of radio waves which are emitted as a neutron star rotates |
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What happens if the core of a star is more than 3 times the suns mass? |
The core will contract until neutrons are formed at which point the gravitational force on core is greater so the core will further collapse into an infinitely dense point. |
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What is escape velocity? |
The velocity that an object would need to travel at to have enough energy to escape a gravitational field. |
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What is a black hole? |
An object whose escape velocity is greater than the speed of light |
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What is the event horizon? |
The distance at which the escape velocity is equal to c, so light has just enough kinetic energy to overcome the gravitational pull. |
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What is the name given to the event horizon of a black hole? |
The Schwarzschild radius |
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What happens inside the Schwarzschild radius? |
Everything has no choice but to travel further into the black hole |
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What is believed to be at the centre of every galaxy? |
A supermassive black hole |
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What is the size of a supermassive black hole? |
more than x10^6 times more massive then our sun |
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What happens as a supermassive black hole consumes stars close to them? |
They produce intense radiation, making the centre of galaxies very bright |
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What is Schwarzschild radius? |
Where: • Rs is the Schwarzschild radius (m) • G is the gravitational constant (Nm²kg-²) • M is the mass of black hole (kg) • c is the speed of light in a vacuum (ms-¹) |
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What is the Schwarzschild radius measured in? |
metres (m) |
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What is the Doppler effect? |
A change in frequency and wavelength as an object changes it's relative position to you. |
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Why does the frequency and wavelength change? |
Because the waves bunch together in front of the source and stretch out behind it. |
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What happens when a light source moves away from us? |
The wavelength of the light reaching us becomes longer and the frequencies become lower, shifting the light we receive towards the red end of the electromagnetic spectrum. |
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What is red shift? |
When the light is being shifted towards the red end of the electromagnetic spectrum |
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What is the effect of red shift? |
The light we receive from a star moving away from us appears redder than the actual light emitted by the star. |
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When does a light undergo blue shift? |
When the light source is moving towards us |
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What does the amount of shift depend upon? |
The velocity of the source |
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When are more waves shifted? |
When the velocity is higher |
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What is recessional velocity? |
The velocity that something is moving away from us with |
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What formula can be used to determine red shift? |
Where: • z is red shift • v is the recessional velocity of the source in line with the observer (ms-¹) • c is the speed of electromagnetic radiation in a vacuum (ms-¹) |
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What does << mean? |
Much less |
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When will the equation show blue shift? |
When red shift is negative, v<0 |
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What is an increase in wavelengths always referred to as? |
Red shift |
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What is an decrease in wavelengths always referred to as? |
Blue shift |
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How does red shift affect a radiation spectrum? |
The entire spectrum, including absorption lines, will be shifted |
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How can you work out how much a star has been red shifted by? |
By looking at the wavelength or frequency of known absorption lines (like the Balmer series) in the observed spectrum and comparing them to the wavelengths or frequencies they should be. |
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What formula gives red shift when comparing frequencies? |
Where: • Δf is the difference between emitted and observed frequencies • f is the frequency of light emitted by the star |
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What formula gives red shift when comparing wavelength? |
Where: • Δλ is the difference between emitted and observed wavelengths • λ is the wavelength of light emitted by the star |
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What is cosmological red shift? |
The idea that space is expanding ans light waves are being stretched along with it. |
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How was the idea for cosmological red shirt developed? |
By the fact that all distant galaxies show red shift, they are moving away from us |
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What is the cosmological principle? |
The idea that no part of the universe is any more special than any other |
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What two ideas make up the cosmological principle? |
On a large scale the universe is: • homogeneous • isotropic |
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What does homogeneous mean? |
Every part is the same as every other part |
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What does isotropic mean? |
• Everything looks the same in every direction - there is no centre |
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What was the steady-state universe theory? |
The universe is infinite in both space and time and static |
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Who was the first scientist to realise the universe is expanding? |
Edwin Hubble |
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How did Hubble realise the universe is expanding? |
By using type 1a supernovae as standard candles to calculate distances to galaxies and measuring their red shift. |
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What does the recessional velocity of a galaxy depend upon? |
How far away they are |
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What does plotting recessional velocity against distance show? |
They are proportional, which suggests the universe is expanding |
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When is a galaxy travelling faster away from us? |
When it is further away |
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What is Hubble's law? |
v=Hd Where: • v is recessional velocity (kms-¹) • H is the Hubble constant (kms-¹Mpc-¹) • d is the distance (Mpc) |
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Why is the universe expanding and cooling down? |
Because it's a closed system |
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What is the main principle of the Big Bang Theory? |
The universe started off hot and dense (perhaps an infinitely hot, infinity dense point) and has been expanding ever since. |
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In what way is the universe expanding? |
Uniformly in all directions |
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What can be used to estimate the age of the universe? |
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What is dark energy? |
A type of energy that fill s the whole of space |
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What is explained by dark energy? |
Why the universe was decelerating but is now accelerating. |
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What is cosmic microwave background radiation? |
Electromagnetic radiation that was produced in the very early universe and, since the wavelength has been stretched with the expanding universe, is now detectable in the microwave region. |
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What are some properties of CMBR? |
• Perfect black body spectrum corresponding to a temperature of 2.73K. • Isotropic and homogeneous which agrees with the cosmological principle. • Tiny fluctuations in temperature due to tiny-energy density variations in the early universe. |
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What does CMBR show? |
The Doppler effect, indicating the Earth's motion through space. |
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What is the Milky Way rushing towards at over a million miles an hour? |
An unknown mass labled the Great Attractor |
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How does the Big Bang model explain the abundance of helium and hydrogen in the universe? |
The early universe was very hot, so it must have been hot enough for hydrogen fusion to occur. Together with the theory of the synthesis of heavier elements in stars, the abundances of all elements can be accounted for. |
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What are spectroscopic binary stars? |
Two stars that orbit each other yet through a telescope can only be resolved as a single star. |
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How can you tell that these stars are binary stars? |
Because the lines in the spectra show a binary star system |
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What is an eclipsing binary system? |
A system whose orbital plane lies almost in our line of sight, so the stars eclipse each other as they orbit. |
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How can the orbital period be calculated? |
By observing how absorption lines in the spectrum change with time. |
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What is the red shift observed when both stars in a binary system are moving perpendicular to our line of sight? |
• There is no red shift • There is only one absorption line for the whole system |
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What is the red shift observed when both stars in a binary system are moving along our line of sight? |
• One star shows maximum red shift • One star shows maximum blue shift • There are two absorption lines in the spectrum, one red shifted and one blue shifted. |
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What happens to the separation between the absorption lines as two binary stars orbit each other? |
It goes from zero to maximum separation and back to zero in half a period. |
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What happens to the apparent magnitude of an eclipsing binary star system? |
The apparent magnitude will drop when the stars eclipse each other. |
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When will the apparent magnitude decrease most in an eclipsing binary star system? |
When the dimmer star passes in front of the brighter star |
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What are quasars? |
The most distant objects seen |
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What are the properties of quasars? |
• Very active radio sources • Brightest objects in the universe • About the size of the solar system |
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What do quasars sometimes do? |
Shoot out jets of material |
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What are active galactic nuclei? |
Very powerful galactic nucleus centred around a super massive black hole which ejects huge amounts of material from its nuclei. |
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What is a galaxy containing an active galactic nuclei known as? |
An active galaxy |
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What is a black whole in an active galactic nucleus surrounded by? |
A doughnut shaped mass of whirling gas falling into it, which emits matter and radiation. |
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What produces jets of matter and radiation streaming out from the poles? |
Magnetic fields |
