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30 Cards in this Set
- Front
- Back
Will the MW galaxy every use up all its gas?
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Yes in star formation...everything thing is being turned into white dwarfs, neutron stars, and black holes)
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What was the first white dwarf found?
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Sirius B (1 solar mass packed into the size of Earth, 10,000x's fainter than Sirius A, but 2x's higher temp) - white dwarf now (came from a 5 solar mass B type star that is older than Sirius A)
-so VERY DENSE - 3 million grams/c^3 Sirius A (2 solar mass main sequence star - brightest star in night sky in visible & infrared light) These stars are visual binary (orbit around one another) |
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The more massive the white dwarf is, the smaller or larger it is in size? If a white dwarf gets smaller as you pile more mass, will the radius ever go to zero?
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The smaller it is! Similar to Jupiter
-So a 1 solar mass white dwarf will be larger than a 1.3 solar mass dwarf Yes, as you pile on more mass, if you go beyond the Chandrasekhar limit, you will have a supernova which will turn into a neutron star |
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Is Sirius B stable?
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YES its a white dwarf, so supported by electron degeneracy pressure
-it is also VERY dense, and ED pressure is independent from temp. -So high density, many electrons looking for unoccupied states, but many more electrons that states, so electrons constantly moving and at high speeds to support the white dwarf |
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White dwarfs in binary stars can receive mass transfer from a larger mass companion star...if this pushes the white dwarf over the C - limit, what are the possible outcomes?
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1) Nova - mass collects on surface of white dwarf, leads to a burst of H-fusion near surface
2) Supernova - turns to a neutron star |
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What is meant by White dwarf supernova being an important standard candle in astronomy?
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A standard candle = an easily identified object which has a known luminosity (white dwarf supernova always has the same peak luminosity cause its always from 1.4 solar mass)
- white dwarfs are standard candles because they always result from the explosion of a 1.4 solar mass white dwarf |
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When a white dwarf super novas...what is left at the end?
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A neutron star (combines the protons and electrons in the white dwarf together to form neutrons)
-supported by ED pressure -about the size of an asteroid |
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How can we detect neutron stars if they are so faint because they are very tiny?
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-Pulsars (pulses of beamed radiation)
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You are watching the MW 11 billion years ago as the first generation of stars was born. In this population of stars, which type of supernova will occur first?
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Type II (Massive Star Supernova)
- have a small life span b/c are very large and luminous and burn through their fuel quickly Type I (white dwarf supernova) -occur later b/c takes time for small older stars to form white dwarfs, mass transfer, and then supernova |
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The white dwarf the sun leaves behind will be composed primarily of?
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A mix of C and O
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Consider two white dwarfs, A & B, with the same temperature, and the same distance from the Earth. 'A' has a mass of 1 solar mass, and B has a mass of 1.3 solar masses, which will be brighter?
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A will be brighter.
-A is bigger b/c it has less mass (doesn't condense as much to increase its density to increase its ED pressure) -Because A is larger, it will be more luminous |
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How is the timing of pulsar pulses determined for neutron stars? Why do these pulses of radio waves exist for neutron stars?
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Determined by the rotation rate f the underlying neutron star
-"ice skater effect" - starts rotating very quickly and slows down rotational energy as it goes on (time b/w pulses grows longer) -The magnetic poles of the neutron star are not aligned with the rotation axis --> this creates beamed radiation |
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All neutron stars will appear as pulsars from Earth?
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FALSE
-it depends on the angle in which the radiation is being beamed at |
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Since neutron stars are supported by neutron degeneracy pressure, is there a limiting mass for neutron stars as there is for white dwarfs? What will happen if it exceeds this limit?
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Yes, but the limiting mass is a bit uncertain as the behaviour of matter in a neutron star is uncertain (~3 Solar masses)
-If >3 solar masses, nothing can stop its collapse, so will collapse without limit into a singularity (all of the mass is contained in a single point of zero volume and infinite density) - black hole |
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Is the singularity at the centre of a black hole real?
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Not clear
The singularity is a prediction of classical theory of gravity (Einstein's General Relativity) |
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What is an event horizon?
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"one way doors" you can enter but can never leave
-hide singularities -event horizons are small (as measured by Schwarzschild radius) |
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Are there any naked singularities? Singularities that do not contain an event horizon?
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Unknown
-Mathematically proven that all event horizons must contain singularities, but not sure about the inverse |
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Why can't Newtonian gravity be used to describe black holes?
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Newtonian gravity says that you can increase escape speed by:
-increasing mass, decrease radius -newtonian gravitation would predict that you can cross the event horizon In order for an object of mass M to reach an escape velocity to equal the speed of light it would have to have a Schwarzschild radius (radius of event horizon) -so can never be small enough to reach escape velocity...may get close but will just fall back down |
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What was Einstein's theory of General Relativity? What can GR theory predict?
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Gravity was not a force exerted by masses, but a curvature of space and time caused by masses
-simpler and more natural way for gravity to work Predicts the deflection of starlight -mass can bend light (like a lens) Ex. Eddington observed a total solar eclipse that showed deflection of star light |
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How has GR theory been tested in the weak field (solar system)? How has it been tested in the strong field limit (black holes)?
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Very well tested in solar system (weak field limit)
-Anomalous orbit of Mercury (wasn't obeying Newton's laws) -Deflection of starlight by Sun -Gravitational time dilation (deeper you are in gravitational field, the slower the clocks will run) Not very tested in strong field limit |
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Why do objects of different mass fall at the same rate?
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GR says that the equivalence b/w gravitational mass and inertia allow for this
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Astronaut Floating in space Example. IF an astronaut is moving in a particular path in space, how can we explain his movement?
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1) Newton - riding on a rocket
-the rocket is receiving instructions on the path to take -kind of like how the Earth is told by the Sun how to orbit around it...doesn't really make sense "action at a distance explanation" 2) Einstein - Gravity as Space (Time Curvature) -riding on a roller coaster -mass of Sun causes space time to curve -freely moving objects (planets) follow the straightest possible path that is allowed by the curvature of space time "local explanation" |
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How much space time is curved is proportional to what?
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The amount of curvature is proportional to the mass enclosed
Black holes are the extreme forms of this curvature |
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What is the meaning of an event horizon?
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Once you cross a black hole horizon, every possible future that you can experience necessarily ends at the singularity
-time appears to run slower closer to the event horizon |
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Could you survive passage through an event horizon?
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Yes if the black hole is sufficiently large.
-a solar mass black hole will rip you apart, but if you have a larger black hole then you have less gravitational difference so it may just stretch you out |
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Are black holes even real? Do we have any evidence that they exist?
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Best evidence: one at the centre of MW galaxy about 2-3 million solar masses (Cygnus X1)
-a blue super giant + x-ray source -blue super giants are too cool to make x-rays -binary system b/c spectral lines shift back and forth every 6 days |
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If an alien is hovering above the Sun and causes it to be instantaneously dark, what will we see on Earth?
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After 8 mins, a dark spot will appear in the center of the Sun, it will grow to consume the Sun in 2 seconds
This tells us that the size of the sun is not larger than 2 light seconds across |
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How does a black hole (Cygnus X1) emit x-rays?
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They don't!
-the gas in the accretion disk around it emits the x-rays |
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The Sun is instantaneously transformed into a one solar mass black hole. What will be the effect on the Earth's orbit?
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Nothing
-The Earth responds to the mass enclosed by its orbit - the size of the central mass does not matter (it will be condensed in the singularity) |
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Will a black hole freely orbiting the MW likely interact (swallow) anything, besides a little bit of interstellar gas and dust?
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NO
-all the ones we know are in binary systems, and get material via mass transfer |