Astronomy Second Midterm - Chapt. 14

Front 1

Will the MW galaxy every use up all its gas?

Back 1

Yes in star formation...everything thing is being turned into white dwarfs, neutron stars, and black holes)

Front 2

What was the first white dwarf found?

Back 2

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)

Front 3

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?

Back 3

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

Front 4

Is Sirius B stable?

Back 4

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

Front 5

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?

Back 5

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

Front 6

What is meant by White dwarf supernova being an important standard candle in astronomy?

Back 6

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

Front 7

When a white dwarf super novas...what is left at the end?

Back 7

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

Front 8

How can we detect neutron stars if they are so faint because they are very tiny?

Back 8

-Pulsars (pulses of beamed radiation)

Front 9

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?

Back 9

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

Front 10

The white dwarf the sun leaves behind will be composed primarily of?

Back 10

A mix of C and O

Front 11

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?

Back 11

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

Front 12

How is the timing of pulsar pulses determined for neutron stars? Why do these pulses of radio waves exist for neutron stars?

Back 12

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

Front 13

All neutron stars will appear as pulsars from Earth?

Back 13

FALSE

-it depends on the angle in which the radiation is being beamed at

Front 14

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?

Back 14

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

Front 15

Is the singularity at the centre of a black hole real?

Back 15

Not clear

The singularity is a prediction of classical theory of gravity (Einstein's General Relativity)

Front 16

What is an event horizon?

Back 16

"one way doors" you can enter but can never leave
-hide singularities
-event horizons are small (as measured by Schwarzschild radius)

Front 17

Are there any naked singularities? Singularities that do not contain an event horizon?

Back 17

Unknown
-Mathematically proven that all event horizons must contain singularities, but not sure about the inverse

Front 18

Why can't Newtonian gravity be used to describe black holes?

Back 18

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

Front 19

What was Einstein's theory of General Relativity? What can GR theory predict?

Back 19

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

Front 20

How has GR theory been tested in the weak field (solar system)? How has it been tested in the strong field limit (black holes)?

Back 20

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

Front 21

Why do objects of different mass fall at the same rate?

Back 21

GR says that the equivalence b/w gravitational mass and inertia allow for this

Front 22

Astronaut Floating in space Example. IF an astronaut is moving in a particular path in space, how can we explain his movement?

Back 22

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"

Front 23

How much space time is curved is proportional to what?

Back 23

The amount of curvature is proportional to the mass enclosed

Black holes are the extreme forms of this curvature

Front 24

What is the meaning of an event horizon?

Back 24

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

Front 25

Could you survive passage through an event horizon?

Back 25

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

Front 26

Are black holes even real? Do we have any evidence that they exist?

Back 26

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

Front 27

If an alien is hovering above the Sun and causes it to be instantaneously dark, what will we see on Earth?

Back 27

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

Front 28

How does a black hole (Cygnus X1) emit x-rays?

Back 28

They don't!
-the gas in the accretion disk around it emits the x-rays

Front 29

The Sun is instantaneously transformed into a one solar mass black hole. What will be the effect on the Earth's orbit?

Back 29

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)

Front 30

Will a black hole freely orbiting the MW likely interact (swallow) anything, besides a little bit of interstellar gas and dust?

Back 30

NO

-all the ones we know are in binary systems, and get material via mass transfer