Intro To Astro. Exam 3

Front 1

Why are binary stars useful?

Back 1

For finding masses of stars – can use Newton’s version of Kepler’s law

Front 2

What is a typical composition of stars?

Back 2

74% H, 25% He, and about 1% metals.

Front 3

What prevents a brown dwarf from undergoing nuclear fusion?

Back 3

Electron degeneracy pressure; brown
dwarfs are not massive enough to over come the degeneracy.

Front 4

What is the determinant stellar property in Spectral classification: OBAFGKM?

Back 4

Temperature

Front 5

Why do O stars have weak Hydrogen absorption lines?

Back 5

Most H is ionized since these stars are really hot

Front 6

Why do M stars have so many absorption lines?

Back 6

They are cool enough for molecular bonds to be intact so molecules produce a lot of absorption lines in these stars.

Front 7

Which type of stars are most common? Least common?

Back 7

Most common – low mass stars

Least common – high mass stars.

Front 8

Which stage lasts longest in star evolution?

Back 8

Main sequence

Front 9

What happens when a star exhausts its core hydrogen supply?

Back 9

The core has to shrink because there is no
more radiation pressure to oppose gravity. As the core shrinks, it heats up. It heats up enough for H to He fusion to begin in a shell around the core

Front 10

When a star exhausts its core hydrogen fuel, the core contracts but the star as a whole expands. Why?

Back 10

The core of a red giant contracts because there is no more hydrogen fusion to heat the core and
raise thermal pressure to resist gravity. However, the shell of hydrogen outside the core heats up to very
high temperatures (hotter than the core during the main-sequence phase), and hydrogen fusion starts
occurring in this shell around the core. The shell fusion provides the star with enough energy to expand.

Front 11

What is a helium flash?

Back 11

The onset of H to He fusion in the core; breaks electron degeneracy pressure.

Front 12

What is the main determinant of where a star will be on the main sequence and how long it will live?

Back 12

Mass

Front 13

How old do scientists estimate the sun to be?

Back 13

4.6-5 billion years old

Front 14

When the Sun dies, what type of star will it become?

Back 14

white dwarf

Front 15

What are white dwarfs made of? What supports them from collapsing?

Back 15

Carbon & oxygen

Electron degeneracy pressure

Front 16

Why is there a maximum mass for white dwarfs?

Back 16

B/c the WD is made up of degenerate
electrons, if its mass was larger than 1.4 Msun, the electrons would jiggle at higher than c, which is not
possible.

Front 17

Why are more massive white dwarfs smaller?

Back 17

The more massive WD have stronger gravity and are able to compress more.

Front 18

What is the size of a typical white dwarf compared to a neutron star?

Back 18

White dwarfs are typically the size of Earth and their mass close to the mass of the Sun. Neutron stars are
the size of a small city and their mass is up to 3 Msun.

Front 19

What is a nova?

Back 19

White Dwarf in a binary where the gas accretes onto the WD and periodically starts
fusion.

Front 20

What is an accretion disk? Why does it heat up?

Back 20

Disk of gas around a WD, Neutron star or Black hole
that is accreting onto one of these objects. It heats up due to friction – tidal forces are stronger closer to
the star and the different layers in the accretion disk “rub” off one another.

Front 21

What are the two types of Supernovae? What remains after each?

Back 21

Type I – WD supernova; nothing
remains afterwards. Type II – end of a life of a massive star; get neutron star if M < 3 Msun; black hole if
M > 3 Msun.

Front 22

What does a star’s interior look like right before it goes supernova?

Back 22

The central part looks like an onion
with different elements fusing in individual shells

Front 23

What causes the supernova explosion?

Back 23

Right before the supernova explosion, the core consists of Fe and it is supported by electron degeneracy
pressure.

However, this does not last long.

Gravity pushes electrons so close together to protons that they combine into neutrons.

At this point gravity wins and the core collapses into a ball of neutrons just a few km across.

Many neutrinos released at this time. The collapse pauses because now the neutrons provide their own degeneracy pressure. However, this halt is very brief. Gravitational collapse of the core releases so much
energy that it drives the outer layers of the star away in an explosion, i.e. supernova.

One part of the supernova explosion mechanism includes that the layers of the star outside of the core try to collapse onto the core and they “bounce” off the core, creating an explosion. In combination with this, the vast amount of neutrinos released produce a shock wave that drives the other layers of the star out into space. Note that in a case of a very massive star, there will be enough mass (i.e. gravity) for the star to overcome the neutron degeneracy pressure. In that case, the core will continue to collapse and will become a black hole. If mass of core > 3 Msun.

Front 24

Which event marks the beginning of a supernova?

Back 24

The sudden collapse of an iron core into a compact ball of degenerate neutrons. Release of neutrinos.

Front 25

What types of objects can be left afterwards?

Back 25

Neutron stars or black holes

Front 26

Which stars on an H-R diagram will someday go supernova?

Back 26

Most massive stars, like O and B type

Front 27

Elements produced during the evolution of a massive star:

Back 27

H, He, C, O, Ne, Mg, Si, Fe

Front 28

Why is iron the last element to be produced through fusion of stars?

Back 28

Because if iron were to fuse into the next element, it would require energy, instead of producing energy.

Front 29

Where are all the other heavier elements that we find on Earth and in the Sun produced?

Back 29

They are all produced during the supernova events. Supernova explosions provide so much energy that elements heavier than iron can also be produced.

Front 30

What is a neutron star? What is its size?

Back 30

Size of a city; as opposed to WD which is size of Earth.

Front 31

What happens when the gravity of a massive star is able to overcome neutron degeneracy pressure?

Back 31

The core will be compressed until it becomes a black hole

Front 32

Can we find planets around pulsars?

Back 32

Yes!

Front 33

Why would it be puzzling to find a 20-Msun main-sequence star and a white dwarf together in a binary
system? (Hint: both stars in a binary system form at the same time.)

Back 33

The more massive star is the 20 Msun
star and it should be the first one to evolve off the main sequence.

Front 34

How can we distinguish between black holes and neutron stars?

Back 34

The minimum mass of a black hole that forms during a massive star supernova is roughly 3 Msun. We can only detect neutron stars and black holes when they are in a binary system (they shine in X-rays when they are accreting matter). From the period of the companion star, we can use Newton’s version of
Kepler’s law and figure out the mass of the unseen companion star. If it is less than 3 Msun, it is a neutron star. If it is more than 3 Msun, it is a black hole.

Front 35

What is Schwarzschild Radius?

Back 35

is the radius of a sphere such that, if all the mass of an object is compressed within that sphere, the escape speed from the surface of the sphere would equal the speed of light.

Front 36

What is a singularity?

Back 36

It is the actual black hole inside the event horizon. Remember that the size of the black hole, or more
precisely the singularity is zero.

Front 37

What would happen to an object that comes close to a black hole?

Back 37

Gets stretched due to strong tidal
forces

Front 38

What would happen to the time when one gets close to a black hole?

Back 38

Slows down – gravitational time
dilation; think of wavelengths stretching (getting redshifted) as they try to escape the BH. If we measure
time based on frequency of light waves, then the time would slow down.

Front 39

What happens to light when it gets too close to a black hole?

Back 39

It looses energy, so it becomes redshifted.
It cannot slow down since it is light, so it can only loose energy.

Front 40

How do we know that black holes exist? (Hint: what observational characteristics do we see around
them?)

Back 40

We detect them in binary systems. Usually through X-ray bursts from accretion disks. If the mass
of the “unseen” companion in a binary system is > 3 Msun, then it is a BH.

Front 41

What makes us think that the star system Cygnus X-1 contains a black hole?

Back 41

It emits X rays characteristic of an accretion disk, but the unseen star in the system is too massive to be a
neutron star.

Front 42

What happens to your perception of time as you fall through the horizon?

Back 42

Your perception of time is the same as it always was. It is the outside observers that see your time slow
down.

Front 43

What does an object falling into a black hole appear like to an outside observer?

Back 43

Object falling into a black hole appears to an outside observer to freeze at the horizon (since its time
slows down closer it gets to the black hole – gravitational time dilation). The object will fade from view
as the light that object emits becomes more and more redshifted.

Front 44

Explain how accretion disks around black holes produce such high-energy photons?

Back 44

See above, friction
in the accretion disk caused by strong tidal forces (gravity)

Front 45

- What is time dilation? Explain how and why your measurements of time will differ from those of
someone moving by you

Back 45

Time runs slower for someone moving at relativistic speeds. Think of the example with Jackie in her spaceship and how you would measure the length of her flashlight bouncing
up and down the spaceship to be longer (slanted so longer) since she is moving. That means that light passed a larger distance, meaning that her clock must be ticking slower.

Front 46

length contraction: How will your measurements of the size of a spaceship differ if the spaceship is
moving by your compared to when it is at rest in your reference frame?

Back 46

The spaceship will look shorter in
the direction of motion. length=speed x time
Your clock records less time than Jackie’s as she passes from one end of your spaceship to the next. So you measure the length of her spaceship to be shorter than she would measure

Front 47

mass increase: How does the mass of an object moving by you compare to its rest mass?

Back 47

The mass of an object moving at relativistic speeds is larger. Think of this as the faster an object wants to
go, the harder it is for it to go so fast because it has a mass.

Front 48

Suppose you could take a trip to a distant star at a speed very close to the speed of light. How does relativity make it possible for you to make this trip in a reasonably short time? What will you find when you return home?

Back 48

Since you are traveling at a speed close to the speed of light, your length’s contract and your time runs
slower, so the distance to a star is shorter. Keep in mind that your perception of time would still be like it
was running as usual.

Front 49

In the rubber sheet analogy, why does the curvature of spacetime near a star depend on both the star’s mass and its size?

Back 49

The more massive stars make deeper funnels in a rubber sheet. Also the more dense the object is, the deeper the funnel.

Front 50

- What is gravitational lensing? According to general relativity, why does it occur?

Back 50

When a massive object bends spacetime around it, a light from a distant object behind it can be viewed and will be magnified.

Front 51

What is gravitational redshift? Can we see it around the Sun? Around a black hole?

Back 51

A strong gravitational field will make light’s wavelength stretch, i.e. it will be redshifted. We see these effects on the Sun and they are stronger (light is more redshifted) around more massive or denser objects like black holes.

Front 52

What is the mass-energy equivalence?

Back 52

It means that mass can be converted to energy and energy can be converted to mass. This is why gravity can bend light, even though light has no mass.

Front 53

Where do stars form? Do they form alone?

Back 53

They form in giant molecular clouds of gas within our galaxy. Stars usually form in groups or clusters.

Front 54

Which wavelengths are best for observing the newly forming stars?

Back 54

Infrared since these wavelengths can penetrate through the gas and dust.

Front 55

What is reddening?

Back 55

When we look at a star through a cloud of gas and dust, it will appear redder. For example, a blue star (hot) when viewed through a cloud of gas and dust, can appear to be orange (cooler).

Front 56

What is a brown dwarf?

Back 56

It is a “failed star.” Brown dwarfs do not have enough mass to ever start hydrogen fusion in the core. Their cores are made of degenerate electrons.

Front 57

Degeneracy pressure

Back 57

the electrons are so closely packed that they start exerting their own pressure
which does not depend on temperature. Degeneracy pressure, therefore, halts the contraction of objects.

Front 58

What are the minimum and maximum masses of stars? Explain why there is a minimum and a
maximum mass for stars?

Back 58

0.08 Msun – brown dwarf; not enough mass to overcome degeneracy of electrons and start fusion of H. Max mass ~ 150 Msun, the radiation pressure of such star would be so strong that it would blow the star apart.

Front 59

How do we measure ages of star clusters?

Back 59

All stars in the cluster are approximately the same age. So, we look for the main-sequence turnoff point. A cluster is young if it still contains massive main-sequence stars (like O and B types). A cluster is old if most of the massive main-sequence stars have evolved off the main-sequence.

Front 60

What do we mean by the main-sequence turnoff point of a star cluster, and what does it tell us?

Back 60

It is the spectral type of the hottest main sequence star in a star cluster, and it tells us the cluster's age

Front 61

What are the two different types of star clusters?

Back 61

Open clusters (young and loose not many stars) and globular clusters (old and dense with over a million stars)