ASTR 364 Final Questions

Lecture Questions

What part of the electromagnetic spectrum is most energetic?

Gamma rays

Why are most telescopes in space?

To avoid atmospheric disturbance

Why do infrared telescopes need to be cooled?

If the telescope is warm, it will emit infrared radiation which will block out infrared being emitted from other objects

If stars are brightest at visible wavelengths, why do we use infrared to search for oldest galaxies?

Universal expansion causes a redshift in the light being emitted

Our pupil diameter is fully dialated about 7mm. Galileo had telescopes with an aperature of roughly 21 mm. How much more light did Galileo’s telescopes collect than the human eye?

9 times more

Galileo had telescopes with an aperature of roughly 21 mm. The Keck telescopes have an aperature of 10m. How much more light can Keck roughly collect compared to Galileo?

250,000 times more

You observe a star with SNR 3. How much longer do you need to observe/integrate to obtain a SNR of 6?

4 times longer

We observe 2 objects in visible light (500 nm), and we are just able to distinguish them (diffraction limit). How much larger/smaller does a radio (1 cm) telescope need to be to separate the 2 sources?

20,000 times larger

What color of visible light is most difficult to observe from the ground?

Blue

Stars that are further away have ___ parallax than nearby stars

Smaller

Why do we use the small angle approximation when measuring distances to stars?

Even the nearby stars are so far away they appear to move only by a small angle on the sky

Why do we observe stars mostly at visible and infrared wavelengths?

Stars emit the most light at visible and infrared wavelengths

How much more/less energy is emitted by a star that is twice as hot?

16 times more

What is the observational technique called that we are using to measure stellar temperatures?

Photometry

The B-V color index of a hotter star is ____ than that for a cooler star

Smaller

Why would we use photometry instead of spectroscopy to measure luminosity?

Photometry observations require less time

The absorption lines from an element have ___ wavelengths as the emission lines from the same element

Same

In a binary system, the more massive star is ___ to the center of mass

Closest

We can measure the distance to a nearby star using

Parallax

For what inclination angles can a binary be a spectroscopic binary?

For all inclination angles except for the parallel to the plane of the sky

The angular separation between Sirius A and B varies betwen 3’’ and 11’’. At 500 nm, the diffraction limit of our eye is about 18”. This means we _____ see both Sirius A and B separately

Cannot

If we want to observe two things that are beyond our diffraction limit, we need to ____ the wavelength or _____ the aperature

Decrease, increase

In an eclipsing spectroscopic binary, the maximal radial velocities measured for the 2 components are 20 km/s and 5 km/s. The orbit is circular, and the orbital period is 5 yrs. Find the mass of each star in solar masses

M2 = 2.4 solar masses
M1 = 0.6 solar masses

The radius of the Sun is 7×10^8 m. The distance to the nearest star is 3.8×10^16 m. If the Sun were the size of a grapefruit (10 cm in diameter), how far away would Proxima Centauri?

3000 kmWh

What happens to the gravitational potential energy E_gr when a star contracts?

It becomes more negative

Can two nuclei in the Sun get close enough together to fuse?

No

What are the units for opacity?

Area/mass

If the opacity is large, photons _____ travel far and the material is ______

Cannot, opaque

If opacity is large, energy transport is very _____ and the stellar surface is then _____

Inefficient, cold

Material in a shell closer to the stellar core will emit blackbody radiation peaking at a _____ wavelength than material in a shell closer to the surface

Shorter (it’s hotter, so it peaks at a shorter wavelength)

What happens to a bubble if its density after rising a distance dr is larger than the density of the surrounding material?

Sinks

The core temperature of main sequence stars varies _____ with stellar mass

Only slightly

If the stellar mass increases by a factor of 100, the effective temperature

Increases by a factor of 10

When most of the hydrogen in the core is converted into helium, nuclear fusion slows down while the star keeps radiating. This causes the core to ____ and temperature to _____

Contract, intcrease

What causes the end of the horizontal branch phase?

Most of the helium in the core is converted into carbon and oxygen

What causes a star leaving the main sequence to lose mass?

Weaker gravity in the surface layers and stronger radiation pressure due to helium shell burning

For massive stars, the helium core will contract _____ and therefore the temperature will increase ____

faster, faster

What particles in an atom will be the first to start behaving like quantum particles?

Electrons

Electrons in the cores of low mass stars on the Red Giant Branch are approximated as quantum particles. Electrons in the cores of intermediate mass stars stay classical. Why?

Temperatures are higher in the intermediate mass stars, allowing the particles to say classical.

What conditions need to be met for electrons to be degenerate?

• The density should be high enough to foce electrons closer than half their de Broglie wavelength

• Temperature should be lower than Fermi temperature

What types of pressure do we need to consider for nuclei (in general)?

• Gas pressure

• Degeneracy pressure

The larger a white dwarf’s mass, the ___ its radius, the ____ its density, the ____ the electron momenta, the _____ electron velocity

Smaller, larger, larger, larger

What is the maximum velocity the electrons could possibly reach?

Speed of light

If the maximum white dwarf mass is 1.4 solar masses, how is it possible that stars with masses up to 8 solar masses end up as white dwarfs?

They lose a large amount of mass on the Asymptotic Giant Branch and as a nebula

Suppose we have a binary with a lower mass star as a red giant and the larger mass star still on the main sequence. Why does this seem to defy our model of stellar evolution?

More massive stars have shorter main sequence liftimes than lower mass stars

In a remnant from core collapse supernovae, what type of pressure may now be able to support the core?

Neutron degeneracy pressure

The density in a white dwarf is ~ 10^6 g/cm³. What is then the density in a neutron star if they are ~500 times smaller?

10^6×500³ = 10^14 g/cm³

What types of stars can be 200 km apart?

Neutron stars

If the core of the Sun (radius ~ 1.4×10^5 km) would colapse to the size of a neutron star, its rotation period would _____ by a factor of _____

Decrease, 10^8

True of False: We can observe radio emission from all neutron stars that are close enough to Earth

False

The amount of mass in stars in the Milky Way is _____ higher than the mass of visible matter in the ISM

50

If you were to observe a sun-like star (G-type luminosity with 1 solar luminosity) without knowing it is behind a cloud of dust, you would think it is

Redder and dimmer

Which stars are most likely to produce radiation capable of ionizing hydrogen?

Very massive stars

The mean free path of photons with energies less than 13.6 eV is 50 pc. This means that

These photons escape the H II region without interacting with other particles, and these photons do not contribute to the heating of the gas

If the cloud collapses, the gravitational potential energy becomes ____ (positive or negative)

More, negative

The cloud will be unstable due to gravitational colapse when

|dE_gr| > dE_th

On what timescale do molecular clouds collapse?

Freefall timescale

For a 1 solar mass clump, the Jeans density is ____ compared to a 1000 solar mass cloud (with the same T and n)?

10^6 times higher

Why is the IMF so hard to observe?

• Low-mass stars are harder to observe but live longer, high-mass stars are easier to observe but live shorter times

• Binaries/multiple star systems can interfere

What allows collapse to keep going once its started?

The molecular cloud can radiate heat away through emission lines, allowing the cloud not to excessively heat up to the point of halting collapse

What timescale does molecular cloud contraction operate on?

Free-fall timescale

When does a protostar become a star?

As soon as hydyrogen fusion stars

Why don’t we see forming stars in sub-mm images of protoplanetary disks?

Stars’ emissions peak in the UV-visible-IR range, with very weak emission at sub-mm wavelengths.

Why may solar system planets be hard to detect?

Our planets are very small, so they have little effects on the star that we would try to observe. Larger planets have extremely long orbital periods, making other methods very ineffective.

Are telescopes always operating at the diffraction limit? If not, why not?

No

• Atmospheric interference

• Optical imperfections

• Resolution can’t always match the limit

Observations of planets furhter away from the star are less affected by stray star light. Why are these planets still sometimes more difficult to observe than planets closer?

The amount of reflected light is then lower because it receives less light - it’s darker and now harder to find

What are the dark regions in the Milky Way?

GIant clouds of gas/dust that block light of stars behind them, and these clouds will later form stars

How do we know what the Milky Way looks like?

Measurements of positions/motions of stars, dust, gas, and comparing it to other galaxies

What wavelength do we use to observe dust?

Infrared/sub-mm

What atoms/moleucles do we look out for to observe gas?

Hydrogen

The Sun is moving at a circular orbit with radius r = 8.3 kpc, so we can estimate the mass internal to the Sun’s distance from the galactic center using ____ law

Kepler’s third law

The Milky Way contains 100 billion stars. We can calculate the chance of 2 stars collding using _____

The mean free path - says that star collisions are possible but very rare

The rotation pattern of the Galxy is driven by gravity and therefore the velocity decreases with radius. This means that over time, the spiral arms would become ____ tightly wound.

More

What wavlengths do we use to observe the Galactic center?

X-rays and radio

How do we know there is a supermassive black hole at the center?

Nased on orbits of stars very close to the center + black hole shadow

What feature of the velocity curve tells us there is dark matte?

We would expect velocity to fall off with radius v ~ r^(-1/2), but it stays flat, indicating that there is more mass than what we see

Why are neutrons stars extremely faint?

They have very small radii

The radius of the Einstein ring is larger when the lensing object is ____ massive

More

How do we know other galaxies are outside of the Milky Way?

Distance to other galaxies are larger than the size of the galaxy

Why can’t we use parallax to measure the distance to Andromeda?

Andromeda is too far away to measure an apparent motion on the sky due to Earth’s orbit around the Sun.

Why can we calculate distance to a star if we know its luminosity?

Because the luminosity is related to the flux, so if you know the luminosity, you can calculate the distance from the measured flux.

The map of ionized gas in the galaxy tells us

Where young high-mass stars are located

Why does material accreting onto a black hole forms a rotating disk before falling into the black hole?

Conservation of angular momentum

For material in the disk to be able to fall into the black hole, the material needs to ____ angular momentum

Lose

The temperature in an accretion disk around a black hole is ____ close to the black hole

Higher

What type of galaxy dominates in galaxy clusters?

Elliptical

The emission of X-rays in galaxy clusters suggest that

The gas in between galaxies is very hot

When the gas in a forming galaxy cools, the pressure _____ causes collapse to _____

Decreases, continues

When 2 galaxies collide, can collisions between individual stars happen?

Yes, but it is rare

Parallax measurements are part of the study

Astrometry

Type Ia supernova are formed in

Low-mass stellar binaries

Due to the expansion of the Universe, light emitted by an object far enough away will be

Redshifted

What is the value of z (redshift) for the current day Universe?

z = 0

A blackbody spectrum is soley characterized by

The temperature of the particles emitting the radiation

When photons frequently scatter of electrons, their mean free path is ____ and therefore we ____ observe these photons

Short, cannot

The universe is roughly 3000 K at the era of recombination. If we were to observe the emission emitted at that time, we would observe a blackbody spectrum corresponding to

A temperature much lower than 3000 K

What is the temperature of the CMB blackbody spectrum?

2.7 K