Astrophysics FINAL EXAM

Flashcards for my astrophysics finals based off the midterms and slides

If a spaceship moves at 0.9c and shines a flashlight forward, how fast does an external observer see the beam move?

At speed c. The speed of light is constant in all inertial frames.

What is observed about the clock of a fast-moving spaceship?

It runs slower than your clock. Time dilation occurs at relativistic speeds.

You travel 5.98 light-years to Barnard’s Star and back at 0.9999c. How long does the round trip take for you?

About 0.169 years. Time is dilated for you, the traveler.

What does the worldline of an accelerating object look like?

A curved line.

Do all observers agree on simultaneity of events?

No. Simultaneity is relative in special relativity.

What kind of geometry has triangle angles adding up to more than 180°?

Spherical geometry.

How do clocks compare at Earth's surface vs far from it?

Clocks closer to Earth tick slower than those farther away due to gravitational time dilation.

What early evidence supported gravitational waves?

Observation of a binary neutron star system with decaying orbits.

What happens to light in a gravitational field?

It bends, and is redshifted. This is predicted by general relativity.

What is the first step of the proton-proton chain?

Two protons fuse to make deuterium.

What can the Zeeman effect measure in the Sun?

The strength of the magnetic field.

What causes granulation on the Sun’s surface?

Convection, not the Sun’s magnetic field.

Why do sunspots appear dark?

They are cooler than their surroundings.

What is the central darkest part of a sunspot called?

The umbra.

Where are most X-rays produced on the Sun?

In the corona.

What happens in the Sun’s radiation zone?

Energy diffuses outward via photon scattering.

What layer is directly above the photosphere?

The chromosphere.

Why does an increase in core temperature reduce fusion rate?

It causes the core to expand and cool.

Who sees whose clock tick faster — you on ground floor or friend on the roof?

You see your friend's clock tick faster; she sees yours slower.

What color are hotter stars?

Blue. Cooler stars are red.

Where are hot, luminous stars on the HR diagram?

Upper left.

Which is the most abundant spectral type?

M-type stars.

What do main sequence stars do?

Fuse hydrogen into helium in their cores.

Do B stars live longer or shorter than K stars?

B stars have shorter lifespans.

What does the main-sequence turnoff point indicate?

The cluster's age.

What are open clusters like?

Loosely packed, found in disk, and young.

What are globular clusters like?

Densely packed, found in halo, and old.

What is a white dwarf?

The dead core of a star where fusion has ceased.

Are Cepheid variables main sequence stars?

No. That’s false.

What happens right after a helium flash?

The core heats up and expands.

What supports a brown dwarf?

Electron degeneracy pressure.

What prevents pressure from increasing during cloud collapse?

Energy is radiated away via photon emission during molecular collisions.

Why were first-gen stars different?

No heavy elements = less cooling, required larger masses to collapse

What happens in a star before a helium flash?

Hydrogen fuses in a shell around an inert helium core.

What is the triple-alpha process?

3 helium nuclei fuse to form carbon.

What triggers a supernova in a high-mass star?

Collapse of an iron core into a neutron star.

If a star moves 3× closer, what happens to apparent brightness?

Increases by factor of 9; absolute magnitude stays the same.

Is a magnitude 5 star brighter than a magnitude 3?

No. That is false — magnitude 3 is brighter.

How do eclipsing binaries show up?

Not only via Doppler shift — that’s a false statement.

What is the most abundant molecule in molecular clouds?

H₂. CO is used to map the clouds.

What powers high-mass stars on the main sequence?

The CNO cycle.

What kind of pressure supports a white dwarf?

Electron degeneracy pressure.

Compared to its progenitor main sequence star, a white dwarf is...

Hotter and dimmer.

What is the upper mass limit for a white dwarf?

About 1.4 solar masses (Chandrasekhar limit).

Will the Sun ever undergo a nova explosion?

No, because it does not have a close binary companion.

What kind of stars end their lives with supernovae?

Stars much more massive than the Sun.

Which object is closest in mass to a neutron star?

The Sun.

Can a star system have more than one nova?

Yes, a nova can happen multiple times if conditions recur.

What is the ultimate fate of an isolated pulsar?

It slows down, its magnetic field weakens, and it becomes invisible.

Why do we think Cygnus X-1 contains a black hole?

It’s an X-ray binary with an unseen companion too massive to be a neutron star.

What is the Chandrasekhar limit?

The maximum mass a white dwarf can have — ~1.4 solar masses.

How far is the Sun from the center of the Milky Way?

About 27,000 light-years.

What indicates the presence of high-mass stars in star-forming regions?

Ionization nebulae.

Which are typically found in the halo of our galaxy?

Globular star clusters.

Why do we think the Sun is not at the center of the Milky Way?

From the distribution of globular clusters (Shapley’s conclusion).

How is atomic hydrogen mapped?

Using its 21 cm spectral line.

Which gas dominates the interstellar medium?

Atomic hydrogen.

What causes a galactic fountain?

Multiple supernovae occurring together.

What evidence shows halo stars are older?

They have fewer heavy elements.

What’s the best evidence for a massive black hole at the Milky Way’s center?

The orbits of stars near the galactic center show a massive central object.

What are the Magellanic Clouds?

Two small galaxies orbiting the Milky Way.

What’s true about elliptical galaxies?

Some contain large amounts of very hot gas.

What’s true about irregular galaxies?

They were more common when the universe was younger.

Which statement is true?

Most large galaxies are spirals or lenticulars.

Why are galaxies in the “blue cloud” blue?

They contain young, hot stars — active star formation.

[Starburst Galaxies] Why are they called that?

They are forming stars so rapidly that the phase is temporary.

What’s the best wavelength to observe starburst galaxies?

Infrared.

Why are white dwarf supernovae good standard candles?

They all have nearly the same peak luminosity.

What two quantities did Hubble compare?

Velocity and distance.

What is a quasar?

The extremely bright center of a distant galaxy.

How do we know quasars are distant?

They have very large redshifts.

What powers active galactic nuclei?

Matter falling into a supermassive black hole, converting gravitational energy.

Where are AGN x-rays produced?

In the hot gas of the accretion disk around the central black hole.

How do we detect cool gas between us and a quasar?

Hydrogen absorption lines at redshifts less than the quasar’s.