ASTR-101-FINAL EXAM

What kind of stars on the Hertzsprung–Russell diagram become white dwarfs when their main sequence life is over?

Low-mass and intermediate-mass stars (less than about 8 solar masses)

Trace the evolution of a star that becomes a white dwarf after it leaves the main sequence.

Core hydrogen fusion stops, the core contracts and heats, outer layers expand into a red giant, helium fusion produces carbon and oxygen, outer layers are ejected as a planetary nebula, and the remaining core becomes a white dwarf

What is the physical size AND mass for the object inside a planetary nebula?

Physical size of Earth; mass of the Sun

Low and intermediate mass stars die as what?

White dwarfs

What supports a white dwarf from further collapse?

Electron degeneracy pressure

What happens to a lone white dwarf after it forms?

It slowly cools and fades over billions of years

What can happen to a white dwarf in a binary system?

It can undergo a nova or a Type Ia supernova

What is a nova?

Hydrogen transferred onto a white dwarf suddenly undergoes fusion, causing temporary brightening

What is the highest mass a white dwarf can have and what happens when the dead star exceeds this mass?

1.44 solar masses; exceeding the limit causes a Type Ia supernova explosion

What kind of stars become neutron stars?

Massive stars greater than about 8 solar masses

How do high-mass stars differ from stars that become white dwarfs?

They fuse heavier elements up to iron in layered shells

Where do elements heavier than hydrogen, helium, and carbon come from?

Fusion in massive stars and supernova explosions

What happens when the core of a dying massive star becomes iron?

The core collapses catastrophically and triggers a supernova

Can fusion create elements heavier than iron inside a stable star?

No, fusion beyond iron requires energy instead of releasing it

What is the mass limit for a neutron star and how large is it?

Up to about 3 solar masses and about 10 kilometers across

What supports a neutron star from collapse?

Neutron degeneracy pressure

What are pulsars?

Rapidly rotating neutron stars emitting sweeping beams of radiation

What can happen to a neutron star in a binary system?

Accreting gas can trigger powerful X-ray bursts

How do scientists distinguish Type Ia and Type II supernovae?

Type Ia lack hydrogen lines; Type II show hydrogen lines

What happens to a dying star whose core exceeds 3 solar masses at death?

Type II supernova explosion leaving behind a neutron star or black hole

What kind of stars become black holes?

The most massive stars

What is the lower mass limit for a collapsing core to become a black hole?

About 3 solar masses

Can anything stop the collapse of a stellar core above this limit?

No known force can stop it

What is the event horizon of a black hole?

The boundary beyond which nothing, not even light, can escape

Can the size of a black hole increase?

Yes, by gaining mass

How do scientists know black holes exist if light cannot escape?

By observing effects on nearby matter such as X-rays and orbital motion

Why are neutron stars and black holes in binaries called X-ray binaries?

Inflating gas becomes extremely hot and emits X-rays

What happens to time near an event horizon?

Time appears to slow dramatically

What happens to light escaping near an event horizon?

It becomes gravitationally redshifted

What causes long-duration gamma-ray bursts?

Collapse of massive stars into black holes

What causes short-duration gamma-ray bursts?

Mergers of neutron stars

What astronomical objects have potential to be lethal for living things at very faraway distances?

Magnetars

How can luminosity be determined?

From apparent brightness and distance

How can surface temperature be determined?

From color or spectrum

How can apparent brightness be determined?

Measured directly with telescopes

How can stellar distance be determined?

Parallax, main-sequence fitting, Cepheid variables, and Type Ia supernovae

What kind of galaxy is the Milky Way Galaxy?

Spiral galaxy

What is the size of the Milky Way Galaxy?

About 100,000 light-years across

What are some structures that compose a spiral galaxy? In terms of distances, how wide are some of these structures?

Disk, bulge, halo, globular clusters, and spiral arms; about 28,000 light-years

Where does star formation occur in our galaxy?

Mainly in spiral arms within molecular clouds

In spiral galaxies, where do we see star-forming regions?

In the spiral arms

How do we know what we see are indeed stars being formed?

Pink and blue stars indicate star-forming regions

What are Population I stars?

Young, metal-rich stars in the disk and spiral arms

What are Population II stars?

Older, metal-poor stars in the halo and globular clusters

Where are globular clusters found?

In the halo around the Milky Way

How do stellar motions differ in the galaxy?

Disk stars move in circular orbits while halo stars move in random elongated orbits

What is the interstellar medium made of?

Gas and dust, mostly hydrogen and helium

What are molecular clouds made of?

Cold dense gas and dust, mainly molecular hydrogen

What is galactic recycling?

Stellar material returned to space through winds and supernovae for future star formation

What is at the center of our galaxy?

A supermassive black hole

What are some reasons why galaxies don’t end up with the shapes we see in spiral galaxies?

Spin/angular momentum and density

What are starburst galaxies?

Galaxies forming stars at much higher rates than normal galaxies

How often do stars form in starburst galaxies compared to others?

Hundreds to thousands per year versus about 4 per year in other galaxies

What are the three galaxy types classified by Edwin Hubble?

Spiral, elliptical, and irregular

What is the difference between spiral and elliptical galaxies?

Spiral galaxies contain gas, dust, and young stars; elliptical galaxies contain mostly old stars and little gas

Where are spiral galaxies mostly found?

In smaller groups and less crowded regions

Where are elliptical galaxies commonly found?

In dense galaxy clusters

What causes irregular galaxies to look strange?

Interactions and collisions with other galaxies

How many galaxies are in the visible universe?

Hundreds of billions

How are distances to nearby stars measured?

Stellar parallax

How are distances to star clusters measured?

Main-sequence fitting

What stars help determine distances to nearby galaxies?

Cepheid variable stars

Why are Cepheid variables standard candles?

Their pulsation periods reveal their true luminosities

What standard candle is used for more distant galaxies?

Type Ia supernovae

What does it mean that distant galaxies move away faster?

The universe is expanding uniformly

What is Hubble’s Law?

A galaxy’s recession speed = Hubble’s constant x its distance

How can galaxy spectra reveal distance?

Redshift measures recession speed used with Hubble’s Law

Why doesn’t Hubble’s Law determine Andromeda’s distance well?

Local motion dominates over cosmic expansion

What does Hubble’s Constant tell us?

The universe’s current expansion rate

How can Hubble’s Constant estimate the age of the universe?

Its inverse gives an approximate age

Why does looking farther into space mean looking back in time?

Light takes time to travel

Why is galaxy evolution tied to cosmology?

Distant galaxies show how galaxies evolved over time

Why do astronomers think galaxies formed after the Big Bang?

Small density variations grew through gravity

What is believed to exist at the center of nearly every galaxy?

A supermassive black hole

Why were galaxy collisions more common in the past?

Galaxies were closer together

Besides initial conditions, what shapes galaxy appearance?

Galaxy interactions and mergers

What are quasars and their power source?

Quasi-stellar radio sources powered by supermassive black holes

How are radio galaxies, AGNs, and quasars related?

All are powered by matter falling into supermassive black holes

What is a quasar?

An extremely luminous active galactic nucleus

What characteristics do AGNs and quasars share?

Strong radiation, jets, variability, and compact energy sources

Where do X-rays from quasars and AGNs originate?

Hot gas in accretion disks near supermassive black holes

What two areas of physics would a Theory of Everything unite?

Quantum mechanics and general relativity

List the four fundamental forces of nature from STRONGEST to WEAKEST.

Strong force, electromagnetism, weak force, gravity

When did the Planck era end?

About 10^-43 seconds after the Big Bang

What is inflation?

A brief period of extremely rapid expansion in the early universe

What problems does inflation solve?

Flatness, horizon, and monopole problems

What Big Bang predictions are supported by chemical composition observations?

Observed abundances of hydrogen and helium

Why do scientists think weak and electromagnetic forces were once united?

High-energy experiments show similarities between them

Why did the era of nuclei end about 370,000 years after the Big Bang?

Electrons combined with nuclei to form atoms

Why is this called matter-energy decoupling?

Light could travel freely without constant scattering

What evidence supports this era?

Cosmic background radiation

What does CBR stand for?

Cosmic Background Radiation

How was the CBR discovered?

Accidentally by Arno Penzias and Robert Wilson

What is the exact temperature of the cosmic microwave background radiation as measured today?

2.725 K ± 0.0006 K

How do small differences in the CBR support galaxy formation?

They reveal early density variations

Why is dark matter important for galaxy formation?

Its gravity helped structures form more quickly

Where did the helium in the universe come from?

Primordial nucleosynthesis after the Big Bang

Why does Olbers’ paradox support the Big Bang?

The universe has a finite age, making the night sky dark

What is dark matter?

Invisible matter detected through gravity

What is dark energy?

A mysterious energy causing accelerated expansion