Physics 110 Astronomy & Cosmology: Stars, Galaxies, and the Universe rutgers Final Exam Eric J. Mendelsohn

Main parts of the Milky Way

Disk (young stars + gas), Bulge (old stars), Halo (old stars, globular clusters, dark matter)

Shape of the Milky Way

Spiral galaxy

Diameter of the Milky Way

About 100,000 light-years

Location of the Sun in the Milky Way

In the disk, about 28,000 light-years from the galactic center

Population I stars

Young, metal-rich stars found in the disk

Population II stars

Old, metal-poor stars found in the halo and bulge

Why halo stars are metal-poor

They formed early before many heavy elements existed

Motion of disk stars

Organized, roughly circular orbits

Motion of halo stars

Random, elliptical orbits

What causes dark lanes in the Milky Way

Interstellar dust absorbing visible light

Three main galaxy types

Spiral, Elliptical, Irregular

Spiral galaxy characteristics

Disk, bulge, halo; active star formation

Elliptical galaxy characteristics

Mostly old stars, little gas or dust

Irregular galaxy characteristics

No defined shape; often active star formation

Which galaxies contain mostly old stars

Elliptical galaxies

Why elliptical galaxies appear red

They contain mostly old, cool stars

What powers a quasar

Gravitational energy from matter falling into a supermassive black hole

What quasars reveal about galaxies

Many galaxies contain supermassive black holes

Cosmic distance ladder

A sequence of distance-measuring methods calibrated from nearby to far away

Closest distance-measuring method

Radar ranging in the solar system

Parallax distance formula

Distance (parsecs) = 1 / parallax angle (arcseconds)

Limitation of parallax

Only works up to about 326 light-years

Cepheid variable key property

Pulsation period determines luminosity

Why Cepheids are standard candles

They follow a period–luminosity relationship

Best standard candle for very distant galaxies

Type Ia (white dwarf) supernovae

Difference between apparent brightness and luminosity

Apparent brightness depends on distance; luminosity is intrinsic

Hubble’s Law

v = H₀D

What Hubble’s Law shows

The Universe is expanding

What H₀ represents

The expansion rate of the Universe

What is expanding in the Universe

The space between galaxies

What is NOT expanding

Galaxies, solar systems, atoms

What redshift indicates

Light stretched to longer wavelengths due to expansion

Difference between Doppler and cosmological redshift

Doppler is motion through space; cosmological is expansion of space

Why all galaxies appear to move away

Space itself is expanding everywhere

Simplified age of the Universe formula

Age ≈ 1 / H₀

Why the age estimate is approximate

Expansion rate has changed over time

If expansion was faster in the past

The Universe would be younger than 1 / H₀

Big Bang theory

The Universe began hot and dense and expanded and cooled

What the Big Bang was NOT

An explosion at a single point in space

What the Big Bang WAS

Expansion of space everywhere

Why the early Universe was very hot

Matter and radiation were extremely close together

What happened to temperature over time

It decreased as the Universe expanded

Cosmic Microwave Background (CMB)

Leftover radiation from the Big Bang

When CMB photons were released

About 380,000 years after the Big Bang

What happened at recombination

Electrons combined with nuclei; light traveled freely

Current temperature of the CMB

2.73 K

Why the CMB is microwave radiation today

Original light was redshifted by expansion

Why the CMB is nearly uniform

The early Universe was almost the same everywhere

What small CMB fluctuations represent

Seeds of galaxies and large-scale structure

Shape of the Universe from CMB data

Flat

Dark matter

An invisible form of matter detected through gravitational effects

Evidence for dark matter in galaxies

Flat rotation curves

Evidence for dark matter in clusters

Fast galaxy speeds, hot X-ray gas, gravitational lensing

What flat rotation curves imply

Large dark matter halos around galaxies

Role of dark matter in structure formation

Provides gravitational scaffolding for galaxies

What dark matter is NOT

Normal gas, dust, or light-emitting matter

Leading dark matter candidates

WIMPs (Weakly Interacting Massive Particles)

Dark energy

An unknown energy causing the expansion of the Universe to accelerate

Evidence for accelerating expansion

Type Ia supernovae observations

What dimmer-than-expected supernovae indicate

Expansion is speeding up

Effect of dark matter on expansion

It slows expansion due to gravity

Effect of dark energy on expansion

It accelerates expansion

Approximate contents of the Universe

~5% normal matter, ~27% dark matter, ~68% dark energy

Ultimate fate of the Universe

Expansion forever (heat death)

Does redshift directly give distance

No, it gives velocity; distance comes from Hubble’s Law

Can we see the Big Bang itself

No, but we observe its afterglow (CMB)

Are dark matter and dark energy the same

No, they are completely different

Are galaxies moving through space away from us

Mostly no; space itself is expanding