Chapter 20: Galaxies and the Foundation of Modern Cosmology

Our deepest images of the universe show a great _____, some of them _____ away

variety of galaxies; billions of light-years

A galaxy's _____ are all closely related

age, its distance, and the age of the universe

cosmology

the study of the structure and evolution of the universe

The study of _____ is thus intimately connected _____

galaxies; with cosmology

Galaxies can be divided into three categories:

spiral, elliptical, and irregular

Why does ongoing star formation lead to a blue-white appearance?

A.Short-lived blue stars outshine the others.

Lenticular galaxy: has a disk like a _____ but much less _____

spiral galaxy; dusty gas (intermediate between spiral and elliptical)

Elliptical galaxy: all _____ component, virtually no _____

spheroidal; disk component

Red-yellow color indicates _____.

older star population

Blue-white color indicates _____.

ongoing star formation

Spiral galaxies are often found in _____ (up to a few dozen galaxies).

groups of galaxies

Elliptical galaxies are much more common in _____ of galaxies (hundreds to thousands of galaxies)

huge clusters

Galaxies tend to fall into two groups, _____ and _____.

the blue cloud; red sequences

Gas and dust are more abundant in _____ galaxies.

spiral and irregular

(How do we measure the distances to galaxies?) Step 1

Determine size of the solar system using radar.

(How do we measure the distances to galaxies?) Step 2

Determine the distances of stars out to a few hundred light-years using parallax

Luminosity _____ each sphere is _____.

passing through; the same

Area of sphere:

4π (radius)²

Divide _____ by area to get _____.

luminosity: brightness

The relationship between _____ and luminosity depends on _____

apparent brightness; distance

We can determine a star's _____ if we know its _____ and can measure its apparent brightness

distance; luminosity

A standard candle is

an object whose luminosity we can determine without measuring its distance.

Which kind of stars are best for measuring large distances?

high-luminosity stars

Cepheid _____ are very _____.

variable stars; luminous

(How do we measure the distances to galaxies?) Step 3

Because the period of Cepheid variable stars tells us their luminosities, we can use them as standard candles.

(How do we measure the distances to galaxies?) Step 4

The apparent brightness of a white dwarf supernova tells us the distance to its galaxy (up to 10 billion light-years)

Cepheid variable stars with _____ have greater _____.

longer periods; luminosities

White-dwarf supernovae can also be used as _____.

standard candles

Before _____, some scientists argued that "spiral nebulae" were entire galaxies like our Milky Way, while others maintained they were smaller collections of stars within the Milky Way until Edwin Hubble _____

Hubble; measured their distances

Hubble measured the distance to the _____ using _____ as standard candles.

Andromeda Galaxy; Cepheid variables

The spectral features of virtually all galaxies are _____, which means that they're all _____.

redshifted; moving away from us

By measuring distances to galaxies, _____ found that_____are related in a special way.

Hubble; redshift and distance

Hubble's law:

Velocity = H₀ × distance

Redshift of a galaxy tells us its _____ through _____

distance; Hubble's law

Distances of the _____ are measured from their _____

farthest galaxies; redshifts.

We measure galaxy _____ using a chain of _____.

distances; interdependent techniques

The expansion rate appears _____ everywhere in space.

to be the same

The universe has no _____ and no _____ (as far as we can tell).

center; edge

_____ is evenly distributed on _____ in the universe.

Matter; very large scales

The cosmological principle has not _____, but it is _____ with all observations to date.

been proven beyond a doubt; consistent

You observe a galaxy moving away from you at 0.1 light-years per year, and it is now 1.4 billion light-years away from you. How long has it taken to get there?

14 billion years

Hubble's constant tells us the _____ because it relates the _____ of all galaxies.

age of universe; velocities and distances

Distances between _____ change while _____.

faraway galaxies; light travels

Astronomers think in terms of _____ rather than distance.

lookback time

Expansion stretches _____, causing a cosmological redshift directly related to _____.

photon wavelengths; lookback time

The Cosmological Horizon marks the

limits of the observable universe.

Why does the observable universe have a horizon?

We cannot see back to a time before the beginning of the universe!