AU Chapter 20

Hubble Deep Field

Our deepest images of the universe show a great variety of galaxies, some of them billions of light-years away

What three things are all closely related?

A galaxy's age, its distance, and the age of the universe

The study of galaxies is thus intimately connected with cosmology …?

the study of the structure and evolution of the universe.

What are the three major types of galaxies?

Spiral galaxy, elliptical galaxy, and irregular galaxies

Disk component:

stars of all ages, many gas clouds

Spheroidal component:

bulge and halo, old stars, few gas clouds

Blue-white color indicates ?

ongoing star formation

Red-yellow color indicates ?

older star population

Barred spiral galaxy:

has a bar of stars across the bulge

Lenticular galaxy:

has a disk like a spiral galaxy but much less dusty gas (intermediate between spiral and elliptical)

Elliptical galaxy:

all spheroidal component, virtually no disk component

Spiral galaxies are often found in ?

groups of galaxies (up to a few dozen galaxies).

Elliptical galaxies are much more common in ?

huge clusters of galaxies (hundreds to thousands of galaxies)

Simple definition: How are the lives of galaxies connected with the history of the universe?

Galaxies generally formed when the universe was young and have aged along with the universe.

Simple definition: What are the three major types of galaxies?

– The major types are spiral galaxies, elliptical galaxies, and irregular galaxies. – Spirals have both disk and spheroidal components; ellipticals have no disk.

Simple defintion: How are galaxies grouped together?

– Spiral galaxies tend to collect into groups of up to a few dozen galaxies. – Elliptical galaxies are more common in large clusters containing hundreds to thousands of galaxies.

How do we measure the distances to galaxies?

Step 1 • Determine size of the solar system using radar. Step 2 • Determine the distances of stars out to a few hundred light-years using parallax. Step 3 • The apparent brightness of a star cluster's main sequence tells us its distance. Step 4 • Because the period of Cepheid variable stars tells us their luminosities, we can use them as standard candles. Step 5 • The apparent brightness of a white dwarf supernova tells us the distance to its galaxy (up to 10 billion light-years)

What is the same in each sphere?

Luminosity passing through each sphere is the same

Area of sphere:

4p (radius)2

How to get brightness?

Divide luminosity by area to get brightness.

The relationship between apparent brightness and luminosity depends on ?

distance

We can determine a star's distance if we know ?

its luminosity and can measure its apparent brightness:

A standard candle is an object whose ?

luminosity we can determine without measuring its distance

Knowing a star cluster's distance, we can determine the ?

luminosity of each type of star within it

Cepheid variable stars are very ?

luminous

Cepheid variable stars with longer periods have greater ?

luminosities.

What can also be used as standard candles?

White-dwarf supernovae

How did Hubble prove that galaxies lie far beyond the Milky Way?

Hubble settled the debate by measuring the distance to the Andromeda Galaxy using Cepheid variables as standard candles

The spectral features of virtually all galaxies are redshifted, which means that they're all ?

moving away from us

Hubble's law:

Velocity = H0 ´ distance

Redshift of a galaxy tells us its … through Hubble's law

distance

Distances of the farthest galaxies are measured from their ?

redshifts

We measure galaxy distances using a chain of ?

interdependent techniques

Simple definition: How do we measure the distances to galaxies?

– The distance measurement chain begins with parallax measurements that build on radar ranging in our solar system. – Using parallax and the relationship between luminosity, distance, and brightness, we can calibrate a series of standard candles. – We can measure distances greater than 10 billion light-years using white dwarf supernovae as standard candles.

Simple definition: How did Hubble prove that galaxies lie far beyond the Milky Way?

He measured the distance to the Andromeda Galaxy using Cepheid variable stars as standard candles.

Simple definition: What is Hubble's law?

The faster a galaxy is moving away from us, the greater its distance: Velocity = H0 x distance

How does Hubble's Law tell us the age of the universe?

The expansion rate appears to be the same everywhere in space.

The universe has no …? (as far as we can tell).

center and no edge

One example of something that expands but has no center or edge is the ?

surface of a balloon

Cosmological Principle

The universe looks about the same no matter where you are within it.

Matter is ? It has no center or edges.

evenly distributed on very large scales in the universe.

Hubble's constant tells us the age of universe because it relates the ?

velocities and distances of all galaxies.

How does expansion affect distance measurements?

Distances between faraway galaxies change while light travels. Astronomers think in terms of lookback time rather than distance

Expansion stretches photon wavelengths, causing a … directly related to lookback time.

cosmological redshift

The Cosmological Horizon marks the ?

limits of the observable universe.

The Cosmological Horizon is a horizon in time rather than space. Since looking far away means looking back in time, there must be a …?

limit – the beginning of the universe

Simple defintion: Why does the observable universe have a horizon?

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

Simple defintion: How do distance measurements tell us the age of the universe?

– Measuring a galaxy's distance and speed allows us to figure out how long the galaxy took to reach its current distance. – Measuring Hubble's constant tells us that amount of time: about 14 billion years.

Simple defintion: How does the universe's expansion affect our distance measurements?

Lookback time is easier to define than distance for objects whose distances grow while their light travels to Earth.