Galaxies and the Foundation of Modern Cosmology

Galaxies and Cosmology

A galaxy's age, its distance, and the age of the universe are all closely related.

The study of galaxies is thus intimately connected with cosmology—the study of the structure and evolution of the universe.

Patterns in Properties of Galaxies

Galaxies can be divided into three categories: spiral, elliptical, and irregular.

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

Short-lived blue stars outshine the others.

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. Red-yellow color indicates older star population.

Irregular Galaxies

Blue-white color indicates ongoing star formation.

Galaxy Groups

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

Galaxy Clusters

Elliptical galaxies are much more common in huge clusters of galaxies (hundreds to thousands of galaxies).

Galaxies tend to fall into two groups

the blue cloud and red sequence.

Galaxy Distances

Brightness alone does not provide enough information to measure the distance to an object.

Determining Distance

1. Determine size of the solar system using radar.

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

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

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

Which kind of stars are best for measuring large distances?

high-luminosity stars

The Puzzle of "Spiral Nebulae"

Before Hubble, 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. The debate remained unsettled until Edwin Hubble finally measured their distances.

The Great Debate

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

Your friend leaves your house. She later calls you on her cell phone, saying that she's been driving at 60 miles an hour directly away from you the whole time and is now 60 miles away. How long has she been gone?

60 minutes

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

Lookback Time

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

Cosmological Horizon

The Cosmological Horizon marks the limits of the observable universe.

It 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!