Introduction to Astrophysics

Lecture 14:

Galaxies

NGC1232

Aims of studying galaxies

There are many things we would like to know about galaxies:

What is their visual appearance?

How do we classify them?

How far away are they?

What physics governs them?

How did they form?

Galaxies come with a wide range of different morphologies. Broadly speaking they are divided into three classes:

Spirals

Ellipticals

Irregulars

Types of galaxy

Spiral Galaxies

Spiral galaxies are perhaps the most spectacular astronomical objects.

They usually consist of a flat disk of stars with spiral arms, and a central bulge.

The most spectacular are known as Grand Design spirals.

Spiral Galaxies

There are two main types of spiral galaxies, regular ones and barred spiral galaxies, though it is not always easy to tell them apart.

Elliptical Galaxies

Elliptical galaxies are blobs of stars, resembling the central bulges of spiral galaxies or globular clusters, though often much bigger.

Irregular Galaxies

Irregular galaxies are the remainder! In many cases their unusual shapes are due to recent interactions/collisions with other galaxies.

The Hubble Sequence

The Hubble Sequence

The Hubble Sequence, sometimes called the `tuning fork diagram’, indicates the different galaxy types. It puts ones of similar properties near one another.

At the junction are lenticular galaxies, such as the Sombrero galaxy shown here.

The Hubble Sequence

The different types of spiral and elliptical are given a number or letter to indicate their general shape. So, for example,

E0 are very spherical ellipticals

E7 are very flattened ellipticals

Along the spiral classifications, the bulge gets smaller as the letter increases, so for example SBc galaxies have hardly any bulge at all.

The Hubble Sequence

Nowadays, things are often made a little more complicated!

The Hubble Sequence

Hubble originally thought that his classification might actually correspond to evolution, with ellipticals later becoming spirals. This view is now seen as oversimplistic, and if anything it is the spirals that become ellipticals through mergers.

Galaxy characteristics

Spirals Ellipticals Irregulars

Mass (in solar masses)

109 to 1012 105 to 1013 108 to 1011

Star content Old and young stars

Only old stars

Old and young stars

Interstellar medium

Gas and dust

Almost no gas or dust

Plentiful gas, variable dust

Note that ellipticals have a much wider mass range than the others.

Distances to galaxies

One of the longest standing problems in astronomy has been to measure the distances to galaxies. As with stars, distances are needed if we are to fully understand their properties, such as their brightness.

Knowing the distances to galaxies is also a crucial part in determining the size and expansion rate of the Universe.

Distance measuring techniques: 1) Standard candles

A standard candle is any object which is believed to have the same absolute brightness wherever it is in the Universe. It also needs to be bright enough to be seen at useful distances. Common examples are:

Cepheid variable stars – these are bright stars whose brightness varies periodically. The Hubble Space Telescope can see them in nearby galaxies.

Type Ia supernovae – caused when a white dwarf accretes mass and goes over the Chandrasekhar limit. Can be seen to very great distances.

Total galaxy luminosity – can be used if desperation sets in.

Distance measuring techniques: 1) Standard candles (continued)

Use of a standard candle is quite simple, because light intensity obeys the inverse square law.

If a cepheid in Galaxy A is four times as bright as one with the same properties in Galaxy B, then

Galaxy B is twice as far away.

Note however that this doesn’t tell us the actual distances, only the relative distances. To get an actual distance we would need an independent measure of how bright the standard candle is. Such estimates have proven very hard to come by.

NB For very distant objects it may be necessary to include the expansion of the Universe in the calculation.

Distance measuring techniques: 2) Empirical relations

Rather than use a standard candle, astronomers have made use of so-called empirical relationships connecting different observables. For example:

Tully-Fisher relation – this says that the true luminosity of a galaxy is related to the rotational velocity of the galaxy, which can be measured.

Surface brightness fluctuations – elliptical galaxies exhibit variations in brightness across their surface, which can be used to estimate the distance to them.

Despite enormous amounts of work over many decades, going all the way back to Hubble himself, the distances to any galaxies other than the most nearby remain quite uncertain.