Astronomy 100


Lectures Table of Contents Astro 100



  1. Galaxy Types
  2. Distances, Sizes, Masses, and Luminosities
  3. Galaxy Clusters

Terms to Know

spiral galaxy
elliptical galaxy
irregular galaxy
barred spiral
Hubble tuning fork diagram
galaxy cluster

Hubble Law

Hubble constant

The sky is literally covered with galaxies. Your fingernail held at arm's length covers a million galaxies , each one containing about 100 billion stars. Many of those galaxies are 10-15 billion light years away, close to the edge of the observable Universe, so we are peering back in time towards the Big Bang. What do they look like? How did they form? What clues do they hold about how the Milky Way came to be, and how Earth and the rest of the Solar System evolved?

1. Galaxy Types

Almost all galaxies belong to one of three major types:
  1. Spiral galaxies, or disk galaxies, like the Milky Way:
    • gas-rich
    • active star formation
    • generally blue due to hot young stars (halo may be red)
    • often dusty
    • disk stars rotate smoothly around center
    • sometimes show bars across their nuclei
  2. Elliptical galaxies
    • gas-poor
    • no active star formation
    • old red stars
    • stars move in haphazard orbits, not smooth flow
  3. Irregular galaxies
    • distorted shapes, sometimes due to two or more galaxies colliding
    • often gas-rich with active star formation
Hubble arranged ellipticals and spirals with and without bars into a diagram, the Hubble tuning-fork diagram, thinking it might help understand how galaxies were formed and how they came to appear the way they do. He thought perhaps ellipticals turned into spirals; now we think exactly the opposite!

2. Distances, Sizes, Masses, and Luminosities

Measuring distances to galaxies is important because only then can we determine their true sizes and luminosities.

How can we measure those distances? Parallax from the Earth's orbit, the way we do with nearby stars? Nope -- galaxies are much too far away. Cepheid variable stars? YES!

Other methods of measuring distances are needed for galaxies further than the closest 100 or so galaxies, because Cepheids become too faint to detect at those distances. Most of those methods also rely on trying to identify some object or class of objects with which we are already familiar -- like using the apparent size of a tree to estimate the distance to a mountain. Some of those methods include:

  • Identifying the brightest globular cluster
  • Finding planetary nebulae, whose luminosities are well-known
  • Searching for a particular kind of supernova that always has the same luminosity at its peak brightness
  • Measuring the galaxy's rotation speed, which is closely tied to its luminosity (bright galaxies rotate faster). Comparing luminosity with apparent brightness yields distance.

The distance to the nearest major galaxy, the Andromeda Galaxy (M31), is about 2 million light years, or 700 kiloparsec (kpc) = 0.7 megaparsec (Mpc). M31 and the Milky Way are sister galaxies. Since their diameters are each about 25 kpc, they are separated by about 700/25 = 28 times their size -- like 2 basketballs 10 meters apart. Note how different this is than the case of nearby stars, which are like grapefruits separated by thousands of miles!

Once we know the distance to a galaxy, we can use that information together with its apparent size to measure its true, physical size. Typical sizes of galaxies can range from much smaller than the Milky Way -- around 1 kpc in diameter -- to much larger -- around 50 kpc. Some dwarf galaxies are barely larger than one of the Milky Way's globular clusters, while some giant ellipticals could swallow the Milky Way up whole.

Masses of galaxies can be difficult to measure accurately, but we can get pretty good estimates by using the same techniques used in the Milky Way: measure the orbital speed of stars and apply Kepler's Laws, just like "weighing" the Sun using planetary orbits. The Milky Way's mass is about 1012MSun, including dark matter. The smallest dwarf galaxies have masses of only 109MSun or so, while the largest giant ellipticals can have up to 1014MSun.

Likewise, the luminosities of galaxies span a huge range, from 109LSun to over 1013LSun. Generally, the most massive galaxies are the brightest and largest.

3. Galaxy Clusters

Just as some stars appear more or less alone while others appear in clusters, galaxies can appear either in isolation or in groups or clusters. Galaxy clusters can include thousands of individual galaxies -- but they still make up only about 10% of the total mass in galaxies today. All the other galaxies live outside of clusters.

Like the stars orbiting in disk galaxies, galaxies in clusters move faster than they should, if the only mass present were the mass we see as stars. What could provide the gravity to make them move so fast? Dark matter! Again! Up to 90% or more of a typical cluster appears to be made up of the mystery substance...what could it be?

The Hubble Law

The Expanding Universe

Edwin Hubble measured the recession velocities of nearby galaxies using the Doppler shift technique and found a remarkable fact: the farther away a galaxy is from the Milky Way, the faster it appears to be flying away! In all directions, galaxies appear to be zooming away from us. The Universe is expanding!

Does this mean that we are at the center of the expansion? NO! Imagine painting dots on a balloon. Now blow up the balloon and measure the distances among 3 or 4 dots. Now blow up the balloon some more. What are the distances now? The greater the separation between two dots, the faster they grow more distant -- and every dot sees exactly the same thing , as if it were at the center of the expansion.

The relation between recession velocity and distance is called Hubble's Law :

Vr = H0  d
  • Vr = the recession velocity (speed away from us) in km/s of any galaxy
  • d = the distance from us to that galaxy in Mpc, and
  • H0 = the famous "Hubble Constant", which tells how fast the Universe is expanding, in km/s per Mpc.

The value of H0 is between 50 and 100, but the exact value is one of astronomy's most hotly contested debates.

Example: say the value of H0 is 75 km/s per Mpc (close to what the experts are now saying). How far away is a galaxy that is flying away from us at 1500 km/s?

Answer: Inverting Hubble's Law, we find d = Vr / H0 = 1500 km/s / 75 km/s/Mpc = 20 Mpc, or about 60 million light years.

We will return to this expanding Universe soon!

The Evolution of Galaxies

Galaxies must have been formed at some point and evolved to the point where we can recognize them today as spirals, ellipticals, or irregulars; what was the process like? And what path did they take to get where they are today? And what is the ultimate fate of galaxies like the Milky Way?

These questions are among the most important in astrophysics today, and many astronomers are trying to answer them in many different ways.

As we look at very distant galaxies, we are looking back in time, because the light takes so long to arrive here. When we look at M31, 2 million light years away, we are seeing it as it was 2 million years ago, when the light left on its journey. That's very little to a galaxy 10 or 12 billion years old.

But when we look 10-15 billion light years away, we are looking at objects as they were when the Universe was only 1-6 billion years old or so, perhaps only 10% of its current age. The "baby galaxies" we can see at those enormous distances -- pushing the Hubble Space Telescope and the 10-meter Keck telescopes to the limit -- seem to be small clumps of galaxies, rather than enormous Milky Way-sized systems. This lends support to the theory that galaxies formed from the bottom up, assembling small pieces together to make larger ones. Distant galaxies also appear bluer than nearby ones, implying that galaxies were forming more stars in the past than they are today.

Even today, many galaxies are colliding together and evidently combining into larger end products. These galaxy mergers were likely more common in the past, and probably played an important role in the formation of the Milky Way.

Lectures Table of Contents Astro 100

Last updated: September 2, 1999 Neal Katz
Houjun Mo Astronomy 100