Summary

In this exercise, you will learn to classify galaxies using the Hubble Classification scheme. You will also find their distances using the Hubble law.

Background and Theory

A galaxy is an assembly of between a billion (109) and a hundred billion (1011) stars. In addition to stars, there is often a large amount of dust and gas, all held together by gravity. The Sun and the Earth are in the Milky Way Galaxy (sometimes referred to as "the Galaxy"). Galaxies have many different characteristics, but the easiest way to classify them is by their shape (or "morphology"), and Edwin Hubble devised a basic method for classifying them in this way. In his classification scheme, there are three types of galaxies: spirals, ellipticals, and irregulars.

• Spiral galaxies were the first to be discovered, because the most luminous galaxies close to the Milky Way are spirals. These galaxies get their name from the spiral distribution of light seen in photographs. A subclass of spirals contains the barred spirals. Ordinary spirals have a nucleus which is approximately spherical, while barred spirals have an elongated nucleus which looks like a bar. Spirals are labeled as Sa, Sb, or Sc; barred spirals are designated SBa, SBb, or SBc. The subclassification (a, b, or c) refers both to the size of the nucleus and the tightness of the spiral arms. The nucleus of an Sc galaxy is smaller than in an Sa galaxy, and the arms of the Sc are wrapped more loosely.
• Elliptical galaxies are classified according to the relative sizes of their apparent major and minor axes. Thus if x and y are these apparent axes, an elliptical galaxy is classed as En where

n=10·(x-y)/x.

All elliptical galaxies have n between 0 and 7.
• Irregular galaxies have no obvious spiral or elliptical structure. It is thought that many irregulars were once spiral or elliptical, but that a close encounter with a larger galaxy disrupted the organization of the material by gravitational forces. Irregular galaxies come in two flavors: Irr I's are resolvable into individual stars, and Irr II's are not.

Not all galaxies are easily classified. Quasars are the bright, superluminal cores of very distant active galaxies. These galaxies are so distant in fact, that the quasars look like stars in most images. However, their redshifts are so high that we know that they can not be stars. These quasars are moving away from us at extremely high velocities. Quasar 3C273, for example, is moving away from us at 43,700 km/s!

The relationship between galaxy types is not clear. Because there is little evidence of star formation in elliptical galaxies, and because they seem to have extremely small angular momentum, it was thought that perhaps elliptical galaxies are much older than spirals. If this is true, then we would expect to see more spiral galaxies as we look farther out into the universe (that is, back in time). Recent observations made by Hubble Space Telescope do show more spirals in distant clusters of galaxies, however, there are also many more distorted galaxies and blue irregulars with enormous star formation rates.

We do know that there is a correlation between the environment and the type of galaxy that formed there. Dense clusters have much higher percentages of elliptical galaxies, indicating that dense galaxy formation regions are more likely to form ellipticals. The entire problem is not yet well understood, and many explanations rely heavily on the postulated existence of dark matter.

In the late 1920's, Edwin Hubble discovered one of the most fundamental properties of the universe, namely that it is expanding in all directions with a speed proportional to the distance. He used the redshift of spectral lines from distant galaxies (calculated by Slipher) whose distances could be determined by other means (for example, by Cepheid variable observations or measuring the angular sizes of HII regions). He interpreted the observed spectral shift as a Doppler shift, and determined that all galaxies (except a few very close ones that are in the same group of galaxies as the Milky Way) are receding from the Milky Way Galaxy with speeds proportional to their distances:

v=H·d,

where d is the galaxy's distance (in Mpc), H is Hubble's constant (with a modern value of about 65 km/s/Mpc), and the speed v is found from the Doppler shift of the galaxy.

Procedure

Print out the worksheet.
1. Examine the images of each of the galaxies listed in the table below. When there is more than one galaxy in the image, use the finding chart to identify the galaxy in question. Identify each galaxy's type. Estimate the subgroup of the spirals, and measure the major and minor axes of the ellipticals so that you can calculate n and find the subclass. Use any scale you like to measure the major and minor axes, but be sure to measure both axes on the same scale. Note: you only need to measure the axes for the elliptical galaxies!

Galaxy images
NGC 1381 NGC 1398 NGC 224 NGC 3031 NGC 3384 NGC 4374 NGC 4435 NGC 4486
NGC 4565 NGC 4594 NGC 4736 NGC 5055 NGC 5194 NGC 5236 NGC 7331 3C273

2. Use the Hubble constant and the formula given in the Background and Theory section to find the distance to each galaxy. Convert the distance from Mpc to light years. (1 Mpc = 3.26·106 l.y.) Converting to light years gives the amount of time the light traveled between leaving the galaxy and arriving at the telescope.
3. Check to make sure that all of your answers make sense. For example, check that none of the galaxies' light has been traveling for more than the age of the Universe. It is often difficult to make astronomical numbers meaningful. For each of the galaxies, indicate what was happening in the Earth's history when the light left that galaxy. For reference, the dinosaurs became extinct about 65 million years ago, Pangaea split into multiple continents about 200 million years ago, the Earth is about 4.5 billion years old, and the Universe is about 15 billion years old.
4. The velocity of NGC224 is negative. What does this mean? What are the implications for applying the Hubble Law to this galaxy?
5. 3C273 is one of the brightest radio sources in the sky. But the type of galaxy 3C273 is impossible to find from these images. Does this make sense? Hint: Look at the distance...
6. Look again at the color image of ngc5194. What color are the arms? What color is the bulge? Explain the colors that you see.