GALAXIES I

Types of Galaxies

There are other types of galaxies in a addition to spiral galaxies. The main galaxy types are:

There are roughly 100,000,000,000 (100 billion) galaxies in the universe. Each contains about 100,000,000,000 (100 billion stars). Therefore, there are about 10,000,000,000,000,000,000,000 stars in the universe! This number reads 10,000 billion billion.

The Hubble tuning Fork was Edwin Hubble's attempt at classifying galaxies and trying to understand their evolution. He thought that one type of galaxy evolved into another. We no longer think this is true.

Elliptical Galaxies

Some giant elliptical galaxies are found in the centers of giant clusters of galaxies. Some of them are so huge that 20 Milky Way galaxies could fit inside them!

Not all elliptical galaxies are "round" in appearance. We measure their long axis and short axis and then compute a number to describe their "out of roundness", called ellipticity.

Spiral Galaxies

Spiral galaxies are classified by the relative sizes of their bulges compared to their disks.

For spiral galaxies, there are clear trends between the bulge components and the disk components.

Irregular and Dwarf Galaxies

Our galaxy has two companions, the Small and Large Magellanic "clouds". These are irregular galaxies. The SMC is not really forming stars anymore, but the LMC is.

Nature makes more small things than large things. Most of the galaxies in the universe are dwarf irregulars and dwarf ellipticals. They do not have many stars in them and so are not vary bright. They do not contribute a lot to the total luminosity of galaxies in the universe.

Distances to Galaxies

Measuring the distances to galaxies is a tricky business. We use different methods for different ranges of distances. This can be tricky because each step going out in distance depends upon how well we know the previous steps. The basic methods are (in order of increasing distance)...