GALAXIES- TYPES

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: denoted "E"

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. We adopt a number ranging from 0 to 9. An E0 designation means that the galaxy appears to us as circular. An E9 degination means that the galaxy appears to us as a very alongated ellipse. The larger the number, the more out of round the galaxy appears to us. From round to elongated is E0 to E9.

Dwarf Elliptical galaxies are simply very small elliptical galaxies- much smaller than the Mily Way galaxy and about the size of the bulge of the Milky Way.

Spiral Galaxies: donoted "S"

For spiral galaxies, there are clear trends between the bulge components and the disk components. Spiral galaxies are subclassified by the relative sizes of their bulges compared to their disks.

Barred sprial galaxies are doneted "SB". S for sprial. B for barred. Like regular spiral galaxies they are also subclassified by the sizes of their bulges compared to their disks. Thus we have SBa, SBb, and SBc barred spiral galaxies.

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.