The Basics: Galaxies
What is a galaxy? Stated simply, a galaxy is a gravitationally bound collection of stars. Sounds pretty small and boring, hey? To expand on that a galaxy is a very large collection of stars sometimes numbering in the trillions and they come in many shapes; elliptical, spiral, irregular, barred, and combinations of the above. Some appear almost colorless and others cover the color spectrum. To date several billion galaxies have been viewed. Think about that for a minute, several billion galaxies each containing hundreds of billions of stars. I’m thinking we are not alone!
A galaxy contains nebula, stars, star clusters, black holes, supernovas, planets, moons, etc. Basically everything we can see is contained in a galaxy except for other galaxies. Our galaxy is formed using the same mechanics as star formation but on a much larger scale. It is thought that this very large complex of gas and dust began to condense in a spiral motion causing the dust/gas to flatten due to the Conservation of Angular Momentum law. Clumps of dust began to form and these clumps evolved into stars via the star formation process I described on The Basics: Nebula page. This process will continue until all available dust and gas are gone. Our galaxy is estimated to be 10 billion years old and some galaxies viewed by the Hubble Space Telescope are estimated to be over 13 billion years old.
Elliptical galaxies have many shapes ranging from circular like a cotton ball to elliptical like the tip of a Qtip viewed from the side. They have a scale describing how elliptical they are, E0 types are the most circular and E7 are the most elliptical. You can see a few elliptical galaxies in my image of the Virgo Galaxy Cluster. They are fuzzy looking and they appear to have a central core. This fuzzy appearance indicates that this type of galaxy does not have a lot of structure such as spiral arms or a bulging central disk. Some classes of elliptical galaxies also display very little hydrogen gas and no dust lanes along with minimal evidence of young stars which would indicate that they are older.
Spiral galaxies are named as such because they look like they have arms that radiate out from a central core. The arms are spiral in shape and it is thought that this is due to the rotation of the galaxy. Our neighboring galaxy, the Andromeda Galaxy is a spiral galaxy and contains about a trillion stars. My Whirlpool Galaxy image shows the spiral arms face on. A spiral galaxy has a well defined core, a distinct bulge at the core, and several spiral arms. Also, a galaxy does not need to be viewed face-on to to determine what type of galaxy it is. A galaxy viewed edge-on will show whether or not there is a galactic bulge, if it does it is a spiral galaxy. Take a look at the Sombrero Galaxy and see what I mean. They are classified using three designations; Sa, Sb, and Sc. Type Sa have the largest galactic bulge plus the tightest and most circular shaped arms. Type Sc have the smallest galactic bulge and typically have loosely spaced and less defined arms.
Barred spiral galaxies are a variation of spiral galaxies. The major difference is the bar shape of the central core and the spiral arms of a barred spiral originate from the end of the bar shaped bulge. This is different from a spiral galaxy because a spiral galaxy core is typically round. The barred spiral galaxies are designated as SBa, SBb, and SBc with each definition the same as spiral galaxies.
Irregular galaxies are galaxies that do not fit into the Elliptical or Spiral category. They do not have the spiral arms or bulging core areas of a Spiral galaxy nor do they have the roundness of an Elliptical galaxy. Irregular galaxies can be classified as Irr I and Irr II. Of the two the Irr I type is most common. Here is a typical irregular galaxy, the Barnard Galaxy.