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• Edwin Hubble was the first astronomer to conceive of a classification for the different types of galaxies. His classification scheme is known as Hubble’s “Tuning Fork”. It is NOT a sequence of galaxy development, it simply shows how the shapes of different types of galaxies are related. An excellent diagram can be found at the Sloan Digital Sky Survey web site: http://cas.sdss.org/dr6/en/proj/advanced/galaxies/images/TuningFork.jpg
• There are four basic types of galaxies: Elliptical, Spiral, Barred Spiral, and Irregular. The first three types are subdivided into subtypes.
◦ Spiral (S) and Barred Spiral (SB) galaxies have a pinwheel pattern of bright spiral arms and a circular central region (galactic nucleus) when viewed face on. When viewed from the side they look like edge on disks with dark dust lanes and bright nebulae indicating the presence of a lot of interstellar matter and star formation. The difference between a regular spiral galaxy and a barred spiral galaxy is the shape of their nucleus. A regular spiral has a round galactic nucleus whereas the nucleus of a barred spiral is longer in one direction making it look more like a bar.
▪ The subtypes are assigned based on the size of the nucleus and the spacing of the spiral arms. Spirals, either barred or normal, are subtype “a” if they have a large bright nucleus and closely spaced arms: Sa or SBa. If a spiral has a small nucleus and widely spaced arms, it is given the subtype c (Type Sc or SBc). Subtype b represents the medium size of the nucleus and spacing of spiral arms.
◦ Elliptical (E) have very little interstellar gas and dust and no young stars. They shaped symmetrically ranging from perfectly round to very elongated similar to a football. The density of stars is greater in the center and becomes less toward the edges.
▪ Their subtypes are based on their overall shape determined by the relationship of their longest axis and their shortest axis. The subtypes range in numbers from 0 to 7 with type 0 of perfectly round shape and type 7 of the most elongated shape. However, these numbers are based on the way they look from Earth. Even a football shape may look round when viewed from one end.
◦ Irregular galaxies (Irr) have rather unusual shapes and many of them contain large amounts of interstellar gas and dust.
• The laboratory exercises include the determination of the relative distances of galaxy clusters. All galaxies in a galaxy cluster are pretty much the same distance away from Earth. We can also make the assumption that the size range of the galaxies in one cluster is very similar to that of any other cluster. We can determine how many times further one cluster is away from Earth than the other by measuring the ratio of the sizes of the largest few galaxies in each. For example, you can measure the longest axes of the five largest galaxies in each cluster. Then, take the average for each cluster. The ratio of this average for the closer cluster (that size average is going to be larger) divided by the size average for the further cluster (that size average is going to be smaller) is going to be the factor of how many times further the further cluster is than the closer one. If the absolute distance of the closer cluster is known, this absolute distance can simply be multiplied by this factor to get the absolute distance of the further cluster.
• The last phenomenon investigated in this laboratory is that of galaxy cannibalism. Unusual shapes of some galaxies are caused by the gravitational interaction of galaxy with another one they are on collision course with. Gravity is an action-at-a-distance force so that the colliding galaxies are already distorting each other’s shapes before their visible matter intertwines.
Note: For questions 1 – 6 including Table I: Classification of Galaxies, you will use http://sites.gpc.edu/astr1020lab/galaxies-lab/ . For the remaining questions you will use the Galaxies Fields file in the table of contents of the Galaxies Lab Content Module.
• Q1: Look at the third image in the first row, Milky Way Galaxy (infrared) taken by the COBE satellite. There are three general types of galaxies: Spiral (specify: regular or barred), Elliptical, and Irregular. When you look at the image, think about what these general types of galaxies might look like from different perspectives. Also, think of the differences in the amount of gas and dust contained in these general types.
• Q2: Again, decide on one of the three general types and specify regular or barred if you think it is Spiral. Pay particular attention to the nucleus of the galaxy.
• Q3: Think about whether or not you would be able to discern the particular shape of the galactic nucleus from the COBE infrared image of the Milky Way Galaxy.