The Milky Way is just one of about 50 billion galaxies that are thought to exist. Just as stars can be classified using an H R diagram, galaxies can also be classified according to certain physical properties. The Milky Way is a spiral galaxy, but most galaxies are not spirals they have other shapes. Edwin Hubble began to classify them in the 1920's and his classification system is still in use today. Types of Galaxies 1) Spiral Galaxies Nuclear bulge and spiral arms (the tighter the spiral arms, the larger the nuclear bulge). Sa tightly wound arms, large nuclear bulge Sb moderately tight arms and medium bulge Sc loosely wrapped arms, tiny bulge Because many galaxies are not face on to us, they must be classified by the size of the nuclear bulge. Page 388 389. May 8 8:38 AM May 8 8:45 AM The Milky Way appears to be a type of spiral galxy called a barred spiral (page 392). Barred spiral galaxies have a straight bar of material passing through their nuclear bulges. The spiral arms radiate out from the ends of the bar, not from the bulge itself. Barred spirals are classified the same way that normal spirals are: SBa, SBb, SBc. All galaxies but one (NGC4622) are trailing arm spirals. Elliptical galaxies have no spiral shape. They are classified according to how round or oval they look (page 393): 1) E0 look very round 2) E7 look very oval Galaxies are classified simply by their appearance as seen from the earth they probably look different from other angles. May 8 9:00 AM May 8 9:04 AM Elliptical galaxies look very bland because they have little gas and so should have few stars forming. Ellipticals come in all sizes from dwarf (common) to giant (rare) the largest elliptical is over 20 times the size of the Milky Way (10 trillion solar masses)! Edwin Hubble produced a "tuning fork" diagram to show how the types of galaxies are related to one another. Dwarf galaxies have so few stars that it is possible to see straight through them, making them difficult to detect. May 8 9:21 AM May 8 9:28 AM 1
Astronomy Notes Chapter 16.notebook Although Hubble's diagram makes it look like one galaxy shape can be produced by another, there is little evidence that galaxies change their classification. Some galaxies do not fit Hubble's classification and are known as irregular galaxies (page 395). There are two classes: 1) Irr I irregular, but with some structure 2) Irr II no structure of any sort Sagittarius Dwarf Galaxy Small Magellanic Cloud Irregulars might sometimes be produced when one galaxy collides with another. May 8 9:37 AM May 10 9:35 AM Galaxies do not occur in isolation, but instead are found in large, gravitationally bound groups called clusters. The galaxies in a cluster orbit one another and sometimes collide. The Milky Way and its neighbors belong to a cluster called The Local Group. The galaxies in the local group are moving toward the Great Attractor, which is located in the direction of Hydra and Centaurus. May 8 9:47 AM May 10 9:41 AM Clusters are bound into even larger groupings called superclusters. The overall appearance of this is like soap suds, which gives clues to the early universe. The clusters in a supercluster are not gravitationally bound they are slowly moving apart from one another. Galactic clusters may be regular or irregular, rich or poor. All superclusters are moving away from one another. Between the superclusters are great, mostly empty spaces called voids. Galactic clusters appear to be found along the surfaces of the voids. May 10 9:53 AM Galaxies sometimes collide, which can throw some stars out into intergalactic space. Collisions also trigger massive amounts of star formation. May 10 9:49 AM 2
When a small galaxy collides with a large one, galactic cannibalism occurs. The smaller galaxy is absorbed into the larger one and may lose its identity. Galactic clusters do not have enough visible mass to stay graviationally bound, but they are. It is felt that dark matter supplies the missing mass. Giant ellipticals may form in this way, continuing to grow over time. May 10 10:02 AM May 10 10:10 AM When the spectra of galaxies were first measured, it was found that the light from most of them was very red shifted. This meant that they were moving away from us at a substantial speed. What does this mean? By comparing the red shift of a galaxy to its distance, Hubble found that the farther away a galaxy is, the faster it is moving away from us. This movement is now called the Hubble Flow. Hubble found that a graph of distance versus reshift produced a nearly straight line. From this, he derived the Hubble Law: Recessional Velocity = H 0 x distance H 0 is the Hubble constant its value is uncertain, but it is thought to be close to 72 km/s/mpc. May 10 10:11 AM May 10 10:16 AM Hubble, then, discovered that the universe is expanding. Will it expand forever or will it someday stop and begin to collapse? Currently, it appears that there is not enough mass to stop the expansion it will expand forever and will eventually suffer heat death. How can the distances to very distant galaxies be measured so that Hubble's equation can be verified? One method is to use supernovae as standard candles. What is a standard candle? Page 405. May 10 10:22 AM May 10 10:25 AM 3
Since the speed of light is finite, the farther out into space we look, the further back in time we are seeing. We have currently seen galaxies over 13 billion light years away this means that we are seeing the universe when it was very young. How can we see things so far away? One method is gravitational lensing where a distant galaxy bends the light of a galaxy even more distant. May 10 10:41 AM Dec 8 12:19 PM 4
Attachments Colliding Galaxies Fornax Cluster Galaxy Zoo Milky Way Andromeda Collision