2. Can observe radio waves from the nucleus see a strong radio source there Sagittarius A* or Sgr A*.

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1 7/7 The Nucleus of the MW its center 1. Can t see the nucleus in visible light too much stuff in the way. 2. Can observe radio waves from the nucleus see a strong radio source there Sagittarius A* or Sgr A*. 3. IR observations show stars to be tightly packed near the center. 4. Distances between stars about 1000 AU not 100,000 AU like out here in the disk. 5. We see a ring of gas that extends from about 6 pc from the center to 25 pc from the center. 6. This gas may have been pushed away from the center by an explosion 100,000 years ago. 7. From the orbital parameters of the stars near the center and Kepler s third law, we have calculated the mass of the nucleus to be 2 million solar masses. 8. The size of the nucleus is no more than 2 AU in diameter we see fluctuations in the nucleus over a time periods of 15 to 20 minutes. The size of the object is the distance light can travel in this time about 2 AU. 9. Something that small and that massive must be a black hole! Other Evidence: 1. Sgr A* does not move stars around it do, but it doesn t. 2. Evidence of black holes at the centers of other Galaxies as well. How did the MW form? We don t know! One scenario has it forming like the solar system as a contracting cloud of gas but doesn t account for the black hole at the center nor does it work numerically. Another possibility: 1. Galaxy formed in the early universe then, the universe was smaller and denser. 2. Those conditions were conducive to the production of massive stars hundreds of solar masses. 3. These massive stars would have evolved very rapidly to become black holes. 4. These black holes would have attracted matter in the form of other stars, other black holes, and gas. 5. Some of this matter would be consumed making the black hole larger and more effective at

2 attracting more matter. 6. Some stars would stay in orbit about the black hole some stars would be formed in the gas around the black hole slowly a galaxy would form around the black hole. 7. Note that the distribution of dark matter in the early universe affected the formation of galaxies - galaxies formed where the concentration of dark matter was large. 8. Galaxies are relatively large compared to the distance between them collisions are common. 9. Part of the evolutionary process involves interactions between galaxies. Other Galaxies A century ago, it wasn t clear if there other galaxies than the MW. Using the 100-inch reflecting telescope on Mt. Wilson, Edwin Hubble was able to image stars in objects then called spiral nebulae. Some of the stars he could image were Cepheid Variable Stars There is a relation between the period of variation and the absolute magnitude of these stars: 1. Variable stars vary in brightness by expanding and getting brighter then contracting and getting dimmer. 2. The larger the star, the more slowly it oscillates longer period of oscillation. 3. But more massive stars are more luminous more intrinsically bright. 4. The longer the period of variable star, the brighter it is. 5. Need only calibrate the variable stars measure the period to find the absolute magnitude. 6. We can always measure the apparent magnitude from these two measurements, we can find the distance to the star. Hubble found that the Andromeda Galaxy was too far away to be part of the MW it was a galaxy in its own right. We can see 100's of billions of galaxies in the universe. Hubble also measured the speeds of the galaxies using the Doppler effect and spectroscopy. He found that, except for the nearest galaxies, all galaxies were moving away from us. The more distant the galaxy, the faster it was moving away. From this we conclude that the universe is expanding.

3 Hubble fit the data and got the following relation: Here v is the speed of the galaxy in km/s and d is the distance to the galaxy in Mpc. Our current best measurement for the Hubble constant is 72 km/s/mpc. Classification of Galaxies Hubble Scheme Three main types spiral, elliptical, irregular. Spiral galaxies are classified by how large their nuclear bulge is and how tightly wound their spiral arms are: Sa, Sb, Sc from largest bulge tightest arms to smallest bulge and loosest arms. Barred spirals are classified the same way: SBa, SBb, SBc MW is an SBb galaxy. Elliptical Galaxies Classified by how elliptical they are from E0 (spherical) to E7 How the number # is calculated in E#: Let a be the largest dimension of the galaxy and b be the smallest dimension. Then rounded to the nearest integer. Irregular Galaxies being irregular, no way to classify. Measuring Galaxies Distance can t use parallax galaxies are too far away. Use distance indicators or standard candles objects whose absolute magnitudes we can deduce.

4 1. Cepheid variable stars M = 7 2. Brightest Red Supergiants M = 9 (We assume that the brightest red supergiants in other galaxies are same brightness as the brightest red supergiants in the MW.) 3. Largest Globular Clusters M = Largest HII regions M = Type Ia Supernovae M = Brightest galaxies in a group M = Redshift and the Hubble law. Measure the size of a galaxy Measure the angle subtended by the galaxy in radians and multiply by the distance to the galaxy to get its diameter. Measure the masses of galaxies using Kepler s third law either by examining the motion of the stars in a galaxy or the motions of galaxies in a cluster. Once we know the distance to a galaxy, we can find its luminosity. Physical Characteristics of Galaxies: Spirals: 1. Gas and dust and star formation in their spiral arms. 2. Masses range from a billion to a trillion solar masses. 3. Diameters range from 15,000 ly to 150,000 ly. 4. Luminosities range from a 100 million to 100 billion solar luminosities. Ellipticals: 1. Very little gas and dust and no star formation. 2. Composed of old stars. 3. Masses range from hundreds of thousands to 10's of trillions of solar masses. 4. Sizes range from 3000 ly to 600,000 ly. 5. Dwarf ellipticals are probably the most common galaxy in the universe. 6. Luminosities range from a million to 100 billion solar luminosities. Irregulars: 1. Contain gas and dust but not as dense as in spirals there is some star formation. 2. Masses range from a 100,000 to 100 million solar masses. 3. Sizes range from 3000 ly to 30,000 ly. 4. Luminosities from 10 million to a couple billion solar luminosities. 5. Probably dwarf ellipticals torn apart by large companions. Lives of Galaxies: Galaxies live in clusters some small such as our own local group containing about 3 dozen

5 galaxies; some large with thousands of galaxies. Unlike stars that form in clusters, galaxies form first and then combine into clusters. Galaxies are large compared to the distances between them collide quite often. We see giant elliptical galaxies at the centers of all large clusters galaxies move to the center and merge. It is possible for a couple of elliptical galaxies to collide and become a spiral and for two spiral galaxies to collide and form an elliptical. Often, star formation is triggered by the collision of two galaxies. Active Galaxies: An active galaxy is one that emits a prodigious amount of energy from its nucleus. Driven by black hole. Three main types: Seyfert Galaxies Most often when taking pictures of a spiral galaxy, we over expose the nuclear bulge so that we can see the spiral arms. When doing so, a Seyfert galaxy looks just like a normal spiral galaxy. In low exposures, Seyfert galaxies have much brighter nuclei than ordinary galaxies. About 2% of all spiral galaxies are Seyfert galaxies. Radio Galaxies generally elliptical galaxies that generate a lot of radio energy from their centers. Look at a radio galaxy from the side and see radio lobes. Matter spiraling into the black hole at higher latitudes gets spun out along the axis of rotation and ejected from the galaxy it emits radio waves to form lobes. It s possible to be oriented so that we are looking down the lobe looks like a different type object. Quasars very far away and very bright may be young galaxies with much matter near their centers matter then is consumed by the black hole with a tremendous outflow of energy. Eventually, the matter in the center is consumed, and the quasar turns off. It then becomes an ordinary galaxy. All active galaxies are probably the same type of object at different stages of evolution and viewed from different perspectives.

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