The Memphis Astronomical Society Presents A SHORT COURSE in ASTRONOMY

1. TheMemphis Astronomical SocietyPresentsA SHORT COURSEinASTRONOMY

2. CHAPTER 11G A L A X I E SDr. William J. BuslerAstrophysical Chemistry 439

3. GALAXIES A. In the Beginning… • For the first moments after the Big Bang, the event which ultimately gave rise to the Universe, there probably existed only energy. • This energy was primarily in the form of very short-wavelength, ultra-high-energy photons known as -rays; however, all other forms of electromagnetic radiation, including visible light, were also present.

4. GALAXIES A. In the Beginning… • After a few seconds, some of this energy was probably converted into matter, primarily hydrogen, deuterium, and some helium. • After a few minutes, lithium and beryllium probably were formed in small amounts. • No other elements were made until much later, after new stars began to synthesize heavier elements from carbon through iron.

5. GALAXIES A. In the Beginning… • The rest of the elements were eventually synthesized in the explosions of supernovae, many millions of years after the Big Bang. • The photons which escaped from the Big Bang now constitute the “Cosmic Background Radiation”, which is blackbody radiation from material at a temperature of 2.73  0.01 K.

6. Cosmic Background Radiation Pasachoff #123

7. GALAXIES B. Formation of Galaxies • As the blobs of matter flew away from the site of the Big Bang, they condensed (as the Universe began to cool) into protogalaxies having various amounts of angular momentum and turbulence. • In each protogalaxy, the matter with the least angular momentum fell into the center and became the nucleus. • Matter with high angular momentum flattened into a rotating disk which would later become the spiral arms.

8. GALAXIES B. Formation of Galaxies • Small blobs of matter with intermediate amounts of angular momentum fell into elliptical orbits randomly oriented around the nucleus, and would eventually evolve into globular clusters. • A small amount of material was left behind to become the galactic halo. • Note the strong parallels to the formation of the Solar System (Chapter 6): • nucleus = Sun, • disk = planets, • globular clusters = comets, • halo = Oort cloud.

9. The Milky Way Galaxy Pasachoff #103

10. GALAXIES C. Types of Galaxies • Despite the fact that all galaxies were formed in the same general way, galaxies appear in a wide variety of shapes. • Edwin Hubble proposed that galaxies evolved along a “tuning fork” diagram; the direction of evolution was debated. • Cf. The “Main Sequence” on the Hertzsprung-Russell Diagram.

11. Hubble “Tuning-Fork” Classification of Galaxies Pasachoff #115

12. GALAXIES C. Types of Galaxies • Not all the new protogalaxies had the same quantity of matter; there was also an uneven distribution of angular momentum and turbulence; hence, not all the resulting galaxies are the same. • This is analogous to the fact that the mass of a star determines its position on the Main Sequence. • Also, recall that the amount of angular momentum in a primordial stellar nebula determines whether it will give rise to a single star, a multiple star system, or a star with planets.

13. GALAXIES C. Types of Galaxies 1. Galaxies with a relatively large proportion of low-angular-momentum matter developed enormous nuclei (or multiple nuclei), almost certainly containing one or more supermassive black holes. Most of the rest of the matter in these galaxies became globular clusters; almost no disk formed. These are the ellipticalgalaxies and appear to be the earliest to form. (Turbulence delays star formation!)

14. GALAXIES C. Types of Galaxies 2. Galaxies with high amounts of angular momentum developed a relatively small nucleus (possibly containing black holes), a moderate number of globular clusters, and a well-defined disk orbiting the nucleus in a flat plane. These are the spiralgalaxies; they rotate the fastest. Those with double nuclei (analogous to binary star formation) might be the barred spirals. 3. Galaxies with a very high degree of turbulence postponed their star formation, and even now have an irregular appearance.

15. GALAXIES C. Types of Galaxies 4. Quasars and Seyfert Galaxies: As the nuclei of the giant elliptical galaxies formed, it is very likely that supermassive black holes were created there. The remaining infalling material became superheated, emitting tremendous amounts of energy in all parts of the spectrum. (Recall the binary star / black-hole system, Cygnus X-1). Those giant elliptical galaxies which are far enough away from us to still be seen in this early stage of their formation are known as quasars.

16. M87’s “Jet” Pasachoff #120

17. GALAXIES C. Types of Galaxies • As the rate of residual material falling into the supermassive nuclear black hole(s) decreased, the energy output fell off accordingly; these are the Seyfert galaxies. • Spiral galaxies like the Milky Way, small galaxies, and turbulent irregular galaxies probably were never quasars or even Seyfert galaxies.

18. GALAXIES D. Evolution of Galaxies 1. Ellipticals had star formation first; now all contain very old Population II stars, mostly in globular clusters, with no gas left. 2. Irregular galaxies are highly turbulent, with new star formation just now getting underway. 3. Barred spirals may be due to the merger of two galaxies – it is possible for a supermassive black hole to be ejected in the process.

19. A. Computer Simulation of Merging Galaxies B. The “Antennae”: NGC 4038-4039 Pasachoff #116

20. GALAXIES D. Evolution of Galaxies 4. Spiral galaxies contain old stars (formed early) in the nuclear bulge and in the halo of globular clusters (Population II). The disk appears to have spiral structure, but that is probably an illusion. The best explanation is that gravity waves from an asymmetric nucleus, or density waves of unknown origin, or shock waves from supernovae, trigger the formation of new stars in a curved pattern radiating out from the nucleus.

21. GALAXIES D. Evolution of Galaxies • These new (Population I) stars act as markers for star formation activity, giving a spiral appearance like curved spokes. • These stars tend to be brighter and bluer than the old Population II stars. • Recall that many spiral galaxies look reddish toward the center and bluish in the spiral arms.

22. .. M31, Galaxy in Andromeda Astrophotograph by David Hanon

23. GALAXIES D. Evolution of Galaxies • It is easy to see the spiral structure in many other galaxies, but since we are inside the Milky Way, we must use indirect means to unravel its maze-like structure. • One way is to use radio telescopes to detect the 21-cm radio frequencies emitted by clouds of hydrogen in the spiral arms, as the electron flips its spin with respect to the proton. • Since hydrogen is found primarily within the arm structure, it serves as a “tracer” for the spiral arms. • The Doppler effect allows us to see whether the hydrogen is moving towards us or away from us.

24. Source of 21-cm Radiation Pasachoff #110

25. Spiral Arms in the Milky Way Galaxy Pasachoff #111

26. GALAXIES D. Evolution of Galaxies • Continuous recycling takes place in spiral galaxies due to supernova explosions and new star formation. • As a result, spirals are the only type of galaxy where solid planets (and therefore civilizations) are possible.

27. T H E E N D