Astronomy 1020 Stellar Astronomy Spring_2014 Day-38

1. Astronomy 1020 Stellar Astronomy Spring_2014 Day-38

2. Course Announcements • SW ch. 19-21 due Mon. 5/5; @ 11:00 am • Lecture-Tutorials will collected on Monday. • Reports due Mon. Apr. 28 • Final Exam & Exam-5 – Monday May 5 1030-1230

4. Concept Quiz— Elliptical Galaxies Which of these is not a characteristic of elliptical galaxies? • Stellar orbits go in all directions. • oval shape • star formation • dwarf and giant types

5. Rotation of the disk will naturally make spiral arms. • Central parts rotate around more quickly than the outer parts. • Disks can be “kicked” to rotate. • The arms cannot be physically confined, or they would be wound to nothing in no time.

6. Instead, spiral arms are spiral density waves: compression waves moving through the disk. • Stars and gas enter and leave the arms. • Waves trigger star formation. • Stars are not affected as much as the gas.

7. Orbital speeds of stars and gas depend on total amount of mass in the galaxy. • Measured by rotation curves. • It turns out that luminous(normal)mattercannot produce all the gravity in a galaxy.

8. According to Kepler’s laws, rotation speeds should decrease with larger radius; they do not! • There must be an additional source of gravity that does not make light, called dark matter.

9. Galaxies are mostly dark matter: About 95 percent of the mass of a spiral galaxy is dark matter. • It is located in a large dark matter halo around the galaxy. • Dark matter dominates elliptical galaxies as well.

10. What is the dark matter? • Probably WIMPs: weakly interacting massive particles (new type of matter). • Not likely MACHOs: massive compact halo objects (unseen clumps of regular matter).

11. Dark MatterLecture Tutorial pg. 143 • Work with a partner! • Read the instructions and questions carefully. • Discuss the concepts and your answers with one another. Take time to understand it now!!!! • Come to a consensus answer you both agree on and write complete thoughts into your LT. • If you get stuck or are not sure of your answer, ask another group.

12. Concept Quiz—Dark Matter How do we know that galaxies are mostly dark matter? • Stellar orbits are faster than expected from the gravity of the stars alone. • We see dark lanes in spiral arms. • Most galaxies have supermassive black holes.

13. Many galaxies have bright active galactic nuclei (AGNs). • Quasars are extremely luminous AGNs. • Luminosity equals about a trillion Suns!

14. Closest quasar is about 300 Mpc away. • Once thought to be an odd form of star, the quasar is a center of violent activity in the hearts of large galaxies. • Quasar stands for “quasi-stellar radio source.”

15. Active Galactic Nuclei and Quasars • Low-luminosity cousins are Seyfert galaxies and radio galaxies. • Only 3 percent of all galaxies contain AGNs. • Luminosity of the AGN can be as large as the rest of the galaxy.

16. Light from AGNs ranges from radio to gamma rays. • Existence of synchrotron radiation means large magnetic fields are present. • Must be about the size of our Solar System as well due to rapid variability.

17. AGNs are made by central supermassive black holes with accretion disks. • “Supermassive” = masses of thousands to tens of billions of solar masses.

18. Jets of material shoot out from the poles. • A dense ring (torus) of dust blocks the central regions. • What we see depends on the viewing angle.

19. CONNECTIONS 20.1 • The unified model of AGN says that the different types of AGNs are partly explained by our viewing angle. • If the AGN’s jets are beamed directly at us, the relativistic beaming of the light produced by the very fast-moving matter overwhelms observations.

20. Supermassive Black Holes • The orbital speeds of gas near the black hole yield its mass. • Supermassive black holes probably exist at the centers of all galaxies. • Normal galactic nuclei do not contain accretion disks. • Material in the accretion disk is an AGN’s source of fuel. Without it, the black hole can only be found by gravitational effects.

21. MATH TOOLS 20.1 • Supermassive black holes have Schwarzschild radii larger than stellar-mass black holes, but have a smaller density. • The radius of a 6.6-million solar mass black hole is:

22. MATH TOOLS 20.2 • Einstein tells us energy is related to mass by E = mc2. • By measuring the amount of energy emitted by an AGN, assuming an efficiency of matter-to-energy conversion of 10–20%, we can find out how much mass is consumed. • If the efficiency is 15% and the luminosity is 5 x 1035 J/s, then the mass consumed per second is:

23. It is unknown what makes a galaxy more suitable for life. • Mergers of galaxies can dramatically change the galactic environment, especially by sparking an AGN and increasing radiation.

24. CONNECTIONS 20.1 • This leads to optical illusions of superluminal motion where the jets appear to move faster than the speed of light. • They are not actually violating relativity’s rules.

25. Concept Quiz—Blue Light You do a big survey and find that extremely distant galaxies give off a lot of blue light. What is going on? • Interstellar dust is affecting the colors. • These galaxies are mainly elliptical galaxies. • The galaxies had a high rate of star formation when their light was emitted. • You must have made a mistake, since the stars in these galaxies would all be very old.

26. PROCESS OF SCIENCE • Wrong ideas can sometimes be useful. Hubble initially created the tuning fork diagram to show his view of galactic evolution, which was wrong. • Astronomers find it useful, however, for classification and teaching.