Astronomy 1020 Stellar Astronomy Spring_2016 Day-38

Course Announcements THE TODAY – 4/25 is THE last day to turn in any labs other than the two being set-up for the make up day. I will collect the L-T books TODAY to get them back to you on Wednesday. FINAL EXAM (and Exam-4): TUESDAY, MAY 3, 1030 LAB MAKE-UP DAY: TUESDAY, Apr. 26 Lenses & Telescopes and Spectrometer ONLY!

 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) matter cannot produce all the gravity in a galaxy.

 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.

 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.

 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).

Dark Matter Lecture 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.

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

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

 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.”

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.

 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.

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

 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.

 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. 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. CONNECTIONS 20.1

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.

 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: MATH TOOLS 20.1

 Einstein tells us energy is related to mass by E = mc 2.  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 J/s, then the mass consumed per second is: MATH TOOLS 20.2

 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.

 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. CONNECTIONS 20.1

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? A.Interstellar dust is affecting the colors. B.These galaxies are mainly elliptical galaxies. C.The galaxies had a high rate of star formation when their light was emitted. D.You must have made a mistake, since the stars in these galaxies would all be very old.

 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. PROCESS OF SCIENCE