Astronomy 1020-H Stellar Astronomy Spring_2016 Day-38

Course Announcements FINAL EXAM (and Exam-4): THURSDAY, MAY 5, 0800 LAB MAKE-UP DAY: TOMORROW – both morning & afternoon – E118 OR B107 Lenses & Telescopes and Spectrometer ONLY!

 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

 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

 There is a lot of dust in our galaxy.  Dust blocks light, making things fainter. They look farther away.  Makes it difficult to find accurate distances to objects that are affected by dust.

 The Milky Way is a barred spiral galaxy, as determined by our observations: SBbc.  The starry disk is evident in the night sky in areas with low light pollution.  We can see dark lanes of dust and gas.

 Globular clusters can help find distances in our galaxy and to other galaxies.  Bound collections of up to a million stars.  Luminous and can be seen at great distances.

 Globular clusters contain very old stars.  Low-mass stars have evolved to be RR Lyrae standard candles.  Shapley used them to find the size of our galaxy and saw that our Sun is not near the center.

Structure of the Milky Way  Globular clusters are spread throughout the spherical halo encompassing the Milky Way disk and central bulge.  The Sun is about 8,300 pc from the center of the Milky Way.

Structure of the Milky Way  Observations of the orbiting speeds of neutral hydrogen gas have revealed a dark matter halo encompassing the rest of the Milky Way, stretching out to a distance of 45,000–50,000 pc from the center.

 Doppler shifts from orbital rotation give the speed of gas and stars in the disk.  The neutral hydrogen gas emits radio waves with = 21 cm.

 These speed measurements show a flat rotation curve for the Milky Way.  About 90 percent of the matter in the Milky Way is dark matter.

 We can apply Newton’s and Kepler’s laws to calculate the mass of the Milky Way inside of the Sun’s orbit.  The Sun’s orbital speed is ~220 km/s, at a distance of 8,300 pc from the center. MATH TOOLS 21.1

 Observing the stars orbiting the Milky Way’s supermassive black hole allows us to calculate its mass.  Star S0-2 has a period of 15.8 years and a semimajor axis of 1.5 x km.  Use Kepler’s third law, and the fact that the star’s mass is negligible to find the mass: MATH TOOLS 21.2

 Globular clusters in the halo are older than the open cluster stars in the disk.  Globular clusters can be 13 billion years old.  We see no young globular clusters.  The processes that formed the two types of clusters must have been very different.

Milky Way Scales Lecture Tutorial pg. 135 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.

 Stars are mostly hydrogen and helium.  Elements heavier than boron must have been formed in stars.  The more massive elements found in a star, the more prior star formation took place.

 New heavy elements are ejected into space through supernovae.  New stars form with some heavy elements.  The abundance of heavy elements records the history of star formation.

 We observe younger stars to have more massive elements than older stars.  Globular clusters can have as little as 0.5 percent of the amount of massive elements that disk stars like the Sun possess.

The First Stars  The Sun has about a 2 percent concentration of massive elements.  Even globular cluster stars have a small amount of massive elements.  There must have been at least one generation of stars that existed prior to globular clusters to process those elements.  Those stars would have been very massive, and therefore would have died quickly.

 Younger stars appear close to the thin disk.  Thicker disk (~3,700 pc thick) has older stars that were either captured from galactic mergers or diffused from the thin disk.