1. Astronomy 1020 Stellar Astronomy Spring_2016 Day-35

2. Course Announcements Observing Reports are due: Mon. 4/18 at class time. APSU Research & Creative Activity Forum – Friday 4/15 1-4pm MUC, Ballroom and 3 rd floor meeting rooms FINAL EXAM (and Exam-4): TUESDAY, MAY 3, 1030 LAB MAKE-UP DAY: TUESDAY, Apr. 26 Lenses & Telescopes and Spectrometer ONLY!

3.  Black holes should lose energy by Hawking radiation: Virtual particles come into existence near it, and one falls into the black hole while the other becomes real and leaves.

4.  Black holes can be found by effects of their gravity.  Can exist in X-ray binary systems.  Rapidly varying X-ray sources require a very small, very massive object.

5.  It is possible that some gamma-ray bursts (GRBs) are given off by the creation of black holes.  The intense amount of radiation would spell trouble for life on a planet in its way.

6.  Earth’s motion produces the aberration of starlight.  Slight change in stars’ positions reveals Earth’s relative motion.  Analogy: falling rain or snow as seen from a moving car. CONNECTIONS 18.1

7.  The boxcar experiment shows that two events that are simultaneous for a stationary observer may occur at separate times for someone in a different frame of reference.  Consequence of the speed of light being constant for all observers, moving or not. MATH TOOLS 18.1

8.  The Lorentz factor tells you how much time dilation or length contraction occurs when something is moving at a given speed.  The Lorentz factor is always ≥ 1.  If you are traveling at a high speed through space, you will make your trip in a shorter distance and shorter time than as observed by someone on Earth. MATH TOOLS 18.2

9.  General relativity does not show that Newton’s laws are incorrect.  Newton’s laws turn out to actually be approximations of the more detailed general theory of relativity when you consider a weak gravitational field, such as Earth’s or the Sun’s.  New theories should incorporate the old, correct ideas. CONNECTIONS 18.2

10.  Scientists check theories by considering their limits.  General relativity reduces to Newton’s laws in the limit of being far from a mass. PROCESS OF SCIENCE

11.  You can find the total mass of two objects in an X-ray binary by considering the general form of Kepler’s third law, knowing their separation and orbital period.  For a blue supergiant and a compact object separated by 0.2 AU with a period of 5.6 days: MATH TOOLS 18.3

12. Concept Quiz—Event Horizon What is the event horizon of a supermassive black hole with a mass of 4 x 10 6 M  ? A.3 km B.12 km C.12 million km D.4 x 10 15 km

13. Concept Quiz—General Relativity According to general relativity, why does the Moon orbit Earth? A.The Moon and Earth have equal gravitational forces, but the Moon’s acceleration is greater than Earth’s. B.The Moon is being acted on by a gravitational force to keep it in the warp in spacetime created by Earth. C.The Moon is in a stable circular geodesic in the warp in spacetime created by Earth. D.General relativity says that Earth is orbiting the Moon.

14.  In everyday life, your perspective is easily related to someone else’s.  In such an inertial frame of reference, the usual laws of physics apply.  Velocities simply add.  Newton’s laws should apply to the universe.

15.  Things are different at high speeds.  All observers will see that light moves at a speed of c, regardless of their motion.  Motion must be analyzed differently, using Einstein’s special theory of relativity.

16.  Usually, perceived speeds depend on objects’ relative motion, but nothing can go faster than c!  Example: If a spaceship is going by Earth at a speed of c/2 and emits light, we’ll still see the light pass by Earth at a speed of c, not c + c/2.

17.  A galaxy is a large, gravitationally bound collection of stars and gas.  A galaxy like the Milky Way contains about 10 billion stars.  There are hundreds of billions of galaxies in the universe.

18.  Great Debate between Harlow Shapley and Heber Curtis.  Are galaxies just nebulae inside the Milky Way or separate “island universes?”  Inconclusive—not enough data, until Hubble.

19.  Hubble settled the debate after 1920 by finding the first accurate distance to Andromeda.  He used Cepheid variable stars.

20. Cosmology  Cosmology is the study of the universe, including: Structure: how matter is arranged. History: how stars, galaxies, and structure change. Origins: conditions at early times. Fate: the ultimate future of the universe.

21.  Cosmological principle: The universe is assumed to be homogeneous and isotropic.  Homogeneous: the same in all places.  Isotropic: the same in all directions.  True on large scales.

22. SDSS – The Northern Hemisphere

23. 2dF-2SLAQ – The Southern Hemisphere

24. 24 Concept Quiz— Cosmological Principle Suppose we observed that there were many more distant galaxies in the northern half of the sky than in the southern half. Which statement would be true about the universe? A.It is homogeneous and isotropic. B.It is homogeneous but not isotropic. C.It is isotropic but not homogeneous. D.It is neither homogenous nor isotropic.

25. The Universe Is Expanding  The universe (the space between the galaxies, specifically) is expanding.  It follows from two observations: Galaxies are moving away from us. Their speeds are proportional to their distances; galaxies farther away are moving away more quickly.  We’ll discuss each consideration at length.

26.  We measure speeds with the Doppler shift.  All galaxies except the nearest have a redshift: observed wavelength > rest wavelength.  Redshifted spectral lines = movement away.

27.  We define a number z for the redshift:  The recession velocity is given by v r = z  c.

28. Speeds and Distance  Galaxies farther away are moving more quickly (have greater redshifts).  If we measure distances and velocities, we find Hubble’s law: v r = H 0  d G  H 0 is a number called the Hubble constant, and measures how quickly the universe is expanding.  d G is the distance to the galaxy.

29.  Determining a galaxy’s redshift allows you to find its recessional velocity and its distance according to Hubble’s law.  Example: observing a spectral line at 379 nm instead of 373 nm. MATH TOOLS 19.1