Astronomy 1020 Stellar Astronomy Spring_2016 Day-34

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!

 If the mass of a neutron star exceeds 3 M , it will collapse into a black hole.  Not even light can escape the gravitational pull of a black hole.  Can form directly from Type II supernova (if massive enough) or from accretion by a neutron star in a binary system.  Makes us question our assumptions about the nature of space and time.

 An event describes something that happens at a specific location and specific time.  Special relativity describes the relationship between events in space and time.  Combines those two aspects into a four- dimensional spacetime.  Something that must be analyzed using special relativity is called relativistic.  Newton’s laws don’t apply to the universe, but they are not wrong; they are contained in special relativity.

 There are many important implications of special relativity.  These apply to many areas of physics, including very small scales.  Here are five important implications: 1. Mass and energy are the same thing. Matter can be converted into energy and vice versa. Special Relativity

2.The speed of light c is the ultimate speed limit. 3.“At the same time” is relative. Perceived information is dependent on relative motion. Special Relativity

4.Time passes more slowly in a moving reference frame  time dilation. 5.An object is shorter in motion than it is at rest  length contraction. Special Relativity

 Human space travel is currently difficult.  Current technology can make ships travel at speeds of 20,000 m/s.  Would take over 50,000 years to get to our nearest neighbor star, Proxima Centauri.  Moving at high speeds takes a lot of energy.

 The general theory of relativity describes how mass distorts the geometry of spacetime.  Asks us not to think of gravitation as a force, but rather the result of the shape of spacetime that objects move through.  The greater an object’s mass, the more it will bend the spacetime around it.

 Special relativity: You can’t tell the difference if you’re in a spaceship at rest or moving at a constant velocity.  Both are valid inertial reference frames.

 General relativity: You can’t tell the difference between being stationary on Earth and accelerating at 9.8 m/s 2 in a spaceship.  Equivalence principle.

 An object in spacetime follows the geometry of spacetime as the object moves.  Imagine spacetime as a rubber sheet.  Path = geodesic (shortest distance between 2 points).  Falling objects have curved geodesics.

General Relativity  There are many consequences of general relativity. Here are four: 1. Mercury’s orbit is not completely stable— it precesses about 43 arseconds per century.

General Relativity 2.Light will also follow curved space and be bent around massive objects.  This gravitational lensing can displace and distort an object’s image.

An Einstein Ring The “8 O'clock Arc”

General Relativity 3.Time runs more slowly near massive objects  general relativistic time dilation.  Results in gravitational redshift of light coming from near those massive objects.

General Relativity 4.Gravitational waves should move through spacetime like ripples through the rubber sheet.  Should move at the speed of light.  Have not yet been observed.  LIGO (Laser Interferometer Gravitational- Wave Observatory) is looking.

General Relativity  A black hole is a singularity—all the matter has collapsed to one point.  Infinitely dense.  It is a bottomless well in the fabric of spacetime.

General Relativity  Once you get too close, no geodesics lead out, not even for light.  The boundary of no return is called the event horizon or the Schwarzschild radius.

 Event horizon of a 1 M  black hole = 3 km.  Extreme tidal forces would rip an object or human apart as it fell in.  Gravitational time dilation and redshift become infinite.

 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.