Ch 12-03 Black Holes

• If the stellar core is more massive than about 3~5 M_⊙ when collapsing, no conceivable forces can resist the strong gravity. The mass will collapse infinitely.
  Einstein's General Relativity also predicts the existence of an event horizon, within which nothing, including photons, can escape to the outside world.
  
  
  [Black holes are just objects with mass confined within the corresponding event horizon, far away from which nothing is unusual.]
  
  
• The Schwarzschild radius - the radius of the event horizon
  
  R_S = 2GM / c² ≈ 3 km (M/M_⊙)
  
  
  
  
• The properties of a black hole is totally determined by its mass (M), angular momentum (J), and electric charge (Q) -- black-hole's no-hair theorem.
  
  
• Travelling towards a black hole
  
  
  [Some key words in this journey: Time dilation; Gravitational redshift; Tidal distortion, friction, and disruption]
  
  
• Where are black holes? Do they really exist?
  
  Recall the mass estimate using binaries, in particular spectroscopic binaries. Some binaries are found to possess a massive member but invisible optically. Most of them, furthermore, are X-ray sources with significant variability.
  
  
  [Black holes may reveal themselves by X-ray emission of their hot accretion disks. Sometimes they also emit radio and gamma-ray emissions from a jet.]
  
  Some stellar-mass black holes
  
  
  
• Besides being formed in supernova events, black holes are also formed at the end stage of very massive stars: Hypernovae and Collapsars - recall the long GRBs.
  
  
• LIGO/Virgo detections of gravitaional waves also suggest that black-hole merger is the source of the detected waves (see Section 4-5 and Section 12-2).
  
  
• Except for stellar-mass black holes, supermassive black holes (10⁶ ~ 10⁹ M_⊙) are often found at the center of galaxies. This, as well as black-hole images (see also Section 6-4), will be discussed in General Astronomy (II).
  
  
• There are also increasing evidences for the existence of intermediate-mass black holes.
  
  
  [M82 X-1 is an Ultra-Luminous X-ray sources (ULX), which is believed to host a 500 M_⊙ black hole.]
  
  
• Primordial black holes, in addition, were proposed by Steven Hawking in early 1970's. Most of those primordial ones may have evaporated due to Hawking radiation (black-hole evaporation).
  
  
  [Jacob Bekestein and Steven Hawking proved that the Hawking radiation can be formulated as blackbody radiation with a certain temperature. The wavelength of maximum radiation is 16 R_S. The temperature of a black hole with 1 M_⊙ corresponds to 6 x 10^-8 K. The temperature is inversely proportional to the mass. Therefore the process is a run-away one: black holes will evaporate! It takes 1.5 x 10¹⁰ years for a 10¹⁰kg black hole to evaporate, 10⁶² years for 5M_⊙, and 10⁸⁰ years for 5 million M_⊙.]