Class 18 : Stellar evolution, Part II Evolution of a 50 M  star… Black holes. Hypernovae. Gamma-Ray Bursts (GRBs)… Observational characteristics of GRBs. Possible explanations.

I: Evolution of a 50 M  star Starts off as before… Forms from gas cloud. Steady H-burning (for 2-3 million years). When hydrogen runs out, get whole succession of nuclear reactions. Get “onion” structure (like 10 M  case). Core collapses and drives supernova. Core tries to form neutron star… … but too much mass. Neutron star crushes under its own weight. Undergo complete collapse – get black hole.

Anatomy of a Black Hole Singularity – central “point” containing all of the mass. Known laws of physics break down. Event horizon – point of “no return”. Everything within this radius is dragged to the singularity by enormous gravity. R Sch = 3 km for the Sun.

18 th century ideas Idea of an object with gravity so strong that light cannot escape first suggested by Rev. John Mitchell in Laplace (1798) – “A luminous star, of the same density as the Earth, and whose diameter should be two hundred and fifty times larger than that of the Sun, would not, in consequence of its attraction, allow any of its rays to arrive at us; it is therefore possible that the largest luminous bodies in the universe may, through this cause, be invisible.”

Both space and time are strongly distorted in the vicinity of a black hole. Light seems to bend, time slows down.

Stellar evolution: Summary Roughly speaking, If the star’s initial mass is… The star’s fate is to become a… < 10 M  White dwarf  10 M  & < 20 M  Neutron star > 20 M  Black hole

Hypernovae Possible that very massive stars may undergo a “hypernova.” 100 more powerful explosion than a supernova. Can be seen across vast distances. May be responsible for “gamma-ray bursts.”

II : Gamma-Ray Bursts Flashes of gamma-rays discovered by military in 1960s. Occur about once per day. Evenly distributed on the sky. Last seconds. Mysterious – until recently we… Didn’t know how far away they are, nor how powerful they are.

Problem Gamma-ray telescopes give blurry images! Difficult to say where GRB was located. Very many possible objects located in “error box.” Need to pinpoint “afterglow.”

Have recently located afterglows of long-duration GRBs… Measurements made at X-ray, visible, and radio wavelengths. They are in distant galaxies! Must be very powerful…  observed energy of a supernova. Still haven’t located short-duration GRBs…

What could cause such powerful explosions? Possibility I: Hypernova of a very massive star… … forms black hole. Possibility II: Collision of 2 neutron stars… … also forms black hole.