Massive Stars (con't.)
Black Hole
G292.0+1.8
Credit: NASA/CXC/A.Hobart
Binary System
G292.0+1.8
Credit: NASA
If the core remnant of a collapsed massive star exceeds 3 solar masses, neutron degeneracy pressure cannot stop the complete and total collapse of the star. The neutrons get pushed into each other until the star becomes a region, or boundary, in space around the black hole, called the event horizon, beyond which we cannot see. The extreme gravitational field within the event horizon emits no radiation; however, it can be indirectly detected by its effects on the spacetime around it - including accretion disks and companion stars. Artist illustrations are usually used to portray these conditions, such as the black hole and binary system shown.
Hypernova in M100
G292.0+1.8
Credit: Y. Chu (UIUC) et al., POSS, ROSAT, MDM, HST
GRB 020813
G292.0+1.8
Credit: Illustration: NASA/CXC/M.Weiss; Spectrum: NASA/CXC/N.Butler et al.
Gamma-ray bursts (GRBs) are among the most energetic and most luminous explosions in the Universe. They occur roughly once a day, last from a few thousandths of a second to a few hundred seconds, and come from all different directions of the sky. Their gamma radiation is more energetic than visible light and can be measured by satellites orbiting the Earth in space. The energy set free by the bursts in just one second is comparable to the energy production of the Sun during its whole life. There is evidence that GRBs are produced during catastrophic explosions which end the lives of extremely massive stars. Two possible candidates for this type of massive explosive event have been discovered in the spiral galaxy M100. The gigantic energy which powers the gamma-ray burst is thought to be provided by rapidly spinning black holes which form when the central core of a very massive star becomes unstable and collapses under its own gravity. The infalling stellar material becomes part of the newly formed black hole, which releases enormous amounts of energy in two jets. The jets expand relativistically, at almost the speed of light, along the rotation axis. Before they break out from the stellar surface, they have to drill their way through thick layers of stellar material, thus getting collimated into very narrow beams with an opening angle of only a few degrees. Recent observations, like GRB 020813, are confirming that the origin of long gamma-ray bursts comes from exploding massive stars.
Cygnus Region
Credit: Jayanne English (U. Manitoba) et al., CGPS, CNRC
Stellar evolution is a fascinating and fundamental topic. We are just beginning to construct the knowledge necessary to understand the processes of star formation and destruction. Ground-based and orbiting spacecraft are imaging stars in all stages of evolution from radio through gamma rays. Images, like the radio image of the Cygnus region shown here, give us fascinating views of stellar evolution - from protostars just emerging from their stellar cocoons to thermonuclear fusion in massive hot, blue stars, to supernovae remnants that result from the catastrophic collapses of stellar cores. Somehow, within this maelstrom of turbulence, intense radiation and ferocious stellar winds, stars and planetary systems form. Technological advances are allowing us to explore the universe in unprecedented detail, and with these dramatic improvements in resolution come the prospect of significant advances in understanding a wide range of cosmic phenomena, including the never-ending cycle of stellar formation and destruction.
Revised: March 19, 2008