Stellar Evolution and Death -- Rejuvenation of White Dwarfs, Neutron Stars, and Black Holes cont.

Stellar Evolution and Death header

4. Rejuvenation of White Dwarfs, Neutron Stars, and Black Holes (Continued)

Accretion Disk

There is one feature that many such mass-transferring systems share -- the formation of an accretion disk around the compact star:

The matter pulled off the secondary star does not fall directly onto the compact star. Instead, it orbits around it and forms an accretion disk.
Friction slows the matter's orbital motion, which causes the matter to spiral through the disk down to the surface of the compact star.
The falling and spiraling of the matter toward the compact star releases large amounts of gravitational energy and heats the accretion disk.
The final impact on the compact star releases additional energy, heats the star's surface, and, in some instances, triggers renewed nuclear burning. In the case of black holes, which have no surface, the heating occurs only in the accretion disk.

Artist's Concept of a Black Hole

Artist's concept of a black hole surrounded by an accretion disk (yellow). Two high-speed jets are ejected in opposite directions from the inner part of the accretion disk.

The line art illustrates the warping of space-time in the vicinity of a rotating black hole. According to Einstein's general theory of relativity, space and time in the vicinity of a black hole are warped. If the black hole rotates, the space-time coordinate grid (blue lines) twists into a spiral pattern as shown here. (Note that only two dimensions of space-time are represented. In reality, there are four dimensions: time and three spatial coordinates.)

Image Credit: Joe Bergeron of Sky & Telescope magazine.

The transfer of matter from the secondary to the compact star is not always as straightforward as just described, as we shall see:

Classical Novae
Dwarf Novae
Supernovae of Type I
X-Ray Binary Systems
X-Ray Pulsars
Black Hole Candidates

Back Next
Back to Contents