Black Holes

Stellar Death

One mechanism of black hole formation is star death. When a star runs out of its fuel storage it explodes into fragments, burning the remainder of its fuel and giving rise to a supernova, a sudden outburst of energy by a star as it dies. A supernova forms when there is a sudden disruption in the ongoing nuclear reactions in the core, which causes an explosion that sends the particles of the star away from it rapidly in a cosmic shockwave. The energy released by a supernova is approximately the amount of energy released by a medium sized star over its entire lifetime. A supernova is highly unstable and exists for about a month before the remaining mass collapses under its own gravity, forming a neutron star. Neutron stars are the densest stars known to exist (though not the theoretically densest possible stars). The eventual fate of this star depends upon its mass. If the star has a mass greater than about 3 times the mass of the sun, it will eventually collapse into a black hole. This limit on how large a neutron star can be is known as the Tolman-Oppenheimer-Volkoff limit.

\[\] Black Hole Accretion Credit: ROSAT/MPE/NASA A compact star in an binary system, specifically, an accreting binary system, may also form a black hole. Accreting binary systems generally form when a lower mass star expands into a more compact star, and is said to accrete more particles, often forming a astronomical structure called an accretion disk. Accretion disks can form around black holes, where they look similar to accretion onto a neutron star or white dwarf.

To the right is a ROAST X-ray image of LMC X-1 (the bright cluster on the left). This bright cluster is an accretion disk in the Large Magellanic Cloud, giving off x-ray emissions in the vicinity of a massive star that can be detected by astronomers. This star isn't visible, but is estimated to have a mass of 5 solar masses or more, and is a candidate for a black hole. On the right side of the image, normal stars with 1 solar mass or lower are visible.

\[\] Bipolar Mass Ejection

One way to detect a black hole is to look for rapidly ejected mass from a local region of spacetime. Some of the material accreting onto a black hole may gain a large amount of angular momentum that propels it above the escape velocity. It then may be thrown off at a high velocity in the direction defined by the black hole's rotation. This process is called bipolar flow. However, ejected mass could also come from neutron stars that are accreting mass. The key difference between a neutron star and a black hole is the magnitude of the mass of the object, and estimating the mass of the unseen object that is ejecting particles is a key means of proving that bipolar mass is associated with a black hole.

\[\] Binary Black Hole Systems

The LIGO experiment, which announced the first-ever detection of gravitational waves on February 11, 2016 [1], observed gravitational waves coming from a binary black hole system consisting of two black holes of about 30 solar masses each. In a binary black hole system, two black holes orbit each other. In this case, the two black holes inspiraled towards each other until they collided, forming a single black hole. This merger of the two black holes generated waves of expanding and contracting spacetime that spread out from the new black hole, that is, gravitational waves. As they passed through the earth, they caused the earth to expand and contract by about the width of an atomic nucleus, sufficiently large to be detected in an interferometry experiment.

A binary black hole system emitting graviational waves, Credit: T. Carnahan NASA GSFC