The second topic in our series of discussions was Black Holes.The members of the club met on the terrace of Faculty Division III,BITS Pilani to conduct the discussion session. Following are excerpts from the discussion.BLACK HOLES
A black hole is a region of space with such a high gravitational field that no matter or radiation can escape from it.Even light cannot escape its gravitational pull and hence, the term “black” is used to describe it.They are usually classified on the basis of their mass into two broad categories: Stellar-mass and Supermassive.
An artist’s rendition of a black hole
Stellar-mass black holes are formed when stars above approximately three solar masses(Tolman-Oppenheimer-Volkoff limit) collapse into a supernova. They grow in size by absorbing mass from the surroundings and merging with other black holes, leading to the formation of supermassive black holes with masses in the range of millions of solar masses. It is believed that the center of almost every galaxy contains a supermassive black hole.
Properties
An interesting theorem, popularly known as the “no-hair theorem”, describes the properties of black holes.It states that once a black hole achieves a stable state it has just three independent physical properties associated with it-mass, charge, and angular momentum. A consequence of this theorem is that once and object falls into a black hole and achieves a stable state, information about every quantity that cannot be measured outside a black hole is lost. This is known as the black hole information loss paradox.The simplest black holes are the ones with no charge and no angular momentum. These are termed as Schwarzchild black holes. They are the only type of black holes that are spherically symmetric.Structure
Structure of a stationary black hole
At the centre of a black hole lies the gravitational singularity, a point of zero volume and infinite density. It is surrounded by the event horizon, the boundary at which escape velocity equals the speed of light and hence nothing can escape. The distance between the singularity and the event horizon is known as the Schwarzchild radius.
Structure of a rotating black hole
For rotating black holes the singularity takes the shape of a ring and the event horizon is surrounded by a region called the ergospehere in which it is impossible to stand still due to the process known as frame-dragging.Evaporation
Contrary to popular belief, the noted theoretical physicist and cosmologist, Stephen Hawking proved that black holes emit radiation in a perfect black body spectrum. This radiation, known as Hawking radiation, is emitted due to quantum effects and causes the black hole to lose mass and evaporate over time.Observing a black hole
Black holes do not emit any direct signals that can be used for detecting them. The Hawking radiation is predicted to be very weak and is thus not useful for observational purposes. Black holes are, therefore, detected indirectly by observing their effects on the matter, energy or space around them.
Black holes accrete matter from their surroundings causing it to gain kinetic energy due to the gravitational pull and heat up. This causes the atoms to ionise and, at temperatures of a few million Kelvin, emit X-rays which may be detected by telescopes.
Black holes are also often found in X-ray binary systems in which they accrete matter from the other star forming accretion disks. These can be easily observed as one of the stars is a regular star.The first strong candidate for a black hole discovered in this way was Cygnus X-1.
A black hole absorbing matter from its companion star in an X-ray binary system forming an accretion disk
Various questions were put up in the discussion session and consequently topics like spherical symmetry of Schwarzchild black holes, Birkhoff’s theorem, the ergosphere, accretion and accretion disks etc. were discussed in more detail.
