A black hole is an object whose gravity is so strong that nothing can escape it, not even light moving at the fastest possible speed in the universe.
Black holes do not emit any light or particles, they are dark and impossible to see directly. However, black holes are frequently detected by indirect means, e.g. through gravitational effects. Many objects are discovered by indirect means in astronomy. One example are exoplanets – we detect a gravitational wobble in a star and infer that the wobble is due to a planet orbiting the star.
The region around a black hole can be very bright if the black hole has an accretion disk. Accretion disk is a thin disk of gas and dust that is spiraling into the black hole (see Figure). Due to very strong gravity near a black hole, the particles in the disk feel friction and radiate high energy X-rays and Gamma-rays.
A black hole can also have strong jets, perpendicular to the accretion disk that look like they are coming straight from the black hole. The material in the jets actually comes from the accretion disk and is generated by powerful magnetic fields surrounding the black hole.
A black hole’s appearance can sometimes be a bit more complex, like in the Figure Below. The example is from an imaginary black hole called Gargantua featured in the movie "Interstellar ." The depiction of the accretion disk in this movie is scientifically accurate. The accretion disk, in this case, is warped by gravitational lensing and it forms two images – one loops under, and the other over the black hole.
You need not worry if you are unfamiliar with gravitational lensing, or warping of spacetime because we will explore such topics throughout the semester!
Black holes are formed from cores of very massive stars that die in supernova explosions. They are made of ordinary matter compressed so much that the resulting object, i.e. the black hole, becomes infinitely dense.
The nearest known black hole, A0620-00, is about 3000 light years (ly) away. However, from our understanding of life-cycles and distribution of stars in the Milky Way Galaxy, we expect there to be another black hole at a closer distance.
There are roughly 1011 (100 billion) stars in the Milky Way Galaxy. About 10-3 or 0.1% (1 in 1000) stars are massive enough to become black holes. Our Milky Way Galaxy has about 108 (10 million) black holes!
In the observable Universe, there are roughly 1011 galaxies. Therefore, we expect to have 1019 black holes in the Universe (# of galaxies in the universe = 1011 , times the number of stars in a typical galaxy that can become black holes = 108 ).
NOTE: During the first few lessons you will see numbers expressed both in scientific notation and in millions, billions, etc. However, for the remainder of the semester we will use scientific notation exclusively.
To get us started on our exploration of black holes, here is scientifically well done video:
One of the objectives of this course is for you to understand the details of movies such as the one above. For this purpose, we first, have to understand concepts of gravity, escape velocity, spacetime, and special and general relativity. These are the topics of the first unit.