One of the most important findings in astrophysics is that when stars die they can collapse to extremely small objects. When their internal fuel sources are exhausted, stars millions of miles across can collapse to roughly 10 kilometers in diameter, known as neutron stars.
In extreme cases, stars can contract even further under their own gravity to an infinitely dense point, or singularity, called a black hole. The discovery of black holes has been a critical confirmation of Einstein's theory of gravity, known as general relativity.
Black holes are among the most commonly studied astrophysical phenomena for the simple reason that they are associated with the brightest known objects and events in the cosmos. Even though black holes are themselves invisible, they tend to capture gas around their margins due to their gravitational pull. The gravity near the black hole is very strong and forces the gas to be compressed into even smaller space. This squeezing increases friction between gas particles, causing the gas to heat up. This heat is observed as radiation by telescopes.
By studying the plasma around black holes, researchers are seeking to test Einstein’s theory of general relativity and discover if there are astrophysical alternatives to black holes in the universe.
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JETS: Jets are one of the most remarkable black hole phenomena. Jets are created when an energetic gas tries to accrete into the black hole, but the hole can’t absorb all of the energy. Some of the energy is ejected back in the form of twin jets or streams of highly energetic particles. The most famous black hole jet system is M87, whose core is shown in yellow below. The blue jet shoots out of the core of M87.
Accretion Disks: Accretion disks are hard to resolve because they are often very small, so often we simply look at the color and timing of unresolved disks. However, in some cases, the outer parts of the disk can be seen, as in the image below showing the outer disk (right) and the jet (on left).
One of the most recent exciting discoveries is that we can measure the spin of black holes – i.e. we can measure how fast they are rotating. It’s known that the faster a black hole rotates, the more energy can come out in their jets. So this is one way to test whether we understand black holes. The image below shows how the disk is different for non-spinning and spinning black holes, where the disk naturally is hotter and closer to the hole for the spinning case. This can be detected by simply looking at the color of radiation in the x-rays.
Another way of understanding black holes is through the use of computers to simulate black holes, the gas around them, and the jets that they produce. Below are some images from simulations of black holes. The central black circle is the black hole, while most of the heavy gas around the black hole is the disk that feeds the hole, and above/below the hole is the outgoing jet.