An Introduction to Black Holes - A supermassive gravitational pull

A black hole is an astronomical object with such a tremendous gravitational pull that nothing can escape it, not even light. The event horizon of a black hole is the boundary where the velocity required to escape exceeds the speed of light, which is the universe's speed limit. Matter and radiation are drawn in, yet they are unable to exit.

There are two types of black holes that have been studied extensively. Super massive monsters weighing 100,000 to billions of solar masses are found in the centers of most big galaxies, including ours. Stellar-mass black holes with three to dozens of times the Sun's mass are found throughout our Milky Way galaxy, while super massive monsters weighing 100,000 to billions of solar masses are found in the centers of most big galaxies, including ours.

The existence of an intermediate-mass black hole, weighing 100 to 10,000 solar masses, had long been suspected by astronomers. While a few candidates have been identified based on circumstantial evidence, the most compelling example to date occurred on May 21, 2019, when the National Science Foundation's Laser Interferometer Gravitational-wave Observatory (LIGO) in Livingston, Louisiana, and Hanford, Washington, detected gravitational waves from the merger of two stellar-mass black holes. A black hole the size of 142 Suns was created as a result of this event, called GW190521.

When a star with more than 20 solar masses runs out of nuclear fuel in its core and collapses under its own weight, it becomes a stellar-mass black hole. The star's outer layers are blown off by a supernova explosion that occurs as a result of the collapse. If the crushed core has a mass greater than three times that of the Sun, no known force can prevent it from collapsing into a black hole. Although the origin of super massive black holes is unknown, we know they exist from the very beginning of a galaxy's existence. Once created, black holes can develop by absorbing anything that falls into them, such as gas from nearby stars and even other black holes.

Figure 1 The Event Horizon Telescope's investigations of the centre of galaxy M87 yielded the first image of a black hole. Light bends in the extreme gravity surrounding a black hole 6.5 billion times the mass of the Sun, forming a brilliant ring.

The Event Horizon Telescope (EHT) – an international initiative that linked eight ground-based radio telescopes into a single Earth-size dish — obtained the first image of a black hole in 2019. It appears as a dark circle silhouetted by a hot, incandescent matter circling disc. The super massive black hole weighs more than 6 billion solar masses and is found at the center of the galaxy M87, which are around 55 million light-years away. Its event horizon is so large that it might span a large portion of our solar system, possibly even beyond the planets.

Another significant black hole discovery occurred in 2015, when scientists discovered gravitational waves, which are ripples in the fabric of space-time predicted by Albert Einstein's general theory of relativity a century ago. The waves from an event known as GW150914, in which two circling black holes spiraled into each other and collided 1.3 billion years ago, were discovered by LIGO. Since then, the gravitational waves produced by black hole mergers have been observed by LIGO and other facilities.

Figure 2 This image of Cygnus A, the brightest radio source in the sky outside our galaxy, was created using data from the National Science Foundation's Very Large Array facility. Long, thin particle jets emitted by the galaxy's center's supermassive black hole connect to wide lobes where speeding electrons trapped by magnetic fields generate radio waves. The structure stretches for half a million light-years from tip to tip. Credits: NRAO/ AUI

These are novel approaches, but astronomers have been studying black holes for decades by measuring the many types of light they emit. Although light cannot escape the event horizon of a black hole, the massive tidal forces in its vicinity cause adjacent matter to burn up to millions of degrees and generate radio waves and X-rays. Some of the material circling even closer to the event horizon may be flung out, generating fast-moving jets of particles that emit radio, X-rays, and gamma rays. Super massive black holes have jets that can travel hundreds of thousands of light-years into space.