What is black hole?

What is a black hole?

Black Holes

Black holes are points in space that are so dense they create deep gravity sinks. Beyond a certain region, not even light can escape the powerful tug of a black hole's gravity. And anything that ventures too close—be it a star, planet, or spacecraft—will be stretched and compressed like putty in a theoretical process aptly known as spaghettification.

There are four types of black holes: stellar, intermediate, supermassive, and miniature. The most commonly known way a black hole form is by stellar death. As stars reach the ends of their lives, most will inflate, lose mass, and then cool to form white dwarfs. But the largest of these fiery bodies, those at least 10 to 20 times as massive as our own sun, are destined to become either super-dense neutron stars or so-called stellar-mass black holes.






In their final stages, enormous stars go out with a bang in massive explosions known as supernovae. Such a burst flings star matter out into space but leaves behind the stellar core. While the star was alive, nuclear fusion created a constant outward push that balanced the inward pull of gravity from the star's own mass. In the stellar remnants of a supernova, however, there are no longer forces to oppose that gravity, so the star core begins to collapse in on itself.







If its mass collapses into an infinitely small point, a black hole is born. Packing all of that bulk—many times the mass of our own sun—into such a tiny point gives black holes their powerful gravitational pull. Thousands of these stellar-mass black holes may lurk within our own Milky Way galaxy.





One black hole is not like the others





Supermassive black holes, predicted by Einstein's general theory of relativity, can have masses equal to billions of suns; these cosmic monsters likely hide at the centers of most galaxies. The Milky Way hosts its own supermassive black hole at its center known as Sagittarius A* (pronounced “ay star”) that is more than four million times as massive as our sun. The tiniest members of the black hole family are, so far, theoretical. These small vortices of darkness may have swirled to life soon after the universe formed with the big bang, some 13.7 billion years ago, and then quickly evaporated. Astronomers also suspect that a class of objects called intermediate-mass black holes exist in the universe, although evidence for them is so far debatable.



No matter their starting size, black holes can grow throughout their lives, slurping gas and dust from any objects that creep too close. Anything that passes the event horizon, the point at which escape becomes impossible, is in theory destined for spaghettification thanks to a sharp increase in the strength of gravity as you fall into the black hole.



As astrophysicist Neil Degrasse Tyson once described the process: “While you're getting stretched, you're getting squeezed—extruded through the fabric of space like toothpaste through a tube.”



But black holes aren't exactly “cosmic vacuum cleaners,” as often depicted in popular media. Objects must creep fairly close to one to lose this gravitational tug-of-war. For example, if our sun was suddenly replaced by a black hole of similar mass, our planetary family would continue to orbit unperturbed, if much less warm and illuminated.

Peering through the darkness

Because black holes swallow all light, astronomers can't spot them directly like they do the many glittery cosmic objects in the sky. But there are a few keys that reveal a black hole's presence.



For one, a black hole's intense gravity tugs on any surrounding objects. Astronomers use these erratic movements to infer the presence of the invisible monster that lurks nearby. Or objects can orbit a black hole, and astronomers can look for stars that seem to orbit nothing to detect a likely candidate. That's how astronomers eventually identified Sagittarius A* as a black hole in the early 2000s.



Black holes are also messy eaters, which often betrays their locations. As they sip on surrounding stars, their massive gravitational and magnetic forces superheat the infalling gas and dust, causing it to emit radiation. Some of this glowing matter envelops the black hole in a whirling region called an accretion disk. Even the matter that starts falling into a black hole isn't necessarily there to stay. Black holes can sometimes eject infalling stardust in mighty radiation-laden burps.

Detection of gravitational waves from merging black holes

On 14 September 2015 the LIGO gravitational wave observatory made the first-ever successful direct observation of gravitational waves. The signal was consistent with theoretical predictions for the gravitational waves produced by the merger of two black holes: one with about 36 solar masses, and the other around 29 solar masses. This observation provides the most concrete evidence for the existence of black holes to date. For instance, the gravitational wave signal suggests that the separation of the two objects prior to the merger was just 350 km (or roughly four times the Schwarzschild radius corresponding to the inferred masses). The objects must therefore have been extremely compact, leaving black holes as the most plausible interpretation.

More importantly, the signal observed by LIGO also included the start of the post-merger ringdown, the signal produced as the newly formed compact object settles down to a stationary state. Arguably, the ringdown is the most direct way of observing a black hole From the LIGO signal it is possible to extract the frequency and damping time of the dominant mode of the ringdown. From there it is possible to infer the mass and angular momentum of the final object, which matches independent predictions from numerical simulations of the merger. The frequency and decay time of the dominant mode are determined by the geometry of the photon sphere. Hence, observation of this mode confirms the presence of a photon sphere, however, it cannot exclude possible exotic alternatives to black holes that are compact enough to have a photon sphere.

The observation also provides the first observational evidence for the existence of stellar-mass black hole binaries. Furthermore, it is the first observational evidence of stellar-mass black holes weighing 25 solar masses or more.

Since then many more gravitational wave events have since been observed.

Proper motions of stars orbiting Sagittarius A

The proper motions of stars near the center of our own Milky Way provide strong observational evidence that these stars are orbiting a supermassive black hole. Since 1995, astronomers have tracked the motions of 90 stars orbiting an invisible object coincident with the radio source Sagittarius A*. By fitting their motions to Keplerian orbits, the astronomers were able to infer, in 1998, that a 2.6 million M☉ object must be contained in a volume with a radius of 0.02 light-years to cause the motions of those stars. Since then, one of the stars—called S2—has completed a full orbit. From the orbital data, astronomers were able to refine the calculations of the mass to 4.3 million M☉ and a radius of fewer than 0.002 light-years for the object causing the orbital motion of those stars. The upper limit on the object's size is still too large to test whether it is smaller than its Schwarzschild radius; nevertheless, these observations strongly suggest that the central object is a supermassive black hole as there are no other plausible scenarios for confining so much invisible mass into such a small volume. Additionally, there is some observational evidence that this object might possess an event horizon, a feature unique to black holes.