How Do We Know Black Holes Exist?

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Black holes are one of the most amazing phenomena in the universe. To get an idea of their magnificence, join your thumb and index finger to form a circle. Now imagine the entire Earth is squeezed down to fit in that circle. The density of the object between your thumb and finger would be the same as the density of a black hole. Due to their huge masses and extremely compact sizes, their gravitational pull is very strong, so strong that matter and even light cannot escape it. If this is true, how do we know that black holes really exist?

Watch Salman Hameed explaining the Evidence for Black Holes [Urdu] in Kainaati Gupshup:

In 1835, Auguste Comte, a French philosopher, said that we’ll never know what stars are made of because they are so far away. Less than two centuries later, not only do we know the chemical composition of stars but also that they have different masses and that the final state of a star depends on its mass. Consider our Sun as an example, four or five billion years from now, it will run out of fuel, expand to form a red giant star and eventually collapse. Only its core will remain. This core remnant is called a white dwarf. A white dwarf is the same size as Earth and about two million times as dense. If the star is eight to ten times larger than the Sun, then its remnant will be an even smaller object called a neutron star. A neutron star is as big as a large city and as dense as the nucleus of an atom. But if the star is even bigger, twenty or thirty times larger than the Sun, it will continue to collapse and form a black hole. Black holes that form as a result of the collapse of a star are called stellar mass black holes. Their masses range from about 5 to several tens of solar masses.

Stellar Mass and Super-massive Black Holes

In 1964, astronomers discovered a blue super-giant star revolving around an invisible X-ray source in Cygnus X-1 system in the constellation Cygnus. The mass of this X-ray source was estimated to be about 14 solar masses. According to the Chandrasekhar limit, the mass of a white dwarf star cannot exceed 1.4 solar masses. Moreover, the Tolman-Oppenheimer-Volkoff limit says that a neutron star cannot be more massive than about 3 solar masses. Therefore, the unseen object could have only been a black hole.

A Stellar Mass Black hole
Cygnus X-1: A Stellar Mass Black Hole. Image: NASA 

Astronomers have detected stars and clumps of gas at the center of our own galaxy, the Milky Way, swirling at very high speeds. The calculations show that they could only be circling around a super-massive black hole. They believe that at the center of every large galaxy, there lies a super-massive black hole, with mass around a million or a billion times the mass of the sun. Due to larger masses, their gravitational pull is even stronger. Last year, astronomers observed a huge black hole, about twenty million solar masses, at the center of the galaxy NGC 3690 ripping a star that had come near it.

Accretion Disks and Jets

The immense gravity of a black hole attracts all kinds of stellar debris. Material that has come close to the black hole but not quite fallen into it, starts spinning around it, forming a flat disk-like structure called an accretion disk. Gravity and friction compress and heat up the material in the disk, resulting in the emission of electromagnetic radiations like X-rays and gamma rays. Interestingly, a small fraction of super-massive black holes also fire off jets of particles. Exactly how these jets are formed is still unknown. The extreme magnetic fields, in the vicinity of these black holes, can collimate the particles, forming jets that can be detected from tens of thousands of lightyears’ distance. In fact, the jets expelled by the supermassive black hole at the center of Centaurus-A, a galaxy about 15 billion light years away from Earth, were observed to be over a million light years long. The observation of radiation from the accretion disks, and expulsion of these jets is a strong evidence for the existence of black holes.

 Jets from Centaurus-A
Jets from Centaurus-A. Image: NASA 

Gravitational Waves

Gravitational waves are ‘ripples’ in space-time produced by massive accelerating objects. Events such as collisions of black holes produce very strong gravitational waves. In 2015, gravitational waves, from two colliding black holes, were detected for the first time by Laser Interferometer Gravitational-Wave Observatory (LIGO). The two black holes, weighing about 29 and 36 solar masses, merged to form a larger black hole. The event occurred 1.3 billion years ago and the energy released was greater than the energy of all the stars in the universe. In this process, a total mass of about three times the mass of sun just disappeared and gravitational waves were produced. The detected variation in space-time was much, much smaller than the size of an atom. But this is a huge evidence for the existence of black holes.

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