Black hole
This artist's concept illustrates a supermassive black hole with millions to billions times the mass of our sun. Supermassive black holes are enormously dense objects buried at the hearts of galaxies. (Smaller black holes also exist throughout galaxies.) In this illustration, the supermassive black hole at the center is surrounded by matter flowing onto the black hole in what is termed an accretion disk. This disk forms as the dust and gas in the galaxy falls onto the hole, attracted by its gravity. NASA/JPL-Caltech

Scientists believe that the Primordial black holes are formed during the early phase of Universe soon after the big bang. These black holes, with very strong gravity, do not allow even the light to escape from them, which could be observed only through specially designed space telescope.

The size and mass of the black holes categorize it to its three known forms. The smallest form of black hole or the primordial black hole is as small as a single atom with huge engulfed mass.

Medium sized black holes known as "stellar" would have 20 times the mass of the sun and the diameter of 10 miles. Supermassive black holes which are the largest in the set would have masses greater than 1 million Suns with the diameter as much as the size of the Sun.

Formation

Black hole
An artist's drawing shows the current view of the Milky Way galaxy. Scientific evidence shows that in the center of the Milky Way is a supermassive black hole. NASA/JPL-Caltech

Stellar black holes form when a massive supernova explodes and collapses into its center. The supernova explosion also sets off huge quantities of mass and energy of the star into space.

Meanwhile, the supermassive black holes are formed at the time of birth of a galaxy. It was believed that every galaxy has a supermassive black hole in its center. The gravitational force of these supermassive black holes plays important role in the birth and the growth of the galaxies.

Detecting black holes

Black hole
The black hole named Cygnus X-1 formed when a large star caved in. This black hole pulls matter from the blue star beside it. NASA/CXC/M.Weiss

Even though black holes cannot be seen, its presence could be detected by studying nearby stars and gases. Scientists study stars which orbit certain points in the sky as the clue for the presence of a black hole.

Scientific instruments could observe the high- energy light formed when a black hole and a star are orbiting close together. Sometimes black hole's gravity would be strong enough to pull all the outer gases of a star to form a glowing disk called accretion disk around it. Accretion disk spirals into the black holes to heat up the gases which release X-rays in all directions.

Telescopes measures these X-ray lights to determine the location and properties of the black hole.

Chances of Sun turning to a black hole

Black hole
The sun emitted a significant solar flare, peaking at 7:28 p.m. EST on Dec. 19, 2014. NASA's Solar Dynamics Observatory, which watches the sun constantly, captured an image of the event. Solar flares are powerful bursts of radiation. Harmful radiation from a flare cannot pass through Earth's atmosphere to physically affect humans on the ground, however -- when intense enough -- they can disturb the atmosphere in the layer where GPS and communications signals travel. This flare is classified as an X1.8-class flare. X-class denotes the most intense flares, while the number provides more information about its strength. An X2 is twice as intense as an X1, an X3 is three times as intense, etc. NASA/SDO

Scientists say that the Sun does not have sufficient mass to turn into a black hole. The Sun during its end stage would become a red giant star and would later turn into a ring of gas called Planetary Nebula when its entire fuel gets used up. The scientists predict that the Sun would turn in to a white dwarf star once it cools down.

Importance of Blackhole studies

Black hole
Using data from Chandra and the Sloan Digital Sky Survey, scientists have found evidence that supermassive black holes in the early universe grew intermittently in the first billion years after the Big Bang. X-ray: NASA/CXC/Univ. of Rome/E.Pezzulli et al. Illustration: NASA/CXC/M.Weiss

Space telescopes like the Chandra X-ray Observatory, the Swift Satellite, and the Fermi Gamma-ray Space Telescope makes intensive studies on the black holes. These research could help the scientists to know about the origin, evolution, and destiny of the Universe.

Chandra X-ray Observatory had recently observed merger of two supermassive black holes. The merger of the black holes had resulted in the release of huge amount of gravitational waves along with gamma and other waves. The gravitational waves from this black hole merger had been the first ever of the sort observed by scientists.
paired black hole
Artist's illustration of supermassive black hole pair. NASA/CXC/A.Hobart