• Every black hole started out as a star. Every star started out as a swirling mass of gasses. As gravity pulled the gas cloud closer, everything pressed together. Pressure built up on the inside, eventually becoming so intense that a nuclear reaction started. The star became a furnace which burned for millions or billions of years, until the furnace ran out of fuel and the nuclear reaction ceased.

• The center of the star began to cool down, and the superheated plasma surrounding the center collapsed and compressed, and then rebounded, like a wave hitting a cliff wall and bouncing back. The pressure pushed the outer layers of the star outward in what’s called a supernova. A supernova has the potential to become a black hole, but it all depends on the size of the original star.

• If the star was 25 to 40 times bigger than our Sun, the supernova eventually collapsed in upon itself, pulling matter in so tightly that gravity will not even allow light to escape. It is now a black hole.

• However, if the star was only ten times the size of the Sun or less, when the supernova contracts, it becomes a white dwarf. This will be the eventual fate of our Sun.

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• If the star is ten to 25 times the size of our Sun, it’s not quite big enough to form a black hole. It becomes a neutron star.

• Black holes come in four different sizes: stellar, supermassive, intermediate, or miniature. Black holes grow by swallowing surrounding matter and by merging with other black holes. There is no limit to their size.

• Approximately one out of every thousand stars is massive enough to become a black hole. Since the Milky Way contains over 100 billion stars, our home galaxy must harbor some 100 million black holes.

• Albert Einstein first predicted the existence of black holes in 1916, with his general theory of relativity. The term “black hole” was coined in 1967 by American astronomer John Wheeler.

• The first black hole ever discovered was Cygnus X-1, located within the Milky Way in the constellation of Cygnus, the Swan. Astronomers saw the first signs of the black hole in 1964, and confirmed its existence in 1971 by noting a blue star circling endlessly around a blank spot, while the blank spot was constantly drawing material off of the star. Cygnus X-1is ten times larger than the Sun.

• The first image of a black hole was captured in 2019. The photo of the black hole at the center of the M87 galaxy 55 million light-years from Earth thrilled scientists around the world. It is 6.5 billion times heavier than the Sun.

• There is a black hole at the center of every galaxy, but black holes exist elsewhere too.

• The supermassive black hole at the center of the Milky Way, Sagittarius A* (pronounced A-star), is 4.3 million times the mass of our Sun. It’s 27,000 lightyears away. This is the only black hole whose mass has been measured directly by observing the full orbit of a circling star. It’s diameter is about equal to the distance between Earth and the Sun.

• A snowball size ball of plasma from the center of the Sun weighs as much as a 10-year-old child. But the same size ball of material from inside a black hole would weigh as much as ten Earths.

• The closest black hole to Earth is dubbed “The Unicorn” and is located about 1,500 light-years away. The nickname has a double meaning. Not only does the black hole reside in the constellation Monoceros (“the unicorn”), but it’s also incredibly low mass, being only about three times that of the Sun, making it rare and one of a kind, like a unicorn.