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What is a black hole, and how can a star become one?

A black hole is a region in space where gravity is so strong that nothing, not even light, can escape its gravitational pull. The boundary surrounding a black hole is called the event horizon, and once an object crosses this boundary, it is trapped within the black hole.

Stars can become black holes through a process known as stellar collapse. This occurs when a massive star exhausts its nuclear fuel and is no longer able to counteract the force of gravity trying to collapse it. The fate of a star depends on its mass:

  1. 1) Low to Medium Mass Stars:

    • For stars with masses similar to our Sun, the gravitational collapse is typically counteracted by nuclear fusion reactions in the star's core, where hydrogen is converted into helium. As these stars run out of nuclear fuel, they expand into red giants and then shed their outer layers, forming a planetary nebula. The remaining core, called a white dwarf, is stable and does not collapse further under normal circumstances.

  2. 2) High Mass Stars:

    • In contrast, high-mass stars (several times more massive than the Sun) have much stronger gravitational forces. When these stars run out of nuclear fuel, gravity overwhelms the outward pressure generated by nuclear reactions, causing a rapid collapse of the star's core.
    • The collapsing core may rebound if the star is not too massive, leading to a supernova explosion. The remnants of the explosion can form a neutron star.
    • However, if the star is extremely massive, the gravitational collapse continues, and nothing can stop it. The core collapses to a point of infinite density, creating a singularity, surrounded by the event horizon—the defining feature of a black hole.

The size of a black hole's event horizon (its "size" in a sense) is determined by its mass. The more massive the black hole, the larger its event horizon. Stellar-mass black holes are typically a few times to tens of times the mass of the Sun. Supermassive black holes, found at the centers of most galaxies, can be millions or billions of times the mass of the Sun.

For More Planetary Facts & Space Animation: Space Balls Videos

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