When a Star Dies: Birth of a Black Hole

When the star dies

A star lives in a precarious balance between gravitational pressure that tends to contract it and thermal pressure from nuclear reactions that tends to make it explode. When fusion reactions stop in the core of a massive star (more than 20 times the mass of the Sun), gravity takes over without opposition. This situation marks the beginning of a catastrophic collapse: the core collapses on itself, forming a gravitational singularity (a stellar black hole).

The event horizon: the point of no return

A black hole is defined by its event horizon, a spherical surface around the singularity where the escape velocity equals that of light. Nothing, not even a photon, can escape from it. To an outside observer, the star appears to freeze and darken as it approaches this horizon, slowly disappearing from the visible universe.

An invisible but detectable transformation

Although the black hole is invisible, its presence is betrayed by its gravitational interactions with its environment. For example, when it sucks matter from a nearby star, the accretion disk surrounding it can emit intense X-rays. This is how the first black holes in our galaxy, such as Cygnus X-1, were detected.

Table: From massive star to black hole

References:
• Chandrasekhar S., On Massive Stars, The Astrophysical Journal, 1931.
• Oppenheimer J. R., Snyder H., On Continued Gravitational Contraction, Physical Review, 1939.
• Misner, Thorne, Wheeler, Gravitation, W. H. Freeman, 1973.
• NASA, ESA, Black Holes (2024).