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Black holes
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Objects so dense that not even light escapes the horizon.
r_s = 2GM/c²A black hole has an event horizon where the escape speed equals c. Anything inside cannot send signals out. Formed by the collapse of massive stars.
Sagittarius A* is 4 million solar masses.
They are not cosmic vacuum cleaners; far away they gravitate like any mass.
In more detail
A black hole is a region where gravity is so strong that nothing, not even light, can escape from inside the event horizon. Black holes are described by mass, spin and charge.
Real-world uses
- Astronomers study black holes at galaxy centres to understand galaxy growth.
- Gravitational wave detections confirm black hole mergers.
- Astronomers detect black holes by the motion of stars orbiting them and by the X-rays from gas falling in.
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Frequently asked questions
Are black holes holes in space?
No. They are regions of extreme density and curvature, with an event horizon.
Can anything escape?
Not from inside the event horizon, though Hawking radiation may slowly reduce a black hole's mass.
Can anything escape a black hole?
Nothing can escape from inside the event horizon, not even light, although Hawking radiation may slowly leak from very small black holes.
Related topics in Gravitation & Orbits
Weighing the Sun and planets · Gravitational lensing · Gravitational waves · Schwarzschild radius · Neutron stars and white dwarfs · Dark matter evidence