What Are They?

• In general: Regions of space where gravity is so strong that not even light can escape (the escape velocity is the speed of light); These regions (but not the hot matter around them!) will then be black.
• Is light affected by gravity? Not according to Newton's theory, but we have lots of evidence (from stars as well as from distant galaxies) that gravity causes light bending, and redshift; To account for this, you need Einstein's theory of gravity, general relativity.
• Can light actually be trapped? Yes; The escape velocity increases if a body is compressed, and at some point an event horizon forms around it at a distance called the Schwarzschild radius, an imaginary surface within which everything is trapped, including light itself; Around it, light can even orbit around in the photon sphere!

Stellar Black Holes

• Star collapse: Most black holes are believed to be relics of very massive stars, whose cores alone have more than 3 solar masses; Their supernova remnants are too massive to become neutron stars and undergo the ultimate form of gravitational collapse, possibly with a flash that can be observed.
• How massive are they? Since the exploding star blows off most of its mass, these "small" black holes probably are no heavier than 15–20 solar masses.
• How large are they? Usually a few miles across or so, depending on their mass (if the Sun could become a black hole, its Schwarzschild radius would be 3 km, and for the Earth it would be 1 cm!).

  Other Kinds of Black Holes

• How they may differ: Only by their mass, rotation rate, and electric charge, if they have one; All other features they may have had are crushed away or hidden inside.
• Tiny black holes? "Primordial" ones could have formed in the very early universe, and in that case may or may not still be around (small black holes evaporate fast).
• Medium size black holes: Hundreds to thousands of solar masses; They may form inside star clusters (two have possibly been seen at the centers of globular clusters).
• Supermassive black holes: At the cores of most galaxies, with up to billions of solar masses, formed from pileup of matter and stars; We are just beginning to understand their role in galaxy evolution.

What Effects Does a Black Hole Have?

• On nearby objects: Outside the event horizon its gravity is like that of a star, only stronger–it does not reach out and "suck you" in like a vacuum cleaner! But it does produce extreme cases of tides (the smaller the black hole, the stronger the stretching and squeezing), and an apparent slowing down of all motion.
• On surrounding matter and space: The accretion disk gets heated up and emits radiation (X-rays, visible light) as well as jets of matter (and accompanying radio waves...); These can be seen and can also have long-range effects in a galaxy; If the black hole rotates, it also drags space along in its rotation.
• Inside the black hole: There is no return after the horizon is crossed; Our current theory predicts that there is a singularity inside...

  Have We Actually Seen Black Holes?

• The main problem: Black holes cannot be seen directly; We have to rely on being able to see their effects on a companion star or a disk of matter.
• Small: We know a few dozen stellar black holes in binaries in our galaxy, like Cygnus X-1 (from their estimated masses and sizes), and 20 in M31.
• Mid-size: Some globular clusters (Omega Cen or NGC 5139, M15 and G1) may have black holes with thousands of solar masses at their centers.
• Supermassive: We know beyond much doubt that the core of our galaxy is a supermassive black hole, and it is likely that the cores of most or all galaxies are black holes, with masses in the millions of solar masses or more (depending on galaxy size); We know of many examples, including one galaxy with two supermassive black holes.

Interesting Issues

• Formation of massive black holes: Did most of them form gradually, going through smaller sizes, or from a sudden collapse of material, as in galaxy mergers?
• Black hole radiation: Black holes in principle can emit particles and radiation, as if they had a temperature, by tearing them out of the surrounding vacuum; as Stephen Hawking put it, "Since black holes behave like black bodies, they are not black."
• Can we see this? The radiation is significant only for tiny black holes; but the SETI people are searching for radio waves from the regular-size ones...