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Are Black Holes Inevitable? Perplexing! - but are we sure they exist? Some Considerations 1. 2. 3. 4. Neutron stars of a few solar masses are already very close to the Schwarzschild radius, so it wouldn’t take much to tip the balance. For the most massive stars, the Schwarzschild radius is already too big. For example, if you wanted to allow a 10-solar-mass star to settle down as a neutron star, about 10 km in diameter, it already inside its Schwarzschild radius and is doomed to collapse! Stars can’t ‘know’ that they should shed a lot of their dangerous mass, to prevent this kind of collapse. And there are lots of stars of 10, 20, or even 50 solar masses! No mysterious new physics can save you. Why No New Physics? - a Special Circumstance Imagine a huge cluster of a billion (109) stars, perhaps near the center of a galaxy. (That’s about 1% of the stars in the Milky Way.) Now Let Gravity Work Allow gravity to draw these stars together, to within a radius of 3x109 km – about the size of the Solar System. (That’s a lot of stars closely clustered together, but not unimaginable. There would be room!) Overall, at this stage the average density is less than that of water. (It is higher in the centres of individual stars, of course, but not dramatically so.) But this huge distribution of material is already within the Schwarzschild radius, and now there is no escaping the inevitable collapse! Even light is trapped. No One Rings a Bell As the stars draw together, no special conditions are encountered, no exotic or extraordinary circumstances. No one tells the material that a special new behaviour has to rise up to save the day. ….but it’s already too late; gravity will win. SuperMassive Black Holes! This reasoning tells us that SMBHs (Super-Massive Black Holes) seem to be particularly inevitable, even if we could find some way to avoid black holes that have the mass of a single star. Most obviously, we’d expect to find SMBHs in the cores of galaxies, where there are many stars that can clump together. This almost certainly explains quasars, for example [more on this later!] Let’s Try This Out! Meet the Milky Way What We See Zooming in on the Center In the Very Core: A Super-Massive Black Hole http://www.physics.queensu.ca/~hanes/Movies/MW-SMBH.mp4 Keep This in Perspective, However! The Super-Massive Black Hole (SMBH) contains only about 1/1000 of one percent of the mass of the Milky Way – it’s big, yes, but hardly dominant. (Moreover, there are globular star clusters of comparable mass, located here and there in the galaxy.) So our SMBH doesn’t ‘control’ the Milky Way the way the Sun ‘controls’ the planets! There are galaxies in which we find evidence for billionsolar-mass black holes!