At the center of most spiral galaxies lies a huge spherical bulge of older stars 10,000 light years across or more. No spiral arms disturb the central bulge, so its stars are old and yellow and red rather than blue and glittering with youth.
And in the very center of almost all galaxies lies a black hole.
When a star dies, its gravity is no longer opposed by the outward pressure of nuclear fusion in its core. It collapses until the atoms in the stellar remnant are packed as closely as possible against one another. In a star like our Sun, gravity will pack the atoms together until electrons can’t get any closer to one another. This is called a white dwarf, perhaps with the mass of our Sun but packed into the size of the Earth. A larger star has enough mass to crush the electrons into the protons in the remnant, forming nothing but neutrons. At this point the resulting neutron star may be only a few miles across, dense beyond our capacity to imagine. The packed neutrons are governed by the strong nuclear force that holds atoms together.
But if a dead star is massive enough, greater than about 10 times the mass of our own Sun, its gravity may be so strong that even the strong force cannot hold the neutrons up. In such cases the matter just continues to fall in and in, deeper and deeper, bending space itself into a tighter ball until dimensionality itself becomes meaningless. This is a Black Hole — a singularity, a point of infinite density of mass and infinite curvature of space and time. The gravity is so intense that even light cannot escape. Hence the name!
The Supermassive Black Holes at the centers of most galaxies may have formed by accumulation and merging of smaller black holes formed by huge dying stars, but many of them are so big that the universe may be too young to have enough time for all that. Nobody really knows how they got there, but they are truly Leviathan. The one in the center of our galaxy is estimated to have 4 million times the mass of our Sun! It’s hidden behind thick clouds of soot and dust, 26,000 light years away in the direction of the constellation Sagittarius. Using special telescopes, scientists have measured stars orbiting the central black hole at ⅓ the speed of light!
