There are tens of thousands of them. Not to be confused with astrology, which is a set of beliefs about the influence of heavenly bodies on human affairs. Astrology is not generally accepted to be a science. The theory that the Universe 'started' with an explosive event that created time and space, between 15 and 20 thousand million years ago. Two stars that revolve around each other.
Binary systems are quite common.
Black holes can't be seen, but can be detected through the gravitational effects they exert on other objects. The best known comet is Comet Halley, which orbits the Sun once every 76 years. If a galaxy is moving towards our Galaxy, or away from it, the light we see coming from that galaxy appears different from what it would be if the galaxies were 'standing still'. As most of the galaxies we see are moving away from ours, this redshift effect is much more common than a blueshift. The Doppler shift applies to radio waves and other forms of radiation as well as to light.