There are an estimated 50 thousand million galaxies in the universe, with the typical galaxy containing 50 thousand million to 100 thousand million stars. It is estimated that there are 10 on 22nd stars in total in the universe.
The brightest astronomical event in historic times was the supernova of 1054, which produced the Crab Nebula. The supernova was far brighter than Venus. It was bright enough to be visible in daylight and to cast a shadow at night. We know of it through the astronomical records of China, Japan, and the Middle East.
The telescope was invented in 1608 when spectacle-maker Hans Lippershey's apprentice was playing games. The apprentice was amusing himself with lenses and found a combination that made things seem closer. When Lippershey was shown this combination, he enclosed the lenses at two ends of a tube.
As late as 1820, the universe was thought by European scientists to be 6,000 years old. It is now thought to be about 13,700,000,000 years old.
The Earth is rotating on its axis at a rate of 460 metres per second at the equator, and is orbiting the sun at a rate of about 30 kilometres per second. The sun is orbiting the centre of the Milky Way at a rate of about 220 kilometres per second. The Milky Way is moving at a speed of about 1000 kilometres per second towards a region of space 150 million light years away called the Great Attractor.
A "light year" is a measure of distance, not time. It is defined as the distance light travels in one year. Light moves at a velocity of about 300,000 kilometres each second, so in one year, it travels about 9,500,000,000,000 kilometres.
The matter in the universe is so thinly dispersed that the universe can be compared with a building twenty miles long, twenty miles wide, and twenty miles high, containing only a single grain of sand.
A massive star has a shorter lifetime than a less massive star. The more massive a star, the more tightly its gravity pulls it together, the hotter it must be to keep it from collapsing, and the more rapidly it uses up its hydrogen fuel.
The reason there are so few really massive stars is that they do not live very long, as little as a million years. For comparison, our sun has an expected lifetime of about 11,000 million years.
About 25% of the universe consists of "dark matter", and about 70% consists of "dark energy", leaving only about 5% of the universe visible to us. The star Betelgeuse, a bright star in the constellation of Orion, is estimated to have a diameter of around 700 million miles. If it were placed at the centre of our solar system, it would extend beyond the orbit of Jupiter.
Alpha Centauri, one of the stars in the system closest to the sun, is never visible in the sky north of about 30° Northern Latitude. Though it is the third-brightest star in the sky, it was not seen by the ancient Greeks nor the chief observatories of the mediaeval Arabs at Cordoba, Baghdad, and Damascus, all located north of the 30° line.
The most distant galaxy ever observed is estimated to be around 13,000,000,000 light-years away. Discovered by the Hubble Space Telescope in 2004, it is located behind the galactic cluster Abell 2218, which bends the object's light. It is a small, energetic galaxy whose light that is seen on Earth now would have set out when the universe was just 750 million years old. This is the most distant object that can be observed consistently; some ephemeral gamma-ray bursts have been observed that are slightly more distant than this object.
The Milky Way has a radius of about 50,000 light years.
Hotter stars emit blue light; cooler stars emit red light.
The closest star we know of that exploded as a supernova is the one that left behind the Gum Nebula (named for Colin Gum, an Australian astronomer who discovered it in 1950). The star was only around 1,500 light-years away. At its peak, that supernova may have shone as brightly as the full moon for several weeks; however, that occurred 11,000 years ago, and only Stone Age humans were around to watch and wonder