Stars are massive, self-luminous celestial bodies made of hot gas and plasma. In our Milky Way alone, there are about 200 billion of them, but
only ~5,000 of them can be seen in the sky with the naked eye.
An important attribute of stars is their temperature. The coolest objects belong to the "brown dwarfs" and reach only a few hundred degrees at
the surface. "Red dwarfs" on the other hand are already 3,000K hot, our yellowish shining sun comes to 5,500K. The hottest are the massive
"blue super giants", whose temperature can be up to 50,000K. They can shine up to one million times as brightly as our central star. The lightest
stars are just 10-15x as heavy as the planet Jupiter, the upper mass limit is about 160-180 solar masses according to the current state.
More than 70% of the stars are part of a double or even multiple star system, in which the stars move around a common center of gravity. So a
single star like our sun is rather the exception than the rule.
When a star dies, stars like our Sun end up with a "white dwarf" that slowly cools and eventually becomes a "black dwarf." Much more massive
stars end their life after a spectacular supernova explosion as a "neutron star" or become a "stellar black hole". In the case of extremely massive
stars, it can even happen that the celestial body is completely torn apart.
