Our universe, including our galaxy, is home to millions of stars, each varying in mass and following a unique evolutionary path. This diversity in stellar development is the reason for the wide range of objects in our universe.  

Stars form from vast clouds of gas (primarily hydrogen) that collapse under their own gravity until nuclear fusion starts in the nucleus due to the extreme pressure. Our galaxy alone holds approximately 250 billion stars. Below are some of the most common types of stars:

Stars with less mass than our sun

Red dwarfs are relatively small stars and have less mass than our sun. They are not as hot as the other stars and do not emit as much energy, giving them their characteristic red color. At the end of their lifetime, when they run out of fuel, they fade away unspectacularly and turn into a black dwarf.

Stars with similat mass to our sun

Stars with masses similar to the sun burn hydrogen (nuclear fusion) for about 10 billion years and then exhaust their fuel and enter the red-giant-stadium: during this stage, hydrogen continues to burn in a shell around the core, causing the layers in the middle of the star to heat up and the whole star to keep bloating. They take on a red-orange color because the expansion decreases the temperature. 

Stars like our sun expand in this stadium to an extend to reach as far as jupiters orbit. 

The red-giant phase lasts about one billion years, after which the star sheds ist outer layers to form a planetary nebula. 

What remains is the core, which becomes a white dwarf. 

White dwarfs do not have any fuel left, no pressure inside and they are very hot in the beginning and cool down over billion of years. They only have the size of the earth and mainly consist of carbon and a gas of electrons inside.

Stars with greater mass than our sun

Massive stars with greater mass than our sun undergo more dramatic transformations. After reaching the red-giant stadium, they continue to burn heavier elements in their cores through nuclear fusion. Fusion stops at the pointI iron is formed because creating elements heavier than iron requires more energy than produced. This heavy core causes the outer layers of the star to collapse under ist own gravity, triggering a spectacular explosion known as Typ || Supernova. The supernova is so powerful that it creates elements heavier than iron, such as gold and silver, which are then ejected into space. 

They form new stars and planets, meaning that the gold in our jewelry originated in ancient supernovae.

For stars with masses between 10-50 times that of our sun, the remnant left behind after a supernova is a neutron star. The iron nucleus of the star causes and immense pressure in the collapsing core which forces electrons and protons to combine, forming neutrons. 

Neutron stars are incredibly dense and despite having a radius of only 10-15km, they contain 1,5 times the mass of the sun.

For those stars with mass more than 50 times that of the sun, the lifecycle ends forming one of the most intriguing objects in our universe. After the red-giant stadium and the supernova explosion, these stars collapse into black holes. Black holes are regions in the universe where gravity is so intense that not even light can escape.

These enigmatic objects put physicist in front of a whole new range of mysteries and exciting new challenges.