Life of a star

Stars are born when a gas and dust cloud subtracts in space by its own gravity. Stars produce energy in fusion reaction of atomic nuclei. It is a nuclear reaction where light elements combine to heavier elements and at same time energy is released. Inside a star there is high enough temperature (about 14 million degrees) and pressure (about 200 billion atmospheres) for fusion to happen.

At the end of its life a star like our Sun produces a planetary nebula. Star has shined 5-10 billion years and the fusion reactions are ending. Now the center of star starts to subtract and the temperature of center rises to new level. At same time the center is unstable and it subtracting and expanding in cycles. The outer parts of the stars are blown to space, as a planetary nebula, in this process. The fusion reactions had produced layers of elements to the star. On top there is still hydrogen and in the core there is oxygen and carbon. At the end a white dwarf star, size of the Earth, is left in center of planetary nebula. The dwarf star fades during billions of years. This kind of end is for a star which has mass under 3 times the mass of the Sun.  

Stars which have mass over 3 times of sun produce in fusion reactions elements all the way to Iron.  Iron does not combine in fusion reactions anymore, so the fusions stop, and the pressure inside the core of the star suddenly drops. Now happens a violent end. Star starts to collapse. Atomic nuclei are  pushed together. Electrons combine with protons, producing neutrons. The event happens in few tens of  seconds and leads to a supernova explosion. The whole outer core explodes to outer space. The supernova  shines with the power of all stars in the galaxy. The energy released in explosion forces in the expanding  shell the Iron to fusion to still heavier elements, producing all the elements up to Uranium.  As examples of supernova explosions are Messier 1, Crab nebula and Veil nebula in Cygnus.

When mass of the star is 3 to 6 solar masses in the center there is left a neutron star,  size about 30 kilometers, mass about 1 solar mass. One tee-spoon of this star weights as much as an oil tanker.

If the mass of star is over 6 solar masses, then the pressure of neutrons is not enough to stop the  collapsing core in supernova explosion. The core collapses smaller than 6 kilometers limit. A black  hole is born. Gravity in this 6 kilometer limit is so high that even light cannot escape. Red dwarf stars have small mass compared to our Sun. Nuclear reactions happen slowly in them and red dwarfs will have a long lifetime, even 100 billion years. About 75% of the stars in our galaxy are red dwarfs. They are mostly so dim, that we do not see them with naked eye.

Universe was born 13.7 billion years ago in Big Bang and after it there was only the simplest elements hydrogen and helium. The heavier elements were produced by first generation of stars. In the deaths of these stars the supernovas and planetary nebulae distributed the heavy elements back to interstellar space. 

Our Sun and Solar system was born 4.5 billion years ago from a gas and dust cloud, to which heavy elements,  needed by life, were distributed by ancient stars. Those atoms that are in you were produced by ancient stars: We are stardust.

Above: Supernova remnant Messier 1, Crab nebula, in constellation of Taurus. Distance 6300 light years. Angular size in sky 6 arch minutes (one fifth of angular size of Moon). Real size is 11 light years.  This exploded as supernova year 1054. The extremely bright star was written to history books by Arabs,  Chinese and Japanese. Visually it was four times brighter than Venus-planet and it could be easily seen even during the daytime. In the core of Crab nebula there is now a neutron star, spinning  around its axis 30 times per second. It is strong roentgen and gamma ray source. Picture was taken 24.10.2020 with Celestron 8F10 telescope. Exposures were 5 x 7 minutes, ISO 1600.

Below supernovas in galaxies M82, M65 and M101.

East part of Veil, Caldwell 33 imaged 16th of October 2023. Telescope TS Photon 200/800, camera Canon Eos 80Da. Exposures 12 x 100 seconds, Iso 800.

Properties of the stars

It is interesting to compare the colors of the stars when they are set side by side. The color reflects the surface temperature. Cool stars are red. Hot stars are white. 

Below is Black hole Cygnus X-1. In constellation of Cygnus, near star eta Cygni, there is a strong X-ray source  Cygnus X-1- It is a black hole orbiting the blue giant star HD 226868. Distance 6070 light years. Image taken 3rd of September 2023.