The lifecycle of stars, a simple overview

We all know that everything started with the Big Bang but how did it form our lovely sky of lights? How did our Sun form and our majestic starry night? How do all the other stars form? I’ll try to give a simple overview but bear in mind there is a lot of grey are in between.

It started with the big bang that expelled endless amounts of matter across this ever expanding universe. This eventually started to form clouds of gas. Out of these clouds of gas stars will form. Keep in mind that stars are nothing more than ‘nuclear fusion reactors’.

The way these clouds of gas can form stars is when two clouds collide or get pressurized by an event like a giant supernova. What also is possible is when two already existing galaxies ‘crash’ into each other, from where multiple stars can be ‘born’. Out of both these situations, so called, protostars emerge.

What now is the decisive factor in the stars lifecycle is it’s starting mass.

The protostar can have;

Low mass

These low mass stars are named red dwarf stars. Red dwarf stars are only 7.5%-50% of our Sun’s mass. Red dwarfs burn very steadily for a very long time. Out of the many types of red dwarf there are some that are convective, so they will to ‘burn-up’ all their hydrogen atoms. So far no red dwarves have been found that have expended all their hydrogen. This is because our universe is ‘relatively young’ and the fact that red dwarves burn slowly, so this event cannot have happened yet. Red dwarves are the most common star in the Milky Way Galaxy and the closest one to us is Proxima Centauri but this star is not visible to the naked eye.

What happens after this star has burnt up all its hydrogen has been calculated with computer simulations. With a mass of 0.25 times our sun our great it can transform into a red giant. Below this mass it will turn into a blue dwarf, that will in turn, turn into a white dwarf and ‘fizz out’. This white dwarf will be very hot when formed but then will turn into a black dwarf as it cools down and ‘dies’.

From left to right, the Sun, α Centauri A, α Centauri B, and Proxima Centauri.

From left to right, the Sun, α Centauri A, α Centauri B, and Proxima Centauri.

Intermediate mass

The intermediate mass stars are named red giants. Stars that have spent all their hydrogen supply and are left with a helium outer shell and an oxygen and carbon core. At that point the core starts to become more and more compact as the outer shell starts to expand and cool. Because of this cooling down of the outer layer, the star will get a more reddish hue and enlarge. If our own Sun would become a red giant its outer shell will be as large as the orbit of Jupiter. As the star now starts to expand it will collapse unto itself. Normal red giants will collapse and ‘eject’ their outer layer leaving a planetary nebula around its core. This core is also a white dwarf.

The Helix Nebula.

The Helix Nebula

The heavy elements these stars emit in their supernovas are also the very same elements that our life is composed of. That is carbon, calcium, oxygen etc. So in theory, a lot of the atoms that you consist of have been ‘forged’ in the heart of a star.

High mass

High mass stars are named red giants as well but these fast burning stars rapidly expand and are transformed into red supergiants. Red supergiants only burn a few hundred thousand years and perhaps up to a million years. At that time their core has built up a lot of iron after ages of nuclear fusion. After this, the red supergiants will cause a supernova.

These super nova’s are the biggest explosions in the known universe and also has two different outcomes depending on the mass of the exploded star.

‘Small’ super novas result in a neutron star. The remnants of the core of a red supergiant. This neutron star can be about 12km in radius. So relatively VERY small, but this little star has about 500.000 times the mass of the Earth! Not only is the mass and size intriguing, the spin time for one rotation of a neutron star 1.4 ms to 30s! This is due to the fact that a gigantic star spins really fast and if it suddenly were to lose 99% of its mass, the rest would keep on spinning very fast. (Comperable if you would to spin around in a desk chair and retract your limbs to the center, you can feel yourself speeding up.)  Density wise, five milliliters of this stuff (a teaspoon) would have the weight of 5.5×1012 kg. The density of this star is so large, it’s the equivalent of all the humans on the planet compressed into a size of a sugar cube.

Neutron star compared to Manhattan.

Neutron star compared to Manhattan

The other result is a black hole. After the explosion a normal core is about 1.4 times our Suns mass. If this mass is too great it will result in a black hole, creating a hole in space. This black hole its gravity is so large it sucks up light and matter. Nothing can come from a black hole, only stuff goes in.


A black hole simulated

The light gets distorted by the gravitational pull of the black hole, which creates a sort of ring around the void. This phenomena is called gravitational lensing.


Universe today - Red Supergiant Star - Fraser Cain 2009

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