Not all stars are the same: when a large molecular cloud slowly contracts under its own gravity, densifications of very different sizes are formed in it.
And this ultimately results in stars of different masses and sizes – which in turn develop in very different ways depending on their mass.
The actual life of a star begins when the pressure and temperature in the center of a gas ball have become large enough to ignite the fire of nuclear fusion:
Hydrogen nuclei fuse to form helium nuclei, and a lot of energy is released in the process.
Since hydrogen is the most abundant element in the universe, stars are also composed of about three-quarters hydrogen.
Thus, with the onset of hydrogen burning, stars have opened up a large but nevertheless limited energy reservoir.
Stars burn faster than a house made of wood
At first sight one could think now that a large and massive star must live longer because it contains more hydrogen and therefore has a larger energy reservoir.
But this is not so. Because the giants among the stars are much more wasteful with their energy reserves than the dwarfs.
The luminosity of a star in the phase of hydrogen burning – astronomers also speak of the so-called main sequence stage – even increases with the 3.5th power of mass.
A star with ten times the mass of the Sun therefore radiates more than 3000 times the energy of our central star in the same time – and consumes its energy supply correspondingly faster.
Overall, the duration of the central hydrogen burning is inversely proportional to the 2.5th power of the star’s mass.
For the Sun, astronomers have calculated a lifetime on the main sequence of about eleven billion years.
For small red dwarf stars with one-tenth of the mass of the Sun, on the other hand, this phase takes almost 3.5 trillion years – a multiple of the previous world age.
So stars that contain much less mass than our Sun cannot be burned out at all.
The situation is completely different for giant stars: A star with ten times the mass of the Sun has used up its central supply of hydrogen after just under 35 million years.
So it is the large, massive stars that breathe out their life the fastest. In fact, it is also these massive stars that go supernova in a massive explosion and light up for a short time as a “new star” in the sky.
There’s a big bang in the end
When all the hydrogen in the central region of a massive star has been consumed, pressure and temperature there continue to increase and further fusion processes occur: Helium burns to form carbon and oxygen, and carbon is turned into even heavier elements, including iron.
But after that, no more energy can be gained from nuclear fusion – the interior of the star collapses and becomes – depending on its mass – a neutron star or a black hole, while the outer layers are catapulted into space in a huge explosion.
All these processes after the main sequence take place within ever shorter times and therefore do not play a significant role in the total lifetime of a massive star compared to the main sequence.
For stars of low or medium mass like our Sun, the end is less dramatic.
If the mass is greater than a third of the mass of the Sun, helium will continue to fuse for several million years, but the pressure and temperature inside the star will not be sufficient for further fusion processes.
The stars inflate to form a red giant, repel their outer hulls and finally collapse into a white dwarf star that slowly cools down over billions of years.
More information about this topic:
https://www.livescience.com/32319-how-long-do-stars-live.html [ Last seen: 01.12.20 ]
https://skyandtelescope.org/astronomy-resources/how-long-do-stars-live-stars-die/ [ Last seen: 01.12.20 ]
https://www.thoughtco.com/what-are-stars-really-3073631 [ Last seen: 01.12.20 ]
https://www.universetoday.com/25160/how-long-do-stars-last/ [ Last seen: 01.12.20 ]
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