Stars are formed in huge gas nebulae.
They contract by their own gravity and start to rotate.
They contract so far that they form a sphere, and the protostar is born.
A protostar is something like a child star. This protostar grows steadily and becomes bigger and bigger.
At some point it has accumulated so much mass that it ignites hydrogen fusion due to the high temperature created in its interior by the mass pressure.
At that time the star is about 10 million degrees hot in its center. Slowly the rest of the dust and gas falls onto the star and the birth shell becomes transparent.
Depending on the amount of matter available to a protostar, it continues to evolve.
If there is little matter, a low-mass star is formed, such as a red dwarf or even a brown dwarf, which does not even have enough mass to ignite its own hydrogen nuclear fusion.
With medium mass a star like our sun is formed and with much mass giant blue stars are formed. They are blue because they are hot on the surface.
Some of these stars have a surface temperature of more than 60000 K. Small stars burn their hydrogen much slower and therefore live much longer than big hot stars. Giant blue stars live only about 1-2 million years, which is an extremely short time for stars.
Life-cycle of a star
The lifetime of a star with 15 times the mass of the Sun looks like this:
10 million years fusion of hydrogen to helium
1 million years fusion of helium to carbon
6000 years fusion of carbon to neon, magnesium, sodium
7 years fusion of neon to oxygen
1 year fusion of oxygen to silicon
3 days fusion of silicon to iron
a few tenths of a second until the iron core collapses
Ejection of the surface and the brightness increases to ten billion times (supernova eruption)
A neutron star remains
After a supernova the so-called supernova remnants remain. There are many supernova remnants in which neutron stars were found, for example in the Crab Nebula in the constellation Taurus. Neutron stars rotate very fast, the neutron star in the Crab Nebula rotates 30 times a second.
Black holes are formed when a neutron star becomes more massive.
In double star systems the neutron star collects mass from its companion.
As a result, it develops such a high attraction at the surface that even light cannot escape. The neutron star has become a black hole.
Smaller stars live much longer. A red dwarf of the M-class lives about 100 billion years, our sun after all 10 billion years (of which 4.5 have already passed).
Also they do not end in a supernova but repel the outer shell which becomes a planetary nebula. The stars end up as an ordinary white dwarf.
Siyavula.com: https://intl.siyavula.com/read/science/grade-9/birth,-life-and-death-of-a-star/27-birth,-life-and-death-of-a-star [ Last seen: 01.12.2020 ]
NASA.gov: https://science.nasa.gov/astrophysics/focus-areas/how-do-stars-form-and-evolve [ Last seen: 01.12.2020 ]
kids.frontiersin.org: https://kids.frontiersin.org/article/10.3389/frym.2019.00092 [ Last seen: 01.12.2020 ]
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