Stars, like people, are born and die. One is reserved decrepit old age in the guise of a dull white dwarf, the other - "afterlife" in the form of a neutron star or a black hole. But how to determine what metamorphosis awaits this or that star, including our native Sun?
©Depositphotos
Astrophysics has already advanced enough in the study of the evolution of stars. Theoretical models are backed up by reliable observations, and despite the presence of some gaps, the general picture of the life cycle of a star has long been known.
BIRTH
It all starts with a molecular cloud. These are huge areas of interstellar gas, sufficiently dense for hydrogen molecules to form in them.
Then an event occurs. Probably, it will be caused by a shock wave from a supernova that exploded nearby, or maybe the natural dynamics inside a molecular cloud. However, the outcome is one - gravitational instability leads to the formation of a center of gravity somewhere inside the cloud.
Surrendering to the temptation of gravity, the surrounding substance begins to rotate around this center and is layered on its surface. Gradually, a balanced spherical nucleus with a growing temperature and luminosity - the proto star, is formed.
The gas-dust disk around the proto star rotates faster, because of its growing density and mass, more particles collide in its interior, the temperature continues to grow.
As soon as it reaches millions of degrees, the first thermonuclear reaction takes place at the center of the proto star. Two nuclei of hydrogen overcome the Coulomb barrier and combine to form the core of helium. Then - the other two cores, then - the other ... until the chain reaction covers the entire area in which the temperature allows hydrogen to synthesize helium.
The energy of thermonuclear reactions then rapidly reaches the surface of the luminary, dramatically increasing its brightness. So the proto star, if it has enough mass, turns into a full-fledged young star.
Active star formation region N44 / ©Depositphotos
NO CHILDHOOD, NO ADOLESCENCE, NO YOUTH
All proto stars, which are heated enough to launch a thermonuclear reaction in their bowels, then enter the longest and most stable period, occupying 90% of the total time of their existence.
All that happens to them at this stage is the gradual burning out of hydrogen in the zone of thermonuclear reactions. A literal "burning of life". The star is very slowly - for billions of years - will become hotter, the intensity of thermonuclear reactions will grow, like the luminosity, but no more.
Of course, events are possible that accelerate stellar evolution - for example, a close neighborhood or even a collision with another star, but this does not depend on the life cycle of a single luminary.
There are some kind of "stillborn" stars that cannot reach the main sequence - that is, they cannot cope with the internal pressure of thermonuclear reactions.
These are low-mass (less than 0.0776 from the mass of the Sun) proto stars - the very ones that are called brown dwarfs. Because of insufficient gravitational compression, they lose energy more than is formed as a result of hydrogen synthesis. With time, thermonuclear reactions in the bowels of these stars cease, and all that they have left is a prolonged, but inevitable, cooling.
Brown dwarf in the artist's view / ©Depositphotos
RESTLESS OLD AGE
Unlike people, the most active and interesting phase in the "life" of massive stars begins towards the end of their existence.
The further evolution of each individual light that has reached the end of the main sequence-that is, the point where there is no hydrogen for thermonuclear fusion at the center of the star-directly depends on the mass of the luminary and its chemical composition.
The less weight a star has on the main sequence, the more prolonged will be her "life", and the less grandiose will be her finale. For example, stars with a mass less than half the mass of the Sun - such as those called red dwarfs - have never even "died" since the Big Bang. According to calculations and computer simulation, such stars, because of the weak intensity of thermonuclear reactions, can safely burn hydrogen from tens of billions to tens of trillions of years, and at the end of their path, they probably die out just like brown dwarfs.
Author's idea of an exoplanet revolving around a red dwarf GJ 1214 / ©Depositphotos
Stars with an average mass of half to ten solar masses after burning hydrogen in the center are able to burn heavier chemical elements in their composition - first helium, then carbon, oxygen and then, how lucky with the mass, up to iron-56 (iron isotope, which is sometimes called "ashes of thermonuclear combustion").
For such stars, the phase following the main sequence is called the stage of the red giant. The launch of helium thermonuclear reactions, then carbon, etc. each time leads to significant transformations of the star.
In a sense, this is a death agony. The star then expands hundreds of times and turns red, then shrinks again. Luminosity also changes - it increases thousands of times, then decreases again.
At the end of this process, the outer shell of the red giant is dumped, forming a spectacular planetary nebula. In the center remains a naked core - a white helium dwarf with a mass of about half the solar and a radius roughly equal to the radius of the Earth.
White dwarfs have a destiny similar to red dwarfs - a quiet burnout for billions of trillions of years, unless, of course, there is a companion star nearby, due to which a white dwarf can increase its mass.
The system KOI-256, consisting of red and white dwarfs / ©Depositphotos
EXTREME OLD AGE
If the star is particularly lucky with the mass, and it is approximately 12 solar or more, then the final stages of its evolution are characterized by much more extreme events.
If the mass of the red giant's core exceeds the Chandrasekhar limit of 1.44 solar masses, then the star does not just drop its shell in the finale, but releases the accumulated energy in a powerful thermonuclear explosion-a supernova.
In the heart of remnants of the supernova scattering the stellar matter with enormous force for many light years around, there is in this case no longer a white dwarf, but a super dense neutron star, with a radius of only 10-20 kilometers.
However, if the mass of the red giant is more than 30 solar masses (or rather, the supergiant already), and the mass of its core exceeds the Oppenheimer-Volkov limit, which is about 2.5-3 solar masses, no white dwarf or neutron star is formed.
In the center of the supernova remnants, something much more impressive appears - a black hole, as the core of the exploding star shrinks so much that even neutrons begin to collapse, and nothing more, including light, can escape from the limits of the newborn black hole - or rather, its event horizon.
Especially massive stars - blue supergiant’s - can pass the stage of the red supergiant and also explode in the supernova.
Supernova SN 1994D in the galaxy NGC 4526 (a bright spot in the lower left corner) / ©Depositphotos
MOREOVER, WHAT AWAITS OUR SUN?
The sun belongs to the stars of medium mass, so if you carefully read the previous part of the article, you yourself can already predict which way our star is on.
However, even before the Sun turns into a red giant, a number of astronomical shocks await humanity. Life on Earth will become impossible in a billion years, when the intensity of thermonuclear reactions in the center of the Sun will be sufficient to evaporate the terrestrial oceans. Parallel to this, the conditions for life on Mars will improve, which at some point can make it habitable.
After about 7 billion years, the sun warms up enough for a thermonuclear reaction to be launched in its outer regions. The radius of the Sun will increase approximately 250 times, and the luminosity 2700 times - will turn into a red giant.
Due to the increased solar wind, the star at this stage will lose up to a third of its mass, but will have time to absorb Mercury.
The mass of the solar nucleus due to hydrogen burning around it will then increase so much that a so-called helium flash occurs, and thermonuclear synthesis of helium nuclei into carbon and oxygen will begin. The radius of the star will decrease significantly, up to 11 standard solar ones.
Solar Activity / ©Depositphotos
However, 100 million years later, the reaction with helium will pass to the outer regions of the star, and it will again increase to the size, luminosity and radius of the red giant.
The solar wind at this stage will become so strong that it will carry out the outer regions of the star into outer space, and they form an extensive planetary nebula.
And where there was the Sun, there will be a white dwarf the size of the Earth. At first, extremely bright, but with time more and more dim.
The illustrations are used in agreement with the Depositphotos photobank
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