All stars in the universe, including our Sun, go through different phases. Each type of star undergoes different processes and, at the end of its life, transform into different types of objects. Scientists call this sequence stellar evolution processes. But what is stellar evolution?
Understand what is stellar evolution
Stellar evolution is the sum of a star’s radical changes during its lifetime, and includes its transformation after collapse. Depending on the star’s mass, it can become a white dwarf, a neutron star or a black hole.
No star lasts forever, because they all have a finite amount of fuel for nuclear fusion — which is how they produce energy to shine. But, contrary to what one might imagine, the smallest stars are the ones that “live” the longest.
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The lifetime of an intermediate star like the Sun is a few billion years, but less massive ones can last up to trillions of years. The most massive, known as blue giants, live only a few thousand years.
How are stars born?
If there is one thing that all stars have in common during stellar evolution, it is the way they are born. They form inside large clouds of gas and dust, known as the star-forming region, or by the nickname of the nursery of stars.
Within these clouds, massive clumps of matter accumulate, so gravity kicks in to push this material toward the center of the clump, until they collapse in on themselves. Then the material starts to heat up and becomes a protostar.
It will still be a long time before the pressure of gravity causes nuclear fusion — the process of fusing the nuclei of the hydrogen atoms present in the cloud — to start happening. When this occurs, the object will be considered a proper star.
At this time when nuclear fusion began to occur, the star entered the so-called main sequence, which is the longest phase of its life. From there, the evolution of each stellar class will be different.
How do stars “catch on fire”? Understand nuclear fusion
The first difference, in addition to the mass and color mentioned above, is in the elements that each type of star will be able to fuse in its core. After the fusion of hydrogen (the simplest atom) into helium, low-mass stars (up to 0.5 solar mass) cannot fuse helium atoms because they are not massive enough to exert the necessary pressure on their nucleus.
Intermediate stars (0.5 to 10 solar masses) are able to fuse helium into carbon, while high-mass stars continue to produce elements such as neon, oxygen, silicon and iron. From there, things get complicated for all of them — no star can fuse iron atoms.
Stellar evolution of low-mass stars
Exactly what happens after a low-mass star’s hydrogen fuel runs out is not yet known. It is that these stars can live for up to trillions of years, much longer than the current age of the universe itself. Therefore, the only way to predict how they evolve is through computer simulations.
In general, they will never be able to fuse helium because there is not enough mass to put pressure on the atoms. However, some can even fuse helium at superheated points in the core, causing an unstable reaction. In such cases, the end of the star is simply to evaporate, leaving behind a brown dwarf. There is also the possibility of becoming a red giant, which will be described below.
Stellar evolution of intermediate-mass stars
After the main sequence phase of intermediate-mass stars, they undergo a major transformation. There are two possible paths to this step: they can become red giant branch stars, whose layers are still fusing hydrogen into helium, or asymptotic giant branch stars, which have a core fusing helium into carbon.
In both cases, the accelerated fusion of hydrogen, in a layer immediately above the core, causes the star to expand faster than the energy output. As a result, the star cools down and becomes redder than when it was on the main sequence (red stars are cooler than orange ones, for example).
With pulsations occurring due to temperature variations, the outer layers of the star are ejected and can form a planetary nebula with a core at the center. This core will cool down and become a compact object known as a white dwarf. This is the fate of stars like the Sun.
Stellar evolution of massive stars
With massive stars, things get even more interesting. First, because the lifetime of these stars is only a few million years, which allows astronomers to witness the end of some of them, which are already reaching the end of their evolutionary cycles. Second, their death can be accompanied by spectacular supernovae.
There is a small chance that massive stars will become red supergiants, but extremely massive ones (more than 40 solar masses) lose mass quickly and tend to rip their envelopes off before they can expand. Because of this, they maintain extremely high temperatures and the color blue (blue stars are hotter than orange and red ones).
The massive ones are the only stars capable of fusing elements all the way to iron, but at that point adding fragments to the cores (each element forged forms a new layer, like an onion) releases less energy than it takes to release them from the parent nucleus. .
If the mass of the core exceeds a critical limit (known as the Chandrasekhar limit), the internal pressure will be insufficient to support the gravitational weight, and the core will collapse. In this, there will be two possibilities: a neutron star will appear or, in the case of cores that exceed the Tolman-Oppenheimer-Volkoff limit, a black hole.
Some of the energy released by this core collapse is converted into a supernova. This process creates materials heavier than iron, including radioactive elements, at least as far as uranium. Red giants can also produce elements heavier than iron, but the amount in supernovae is much higher.
The abundance of heavy elements produced by supernovae is important for enriching planetary systems that form from other stars, such as our Solar System. No single supernova can produce the amount of heavy elements that exist in our cosmic backyard, so scientists conclude that they came from ejection in red giant stars and distant supernovae.
Finally, the explosion of a giant star in supernova leaves behind a cloud of debris, called a “supernova remnant”. The images of these remnants are always spectacular and can be recorded at various wavelengths through telescopes that “see” different types of light.
