The stars are ever changing spheres of hot plasma, gaining energy from the nuclear fusion taking place within them. A human lifetime is only a blink of an eye in comparison to the life of a star. This page discusses the life cycle of stars, a process also known as stellar evolution.
Stars begin their life cycle in stellar nurseries of interstellar gas known as nebulas. Nebulas are areas of mostly hydrogen, helium, and cosmic dust particles. Gravity acts to pull matter together, and often times compression waves from the galaxy's spiral arms or a nearby supernova explosion can cause the nebulas to become denser, allowing gravity a better chance of pulling the matter closer.
Nebulas come in a variety of different forms. One form is known as a molecular cloud. These are dark, cool areas of gas and dust. They can sometimes be seen because they silhouette the background of stars, or because light from an especially bright star happens to reflect from one. If a nebula is close to star(s), or is in the process of compression and heating, it may become an emission nebula. Emission nebulas often glow with shades of red or green due to ionized hydrogen and/or oxygen.
As matter is pulled together, it heats up. Eventually a large nebula can break up into smaller and smaller nebulas which become denser and warmer. We can see this process taking place at the Orion Nebula, the Lagoon Nebula, and at many other locations in the sky. A large nebula can spawn thousands of new protostars. As a protostar begins to form, it gains in size and builds up mass from its surroundings. Eventually the core will become so hot (tens of millions degrees Celsius) and dense that hydrogen will fuse into helium, which releases tremendous amounts of energy.
After the fusion process begins, a star is able to stabilize and will remain relatively stable until its supply of hydrogen in the core has run out. A young to middle age star is known as a main sequence star, primarily gaining energy and stability because it fuses hydrogen into helium. A protostar must be at least around 8-9% of the mass of the sun in order to gain the necessary pressure and heat to begin nuclear fusion in its core. If an object does not have enough mass, it is known as a substellar object, to big to be a planet and too small to be a star. Substellar objects are often called brown dwarfs, because they glow with a soft, cool, and dim reddish light and gain heat from the contraction process.
A star fuses hydrogen into helium in its core will do so at a rate which increases if its mass is higher. More massive stars use their fuel faster than less massive stars. A small red dwarf fuses at a rate that would take hundreds of billions of years to complete, while very massive stars are only capable of being main sequence stars for a period of a few million to tens of millions of years. Stars only 20% the mass of the Sun, or 0.2 solar masses, will glow with a soft reddish-orange light and have a low surface temperature of approximately 2500K, while a star five times the mass of the Sun would shine with a hot, bluish light and have a surface temperature of approximately 13000K. (The temperature at the sun's surface is approximately 5800K).
Most stars in the universe have less mass than the sun. The most common stars are between 15%-80% the mass of the sun. Despite that fact, most of the stars visible in the night sky to the unaided eye are of equal or greater mass than the sun. This is because they are much brighter and can be seen from greater distances.
Overall star color is dependant upon temperature. The colors of stars shown in textbooks are often misleading. Stars, regardless of mass, release energy across the electromagnetic spectrum (including across the visible spectrum), with absorption and emission lines slightly skewing the overall star color. "Red" stars are considered red because the majority of their visible light is from the red part of the spectrum, and most "red" stars actually look more orangish (like a sodium vapor streetlight) or pink than "red". (Although even cooler brown dwarfs would look very reddish). "Blue" stars don't appear sky blue, but rather an intense bright bluish to violet, more like a mercury vapor streetlight or an intense welding arc. Although the sun is generally described as "yellow", it is in fact a white star when seen from outer space. Our perception of it as being yellow is due to atmospheric scattering of its light, which appears to shift it to yellow.
Eventually the star's period of relative stability will come to an end as its core has lost its supply of hydrogen. The equilibrium of gravity pushing in and the energy from nuclear fusion pushing out which kept the star stable is no longer in place. The core is forced to contract and in doing so, its temperature soars. The added heat and pressure will cause helium to fuse into carbon, which is a much more vigorous and shorter lived reaction compared to its previous state as a main sequence star. There may also be a layer on the outer core still fusing hydrogen to helium above the main part of the core. The added heat causes the outer layers of star to expand outward (after a brief variable stage) and cooling as they do so, and they glow with a reddish orange color. The star is now a red giant. A new equilibrium is restored, but it will not last as long as the previous one. A red giant star is hundreds of times larger in regards to the amount of space it takes up, but the mass has remained basically the same.
If a white dwarf is close to another star in a multiple star system, it can often pull matter away from the one other star onto its surface. Sometimes enough hydrogen accumulates on the white dwarf to trigger fresh fusion reactions of the hydrogen. The white dwarf brightens significantly, becoming a nova. A nova can remain at a high level of brightness for years as the reaction declines, and afterwards the white dwarf returns to normal. The process of a white dwarf gaining mass from a companion star can also lead to its total destruction. A white dwarf can pull enough matter off its neighboring star to become unstable. It is also possible that white dwarfs can collide in a binary system, due to orbital decay and drag forces from their previous time periods as red giants. If the mass of a white dwarf becomes greater than the Chandrasekhar Limit (about 1.38-1.44 solar masses, depending upon rotation and composition), the gravity becomes so strong that it overcomes the resistance of electron repulsion and the star collapses. It would theoretically become a neutron star (which I will get to later), but unlike what would form a neutron star, the white dwarf contains matter that could be used for nuclear fusion, but never reached the temperatures necessary to do so. The collapse results in a sudden rise in temperature, causing an uncontrollable reaction of carbon fusing into heavier elements and causing the star to completely blow up. This is known as a Type Ia supernova explosion.
For stars more massive than about eight solar masses in the red giant stage, the core continues to produce elements heavier than carbon in its core. Carbon fuses into oxygen and neon, then oxygen to silicon and sulfur, then finally silicon and sulfur fuses to iron. Each reaction occurs in quicker succession, with layers of previous reactions still happening on the outer parts of the core. The supergiant swells up more and more during the final stages in its life, often becoming variable in brightness and throwing off material explosively. The star color may change also, varying from being a blue supergiant to being a red one, depending upon speed and amount of stellar wind outflow. The last reaction of silicon and sulfur fusing to iron takes less than a week to complete. Once the core has been converted to iron, the star no longer has a way to hold back gravity.
Type Ib, Ic, or Type II Supernovas
The star cannot use iron to sustain itself or hold back the pressure gravity is exerting. For atoms the size of or heavier than iron, fusion absorbs instead of releases energy. (This is the same reason why we can get energy out of splitting uranium nuclei in a process called fission.) Gravity causes the core to collapse catastrophically and the outer layers cave in. Then these layers are ripped apart by an immense shock wave. This is usually known as a Type Ib, Ic, or Type II Supernovas. Neutrons set free by the collapse bombard material forming elements heavier than iron, such as gold, silver, bromine, iodine, mercury, lead, and uranium. The outrushing material sends shock waves through space which can ignite further star formation in nearby nebulas. The supernova becomes as bright as an entire galaxy for a period of weeks.
A neutron star is generally the remnant produced from this type of supernova. The neutrons prevent gravity from further contracting the star. Protons and electrons were forced to merge to become neutrons as a result of the collapse of the core. Neutron stars are much denser than white dwarfs. A cup of neutron star material would weigh as much as a small planet! Between 1.38 to about 2.2 solar masses of material is compacted to a sphere less than 20 miles in diameter. Neutron stars often have very strong and distorted magnetic fields, often causing them to have an unequal amount of energy output into space. From afar, it may look like the star is blinking on and off. Such neutron stars are known as pulsars.
If the supernova leaves behind more than about 2.2 solar masses of degenerate material, it will become a black hole. Matter is forced into zero volume because gravity is able to overcome any resistance that matter gives to further compression. The escape velocity required to escape such an area is so great that light, moving at approximately 186,000 miles per second, cannot leave. Often black holes can be detected when they pull matter off of nearby star(s), which is seen emitting x-rays and gamma rays, heating up before it falls onto the black hole.
Gamma Ray Bursts
Extremely massive stars may have cores that collapse directly to become black holes, leading to an intense burst of directed energy known as gamma ray bursts. Gamma ray bursts have been observed in other galaxies and their exact mechanism remains a subject of debate.
Conclusion
The process of stellar evolution is a complex and intracate topic. The life cycle of stars is a fascinating concept to learn about and understand. One can gain a sense of wonder understanding the complex processes and how they interact together.
My space art galleries:
GALLERY I
GALLERY II
GALLERY III
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