There are around 1024 stars in the universe, and each one is a bit different from the others. But what every star has in common is their life cycle. From a huge molecular cloud to a main sequence star, to a giant, and then, finally, a supernova, each star lives an exciting life. Read on to find out more!
Stage one: Molecular Cloud
A molecular cloud is a giant accumulation of gas and dust (mostly molecular hydrogen) in outer space. Molecular clouds are quite cool as compared other parts of outer space, at around 10 - 30 degrees kelvin (around -230oC). Molecular clouds can be up to 600 light years in diameter, and their mass can be up to a few million solar masses (around 0.7x1034 kilograms). These clouds aren’t totally homogeneous- they contain areas of higher gas and dust density, known as knots, or cores. It is in these knots where stars begin to form.
Stage two: Protostar
A protostar is the first stage in a star’s evolution. First suggested by Chushiro Hayashi in 1966, a protostar is formed when one of the knots in a molecular cloud begins to contract, and then gain mass, which in turn causes them to contract further due to increase in gravity. The resulting clump of hot gas and dust is a protostar, which is around the size of a solar system. As the protostar gains more gravity, it continues to heat up and attract material, which is pulled into a disk around the star. In stars the size of our sun this stage last for about a million years.
Stage three: Proplyd
The proplyd is a large (around 100 AU across) pancake-shaped disk of gas and dust that forms around a million years after the protostar, which has at this point shrunk to around one AU (astronomical unit) in diameter. The proplyd is thought to be what planets eventually form from, with the dust compacting to form planets orbiting the star. These proplyds are strong infrared sources, with hot and active centers, which gradually cool off and bulge towards the edges. Proplyds are also known as protoplanetary disks.
Stage four: T. Tauri star and Bipolar Molecular Outflow
After another million or so years, the protostar has again shrunk to form what is known as a T Tauri star, which is the name of the first star of this type discovered. T Tauri stars are highly active young stars, generally a few times the diameter of our sun, which means they appear brighter than other stars of the same mass. T Tauri stars spin rapidly, and they are characterized by their large solar flares caused by intense magnetic activity, which also causes huge sunspots to form over large areas of the star.
Bipolar molecular outflow
As a T Tauri star pulls gas from it’s protoplanetary disk, it ejects it outward in two particle beam jets perpendicular to the disk, which light up when they interact the gases surrounding the star. These beams form bright nebulae known as Herbig-Haro objects.
Stage Five: Main Sequence Star
After around 10,000 years of bipolar molecular outflow, the T Tauri star gets hot and massive enough to initiate fusion in its core. At this point the star is, so to speak, fully grown, or a main sequence star. It is estimated that around 90% of stars in the universe are main-sequence.
Fusion is the process wherein hydrogen atoms combine to form helium, releasing 0.7 percent of their mass as pure energy in the form of heat and radiation. This doesn’t sound like much, but according to Albert Einstein’s famous equation, E=mc2, one gram of mass converted into energy would be around 900,000,000,000,000.0 joules. One joule is approximately the energy used to lift an apple, so you could lift 0.9x.1014 apples with this much energy. Technically.
This means that our sun (an average-sized star) produces approximately 0.4x1014 joules of energy, which is equivalent to the energy produced by about a trillion tons of TNT going off every second.
The lifetime of a main sequence star depends heavily on its size. A small star can last around 80-100 billion years, while a large star (around 100 times the mass of our sun) may last for only around twenty million. The fact that large stars have such short lives seems counterintuitive, but is caused by the fact that they use up their fuel (hydrogen) at a much faster rate than smaller stars.
Stage six: Giant
Once all of a stars hydrogen is converted to helium, a star’s core will collapse upon itself, causing the outer layers to expand forming a subgiant, and then finally, a red giant, the red colour being caused by lower surface temperature. Red giants are generally 0.3 - 8 solar masses, their size depending heavily on the original size of the star. Extremely massive stars can sometimes even form into an even larger class of giant, known as supergiants, whos size also depends on the size of the star, but is estimated that a large supergiant could be up to 2,600 times the size of our sun.
Stage seven: Core Fusion
As the star expands and the core shrinks, it starts to fuse the helium inside it. This forms other atoms, which also start to fuse, while the core keeps shrinking. This process keeps repeating until iron is formed, which is the first element that takes more energy to fuse then it makes. Since the star has at this point stopped making energy, the fusion stops.
Stage eight: core collapse supernova
When the core of a massive star stops producing energy, the outer layers of the core collapse, which creates a massive explosion called a supernova.
According to Dr. Donald Spector, a Hobart and William Smith Colleges physicist, “However big you think supernovae are, they’re bigger than that.” although this claim is obviously and irreparably paradoxical, it is a good rule of thumb when estimating supernova-related numbers.
Dr. Spector’s rule certainly applies here. The amount of energy produced by a normal-sized supernova is around 1044 joules. To put that into context, that is equivalent to 2.39x1060 megatons of TNT. A supernova can outshine an entire galaxy.
And that is how a star dies.
After Death: Dwarfs, Neutron Stars and Black holes
Not all stars produce supernova. Medium to small stars, such as our own sun, simply shrink to become what is known as a white dwarf. White dwarfs are extremely dense. A white dwarf with the mass of our sun would only be around as big as the Earth. White dwarfs don’t produce fusion; they glow purely from their thermal energy, and last for billions of years.
When larger stars (between 10 and 29 solar masses) collapse as a supernova, they often form an incredibly dense Neutron stars. These stars are even denser and smaller than white dwarfs, with stars around ten kilometers wide weighing twice as much as our sun. These stars are believed to be made almost entirely of neutrons, and there are a few different types of them. Most neutron stars are known as Pulsars, due to their pulsating radio signal. This is caused by a combination of extremely rapid rotation and a beam of radiation jetting out of the magnetic poles. Another kind of neutron star is the Magnetar. These neutron stars have a huge magnetic field, around a thousand times the strength of an ordinary neutron star.
The largest supernovae can form Black Holes. These strange objects are so massive that they collapse into a dimensionless point known as a singularity, which has infinite mass. Almost nothing escapes a black hole’s gravity. Not even light, or electromagnetic radiation. Matter can only escape a black hole in the form of hawking radiation, which is still purely theoretical. There is believed to be a supermassive black hole at the center of our galaxy.
And that is a star’s life.
A star’s life
From birth as a protostar, to the formation of a protoplanetary disk, to the transformation into a red giant, then finally, to death as a supernova. Every stage of a star’s life cycle is awesome. To learn more, see the sources page below.
REFERENCES
Molecular clouds
http://www.sun.org/encyclopedia/molecular-clouds-and-dark-nebulae
Energy from sun
http://archive.boston.com/news/science/articles/2005/09/05/how_much_energy_does_the_sun_produce/
Main sequence stars
https://www.space.com/22437-main-sequence-stars.html
Amount of zeros because I don’t know for some reason
https://www.quora.com/How-many-zeros-are-in-1-million
Main stages of a star’s life
https://sciencing.com/7-main-stages-star-8157330.html
Energy conversions
https://www.unitjuggler.com/convert-energy-from-J-to-MT.html?val=1.0E-44
Dr. Spector
http://people.hws.edu/spector/
Info on white dwarfs
https://en.wikipedia.org/wiki/White_dwarf
Info on neutron stars
https://en.wikipedia.org/wiki/Neutron_star
Info on black holes
https://en.wikipedia.org/wiki/Black_hole
Dr. Spector quote:
What if? By Randall Munroe
Info on star life cycle:
The Illustrated Atlas of the Universe By Mark A. Garlick and Wil Tirion
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