Life Cycle of a Star
Deep in the cosmic horizons of our universe, a dusty cloud of space ingredients is getting a bit sick & tired of being nought but a dusty cloud. “What if..” the dusty cloud thought “, instead of being a gnarly musk of dust, I become a glowing orb of plasma?” and so it came to be that a star was born.
Those great big balls of plasma exude light which travels at roughly three hundred kilometres per second across time and space to slam into your eyeballs so that you can see them. Undoubtedly, the sun plays a major role in life as we know it and has been a source of inspiration and fear across the vast civilisations of history. The Aztecs sacrificed people daily to ensure the sun would come up tomorrow; the greeks decided that a titan called Helios was the only way to describe the sun's grandeur. But what is that yellow orb draped in blue, and how did it come to be?
Brown Dwarfs & Baby Stars
We shall begin at the most logical place; the beginning. When the universe first came into being for a long while, it was just a hot dense soupy cloud of things and stuff, specifically hydrogen and helium (the lightest elements). Over time, gravity takes its toll, causing the cloud to collapse in on itself under its own weight; however, the pressure of the atoms stops this, and eventually, a sphere is formed where weight and pressure are equal (hydrostatic equilibrium).
As you keep adding mass to the sphere, it slowly stops getting bigger and starts getting denser instead. An example of this is the difference between Jupiter and Saturn; Jupiter has three times the mass of Saturn, but it's only 15% bigger because its density is double that of Saturn. Essential density is a measure of how many atoms are in a given area; the more atoms squeezed in, the greater the mass and the greater the density. The inside of a deodorant can is far denser and has more mass than a jar of equal size and volume that contains air. Add enough volume, and you eventually get a brown dwarf, an intermediary stage between a gas giant (planet like Jupiter) and a star. On average brown dwarves are between 0.013-0.075 of the Sun's mass, which may sound very small indeed; however, that's between 4327 and 23964 times as massive as the Earth.
If a brown dwarf doesn’t produce enough heat, it’ll stay all sad and brown; however, if it becomes hot enough, nuclear fusion will start, and the brown dwarf becomes a main-sequence star.
Main-Sequence Stars
The sun is currently in its main-sequence phase and has been chilling (not literally) this way for about 4.6 billion years. How long a star is in this state and how long a star lives, in general, depends on the star's size; a star bigger than the sun will burn brighter but also quicker. This allows astronomers to measure the size of a star based on its luminosity (brightness). Our sun will spend another 5 billion years in this state, converting hydrogen into helium in its core until all the hydrogen is used up through this nuclear fusion. For this to take place, the Sun's core is a brisk 15 million degrees Celsius and sends waves of radiation and light at us in the form of solar winds.
In this state, and the following Red Giant phase, stars are essentially the universe's factories; they progressively form heavier and heavier elements in their bellies, with the heaviest element produced by stars being iron. Stars heavier than 1.5 times the sun can fuse carbon, nitrogen and oxygen.
In 5 billion years, when the sun is senile and old, it’ll get really fat. This takes place because the sun will have used up all available hydrogen and will convert to using helium as fuel for the next billion years as a Red Giant.
Red Giants and Supergiants
Red giants are far dimmer but much larger stars than main-sequence stars. The largest known star is a red hypergiant called UY Scuti, with a radius of 1188300000km (1.1883 billion) and a volume 5 billion times greater than the sun.
The size and nature of a star changes because the nuclear engine within essentially restarts. Once the hydrogen has run out, nuclear fusion ends, the forces of which were in balance with gravity; now that nuclear fusion is no longer taking place, gravity dominates, and the star begins to implode. However, before the star is defeated, the core gets so hot and dense from the collapse nuclear fusion restarts with helium as the fuel, causing the star to expand far beyond its previous extent. When the sun becomes a red giant, the sun’s growing surface (photosphere) will cause oceans to boil. Life on our pale blue dot will end. Helium fusion causes a build-up of oxygen and carbon; eventually, the star’s outer layers are blown off to form a cosmic entity known as planetary nebulae. The star will now have to make a decision.
Would you rather be a small white chap or explode?
Depending upon if the star is a particularly fat boi or not, it will either peacefully go out or, juxtaposingly, explode. Our Sun will follow the former path once it has expelled its outer layers; only its core shall remain as a very hot, very dense, glowing white orb → A white dwarf. If there were a white dwarf, the Earth's size, it would be 200 times denser than the earth itself. It would also be over 100 000 kelvin or 99726.85 degrees C. That certainly is one toasty guy! Eventually, as the radiation wastes away, the star will go out and fade into obscurity as a black dwarf; such is the fate of our Sun.
Secondly, instead of the outer layers being blown out and a white dwarf peacefully fading away, the star can explode in a supernova event. Our sun will not go supernova as it’s too small. In a binary star system (solar system with two stars), a white dwarf can steal material from its companion until it becomes so massive it explodes as a supernova.
For a solitary solar system, such as our own, the star would have to be much more massive. As this heavy star ages, material from its outer layers flows into the core until the core becomes so heavy it collapses under its own gravity and implodes. When a supernova occurs, it can be brighter than entire galaxies and sends out huge amounts of gamma and x-rays and is around 55,555,555, 537 (55 billion) degrees C. While stars like our sun end their life cycles at the white dwarf stage, more massive stars that go supernova have two new possibilities.
Neutron Stars & Black Holes
First, I will describe what a neutron star is. A neutron star is an incredibly dense object; imagine crushing the sun down to a city's size. One sugar cube worth of neutron star weighs 1 trillion kg. As a supergiant goes supernova, if the core is between 1–3 solar masses (mass of our sun), the star can withstand the collapse to form a neutron star. Many neutron stars do not emit enough radiation to be detected; however, specific neutron stars known as pulsars can be.
A pulsar is a neutron star that pulses with radiation in a regular pattern. This radiation is fired out as jets from the north and south pole due to the incredible strength of the stars magnetic field, as shown below. Due to the need for a magnetic field, neutron stars have to be rotating for this to happen.
Finally, if the star's core is hefty, it will collapse into an infinitely dense point known as a singularity. Here, gravity is so strong that no light can escape forming a black hole. A black hole is so heavy that it literally bends time and space around it. Comparing someone living next to a black hole with someone living on Earth, the person living near the black hole would age slower; this is known as time dilation, which I will now attempt to explain (wish me luck):
Imagine putting a bowling ball on a suspended bit of carpet; where the ball is located, the carpet will droop downwards, causing the fabric to stretch. You have increased the surface area of the carpet by stretching it. An ant on the droop's surface would now take longer to travel across its surface compared to an ant away from the droop. In the analogy, time & space is the carpet, the black hole is the ball, and we are the ant. The black hole resident would take longer to travel through time than the person living on Earth because time is stretched.
Black holes are insane objects to imagine or quantify, but they also come in varying shapes and sizes. For example, in the CERN supercollider, some theories suggest it would be possible to create microscopic black holes and out in the universe galaxies, including the milky way orbit around supermassive black holes. The largest black hole known is Holmberg 15A and is 44 billion times the sun's mass; however, there are larger black holes, but the mass’ for these are only estimates.
While black holes seem perpetual in nature, they can die. Black holes emit a special kind of radiation called Hawking radiation (after Stephen Hawking) and eventually lose their energy and die. However, a black hole with the sun's mass would take 10x⁶⁷ years to die, which is many times older than the age of the universe.
Conclusion
There we go; I have now described the life cycle of a star. Thank you for making it through this lengthy article. Obviously, there is far more complex science involved and many different types and subtypes of stars which I couldn't hope to understand or describe in a single article. Still, I hope this provided a fairly brief and concise overview because I really enjoyed writing it.
If you want to know more, please follow the embedded links to the NASA gallery, which will allow you to explore more photos and descriptions. If I have made errors, grammatically or scientifically, please let me know on Twitter the link for which can be found on my homepage. If you liked this, I have a small catalogue of articles for you to explore at your leisure.
Have a good day & thanks for reading!
