THE BEGINNING OF THE END
The end of a star’s formation process is marked by the event of solar winds blowing away the excess gas, debris, and other byproducts of the star’s formation. From there, the star will spend most of its life on the main sequence.
The end of a star’s life begins with its helium core compressing. The more dense it becomes, the fewer places there are for electrons inside the star to go. The shrinking space builds up a pressure inside of the star known as “degenerate pressure.” Once the degenerate pressure is high enough, helium begins to fuse. Helium fusion occurs at around 100MK.
When helium fuses, a tremendous amount of energy is produced in a matter of a few seconds. This event is known as a “helium flash.” After the helium flash, thermal pressure quickly dominates over degenerate pressure.
At this point, the star expands into a red giant and officially leaves the main sequence. It travels to the top of the diagram for the helium flash, and then drops back down to the region known as the “horizontal branch,” where the star experiences an internal build-up of carbon.
Up until this point, every star behaves similarly during its death. Everything that happens after this point in the process is pre-determined by the mass of a star’s core.
LOW-MASS STAR DEATH
A low-mass star will expand once more into a red giant, but only once and never again. At this point, a low-mass star does not have enough helium, carbon, or pressure required to repeat the fusion process.
There may not be enough pressure to fusee helium, but there is enough pressure called “radiation pressure” which blows the outer layers of the star right off of its core. The outer layers of gas and other debris scattered around the star are collectively known as a planetary nebula, because the materials within this cloud may collide with each other and slowly build up until they form planets. The planetary nebula is subject to spherical symmetry.
The core left behind is very small and very hot, and mostly composed of leftover carbon. This is known as a “white dwarf.” The white dwarf will spend the rest of its days as a dormant lump of carbon until it eventually deteriorates into nothing.
If there is a nearby binary companion star, the white dwarf’s gravitational attraction will begin to pull material away from the companion star in a process known as “accretion.” Once the white dwarf accretes enough material from the companion star, one of two possibilities remain.
In one possibility, a controlled fusion reaction known as a “nova,” the white dwarf will blow off the outer layers of the accreted material. This reaction leaves the white dwarf completely unchanged and unharmed. Another possibility is an unhinged, uncontrolled fusion reaction known as a “type 1a supernova,” in which the material and the white dwarf explode. This completely destroys the white dwarf and leaves no remnants of it behind whatsoever.
HIGH-MASS STAR DEATH
In addition to the proton-proton cycle, high-mass stars also undergo several other fusion reactions, including the CNO cycle and helium capture. This helps to create energy more quickly, and creates helium and carbon in other, more efficient ways. Other materials produced include O, Ne, Mg, Si, S, and Fe. Nothing heavier than Fe is produced inside of the star, but heavier elements will be seen later in the star’s death. Every element found on the periodic table is produced either within a star or as a result of a star’s death.
Iron begins to build up within the star’s core, growing more and more dense as more layers are accumulated. Eventually, the radiation pressure loses to the force of gravity. At this point, the star is massive.
Once gravity overcomes radiation pressure, fusion stops, and all materials are pulled into the center of the star very quickly. The materials all collide with the core head-on at great speed and pressure, causing an explosion. This explosion results in a type II supernova, a supernova in the sky. Possibly even a champagne supernova in the sky…
Within this explosion and the creation of a supernova, we find all the elements heavier than iron. The remnants of the supernova can last anywhere between days to millennia. Usually, the very small and very hot core of the star is left behind.
At this point, the supernova becomes either a neutron star or a black hole, depending on the mass of the star’s core. The smaller of the high-mass stars become neutron stars, while the larger ones become black holes.
Neutron Star Formation: The supernova explosion sometimes leads to the implosion of the star’s core. The supernova remnants expand infinitely into space. The core post-implosion is composed of mostly/all neutrons, and this is known as a neutron star. Not much is known about neutron stars because they are difficult to study. Scientists are unsure of what materials lie in the very center of a neutron star’s core. The neutron star is left in a state known as “ground zero.”
Black Hole Formation: A type II supernova explosion may also result in the creation of a black hole. This occurs when gravity overcomes degenerate pressure, and material is sucked towards the center of the explosion so rapidly that a black hole results. Black holes are extremely difficult to study because light cannot pass through them, so we cannot see them.
If a binary companion star is near a black hole, the black hole’s gravity will suck material away from the companion star in a process known as accretion. The accreted material then rotates around the black hole, increasing its overall temperature. The rapidly-moving material emits large amounts of x-rays, which in turn can be studied by scientists and be used to learn more about the mystery of black holes.
STAR MASS GUIDE
The mass of a star’s core is compared to the mass of our own sun’s core in order to determine its ultimate fate. If a core has a mass less than 1.4 of our sun’s cores, it will end up a white dwarf. If a core has a mass greater than 1.4 but less than 3 of our sun’s cores, it will end up a neutron star. If a core’s mass is greater than that of 3 of our sun’s cores, gravity will overcome degenerate pressure and it will end up as a black hole.
CONCLUSION – A VERY UNIQUE WHITE DWARF
An article I found on Cosmos Magazine’s website titled “An old star learns new tricks” describes the recent discovery of a very unique white dwarf star.
Dwarf stars are typically thought to do nothing once they become white dwarfs because they no longer have enough pressure or materials to engage in any more fusion cycles. As described above, they typically either explode into nothing or waste away into nothing.
However, the white dwarf discovered in the article refuses to go down without a fight. Apparently, the dormant star is no longer dormant, occasionally spitting out light beams and even rotating rapidly, oddly enough behaving quite similarly to a neutron star. Scientists cannot figure out why or how the white dwarf is able to do any of this.
This article connects to what we learned in class because it is an example of a white dwarf doing exactly the opposite of what we learned in class. This is important because it is a great reminder that no matter how much we think we know about space, there are always exceptions and there is always something new to be learned, even when we think we have a concept completely nailed-down.
REFLECTION: I really liked the article because I thought that the phenomenon was bizarre and intriguing. As I stated above, it is a reminder about the infinite amount of things we do not yet know about space. It seems especially absurd that the white dwarf can randomly shoot out beams of light for no apparent reason, and I really wish I knew why and how. Maybe there is something in all white dwarfs besides carbon, or maybe just this specific one is special and contains something other than carbon. Either way, it is quite fascinating.