Stellar Remnants
If I were to immediately define “stellar remnants,” I would be beginning at the end. Instead, let’s begin at the beginning: the life cycle of a star, or stellar evolution.
After formation in a nebula, stars begin to live via nuclear fusion in their core. But eventually, the atoms needed for nuclear fusion -- hydrogen -- run out, and the death of the star begins. This death involves the fusion of new elements, resulting in either a large supernova explosion or a planetary nebula. After either comes our subject: compact stars, or stellar remnants.
Stellar remnants are the final stage of the star life cycle and are made up of the final material leftover at the death of a star. This material can take the form of a white dwarf, a neutron star, or a black hole, based upon its mass (see the Chandrasekhar limit).
A white dwarf, or degenerate dwarf, shown in the above image, is the result of the death of an average sized star. That is, someday our own Sun will become a white dwarf. It is very bright and very hot (as seen above), and composed mostly of helium, carbon, and oxygen. White dwarfs are also called degenerate dwarfs because they are made up of degenerate matter. Degenerate matter exists due to the Pauli Exclusion Principle, which states that electrons cannot occupy the same space. Therefore, electrons are forced to make up the energy state of an atom, and end up filling all energy levels of the atom, making it degenerate. This degeneracy halts the collapse of the white dwarf and its ability to perform nuclear fusion. Afterwards, white dwarfs continue to contract due to the force of gravity, which is why it is so hot! However, this heat is released by the white dwarf shining over time, and the white dwarf is not hot enough to replace that energy with nuclear reactions. Thus over time white dwarfs cool down and become black dwarfs, but this can take trillions of years -- longer than the current age of the Universe -- and so none exist today. White dwarfs are also capable of becoming supernovae if they consume a nearby star, often one that's part of a binary system. This can happen due to the white dwarf's extreme gravitational pull due to its large density -- a density so large that only a teaspoon of white dwarf matter would weigh 5.5 tons -- as much as an elephant here on Earth~
Credit: http://www.nbcnews.com/id/20360861/ns/technology_and_science-space/t/astronomers-spot-nearest-neutron-star/
The above image displays an artist's rendition of a neutron star -- actual images neutron stars are difficult to capture due to their small size. A neutron star occurs when the mass of the dying star exceeds the Chandrasekhar limit. Electron degeneracy can’t support the star’s mass, so collapses both protons and electrons into neutrons (hence the name!). Neutron stars are special in a couple of ways, and their existence actually affects human beings directly. Due to their high spin rate and electromagnetic radiation, the pulsing light emission of neutron stars can be seen from earth. The first observation of this was in 1967 by a woman named Jocelyn Bell. Though she suspected something complex was happening, her thesis advisor didn’t recognize the pulses (now known as Pulsars) and mistook the light as attempted contact from alien civilization. The pulses of light are actually electromagnetic radiation. Some neutron stars are the most magnetic objects in the Universe -- these especially magnetic stars are called Magnetars. Magnetars are so strong because the star's collapse and subsequent compression of magnetic field lines trap radiation in all but breaks at the magnetic poles. Like Earth, these magnetic poles are not directly aligned with the physical North and South poles.
Pictured above is perhaps the most (subjectively) mythic of stellar systems -- the black hole. Black holes are the results of the death of the largest stars discussed in this article, and as a result have an immense density. They can be difficult to discover because black holes can’t be seen. However, we are able to see the accretion disks surrounding black holes. These are made up of a spiral of hot gas from a nearby star being pulled towards the black hole by its gravity, and can be seen at X-ray wavelengths. The binary star system of the star and the black hole is called an X-ray binary. The incredible density of the black hole contributes to such a large gravitational pull that the speed needed to escape is faster than the speed of light -- thus, not even light can escape a black hole. This impossibility of escape means that very little exploration of black holes can be done, but what is observed from the outside is strange enough to increase curiosity. For example, around the Event Horizon of a black hole time appears to slow. If you were to sit at the event horizon of a black hole, you would be able to use it as a time machine to see back through the entire evolution of the universe. You could even witness the future! Black holes are also infinitely dense, and because of this, the surrounding spacetime continuum is flexible. So flexible, in fact, that black holes can bend light from distant objects into a circle, which is called an Einstein ring.
TL;DR -- Even post mortem, stars continue to do some pretty fascinating things. We still have a lot to explore!
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