Stars die, and as they do, they leave behind remnants, usually compact objects of some sort, like a white dwarf, or a neutron star. What kind of remnant it is depends on the initial mass of the star, and how it met its' death.
Theoretically, this is what a low mass star will become when it has stopped fusing hydrogen into helium. Brown dwarfs will also become black dwarfs - relatively small objects that do not emit light. It may take trillions of years before a sub-/stellar object cools down so much that it doesn't emit light anymore. Hence, none are known to exist today: The universe is not old enough.
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White dwarfs are the end result of a star approximately 8 MSun. It is a small, earth-sized object, and a mass of maximum 1.44 MSun, which is called the Chandrasekhar Limit, after the indian astronomer Subrahmanyan Chandrasekhar. The white dwarf is an incredibly dense object
with densities of about 1 000 kg per cubic-centimeter, or 109Kg/m3. Their surface temperature may initially be as high as 150 000 degrees Kelvin, but they will cool down and eventually become black dwarfs.
White dwarfs consist mostly of helium, carbon and oxygen, depending on its' mass and the matter is in the state of plasma: degenerate matter, which is a strange kind of matter. In white dwarfs electrons are forced to fill up all energy states available in an atom, and since it is impossible for two fermion particles (the electron is one)
to occupy the same energy state electrons are eventually forced to occupy higher energy state levels until the atom is full of electrons. This principle is known as the Pauli Exclusion Principle. If all energy levels are full, then the matter is said to have become degenerate. Degenerate matter has unusual properties:
The more massive a degenerate white dwarf is, the smaller it becomes. If you add heat to these compact objects, then it does not expand, which is the opposite of normal gas! White dwarfs are so luminous because they have trapped heat since earlier stages in their evolution.
If the white dwarf is located on a close binary star system, where it is able to rip off gas from its' companion, then the system is called a Cataclysmic Variable. The matter (mostly hydrogen) falling down on the white dwarf may become so hot that it ignites a fusion reaction on the surface of the white dwarf and turns the hydrogen into helium, releasing tremendous energy.
This release of energy (an explosion) is called a nova (nova from latin, meaning "new"). Astronomers in earlier centuries thought of these events as a new star on the night sky was born, since the explosion made the white dwarf much brighter and may have been unable to be viewed before the outburst. The white dwarf will survive this explosion, and such outburst may continue until the donor
star is out of gas.
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If a stellar remnant has more mass than the Chandrasekhar limit, then the electron degeneracy cannot support the mass of the object and so it collapses, creating neutrons out of protons and electrons. Now, the object is no longer supported by electron degeneracy, but rather neutron degeneracy. In the process the object acquires a radius of about 20 km and
a density in the order of 1017 kg/m3. In other words, an incredibly dense object! This object is known as a neutron star (mass between 1.44 and 3 solar masses), and is created by stars that have a larger initial mass than 8 MSun. Beyond 3 solar masses the neutron degeneracy fails and the neutron star collapses, into a black hole.
The first observation of a neutron star was made by Jocelyn Bell, in 1967. This neutron star was emitting light in very regular pulses, so she first thought that an alien civilization was trying to contact her. Similar sources were greater in other parts of the sky, so the alien theory was ruled out.
Besides, some sources emitted pulses several thousand times a second. Objects that emit pulsing signals with a duration of 0.001 seconds can't have a diameter any greater than 0.001 light-secs, which is 300 km. Such a small size ruled out white dwarfs, pointing towards the hypothesized neutron star as the explanation for these Pulsars, as they came to be known.
The reason why we see pulses is because a beam of electromagnetic radiation from one of the star's electromagnetic poles hits us each time the star spins around its' axis. The magnetic and rotational axes of a pulsar are misaligned for a similar reason that "true north" and "magnetic north" are different on Earth.
The high spin rate (sometimes thousands of revolutions per second!) of a neutron star is obtained from the original starís spin as a result of angular momentum conservation.
A special kind of a neutron star is a Magnetar, which is an extremely powerful magnetic star. In fact, they are the most magnetic objects known in the universe. They do not last long as magnetars, maybe only 30 000 years, after that the magnetic field fades. However, while a magnetar is active, it can produce magnetic fields with the intensity of 1015 Tesla. The strongest magnet produced on Earth is only 40 Tesla.
Strange things go on around magnetars..
Astronomers are only aware of 15 or so magnetars' existence.
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As the name indicates, these objects are black, because they have such strong gravity that the escape velocity from a black hole exceeds the speed of light. Therefore, nothing, not even light can escape it. Proving their existence was difficult at first, but now astronomers believe they have enough evidence tho be certain that they exist. One way to discover a black hole is if it is located in a close binary star system.
Such is the case of Cygnus X-1, which was the first likely candidate for a black hole that was discovered. Cygnus X-1 is a migh mass binary starsystem with a blue star, and a black hole containing about 8.7 solar masses, which are compressed within an object with a 26 km radius. These measurements alone rule out a neutron star as the companion. The system holds a separation of 30 million kilometres away from each other, and
matter falls from the donor star down to the black hole, in an accretion disc, which is heated up to millions of degrees. The accretion disc is so warm it radiates x-rays!
Strange things happen around black holes. As one gets closer and closer to a black hole's Event Horizon, time appears to slow down to an outside observer. The event horizon is a region, usually spherical (if the black hole isn't rotating), which marks the outer boundary of a black hole. Inside, the gravity is so strong that nothing can escape.
Mathematically, black holes are considered as Singularities, which refer to objects with infinite density, infinite mass and infinite curvature of spacetime.
In order for an object to become a black hole, it must first collapse to a point where it reaches The Schwarzchild Radius. This radius depends solely on the mass of the object. For the sun to become a black hole, the entire mass of it must be compressed within a spherical object with a radius of no more than 3 kilometres! For the earth, this radius is only 0.8 centimeters!
The minimum mass for a star to leave a black hole behind is unknown, but some astronomers believe it is around 25 solar masses. The remnant will contain atleast 3 solar masses, but the upper limit to the mass of black holes is unknown, perhaps it's infinite. Super massive black holes, which are located at the center of galaxies have been found to contain up to 3 billion solar masses!
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Neutron Stars - Illustration (P)
Above: A Magnetar, with a disc of gas surrounding it.
This illustration is available upon request, as a print (5000x3000 pixels, 300 dpi), and as a .PSD file, customizable.