The Smallest Stars

Hey ... we have finally arrived at perhaps the most interesting page on stars. This page will deal with the remnant of the supernova explosion ... not the exploded cloud of dust and gas that provided some very beautiful pictures for telescopes ... but with the collapsing core inside the maelstrom. There are two possible outcomes for the stellar core of a massive Red Supergiant, Blue Supergiant, or Red Hypergiant, all of which depend on the mass of the core, and the presence or absence of a companion star. There is a very exciting outcome for a White Dwarf that somehow is accreting material from a companion star ... the Type Ia Supernova that is one of the most incredible energetic event known. In each of these scenarios, the mass of the core remnant determines the fate, and it is impossible to reproduce any of these scenarios in a lab. We have learned about the outcome possibilities by studying the effects of the abnormal, highly evolved core remnants on companion stars.

As we learned from a pervious chapter, the outward pressure from degenerate electrons keeps a stellar core from collapsing under the inward pressure of gravity. It is as if all of those negatively charged, free-swimming electrons can be compressed into a ball only to a point where the negative charges repel each other with such force that further collapse is not possible. The star formed under such circumstances is called a White Dwarf, and this object is typically the size of the Earth. Indian physicist Subramanyan Chandrasekhar theorized that such a White Dwarf's degenerate electron pressure can prevent further gravitational collapse up to a particular mass point. He theorized that if a White Dwarf has a mass greater than 1.4 solar masses, gravity will overcome the electron pressure and crush the negatively charged particles into the positively charged protons, creating neutrally charged neutrons. With no degenerate electron pressure remaining, the core suddenly collapses until those neutrons cannot be packed into a ball any smaller than a small city, perhaps with a diameter of 20-30 km. Imagine shoving 1.5 Sun's material into an object the size of Minneapolis! As it is impossible to squeeze a bag of marbles any smaller than those marbles will permit, this star of pure neutrons cannot be crushed any smaller than the proximity of the touching neutrons will permit. The gravity at the surface is tremendously strong, but the neutrons are prevented from collapsing any further by this "degenerate neutron" outward pressure. Scientists have theorized that if the neutron star has a mass greater than 3 solar masses, gravity will defeat even degenerate neutron pressure and crush the star to a point of zero volume. With no physical volume, the star disappears from sight, leaving behind only the gravitational evidence of its presence. This object has messed up so greatly with the formula for escape velocity, owing to its zero radius, that nothing can escape ... not even light. This object has essentially disappeared from our vision and left behind a "Black Hole."

White Dwarfs, Neutron Stars, and Black Holes are the subjects of this final chapter on stars. I hope you can enjoy a look into these smallest of the stars and the most exotic objects in the Universe.

Please proceed first White Dwarfs, then to Neutron Stars, and then on to Black Holes. There is also a really interesting lab that allows you to look more closely at Supernova 1987A and determine what type it is.

I make no apologies for the depth of this material, and I recognize that much of it may be well above many of the heads of my readers, as well as my own. However, if you complete this course to the standard level, and later have time to dig into this stuff more deeply, then here it will reside ... waiting for you, or just go to the Syllabus .


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