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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