Chandrasekhar Limit --- A white dwarf can't have indefinitely high
mass. As the density increases, the degeneracy pressure saturates. Formally,
the radius of the star could become zero. The mass at which this happens is
the Chandrasekhar Limit, about 1.4 solar masses for a white dwarf of normal
composition.
Upper limit to neutron star mass --- is less well determined, but is probably
about 2 -3 solar masses.
Mass-Transfer Binaries --- Close binary stars can evolve so that
their outer envelopes affect one another. Matter can transfer from the more
quickly evolving star of the two when it becomes a red giant and "fills its
Roche lobe" (expands to where it feels the influence of the second star).
Matter can be transferred to the companion. If the companion in very compact,
this transferred material heats up to x-ray emitting temperatures as it is
tranferred because of the extremely strong gravitational field. X-ray
telescopes in the 70s and 80s discovered many variable sources now detected in
the optical and thought to be mass-transfer binaries.
Novae --- are thought to be a result of mass transfer from an ordinary star
onto a white dwarf binary companion. If the material accreting onto the white
dwarf get hot enough it can initiate a burst of nuclear fusion, causing the
white dwarf to brighten. Novae are an example of "cataclysmic variable"
stars.
Type I Supernovae --- (SNeI) are thought to be the sudden collapse of a
white dwarf accreting in a mass-transfer binary when the mass of the
W.D. exceeds the Chandrasekhar limit. Most of the five historical SNe in the
Galaxy were probably Type~I rather than Type~II.
Supernova light curves --- are a plot of the luminosity of the supernova with
time. Type I supernova light curves are very similar to each other,
reflecting the fact that all type I supernovae are thought to come from a very
similar process --- the collapse of a white dwarf that just exceeds the
Chandrasekhar limit. In contrast, the progenitors of type II supernovae are
massive stars that differ in their properties (mass, chemical composition,
rotation rate, ...) and this heterogeneity is reflected in their light curves.
Type I supernovae are the most recent example of a "standard candle" --- a
bright source of known luminosity that can be used to measure the overall
geometry of the Universe.
Pulsars and supernova remnants --- Supernovae leave behind evidence of the
explosion in the form of a "supernova remnant" (SNR), (yet another) nebula in the
interstellar medium. Some examples int he Galaxy are the Crab Nebula, Cygnus
A, Tycho's SNR (the remnant left behind by the SN that Tycho Brahe observed).
A SNR is a circular nebula, the source of the light being in this case the
effects of the supernova blast on the surrounding gas. SNR can be observed to
be expanding, and extrapolating back produces plausible association with
observed supernova events. There are about 150 known SNR in our galaxy.
Three of these have pulsars in the middle of them, so in at least a few cases
there is a direct link between the pulsar and the type of event thought to
give rise to neutron stars. Why don't all SNR have associated pulsars? Maybe
not all type II supernovae leave behind neutron stars; we have to be in the
"beam" of a pulsar before we see it; SNR expand and fade away; pulsars slow
down and fade away.
Black Holes --- The upper limit for neutron stars is about 3 solar masses. A
stellar core which has 'burned' all the way up to iron and has a mass of more
than about 3 solar masses is thought to collapse to become a black hole, an
infinitely small object that 'curves' space up around it so that nothing --
not even light --- can excape from a region around the so-called
'singularity'. The relevant `size' of a BH is the Shwarzschild radius, the
distance from the hole from within which nothing, not even light, emerges
because of the extremely strong gravitational field (event horizon). Or, the
horizon is that surface from which the escape speed is the speed of light.
This radius, R, is = 2G(mass of black hole) / (square of speed of light) [the
same formula as describes excape speed, although a black hole is not described
by Newtonian gravity]. If the Sun were compressed to become a BH it would
have a Shwarzschild radius of about 3 km. Because black holes have such
strong gravitational fields, they must be described by General Relativity, not
Newtonian mechanics.
Detecting black holes --- The best guess is that solar-mass type black holes
can be found as members of certain x-ray binaries.
These are single-lined
spectroscopic binaries in which the mass of the unseen compact companions can
be inferred to be more than the upper limit of 3 solar masses that would make
such a dark remnant a neutron star. Cygnus X--1 is thought to have a mass of
about 7 solar masses, and therefore could be a Black Hole.
Star formation and the Milky Way galaxy (preview) --- We see plenty of
examples of high-mass main sequence stars relatively close to the Sun, and yet
we know these stars have short MS lifetimes. In other words, star formation
must be an ongoing process, at least in our star system, i.e. our "Galaxy". I
showed a very short movie sketch that illustrated some of the important ideas
that we think relate star death (supernovae), star birth, and the most
prominent feature of a galaxy like ours, its "spiral arms". Briefly, star
formation can happen when something triggers a cloud of gas and dust to
collapse and ultimately start fusion reactions and become a star. That
"something" might be a nearby supernova explosion. This process happens in
the "spiral arms" of a "spiral" galaxy, and luminous new stars help to outline
the spiral arms. The key feature in this scenario is the presence of gas and
dust between the stars, the so-called "interstellar medium".