From: Henry Spencer <henry@zoo.toronto.edu>
Newsgroups: sci.space.science
Subject: Re: Detecting a SuperNova
Date: Sun, 10 May 1998 05:28:30 GMT

In article <3552fd6d.0@news.IsraCom.net.il>,
cabot <caboth@isracom.co.il> wrote:
>Is there any way of determining when a star will go nova?
>If there is then with what accuracy?

First, a fine point:  a nova and a supernova are very different things.

A nova is a variable star that produces a spectacular explosive outburst.
It's bright but not immensely so.  It does not destroy the star, which
eventually returns to more or less its previous state.  Some novas have
repeated their outbursts, some more than once; it seems likely that all
novas repeat -- that they are really just highly variable stars -- and we
simply haven't been watching long enough to be sure of this.

Last I heard, there are no obvious signs that a repeating nova is about to
repeat, and nobody has managed to get detailed pre-outburst observations
of a nova that is not a known repeater.  It's thought that they may be
neutron stars in binary systems; the neutron star would gradually
accumulate surface gas captured from the ordinary companion star, and
every now and then the surface accumulation would get hot enough to ignite
and burn in a big nuclear flash.

A supernova is a far larger explosion that destroys the original star.
The explosion is briefly as bright as an entire galaxy.  There are several
types; some leave a neutron star or black hole behind, others destroy the
star utterly and leave no trace.

Nobody has gotten detailed observations of a pre-supernova star.  There
have been no supernovas in the visible parts of our galaxy for several
centuries, although they are observed regularly in other galaxies (most
notably Shelton's supernova in the Large Magellanic Cloud a decade ago).

Supernovas that leave a remnant behind are almost certainly giant stars,
which are thought to inevitably end their lives in such an explosion as
their fuel-depleted core collapses, releasing enough energy to blow off
the outer layers.  The details are not yet entirely understood.  It
surprised people when Shelton's supernova turned out to have been a blue
giant before it exploded -- folks had expected that a pre-supernova star
would be a red giant.

Supernovas which don't leave a remnant are probably white-dwarf stars in
binary systems.  The white dwarf burns its hydrogen to helium and its
helium to carbon, but never gets hot enough to burn carbon much; left to
itself it would cool into quiet obscurity.  But gas captured from the
ordinary companion star accumulates on the surface, raising the white
dwarf's mass until the temperature and pressure at its core are high
enough to start carbon burning.  And when some of it starts to burn, that
raises the temperature and pressure still more, and there is no stopping
it -- a whole star full of fusion fuel burns in one stupendous flash.

We know *generally* what to look for in all these cases, assuming we
haven't overlooked something important.  But it's possible that none of
them give any clear warning when the explosion is imminent.  All of them
probably teeter on the brink of exploding for a long time before they
actually blow.  It may well be that a supernova could explode at any time
in a million-year range, depending on precise details of conditions deep
within the star.
--
Being the last man on the Moon                  |     Henry Spencer
is a very dubious honor. -- Gene Cernan         | henry@zoo.toronto.edu

From: Henry Spencer <henry@zoo.toronto.edu>
Newsgroups: sci.space.science
Subject: Re: Detecting a SuperNova
Date: Mon, 11 May 1998 00:05:43 GMT

I wrote:
>...It may well be that a supernova could explode at any time
>in a million-year range, depending on precise details of conditions deep
>within the star.

Also, a P.S. to this:  if you want to worry about violent astronomical
events, worry not about novas and supernovas but about gamma-ray bursts.
It is now pretty much settled that they are cosmologically distant, mostly
in the far reaches of the universe, and they are nevertheless prominent in
the gamma-ray sky.  (They were first noticed because they set off
gamma-ray detectors in satellites designed to detect violations of the
Test Ban Treaty, i.e. near-Earth nuclear explosions.)  They are colossally
energetic events, making supernovas look insignificant, and a lot of their
energy comes out as highly penetrating radiation.  Having one of those
happen anywhere nearby would be extremely bad news, and we have no idea
what their pre-explosion state looks like.  (The current hot theory is
colliding neutron stars, but the latest observations suggest that the
bursters are too energetic (!!) for that model to work well.)
--
Being the last man on the Moon                  |     Henry Spencer
is a very dubious honor. -- Gene Cernan         | henry@zoo.toronto.edu