Timeline for Supernova 1987A
Feb 19, 1998
From a posting to sci.astro.amateur
Okay, here is a brief synopsis of what we think is the history of
SN87A, starting some 10,000's of years before it blew:
For a long time, it was a simple main sequence (but very
massive and hot) star.
- It became a red supergiant (RSG), and started to blow a
slow, dense wind. This wind was not spherical,
but had some degree of axial symmetry
(in other words, it may have not been exactly
spherical, but squashed like an oblate spheroid). This was
most likely *not* caused by simple rotation of the star,
since RSG's rotate slowly. Perhaps it had a companion, like
a close binary, or (my pet theory) a closeby superjovian
which spun up the star when the RSG expanded around it.
Anyway, this wind was more dense along the equator,
and less towards the poles. This is important later.
- Sometime later the star turned into a blue supergiant (BSG).
The wind became much faster and less dense. This wind catches
up with the older RSG wind. Since the old wind is less dense in
the polar direction, the BSG wind moves faster that way.
It is slower in the equatorial direction, so you get a
"bipolar outflow"; a funny shape like an hourglass. Two lobes
form up and down, and it gets pinched in the middle. The middle
pinch forms the inner ring we see. The gas gets compressed
by the shock of the BSG slamming into it, so the hourglass shape
has some finite thickness. It is still pretty thin, compared
to its overall size. Its length is about 2 parsecs.
- Think about each lobe now. It is very roughly spherical, but squashed
a bit. Somehow (the exact mechanism is unknown right now) each
lobe gets a slight overdense region around *its* middle. These
form the so-called "outer rings". The stage is now set, but note that
this is still some 10,000 years before the star explodes.
- 1987: KABOOM! Saduleak -69 202 transforms itself into SN1987A.
10^57 or so UV photons scream out from the inferno. Some small fraction
hit the ring system, causing it to glow. The gas in the
hourglass is not terribly dense, nor is it thick, so it glows
feebly. The rings, however, are denser, and the glow is more
pronounced. 50,000 parsecs away, we see three rings glowing
with almost no trace of the hourglass nebula itself.
- 1987-1990: For a short period of time (about two years), light travel
time effects complicate matters here on Earth. Those effects are
complicated and it would be difficult to explain them here. Luckily
for this discussion they are not too important.
- Present: The rings fade as the gas recombines.
This fading has helped us quite
a bit in finding the temperature, density and even the structure of
- Now + a short time, maybe three to five years: The expanding debris from
the explosion, moving outwards at perhaps 10% the speed of light,
slam into the inner ring. The ring starts to glow as the debris
heats it up. These fireworks may be bright enough to light up the
the outer rings as well. Some hundreds of years later the debris
hits the outer rings as well.
[Note: this part is now superceded
by the WFPC2 and STIS observations!]
- The future: Eventually, we get something that may look quite
a bit like the Crab Nebula. There is some evidence that the Crab
has a ring of emission in the middle, which you might expect to be the
leftover ashes of the inner ring as it shaped the expanding debris.
Incidentally, we are talking hundreds, or even a thousand, years
in the future. Don't hold your breath!
* * * * * The Bad Astronomer * * * *
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