Supernova 1987A (2) ...
The unusually small size of the SN 1987A progenitor is illustrated in the figure below,
in comparison with the calculated size of a standard Type II-P supernova progenitor star.
Whereas we normally expect a Type II supernova to result from a red supergiant with a
radius comparable to 1 AU or larger, we see that when SN 1987A blew up, the initial star
was only about 20% of that size.
The small size of the 1987A progenitor
could have influenced the light curve.
Because the star was much more compact than a typical red supergiant at core collapse,
more explosion energy had to be used to expel the envelope than normal
(just as it takes more energy to climb to the top of a mountain if you start near the
middle rather than near the top). This, in
turn, reduced the energy left to power the emission of light after the explosion.
The images shown below indicate that the star
had indeed
ejected mass in an earlier stage, since we are now seeing the ejected matter being heated
by ultraviolet radiation
from the supernova.
The Plateau in SN II-P Light Curves
It is believed that the late peak in the SN 1987A light curve is in fact closely related
to the plateau
normally seen in the light curve for a Type II-P supernova. The connection between the
two is indicated by the dashed vertical gray line in the light curve figure
shown above.
Thus, prevailing opinion is that Supernova 1987A was a relatively normal Type II
supernova in that it was generated by the core collapse of a massive star.
However, the initial star was smaller than usual for such a supernova at
the time of explosion. This altered the initial part of the light curve substantially
and reduced the total energy available for producing light, which made the overall light
curve weaker than normal for a supernova.
Detection of the Neutrinos
Supernova 1987A was the first "nearby"
supernova since the invention of the telescope more than three centuries ago.
In addition to the
light show, 19 neutrinos
(the detectors observed electron antineutrinos, to be more precise)
were detected in a burst about 12 seconds in duration.
The burst of neutrinos
preceded the first sighting of the supernova's light by 3 hours, as
expected from core-collapse
supernova theory. The neutrinos were emitted almost promptly when the core collapsed,
but the shock wave took some hours to reach the surface and produce the first increase in
light observed for the supernova.
These observations are rather conclusive evidence for the
correctness of the core collapse mechanism, since it would be difficult to produce
the observed neutrino burst by any other means than the gravitational collapse of a massive star core.
It is estimated that
for an instant in 1987 on the Earth
the neutrino flux of SN 1987A was as large as the
visible-light flux of the entire universe. Unknown to you, in February of 1987,
tens of trillions
of neutrinos from this supernova passed through your body in a few seconds time.
Since neutrinos
pass so easily through matter at normal densities,
it is unlikely that even a single one of these ghostly particles
interacted with you, though.
The Mysterious Rings
The adjacent figure
is a 1994
image of the region surrounding
SN 1987A.
The supernova is in the center. The two bright stars just happen to lie in the
field of view and are not associated with the supernova. The bright yellow
ring around the center
is thought to be gas and dust ejected by the star well before it
became a supernova. It is glowing now because it is
being heated by the light from the supernova (the expanding shell
of the explosion that eventually will produce the supernova remnant
is still too small to be seen in this image; we shall see it in a later image).
The two large red
rings are not completely understood.
It is rather
certain that the rings result from something that the star did before
it became a supernova. They are probably associated in some way
with matter ejected by strong stellar winds from
the pre-supernova star that is being illuminated now by the supernova.
