Supernovae

Supernovae 1987A

Astronomers see numerous supernovae every year, but these are usually in very distant galaxies.  Supernovae are rare, occurring in any one galaxy once every fifty years or so.  This means that to see large numbers of supernovae, one must search numerous galaxies every day, which means looking far out into space.  On occasion, however, a supernova occurs in a nearby galaxy, providing astronomers with a close-up view of an exploding star.

The most important of the nearby supernovae is a supernova seen in February of 1987, named SN 1987A.^[1]  It occurred in the Large Magellanic Cloud, which is 50 kpc away from Earth, so it occurred only (!) 163,000 years ago.  No other supernova observed since the start of the space age has occurred closer to Earth.  SN 1987A was observed with ground-based and space-based instruments, as well as with neutrino detectors buried deep under the Earth.  It occurred in a highly-visible region of the Southern Hemisphere sky, unobscured by dust.  It's source was a star that had been studied before the supernova occurred, with the final observation of the star occurring just hours before the explosion.  This supernova proved the theory that the core-collapse of a massive star produces a supernova, but it also showed that, contrary expectation, not all stars that explode in a supernova are red supergiants?sometimes blue supergiants also explode.

The blue supergiant star Sk -69 202 (for star #202 in the -69 declination band of the Sanduleak catalog),^[2] which is a type B3 I star, created SN 1987A.  Like all blue supergiants, it was extremely luminous, with an absolute visual magnitude of -6.3, but it was too faint to see with the unaided eye, having at 50 kpc distance an apparent visual magnitude of 12.2, which is at the limit of the largest portable telescopes.  Because of its high luminosity, it was regularly observed, with the last observation occurring about 5 hours before a neutrino burst released by the supernova arrived at Earth.  Three more observations were made in the following 6 hours.  Subsequent observations, made less than 24 hours after the neutrino burst, finally alerted the astronomical community that a supernova had occurred.  The blue supergiant brightening from 12th magnitude to 6th magnitude, a factor of 250 increase in power radiated as visible light, in the first-three hours after the neutrino burst.  This brightening accounts for most of the brightening of the supernova.  Once the supernova faded, and the supernova shell expanded sufficiently to become transparent, astronomers found that Sk -69 202 no longer exists.

SN 1987A is classified as an unusual type II supernova.  It is type II because it has hydrogen lines in its spectrum.  It is unusual because the doppler shift of those lines suggests an expansion of around one-tenth the speed of light (twice the expansion speed of a typical type II supernova) and because it is much less luminous than a typical type II supernova, although the total amount of energy released in the explosion is similar to that released in a typical type II supernova.  SN 1987A is also unusual in brightening in only 3 hours, rather than over the several days that is more typical of type II supernovae.  These unusual features are directly tied to the small radius of the exploding star.  The rapid brightening of the star directly reflects this small radius; more time is required for the energy released by the collapse of a star's core to travel to the photosphere of a red supergiant than to that of a blue supergiant, because the red supergiant is physically much larger than the blue supergiant.  The remaining-two characteristics?the high velocity and the low luminosity?are set by the star's radius through the thermodynamics of a supernova.

Like an internal combustion engine, a supernova explosion is a heat engine that converts heat into kinetic energy.  Just as the motion of an engine's pistons convert the heat released when fuel is burned into the kinetic energy that propels a car, the expansion of a star during a supernova explosion converts the heat released by the collapse of the star's core into kinetic motion of the outer regions of the star, and as with an internal combustion engine, the efficiency of this conversion depends on the compression ratio of the system.  The higher the compression ratio in an internal combustion engine, meaning the higher the ratio of the final volume in a piston cylinder to the initial volume in the cylinder, the more efficient the conversion of heat into kinetic energy.  For an exploding star, a high compression ratio is achieved by making the radius of the star that explodes as small as possible, because the point at which the supernova shell becomes transparent and releases its remaining heat is independent of the initial radius of the star.  This means that the supernova of a blue supergiant converts much more of the supernova energy into kinetic energy than does the supernova of a red supergiant of equivalent mass; the former is a more efficient heat engine than the latter, because a blue supergiant has a much smaller radius than does a red supergiant.  The consequence of this efficiency is that the velocity of the supernova shell is higher, and the temperature of the shell is lower, in a blue supergiant supernova than in a red supergiant supernova.

SN 1987A is not the only supernova with a previously-observed star.  Generally, a supernova must be closer than about 30 megaparsecs from Earth to have its progenitor identified.  A handful of other known stars in nearby galaxies have exploded, and most of these stars are red supergiants, as expected from theory.^[3]  They behave as typical type II supernovae.  The supernova of a blue supergiant is simply a rare event, which is reflected in the rarity of its type:  under-luminous type II supernovae account for less than 3% of all core-collapse supernovae.

Two other atypical features of SN 1987A are clues that motivate the many theories as to why the star Sk -69 202 exploded while it was in a blue supergiant state.  The first is the unusual chemical composition of the supernova shell, and the second is the odd triple-ring remnant left by the supernova.

 The chemical composition of the star Sk -69 202 is evident in the spectrum of the supernova.  At the start of a supernova, the spectrum shows the composition of the star's outer layers.  The spectrum of SN 1987A showed an overabundance of nitrogen relative to oxygen and carbon as compared to the Sun, which is a signature of the CNO hydrogen fusion cycle.  The appearance of this overabundance at the surface of Sk -69 202 suggests either that the star over its lifetime had lost most of its outer hydrogen layer, revealing the layers that had experienced thermonuclear fusion, or that convection transported the products of thermonuclear fusion to the star's outer layers.^[4]  A companion star can cause either of these outcomes, while a rapid spin could have caused convection in Sk -69 202.

The unusual remnant of SN 1987A consists of three rings aligned along an axis of symmetry; the typical supernova remnant is a simple shell centered on the point of the explosion.  The interpretation of SN 1987A's remnant is that the axis of symmetry is aligned with either the rotation axis of the exploding star or the orbital axis of that star around a dimmer companion star.  The rings themselves are regions of high gas density in the star's stellar wind that were flash-ionized by ultraviolet radiation from the supernova.  The center ring is interpreted as a high-density ring in the plane of the exploding star.  The remaining two rings, one below and one above the plane defined by the center ring, have a variety of interpretations.  The most important aspect of the axisymmetric remnant, however, is that it supports the belief that the star Sk -69 202 was either spinning rapidly or was orbiting a companion star.

Together, the chemical composition of Sk -69 202 and the axisymmetry of its remnant suggest that either its rapid spin or its orbit around a companion star altered the evolution of Sk -69 202, causing it to be in a blue supergiant stage rather than a red supergiant stage when its core collapsed.