Supernova 1987 A

On July 7, 2011, the ESA and NASA announced that new observations from the infrared Herschel Space Observatory indicate that the 1987 supernova has expelled the equivalent of between 160,000 and 230,000 Earth masses (~0.4 to 0.7 Solar-mass) of fresh dust into surrounding interstellar space (Matsuura et al, 2011). Such "cosmic dust" is made of various elements, such as carbon, oxygen, iron and other atoms heavier than the abundant hydrogen and helium created in the Big Bang. The enormous amount of dust found around Supernova 1987a indicates that supernovae may have produced much of the dust detected in observations of the early Universe. In today's Universe, old red giant stars are believed to be the major dust producers, with dust grains condensing as warm gases flow away from the star, but such stars did not yet exist in the early Universe although there was already large masses of dust. Supernovae, however, can be produced by the most massive stars which die out within a few million years (ESA news release; news release; Herschel news release; JPL news release; Nancy Atkinson, Universe Today, July 7, 2011; and Matsuura et al, 2011).

In February 1987, light from a supernova explosion more than 179,000 light-years (ly) away finally reached the Solar System (adjusting for the HIPPARCOS-corrected, Cepheid distance scale derived in 1997). The explosion was located in the Large Magellanic Cloud (LMC), a small irregular galaxy and orbiting satellite of the Milky Way visible to naked-eye observers from Earth's Southern Hemisphere. Relatively close to the more distant, Small Magellanic Cloud (NGC 292) to which it may not be gravitationally bound (Demers and Irwin, 1993), the LMC is located in the eastern part (5:23.6-69:45, ICRS 2000.0) of Constellation Dorado, the Goldfish -- south of Delta and Beta Doradus, east of Theta Doradus, west of Delta Volans, and southeast of Alpha and Gamma Doradus. The brightest and largest of the Milky Way's satellites at 50,000 ly wide or more, the LMC has around 20 billion stars which eventually be torn apart and integrated gravitationally into the Milky Way within a few billion years (more about LMC, galaxies in general, and the absorption of satellites and their globular clusters). Gravitational tides induced by the much massive Milky Way have already distorted the LMC's original shape by moving around its gas, dust, and stars, which may help to foster stellar formation in nebulae like the Tarantula.

Photo by David Malin,
© Anglo-Australian Observatory



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The Large Magellanic Cloud
with the Tarantula Nebula at
center left (more at Astronomy
Picture of the Day and AAT).

Supernova 1987 A actually took place within the Tarantula Nebula (also known as 30 Doradus and NGC 2070). This massive emission nebula sprawls across more than 2,000 ly, (Astronomy Picture of the Day) and encompasses open cluster R136, which contains more than 3,000 stars -- many hot and blue, including some of the largest, brightest, and most massive stars known (more from STScI; and Hunter et al, 1996). It also contains the young, open star cluster Hodge 301, which formed together tens of millions of years ago and whose most massive members are quickly exhausting their nuclear fuel -- like its three, swollen red supergiants that will soon explode in supernovae (more at STScI).

Photo by David Malin,
© Anglo-Australian Observatory


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The Tarantula Nebula and
Supernova 1987 A at lower
right (more at Astronomy
Picture of the Day and AAT).

Within the Tarantula, the supernova remnant is surrounded clouds of gas and dust. There are also many bright blue stars nearby that are very massive, each exceeding six Solar-masses. Only about 12 million years old, they are young but short-lived members of the same generation of stars as the star (Sanduleak -69°202) that went supernova in 1987. The bright gas clouds (emission nebulae) in this region is another sign of the youth of this region, which still appears to be fostering the birth of new stars (Astronomy Picture of the Day).

STScI, AURA, NASA



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The supernova remnant is
located in a region of the
Tarantula with abundant gas,
dust, and massive, hot and
young blue stars (more).

Very quickly, the supernova dimmed by so much that only powerful optical instruments like the Hubble Space telescope could see it from Earth.

Robert P. Kirshner
and Peter Challis,
Harvard University,
STScI, NASA



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Supernova remnant 1987 A
among the young stars of
the Tarantula (more).

Supernova 1987 A (SN 1987A) was the brightest supernova in Earth's night sky since Kepler's "star" of 1604. It was a photometically and spectroscopically, atypical type-II supernova was underluminous with a slow decline and unexpected color evolution, which may have been due to its progenitor star being from a low-metallicity stellar population (L. A. L. Da Silva, 1990). The supernova lies at the position of a former, blue supergiant star of a spectral and luminosity type B3 I (5:35:28.3:-69:16:13, J2000; and 5:35:28.26-69:16:13.0 ICRS 2000.0), star 1 in a stellar trio known as Sanduleak -69° 202 (CXC; Phillips et al, 1987; Herald et al, 1987; Kunkel et al, 1987; Sonneborn et al, 1987; and Nicholas Sanduleak, 1970). It had around 20 Solar-masses and a diameter about 40 times larger than Sol's (Podsiadlowski et al, 2007; Tuchman and Wheeler, 1989; and Achim Weiss, 1988). The star was first listed by Nicholas (Nicolae) Sanduleak (1933-1990) as part of a deep optical survey of the Large Magellanic Cloud. A useful catalogue number for the progenitor star is: Sk -69 202. Useful catalogue numbers and designations for the supernova are: LMC SN, SN 1987A, and LMC 264.

Photo by David Malin,
© Anglo-Australian Observatory



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The supernova is superimposed
here on an image of its progenitor
star Sanduleak -69° 202 (more).

Based on theoretical modelling that reproduces the triple-ring structure of SN 1987A, the star probably transitioned from a blue to a red, then back to a blue supergiant just before its explosion (Tanaka and Washimi, 2002; Tuchman and Wheeler, 1989; and Saio et al, 1988). However, theoretical models of the evolution of massive stars would predict that these stars should end their evolution as red supergiants (rather than as blue supergiants) with a radius atleast 1,000 times the radius of the Sun. Moreover, the composition of the star that explodedwas very unusual; in particular, the outer layers of the star had an abundance of helium that was about a factor of 2 larger than the expected abundance, as if part of the material from the core (where helium had been produced as the star evolved) was somehow mixed into the outer layers (Podsiadlowski et al, 2007).

Podsiadlowski et al, 2007; and
Christopher Burrows, STScI, NASA

Larger image comparison.

The triple-ring pattern may have
been created from a stellar merger
that created the progenitor star
of the supernova around 20,000
years previously (more).

On February 22, 2007, astronomers Thomas Morris and Philipp Podsiadlowski announced simulation results which support a common hypothesis that the merger of two stars and the orbital movements that preceded it may have generated the distinctive triple-ring system of Supernova 1987A. The expansion velocity of the rings suggests they were produced about 20,000 years before the supernova explosion itself, possibly the result of a star with five Solar-masses being merged into a larger 15-20 Solar-mass, red giant star and colliding with its core. As smaller star spirals in towards the red giant's core, it stirs up the red giant's atmosphere and gives more angular momentum of the red giant's gas which helps to flatten it into a disk. The friction of the smaller star's through the red giant also heats up the star's atmosphere in its vicinity, which causes it to expand outwards in all directions although much of this outflow is deflected by the outer part of the disk and channelled to eventually produce two large rings above and below it. Moreover, the stellar merger produces a single star spinning so fast that it throws off gas at its equator to form a third, inner ring (Podsiadlowski et al, 2007; simulation images; and David Shiga, New Scientist, February 22, 2007).

NASA Observatorium


See a discussion of
"the Burning of Elements
Heavier than Helium" and
"Supernova Explosions"
as part of stellar
evolution and death.

Since star 1 of Sanduleak -69° 202 (Sk -69 202) had more than eight Solar-masses, its core nuclear burning progressed beyond helium (He) fusion. After acquiring fusing much of its helium into carbon (C) and oxygen (O) "ash," the core contracted and heated to even higher temperatures until its carbon and oxygen ignited. Their fusion yielded such elements as neon (Ne), magnesium (Mg), silicon (Si), and sulfur (S). Eventually, core silicon and sulfur ignited to form iron, nickel, and other elements of similar atomic weight, so that the star's internal structure resembled an onion, which deeper shells burning heavier elements, until a central core of iron is created. Unfortunately, iron does not burn (undergo fusion).

ESO





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Light curve of the
supernova over over
more than 12 years
(~4,500 days) (more).

Eventually the core gets so "cold" that it cannot withstand the gravitational pressure of the star's outer layers and they collapse towards the core. This implosion is so violent that it generates an enormous burst of energy that rebounds as neutrinos back outward and blows the star apart as a supernova. (For a supernova to do injure life in the Solar System, it would have to explode at a distance of less than about a hundred light-years, more than a thousand times closer than SN 1987A.) This blast wave of neutrinos heats the star's inner layers to 10 billion degrees F (around 5.6 billion degrees C) and induces explosive nuclear burning to create even heavier elements than iron. Although most of the star is blown off as a rapidly expanding, gaseous shell, a compact remnant is left behind as a neutron star (like the "pulsar" found in the Crab Nebula) or a black hole. In the case of SN 1987A, astronomers have not as yet determine which type of stellar remnant was left behind because of obscuring debris from the supernova explosion.

CXC, SAO, NASA,
(Burrows et al, 2000)



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Supernova shockwave lights up
gas expelled thousands of years
ago by its progenitor star
Sanduleak -69° 202 (CXC).

During the 1990s, optical observations of SN 1987A with the Hubble Space Telescope revealed gradually brightening hot spots from a ring (actually a shell) of matter that was ejected by the progenitor star thousands of years before it exploded. A subsequent X-ray image showed that a shockwave, traveling at a speed of around 2,800 miles (4,500 km) per second -- or 10 million miles per hour -- is smashing into parts of the optical ring. The gas in the expanding shell has a temperature of about 10 million degrees C (18 million degrees F), but is visible only with an X-ray telescope. As the X-ray emission peaks close to the optical hot spots, the the wave was apparently beginning to hit the ring (Sugerman et al, 2002). Over the next few years, the shockwave should have ignited more material in the ring, with an inward moving, or reverse, shockwave heating the material ejected in the explosion itself. Today, much of the inner reaches of Sn 1987A has cooled down (to a few hundred degrees F/C) and the debris is now heated by nuclear energy from the decay of radioactive nuclei produced in the initial supernova explosion.

P. Challis, R. Kirshner, and
B. Sugerman; CfA, STScI, NASA


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As the supernova blast wave continues
to plow into a pre-existing, one light-
year-wide ring of material previously
expelled by the progenitor star, hot
spots heated to millions of degrees
began to appear like pearls in the mid
1990s (more at APOD and STScI).