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Rethinking last century's
closest, brightest supernova
PRESS RELEASE
Source:
University of California Berkeley
BERKELEY Twenty years ago next month,
the closest and brightest supernova in four centuries lit up the
southern sky, wowing astronomers and the public alike.
A Luminous Blue Variable star named HD168625, located in our Milky
Way Galaxy, is surrounded by a bipolar nebula that is similar to the
one around SN1987A, a supernova that exploded in 1987 in the Large
Magellanic Cloud and was the nearest supernova to Earth in 400
years. (Image credits: NASA, JPL-Caltech, Nathan Smith/UC Berkeley)
Ongoing observations of the exploded star, called supernova 1987A,
provided important tests for theories of how stars die, but it also
raised some new questions. Principal among these was how a bizarre,
triple-ring nebula surrounding the supernova - ejected by the star a
few thousand years before it exploded - originated. Astronomers
devised a complicated theory that, within a relatively short period
of time, the original star, a red supergiant, merged with a
companion and started spinning rapidly, then underwent a transition
to a blue supergiant, and finally exploded.
University of California, Berkeley, astronomer Nathan Smith has
proposed a different theory for the origin of the nebula, arguing
instead that SN1987A's progenitor star may have been in a class of
unstable blue supergiant stars, called luminous blue variables,
which eject material from their surfaces in recurring, volcano-like
eruptions before they finally die in a supernova explosion.
Smith recently discovered two such blue supergiant stars with
nebulae closely resembling the peculiarly shaped cloud of dust and
gas around SN1987A. A third such nebula was already known. "Taken
together, the three closest analogs of SN1987A in our galaxy are all
around blue supergiants; two of them have not gone through a red
supergiant phase at all, and one was ejected as a luminous blue
variable," said Smith, a UC Berkeley postdoctoral researcher. "This
makes a pretty solid case that we should rethink models for how the
rings around SN1987A were formed.
"If these other stars with rings are likely to explode, it may hint
that LBVs and blue supergiants can explode even before becoming red
supergiants, which would be a bit of a shock to our understanding of
stellar evolution."
Smith will present his findings today (Tuesday, Jan. 9) at a 10:30
a.m. press conference and an all-day poster session during the
American Astronomical Society (AAS) meeting in Seattle. The
proximity of SN1987A, only 168,000 light years away in the Large
Magellanic Cloud, and the availability of pre-existing data provided
the first chance for astronomers to posthumously identify the star
that exploded. Astronomers were surprised to find that it had been a
hot blue supergiant - not a cooler red supergiant, as most theories
predicted at the time.
Adding to the mystery, images taken in the early 1990s by
instruments like NASA's Hubble Space Telescope revealed a bizarre,
triple-ring nebula. The origin of this nebula and its shaping
mechanism are still difficult to understand. The merger theory with
conversion from red supergiant to blue supergiant before exploding
has become the prevailing view because it accounts for both the blue
supergiant and the shape of the nebula.
Diagram explaining the bipolar nebula around the Luminous Blue
Variable HD168625, which has a geometry that makes it a near twin of
the famous nebula around SN1987A. Rings near the equator are
sometimes seen around stars that shed mass from their surfaces, but
the larger rings above the poles are very rare. Tipped toward Earth
and illuminated by the star, the rings look like ellipses in images
taken with NASA's Spitzer Space Telescope.
The surprise, Smith said, is that analysis of these new objects in
our galaxy that resemble SN1987A provide good reasons to suspect
that they ejected and shaped their nebulae while they were still
blue supergiants, and not in the transition from red to blue as has
been proposed for SN1987A. Furthermore, none of the three stars is
spinning rapidly, as one might expect if it had recently merged with
a close orbiting companion star. A merger and the subsequent
red-to-blue transition are the key ingredients in the prevailing
explanation for the nebula around SN1987A, but the three stars
discussed by Smith apparently formed similar nebulae without either
mechanism.
"We are seeing these nebulae before the stars blow up, and they look
quite similar to the nebula around SN1987A," said Smith. "The
trouble is, they may contradict how we think the nebula around
SN1987A was formed."
According to Smith, the unusual nebula around SN1987A, looking like
a figure 8, was originally interpreted to mean that the star had
recently been a red supergiant that had shed its outer envelope in
an expanding shell, but then turned into a blue supergiant before
exploding. The blue supergiant generated a faster wind that overtook
the earlier wind and became distorted.
"In that picture, the equatorial ring formed because the slow wind
of the red supergiant had more material in the equator, so the waist
of the blue supergiant wind was pinched," Smith said. "The fly in
the ointment is that in order to get the enhanced density in the
equator of the red supergiant, you need it to be spinning rapidly -
but red supergiant stars don't do that because they are so big. So
the only solution would be if the progenitor of SN1987A swallowed a
companion star and the two merged, while the added angular momentum
made the red supergiant spin to make a disk." "This requires that
the nearest and best observed supernova in modern history just
happens to also be a freak, resulting from a coincidental merger
event," he added.
While looking through images taken by NASA's Infrared Array Camera
on the Spitzer Space Telescope, however, Smith noticed a similarly
weird nebula around a nearby star designated HD168625. This star is
a luminous blue variable (or LBV), an unstable massive star that
burps from time to time and ejects a bipolar nebula as a blue
supergiant, not a red supergiant. A well-known LBV is Eta Carinae,
the brightest and most massive star in our Milky Way galaxy,
weighing in at more than 100 solar masses.
"This new twin of the SN1987A nebula around an LBV gives us an
alternative to the binary merger hypothesis for how these form,"
Smith said. "It hints that SN1987A may have ejected the nebula as a
blue supergiant or an LBV, and not as a red supergiant."
Later, Smith identified a second ring nebula, identical in size to
the equatorial ring around SN1987A but surrounding another blue
supergiant in our galaxy. He found this in the Carina Nebula in the
southern Milky Way in data taken by the 4-meter Blanco telescope at
Chile's Cerro Tololo Inter-American Observatory, part of the
National Optical Astronomy Observatory, and in images taken by one
of two 6.5-meter Magellan telescopes in Chile.
The second star, called SBW1, has almost the same spectral type as
the progenitor of SN1987A, but the chemical abundances in the nebula
imply that it has not yet been a red supergiant. This directly
contradicts the old picture for how the rings around SN1987A were
formed, he said. A third similar object in our galaxy, called Sher
25, was already known, and it has chemical abundances that also
suggest it has not yet been a red supergiant.
Smith's research was supported by NASA.
NASA's Jet Propulsion Laboratory (JPL) in Pasadena, Calif., manages
the Spitzer Space Telescope mission for NASA's Science Mission
Directorate in Washington, D.C. Science operations are conducted at
the Spitzer Science Center at the California Institute of
Technology, also in Pasadena. Caltech manages JPL for NASA.
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From our TPOD "Electric
Supernovae":
In the conventional view, a
supernova is an exploding star. Because gravity is the only
force available to explain the organization of matter into
stars, stars are isolated and autonomous objects that must get
the energy they radiate from internal sources.
The explosive release of
abnormal amounts of energy in a supernova must come from the
same (or similar) internal sources. When telescopes observe
high-energy radiation and fast-moving particles, the cause can
only be heating and acceleration by shock waves. The intensities
required must demolish the star.
These are constraints imposed
by theory, not empirical limits from observing actual
supernovae.
In the Electric Universe view,
a supernova is also an exploding star. But an electric star is a
power-consuming pincha
loadin a
galactic circuit of
Birkeland currents. The circuit drives the pinch,
just as circuits in a house drive the electric lights.
Also from our TPOD
Deep Space Explosion Baffles Astronomers
Perhaps Supernova 1987a
provided a clue. As reported by Wallace Thornhill, this earlier
observed explosion defied expectations of astronomers while
exhibiting all of the peculiar features expected of a powerful
plasma "Z-pinch. Direct observation thus suggests an electrical
cause for supernovae, and the more recent deep space explosion
should be examined for electrical signatures as well.
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