Supernova 2006gy

On September 18, 2006, astronomer Robert M. Quimby detected the brightest and largest supernova ever recorded by contemporary astronomers, using the ROTSE-IIIb telescope at McDonald Observatory (Robert M. Quimby, 2006; and Katie Humphrey, Austin-American Statesman, May 9, 2007) -- but became second brightest on October 10, 2007 after twice-as-bright Supernova 2005ap (see APOD; and Quimby et al, 2007). The explosion reached a peak magnitude of -22 and was designated Supernova (SN) 2006gy. Its characteristics appear to be consistent with theoretical predictions made about four decades ago, and so SN 2006gy may become the model for a new class of supernovae associated with the most massive (and possibly the first) stars born in the universe (NASA news release; CXC news release; UC Berkeley press release; CfA press release; APOD; Smith et al, 2007; Ofek et al, 2007and Prieto et al, 2006 -- more below). On August 13, 2007, two astronomers submitted a paper suggesting that SN2006gy underwent a subsequent neutron star to quark-nova stage that can explains its rise to extreme brightness, which may be slower than should be typical of pair-instability supernovae (Leahy and Ouyed, 2007; David Shiga, New Scientist, August 20, 2007 -- more below). Between October 17 and November 13, 2007, two other groups of astronomers announced alternative theories for the unusual brightness of this supernova involving multiple stellar collisions in the dense core of a star cluster and successive stellar explosions colliding with older gas shells (Govert Schilling, New Scientist, November 14, 2007; and Woolsey et al, 2007). On April 10, 2008, two astronomers submitted a revised paper that the supernova's lightcurve matched that of a theoretical neutron star implosion ("quark nova") into a quark star (Leahy and Ouyed, 2008).

Robert Lupton,
SDSS Consortium
(Used with permission)



Larger image.



SN 2006gy and its
host galaxy NGC
1260 are located in
the Perseus Cluster
of galaxies, part of
the Pisces-Perseus
Supercluster (more).

The explosion was located in NGC 1260 about 240 million light-years away (03:17:27.2+41:24:19) in Constellation Perseus. NGC 1260 is an early-type galaxy and a member of the Perseus Cluster of galaxies. When viewed in X-rays, Perseus appears to be the brightest galactic cluster in Earth's sky. It is a major component of the Pisces-Perseus Supercluster.

Joshua Bloom,
PAIRITEL,
UC Berkeley




Larger infrared
image.




Located some 240
million light-years
away, NGC 1260
is an early-type
galaxy with a dust
lane, a few young
massive stars in
the HII region
where the
supernova was
found (more).

NGC 1260 has mostly old stars but is enriched with elements heavier than hydrogen near Sol's abundance. The galaxy has a dust lane (passing some 300 parsecs from the supernova) as well as an H II region near the supernova. Very sensitive observations of the galaxy's core also show that it does have young, massive stars, some near the supernova (Ofek et al, 2007).

Joshua Bloom
and C. Hansen,
Lick Observatory,
UC Berkeley/



Larger image.



In visible light, the
supernova was
brighter than the
core of its host
galaxy NGC 1260,
at left (more).

SN 2006gy had a luminosity (or intrinsic brightness) equal to that of some 50 billion suns -- around 10 times brighter than host galaxy NGC 1260 -- before beginning a slow decline. It was around 100 times brighter than typical supernovae, and unlike typical supernovae that reach a peak brightness in days to a few weeks and then dim into obscurity a few months later, SN 2006gy took 70 days to reach full brightness and stayed brighter than any previously observed supernova for more than three months. After nearly eight months, it was still as bright as a typical supernova at its peak.

Smith et al, 2007;
CXC, NASA



Larger illustration.



SN 2006gy was
brighter and
stayed brighter
far longer than
past supernovae
(more).

Classified as a peculiar Type-IIn supernova (with narrow hydrogen lines early on), astronomers believe that SN2006gy was created by the explosion of an extremely massive progenitor star, that may have been near a hypothesized, modern-universe observational "cutoff" of 150 Solar-masses, which was recently derived from measurements of relatively metal-rich stars in the Arches Cluster near the center of our own Milky Way galaxy (Donald F. Figer, 2005). Today, such massive stars (such as R136a1, which was discovered in the Large Magellanic Cloud) are so rare that galaxies like the Milky Way may contain only a dozen out of a stellar population of 400 billion. However, such massive stars have been hypothesized to have been far more common in the early universe, where a lower abundance of elements of elements heavier than hydrogen could have fostered the development of such stellar giants.

The star that produced SN 2006gy exploded earlier than expected, during the evolved but still extremely massive and now rare, luminous blue variable (LBV) stage when it still had not completely shed its massive envelope of hydrogen gas. Observations of SN 2006gy with optical telescopes have determined that the bulk of supernova's debris is moving outward at around 15 million kilometers (or 9.3 million miles) per hour (kph or mph) into a circumstellar nebula or "cloud" of hydrogen gas that is coasting along at a leisurely 700,000 kph (or 430,000 mph). This cloud was presumably ejected by the doomed star prior to the explosion.

Melissa Weiss,
CXC, NASA



Larger illustration.



SN 2006gy's
progenitor star
is hypothesized
to have been a
luminous blue
variable star
with close to
150 Solar-
masses (more).

Like observed LBVs that suffer multiple giant explosions to shed several Solar-masses with a few years, SN 2006gy's progenitor star did apparently expel a large amount of mass prior to exploding. Indeed, the inferred mass loss rate of the progenitor star was "stupendous," averaging as much as 0.5 Solar-mass per year over a decade preceding the explosion (Smith et al, 2007; and Ofek et al, 2007). Such a high mass-loss rate can be associated with a binary-star ejection of their common envelope (Ofek et al, 2007), and some LBVs are suspected of being binary stars. This large mass loss is similar to that seen from evolved LBV stars like the Pistol Star and Eta Carinae, a massive star and potential binary in the Milky Way which may be poised to explode as a supernova within the Milky Way galaxy but would be only about 7,500 light years away from Earth (Smith et al, 2007).

SN 2006gy was too bright in visible light for Type-II supernovae produced by most massive stars (e.g., those known to have less than a 100 or so Solar-masses). Moreover, it produced relatively few x-rays for Type Ia supernovae involving white dwarfs exploding into a dense cloud of hydrogen (Dennis Overbye, New York Times, May 7, 2007). If was a Type Ia supernova, then 2006gy should have been around thousand times brighter in X-rays than was detected. Hence, astronomers believe that 2006gy may have originated from an extremely massive star, that had 140 to 260 Solar-masses in theory. The features of SN2006gy imply that the most massive stars can explode earlier than expected during the LBV phase, so that they never get a chance to completely shed their massive envelope and become less massive Wolf-Rayet stars, and that can create brilliant supernovae instead of dying inconspicuouly through direct collapse to a black hole (Smith et al, 2007).

Smith et al, 2007;
CXC, NASA



Larger image.



In x-ray emission,
SN 3006gy was also
nearly as bright as
the core of host
galaxy NGC 1260,
but not bright
enough for a Type-Ia
supernova (more).

Stars that have at least 10 Solar-masses destroy themselves after fusing hydrogen to helium, helium to carbon, and on to larger elements until they reach iron, when fusion fails. Towards the end of this process, the energy produced in the core of the star becomes insufficient to support the outer layers, which collapse inward under gravitationAL pressure, ending fusion after creating some even heavier elements, and crunching the core to a neutron star or black hole. The rebound from the core implosion blows away the outer layers of the star as a bright supernova.

Melissa Weiss,
CXC, NASA



Larger illustration.



Stars evolve at a rate
depending on their mass,
but only the largest stars
and some white dwarfs (with
companions donating mass)
die as supernovae (more).

For much more massive stars with 140 to some 260 Solar-masses, core temperature becomes so great at several billion degrees that, before the fusion progression theorized for less massive stars is complete, high-energy gamma rays in the core begin to annihilate one another and create matter-antimatter pairs (mostly electron-positron pairs). Hence, instead of mass being converted to energy in the star's core (via Albert Einstein's famous equation: E = mc2), energy is being converted to mass. Since gamma radiation provides the energy preventing gravitational collapse of the outer layers of the star onto the core, at some point the loss of this energy (through so-called "pair instability") causes violent pulsations that eject a large fraction of the outer layers of the star and eventually a star's outer layers to collapse inward to create a thermonuclear explosion that, in theory, would be brighter than previously detected supernova. In this type of pair-instability supernova, the star is blown to bits without creating a black hole. For stars with greater than around 260 Solar-masses, the pulsations would be overwhelmed by gravity, and so the star would collapse to form a black hole without an explosion. Currently, the favored explanation for SN 2006gy unusual features is derived from the so-called pair-instability model for supernova creation.

In theory, pair-instability supernovae should produce a relatively greater abundance of heavy elements. For stars with initial masses above about 200 suns, pair-instability supernovae would produce an abundance of radioactive nickel. According to some astronomers, the radioactive decay of nickel-56 produces most of the light of a supernova, and SN 2006gy produced about 22 Solar-masses of nickel (Smith et al, 2007), compared to maybe 0.6 solar masses in a Type Ia supernova created by a white dwarf (and stolen mass from a companion star). Since astronomers believe that a large proportion of the universe's first stars were supermassive stars like SN 2006gy's progenitor, such supernovae should have dispersed large quantities of newly synthesized elements heavier than hydrogen, instead of collapsing into black holes. In addition, as these pair-instability supernovae are so bright, astronomers hope to detect similar explosions from the first stars in the universe over 13 billions years ago with more powerful observatories, such as the upcoming James Webb Space Telescope.

On August 13, 2007, two astronomers submitted a paper suggesting that SN2006gy underwent a subsequent neutron star to quark-nova stage that can explains its rise to extreme brightness, which may be slower than should be typical of pair-instability supernovae involving the first supermassive stars born after the Big Bang (Leahy and Ouyed, 2007; David Shiga, New Scientist, August 20, 2007). They present a scenario where an explosive transition of the newly born neutron star to a quark star results in a delayed second explosion that occurs inside the ejecta of a more common, core-collapse supernova. As a result, the earlier supernova ejecta is reheated to radiate at a higher level of energy for a longer period of time.