SN 2006gy


This story is getting bigger and bigger (pun intended).

At the bottom of this post, I have a set of links that I’m updating as I find more. [Last update 5/19/2007]

I first saw a mention of the supernova SN2006gy, in a post over at Tom’s Astronomy Blog. SN2006gy is now being called ‘the largest supernova ever observed’, and it may be the first observation of a specific type of supernova (see below for details). This supernova occurred in NGC 1260, a galaxy about 240 million light years away. Observations of the very unusual light curve indicate that the progenitor star (i.e., the star that exploded) was a hyper-massive star between 140 and 250 Solar Masses (M⊙). Phil Plait (The Bad Astronomer) has a post about it on Astronomybuz.com, and I have posted a followup link swarm over there as well.

This massive supernova was first observed by University of Texas graduate student Robert Quimby on September 18, 2006. It was detected by an optical robotic telescope as part of the Texas Supernova Search project. NGC 1260 is a very faint (Mag 14.1) spiral galaxy located in the constellation Perseus, about 1° 48 ‘ 13″ away from Algol in the opposite direction of Triangulum. It’s not visible to the naked eye, and you’ll need a pretty good scope to pick it out. There is a cluster of faint galaxies in the middle of Perseus, and NGC 1260 is just one of them.

The apparent magnitude at its peak was 15, making it a very faint object, visible only with a powerful telescope. However, due to the distance, the absolute magnitude is calculated at -22. In over-simplified terms, the absolute magnitude is the calculated brightness of an object if viewed from a fixed reference distance. That distance is called the ‘standard luminosity distance’ and is about 10 parsecs or 32.616 light years. Our sun has an absolute magnitude of 4.83, but a much larger apparent magnitude of −26.73 because we are so close to it. The apparent magnitude of the full moon is −12.6. If Eta Carinae (another hyper-massive star about 7,500 light years away in the constellation Carina) were to explode in the same way, it would quite possibly be bright enough to be seen during full daylight, and to read by at night, because it is much closer to us (7,500 light years instead of 240 million light years). It would have a much higher apparent magnitude than SN2006gy.

So, since this supernova was observed in September 2006, why is it making news now? It turns out there are several very good reasons.

First of all, it is behaving very differently than a typical supernova. Typically, supernovae reach their peak brightness in a few days to a few weeks. They then fade into obscurity a few months later. SN2006gy took 70 days to reach it’s full brightness (see the light curve above). And boy, was it bright. It was brighter than any previously observed supernova. Then, it stayed brighter than any previously observed supernova for more than three months. Nearly eight months later, it still is as bright as a typical supernova at its peak, outshining its host galaxy.

Second, this object is so unusual, there was a lot of debate about what it actually is. Many scenarios were considered and rejected. It did not behave according to the usual ways that stars evolve. Some astronomers initially thought it may be an outburst from an active galactic nucleus (AGN), however further observations showed that there was a clear separation between the nucleus of NGC1260 and SN2006gy. It did not show the typical spectral lines of a Wolf-Rayet star (also see “The Physical Properties of Wolf-Rayet Stars“). They could tell early on that it was embedded in a cloud of matter, and there was debate as to if this was a dense region of the inter-stellar medium (ISM), or if it was the outer layers of a super-massive progenitor star similar to Eta Carina that had been previously ejected to form the circum-stellar medium (CSM). However, it ejected too much mass to be a type Ia supernova that exploded in a hydrogen-rich cloud. It was suggested early on that it was something else, something new and unusual.

Now, it appears that it is that ‘something else’, specifically a different kind of supernova, theorized but never before observed; a ‘‘pair instability’ (aka ‘pair production’, ‘pair-producing’ or ‘pair production instability’) supernova‘. To save wear and tear on my fingers, I’m going to adopt the abbreviation ‘PISn’ for the rest of this article. This type of supernova was first theorized in the 1960’s, but has never been observed, until (perhaps) now. This type of supernova requires an immensely heavy progenitor star, somewhere between 140-260 M⊙. Theoretically, a PISn can only occur in a supermassive but low metallicity star (in astronomy parlance, everything heavier than helium is ‘metal’), in other words star very much like a Population III star. Population III stars were the first generation of stars that condensed out of the Big Bang. They were composed of only hydrogen and helium, virtually no heavier elements at all. (NOTE: The progenitor of SN20006gy was not a Population III star, it is far too recent. Supermassive stars burn out quickly, within a million years or so as opposed to 10 Billion years for yellow dwarf stars like our Sun.) Stars without the right amount of mass (either too much, or too little) don’t turn into this kind of supernovae, and stars with too high metallicity don’t get big enough to cause this kind of supernova.

It turns out that PISns are very important to our understanding of the early phases of the universe. As noted above, the Population III stars had very low metallicity, almost no elements heavier than Helium. Because of this, they could form so-called “supermassive” stars (stars greater than 100 M⊙) more readily than today. Supermassive stars are not formed as frequently now as they were in the early universe, and when they do form, they tend to rip themselves apart, shedding huge amounts of mass in outbursts like Eta Carinae. (Eta Carinae was the site of a giant outburst about 150 years ago, when it became one of the brightest stars in the southern sky. Though the star released as much visible light as a supernova explosion, it survived the outburst. The outburst produced two huge ‘blobs’ of ejected material and a large thin equatorial disk, all moving outward at about 1.5 million miles per hour.) Supernovae in general, including PISNs, were important vehicles in forming the heavier elements (meaning everything heavier than helium) and spreading them across the universe.

The observations of SN2006gy showed that a huge amount of energy was being released; 3×10^44 ergs / second (in contrast, our sun outputs about 3.86 x 10^33 ergs/second or 386 billion billion megawatts), which indicated a dense CSM. From this, scientists (See Ofek, et. al below) have inferred a mass loss rate of the progenitor is was about 1 tenth of a solar mass per year over a period of about 10 years prior to explosion. The total radiated energy in the first two months was about 1.1 × 10^51 ergs, which is only a factor of two less than that available from a super-Chandrasekhar Ia explosion (Another recently observed oddball type of supernovae). Therefore, given the presence of a star forming region in the vicinity of the SN and the high energy requirements, a plausible scenario is that SN 2006gy is related to the death of a massive star (e.g., a PISn) (See Smith, et. al below).

In the theoretical model of a PISn the temperature at the core becomes so great so quickly, that before the typical supernova fusion cascade (hydrogen -> helium -> carbon etc.) can complete, gamma rays in the core of the star become so energetic that they begin interacting with each other and with the core material, creating matter-antimatter particle pairs. Gamma radiation is the energy that provides the most of the radiation pressure which prevents collapse of the outer layers of the star. The conversion of gamma radiation into particle-antiparticle pairs in the core removes the radiation pressure (the energy is held in the core instead of radiating out of it). Once that gamma radiation starts to disappear, the outer layers of the star fall inward. This creates more pressure in the core, which creates even more energetic gamma rays, which create even more particle-antiparticle pairs. This is where the name ‘pair production instability’ comes from. This process occurs sooner in the lifetime of the star than the typical fusion cascade would complete. The result is a runaway thermonuclear explosion that, theoretically, would be brighter than any typical supernova.

Why would it be brighter? Most of the light of a supernova is generated by the conversion of lighter elements into a radioisotope of nickel, nickel-56. Nickel-56 has a half-life of just over 6 days, and is produced in large quantities in type Ia supernovae. As it decays into other elements, it gives off significant amounts of radiation. The shape of the light curve of type Ia supernovae corresponds to the decay of nickel-56 to cobalt-56 and then to iron-56. In most supernovae, a significant amount of mass is ‘locked up’ in the form a stellar remnant, either a black hole or a neutron star. However, in a PISn, the star is completely annihilated, blown apart from the inside, leaving behind no black hole or neutron star remnant. Instead the entire mass of the star is spewed out in the explosion with enough velocity that it does not fall back to form a neutron star or black hole. The result is huge amounts of ejecta in the form of heavy elements. In fact, the amount of matter ejected from the explosion would pretty much match up with the total amount of matter that would be theoretically available in the star. By studying the light curve of the explosion, it is estimated that SN2006gy ejected about 20 M⊙ of nickel-56 alone, a huge amount. That’s why it was so bright.

It’s not very often that a single event propels a section of theory from the purely theoretical to observed instance, but this event appears to be doing just that. Check out this new stellar evolution graphic courtesy of the Chandra team.

After analyzing the images from the Chandra X-Ray Observatory and ground-based observations from observatories like James Lick and the Keck Observatory, the scientists have determined that:

Of all exploding stars ever observed, this was the king… We were astonished to see how bright it got, and how long it lasted.”

Here’s a link swarm for SN 2006gy:

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6 thoughts on “SN 2006gy

  1. Pingback: AstroGeek I'm humbled «

  2. Pingback: AstroGeek SN2006gy and Eta Carinae Update «

  3. Pingback: Itty Bitty SuperNovae « AstroGeek

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