Former Supergiant Star In Whirlpool Galaxy Goes Missing

Supernova 2011dh photographed in a spiral arm of the Whirlpool Galaxy on July 4, 2011 from Port Wing, Wis. The other stars you see in the photo are foreground stars in our own Milky Way galaxy. Credit: William Wiethoff

On May 31, 2011 a supernova suddenly appeared in M51, a bright spiral galaxy near the end of the Big Dipper’s handle. Better known as the Whirlpool Galaxy, M51 is one of the most picturesque galaxies in the sky and the first in which spiral structure was seen. The Whirlpool is on every amateur astronomer’s “must see” list because it’s bright, close (23 million light years) and one of the few galaxies that shows a spiral shape in smaller telescopes.

A supergiant star collapses and explodes when it runs out the fuel needed to keep gravity at bay. Often the remnant core further collapses into a neutron star or a black hole. Credit: NASA

The supernova, dubbed 2011dh, peaked in brightness several weeks later and then gradually faded from view. Astronomers determined it was a Type II explosion. Type II supernovae occur in supergiant stars at least 8 times more massive than the sun that burn through the nuclear fuel in their cores until it’s exhausted. When the burning stops, so does the heat pressure that counteracts the ever-present force of gravity. Result: the star collapses in upon itself, creating shock waves that blast it to bits in a titanic explosion. The enormous energy released makes the former supergiant suddenly brighten by millions of times.

Relative size of a supergiant down to a black hole. Exploding supergiant stars sometimes leave a remnant neutron star or black hole in their wake. The crushing forces of collapse push electrons into protons to create neutrons, hence the name.

Often the core continues collapsing into a tiny, city-sized neutron star or takes the final plunge and squeezes itself into a black hole. This weekend a team of astronomers using the Hubble Space Telescope report that the yellow supergiant star that went supernova two years ago has vanished. Gone.

Seems obvious, so what makes it interesting? When a star becomes a supernova, one of the first things astronomers do is go back and look at old pictures of the galaxy in which the supernova occurred to identify the original star called the “progenitor”. Because stars in distant galaxies are extremely faint and difficult to separate from others in their neighborhood, they can be hard to identify.

NASA supernova animation

Now that the suspected supergiant star has disappeared,  we’ve clinched the identity of the star before the explosion. That key data point helps astronomers unravel the evolution of supernova 2011dh from a yellow, hydrogen-burning behemoth to its present state as an expanding shock wave riddled with the former star’s innards.

It also throws a bit of a wrench into our understanding of how stars evolve. The progenitor star began its life with 13 times the sun’s mass and became 100,000 times more luminous than the sun by the time it blew. Yellow supergiants aren’t typical supernova candidates unlike the red supergiant class, whose most famous member is Betelgeuse in Orion. That means that once again astronomers will need to reexamine theories. As for remnants of 2011dh, if there are any, they’ve yet to be found.

Pre-supernova image (left) of 2011dh taken by the Hubble Space Telescope and Gemini NIRI post-supernova image (right). Credit: NASA/ESA and Gemini Observatory

As with people, so with stars – to understand the adult, know the child. The animation shows the whole process from supergiant to supernova followed by the expanding blast wave-gas shell and finally a rapidly-rotating neutron star beaming jets of particles into space.

The moon will shine above Antares tomorrow morning March 4, 2013. The map shows the sky facing south around 5:30 a.m. local time. Stellarium

We have several supernova candidate stars easily visible in the sky from Earth. Betelgeuse in Orion is one, so is Antares, the brightest star in the constellation Scorpius the Scorpion. Both are red supergiants. Tomorrow morning March 4 the waning moon pays a visit to Antares in the wee hours before dawn. Take a look if you happen to be out around 5-6 a.m. and consider its fate.

10 Responses

  1. What you may have forgotten (as had I, though I had written about it then 🙂 is that while astronomers had soon figured out that the yellow supergiant was precisely at the position of the supernova, several clever arguments were made in the scientific literature at the same time that it could *not* be the progenitor and thus had to be a companion to another star that actually exploded. Prepare for another round of papers now …

    1. astrobob

      I remember now that you bring it up. That’s great – they were right in the first place. They’re probably buying rounds of beer.

        1. astrobob

          Some supergiants like Betelgeuse, Rigel and Antares are bright and visible with the naked eye. Supergiants in other galaxies require very large telescopes and long time exposures using sensitive cameras.

          1. astrobob

            Astronomers use the Hubble Space Telescope, the twin 10-meter Keck Telescopes in Hawaii and many others.

  2. Edward M. Boll

    I first noticed Antares in May of 86 when it was a little below Saturn. Halley was then getting faint. Panstaars is now at 2.0 magnitude.

  3. H.Bob

    Astro-Bob, are there other examples of a star that has gone missing? It strikes me as an incredibly rare event for us humans to catch given the vastness of time. I can appreciate that across the universe as a whole they might be popping out like firecrackers, but in the narrow slices of the universe we can see, and in the extreme narrowness of a couple of decades of decent astronomy, it must be so rare to actually witness a light going out (over weeks or years etc) ?

    1. astrobob

      There are probably a lot of “missing” stars. When a star becomes a supernova it blows up. If we’re lucky it leaves a neutron star remnant we can detect. Supernovas happen all the time – to find one both professional and amateur astronomers photograph thousands of galaxies all the time.

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