Polyjuice. Once swallowed, the magic potion of the Harry Potter movies allows the user to assume the appearance of any person for just one hour. Starting in 2009, astronomers have witnessed something of this “polyjuice factor” in a star in a galaxy 66 million light years away. With one difference. The change appears permanent.
On August 26, 2009 the team of astronomers at the CHilean Automatic Supernovas sEarch (CHASE) discovered what they first thought was a brand new supernova in the galaxy NGC 7259 in the constellation Piscis Austrinus the Southern Fish. It even got one of those cool supernova names – SN 2009ip. But further study of older pictures taken by the Hubble Space Telescope as well as follow up observations revealed that the speed of the material blasted away by the star as well as its intrinsic brightness didn’t match that of a supernova. Astronomers realized they were seeing an outburst of a bright blue star called a luminous blue variable or LBV instead.
LBVs are brilliant, massive, extremely hot supergiant stars that burn their fuel rapidly and live short lives of several million years compared to the sun’s ~10 billion year lifetime. Just to give you an idea, our supernova imposter packs away 50 to 80 times the mass of the sun and blazes hundreds of thousands of times brighter. Radiation raging from the interiors of LBVs causes the stars to sputter and spew portions their atmosphere into space. Some slough off the equivalent of the sun’s mass in just 10,000 years. Spendthrifts of the universe.
Along with losing mass, an LBV’s light varies almost continuously from day to day with bigger variations over longer time spans caused by large outbursts that release lots of matter into an ever-expanding envelope centered on the star.
Continued observation of SN 2009ip turned up a second outburst in 2010. This was further confirmation that the star didn’t destroy itself in a supernova explosion after all but returned to shine another day.
Then in July It re-brightened again, but this time astronomers at several observatories, using spectrographs to tease apart the star’s light, discovered dramatic changes in its appearance. You know what they say – the third time’s the charm.
Not only was it much brighter than the earlier outbursts, but the hotheaded sun was blasting out matter at supernova-style speeds of over 8,000 miles per second. Additional details of the star’s spectrum or light fingerprint all pointed to one thing: 2009ip appears to have become a real supernova right before our eyes!
Its rapid surge in brightness indicates that high-speed material flung from the host star has crashed into the older envelope of matter shed by previous eruptions. What’s driving all this new activity? As described earlier, big stars are the gas-guzzlers of the universe, rapidly consuming every drop of nuclear fuel in their cores. The pressure from burning keeps gravity at bay, but when 2009ip’s needle hit empty, the fire went out and its core caved in to the force of gravity, creating a shock wave that ripped the behemoth to shreds. Bada boom! A star dies, a supernova is born.
Supernovas located in galaxies in the far southern sky are not easy to see from a northern place like Duluth, Minn., yet this one grew bright enough earlier this month to stand out clearly in my 15-inch scope. SN 2009ip is still hanging in there at magnitude 13.9, making it a fine showpiece for enterprising amateurs. To keep tabs on it and see more photos, I recommend a trip to David Bishop’s special page on the object. If you’d like to read a detailed analysis, check out this paper on the topic by a team of astronomers at the Universities of Arizona and California.
Since stars live exceptionally long lives far in excess of the human time scale, we normally have to observe thousands of them and piece the observations together using computer models to build a full picture of a stellar lifetime. Seeing 2009ip flirt with destruction and then finally detonate in the space of three years offers astronomers – amateur and professional – a rare glimpse of stellar evolution in real time.