Thursday, January 9, 2014

Astronomers Discover a Double Take of Exploding Stars in a Distant Galaxy

Hubble Space Telescope image of the spiral galaxy NGC 6984. The bright orange dot to the lower right of centre marked with an arrow is a recently discovered exploding star

Dairy farmer and amateur astronomer Stuart Parker from New Zealand has discovered a giant star exploding in a distant spiral galaxy named NGC 6984.  The stellar explosion, called a “supernova” by astronomers, was observed for the first time in July 2013. Supernovae are very common, however this explosion was remarkable in that it appeared to occur in virtually the same position as a supernova named SN 2012im that had been found the year before in the same galaxy. Following his discovery, Parker immediately informed professional astronomer Dan Milisavljevic at the Harvard-Smithsonian Center for Astrophysics (CfA) who initiated a cascade of follow-up observations using international observatories to confirm the discovery and study the stellar explosion. The Southern African Large Telescope (SALT) located in the northern cape just outside Sutherland was the first professional telescope facility in the world to respond, within an hour of the original discovery,  and has confirmed the reality of the supernova which has been officially named as SN 2013ek by the International Astronomical Union.

Every couple of seconds a star explodes somewhere in the universe. These tremendously energetic events called supernovae shape the galaxies they live in and seed the cosmos with the raw materials of life. During the explosion the supernova is incredibly bright and can even be brighter than its host galaxy. However, the supernova quickly fades and may no longer be visible weeks to months after the initial explosion.

Telescopes located around the world - from remote, high altitude professional observatories to the backyards of amateur enthusiasts - find and study as many supernovae as possible. Sometimes it is possible to find two or more supernovae in random locations of the same galaxy over the course of years or decades. This past year,
however, astronomers were surprised to learn that sometimes supernovae can even be found right on top of one another!

The question astronomers had to answer in this case was were the two explosions related? Or was this a chance alignment of two completely independent supernovae? 

Milisavljevic believed that the rare discovery of two supernovae located so close to one another warranted a closer look with the Hubble Space Telescope (HST). He and collaborators asked for special permission to interrupt regular operations with HST to take a closer look at the supernova while it was still bright. Their request was approved and the resulting images obtained in August 2013 showed two separate objects in the region of Parker's discovery. One source was identified as the newly discovered SN 2013ek, but the other source was something unknown and possibly the previous supernova SN 2012im. 

Though the offset between the two sources in the images is very small, they are separated by an angle of only a thousandth of a degree, at the distance of the host galaxy (approximately 200 million light years away) it implies a large physical distance that makes an association between the two explosions nearly impossible.

Milisavljevic stresses that the nature of the second source is not certain and could easily be an unrelated nearby cluster of stars. Another observation with the Hubble Space Telescope could firmly resolve whether the second source is actually SN 2012im. He plans to observe the galaxy again this year.  "By waiting a year to take another set of images with the Hubble Space Telescope," says Milisavljevic, "we can do a nose count to see what is left behind." 

Milisavljevic is presenting the results of his study of supernova SN 2013ek at the 223rd annual meeting of the American Astronomical Society during January 5-9, 2014.

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