At left is a "before" photo of M101, also called the Pinwheel Galaxy, with no supernova visible. At right is a picture taken with a 20" telescope on August 25 showing the new supernova. The new object is located southwest of the galaxy's core. Credit: Jim Misti (left), Joseph Brimacombe (right)
One of the closest supernovas in years was discovered at a very early stage in the bright galaxy M101, the Pinwheel Galaxy, in Ursa Major. The galaxy can be found just above the Handle of the Big Dipper in the northwestern sky at the end of evening twilight. The supernova, dubbed PTF11kly, was discovered only two days ago by the Palomar Transient Factory, an automated wide-field sky survey looking for “optical transients” or things that pop up out unexpectedly like supernovae.
This map shows the sky facing northwest at the end of evening twilight. M101 is an 8th magnitude milky smudge above the end stars in the Handle. You'll need a 6-inch or larger telescope to see it well. You can use the closeup photo at top to navigate your way to the supernova. I'll provide a more detailed finder map tomorrow. Created with Stellarium
At discovery, the exploding star was very faint, but quickly rose in brightness. In just one night’s time, it shot from a dim 14.8 magnitude (Aug. 25) all the way up to 13.7 last night. That’s better than a full magnitude or a factor of 2 1/2. Through my 15-inch scope, it was plainly visible.
While the supernova won’t become bright enough to see with the unaided eye, it almost certainly will be visible in small amateur telescopes within the next few days. That’s because the host galaxy is only 25 million light years from Earth — close by galaxy standards. With no moon in the sky for the next week or so, professional and amateur astronomers around the world will have PTF11kly in their crosshairs.
In a Type Ia supernova, a white dwarf (left) draws matter from a companion star until its mass hits a limit which leads to collapse and then explosion.
Since there’s more than one way for a star to explode, supernovae come in several varieties. This supernova in M101 is a Type Ia (one-a), a rather dry term describing one of the most catastrophic events in the universe. Here a superdense white dwarf, a star only about the size of Earth but with the gravitational power of a sun-size star, pulls hydrogen gas from a nearby companion down to its surface where it adds to the star’s weight. When the dwarf packs enough pounds to reach a mass 1.4 times that of the sun, it can no longer support itself. The star suddenly collapses in upon itself and detonates in an explosion that blows it to bits. What we see here on Earth is the sudden appearance of a brand new star within the galaxy’s disk. Of course, it’s not really a new star, but rather the end of one.
Artist's view of a white dwarf exploding as a supernova. Credit: ESO
You might recall the other bright supernova earlier this spring in the Whirlpool Galaxy not far from the Big Dipper. That one was a Type II variety, where a supergiant star runs out of nuclear fuel, collapses and then explodes. What’s unique about the Pinwheel Galaxy supernova was that it was caught in the very early stages.
Since most supernovae are in distant galaxies, we rarely get to see the start of a supernova event, because the star is too faint at the beginning of the explosion. Only when it’s well along, does it finally get bright enough to be picked up through a telescope.
M101 is practically a next door neighbor at just 25 million light years, allowing us to study a Type Ia supernova from almost the very start. Astronomers are eager for all the information they can get on this event to help them and us better understand the supernova process. For more photos of the “new star”, check out Dave Bishop’s Latest Supernovae website.