Thursday, August 25, 2011

A Supernova in Beautiful Galaxy M101

After starting my series of beautiful galaxies with Messier 100 I was amused to discover this morning that M101 (The Pinwheel Galaxy) is in the astronomical news after a possible Type 1a Supernova was discovered in it yesterday. So following M100 I obviously have to do M101 this morning!

M101 as seen in a mosaic of Hubble Space Telescope images added to some ground based data. Credit: NASA and Robert Gendler. For more info see APOD
M101 was classified in the Hubble Atlas as an Sc galaxies with a weak bar and a partial ring around the bar before the arms emerge (SAB(rs)c). It's illustrated twice in the atlas, with the largest illustration accompanied by text which describes it as the prototype of multiple armed Sc galaxies, what we might today call a "flocculent" spiral.

 According to the "Atlas of the Messier Objects", M101 is the third largest galaxy (in angular size) in the Messier list, and is visible as an extended object even through 10x50 binoculars (presumably only from a very dark site though). It definitely seems to be a popular deep sky object for amateurs, and following the SN1a discovery the AAVSO have put out a call for their members to get observing. Some predictions suggest the supernova is still brightening and could reach 10th magnitude - only 2 magnitudes fainter than Neptune (at its brightest).

 M101 has already been an important calibrating object for the extragalactic distance ladder having had its distance measured using many of the well known methods (e.g. the Leavitt Law (Cepheids), Tip of the Red Giant Branch, the Tully-Fisher relation). The most reliable distances have ranged from 6-8 Mpc (18-24 million light years or so) making the galaxy almost twice as large as our own Milky Way.

 The addition of a Type 1a Supernova in M101 would obviously be extremely exciting as a way to further link commonly used distance indicators. SNIa are an important type of object in astronomy - they are extremely bright, and therefore observable at very large distances. Much of the evidence for the acceleration of the expansion of the universe (which indicates the need for "dark energy") comes from measurements of SN1a at very large distances which suggest that in the past the universe was expanding more slowly than it is today!

 It will be interesting to see the result of the many follow-up observations of the SN in M101 - and exciting to think they are happening (on the other side of the world of course) as I write this.