An exploding star in M74 in Pisces, discovered July 25th, remains just in reach of amateur telescopes.
|Update September 5th: Supernova 2013ej held at its peak magnitude of 12.5 for the first half of August even as it gradually reddened (or more accurately, yellowed). As of September 5th it had faded to V magnitude 13.2 and had reddened much further. See up-to-date light curve.
Messier 74 is a relatively bright (10th magnitude) spiral galaxy some 30 million light-years away. Located in Pisces, it's well up in the eastern sky by midnight or 1 a.m. as of mid-August.
The supernova lies 92 arcseconds east and 135 arcseconds south of the galaxy's core. The coordinates are right ascension 1h 36m 48.16s, declination +15° 45′ 31.0″. For judging its brightness yourself, print out this ½°-wide comparison-star chart from the American Association of Variable Star Observers (AAVSO).
Spectra taken soon after the discovery show a blue continuum with distinct Balmer emission lines from hydrogen atoms that are typical of a young Type II supernova. This means the blast resulted when a single massive star collapsed inward onto its unstable core and exploded.
Type II supernovae typically shine with roughly 1 billion times the Sun's luminosity for a couple of weeks. At M74's distance, that would be magnitude 11.5. However, the peak luminosities of Type II supernovae can differ by several magnitudes.
It turned out that M74 had been targeted more than once by the Hubble Space Telescope, and images taken in November 2003 and June 2005 appear to show the progenitor: a 25th-magnitude star that was particularly bright at near-infrared wavelengths and likely an M-type red supergiant before it destroyed itself.
Rubab Khan (Ohio State University) also reported that the Spitzer Space Telescope showed a candidate star in the supernova's location that (in the infrared, at least) outshines the Sun by up to 30,000 times.
Located at Lick Observatory, the KAIT telescope is a 0.76-m dedicated reflector that scans the sky each clear night for exploding stars. It revisits each of roughly 20,000 galaxies every 2 to 12 days. KAIT's singleminded searches have been highly successful: since 1998, this telescope has discovered nearly 900 supernovae.