A big star exploded as a supernova in the lovely face-on spiral M101 in Ursa Major this month. At a distance of 20 million light years, this is the closest supernova in five years and the first in this galaxy since 2011. The new supernova isn’t close enough to see with the unaided eye, alas, but it lies within reach of a 5” or larger telescope for visual observers (as of the end of May 2023) and it offers an easy target for imagers.
Catalogued as SN2023ixf, the supernova was discovered by Japanese amateur astronomer Koichi Itagaki on May 19 when it had brightened to magnitude 14.9. Within less than 12 hours, the star continue to brighten to magnitude 13.5, an increase of near four times. Astronomers examined data collected by the automated Zwicky Transient Facility (ZTF) and found evidence of the supernova two days before Itagaki’s discovery, but it went unnoticed for several days. As of May 31 the supernova appears steady at magnitude 11 and may have reached its peak brightness.
The spectra of the star show absorption lines of hydrogen gas in its outer shell. This means it’s a Type II supernova, one caused by a massive star that’s run out of fuel in its core. The star could no longer hold itself up against the pull of its own gravity. The outer hydrogen-rich layers of the star suddenly collapsed towards the core then rebounded in an immense explosion that violently ejected most of the star’s mass along with light and neutrinos. Initial searches for the progenitor star in past images of M101 suggest it was a red supergiant of about 15 solar masses, consistent with our understanding of how these stars expire.
While it’s an exciting time for earthbound observers, it’s a bad time indeed for anyone in the vicinity of the supernova. Estimates suggest any earth-type life within about 160 light years of such an explosion would feel its damaging effects.
You can find M101, also known as the Pinwheel Galaxy, in Ursa Major just above the handle of the Big Dipper asterism. It forms a nearly equilateral triangle with the stars Mizar and Alkaid. The supernova itself lies near a little knot of nebulosity within the galaxy catalogued as NGC 5461. If you have a computerized telescope, you can dial in its exact coordinates which are RA 14h 03m 38.58s and Dec 54°18’42.1”. SN2023ixf lies about 3.8’ east and 2.2’ south of the galaxy’s nucleus.
M101 is a beautiful object in dark sky, but urban observers struggle to see much in light-polluted skies. But the supernova, a point source of light, is a much easier visual target. It’s also easily captured in images or with quick EAA snapshots. Because of its northerly declination, the supernova is not visible to observers in the deep-southern hemisphere.
SN2023ixf will slowly fade from view over the coming weeks, although at an uncertain rate. Type II supernovae typically fade at a rate of about 0.008mag/day. As it fades, it will leave behind, perhaps, a tiny nebula and a neutron star similar to the Crab Nebula (M1), itself a remnant of a Type II supernova in 1054.
And here’s a fun thing to think about: light from more supernovae which have already exploded are on the way to us from M101 and millions of other galaxies. As many as 50,000 supernovae have exploded in the past 2 million years in the Andromeda Galaxy (M31) alone, but their light is still in transit and awaits detection by future astronomers on Earth.
The last supernova visible without optics appeared in 1987 when a star exploded near the Tarantula Nebula in the Large Magellanic Cloud. No supernova have been detected in our Milky Way since 1604. We’re due for one soon, don’t you think?
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