One of the special joys of getting out under a dark rural sky this time of year is seeing the gossamer beauty of the surprisingly expansive star cluster called Melotte 111, also known as the Coma star cluster. Mel 111 makes up a large part of the constellation Coma Berenices, “Berenice’s Hair”. This constellation, which entertains the North Galactic Pole as well as a gaggle of galaxies, can be found about midway between Denebola (some call the Coma star cluster the end of the “tail” of Leo the Lion) and Arcturus, as well as midway between Spica and the Big Dipper. Coma Berenices is transiting the meridian this week as evening twilight ends. At a distance of just 284 light years, the Coma star cluster is the third nearest star cluster to us, surpassed only by the open cluster remnant Collinder 285—the Ursa Major association (80 ly)—and the Hyades (153 ly).
There is a supermassive binary star in our own Milky Way Galaxy that has the potential to create a super-supernova (hypernova?). It could go off tomorrow—or a million years from now. The star system’s name is Eta Carinae. Currently 4th-magnitude and located some 7,500 ly away in the direction of the southern constellation Carina (“The Keel”), Eta Carinae consists of a 100-200 M☉ star and a 30-80 M☉ star in a highly-eccentric 5.54y orbit with the more massive star undergoing prodigious mass loss. Eta Carinae never rises above the horizon unless you’re south of latitude 30° N. So, if Eta Carina ever does go supernova while humans still walk the Earth, you’ll have to travel at least as far as southern Texas or southern Florida to see it. And it will be an impressive sight, easily visible during the daylight hours.
Closer to home, there are seven prime candidates for the next relatively nearby supernova. The nearest of these currently is IK Peg. Keep in mind that over hundreds of thousands of years, stars move quite a lot, so what is close to us now will not necessarily be close to us when a supernova event finally does occur.
IK Pegasi, a binary system comprised of a white dwarf already near the Chandrasekhar limit, and a close-by soon-to-be-giant main-sequence star, lies just 147 to 155 ly away in the direction of the constellation Pegasus, the Winged Horse. IK Peg appears to us visually as a 6th magnitude star located roughly ⅓ of the way from Delphinus to the Square of Pegasus. As the giant star expands into the vicinity of the white dwarf, the white dwarf will accumulate enough material to put it over the Chandrasekhar limit, and a Type Ia supernova will ensue.
Spica (α Vir), located at a distance between 237 and 264 ly, is a massive binary system (10 M☉ and 7M☉), with the two stars orbiting each other every four days.
Alpha Lupi (α Lup) is a massive star (~10 M☉) located between 454 and 476 ly from our solar system.
Antares (α Sco) is a massive star (~12 M☉, the supernova progenitor) orbited by another massive star (~7 M☉). However, their orbital period is at least 1,200 years. The Antares system lies between 473 and 667 ly from our solar system
Betelgeuse (α Ori) is a massive star (~12 M☉) between 500 and 900 ly away. Incidentally, there is a lot of uncertainty about the distance to Betelgeuse, primarily because it’s angular size (44 mas) is an order of magnitude larger than its parallax (4.5 mas) (Harper et al. 2017).
Rigel (β Ori) is a massive star (~23 M☉) between 792 and 948 ly distant.
Gamma2 Velorum (γ2 Vel) is a binary system 1,013 to 1,245 ly distant containing two stars which will go supernova in the not-too-distant future. The system consists of a 28.5 M☉ O7.5 giant star and a 9.0 M☉ Wolf-Rayet star (the nearest, incidentally) orbiting each other every 78.5 days. The Wolf-Rayet star will be the first to supernova, followed later by the O giant star.
Tomorrow—or a million years from now? We have no way of accurately predicting. But rest assured, in the unlikely event that any one of these stars goes supernova during our lifetimes, none will be close enough to harm us. Instead, for a time, we will be treated to a object comparable to the Moon in brightness and visible both day and night.
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