Milky Way Supernova Candidates

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 MO7.5 giant star and a 9.0 MWolf-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.

Firestone, R.B., 2014, ApJ, 789, 29
Harper, G.M., Brown, A., Guinan, E.F., et al., 2017, AJ, 154, 11
Richardson, N.D., Russell, C.M.P., St-Jean, L., et al., 2017, MNRAS

Brightest Event Ever Observed

On June 14, 2015, perhaps the intrinsically brightest event ever recorded was detected at or near the center of the obscure galaxy APMUKS(BJ) B215839.70−615403.9 in the southern constellation Indus, at a luminosity distance of about 3.8 billion light years.

ASASSN-15lh (All–Sky Automated Survey for SuperNovae), also designated SN 2015L, is located at α2000=22h02m15.45s, δ2000=-61° 39′ 34.6″ and is thought to be a super-luminous supernova—sometimes called a hypernova—but other interpretations are still in play.

Let’s put the brightness of SN 2015L in context.  Peaking at an absolute visual magnitude of -24.925 (which would be its apparent visual magnitude at the standard distance of 10 parsecs), SN 2015L would shine as bright as the Sun in our sky if it were 14 light years away—about the distance to van Maanen’s Star, the nearest solitary white dwarf.  SN 2015L would be as bright as the full moon if it were at a distance of 8,921 light years.  SN 2015L would be as bright as the planet Venus if it were at a distance of 333,000 light years.  Since the visible part of our galaxy is only about 100,000 ly across, had this supernova occurred anywhere in our galaxy, it would have been brighter than Venus.  If SN 2015L had occurred in M31, the Andromeda Galaxy, 2.5 million light years away, it would take its place (albeit temporarily) as the third brightest star in the night sky (-0.47m), after Sirius (-1.44m) and Canopus (-0.62m), but brighter than Alpha Centauri (-0.27m) and Arcturus (-0.05m).

The Open Supernova Catalog (Guillochon et al. 2017) lists three events that were possibly intrinsically brighter than SN 2015L.  Two events were afterglows of gamma ray bursts GRB 81007 and GRB 30329: SN 2008hw at -25.014m and SN 2003dh at -26.823m, respectively.  And the other event was the first supernova detected by the Gaia astrometric spacecraft, Gaia 14aaa, 500 Mly distant, shining perhaps as brightly as -27.1m.

Chatzopoulos E., Wheeler J. C., Vinko J., et al., 2016, ApJ, 828, 94
Dong S., Shappee B. J., Prieto J. L., Jha S. W., et al., 2016, Science, 351, 257
Guillochon J., Parrent J., Kelley L. Z., Margutti R., 2017, ApJ, 835, 64