Where were the Supernovas?

The extraordinary red supergiant star Betelgeuse

(figure from Uranus, Neptune, Pluto)

has been our topic several times: its size, its name, and most recently the excitement over its unprecedented dimming and then recovery to even brighter than before, provoking speculation that it may be about to go supernova. This will happen – some time in the coming 100,000 years, a small fraction of its 10-million-year life. I guess the chance of its happening in, say, 2024 is 1 in 100,000.

(Plural supernovae or supernovas? I see I’ve been inconsistent.)

As explained in the Astronomical Companion‘s opening “Overview”:

Some stars undergo explosions which we see as the novae, throwing off shells of matter which become the ring-shaped planetary nebulae.  Finally the more massive stars explode as supernovae.  The matter they recycle into space, endowed with the heavy elements now formed, can give rise to another generation of stars and planets.  What is left of a star at the end of its active life collapses to form a dense white dwarf; or, if it is more massive, as a denser and smaller neutron star (detectable as a pulsar) or, more massive still, an almost infinitely dense black hole.

The terminology began with Tycho Brahe’s book about the nova  stella, “new star,” that he and others noticed in the sky in 1572, and which was what we now call a supernova.

Provoked by Betelgeuse, I have spent the past five days constructing a chart of the recorded supernovas. I’ve ended up plotting it in the galactic framework, that is, the two dimensions are parallel to and perpendicular to the midline of the Milky Way. Most of the observed supernovae were in our Milky Way galaxy but far from us, therefore appearing near the galaxy’s plane. A chart in equatorial coordinates (right ascension and declination)is much less compact vertically. The chart is still wide, since it has to go all around the sky, so I’ve broken it into two halves: centered on the center of the galaxy in Sagittarius, and on the anti-center, on the Auriga-Taurus-border.

To indicate the more familiar equatorial layout, the celestial equator is shown; and the star and constellation names are oriented east-west.

Supernovae look like stars, and they are marked with symbols sized proportionally to their apparent magnitude in the same way as the stars. The brightest seems to have been that of 1006 (around six times brighter than Venus, visible in daylight). This is their brightness as seen from Earth. Their absolute magnitude is something else; their distance from us ranged from 2,000 light-years for the supernova of 386 AD to 2,600,000 LY for that of 1885, which was in the Great Andromeda Galaxy, and 10,300,000 LY for that of 1895, which was in the galaxy NGC 5253.

The distance in light-years is the time in years between when the supernova exploded and when we see it. So the nova that Tycho saw in 1572 had actually happened 8,000 years earlier.

Observatories now record other supernovae in distant galaxies, which is why our list includes designations like SN 1987A.

The supernovae have four kinds of status: certain (definitely seen); uncertain (they may have been comets, or ordinary novae, or some other kind of stellar outburst); unobserved (a supernova remnant marks the spot, and its age and distance betray the date when it should have been observed); and future! Betelgeuse is the only future one I’ve attempted to indicate. Being much nearer than any of the others, it will probably be the brightest.

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