When a new supernova is discovered, we can take that as a reminder that we live in a terribly hostile Universe. Sometimes stars just explode and devastate a corner of a galaxy. On average, a supernova goes off twice a century in a galaxy the size of the Milky Way. Since there are potentially hundreds of billions of galaxies out there, dozens of supernovae are detonating every second in the observable Universe.
The last bright supernova was SN 1987A, located in the Large Magellanic Cloud, about 168,000 light years away. Even though it was far, it exploded with so much energy it was visible to the unaided eye. That one wasn’t even in our galaxy. The Milky Way’s most recent supernova that we know of was G1.9+0.3, recently confirmed by the Chandra X-Ray Observatory. It would have been visible from Earth about 100 years ago, but it was located in the dusty regions of the Milky Way and obscured from our view. The last bright supernova was discovered in 1604 by the astronomer Johannes Kepler. This was a naked-eye supernova, in fact, at its peak, it was brighter than any other star in the night sky and for a few weeks it was even visible during the day.
So, which star is likely to explode next? Can we even know that?
Here are three stars, two visible in our night sky and one too dim to see but potentially very dangerous, that in the future are going to explode.
At no.3 we have Antares. It is on average the fifteenth-brightest star in the night sky, and the brightest star in the constellation of Scorpius, which is expected to go supernova sometime after about 10,000 years from now, though estimates there vary. It is the brightest, most massive, and most evolved stellar member of the Scorpius–Centaurus Association, that has long been gazed at by astronomers throughout history because while it is not only bright, it is also visibly reddish in color unlike most other stars. Betelgeuse, like Antares, is too distant to pose a threat, but one very unusual even more distant star could go sometime within the next few hundred thousand years.
Fig 1.1: Complete 105 Hubble orbits
Image Credit: NASA, ESA, M. Robberto
Oddly, the universe is full of red stars, but the naked human eye is not great at detecting the colour red if it’s from a dim point source like a star. As a result, we see astronomical objects with red in them, such as in fig 1.1, we see the great Orion nebula; a diffuse nebula situated in the Milky Way, being south of Orion’s Belt in the constellation of Orion, located at a distance of 1,344 ± 20 light years and is the closest region of massive star formation to Earth; as bluish-green or white until we either have a telescope large enough to bring out the reddish hues or we have a red object that’s really bright.
Antares is one such object. But Antares is living on borrowed time and is expected to explode with enough intensity to reach or exceed the brightness of the full moon as viewed here on earth, perhaps enough to cast shadows at night before dimming back down. And while it will be spectacular for future observers, it should pose no threat to Earth due to Antares being fairly distant at about 600 light years.
Fig 2: Composite color image of the Herschel PACS 70, 100, 160 micron-wavelength images of Betelgeuse.
Image Credit: ESA/Herschel/PACS/L. Decin et al
There is an even more prominent star that’s set to go supernova. The constellation of Orion is dominated by yet another red star, bright Betelgeuse. In the night sky, Betelgeuse is easy to spot with the naked eye owing to its distinctive orange-red color. In the Northern Hemisphere, beginning in January of each year, it can be seen rising in the east just after sunset. It’s the 9th brightest star in the sky, and its name, though weird, is actually a distortion of what Arab astronomers called it; Ibṭ al-Jauzā; long ago to denote it as, well, what it is: the armpit of Orion the hunter.
Betelgeuse, like Antares, is too distant to pose a threat, but one very unusual even more distant star could go sometime within the next few hundred thousand years. I’ve been looking at it since I was a kid, and still gaze up at it to this day on cold, dark winter nights, usually while fumbling with my keys. Classified as a red supergiant of spectral type M1-2, the star is one of the largest and most luminous stars visible to the naked eye. If Betelgeuse were at the centre of the Solar System, its surface would extend past the asteroid belt, wholly engulfing the orbits of Mercury, Venus, Earth, Mars, and possibly Jupiter. Betelgeuse also happens to be one of the few stars we’ve been able to resolve and directly image its disc. This is due to the star appearing absurdly huge due to it tossing off layers of itself into space. It is a roiling, crazy star that is known to have shrunk by 15 per cent in less than two decades. And it doesn’t have long to live, probably less than 100,000 years though it could go at any moment, and when it does, like Antares the explosion will be prominently visible from earth.
Fig 2.1: Showing Betelgeuse and the dense nebulae of the Orion Molecular Cloud Complex (Rogelio Bernal Andreo). The red crescent shape is Barnard’s Loop.
But signs Betelgeuse’s end are apparently already evident. The 2nd Century Astronomer Ptolemy working in the then Roman controlled city of Alexandria described the star in the Greek of the period as hypokirhhos, meaning ruddy or reddish. It looks like that now. But not so to Chinese astronomers three centuries before Ptolemy. They made it clear that the star was yellow. And it is indeed possible that the star was, at the time, a yellow supergiant that has since evolved.
1. WR 104
Fig 3: WR 104
Image Credit: NASA
This star is called WR 104, and it’s too dim to see with the naked eye. WR 104 is part of a triple star system. It’s a type of star known as a Wolf-Rayet star, abbreviated as WR, with a B0.5 main sequence star in close orbit with another more distant fainter companion located about 7500 light years away.
We don’t yet have a solid understanding of what a star of this type would do during a supernova leaving open a small chance that it could do damage to earth. It comes down to gamma ray bursts. Some scientists think that a supernova from this star could produce one, and if the star’s poles are aligned with earth, we could receive a dose of gamma radiation that could cause a mass extinction on earth, though the odds are very low for this happening. There is no consensus on this star, and other scientists think that it’s unlikely for a gamma ray burst to be produced at all, and the odds of the star’s poles being aligned with us are low.
Plus, at that great distance, the geometry of the burst would play a major role. If the jet of gamma rays is wide, it will diffuse over distance and won’t do any damage even if it is pointed at us. If it’s more focused, it could pose a threat, but the narrower the beam, the less chance there is of it lining up with earth. The chances are low, but it is possible, that WR 104 could damage our planet sometime over the next few hundred thousand years. That may seem like a long time, but for perspective, in 100,000 years certain species of earthworms on the North American continent will have just recovered their former range in Canada after having been driven out by the advances of the Laurentide Ice Sheet thousands of years ago.