I like to say that the cosmos is like a clock, many objects and events undergo cycles that can be measured and understood. Our calendars and clocks are ultimately based on astronomical processes, such as the rotation of the Earth and its orbit around the sun.
Other objects also store a calendar, but may not check the clock as often as they should. They go late.
This appears to be the case for the T Coronae Borealis or T Cor Bor star system. It brightens dramatically every 80 years or so, going from obscurity to one of the 200 brightest stars in the sky in a matter of hours. That cadence really makes each of his flare-ups a “once in a lifetime” event. The last time it did it was in 1946, so you can expect it won’t happen again until 2026, two years from now. This particular object began showing signs of an explosion more than a year ago, however, and astronomers updated their appointment books to reflect it.
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And then nothing, at least not yet. It’s going to blow up, we’re sure, but it might not be for another year. Or he could go tonight.
T Cor Bor is a binary star, or two stars orbiting each other. One, usually the brighter of the two, is a red giant, a star slightly more massive than the sun the end of his life. Complicated processes at the core of stars cause the outer layers to grow and cool. It becomes much brighter as it grows—it emits much more light—but the cooler gas in its expanding outer layers turns the star red. It is estimated to be approx 75 times wider than the SunAt more than 100 million kilometers in diameter, it is large enough that if it were replaced by our star, it would stretch almost to the orbit of Venus.
The other star is much deader. It started out like the sun and went through the red giant phase. Over time it blew away its outer layers, revealing a white-hot core: a white dwarf. Only the size of the earth, but more massive than the sun terribly warm and dense, but its small stature makes it much weaker than its bloated companion.
Despite its Lilliputian nature, the white dwarf’s density gives it immense gravity. The two stars are so close, only about 75 million kilometers apart, that the white dwarf can physically pull material away from the red giant. This puts T Cor Bor in a second star category: it is not a binary star system, but also a symbiotic one.
The siphoned material from the red giant moves towards the white dwarf, but cannot sink into it. Because the two stars orbit each other, the falling material has angular momentum, the tendency of a spinning object to keep spinning. As it moves toward the smaller star, it speeds up that sideways motion, just as water accelerates as it flows down a bathtub drain. This material forms a flattened disk around the white dwarf called the accretion disk. Matter, mostly hydrogen, falls onto the surface of the white dwarf from the inner edge of the disk.
But all that added material comes with a problem. Over time, hydrogen diffuses and accumulates on the surface of the white dwarf. The amount that falls is small in astronomical terms, only a few billionths of the sun’s mass each year (more humanly speaking, about one seventh of the mass of the moon!). But remember that the gravity of a white dwarf is strong, 100,000 times that of Earth. As the hydrogen accumulates, it heats up tremendously and eventually becomes so compressed that it undergoes catastrophic nuclear fusion. This is it explodes like a thermonuclear bomb—or, really, several trillion of them.
The blast from the explosion spreads rapidly, releasing a large amount of energy. At its peak the explosion emits 1,000 times more light than the two stars combined, and they’re already hundreds of times brighter than the sun, so it’s a big deal.
Seen from Earth, the result is a “new” star, called a nova, that suddenly shines in the sky. But there is more. After the explosions subside and the white dwarf settles, the process repeats itself. The red giant begins to feed on the white dwarf, and the matter accumulates, squeezes, and explodes again: foam, rinse, repeat.
Astronomers have seen T Cor Bor erupt its peak twice in the past, in 1866 and 1946. (There are also previous less conclusive reports 1217, as well as in 1787 of a remarkably bright star appearing suspiciously close to the location of T Cor Bor.) This repetition makes a subclass of the new system called a repeating nova.
But T Cor Bor has some behaviors that frustrate the best predictions astronomers can offer today. In 1938, about eight years before it last exploded, the system became slightly brighter, entering what astronomers sometimes call an excited state. This happened again in 2015, pushing the expected eruption date to 2023. In 1946 it dipped in brightness for a little more than a year before blowing upand the same jump was seen last year. Given this, astronomers adjusted the predicted date to early 2024, possibly as late as September.
Well, as I write this in October, I’ll speak for all the astronomers involved: hooray. But we really have nothing to apologize for: the estimate of the explosive brightness of T Cor Bor is statistical in nature, so it has a high degree of uncertainty. It could just as easily explode before the end of the year or maybe as early as next year. Either way, it should happen soon.
And when it does, astronomers will point telescopes from the ground and into space during the event, hoping to gather as much information as possible to better understand how and why T Cor Bor brightens and then darkens before it explodes.
How bright will he be when he finally decides to deliver? It’s hanging around right now magnitude 10.0too faint to see without large binoculars or a telescope. It should brighten to magnitude 2.0, about the brightness of the star Ursa Major. This would make it easy to see even in light polluted skies.
How can you tell when it finally blows? It is in the constellation Corona Borealis, the crown of the north. This is far enough north in the sky that everyone in the Northern Hemisphere can see it. Over the next month or so, you can find the constellation in the dark and looking west after sunset. The bright orange star Arcturus will be low on the horizon. The Corona Borealis will be a curved arc, like the letter C, about 20 degrees above it (twice the length of your outstretched fist). When T Cor Bor hits, it should be brighter than any star in the immediate constellation, outside the star curve.
Unfortunately, starting in November, the Corona Borealis will be below the horizon at sunset. To see it, you will have to get up before sunrise, around 4 am, when it will be low on the horizon in the north-northeast. As time goes on, it will move higher before sunrise, making it easier to see.
I will note that many of the news reports I have seen about T Cor Bor give the impression that this star will rise to glory and be a gasp-worthy sight in the night sky. It will actually be as bright as an average star. This is still cool and worth checking out though! Only a handful of recurring novae are known in our galaxy, and even fewer are bright enough to detect without optical aid. So while you may not rival Venus in the sky, knowing the reality behind what you’re seeing—two stars locked in a dance of death and dying—results in a soul-steaming explosion that surpasses anything you can imagine. look at
