Bacchus and Ariadne – Guido Reni (Italy, Bologna, 1575-1642) circa 1619-1620. (Crop and constellation emphasis is mine.)
Introduction
The image in the header shows the Greek god Dionysus (Bacchus to the Romans) consoling princess Ariadne with a constellation of her own. The constellation, called Corona Borealis (Northern Crown), is the site of a long-anticipated brightening of a recurrent nova explosion from one of its stars. I’ve delayed this blog entry too long and hope it goes online before the nova explosion occurs. It’s late August as I write, and so far so good. ;-)
The Blaze Star, AKA T Cor Bor or T Coronae Borealis, will be just the ticket for smart scope users. T Cor Bor is due to brighten some eight magnitudes in a nuclear flash sometime this year or next. Astronomers classify it as an RNe or recurrent nova.
Don’t confuse a nova event with a supernova. Supernovas only flash brilliantly once at the time of their death. They then either leave some sort of stellar corpse or totally annihilate themselves. Supernovas are relatively rare and intrinsically much brighter than a nova. About 25-75 novas go off in our galaxy each year; whereas supernovas occur only once every fifty years or so for a galaxy like ours. Novas occur in close double star systems that consist of a relatively bright red giant orbited by a white dwarf. That system brightens dramatically due to a nuclear explosion on the surface of the small white dwarf companion. Novas don’t destroy themselves. A recurrent nova, like T Cor Bor is one that has been seen to flash more than once.
For amateur astronomers, T Cor Bor is relatively bright before and during an event. It puts on its fireworks show once ever 79 years. This upcoming performance will be the only one in many of our lifetimes, unless we were around in the mid-1940s. Our smart scopes will be perfect for recording the action because they can easily find and image the 10thmagnitude “quiet” state of the star system as well as the 2nd magnitude nova flash. I think all the smart scope manufacturers have put it into their observation lists too.
No doubt everyone reading this has seen some online alert about the coming event. This blog will try to fill in information gaps in two ways. First, it will show how the story of the oenophile god, shown above, will connect us with the astronomy of the archaic Greeks. Coincidentally, the light reaching us right now from the star occurred long ago (~2,600 years ago), at the end of the 7th Century BCE and will just be reaching us soon. (When I worked on sky shows, I learned to include a cultural or historical connection, so I will include a couple here for your enjoyment.)
Second, this blog should help you get primed and on alert for the outburst. Within a couple of hours, a nova like this can rise by 8 magnitudes in brightness (~factor of 1,600) to be as bright as the North Star. It will then dim to below our ability to see with eyes alone in less than two weeks. So we’ll want to know exactly where to look and when to get outside with our scopes.
I urge you to take a “before” shot as soon as you have a dark, clear night sky. You’ll want one or more images to be able to compare to just how bright it will get. Near the end of October, we won’t be able to see the constellation until sometime in early December because it will be too close to the Sun in the sky; technically, they will be in conjunction.
Background on the Constellation Ariadne’s Crown
Corona Borealis is a quaint little constellation that transits high for northerners in late spring evenings. It’s not as conspicuous as many other constellations, since its brightest star, Alphecca (AKA Gemma) is just 2nd magnitude, and the rest of the main seven stars average about 4th magnitude. Once you learn to spot Ariadne’s Crown, however, you’ll see that it’s one of the only constellations that looks like its namesake. The red circle in the following image shows where T Coronae Borealis is located.
This cosmic “crown” was presented to Ariadne by the god Bacchus in consolation for stealing her away from her original lover, Theseus. In Greek mythology, the Athenian Theseus slayed the dreaded Minotaur in his labyrinth on the island of Crete. The Minotaur, half-man and half-bull, terrorized Crete. Ariadne, the daughter of Minos, King of Crete, had taken a liking to Theseus. She gave him a roll of thread to unwind as he journeyed into the Minotaur’s lair and back out again. With her help, Theseus survived and slay the dreaded beast. He then sailed home to Athens with his bride-to-be, Ariadne. They made a mistake, however. They took a layover on the island of Naxos.
Those of us who travel know that layovers can be problematic. It’s better to travel direct. As it turns out, Naxos was where Bacchus lived. He took an immediate liking to Ariadne, pulled rank on Theseus, and shooed him back to Athens alone. Bacchus’ gift of the constellation crown impressed Ariadne. She went along with the wine god and eventually became immortalized herself.
There’s really nothing to be drawn from this story for our modern scientific age except that if you turn out to be the first person to spot the rise of T Cor Bor to its intermittent blaze state, then you too might be immortalized. Maybe not the way the astronomer Tycho Brahe was when he spotted a new star, but at least you’ll be a big shot online for a few days.
Tycho’s first edition of De Nova Stella -- Harvard Houghton Library
Tycho and Novas
That a new star (a “nova,” or in Tycho’s case, a “supernova”) could appear was a radical idea in Western science in the 1570s. Tycho Brahe, was a famous Danish astronomer and mentor to Kepler. Tycho saw a new star appear in the constellation Cassiopeia. We now know it was what we call a supernova and contemporary astronomers can see its debris primarily at radio and X-Ray wavelengths. Of course, Tycho couldn’t have known it was a nuclear explosion. It was enough to see a brilliant new star in the sky.
Tycho first spotted his “new star” on 11 November 1572. That event is now called SN1572. It brightened rapidly to magnitude -4, which is close to the brightness of Venus at its brightest. It was in the constellation of Cassiopeia, high in the sky at that time of year. It would have been stunning and hard to miss for anyone. It stayed brighter than 1stmagnitude for about three months. Within less than a year and a half, it dimmed and became too faint to be seen by the unaided eye.
At the time, Tycho’s new star shattered the eternal sphere of fixed stars of the Ptolemaic System. Not only was the world struggling to accept the new Copernican System with its expanded view of the universe, now it seemed that the various star “suns” were not fixed and eternal either. It would take a while for the ramifications to sink in, but Tycho’s nova was a milestone in the history of western astronomy.
Having spent a few years working on the world’s largest astronomy museum in Shanghai, I would be remiss if I didn’t mention SN185 and SN1054, which were not, for some reason seen in the West. SN1054 (seen first on 10 July 1054) gave birth to the Crab Nebula, M1, another wonderful target for smart scope users.
What’s going on at T Cor Bor?
T Cor Bor is a double star system. One component is a red giant, and the other is a tiny white dwarf. They are bound together by their mutual gravity, only 0.5 AU away from each other. (AU= astronomical unit, the distance from the Earth to the Sun.) They complete an orbit in just 227 days. At their distance of 2,630 light years, no telescope on Earth could ever show the two stars as separate images, so we study them by studying their spectra. When T Cor Bor novas, the system will be nowhere as bright as SN1572. Nevertheless, going from 10th magnitude to 2nd magnitude is no mean feat. This degree of stellar brightening can only be due to a nuclear explosion. It may not be as catastrophic as Tycho’s, but it will still be intense. So, how does a star support a nuclear explosion without exploding itself?
The explosion will occur on the surface of the white dwarf star involved with T Cor Bor. This white dwarf is an evolved and compact companion to a red giant star of type M3, which is relatively cool (3500K, about 60% as hot as the Sun). It is so cool that the peak light output is at the very edge of the red end of the visible spectrum. We only see the giant most of the time because it is 6.7 times brighter than its companion white dwarf during its normal quiescent state. This is why, when you image the system ahead of time, you will see that it appears orangish red. That will help you identify it when you’ve imaged it.
Red giants like the one in this system are rapidly losing their atmosphere. The white dwarf component acts like a vacuum cleaner because of its proximity and strong gravity. Around 3x10-8 solar masses fall onto the dwarf every year. Allowing for 78 years of accumulation, the “period” between explosions, it takes only about 2 millionths of a solar mass of the red giant’s hydrogen-enriched envelope to provide the fuel for the nova’s detonation.
As I was preparing this blog, I noticed that the mass of the T Cor Bor’s white dwarf component was 1.37 solar masses. That’s very near the Chandrasekhar Limit for the maximum mass of a white dwarf before it collapses. Might T Cor Bor supernova, I wondered? Probably not; but if it were to detonate totally as a collapsing white dwarf star, becoming what’s called a Type I supernova, then it would be as bright as the first quarter or full moon in our sky. Don’t worry; I checked and even if it did supernova, it would still not be close enough to threaten life on Earth. Nevertheless, if it indeed brightens and blows past second magnitude all the way to magnitude -10, we’ll know it has become a supernova.
What follows is a copy of one of the most important illustrations from Subrahmanyan Chandrasekhar’s research. He’s one of my all-time heroes. Don’t be put off by it. The relativistic (electrons) Fermi gas curve (in green) describes theoretically how the size of a white dwarf would change as more and more mass is added to it. Just notice that as one makes more massive white dwarf star it gets smaller and smaller because of the extreme gravitational pressure. Until, that is, it approaches 1.425 solar mass. At that point the relativistic electrons holding it up give out and it collapses. In actuality it winds up causing the carbon nuclei with it to detonate and we get a supernova of Type I. N.B. If you were able to have beaten Chandra to this discovery you might have won his Nobel Prize. He made the prediction just after he turned 20!
Plot Showing Chandrasekhar Limit =1.425 solar masses from Chandra’s book, Stellar Structure.
Smart Telescopes Make Finding T Cor Bor Easy
I have no idea why the media thinks that their average astronomy readers will be able to spot this blaze event. In light-polluted skies, it’s very hard to find, much less identify the constellation. And, even if they could, skilled observers in dark skies would find that T Cor Bor in its quiescent state is right at their limit. With binoculars providing a five-degree field of view, about 275 stars will be visible. Which would it be?
When the nova is at its maximum, however, if you knew the constellation, then you might recognize that it sprouted a second star to rival Gemma. But only those who know the constellation will see this effect, and believe me, it will not knock anyone’s socks off.
The explosion and brightening will happen very quickly, in less than a day. Then, it will dim by half in a week and return to a “normal” 10th magnitude within a couple weeks. So, it’s important to get on it as soon as it blows and take a few pictures.
The really good news for us, however, is that this challenge is a sitting duck for smart scope users. Here’s how to experience the event.
Take at least one before picture.
You won’t really appreciate the difference in brightness unless you have an image of the “before” state. Here are two of mine done with my eVscope on two dates months apart. T Cor Bor is the slightly reddish star in the center. The reason the field is rotated is because they were shot when the star was at different locations in the sky. [Yes, and I know I need to improve the collimation a bit…. ;-)]
I did a good job with the Seestar, but when I took this one, I had to target the elliptical galaxy IC4587. I’m not sure that you can still find it in the Seestar app’s data base. It is in Unistellar’s as well as in Singularity’s. (I haven’t really tried with my Vespera yet.)
The pre-nova T Cor Bor is circled in the following image. It was too short an exposure to show the galaxy, however.
Even once you’ve imaged it, you can use my images as a guide. T Cor Bor is slightly reddish. Here’s another finding chart I’ve adapted to an AAVSO chart that helps in the identification. The circle is the size of my eVscope’s field of view, which is about 30 arc minutes. Also note that north is up in this image. The numbers next to the stars are their magnitudes without the decimal points.
What can we expect?
1. We’re not precisely sure when the star will go off. I would even hazard to say that we don’t have good enough information to give a prediction within a year. The timescales for astronomical events are generally far longer than a human lifespan. We can’t simply rely on the past time frame that we’re sure about: 1946-1866, which would put it in 2026. The current refined prediction is 2025.5+/-1.3 (see reference at end of the blog). In actual months and days, that’s between February 11, 2024, and September 18, 2026.
2. In any case, I urge you to grab a “before” shot ASAP. You’ll want one for comparison when it finally does go. It’s also because it will be impossible to image Corona Borealis when it gets too close to the Sun in the sky. By my estimate, it will be hard to image between approximately October 20th and December 15th. Let’s hope it doesn’t go off during this time period. ;-)
3. Why is that a difficult time period? Because T Cor Bor goes into conjunction with the Sun’s right ascension around November 1. Although it will still be about 40 degrees north of the Sun, you’d have to be above latitude 80 degrees for it to be high enough (15 degrees) to observe easily. In the high Arctic, it would be available all the time, but I can almost guarantee the weather won’t cooperate.
4. You need to be on alert too. Even if you have your “before” image, realize that it will only be second magnitude for a day or two and naked-eye bright for about a week. Pay attention to social media, but also connect with the AAVSO (American Association of Variable Star Observers). They’re a fantastic organization that is critical for stellar researchers. Here is an image of the light curve from the past outburst in the 1940s taken from AAVSO data:
AAVSO and Other Links
It’s really worth checking out AAVSO if you want to keep up with this nova and others.
I’ll let you digest how to sign in and get information in a way that works for you. MyNewsFlash is one I have signed up for. Read the page, and see how you might want to be alerted. I get notifications daily with all the current observations; but be warned, it’s a long text alert with many observations. I’ve learned how to scan it to see if the visual magnitude has changed from the previous day.
I’m not sure if the supernova hunters will keep track of this local event. I sometimes check them as well. In any case, it’s fun to watch this site because it will alert you of supernovae in external galaxies too.
Conclusion: Back to the Greeks…. Pythagoras and Mathematics
Pythagoras Doing his Math Homework by Raphael
If and when the light from the nova explosion at T Cor Bor reaches us, it will have traveled for over 2,600 light years. That means it left within a generation of when Pythagoras developed his unique view of philosophy – a view emphasizing the use of mathematics and experiment to discover truth in the world. He was ahead of his time, but definitely in synch with the way we do science now. He would have approved of trying to reckon the behavior and predicting the flare ups of T Cor Bor, so I am personally dedicating this upcoming outburst to him!
While I’m at it, I promise to give a nod to Bacchus too, by quaffing a glass of red wine when T Cor Bor finally blows. At first, I thought about a good vintage from 1946, the last time the nova flashed. I discovered a highly rated 1946 Chateau Lafite Rothschild, but unfortunately it would cost over $4,000 before tax and import fees! Alas that is way out of my budget – about as many orders of magnitude as the star will brighten. For that money, I could buy a really nice new telescope (which I really don’t need).
Happy (blaze)-stargazing to all. And, as the Greeks would say:
Opa!
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Special Note: I’d like to thank my good friend and mentor Maurice Givens for pointing out the article by Bradley E. Schaefer with the most accurate prediction of the eruption date range. This article also summarizes most of what is known about this amazing star:
MNRAS 524, 3146-3165 (2023).
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