Will 2024 Be the Year T Coronae Borealis Erupts?

Nova Cygni 1975

On the evening of August 29, 1975, I looked up and saw an out-of-place star in the constellation Cygnus. Though the star wasn’t quite as bright as the three stars that form the Summer Triangle (Vega, Deneb, and Altair), it made the Summer Triangle look more like a summer quadrilateral. For a couple hours, I kept looking at this interloper, trying to figure out what it was. I didn’t have a star chart handy, so I couldn’t check to see if it was just a star that I hadn’t noticed before. It wasn’t until a couple days later when I heard on the news that there had been a nova that I figured out what this star was, but by that time, the nova had faded from naked eye visibility, at least in most cities.

The word nova comes from the Latin word for new. In ancient cultures, novae (the plural of nova) were viewed as new stars. The fact that novae soon faded and disappeared didn’t seem to bother ancient cultures. This nova was given the name Nova Cygni 1975. It also was given the variable star designation V1500 Cygni. It turned out that Nova Cygni 1975 was the brightest appearing nova of my lifetime, at least so far. I’m glad that I didn’t miss it, even if I didn’t know what it was at the time.

The Physical Explanation of Novae

What are novae? Historically, only classical novae, such as Nova Cygni 1975, were known, but astronomers now recognize classical novae as the brightest of novae. Novae of all types often are called cataclysmic variables. A cataclysmic variable is a binary star system in which one of the two stars is a white dwarf. Binary stars are two stars orbiting one another due to their mutual gravity, much as the planets orbit the sun. As stars go, white dwarfs are quite small, about 1–2 times the size of the earth. Most white dwarfs have masses more than 150,000 times that of the earth. Therefore, the densities of white dwarfs are thousands of times greater than the density of the earth (what that means is an entirely different story that I don’t have time to go into right now). The binary companion to the white dwarf is a normal star. The stars are so close together, and the white dwarf is so faint that the cataclysmic variable appears as a single star.

The two stars in a cataclysmic variable orbit close enough to one another so that the gravity of the white dwarf lifts matter from its companion and pulls the matter toward the white dwarf. Because of angular momentum, the transferring matter does not fall directly onto the white dwarf. Rather, the matter settles into a disk above the equator of the white dwarf. The gas orbiting in the disk generates friction, which robs the gas of orbital energy, so the gas slowly spirals down onto the white dwarf. Hence, we call the disk of gas around the white dwarf an accretion disk because the matter accretes onto the white dwarf.

Most of the gas that transfers to the white dwarf is hydrogen, something that is absent in white dwarfs. As the hydrogen builds up on the surface of the white dwarf, the temperature on the bottom of the hydrogen layer increases. Eventually, the temperature and density of the hydrogen are high enough to set off nuclear fusion of hydrogen to form helium, the same process that we think powers the sun and other stars. The fusion releases heat, which causes more fusion, which leads to a thermonuclear runaway. The energy suddenly released causes the system to brighten, an event that we call a nova. The hydrogen fuel is soon consumed, the eruption ends, and the system fades to its pre-event level of brightness. Hydrogen fuel builds up once again, and the process repeats. Therefore, all novae repeat their outbursts.

The length of time between outbursts of novae varies tremendously. For classical novae, the time between outbursts is thousands of years. So Nova Cygni 1975 probably erupted before and will do so again, though I won’t be around the next time, just as I wasn’t around during the previous eruption. For dwarf novae, outbursts may repeat after only a few days. The increase of brightness during outburst is related to how long it takes to repeat, with the greatest increase in brightness corresponding to the longest time between outbursts.

T CrB

T Coronae Borealis (T CrB) is the best example of recurrent novae, a class of novae with only a handful of members. Recurrent novae erupt on a scale of several decades. T CrB normally is magnitude 10, nearly 100 times fainter than the dimmest stars the human eye can see on a dark, clear night. However, in 1866, T CrB suddenly brightened to second magnitude, about the brightness of Polaris, the North Star. After several days, T CrB faded to below naked-eye visibility. A similar eruption happened again in 1946, 80 years after the 1866 eruption.

Now we are 78 years after the 1946 eruption. Are we due for another eruption and so might we expect another outburst of T CrB? Perhaps. Eight years ago, in 2015–2016, T CrB brightened by more than a magnitude but still well below naked-eye visibility. A similar thing happened in 1938, eight years before the 1946 outburst. Then a year ago, T CrB abruptly faded by three magnitudes, making it much fainter than normal. Again, a similar thing happened a year before the 1946 outburst. If this is the kind of behavior that T CrB exhibits prior to an outburst, then we can expect T CrB to erupt again in 2024.

There is still much that we don’t know about recurrent novae.

Or not. There is still much that we don’t know about recurrent novae. Do they repeat their behavior so closely each time they erupt, or is each outburst unique in how it develops? We have only witnessed two eruptions, so we don’t have enough information to know for sure whether the pattern repeats each time. We should keep in mind the disclaimer of investment opportunities: “Past performance is no guarantee of future results.” Still, you might want to keep an eye on the constellation of Corona Borealis (the Northern Crown) this year to catch this relatively rare celestial event—it probably won’t happen again in your lifetime.

For help in finding Corona Borealis and the location of T CrB within the constellation, use the stars chart in the article linked above. Corona Borealis forms a nice little semicircle of stars (it looks like a tiara to me) between the constellations Hercules and Boötes. Alphecca is the brightest star in the tiara, representing a glittering jewel in the crown. T CrB is just below the third star to the left of Alphecca. If you continue the line formed by Alphecca and the two stars to its left, that line nearly intersects the location of T CrB. These stars will be in the evening sky throughout spring and summer, so you may want to look for them every clear evening. If you familiarize yourself with the Northern Crown and if T CrB were to erupt, then you would easily spot the brief interloper. Or you can wait another 80 years.