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Today I want to talk about something cool happening in the sky soon. Sometime between the time of publishing this article and September of this year, a star will become visible where there wasn’t one before. This star is called T Coronae Borealis (T CrB), or the Blaze star. T CrB is classified as a recurrent nova star. Today, I’d like to explain what they are and how they work, to explain why we’ll be seeing a “new” star appear.
The Latin word for “new” is nova, which is where the term arises from. When astronomers first saw a nova, they believed it was a new star being born. However, we now know that this was incorrect. The star there was just so dim previously that you couldn’t see it with the naked eye. Novae can get more than 10 magnitudes brighter than they were, or 10000x brighter!
Additionally, these suddenly bright spots in the sky are not one, but two stars, in a binary system. One of these stars is a bigger, gaseous star. This could be a main sequence star like our sun, a subgiant, or a red giant. The other star is always a hot, dense white dwarf. As these two stars orbit each other, the incredible density of the white dwarf causes it to pull matter from the bigger star towards it. The white dwarf accretes more and more matter around it in a disc (see image below), which then heats up due to the heat of the white dwarf. Eventually, so much matter has gathered around the star that it reaches a critical temperature, around 20 million Kelvin, and fusion starts in the accreted matter. This causes a rapid outburst of energy and matter, leading to a sudden increase in light.
Since the fusion only occurred in the accreted layer around the white dwarf, it is generally unharmed, and so the process can repeat itself. For recurrent novae like T CrB, this process can repeat on timescales less than 100 years. T CrB has a cycle length of around 80 years, and its last eruption was in 1946. You might say this means it should erupt in 2026, but we know that it will explode in the next few months since its brightness dipped in around March of last year, which, if this star acts the same as it did in 1946, indicates an eruption between 1 and 1.5 years later, leading to the September estimate. This graph shows the light curve of T CrB around its 1946 eruption. As you can see, it rises incredibly fast and fades nearly as rapidly. You can also see the dip I mentioned earlier, pointed to in red.
If you have access to the stars from where you are, one of the coolest ways to experience this event would be to take a photo of the region of the sky where T CrB will appear right now (use a starfinding app to find that), then after it erupts, take another photo. By comparing the two, you will be able to see the “new” star! This is a better way to experience it than just looking at it after it erupts, since at that point, unless you’ve memorized the sky, it will just look like another star in the sky. This is a super cool opportunity to see a cosmic event from your own house, and when it does erupt, I’ll publish a shorter article to let you know!