Starburst: Skywatchers on the lookout for an impending nova explosion

This year, fireworks season is continuing deep into summer.

The big, upcoming highlight is a single explosion, a nova in space that will be so bright it will be visible in the night sky with the naked eye.

Sometime between now and early autumn — astronomers don’t know exactly when — stargazers will see an explosion originating from T Coronae Borealis (T CrB), a binary system known as the “Blaze Star” which is located nearly 3,000 light-years from Earth. The T CrB system contains a white dwarf star that is continuously pulling hydrogen to its surface from a nearby red giant.

Roughly every 80 years, the accumulating hydrogen provokes a thermonuclear explosion, after which the cycle begins anew.

Over the past 10 years, astronomers have noticed a pattern of brightening and dimming from T CrB that mirrors its behavior prior to its most recent explosion, in 1946.  Scientists estimate a new event will occur this year — possibly by September — leading amateur and professional astronomers alike to keep their eyes on the sky.

Yale News asked Sarbani Basu, the William K. Lanman, Jr. Professor of Astronomy in the Faculty of Arts and Sciences and a leading expert on the structure of stars, to share her thoughts on the upcoming nova.

How is a recurrent nova different from a supernova?

Sarbani Basu: While one might be tempted to believe that a supernova is simply a gigantic nova, they are two completely different phenomena. Supernovae are the death throes of massive stars. They also occur when dead remnants of low-mass stars, like the sun, which are known as white dwarfs, collide, producing an explosion. In both cases, the stars and the remnants get destroyed.

A nova, on the other hand, is a much smaller explosion, and the star remains intact. A nova occurs in systems where a white dwarf and an old large star — called a red giant — revolve around each other. White dwarfs can be as heavy as the sun or even slightly heavier, but all that mass is concentrated in a sphere about the size of the Earth, which means its gravity is very large. This large gravity, combined with the fact that the outer layers of a red giant are not tightly bound to it, causes material from the red giant to fall onto the surface of the white dwarf. The material from the red giant accretes on the surface of the white dwarf, causing a buildup of pressure and heat. Eventually, it triggers a thermonuclear explosion that destroys the accreted material, but not the white dwarf. The explosions are usually called “eruptions” or “outbursts” to avoid confusion. In a recurrent nova, this process repeats itself.

The nova that is making news, T Coronae Borealis or T CrB to astronomers, is an example of a recurrent nova. The first recorded observation of the system in its bright phase dates back to 1866, though there is some evidence that the system was observed as early as 1217 and also in 1787.

Does the distance between Earth and T CrB affect our ability to see the explosion?

Basu: The T CrB system is nearly 3,000 light-years away. In other words, light from the system takes about 3,000 years to reach us. Thus, the outburst may have already occurred, but the light from the system has not reached us yet. Distance does affect our ability to see the outbursts without telescopes, since the farther away the system is, the dimmer it will appear even during the outburst. However, 3,000 light-years is not far by astronomical standards.

Recurrent novae are not like clocks and cannot be relied on to act as we expect them to, which makes the exact timing of this event difficult to predict.

Where in the sky should we look for it?

Basu: T CrB is a star in the constellation Corona Borealis or the Northern Crown. The Northern Crown is a U-shaped constellation west of the Hercules constellation and east of the Boötes constellation.

To locate it, first, find the Big Dipper (Ursa Major) and follow the handle eastward to find the kite-shaped constellation Boötes. This constellation has a very bright star, Arcturus, at the tail of the kite. Go eastward of Boötes to find the U-shaped Corona Borealis. T CrB is not on the easily visible U, but slightly below the easternmost bright star on the U.

Will the explosion make T CrB as bright as the North Star?

Basu: Yes, after outburst T CrB is expected to be about as bright as Polaris, the “pole,” or “North” star. However, it will quickly dim again, in about a week, and remain dim until the next outburst.

Do you plan to look for it?

Basu: I will try! As a researcher in stellar astrophysics, I am more interested in the data that will be collected than in what the nova will look like. I am curious to learn what it tells us about the accretion of matter from the evolved star to the white dwarf. For these answers, I will look to my colleagues’ results, as some of them are experts in interpreting data from such events, which I am not.

Unlike in 1946, we have many more eyes in the sky today. NASA’s Fermi [telescope], the James Webb Space Telescope, and the European Space Agency’s INTEGRAL [observatory] are all ready to observe the event. On the ground, the Very Large Array [one of the world’s foremost astronomical radio observatories, located in New Mexico] will also observe the event. These missions and facilities will observe the star at its peak outburst and its decaying phase. Citizen scientists are on the lookout to observe the rising phase of the nova. All this data should help us learn about the specifics of the eruption process. Most novae are distant, but this one is relatively close by, making it easier to study.