Visible to the naked eye! Is the nova 2,600 light-years away from Earth going to be "in a bad mood" again?

Author | Zhou Yihao

Review | Huang Jian, Zhao Jingyuan, Zou Qianyi

Editor | Zhao Jingyuan

Classic Rising Stars and Rising Stars

There are many star systems in the magnificent universe. Among them, there is a close binary star system, which consists of a white dwarf and a normal star. It will produce a violent explosion, which greatly increases the brightness of the white dwarf, and then slowly dims, and returns to its previous brightness after several months to decades. This is a nova. Nova can be roughly divided into two categories: Classical Nova, which is observed only once, and Recurrent Nova (RN), which is observed at least twice.

Artistic image of a new star system (Image courtesy of NASA)

Current research has found that the mass of white dwarfs in some recurrent nova systems is gradually increasing, which means that they will one day reach the Chandrasekhar limit and embark on the path of becoming a Type Ia supernova (for carbon-oxygen white dwarfs) or an accretion-collapse supernova (for oxygen-neon-magnesium white dwarfs). It can be seen that recurrent nova systems are crucial for studying the evolution of stars and binary star systems, but their number is very small. There are only 10 known in the Milky Way, and T Coronae Borealis (T CrB) is one of them.

The position of T. Borealis in the night sky. In the picture, the north is up, the south is down, the east is left, and the west is right (picture from Stellarium)

Outbreak Mechanism

The explosion mechanism of T. Borealis is closely related to the composition of its binary star system. The system consists of a white dwarf with a mass of 1.37 times that of the sun and a red giant with a mass of 1.12 times that of the sun, with an orbital period of 227.6 days. The white dwarf accretes the matter (mainly hydrogen) of the red giant companion through its powerful gravity. These materials accumulate on the surface of the white dwarf and are compressed by the gravity of the white dwarf, which increases their temperature. Once the Fermi temperature (about 7×10^7K) is reached, a violent nuclear fusion reaction will be quickly triggered on the surface of the white dwarf, releasing a large amount of energy, causing the white dwarf to brighten rapidly and produce a nova explosion. Subsequently, the binary star system of T. Borealis will gradually return to its state before the explosion until another explosion occurs about 78 years later.

Left: Before the explosion of T Borealis; Right: After the explosion of T Borealis (simulated image) (Image from reference 7)

History

T. Borealis is about 2,600 light-years away and is the closest recurring nova to us. When not erupting, it is about 10th magnitude, which is the brightest among all recurring novae. Before 2023, people only knew that T. Borealis had erupted twice, on May 12, 1866 (discovered by British astronomer John Birmingham) and on February 9, 1946 (discovered by British astronomer Norman Knight and American astronomer Armin Deutsch). During these two eruptions, T. Borealis brightened more than a thousand times, reaching a maximum brightness of about 2.5 magnitude (which is also the brightest among recurring novae), making it a celestial body visible to the naked eye in the night sky. During the outburst of T. Borealis in 1866, observers around the world obtained relatively rich observation data, which was the first time in the history of nova observations. British astronomer William Huggins and chemist William Miller were the first to obtain the spectrum of T. Borealis, which was the first spectral observation of a nova by humans.

Spectrum of T Coronae Borealis taken by Huggins and Miller on May 16, 1866 (from reference 15)

Historical image of the 1946 T. Borealis outburst. The bright star in the central circle is T. Borealis. The image was taken by Harvard Observatory on February 12, 1946, three days after the outburst of T. Borealis. At this time, it had dimmed to magnitude 4.3 (Image source: Star Glass)

In 2023, American astronomer Bradley Schaefer found records of two other outbursts of T. Borealis in historical documents, which occurred in October 1217 and December 1787 respectively, bringing the number of recorded outbursts of T. Borealis to four.

Signs before an outbreak

Since 1920, American amateur astronomer Leslie Peltier has frequently observed T. Borealis, hoping to discover its next outburst. In 1935, he noticed that T. Borealis brightened slightly, and then entered a 10-year plateau period; in 1945, he observed the dimming of T. Borealis. In those days, people knew little about re-emitted novae, but Peltier made a bold prediction: starting in 1945, T. Borealis continued to dim, indicating that it was about to erupt again. Sure enough, on February 9, 1946, T. Borealis erupted again after 80 years, but Peltier was not happy, because after being woken up by the alarm clock in the early morning of that day, he found that he might have symptoms of a cold, so he went back to bed to rest, and did not observe T. Borealis as usual, thus missing the discovery. Pelletier recounted the experience in his book Starlight Nights: The Adventures of a Star-Gazer, writing regretfully: "I am responsible for my own failure, but I still feel that T. Borealis should have been more considerate of me. We have been friends for many years; I have watched over it many nights while it slept... I still watch it now, but with a wary eye. There is no warmth between us any more."

Light curves before and after the 1946 explosion (Image source: Wikipedia)

The dimming before an outburst seems to be a phenomenon unique to T. Borealis. According to observational data from the past 10 years, T. Borealis slightly brightened again in 2015, and then entered a plateau period; from March to April 2023, T. Borealis dimmed as expected. Does this mean that it will erupt again soon? Astronomers predict that, assuming that the light changes before and after each outburst are similar, there is a 68% probability that T. Borealis will erupt between February and September 2024, and a 95% probability that it will erupt between December 2023 and December 2024.

The coming outbreak

The figure below shows the historical light curve of T Coronae Borealis, with a time range of June 10, 1842 to March 30, 2024. The two large brightenings correspond to the outbursts in 1866 and 1946. It can be seen that the recent light curve is rapidly dimming from the plateau, which is very similar to the period before the 1946 outburst, so the previous prediction is not unreasonable.

The light curve of T Coronae Borealis. The ordinate is the magnitude, and the abscissa is the Julian day; red represents the V-band limiting magnitude, green represents the V-band magnitude, and blue represents the B-band magnitude (data from reference 10, AAVSO)

If the upcoming outburst follows a similar pattern of brightness changes as previous outbursts, we can also make the following predictions: after the outburst, T Coronae Borealis will reach a peak brightness of about 2 magnitude within a day (or possibly within a few hours), visible to the naked eye, becoming the brightest nova since V1500 Cygni (V1500 Cygni erupted in 1975 with a maximum brightness of about 1.9 magnitude); then T Coronae Borealis will begin to dim, remaining visible to the naked eye (brighter than 6.5 magnitude) for about a week; one month after the outburst, T Coronae Borealis will return to its pre-outburst brightness of about 10 magnitude; three months after the outburst, T Coronae Borealis will brighten again, reaching a brightness of 8 magnitude, and then slowly dim, a process that will last for four months; thereafter, T Coronae Borealis will remain in a plateau until it falls back to normal brightness eight years later. This is also the general situation of the outbursts in 1866 and 1946. It can be seen that the brightness changes of T Coronae Borealis are very complex. What causes these unique phenomena? What is the relationship between them and the outburst? Astronomers are divided over this. Professor Schaeffer, who specializes in nova research, commented: "Five theorists will give five completely different explanations."

The light curves before and after the 1866 and 1946 explosions are very similar (Image from Reference 10)

Continuous monitoring

For novae, T. Borealis is relatively bright (both before and after the eruption), so it is very suitable for astronomy enthusiasts to monitor. As my country's first amateur observatory engaged in sky surveys and discoveries, Xingming Observatory has begun spectral observations of T. Borealis since the summer of 2023, and later began multi-band photometric observations, hoping to depict a complete picture of T. Borealis before and after the eruption. Fellow enthusiasts can also pay close attention to the position of T. Borealis in recent months. There are many ways to observe it: spectral observation, mobile phone/camera photography, telescope visual/photography, and even naked eye observation.

While monitoring, we should also pay attention to the fact that there is relatively little observational data before each outburst of T Borealis, and astronomers are still unclear about how the complex light changes before and after its outbursts occur. In addition, the composition of the white dwarfs in the T Borealis system is also a mystery. These uncertainties make the confidence in predicting the time of the outburst not high.

Fortunately, with the advancement of science and technology, humans can now conduct multi-band, multi-messenger observations, enabling us to have a deeper understanding of the properties of T Coronae Borealis. These mysteries are expected to be solved in the upcoming outburst.

References

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(10) Bradley E. Schaefer, The B & V light curves for recurrent nova T CrB from 1842–2022, the unique pre- and post-eruption high-states, the complex period changes, and the upcoming eruption in 2025.5 ± 1.3, Monthly Notices of the Royal Astronomical Society, September 2023, 3146–3165.

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(12) Bradley E. Schaefer et al. Announcing T CrB pre-eruption dip: Recurrent nova T CrB has just started its Pre-eruption Dip in March/April 2023, so the eruption should occur around 2024.4±0.3, AAVSO, https://www.aavso.org/news/t-crb-pre-eruption-dip

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(14) Robin George Andrews, A Dead Star Will Soon Spark a Once-in-a-Lifetime Display in Earth's Skies, Scientific American, https://www.scientificamerican.com/article/this-nova-will-soon-erupt-as-a-once-in-a-lifetime-new-star-in-the-night-sky

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