Stars are not fixed and unchanging, as many ancient people thought. Sometimes a star appears where it was not before, and then disappears within a few days or weeks.
The earliest record of such a “guest star,” as it was called by ancient Chinese astronomers, is that of a star that suddenly appeared in the skies around the world on July 4, 1054. It quickly brightened and was visible even during the day for the next 23 days.
Astronomers in Japan, China, and the Middle East observed this event, as well as the Anasazi in present-day New Mexico.
In the second half of 2024, a nova explosion in a star system called T Coronae Borealis, or T CrB, will once again be visible to humans on Earth. T CrB will appear 1,500 times brighter than normal, but will not be as spectacular as the event in 1054.
I am a space scientist with a passion for teaching physics and astronomy. I love photographing the night sky and astronomical events, including eclipses, meteor showers, and unique astronomical events like the T CrB nova. At best, T CrB would be the 50th brightest star in the night sky – only half as bright as the stars in the Big Dipper. It may take some effort to find, but if you have the time, you will witness a rare event.
What is Nova?
Famous Danish astronomer Tycho Brahe observed a new star in the constellation Cassiopeia in 1572. After reporting the event in his work “De Nova Stella” or “On the New Star”, astronomers began to associate the word nova with stellar explosions.
Regardless of their size, stars spend 90% of their lives fusing hydrogen into helium in their cores. But how a star ends its life depends on its mass. Very massive stars — those more than eight times the mass of our Sun — explode in dramatic supernova explosions, such as those observed by humans in 1054 and 1572.
In lower-mass stars, including our Sun, when the hydrogen in the core is depleted, the star becomes what astronomers call a red giant. A red giant is hundreds of times its original size and is less stable. Eventually, all that remains is a white dwarf, a remnant of carbon and oxygen about the size of Earth. White dwarfs are a hundred thousand times more dense than diamonds. Unless they are part of a binary star system, in which two stars orbit each other, their brightness gradually diminishes over billions of years and they eventually disappear from view.
T CrB is a binary star system – it consists of a red giant and a white dwarf that orbit each other every 228 days at a distance about half the distance between Earth and the Sun. The red giant is nearing the end of its life, so it has expanded significantly and is feeding material into a disk of matter surrounding the white dwarf.
Matter from the accretion disk, mostly hydrogen, spirals in and slowly accumulates on the surface of the white dwarf. Over time, this blanket of hydrogen becomes thicker and denser until its temperature exceeds 18 million degrees Fahrenheit (10 million degrees Celsius).
A nova is a runaway thermonuclear reaction, similar to the explosion of a hydrogen bomb. When the accretion disk becomes hot enough, a nova occurs, in which hydrogen ignites, is ejected outward, and emits bright light.
When will it happen?
Astronomers are aware of 10 recurrent novae—stars that have gone nova more than once. T CrB is the most famous. It explodes on average every 80 years.
Since T CrB is 2,630 light-years from Earth, it takes light 2,630 years to travel the distance from T CrB to Earth. The nova we will see later this year occurred more than 2,000 years ago, but its light will reach us later this year.
The accumulation of hydrogen on the surface of a white dwarf is like sand in an 80-year hourglass. Each time a nova occurs and hydrogen ignites, the white dwarf itself is not affected, but the surface of the white dwarf is cleared of hydrogen. After a short while, hydrogen begins to accumulate again on the surface of the white dwarf: the hourglass turns over, and the 80-year countdown to the next nova begins again.
Careful observations during its last two novas, in 1866 and 1946, showed that T CrB became slightly brighter about 10 years before the nova became visible from Earth. Then, it briefly dimmed. This pattern repeated with a brightening in 2015 and a dimming in March 2023, although scientists are not sure what caused these brightness changes.
Based on these observations, scientists predict that the nova could be seen around 2024.
How bright will it be?
Astronomers use a magnitude system, first devised by Hipparchus of Nicaea over 2,100 years ago, to classify the brightness of stars. In this system, a difference of 5 in magnitudes corresponds to a change in brightness by a factor of 100. The smaller the magnitude, the brighter the star.
In dark skies, the human eye can see stars as faint as magnitude 6. Normally, the visible light we get from T CrB comes entirely from its red giant, a magnitude 10 star that is barely visible with binoculars.
During the nova event, the white dwarf’s exploding hydrogen envelope will reach magnitudes 2 or 3. It will briefly become the brightest star in its constellation Corona Borealis. This maximum brightness will last only a few hours, and T CrB will be invisible to the naked eye within a few days.
Where to look
Corona Borealis is not a distinct constellation. It is located in the northern sky above Bootes and west of the Great Bear, home of the Great Bear.
To find the constellation, look due west and locate Arcturus, the brightest star in that part of the sky. Then, at 10 p.m. local time in North America, look about halfway between the horizon and the zenith—the point directly above you.
Corona Borealis is about 20 degrees above Arcturus. That’s about the span of a hand, at arm’s length, from the tip of the thumb to the tip of the little finger. At its brightest, T CrB will be brighter than all the stars in Corona Borealis, but not as bright as Arcturus.
You can also find the constellation using an interactive star chart like Stellarium or one of the many apps available for smartphones. Familiarizing yourself with the stars in this area of the sky before the nova occurs will help you identify the new star when T CrB brightens.
Although T CrB is too far from Earth for this event to compete with the 1054 supernova, it is still an opportunity to observe a rare astronomical event with your own eyes. For most of us, this will be a once-in-a-lifetime event.
But for children, the event could spark a passion for astronomy, and eighty years later, they can look forward to observing it once more.
This article is republished from The Conversation, a nonprofit, independent news organization that brings you facts and trusted analysis to help you understand our complex world. By Vahe Peroomian USC Dornsife College of Letters, Arts and Sciences
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Vahe Peroomian has previously received funding from the National Science Foundation (NSF) and the National Aeronautics and Space Administration (NASA) for his work on space weather and geomagnetic storms.