The stars are not fixed and unchanging, contrary to what many ancient people thought. Every now and then a star appears where there was none before, and then it disappears again within a few days or weeks.
The earliest record of such a “guest star,” so called by ancient Chinese astronomers, is a star that suddenly appeared in the sky all over the world on July 4, 1054. It rapidly brightened and was even visible during the day for the next 23 days.
Astronomers in Japan, China, and the Middle East observed this event, as did the Anasazi in present-day New Mexico.
In the second half of 2024, a nova explosion in the galaxy T Coronae Borealis, or T CrB, will once again be visible to people on Earth. T CrB will appear 1,500 times brighter than normal, but it will not be as spectacular as the event in 1054.
I am a space scientist with a passion for teaching physics and astronomy. I enjoy photographing the night sky and astronomical events, including eclipses, meteor showers, and once-in-a-lifetime astronomical events such as the T CrB nova. At its best, T CrB will be the 50th brightest star in the night sky—brighter than only half 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 a nova?
In 1572, the famous Danish astronomer Tycho Brahe observed a new star in the constellation Cassiopeia. After describing the event in his work “De Nova Stella” or “On the New Star”, astronomers came to associate the word nova with stellar explosions.
Stars, regardless of size, spend 90 percent of their lives fusing hydrogen into helium in their cores. How a star’s life ends, however, depends on its mass. Very massive stars—those more than eight times as heavy as our sun—explode in dramatic supernova explosions, like the ones humans observed in 1054 and 1572.
In lower-mass stars, such as our sun, the star expands into what astronomers call a red giant once the hydrogen in the core is exhausted. The red giant is hundreds of times larger than the original star and is more unstable. Eventually, all that remains is a white dwarf: an Earth-sized remnant made of carbon and oxygen. White dwarfs are a hundred thousand times denser than diamond. Unless they are part of a binary star system, where two stars orbit each other, they slowly fade in brightness over billions of years and eventually disappear from view.
T CrB is a binary star system – it consists of a red giant and a white dwarf, which orbit each other in 228 days at about half the distance between Earth and the sun. The red giant is nearing the end of its life, so it has expanded dramatically and is feeding material into a rotating disk of matter, called an accretion disk, that surrounds the white dwarf.
Matter from the accretion disk, which is mostly hydrogen, spirals inward and slowly collects on the surface of the white dwarf. Over time, this blanket of hydrogen becomes thicker and denser, until the temperature exceeds 18 million degrees Fahrenheit (10 million degrees Celsius).
A nova is a runaway thermonuclear reaction similar to the detonation of a hydrogen bomb. When the accretion disk gets hot enough, a nova occurs, igniting the hydrogen, blowing it outward and emitting bright light.
When will it happen?
Astronomers know of 10 recurring novae – stars that have undergone a nova explosion more than once. T CrB is the best known of these. It erupts on average every 80 years.
Because T CrB is 2,630 light-years away from Earth, it takes 2,630 years for light to travel from T CrB to Earth. The nova we will see later this year occurred over 2,000 years ago, but its light will not reach us until later this year.
The accretion of hydrogen onto the surface of the white dwarf is like sand in an 80-year hourglass. Each time a nova occurs and the hydrogen ignites, the white dwarf itself is not affected, but the surface of the white dwarf is cleared of hydrogen. Shortly thereafter, hydrogen begins accreting onto the surface of the white dwarf again: the hourglass tilts, and the 80-year countdown to the next nova begins again.
Careful observations during the last two novae in 1866 and 1946 showed that T CrB brightened slightly about 10 years before the nova was visible from Earth. It then briefly dimmed. Although scientists are not sure what causes these brightness changes, this pattern has repeated itself, with a brightness change in 2015 and a dimming in March 2023.
Based on these observations, scientists predict we will see the nova sometime in 2024.
How light 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 5-fold change in magnitude represents a 100-fold change in brightness. The smaller the magnitude, the brighter the star.
Under dark skies, the human eye can see stars as faint as magnitude 6. Normally, the visible light we receive from T CrB comes entirely from its red giant, a magnitude 10 star that is barely visible in binoculars.
During the nova event, the white dwarf’s exploding hydrogen shell will brighten to magnitude 2 or 3. It will briefly become the brightest star in its home constellation, Corona Borealis. This peak brightness will last only a few hours, and T CrB will fade from view with the naked eye within a few days.
Where to look
Corona Borealis is not a prominent constellation. It is located above Bootes and west of Ursa Major, home of the Big Dipper, in the northern sky.
To find the constellation, look due west and locate Arcturus, the brightest star in that area of the sky. Then look about halfway between the horizon and the zenith—the point directly overhead—at 10 p.m. local time in North America.
Corona Borealis is about 20 degrees above Arcturus. That’s about the span of one hand, from the tip of the thumb to the tip of the pinky, at arm’s length. At its brightest, T CrB is brighter than any of the stars in Corona Borealis, but not as bright as Arcturus.
You can also use an interactive star chart, such as Stellarium, or one of the many apps available for smartphones to find the constellation. Familiarizing yourself with the stars in this area of the sky before the nova occurs will help you identify the new star as T CrB brightens.
Although T CrB is too far from Earth to match the supernova of 1054, it is still a chance to witness a rare astronomical event with our own eyes. For many of us, this will be a once-in-a-lifetime experience.
For children, however, this event can spark a passion for astronomy, and in eighty years they can look forward to observing it again.
This article is republished from The Conversation, a nonprofit, independent news organization that brings you facts and reliable analysis to help you understand our complex world. It was written by: Vahe Peroomian, USC Dornsife College of Letters, Arts and Sciences
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Vahe Peroomian 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.