The stars are not fixed and unchanging, unlike many ancient people thought. Every once in a while, a star appears where there wasn’t one before, and then fades within 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 skies around the world on July 4, 1054. It brightened quickly, becoming visible even during the day. for the next 23 days.
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Astronomers in Japan, China and the Middle East observed this event, as did the Anasazi in what is now New Mexico.
In the second half of 2024, a nova explosion in the star system called T Coronae Borealis, or T CrB, will again be visible to people on Earth. T CrB will appear 1500 times brighter than usual, but it will not be as spectacular as the event at 1054.
I am a space scientist with a passion for learning physics and astronomy. I enjoy photographing the night sky and astronomical events, including eclipses, meteor showers, and once-in-a-lifetime astronomical events like the T CrB nova. T CrB will become, at best, the 50th brightest star in the night sky – brighter than only half the stars in the Big Dipper. It may take a few tries to find it, 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 reporting 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 the star’s mass. Very massive stars — those more than eight times the mass of our Sun — explode in dramatic supernova explosions, like the ones people observed in 1054 and 1572.
In lower-mass stars, including our Sun, once the hydrogen in the core is depleted, the star expands into what astronomers call a red giant. The red giant is hundreds of times larger than its original size and more unstable. Eventually, all that’s left is a white dwarf—an Earth-sized remnant composed 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.
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T CrB is a binary star system – it consists of a red giant and a white dwarf, which orbit each other every 228 days at about half the distance between the 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 spinning disk of matter called an accretion disk, which surrounds the white dwarf.
Matter from the accretion disk, which is mostly hydrogen, swirls in and slowly accumulates on the surface of the white dwarf. Over time, this hydrogen blanket 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. Once the accretion disk gets hot enough, a nova occurs where the hydrogen ignites, is blown outwards and emits bright light.
When will it happen?
Astronomers know about 10 recurrent novae – stars that have undergone nova explosions more than once. T CrB is the most famous of them. It erupts on average every 80 years.
Because T CrB is 2,630 light-years from Earth, light takes 2,630 years to travel the distance from T CrB to Earth. The nova we’ll see later this year happened more than 2,000 years ago, but its light will reach us later this year.
The hydrogen filling on the white dwarf’s surface is like the sand in an 80-year-old hourglass. Whenever a nova occurs and the hydrogen ignites, the white dwarf itself is not affected, but the surface of the white dwarf is swept away by the hydrogen. Soon after, hydrogen begins to accumulate on the white dwarf’s surface again: The hourglass flips and the 80-year countdown to the next nova begins again.
Careful observations during its last two novae in 1866 and 1946 showed that T CrB became slightly brighter about 10 years before the nova was visible from Earth. Then it faded for a short time. Although scientists aren’t sure what causes these brightness changes, this pattern has repeated, with a brightening in 2015 and a dimming in March 2023.
Based on these observations, scientists predict that the nova will be visible to us sometime in 2024.
How bright will it be?
Astronomers use a magnitude system first devised by Hipparchus of Nicaea more than 2,100 years ago to classify the brightness of stars. In this system, a change of 5 in magnitude means a change of a factor of 100 in brightness. 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 10th-magnitude star barely visible in binoculars.
During the nova event, the white dwarf’s exploding envelope of hydrogen will glow at a magnitude of 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 unaided visibility 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 skies.
To find the constellation, look west and find Arcturus, the brightest star in that region of the sky. Then look about halfway between the horizon and the zenith—the point directly above you—at 10:00 p.m. local time in North America.
Corona Borealis is approximately 20 degrees above Arcturus. This is approximately the length of one hand, from the tip of the thumb to the tip of the pinky, at arm’s length. At its brightest, T CrB will be brighter than all the stars in the Corona Borealis, but not as bright as Arcturus.
To find the Corona Borealis, find Arcturus and then look for an outstretched hand above.
You can also use an interactive star chart like Stellarium, or one of the many apps available for smartphones, to locate the constellation. Familiarity with the stars in this region of the sky before the nova occurs will help identify the young star once T CrB brightens.
Although T CrB is too far from Earth for this event to rival the supernova of 1054, it is nevertheless an opportunity to observe a rare astronomical event with your own eyes. For many of us, this will be a once-in-a-lifetime event.
For children, however, this event can spark a passion for astronomy. Eighty years in the future, they can look forward to observing it once more.
This article is republished from The Conversation under a Creative Commons license. Read the original article.