by The stars are not fixed and unchanging, unlike what many ancients thought. For a while, a star appears where there was none before, and then disappears in days or weeks.
The earliest record of such a “guest star,” named by ancient Chinese astronomers, is a star that suddenly appeared in the skies around the world on July 4, 1054. It brightened rapidly, becoming visible even during the day for the following 23 days. .
Astronomers in Japan, China and the Middle East noted 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 known as 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 not 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 sun and astronomical events, including eclipses, meteor showers and astronomical events such as the T CrB nova. T CrB will, at best, be the 50th brightest star in the night sky – brighter than half the stars in the Big Dipper. It might take some effort to find it, but if you have the time, you will see a rare event.
What is nova?
In 1572, the famous Danish astronomer Tycho Brahe saw a new star in the constellation Cassiopeia. After he reported 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% of their lives fusing hydrogen with 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 those observed by humans in 1054 and 1572.
In lower mass stars, including our Sun, once the hydrogen in the core is exhausted, the star expands into what astronomers call a red giant. The red giant is hundreds of times its original size and more unstable. Eventually, all that remains is a white dwarf – an Earth-sized remnant composed of carbon and oxygen. White dwarfs are a hundred thousand times denser than diamonds. 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 every 228 days about half the distance between the Earth and the Sun. The red giant is nearing the end of its life, so it has expanded greatly, and is feeding material into a rotating disk of material called an accretion disk, which surrounds the white dwarf.
Material from the accretion disk, made mostly of hydrogen, spirals 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 detonation of a hydrogen bomb. When the accretion disk becomes hot enough, a nova occurs when the hydrogen ignites, is blown out and emits a bright light.
When will it happen?
Astronomers know about 10 recurrent novae – stars that have undergone Novae explosions more than once. The most famous of these is T CrB. It erupts on average every 80 years.
As T CrB is 2,630 light years from Earth, it takes 2,630 light 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 direct light will approach us later this year.
The accumulation of hydrogen on the surface of the white dwarf is like sand in an 80-year time glass. Each time a nova occurs and the hydrogen is ignited, the white dwarf itself is not affected, but the surface of the white dwarf is cleaned of hydrogen. Soon after, the hydrogen begins to accumulate again on the white dwarf’s surface: The hourglass disappears, and the 80-year countdown to the next nova begins.
Careful observations during the two previous novae in 1866 and 1946 showed that T CrB was slightly brighter about 10 years before the nova was visible from Earth. Then, it dimmed briefly. Although scientists are not sure what causes these brightness changes, this pattern has repeated itself, with a brightness in 2015 and a dimming in March 2023.
Based on these observations, scientists are predicting that the nova will appear sometime in 2024.
How bright will it be?
Astronomers use a system of magnitude first devised by Hipparchus of Nicaea more than 2,100 years ago to classify the brightness of stars. In this system, a difference of 5 in size means a change by a factor of 100 in brightness. The smaller the size, 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 barely visible to binoculars.
During the Nova event, the white dwarf’s hydrogen envelope will explode to magnitude 2 or 3. It will briefly become the brightest star in its home constellation, Corona Borealis. This maximum brightness will last only a few hours, and T CrB will fade from visibility with the naked eye in a few days.
Where to look
Corona Borealis is not a visible constellation. It is located above Bootes and west of Ursa Major, home to the Big Dipper, in the northern skies.
To find the constellation, look straight back and find Arcturus, the brightest star in that region of the sky. Then look about halfway between the horizon and zenith – the point directly above you – at 10 pm local time in North America.
Corona Borealis is about 20 degrees above Arcturus. That’s because of 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 will be brighter than all the stars in the 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. Knowing the stars in this region of the sky before the Nova occurs will help identify the new star when T CrB brightens.
Although T CrB is too far from Earth for this event to compete with supernova 1054, it is nevertheless an opportunity to observe a rare astronomical event with your own eyes. For many of us, this will be a one-time event.
For children, however, this event could ignite a passion for astronomy. Eighty years from now, they might look forward to watching it again.
This article is republished from The Conversation, a non-profit, independent news organization that brings you reliable facts and analysis to help you make sense of 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 work on space weather and geomagnetic storms.
