A New Star in the Sky

Stars are a relative constant in the sky, unlike planets which change their positions nightly, they rarely change much even over hundreds of years. However this is not to say that the stars never change, sometimes a new star can appear in the sky, something which astronomers believe is due to happen sometime before September this year.

Corona Borealis, or the Northern Crown, is a constellation visible in the northern hemisphere, in fact it is visible from Ireland for some part of very night of the year (when it’s not cloudy of course). It is home to the star T Coronae Borelais, a dim binary star (two stars orbiting each other), which is usually of magnitude 10 which invisible to the naked eye, and only just visible when using binoculars. However about every 80 years the system explodes in a nova and for a few days it becomes significantly brighter, at magnitude 2, about as bright as Polaris, the North Star (smaller numbers are brighter when talking about magnitude). This will make it easily visible to the naked eye, even in in Dublin where there is a lot of light pollution. The last time this happened was 1946 and the next time likely won’t be until the 22^nd century so it truly is a once-in-a-lifetime occurrence.

The area around Corona Borealis in the sky with the nova T Coronae Borealis circled in red. Image created using Stellarium.

This star system is very rare, it is called a recurrent nova (‘nova’ simply means new star) and is one of only about ten of its type known in the Milky Way. The two stars are both a little more massive than the Sun, but are very different from both the Sun and from each other. One is a type of star known as a red giant, a star about the same mass as the Sun which is nearing the end of its life and has swollen up to many times its original size. Because it has swollen up to be so large it only has a tenuous grip on its outer layers of hydrogen gas and can easily lose some of it if, for example there was something else orbiting very close to it, and in fact this is exactly what is happening.

Its companion is what is called a white dwarf, the remains of a star initially about the mass of the Sun which has reached the end of its life and is slowly cooling over billions of years. While the star is more massive than the Sun it is only as big as the Earth, so it is very dense. In fact it is so dense that it is able to steal hydrogen from the outer layers of its companion. This hydrogen builds up on the surface of the white dwarf and heats up before it eventually starts a chain reaction leading to a nuclear explosion on the surface of the star.

An artist’s concept of this scenario, the larger red giant loses material which is collected by the white dwarf. This material builds up and eventually leads to an explosion on the surface of the white dwarf. Image credit: NASA.

This does not destroy the star, and so the cycle repeats itself. However it is not known how much mass the white dwarf keeps and how much is blown off in the nova – although it does appear to gain some amount of mass overall.

These novae produce elements essential for life on Earth such as carbon, nitrogen and oxygen. As such it is essential to understand novae in order to understand how life began in the universe. They are also thought to be the sources of type Ia supernovae, an important tool used in measuring distances in the universe, so both professional and amateur astronomers are ready and waiting to point their telescopes of all kinds, from x-ray to optical to radio, towards the explosion when it happens in order to gather as much information from it as possible.

This nova is not typical of the other recurring novae that we know of, about a year before the nova the brightness of the star actually dips (this is how it is known that the nova is imminent) for reasons that are not yet understood. It is hoped that observations of the nova later in the year will allow astrophysicists to better understand this as well as a few other mysteries unique to this system. The first of these is that for about a decade either side of its eruption the system is at a moderate brightness. The other mystery is that a few months after the initial nova there is a second smaller eruption, making the star brighter for a limited time, but still not as bright as the main nova.

The light curve of T Corona Borealis for its 1946 nova, the pre-eruption dip a few months before the nova as well as the secondary eruption a few months after the nova. Image credit: Created by PopePompus on Wikimedia, based on data from the American Association of Variable Star Observers, licensed under the Creative Commons Attribution-Share Alike 4.0 International license.

This is a once-in-a-lifetime experience for these astronomers, and for everyone. It isn’t every day that you get to see a new star appear in the sky, however briefly.