The bright star just to the right of center of this photograph taken in Australia in December 2013 is a nova in the constellation Centaurus, which is visible in the Southern Hemisphere. Nova Centauri 2013 increased in brightness from magnitude 15 to magnitude 3.3 in a matter of days. It has since settled back to magnitude 11, invisible even with binoculars. Photo courtesy of NASA

When it was observed long ago that there is nothing new under the sun, it was also thought there is nothing new beyond the sun, either.

The stars revolve in predictable nightly and yearly paths. In the fall, you will be able to go outside about 10 p.m. and see the stars of Hercules setting on the western horizon and the stars of Cassiopeia high in the northeast. And the fall after that. And after that. And so on, and on. We measure time by that turning.

But in 1572, when the scientific view of the universe was getting ready to call all the old philosophy in doubt, there was a sudden, dramatic change: A new star appeared in Cassiopeia. It was there for about a year and then faded away. In 1604, it happened again in the constellation Ophiuchus.

They were “new” stars, although they did not shine forever like the rest and were called “novas,” the Latin meaning “new.” As telescopes got better after their invention around 1608, more novas were spotted, but it was a long time before astronomers generally agreed on what was causing them.

A nova — to say it succinctly — is an exploding star. Eventually, it became clear that there are two kinds of exploding stars: nova and supernova. Novas are relatively commonplace in our galaxy, while supernovas are rare. Thirty or 40 novas a year are visible (through telescopes) from Earth.

It’s estimated that more than 80% of all stars are members of a multiple star system, in which two or more stars are orbiting each other. In some binary systems, the two stars are close enough together to exchange matter between them. This can seriously destabilize the processes of the smaller star in the pair.

If enough matter accumulates at its surface, the smaller star can get so hot and dense that it reaches a critical point where its outer layers suddenly explode. The explosion is mind-bogglingly huge. The star blows off so much energy that it can become up to 10,000 times brighter than its normal shine. Some are so bright they’re seen from Earth.

Normally, star processes are described as occurring over millions and billions of years. But novas explode in just a few Earth days. A nova in the constellation Aquila went from 11th magnitude (invisible even with binoculars) on June 3, 1918, to 6th magnitude (easily seen with binoculars) on June 7, and to minus 1.4 (brighter than Mars is in the evening now) on June 9. By the end of that month, it had diminished to 4th magnitude. By March 1919, it was at 6th magnitude.

The star RS Ophiuchi went off in August 2021 and at brightest reached about magnitude 4.3, about the brightness of the dimmer stars in the Little Dipper and much brighter than its normal magnitude 12. RS Ophiuchi is a recurrent nova, meaning that after its disruptive explosion, it returns to its original state, resumes the process of being dumped on by its companion star, and eventually explodes again, and again. RS Ophiuchi has been seen to explode every 10 to 30 years or so in the past about 100 years.

Supernovas differ from novas in their processes and especially in their size. Their explosions can be 10,000 times more powerful than novas, so huge that the star does not survive. More on supernovas next time.

Our sun, calculating from its size, makeup and activities — including the fact that it is an unusual solitary star — is unlikely to explode. In about 5 billion years, after it burns up its fuel, it will expand and the Earth probably will be swallowed up. It then will cool and fade, and true to the reliabilities and fixities of the sky, probably no one will notice. What has been will continue to be.

Dana Wilde lives in Troy. His writings on the stars and planets are collected in the e-book “Nebulae: A Backyard Cosmography,” available by download from online book sellers. Backyard Naturalist appears the second and fourth Thursdays each month. You can contact him at dwilde.naturalist@gmail.com.