AMATEUR astronomers have a new date for their diaries. In 2022, in the constellation of Cygnus, they will be treated to the sight of a nova, or “new star”. By themselves, novas are not particularly noteworthy. Several dozen a year happen in Earth’s home galaxy, the Milky Way, alone. But this one will be special for two reasons.
One is its intensity: provided you are somewhere reasonably dark (in the countryside, in other words, rather than a big city) it will be bright enough to be seen by the naked eye. The second is that it will be the first nova whose existence was predicted before the fact. Assuming everything goes according to schedule, the credit for that will belong to Lawrence Molnar, an astronomer at Calvin College, in Michigan, and his team, who have set out their predictions in a paper to be published soon in the Astrophysical Journal.
It is a tale of scientific serendipity. “Nova”, which is Latin for “new”, comes from the title of a book (also the title of this article) published in 1573 by Tycho Brahe, a Danish astronomer. This recorded what would now be called a supernova that had happened the year before. By proving that the “new star” in question was a very great distance away—at the least, further than the Moon—Brahe dealt a mortal blow to the Aristotelian belief, widespread in Europe at the time, that the heavens were perfect and unchanging.
But the name is a misnomer. Novas are not new stars. Rather, they are explosions that take place on existing ones, drastically but temporarily increasing their brightness. There are several kinds, but Dr Molnar’s nova will be caused when one member of a two-star system collides with the other, causing an enormous and violent outpouring of energy.
Dr Molnar’s interest was piqued at a conference in 2013, when Karen Kinemuchi, another astronomer, presented some puzzling findings on a particular star seen by Kepler, a space telescope designed chiefly to hunt for exoplanets. When Dr Molnar and his team observed the star—named KIC9832227—they discovered that it was a “contact binary”, a pair of stars so close together that the smaller orbits within the atmosphere of the larger.
They also found that the smaller star was orbiting more quickly—and thus closer to its bigger companion—than it had been when Dr Kinemuchi made her measurements. Further observations confirmed that the smaller star was indeed spiralling towards its companion. Based on observations of another contact binary, V1309 Scorpii, which became a nova in 2008, the researchers were able to offer a prediction of the time of impact that, they hope, should be accurate to within about seven months. (The most likely date is a fifth of the way through 2022—ie, mid-March.)
Successfully predicting a nova will be of interest to more than just amateur skywatchers. Astronomers have built mathematical models to describe what happens during such events, but testing them against reality is hard. All previous novas have been detected after the fact. Anyone wanting to study what happens before the explosion must therefore sift back through old observations, hoping that some information about the pre-nova star will have been recorded by chance. Armed with Dr Molnar’s prediction, though, astronomers will be able to watch the build-up as well as the denouement.