This year could be a once-in-a-lifetime opportunity to see a Nova

Astronomers regularly check on a galaxy that currently requires a telescope to see, as they expect it will soon be bright enough to see with the naked eye. According to one prediction, this could happen anytime until September, although there is reason to believe we may have to wait another year.

One of the wonders of astronomy is the way stars, very occasionally, suddenly brighten spectacularly. When the star in question was previously too faint to see, it can appear as if a new star has appeared, and before the invention of the telescope these were known as novae from “The Nova Stella”, Latin for new star.

Such a brightening is expected two and a half thousand light-years away, towards the constellation Coronae Borealis.

The event will occur because what is known as T Coronae Borealis (T CrB) is not one star, but two. Although double stars are the most common arrangement of stars, this pair is a dangerous combination of a red giant and a white dwarf, both of which are modestly more massive than the Sun. Over time, the white dwarf’s intense gravity pulls material from the red giant, creating an accretion disk that looks a bit like one orbiting a black hole.

When some of this material comes close enough to the white dwarf, it is heated to the point where fusion ignites, leading to an increase in brightness. Most such brightenings resemble those of an ordinary variable star. For example, in 2016, the brightness of the T CrB system roughly tripled. However, since it was still barely visible with binoculars, few people cared.

In 1866 and 1946, on the other hand, the wave was something completely different: it increased thousands of times, so that it was easily visible to the naked eye. It’s an event like this that astronomers eagerly await.

Some white dwarfs that subtract material from their companions do so erratically, glowing only once in our observations. Others, known as recurrent novae, keep to a regular schedule. You may not have heard of it before, because most recurring novae are so far away that even as they brighten, we can’t see them unaided, which makes T CrB almost unique. At its peak in 1866 and 1946, it was almost as bright as Polaris, with more than a few hundred stars. Only one other returning nova is visible at its brightest with the naked eye, and that one barely.

The gap between the two well-studied T CrB events was just under 80 years. If that were to be repeated exactly, we would expect the next event in late 2025. Unfortunately, you can’t set your watch on a returning nova, so a few years on either side wouldn’t be surprising.

However, last year, Louisiana State University professor Bradley Schaefer noted that T CrB underwent a remarkable eclipse prior to the 1946 event, and has now done something similar. Based on the timing from dip to peak, Schaefer predicted we could expect a show between February and September of this year. Schaefer also did impressive detective work to find evidence of what appear to be earlier sightings in 1787 and 1217.

While both previous reports could be about something else that happened to be in the same part of the sky, Schaefer strongly argues that these were most likely previous eruptions of T CrB, in which case we can be more confident to expect this soon.

Full view of what T Coronae Borealis might look like, from the light of the red giant-dominated system to the white dwarf explosion.

Full view of what T Coronae Borealis might look like, from the light of the red giant-dominated system to the white dwarf explosion.

Image credit: NASA/Conceptual Image Lab/Goddard Space Flight Center

Nevertheless, Schaefer’s predictions about timing came with a dose of uncertainty, which seems to have been lost in some recent reports announcing that we should see the next peak in September.

T CrB is located at a declination of 25 degrees north latitude, meaning it is visible from Europe and North America most of the year. The exception is around September-November, when the sun is annoyingly close in the sky, so we prefer the explosion not to be timed at that time. Observers in the Southern Hemisphere have a significantly narrower viewing window.

When the explosion comes, NASA predicts that T CrB should be visible to the naked eye for a few days, followed by about a week during which binoculars should still suffice. To find it, look between the very bright stars Vega and Arcturus, a little closer to the latter, and about seven degrees from Alphecca, which should have a similar or slightly greater brightness.

Leave a Reply

Your email address will not be published. Required fields are marked *