Stargazing in June: A nova explosion

During the next few weeks or months, we’ll be treated to a rare astronomical sight, writes Nigel Henbest

Cosmic hydrogen bomb – a nova explosion (Nasa)

Watch out for the once-in-a-lifetime nova!

During the next few weeks or months, we’ll be treated to a rare astronomical sight – one to check off your bucket-list of celestial sights. Admittedly not as spectacular as an auroral display, a comet or a total solar eclipse of the Sun, it’s your once-in-lifetime chance to catch a nova.

The name is Latin for ‘new,’ but a nova is not a star being born. It’s a previously obscure star that suddenly flares up in a brilliant outburst. The specimen I’m focusing this month – T Coronae Borealis – is actually visible through a telescope at any convenient time, but it’s almost 100 times too faint for you to see with the unaided eye.

But in May 1866, Irish astronomer John Birmingham was amazed to find this star shining over a thousand times more brilliantly, rivalling the Pole Star. T Coronae Borealis subsided back to its normal dimness; only to erupt again in 1946. Astronomers have nicknamed it ‘the Blaze Star.’

By analysing the star’s light when in its normal faint state, astronomers have found that it’s a red giant star, 75 times wider and 650 times brighter than the Sun, and lying 2600 light years away from us. And that’s not all. Circling the red giant every seven-and-a-half months is a dim white dwarf, the core of an old star that’s collapsed on itself. Smaller than the Earth, this companion contains more matter than the Sun, squeezed to such a high density that a teaspoonful of its material would weigh as much as an elephant.

The white dwarf’s gravity is gradually siphoning gas from its bloated companion. Over decades, this cannibalistic feast accumulates on the white dwarf’s surface. It’s mainly composed of hydrogen; and when it reaches a critical mass, this stolen material erupts – literally as a hydrogen bomb.

That’s why the star blazed in our skies in 1866. With the stolen gas blasted away into space, the star began to accumulate fresh supplies of hydrogen from the red giant. After 80 years – in 1946 – it again reached critical mass, and underwent another thermonuclear reaction.

Wind forward another 80 years, and you’d expect an eruption at some point in 2026. But the star is already starting to behave oddly, experiencing a slight fading that astronomers previously observed just prior to its 1946 outburst. So we’re now predicting that the next outburst will occur this year, between now and September.

To find the nova, first locate the lovely little circlet of stars called Corona Borealis (the northern crown) lying near the bright star Arcturus. On the starchart, I’ve marked the position of the Blaze Star with an open circle labelled T CrB. When the star erupts, it will be as bright as the Pole Star, Polaris.

Not only is it 78 years since the Blaze Star last erupted, almost 50 years have passed since we’ve seen any nova as bright as this: Nova Cygni, which appeared at a similar brightness in the constellation Cygnus (the swan) in 1975. So make summer 2024 even more memorable by observing this astronomical event of a lifetime!

What’s Up

It’s a pretty poor month for spotting any of the planets. You’ll need to be up before dawn to see any of these worlds, and for most of June the roll-call will be just Mars and Saturn, with Jupiter appearing towards the end of June.

The night sky at around 11pm this month (Nigel Henbest)

In fact, the evening sky this month is more spectacular than the dull pre-dawn “planet parade”, with a selection of brilliant stars on display. High in the south is orange Arcturus, the jewel in the kite-shaped constellation of Boötes, which traditionally depicts a herdsman. To its lower right, blue-white Regulus marks the heart of the celestial lion, Leo, traced out by a crouching-feline outline of stars.

To the left, its twin Spica is a head of corn held by Virgo, the virgin, whose stars form a large Y-shape in the southern sky. And low on the horizon, blood-red Antares – its name means "rival of Mars" – is the heart of Scorpius, a giant scorpion whose tail dangles southwards, to end in a pair of stars representing its lethal sting.

Diary

6 June: 1.37pm: New Moon

8 June: Moon near Castor and Pollux

11 June: Moon near Regulus

14 June, 6.18am: First Quarter Moon

16 June: Moon very near Spica

20 June, 9.51pm: Summer Solstice

22 June, 2.07am: Full Moon

27 June, before dawn: Moon near Saturn

28 June, 10.53pm: Last Quarter Moon near Saturn

Nigel Henbest’s latest book, ‘Stargazing 2024’ (Philip’s £6.99) is your monthly guide to everything that’s happening in the night sky this year