Astronomers expect eruption of the Blaze Star in the Northern Crown

Alan Pickup

Author: Alan Pickup

The maps show the sky at 01:00 BST on the 1st, midnight on the 16th and 23:00 on the 31st.

In a month when none of the planets are easily visible in our sky, and the nights grow increasingly twilit, astronomers are keeping a close eye on a dim star in the constellation of Corona Borealis, the Northern Crown.

Called officially T Coronae Borealis, but popularly known as the Blaze Star, it flared to naked-eye prominence in 1866 and again in 1946. On each occasion, and probably in earlier centuries too, it brightened to the second or third magnitude to rival the magnitude 2.2 of Gemma, also named Alphecca, which is the brightest normal star in Corona. Between these times it has lingered near the tenth magnitude, meaning that we need a telescope to spot it.

Observations show that T CrB, to use its abbreviated name, is now flickering rapidly and behaving just as it did prior to its 1946 outburst. The supposition is that the next flare-up is imminent and could come at any time between now and September. When it does, the neat semicircular shape of Corona will be disrupted by its presence just below and to the left of the usual arc of stars. We need to be on the ball, though, for the spectacle may be brief, with it fading back below naked-eye visibility within a few days.

Our star chart plots Corona Borealis and T CrB’s position high in our southern sky, about 40% along an imaginary line from Arcturus in Bootes, the brightest star high in the south, to Vega in Lyra, almost as high in the east and marginally fainter.

T CrB lies about 2,600 light years away and belongs to a class of variable stars called recurrent novae which consist of a close pair of stars, one a small but hot white dwarf star and its companion a large, cooler and slowly-swelling red giant star. In T’s case the two orbit each other every 228 days at about half the distance of the Earth from the Sun, with hydrogen being drawn from the giant, spiraling around the dwarf and falling to its surface where it accumulates over several decades.

Eventually the point is reached that rapid and runaway nuclear fusion is triggered in the hydrogen, releasing huge amounts of energy and boosting the combined light by 1,500 times or more. Much of the accumulated material is blasted away into space and the whole process starts again.

Most ordinary novae are thought to have a similar origin, but with such long intervals between outbursts that we have yet to see them repeat. Of course, novae are distinct from the more spectacular supernovae which are much rarer and result in the terminal destruction of a star – no supernova has been witnessed in our Milky Way galaxy since 1604 when one exploded in the far south of Ophiuchus.

Before we leave Corona, we should use binoculars to check on R CrB (see chart) which is 1,400 light years away and shines usually at around the sixth magnitude as the brightest star within the semicircle. Every few months or years, though, it plunges in brightness to be visible only in telescopes when obscuring clouds of carbon, essentially soot, condense in its atmosphere. The latest such dip occurred a year ago this month, and it took until November before the star’s radiation drove the last of the carbon away, allowing its light to recover.

Edinburgh’s sunrise/sunset times change form 05:28/20:53 BST on the 1st to 04:36/21:46 on the 31st. The Sun’s northerly progress means that its dip below our northern horizon in the middle of the night is increasingly shallow. Indeed, by the 31st, it dips by less than 12° and what is officially termed nautical twilight persists throughout the night – not good news if we wish to spot the fainter stars and galaxies.

The Moon reaches last quarter on the 1st but is so low in the south-east in our predawn twilight that we may struggle to spot it. It is even lower on the 3rd, but might help us to spot Saturn which stands 12° to its left and shines at magnitude 1.2. All the other planets are lower and deeper in the twilight and unlikely to be seen from Scotland this month.

Moon at first quarter, taken with ASERO in Spain, processed by John Briggs.

The Moon is new on the 8th and might just be glimpsed as the slimmest of crescents hugging our north-western horizon at about 22:00 on that evening. It will be higher and easier on the 9th, though still only 4% illuminated and strongly earthlit. Earthshine will still be obvious on the 10th when it lies 4° left of the star Elnath in Taurus. Catch the Moon below Pollux in Gemini on the 12th and, using binoculars, less than 4° above-right of the Praesepe star cluster in Cancer on the 13th.

First quarter occurs on the 15th with the Moon above Regulus in Leo which is dipping towards our western horizon by our map times. Moving into Virgo on the 17th, the Moon lies above-right of Spica on the 19th and to its left on the 20th. Late on the 21st, it sits 5° below and right of wide double star Zubenelgenubi in Libra (see chart). The latter’s component stars, of magnitudes 2.8 and 5.2, lie 75 light years away and may or may not be gravitationally bound together.

M5 Mark Phillips 250mm f4.8 Newtonian, MPCC QHY168C @-15°C, IDAS D3

M5 by Mark Phillips

To the north of Libra is the globular star cluster Messier 5 in Serpens, plotted as M5 on our chart. When moonlight is not swamping the area, binoculars show this as a hazy round sixth-magnitude blob rather smaller than the Moon. Its many thousands of ancient stars lie 25,000 light years away, just a little further than the similar globular M13 in Hercules, about 15° to the left of Corona.

Full moon is on the 23rd, hours before it encounters Antares in Scorpius very low in in the south at midnight. Last quarter comes on the 30th, with the Moon low in the east-south-east, and below-right of Saturn, on the next morning.

As we are with the planets this month, Scotland is too far north to have a good view of the Eta-Aquarids meteor shower. This lasts for most of May but peaks late on the 5th with meteor rates that put it in the top four showers of the year for observers in the southern hemisphere.

For Scotland, though, the point in the sky from which Eta-Aquarid meteors appear to diverge, the radiant, only rises in the east at about 03:00 BST so we see none of its swift meteors before this time each morning. At least there is no moonlight interference this year, but, with the radiant so low, and with only a brief window before our morning twilight takes over, even dedicated watchers may spot very few. Sadly, The National newspaper assures its unfortunate readers that this extraordinary meteor shower is set to dazzle Scottish stargazers and be visible all night. Like the Orionids shower each October, the meteoroids that bring us the Eta-Aquarids shower originate from Halley’s Comet.

Diary for 2024 May

Times are BST

  • 1st 12h Last quarter
  • 3rd 23h Moon 0.8° S of Saturn
  • 4th 20h Moon 0.3° S of Neptune
  • 5th 03h Moon 0.2° N of Mars
  • 5th 22h Peak of Eta-Aquarids meteor shower
  • 8th 04h New Moon
  • 8th 12h Mars at perihelion, closest to Sun (207m km)
  • 9th 22h Mercury furthest W of Sun (26°)
  • 13th 00h Moon 1.6° S of Pollux
  • 13th 10h Uranus in conjunction with Sun
  • 14th 00h Moon 4° N of Praesepe star cluster
  • 15th 13h First quarter
  • 15th 20h Moon 3° N of Regulus
  • 18th 20h Jupiter in conjunction with Sun
  • 20th 11h Moon 1.4° N of Spica
  • 23rd 15h Full Moon
  • 24th 04h Moon 0.4° N of Antares
  • 30th 18h Last quarter
  • 31st 09h Moon 0.4° S of Saturn

This is an extended version, with added diary, of Alan’s article published in The Scotsman on 30 April 2024, with thanks to the newspaper for permission to republish here.