Sky notes for 2025 June & July

2025 June 1

Nick Hewitt

(Written for 23:00 BST in the UK on July 1)

Midsummer is hard work for astronomers, except for those focused on solar observations. Deep-sky enthusiasts must wait until normal bedtime to begin, while planetary monitors must get up before a very early dawn to catch the outer gaseous giants emerging into the morning sky. With the summer solstice on Jun 21, it is tempting to take a break for a week or two either side of the longest day.

The deep sky

Once darkness finally falls at midsummer, the meridian is dominated by three of the largest of the 88 recognised modern constellations. Hercules is fifth in size, occupying 3 per cent of the sky by area. To its south – and head-to-head with the strongman – lies the more peaceful healer, Ophiuchus, taking up 2.3 per cent, at eleventh on the list. To the north of Hercules sprawls Draco, the Dragon, which occupies the zenith.

Flanking this huge trio lie two much smaller constellations. To the west lies Corona Borealis, the Northern Crown – 73rd of the 88, occupying 0.4 per cent of the sky. To the east is Lyra, ranked 52nd and covering 0.7 per cent.

At the time of writing, Corona Borealis is attracting particular interest of the ‘will it or won’t it’ variety – will T CrB, the ‘Blaze Star’, burst into life as predicted? Perfectly placed throughout June and July, it would be most generous if it were to oblige with a performance. While awaiting this rare event, it is worth monitoring nearby R CrB, in case it dims due to a sooty outflow.

Northwest of the Northern Crown lies sigma Corona Borealis, a fine binary for small telescopes. Discovered by F. G. W. Struve in 1827 with a separation of just 1.3 arcsec, the pair is widening and is currently about 7 arcsec apart. The orbital period is estimated at 726 years, so we have time to enjoy it. Sigma B is a spectroscopic binary that was resolved by the Center for High Angular Resolution Astronomy array, and there is a common proper-motion companion, 635 arcmin distant, that we now know to be a pair of red dwarfs. Sigma A is magnitude 5.6, sigma B is 6.4, while distant sigma C is 12.2.

Hercules lies to the east of the Northern Crown and being huge – as befits the hero – it contains a wealth of interest. Its three famous globular clusters – Messier 13 (M13) and M92, plus NGC 6229 – attract the most attention, but other enjoyable targets await, even if they are trickier in midsummer. The planetary nebulae in Hercules present interesting challenges, with NGC 6210 having sufficient surface brightness to be within reach. NGC 6058, at 12th magnitude, is considerably smaller and fainter. (See Jim Latham’s Observers’ Forum on page 212 of this issue.) For experienced imagers, the faint, old, spherical planetary nebula Abell 39 (Figure 1) can be very satisfying. Thought to be around 1.4 light-years in diameter, it is low surface brightness but 155 arcsec in apparent diameter, with a 15th-magnitude central star.

If the sky is still too bright to enjoy extended objects, turn instead to interesting point sources – be they single, double, or multiple stars. Hercules boasts many enjoyable double stars to while away the late summer evenings, and a small selection includes alpha Herculis (Rasalgethi, ‘the head of the kneeler’). One of the largest stars known, this M-type supergiant and semiregular variable (ranging from magnitude 2.7 to 4) sits on the asymptotic giant branch of the Hertzsprung–Russell (HR) diagram. Its pulsations cause it to swell from 264 to 303 solar radii. It is a binary system, with a fifth-magnitude yellow giant at 4.6 arcsec that is often described as greenish – a contrast effect. The companion is a spectroscopic binary. Delta Herculis (Sarin) is a very nice double at magnitude 3.1, located southeast of the central ‘Keystone’ asterism in Hercules. The fainter companion lies at 8.5 arcsec and with magnitude 8.7 it is straightforward in small telescopes. Another easy pair is 100 Herculis: a couple of evenly bright white stars of magnitude 5.8, separated by 8.5 arcsec. One for larger apertures, good eyesight and the steadiest nights is 86 Herculis, a yellow star accompanied by a tight pair of red dwarfs separated by a mere 1 arcsec.

To the east of Hercules, Lyra signals the arrival of the dominant Summer Triangle with its Milky Way riches that will grace high summer and autumn nights. Although tiny compared with the oversized gentlemen to the west, it is easy to pick out and contains much of interest. The famous Ring nebula (M57), so neatly placed between Sheliak (beta Lyrae) and Sulafat (gamma Lyrae), is generally one of the first targets for the embryonic deep sky observer.

Tearing ourselves away from this spectacular smoke ring of a planetary nebula, we can observe other important stars and deep sky objects. Beta Lyrae (Figure 2), the prototype of the β Lyrae class of eclipsing binaries, consists of two giant stars so close to one another that they distort into egg-like shapes. Their variability was noted by John Goodricke in 1784, and their light fluctuates between magnitude 3.25 and 4.36 with a 12.913834-day period. It is that precise! Almost due south of Vega, beta Lyrae can be compared with gamma Lyrae, which shines steadily at magnitude 3.25, meaning they are of equal brightness when beta Lyrae is at maximum. It is one of the most studied stars but hard to decipher; current thinking is that they are a pair of B-types of unequal mass and the more massive evolved faster, became a red giant and then deposited much of its mass on its less-evolved companion. This has resulted in a thick disc of matter torn from the more evolved star, obscuring it and resulting in the smooth light curve. Another beginner’s star of great interest.

A degree and a half east of Vega lies epsilon Lyrae (Figure 3), the famous ‘Double Double’ – a pair of 5th-magnitude stars that are themselves double. The wide pair, at 3.5 arcmin apart, is striking through a finder or binoculars, and is sometimes regarded as a severe test of naked-eye acuity. Closer scrutiny reveals each of these stars is a close but splittable binary. Discovered by William Herschel in 1779 and first measured by F. G. W. Struve in 1831, the system has been closely monitored ever since. The components of epsilon¹ (the western pair) are of magnitude 5 and 6, and separated by 2.8 arcsec. With an orbital period of around 1,000 years, the pair is slowly closing. Epsilon², the more southerly and easterly component, consists of stars 2.6 arcsec apart of magnitude 5.1 and 5.3. Their period is also around 1,000 years. The whole system shares a common proper motion and is considered a quadruple unit.

Much fainter, but of great importance in mapping the Galaxy, is the famous variable star RR Lyrae (Figure 4; p. 216). It is the prototype of the ‘Cluster Variables’, so-called because so many are found in globular clusters. These are pulsating stars that in some respects resemble Cepheids, although they are fainter, have shorter periods and display smaller amplitude variations. RR Lyrae is the brightest of its kind, varying from magnitude 7.1 to 8 over 0.566 days, so its whole cycle can be followed in a single night – although not in June or July! To locate it, start at Vega, and pass through epsilon Lyrae towards delta Cygni; RR lies just south of a shallow arc of three 7th-magnitude stars. It was through the study of Cepheids and RR Lyrae stars that Harlow Shapley reshaped our understanding of the Milky Way, relegating the Sun to the suburbs.

Two contrasting clusters in Lyra should be easy enough to observe. NGC 6791 is a large, rich open cluster, made up of faint stars lying on the border with Cygnus, just south of theta Lyrae, forming a triangle with two 6th-magnitude stars. Spanning over 10–15 arcmin, and with good contrast, the hundreds of 11th- to 13th-magnitude members are rewarding. The under-observed globular cluster M56 (Figure 5, p.217) lies between Sulafat (gamma Lyrae) and Albireo (beta Cygni). Ironically, this representative of globular clusters in Lyra is devoid of RR Lyrae stars, making its distance more difficult to determine. At magnitude 8.4, it appears as a fuzzy star in a rich starfield when viewed through a finder and it is difficult to resolve in small telescopes, despite being classified as Shapley Class X (loose). It is also quite small, at 7 arcmin across.

By the end of July, UK nights slowly lengthen again, and the northern Milky Way is at its most spectacular. The southern aspect is fine too, but diminished by its low altitude. If you can travel south – perhaps to the Mediterranean or beyond – the centre of our Galaxy comes into view, and from a dark site it is breathtaking. If you go, take binoculars at least, or one of these new-fangled smart scopes. Scorpius is visible in its entirety – an impossibility from Britain. Antares’ ruddy hue is unmistakable, but its greenish companion requires a telescope, steady atmosphere and luck. Although magnitude 6.5, it lies only 3 arcsec from the glare of the huge supergiant.

The Scorpion is littered with both open and globular clusters, as well as dark and bright nebulae. M4 (Figure 6) is the closest globular and loosely packed, thus easily resolved. Lying just west of Antares, it is visible from Britain, unlike the splashy open clusters M6 and M7 (Figure 7). They lie near the scorpion’s ‘sting’ and are superb in binoculars from further south.

A favourite asterism here is the ‘false comet’ (Figure 8) – although to me, it looks like a gecko clinging to the curved vault of heaven. It comprises several parts: the head and eyes by a trio of 3rd-, 4th-, and 5th-magnitude stars – zeta¹ and zeta² Scorpii – and to the north is the superb bright open cluster, NGC 6231, which forms the reptile’s beating heart. The tail is formed by a stream of stars, along with the cluster associated with the emission nebula IC 4628.

The solar system

Sun & Moon

The UK summer months are not the best time to observe the Moon, as it hugs the southern aspect of the ecliptic.

In contrast, activity on the Sun (Figure 9) is at its maximum and has recently been displaying multiple sunspots and prominences. The long summer days are ideal for solar observation – but also a good way to get sunburn or even sunstroke. Take care not only with your eyes, but also with your skin and hydration. Early morning is usually the best time to observe, when the seeing tends to be most stable.

There are no solar or lunar eclipses during high summer in 2025.

Planets

Mercury reaches superior conjunction on May 30. It makes a brief appearance in the evenings during late June and early July, before moving to inferior conjunction on Jul 31.

Venus reaches western elongation on May 31, making it brilliant in the early morning sky. It climbs higher as June progresses, reaching a good elevation before dawn in July.

This is the last chance to observe Mars, which remains an evening planet in June, lying low in Leo. By July, however, it is lost in summer twilight, with a poor magnitude of +1.6 and a 4-arcsec disc.

Jupiter is in conjunction with the Sun on Jun 24, so patience is needed while we await its reappearance in late July mornings.

Saturn appears in the early mornings at the end of June, becoming more accessible by late July. Now in Pisces, it reaches its highest declination for many years, although it will be best observed at the end of summer, reaching opposition on Sep 21. While we missed the ring plane crossing when Saturn was behind the Sun, we will begin to see slightly more of the southern hemisphere from now on.

Uranus moves slowly within Taurus in late June but is a very tricky target in a bright sky.

Neptune lies just north of Saturn at the end of June and through July. However, it is  then a very challenging object, so it is best to wait until September for better visibility.

Minor and dwarf planets

(2) Pallas is well placed in Delphinus as it approaches opposition in early August. At 9th magnitude, it may prove difficult to identify amidst the rich starfield.

(3) Juno is a 10th-magnitude dot in Serpens Caput. To its west, try for (4) Vesta in Virgo (a Vestal Virgin?), the brightest of the trio at magnitude 6.

(134340) Pluto is now in Capricornus, where it is marginally higher than in recent years, but at magnitude 14.4 and stellar in appearance, it may still fall into the ‘too difficult’ category.

Meteors

The June Boötids peak on Jun 27, when the Moon is a thin crescent, setting early. Watch for any enhanced activity, as occurred in 1916 and 1998. These slow meteors originate from comet 7P/Pons–Winnecke.

The Alpha Capricornids are not affected by the Moon this year. Their peak is on Jul 30. These meteors can be colourful, although they are infrequent, and occasionally produce a fireball. The shower is now thought to originate from periodic comet 169P/NEAT.

The Southern Delta Aquarids should be very favourable at their maximum on Jul 31, with no lunar interference – the crescent Moon sets early. It is worth observing for several days either side of the peak, as there can be a steady stream of faint meteors. The shower may originate from comet 96P/Machholz, discovered in 1986.

Comets

There are no bright comets anticipated during this period at the time of writing.

Noctilucent clouds

Between May and the end of July is the season for noctilucent clouds – or polar mesospheric clouds – the ghostly bluish formations seen low in the northern sky. Forming about 80 km up in the atmosphere, they can often be spectacular after twilight if conditions are right.