Sky notes for 2023 August & September

2023 August 1

Nick Hewitt

The autumnal equinox on Sep 23 heralds longer evenings for observing, and we now have outer planets to enjoy on warm late-summer evenings. The year’s most popular meteor shower is very favourable too. The seasonal guide of the Summer Triangle is overhead, its three first-magnitude stars prominent as darkness falls. Vega in Lyra is first to appear, followed by Altair in Aquila, then Deneb in Cygnus. The rich Milky Way flows through Aquila and Cygnus and is a fine sight from a dark location. Rich in clusters and nebulae of all varieties, the season is full of enough delights to last a lifetime.

Lyra is a little off the main band of the Milky Way, being just to its west. Much further south, Capricornus is also just too far east of the Milky Way to contain significant deep-sky quarry and is often overlooked. Although the 10th zodiacal constellation, it has not that much of interest now that Jupiter and Saturn have deserted it. The sea-goat is the 40th constellation in order of area, but what the Babylonians were seeing in this pattern 4,000 years ago stretches the imagination, with its goat’s head and fish’s tail. Its stars are not brilliant either, the brightest being delta (2.8), lying on the eastern border. (On 1846 Sep 25, Neptune was tracked down just four degrees north-east of delta by J. Galle in Berlin.)

However, the more westerly stars are fun for the binocular or rich-field telescope user. Al Giedi (‘The Goat’) is the alpha star (magnitude 3.15): a wide naked-eye double separated by 376 arcseconds (ʺ). Both components are G-type stars with fainter companions. A² is brighter at 3.56; the companion is 11th magnitude and 6.6ʺ distant. A¹ is magnitude 4.24, and the companion 9th magnitude, at 44ʺ. All are optical doubles. Dabih (beta) is another wide common-motion pair, with magnitudes of 3.1 and 6.6 and a separation of 205ʺ. Although the brighter star is a spectroscopic triple, the interest here is in the fine colour contrast of yellow and blue. Other double stars within Capricornus worth pursuing are pi (magnitudes 5.3 and 8.9, separated by 3.2ʺ, white and blue) and omicron (6.1 and 6.6, separation 21.9ʺ, white and blue).

By far the best deep-sky object in the constellation is Messier 30 – a globular cluster and one of the more southerly in Messier’s catalogue (although several in Sagittarius are lower). At 7th magnitude, it is a finder object, but quite small. Although said to be 11 arcminutes in diameter, typical amateur telescopes will only see half of this easily. The tight, bright core is difficult to resolve, but the looser periphery surprisingly easy. On the Shapley–Sawyer scale of globular-cluster concentration, it is an intermediate class V.

At the other extreme, much further north, we have Cepheus: the mythical spouse of Cassiopeia and father of Andromeda. Less showy than his brazen queen, he is miserly with his riches, but exploration of his treasury is rewarding. Cepheus is a northern circumpolar constellation, the 27th largest of the 88, but it does not boast many bright stars. The main pattern resembles a child’s drawing of a house, with the apex of the roof represented by third-magnitude Errai (gamma Cephei) at 77°N. The brightest star is Alderamin (alpha Cephei), a white A-type star of magnitude 2.45 that lies in the south-west of the constellation. With zeta (3.35), it forms the base of the ‘house’.

Zeta itself forms a small triangle with epsilon and delta, the latter being one of the most significant stars in the sky. It is the prototype star of its class, the Cepheid variables. These pulsating giants and supergiants are used as standard candles for estimating the distance scales in the local universe, as their period of variation gives their luminosity – the longer the period, the greater the luminosity. Delta varies from 3.48 to 4.37, with a period of 5.366249 days. Its distance is now accurately known to be 890 light-years and is an important calibrator of the period-luminosity relationship, fundamental to estimating the distances to globular clusters and nearer galaxies. Compare its brightness with zeta and epsilon over five days and see the rapid rise and more leisurely fall. It is also a pretty double star, having a 6th-magnitude companion 41 arcseconds away, and the pair are of contrasting yellow and blue, as if a subdued Albireo (beta Cygni).

Between Alderamin and delta Cephei lies one of the ruddier stars in the heavens: mu Cephei, or Herschel’s Garnet star. This very red supergiant is a semiregular variable that veers erratically from magnitude 3.4 to 5.1. It is one of the largest stars known, although the exact size is uncertain. Some 1,500 solar radii would be in the correct ballpark. This glowing coal lies on the edge of a huge faint complex of emission and dark nebulae beloved of imagers, IC 1396. Although the large nebula has no nickname, it contains the ‘Elephant’s Trunk’: a dark, dense globule silhouetted against a faint glow.

Returning to Alderamin, hop east to eta Cephei, then further south-east to find a neat galaxy-cluster duo. The open cluster is NGC 6939: an average and somewhat unexceptional cluster but better known than many, due to being 39 arcminutes northwest from the Fireworks Galaxy, NGC 6946. The Fireworks name has two origins: it is a very busy star-forming intermediate spiral, and this has led to 10 supernovae being seen in the last 100 or so years – a record for an individual galaxy to date.

Travel north from Alderamin to Alfirk (beta Cephei) and use this as a start to locate the beautiful reflection nebula known as the Iris, NGC 7023. It is a little east of the fascinating variable nebula noted in 1977 by Armen Gyulbudaghian. This faint nebula and its young stellar object PV Cephei are currently difficult, but variation occurs on short time scales, so it is monitored by some of the BAA’s most skilled observers.

While in this region, find and monitor T Cephei, the BAA’s Variable Star of the Year for 2023. This bright Mira-type variable is just east of the Iris Nebula and is most suitable for binocular users who wish to contribute to the Variable Star Section. (See 2023 Handbook of the BAA, P105–7.)

Other fascinating variables are now slipping into the west at the time of writing (June). R Coronae Borealis is fading from its sixth-magnitude norm – will it have recovered by late summer? It has a sooty belch from time to time and can fade as low as 15th magnitude. The 2007 fade was unusually protracted. The recurrent nova T Coronae Borealis has been hinting at a flare-up.

In the east autumnal sky, watery constellations are rising. Aquarius is well placed by late September and now contains Saturn. Then Pisces and Cetus follow, as summer gives way to autumn and nights are longer than days.

The solar system

August has this year’s ‘Blue Moon’ as we have two full Moons, on Aug 1 & 31. Blue Moons are fairly infrequent, the next being on 2026 May 31 and thereafter 2028 Dec 31.

The Sun reaches the autumnal equinox on Sep 23. Its northern hemisphere remains busy, and we should see many active regions into the autumn.

There are no lunar or solar eclipses during August or September.

Mercury reaches eastern elongation on Aug 10, but will not be visible from the UK through August except during daylight. After inferior conjunction on Sep 6, it has its best morning apparition of the year later in September, reaching western elongation on Sep 22 when 18 degrees from the Sun and almost 15 degrees above the horizon before dawn. Its seven-arcsecond disc is then half phase and shining at –0.5, so while difficult, it is a good opportunity to see this speedy planet.

Venus is at inferior conjunction on Aug 13, then slips into the morning sky to become brilliant before sunrise at magnitude –4.5. Moving from Cancer to Leo during September, Venus is well up in the south-east before dawn while still showing a waxing crescent.

Mars is lost in evening twilight and essentially unobservable for the rest of 2023.

Jupiter begins its apparition rising before midnight in early August and is impressive by the end of September, shining brightly at –2.7 in Aries.

Saturn is at opposition on Aug 7, so well placed throughout this period. Although still south of the celestial equator in Aquarius, the ringed planet is at its best altitude for many years. The rings are closing up now, although the next ring-plane crossing is still two years off. Nevertheless, the Saturnian system of moons now lies more comfortably for identification as they are in the ring plane. On the night of opposition, 8th-magnitude Titan is easily spotted to the east, 11th-magnitude Iapetus quite a way out to the west, and the other brighter moons lined up nicely in between. Titan will then pass south of Saturn five days later, while Iapetus on its distant orbit seems very far from the parent planet.

Uranus is just visible with the naked eye from a dark site, at magnitude 5.6. It has a greenish disc of 3.8 arcseconds. In eastern Aries, it becomes better positioned by the end of September.

Neptune lies south of the Circlet of Pisces, reaching opposition on Sep 19. It is a tiny bluish disc of 2.4 arcseconds and magnitude 7.8. Drop a line from iota Piscium through lambda Piscium to find remote Neptune at a similar distance south of lambda.

None of the brighter dwarf planets or asteroids are available, although Pluto, at magnitude 14.4, is in eastern Sagittarius.

Meteors

The Perseids peak on the night of Aug 12/13 and this reliable shower is spared lunar interference this year. Many meteors can be seen either side of the peak of course, as the shower ranges from mid-July until late August. Associated with Comet 109P/Swift–Tuttle, the Perseids have been known for two millennia and are sometimes dubbed the ‘Tears of St Lawrence’. While not the richest shower, its timing in the balmy August nights make it a favourite.

The Kappa Cygnids are much less rich but not affected by the Moon when they peak on Aug 17. As the shower overlaps with the Perseids, meteors not emanating from Perseus but from kappa Cygni may not be sporadics. The shower is often absent, but there are occasional bright fireballs. The parent body remains unknown.

Alpha Aurigids are at their maximum around Aug 31, but the Moon interferes as it is full that night. This shower is rather an anticlimax after the Perseids, as it is weak and although occasional outbursts have been recorded, these will be hard to detect this year. The meteors derive from comet C/1911 N1 (Kiess).

Comets

C/2023 E1 (ATLAS) is well placed in Cepheus in early August, but past its best. See ‘Comet of the Month’ at bit.ly/3K4P6Wu.

Periodic comet 2P/Encke appears in the morning sky throughout September, beginning in northern Gemini and rapidly moving into Leo by the end of the month. It is at perihelion on Oct 23 but will be unobservable by then. However, it may brighten to be a binocular target during September.