Sky notes for 2024 December & 2025 January

2024 December 1

Nick Hewitt

(Written for 21:00 GMT in the UK on January 1)

After last winter’s disappointing weather, can we hope for pristine sparkling nights full of freshly washed stars? Let us hope so. The winter solstice falls on Dec 21 and many hours of deep darkness can be enjoyed, with the outer planets available and the inner planets improving after a difficult autumn. Orion, as ever, is our winter guide, although Pegasus remains available to help navigate the autumnal constellations as they sink into the west.

The deep sky

On the cusp of autumn and winter lies one of the grand constellations – a major player in the Andromeda rescue thriller, starring the Hollywood action hero of the sky: Perseus (Figure 1). Being immersed in the winter Milky Way, the pattern is a treasure trove for the deep-sky enthusiast. The constellation has represented a heroic figure in many cultures over the centuries: Khem to the Egyptians, Mithras to the Persians, David (with the head of Goliath) in biblical times, and it is now best known as the Greek Perseus, ‘the Rescuer’ or ‘the Champion’.

Although there are no first-magnitude stars in Perseus, it boasts several of second and third magnitude, giving it a richness. Nevertheless, the pattern may be elusive at first. The alpha star is Mirfak, and at magnitude 1.8 it commands the Alpha Persei Moving Cluster, also known as Melotte 20 or Collinder 39. This very loose cluster subtends some five degrees and is easy to spot with the naked eye but superb through binoculars. Many of its constituent stars are hot, bright O and B spectral types, forming the Perseus OB3 association, estimated to be around 560 light-years distant. Mirfak itself is a supergiant of spectral type F5 and sits at the centre of a shallow curve of stars in the upper third of the constellation. This arc stretches from eta Persei in the northern section, through gamma Persei, then onto Mirfak, and continues toward delta and mu Persei.

From Mirfak, a sweep slightly southwest finds a very famous second-magnitude star: Algol, or beta Persei. Its variability spooked the ancients, and over the centuries it has had many names. ‘Algol’ derives from the Arabic Al Ras al Ghul, meaning ‘Demon’s Head’, although the Greeks referred to it as the severed head of the Gorgon Medusa, she of serpentine-coiffure and petrifying-gaze fame. Its variation in magnitude from 2.1 to 3.4 and back to 2.1 takes 2 days, 20 hours, 48 minutes, and 56 seconds. This was known but not understood until John Goodricke in 1782 suggested that the fading was due to a partial eclipse by a dark companion. Just over 100 years later it was proven to be a spectroscopic binary by H. C. Vogel, and it remains a great starter target for embryonic variable-star observers. First, locate Algol, then find Almach (gamma Andromedae), the brightest star to its west. At magnitude 2.2, Almach is normally nearly as bright as Algol but is clearly brighter during Algol’s regular fade.

Returning to Mirfak, sweep 7 degrees east to some of the second-best open clusters in the constellation. The trio of NGC 1513, NGC 1528 (Figure 2), and NGC 1545 lies within a binocular field along with 4th-magnitude lambda Persei. NGC 1545 is a triangular-shaped cluster adjacent to b Persei, featuring a 7th-magnitude orange star at one apex, an 8th-magnitude yellow star at another, and a 9th-magnitude blue-white star at the remaining angle. NGC 1528 is a rich cluster composed of faint stars that collectively shine at 6th magnitude.

Of course, the best-known open clusters in Perseus form the stunning Double Cluster (Figure 3), which represents the sword handle of our hero. Easily spotted on the border with Cassiopeia, these clusters must be amongst the first deep-sky objects the newcomer observes, and the pair remain heavily imaged. NGC 869 and NGC 884 lie within a 1-degree field and are both filled with brilliant white stars. NGC 884, less compact but larger, contains several red supergiants, most of which are variable.

Southeast of the Double Cluster, find Miram (eta Persei), an easy double star with components of magnitude 3.8 and 8.4 separated by an easy 28 arcsec. Unusually, the name is not ancient; it appeared just in the last century and was only approved in 2017!

In the far south of the constellation lies a visual challenge for those equipped with large binoculars and pristine, dark, moonless skies. The California Nebula (NGC 1499; Figure 4) is a huge, low-surface-brightness emission nebula that spans 2.7 × 0.7 degrees, lying just north of 4th-magnitude Menkib (xi Persei). Too large to fit within the field of view of most telescopes, a hydrogen-beta filter is said to make the features of this nebula more apparent.

One of winter’s less well-known variable nebulae lies within Perseus and is a small gem that contains reflection, emission, and dark nebulosity, all within a compact 6 × 5 arcmin patch that lies in the north of the constellation. NGC 1333 is the bright reflection nebula situated on the dusty border with Taurus, although it is not part of the Taurus Molecular Cloud. Instead, it resides more than twice the distance away, at around 1,000 light-years, on the western edge of the Perseus Molecular Cloud – a very active star-forming region. The variable nebula is a small, fan-shaped portion of the whole, sometimes designated as the RES nebula after the Swiss team (Christian Rusch, Egon Eisenring, Rainer Spaeni) who noted a fade in brightness of the fan near stars HBC 340 and 341, associated with NGC 1333, in 2014. They alerted professionals, who reviewed Palomar Transient Factory images from 2011 and confirmed a distinct minimum between 2014 Aug and Sep. The nebula had recovered by 2014 Dec. Could it fade again?

For galaxy enthusiasts, Perseus has riches too. In addition to the Milky Way, there are several galaxies within its boundary, including the fine lenticular NGC 1023. This is one of a group, of which the best known is NGC 891 in Andromeda. However, for a real challenge, examine the Perseus Galaxy Cluster (Abell 426; see image at top of p.454) to the east of Algol, of which NGC 1275 is the brightest member. The latter was oddly missed by William Herschel, although he recorded others in the group. Harbouring the powerful radio source Perseus A, NGC 1275 was classified as a Seyfert galaxy due to its peculiar emission-line spectrum. With a visual magnitude of around 11.9, it is not easy to spot and at 300 million light-years, this is not surprising. Around 40 or more members of the Perseus Galaxy Cluster can be imaged buzzing around NGC 1275, although the majority are much fainter. The cluster is thought to have some thousand members, making it one of the most massive in the Universe.

Finally, escaping Perseus, we nip into the realm of his mother-in-law-to-be, Queen Cassiopeia, mother of Andromeda. Cassiopeia is another rich constellation, choc-a-bloc with clusters and nebulae. These include a pair of large emission nebulae that lie northeast of the Double Cluster and have become popular since the introduction of interference filters. They are IC1805 and 1848, nicknamed the Heart (IC 1805) and Soul (IC 1848) Nebulae (Figure 5), although IC 1848 looks more like the sole of a shoe than anything spiritual.

The solar system

Moon & Sun

There are no lunar or solar eclipses during this period. However, January brings two notable lunar events involving the crescent Moon: one sees it close to Venus and the other is an occultation of Saturn. Details are provided below.

Planets

Mercury is at inferior conjunction on Dec 6, then moves into the morning sky for a reasonable apparition in Scorpius. It reaches an altitude of 9 degrees on Christmas morning when its 6-arcsec disc shines at magnitude −0.4. In January, it is very low in the morning sky before superior conjunction on Feb 9.

After an autumn spent scraping the horizon, Venus begins to improve in altitude and its gibbous disc swells from 17 arcsec at the beginning of December to 22 arcsec as the year ends. This coincides with a slight brightening, with the planet reaching magnitude −4.4 at Christmas, but it will achieve a much higher altitude thereafter. The crescent Moon lies very close by on Jan 3 and the pair should be a pretty sight in the early evening. Dichotomy is on Jan 11. By mid-January, Venus lies just north of Saturn which is 5 magnitudes fainter. The end of January sees Venus as a beacon at −4.7 and nearly 30 degrees above the southwestern horizon, by now a waning crescent 31 arcsec in diameter.

Mars (Figure 6) begins its retrograde motion on Dec 8 while in Cancer, manoeuvring into Gemini. By mid-December, the Red Planet shines at magnitude −0.8 and subtends a disc of 13 arcsec. By opposition on Jan 16, it is 14.5 arcsec and magnitude −1.4, the small diameter being compensated for by its favourable altitude just southeast of the Twins, Castor and Pollux. Whether the dust activity of this autumn will continue we shall have to see. It is spring in the Martian northern hemisphere, and the polar cap is shrinking. By mid-evening on the night of opposition, the most striking feature is on display: the India-like Syrtis Major. By midnight, Sinus Meridiani (the ‘Greenwich’ of Mars) should be near the meridian. Enjoy viewing the planet this year, as at opposition on 2027 Feb 19 it will be even smaller (13.8 arcsec) and at a lower altitude in Leo. See Dr Richard McKim’s Mars blog at: tinyurl.com/mrx2sf87.

Jupiter reaches a spectacular opposition on Dec 7 when it dominates the winter sky all night at magnitude −2.8. High in Taurus, it should be available for detailed observation both visually and for imagers. Following the dance of the Galilean moons around the parent body is endlessly fascinating, even with binoculars.

Saturn is well past its September opposition but is still available in early evening in December. On Jan 4, we can enjoy another occultation of the planet (Figure 7) by the crescent Moon. This is an early evening event beginning around 17:21 (depending on the observer’s exact location), with Saturn disappearing past the dark limb only to reappear on the bright limb an hour and a quarter later. Titan follows Saturn, being occulted four minutes later. Note that the 6th-magnitude 85 Aquarii is closer to Saturn that night and brighter than 8th-magnitude Titan. It will also be occulted.

Uranus is very well placed on the Taurus/Aries border at magnitude 5.7. Due to its extraordinary tilt, the five ‘Shakespearean’ moons orbit face-on to us, and on Jan 5 in the early evening they lie in a line to the north of the planet, although they move remarkably rapidly.

Neptune is past its best, although it remains available in the early evening, when it can be found in western Pisces at magnitude 7.8.

Meteors

The peak on Dec 14 of the best of the annual showers, the Geminids, is spoiled by the Moon which is full the following night.

The Quadrantids fare much better for those that brave the post-midnight cold to watch for them as they radiate from the Draco-Boötes border. The peak on Jan 4 is narrow but could be very busy. The parent body is thought to be 2003 EH1.

The Ursids peak on Dec 22 and are rarely abundant. Although under-observed, they are prone to outbursts. The radiant is near Kochab (Beta Ursa Minoris) and the meteors are thought to derive from comet 8P/Tuttle.

Comets

By early December, comet C/2023 A3 Tshuchinshan–ATLAS has faded considerably and remains low in Aquila after sunset, eventually becoming obscured by the twilight glow. In January, still in Aquila, the much-diminished comet appears just above the eastern horizon at dawn, but is likely to be very difficult from the UK.

Although 333P/LINEAR is unlikely to be bright following its perihelion on Nov 29, it is well positioned for observation as it speeds past Mizar in early December. The comet then moves through Draco and appears to slow down as it enters western Pegasus by the end of January. It may peak at magnitude 10. Discovered in 2007, it is a Jupiter-family comet which is in a retrograde orbit.