Sky notes for 2024 April & May

2024 April 1

Nick Hewitt

As the days become longer and warmer, we approach the ‘silly season’ for UK amateur astronomers, with observing opportunities limited to later evening for most. Adding to the frustration, the planetary scene is disappointing, with no major solar-system objects other than the Sun and Moon making a worthwhile appearance during April and May. We must wait until autumn for evening observations of our relative neighbours.

Deep sky

There is always the deep sky of course, and in April it is still open season on the local supercluster of galaxies, with groups in Ursa Major, Canes Venatici and Coma Berenices high in the sky. By May, even the bright late-spring skies cannot hide some of our Galaxy’s spectacular globular clusters – Messiers 5, 13, and 92 being favourites. Despite British Summer Time, April nights are still long enough to enjoy the deep sky at a reasonable hour and the Moon does not interfere until mid-month. By May, the brighter skies make hunting relatively faint quarry more challenging. With Ursa Major at the zenith, it becomes our guide.

The prominent tail – not present in modern ursids – sweeps a curve to the brilliant warmth of Arcturus, the brightest star in the northern hemisphere, and a further sweep south brings Spica (alpha Virginis) into view in the southeast. Spica is a B-type star, thus blue-white and magnitude 0.95. It does not attract too much attention from amateurs, but it is a spectroscopic binary with a rapid four-day period, and the pair are so close they are distorted into egg-shaped stars. The primary is a beta Cephei variable whose pulsations result in rapid but shallow brightness variations. Spica is the southernmost member of the unofficial ‘Spring Triangle’, with Arcturus to its northeast and Denebola (beta Leonis) to its northwest; within this asterism lies the Realm of the Galaxies – the core of our local supercluster.

The Coma–Virgo Cluster is extraordinarily rich, if a bit confusing at first, even with GoTo technology. Star- and galaxy-hopping are recommended if you are a visual observer as so many skills develop by using those methods. Although many galaxies at the centre of the cluster are around 53 million light-years distant, several are visible in a good large finder from a dark location. Commence exploration by sweeping slowly southeast from Denebola towards epsilon Virginis (Vindemiatrix) and several 8–9th-magnitude galaxies will appear. Identifying the pair of giant ellipticals, Messiers 84 and 86, is a great place from which to launch your voyage. This pair represent the western aspect of a swathe of galaxies known as Markarian’s Chain (Figure 1; page 150).

A little to the southeast of the Chain lies the extraordinary beast Messier 87 (Figure 2), the most massive galaxy in the Virgo Cluster. It boasts a supermassive black hole (SMBH) at its centre, recently imaged by the Event Horizon Telescope. This monstrous galaxy comprises a sphere of older stars, 130,000 light-years in diameter and of an estimated 2.7 billion solar masses. Although its diameter is not much larger than that of our Milky Way, being spherical, it packs a much vaster volume. It may be the most massive galaxy known to us. The jet generated by the SMBH is detectable by modern amateurs. First noticed in 1918 by Heber Doust Curtis at Lick Observatory, it is 65,000 light-years long and can be resolved into knots using giant telescopes.

From this position, many Messier galaxies can be tracked down nearby (Figure 3), including Messiers 89 and 90 to the east, and 58 and 60 a little further south. Once familiar with the area, branch out to find the slightly more peripheral Messiers 98 and 85 to the north, and 49 and 61 to the south. Of course, there are numerous NGC galaxies milling about too, just to make the search more challenging, and fun of course.

Talking of jets, just north of the celestial equator is 3C 273, arguably the most historically important quasar (Figure 4a). Radio astronomy took off in the late 1950s and 3C 273 represents the 273rd entry in the 3rd Cambridge Catalogue of Radio Sources from 1959. In 1962, 3C 48 and 3C 273 were occulted by the Moon, and British astronomers Cyril Hazard and John Bolton used Murriyang, the Parkes radio telescope in Australia, to pinpoint the sources accurately. This enabled Dutch astronomer Maarten Schmidt to optically identify 3C 273 and obtain a spectrum using the 200-inch Hale Telescope on Mount Palomar in 1963. The result was unexpected, with redshifted emission lines suggesting the object is receding at 47,000 km/s, corresponding to a redshift of 0.158, now regarded as a distance of 2.5 billion light-years. Today, we know 3C 273 is a radio-loud quasar and the giant elliptical galaxy has been imaged by amateurs, appearing as a faint smudge. At a typical magnitude around 12.8, it is one of the most distant objects visible in modest telescopes, although it only ever appears stellar. Nevertheless, it is possible to capture its enormous jet (Figure 4b). To find this distant powerhouse is not easy, as it lies in a slightly dull field, but track west from Porrima (gamma Virginis) towards 16 Virginis and then use a chart (as in the 2003 BAA Handbook) to locate the faint but important beast.

Porrima (Figure 5) itself is the superb binary that lies at the base of the bowl (see the star chart) formed by Vindemiatrix (epsilon), Minelauva (delta), Zaniah (eta), 3 Virginis, and Denebola in Leo. Within this vessel many galaxies can be found, especially near the rim of the glass. Porrima is a binary that currently can be followed with interest. The components are very similar in magnitude (3.4 and 3.5) with a position angle of 352 degrees and separation of 3.37 arcseconds, although this will change quite rapidly over the next few years. The distance is now widening from periastron in 2005, when the separation was 0.9 arcseconds and impossible to split by amateurs. The period is 168.93 years and the stars have been followed through one full orbit.

As Boötes climbs higher in the east, it drags the Northern Crown (Corona Borealis) in its wake, containing the recurrent nova T Corona Borealis. This fascinating variable is predicted to flare this year (see p. 88) – it may even have done so by the time this is published. The star, which usually hovers around magnitude 10.8 has dramatically brightened on two occasions: by eight magnitudes in 1866 and seven in 1946. In 1938, it brightened by a magnitude then faded slightly before the latter outburst and recently it has been doing this again. Predictions are fraught with difficulty but try to observe this rare recurrent nova as often as possible this year, flaring or not.

The solar system

Sun

The active Sun is now high up and observation possible for many hours.

On Apr 8, a total solar eclipse may become the most observed in history. The path of totality begins in western Mexico and travels over many well-populated cities, including (in order) Mazatlán, Torreόn, Dallas, Detroit, Niagara Falls, Toronto, and Montreal, before terminating on Canada’s east coast. The best weather prospects are in the west, deteriorating the further east one goes. In the UK, only a partial eclipse will be visible at sunset, and nothing will be seen in the south and southeast of England.

Venus will be obvious to the west of the eclipsed Sun during the late stages before second contact, and Jupiter should be easy to spot in the east. Saturn and Mars will be close together in the west, the pair being rather over twice as far as Venus is to the eclipsed Sun.

The day before the eclipse, those in Mexico and the USA will have a daytime challenge of witnessing an occultation of Venus by the Moon in daylight. Clearly the Moon will be a very thin waning crescent at this point. Timings vary across the continent.

Planets

Mercury is at inferior conjunction on Apr 11 and is difficult to glimpse in the evenings before that. Emerging in the morning sky thereafter, it is too low throughout May despite western elongation on May 9.

Venus is not visible in the morning sky as it moves towards superior conjunction on Jun 4. Although technically observable in the daytime, its small disc (around 10 arcseconds) makes it unrewarding and locating it potentially dangerous.

Mars is low in the predawn sky. It is small at around 4.6 arcseconds and difficult to glimpse, at magnitude 1.1.

Jupiter is well past its best, being low in the early evening in April before conjunction with the Sun on May 18.

Saturn is low in the bright pre-dawn May sky.

Uranus remains near Jupiter but is very difficult to observe in April’s bright evening twilight. It is lost by May.

Neptune is just past its March solar conjunction so not available in April or May.

Minor and dwarf planets

If it is a barren time for the major planets, it is quite the reverse for enthusiasts of the brighter asteroids. (4) Vesta is the brightest on display at magnitude 8. It sweeps eastwards, passing 20 arcseconds north of the grand open cluster Messier 35 in early April, and bypasses epsilon Geminorum at the beginning of May. (3) Juno is well placed close to Regulus at magnitude 9.9, and (2) Pallas is some 13 degrees due south of the Great Hercules globular cluster in early May. It is magnitude 9 and at opposition on May 19.

Meteors

The eta Aquarids represent the best meteor shower of this period. Maximum falls between May 5–7 but it can be active from Apr 19 through to the end of May. The shower is due to debris from Comet 1/P Halley and is the spring equivalent of the autumnal Orionids. It is best seen an hour before morning twilight when the Moon will be a thin crescent while the shower is at maximum.

The April Lyrids may be better placed as the radiant (now in Hercules) is high in the midnight sky, but the full Moon will make observation difficult this year. Although the Lyrids have a typical zenithal hourly rate (ZHR) of 15, the shower can occasionally surge to a ZHR of 100. Dust from long-period Comet C/1861 G1 Thatcher is responsible. It is thought to be the longest known meteor shower, with records dating from 687 BCE.

Comets

The rather excitable comet 12P/Pons–Brooks remains visible but by the beginning of April is a low evening object to the west of Jupiter in Aries.

C/2023 A3 Tsuchinshan–ATLAS is still months away from its perihelion on Sep 28, but it is well placed, traversing Virgo from east to west throughout April and May. During this time it should brighten and be visible in small telescopes at around 10th magnitude. We shall have to see.

The faint, periodic comet 13P/Olbers begins April low in the western sky in Taurus but climbs higher through the month, towards Auriga. It will then essentially be lost in the bright sky. Perihelion will be on Jun 30.