Powerhouse Museum - Sky Guide April 2025

Powerhouse Museum - Sky Guide April 2025

Canis Major the Greater Dog is the companion to Orion the hunter. The brightest star in the constellation, Sirius (also known as the Dog Star), is also the brightest in the night sky as it is close to us — only 8.7 light years away or about 82 million million kilometres and 25 times brighter than the Sun. In about 64,000 years it will be seen as the southern polar star due to the Earth’s wobbling axis of rotation and the star’s proper motion.

Canis Minor the Lesser Dog is an obscure and small constellation usually ignored in the search for its dominant companion, Canis Major, the Greater Dog. To find its one bright star, face north and look for Sirius in Canis Major. Roughly one hand span (with your hand at arm’s length) below Sirius is the bright star Procyon. And Procyon is just about all there is to the Lesser Dog! Like many constellations it looks nothing like its name. The star’s name comes from the Greek Prokyon meaning ‘before the dog’ and indeed it does rise before bright Sirius and Canis Major from the latitudes of ancient Greece. Canis Minor and Canis Major together are Orion’s hunting dogs.

Carina the keel and the nearby constellations of Vela (the sails) and Puppis (the stern) were part of the huge constellation Argo Navis, which in Greek mythology carried Jason and the Argonauts in search of Aries the Ram’s Golden Fleece. In 1756, Nicolas Louis de Lacaille published his catalogue of the southern stars showing Argo Navis divided into the three constellations we see today. Canopus, the second brightest star in the night sky, can be found in Carina and is a white supergiant star about 313 light years away. It is best seen from February to April high in the south.

Centaurus the half man half horse represents the scholarly Chiron, tutor of many of the Greek gods and heroes including Heracles, Achilles and Jason. Its two brightest stars of Alpha Centauri — the closest star system to the Sun — and Beta Centauri make up the front legs of the centaur. When used with Crux, they help find south. It also contains the most spectacular globular cluster of all, Omega Centauri (NGC 5139). Centaurus was one of the original 48 constellations as mapped by Ptolemy and originally included Crux before the latter was identified separately sometime in the 17th century CE.

Crux or the Southern Cross is the smallest of all 88 western constellations and consists of four bright stars. Along with the nearby pointers of Alpha and Beta Centauri, Crux can be used to find south. Draw an imaginary line from the top of the cross shape though the bottom and across the sky. Midway between the pointers draw a line perpendicular to the line that joins them. Where these two longer lines intersect is close to the southern celestial pole. From this point drop a vertical down to the horizon to locate south. This technique works at any time of the night, any time of the year. The superb open cluster NGC 4755 (Jewel Box) appears very close to the second brightest star, Beta Crucis.

Gemini the twins with its two bright stars, Caster and Pollux, sits low in the northern sky between Taurus and Cancer and lacks any bright objects of note. It is best known for the Geminid meteor shower that peaks in the early hours of 13–15 December each year and for hosting both Uranus and Pluto when they were discovered in 1781 and 1930 respectively.

Leo the lion looks more like an upside-down question mark when seen from the Southern Hemisphere. Its brightest star Regulus (meaning ‘Little King’) is a system of four tightly bound stars 79 light years away. It was once referred to as one of the four Royal Stars, or guardians of the heavens, because of its proximity to the point in the sky that marked the northern Summer Solstice as seen by Persian astronomers approximately 5000 years ago. Precession of the equinoxes has now moved this point to Cancer the crab. It is host to numerous bright galaxies that can be seen through a modest telescope including the Leo Triplet of galaxies M65, M66 and NGC 3628.

Orion the hunter is one of the original 48 constellations mapped by Ptolemy and strides across the celestial equator, making it easy to see from both hemispheres. Its likeness to a male figure was probably obvious to the earliest humans. Orion’s brightest star Betelgeuse is one of the few stars to show an intense red colour due to its age and size. The three stars of Orion’s belt at his waist separate his head and shoulders to the north from his sword and knees to the south. The well-known saucepan asterism (a pattern of stars that is not a constellation) consists of a base (Orion’s belt) and a handle (Orion’s sword) plus one additional star at the rim. With binoculars you will see a small hazy glow in the middle of the saucepan’s handle: this is the Orion Nebula, a star birth cloud at a distance of about 1350 light years. The mythology of Orion is complex and often contradictory, though he usually represents a tall, strong and handsome hunter.

Virgo the maiden is the second largest of all constellations. Virgo rises in the east in April and sets after sunset by mid-August. It can be most easily found by locating the constellation’s brightest star, Spica. The rest of the figure is composed of relatively faint stars and looks a little like a maiden. It is one of the two constellations in which the ecliptic and celestial equator cross – the other being Pisces. This one marks the current position of the September equinox, the beginning of Spring in the Southern Hemisphere. It is home to a cluster of galaxies at a distance of around 60 million light years — the largest being M87 with a central black hole at least 7 billion times the mass of the Sun. Virgo’s one bright star, Spica, is the 16th brightest in the night sky and about 250 light years away.

Large and Small Magellanic Clouds (LMC/SMC) are the satellite galaxies of our own Milky Way. The Large Magellanic Cloud (LMC) is in the constellation of Dorado and is approximately 163,000 light years away. It consists of around 30 billion stars and hosts one of the largest nebulae detected, the Tarantula nebula. The Small Magellanic Cloud (SMC) is in the constellation of Tucana and is approximately 200,000 light years away. It has around 3 billion stars. The LMC will merge with our galaxy in around 2.4 billion years.

Omega Centauri (NGC 5139) is the brightest and largest of approximately 150 globular clusters orbiting the Milky Way. It is so bright it was labelled as a star on early sky charts by Ptolemy and is one of the few objects in the sky that carries both a star designation and an object catalogue designation. Omega Centauri shines with the luminosity of a million suns and is relatively close to us, only 15,800 light years away. It contains approximately 10 million stars and some theories suggest it could be the remnant core of a galaxy that is merging with the Milky Way.

The Sombrero Galaxy (M104 or NGC 4594) is an almost edge on galaxy about 30% the size of the Milky Way 28 million light years away. It is either a spiral galaxy or an unusual elliptical one. Its central black hole is about one billion times the mass of our Sun, which makes it one of the largest black holes found so far in the heart of a galaxy. The Sombrero Galaxy is situated within the constellation Virgo on the border with Corvus. It requires a moderate sized telescope and will appear as a small smokey smudge.

The Tarantula Nebula (NGC 2070) is a large hydrogen gas cloud approximately 1000 light years in diameter and part of the LMC at about 160,000 light years. The name ‘Tarantula’ comes from the spider-like appearance of the nebula in telescopes and photographs. At the centre of the nebula is the open cluster R136, which contains approximately 500,000 stars, including some of the hottest and most massive supergiant stars known. In 1987 the first naked eye supernova (SN1987A) since the invention of the telescope occurred in this part of the sky.