Sand and Stars

The Big 5 of the African Sky

Image credit Naskies at en.wikipedia

The famous “Big 5” (leopard, lion, rhino, buffalo, elephant) are Africa’s undisputed superstars and seeing any of them in their natural habitat is at the top of most people’s safari tick list. But do you know that there is another Big 5 and you don’t have to be in a game reserve to spot them? 

They’re the “Big 5 of the African Sky” and their habitat is up there… wheeling overhead in our southern night skies.

With charming whimsy the ASSA (Astronomical Society of Southern Africa) assigned a ‘special task force’ to select the best representations of each type of deep-sky class – a dark nebula, a bright nebula, an open cluster, a globular cluster, and a galaxy. (A deep-sky object is an astronomical object outside our Solar System.)

These Big 5 are visible anywhere in the Southern Hemisphere but they are particularly stunning when viewed from a dark, unpolluted sky. All of them are visible to the unaided eye and are absolutely stunning in binoculars. Indeed, you’ll be surprised at how much detail you see through binoculars (and in fact, two are best seen through binoculars, not a telescope).

So if you’re ready for a visual feast… step outside and look up!

The Big 5’s location 

This star chart shows the positions of the Big 5. Start by locating the Southern Cross (Crux), with its four bright and distinctive stars and the two pointers pointing at the head of the cross. Depending on the time of night the cross may appear the right way up or lying on its side. When you have found it orient the star chart so that it approximately matches the orientation of the Southern Cross in the sky. (Be careful you don’t mistake either the False Cross or the Diamond Cross for the Southern Cross.)

1. Dark Nebula: The Coal Sack

Image credit: ESO/S Brunier
What it is…

A dark nebula (nebula is Latin for “cloud”) is intriguing because it appears to be a hole in the bright, starry band of the Milky Way. While it may seem like you are staring into nothingness when you look at the Coal Sack, nothing could be further from the truth. You are staring into an immense cloud of very cold gas and cosmic dust, much of it made from the remains of exploding stars. The dust is so dense that it blocks most of the visible light from stars behind the cloud, making it appear inky black when viewed against the starry cosmos.

Stars are born in these giant clouds of dust and gas and millions of years hence the Coal Sack’s dark days will come to an end when it collapses under incredible gravity and recycles its dust and gas into a sparkling display of hot, new stars.

Where it is…

The Coal Sack is located in the small constellation Crux – the iconic Southern Cross – just east of Acrux, the bright star at the foot of the cross. (It overlaps into the neighbouring constellations Musca, the fly and Centaurus, the centaur). It lies 500 light years away, which makes it one of the nearest dark nebulae to Earth and it is 50 light years in diameter.

What to look for…

The Coal Sack is best seen with the naked eye in dark skies. It forms a conspicuous silhouette against the bright, glittering band of the Milky Way. The edges of the nebula are not clean-cut but somewhat tattered-looking and they seem to simply melt into the brightness of the surrounding Milky Way. It offers a very beguiling exploration in binoculars. At first it seems like a dark pool of nothingness with tiny pinprick stars scattered across it (these stars are all lying in front of the nebula) but keen eyed observers can see a rib-like structure in the blackness and tendrils of blackness flowing southwards. Close to Acrux you can see a gorgeous, charred-orange star (BZ Crucis), set against the blackness of the Coal Sack like the last dying ember in a campfire. It is the brightest member of the cluster NGC 4609 and the rest of the cluster appears as a brightish fuzziness just to its north-west. The cluster lies a further 800 light years behind the Coal Sack, yet the incredibly hot, young stars pack enough punch to remain visible through this dense cloud of dust!  

2. Open Cluster: The Southern Pleiades

Image credit Digitized Sky Surveys/Anglo-Australian Observatory (AAO)
What it is… 

An open cluster is a group of stars of similar age and all born within the same giant cloud of gas and dust. They number anywhere from a few dozen to a few thousand stars. Open clusters typically only last for a few hundred million years because their stars are only loosely bound by gravity and eventually wander off from their birth cluster into the expanses of our galaxy, some travelling thousands of light years from where they were first formed. (Our Sun wandered away from the open cluster into which it would have been born – and as it is about 4.57 billion years old it’s had plenty of time to wander far, far away from its siblings!) The Southern Pleiades is a very young cluster, about 13.7 million years old. It is 7 light years across and has 74 stars.

Officially designated IC 2602, the cluster was nicknamed the Southern Pleiades because it reminded its discoverer, French astronomer Nicolas Louis de Lacaille, of the famous open cluster, the Pleiades. (He discovered IC 2602 in 1751 when he was at the Cape of Good Hope charting the southern skies.)

Where it is…

The Southern Pleiades is located 525 light years away in the constellation Carina (the keel of the enormous constellation Argo Navis, the mythical ship used by Jason and the Argonauts as they sought to recover the Golden Fleece, that was broken up into three separate constellations). It marks the north-eastern end of the Diamond Cross.

What to look for…

The Southern Pleiades is an easy naked-eye cluster surrounding the bright star Theta Carinae. And it is nothing less than dazzling in binoculars! Its stars look like diamonds loosely sprinkled on black velvet. See how many stars you can count in the cluster… in 10×50 binoculars and a dark sky you ought to be able to resolve up to 20 stars. At its heart is blue supergiant Theta Carinae. Blue supergiants are the rock-and-roll stars of the universe – they live fast and die young! Stars don’t get more massive or hotter than blue supergiants and they end their lives in spectacular fashion… by blowing them themselves to smithereens in a supernova.

3. Bright Nebula: Carina Nebula

Image credit ESA/N. Smith and NOAO/AURA/NSF
What it is…

This is a spectacular example of an active star birth cloud – a vast nebula of dust and gas where stars are bursting to life, their intense radiation illuminating the cloud and making it glow so magnificently. Spanning a distance of around 260 light years, it is one of one of the largest star-forming regions in our Milky Way Galaxy. The nebula is host to at least a dozen stars that are each between 50 to 100 times the mass of our Sun and indeed, the nebula is named after its most famous stellar member: the massive star Eta Carinae. Despite being a vast star birth cloud, it is also a star-death cloud for x-ray observations show that some of its most massive stars have already gone supernova.

Where it is…

The immense nebula lies 7,500 light-years away in the constellation Carina.

What to look for…

Little is more wondrous than looking at the silvery glow of a nebula churning gas and dust into newly minted stars and the Eta Carinae Nebula is easily visible to the naked eye as the brightest frosty patch in an exceedingly star-rich portion of the Milky Way. It is a fabulous sight in binoculars – a beautiful glowing cloud of silky nebulosity the colour of moonlight and cut across by its famous dark chevron-shaped lane of cosmic dust that divides the nebula into one-third, two-third portions. The nebula contains eight open clusters and although you can’t make them out in binoculars you can certainly see the brighter stars embedded in the glowing nebula.

The gorgeous orange star towards the middle of the nebula is Eta Carinae. This extraordinary star is among the most luminous and most massive stars known to exist at about 4 million times the brightness and 120 times the mass of our Sun. Eta Carinae belongs to a rare class of stars called Luminous Blue Variables; it is an extremely unstable star and is in the final stages of life and will soon blow itself to pieces in a catastrophic supernova (although in astronomical timescales “soon” could mean tomorrow or a million years from now; it’s impossible to know. So watch this space!)

Interestingly, the Eta Carinae Nebula was selected as one of five cosmic objects observed by the James Webb Space Telescope as part of the release of its first official science images in July 2021. A detailed image was made of an early star-forming region of the nebula known as the Cosmic Cliffs.

4. Globular Cluster: Omega Centauri

Image credit Hubble/ESO
What it is…

Globular clusters are the largest and most massive star clusters, containing hundreds of thousands to millions of very old stars tightly bound by gravity, which gives them their roundish, spherical appearance. The stars in globular clusters are the galaxy’s most ancient inhabitants being between 11 and 13 billion years old, making them almost as old as the universe itself (13.7 billion years). They were born when the first generations of stars and galaxies formed, and likely collapsed from clouds of gas too small to form a galaxy but too large to form an open cluster. There are over 150 globular clusters in our Milky Way Galaxy (with perhaps more, hidden by galactic dust) and they inhabit our galaxy’s outer halo, so far out that it takes them a few hundred million years to complete a single orbit.

Omega Centauri is our galaxy’s most enormous globular cluster. It packs an unbelievable 10 million stars into a diameter of 300 light years. This means that the average distance between the stars in the core is probably around half a light year. When you consider that our nearest star – Alpha Centauri (the bottom Pointer) – is 4.2 light years away, you can imagine what an overwhelmingly brilliant night sky you’d see if you lived on a planet in Omega Centauri!

Where it is…

Omega Centauri lies 16,000 light years away in the constellation Centaurus, the centaur.

What to look for…

Omega Centauri appears as a fairly faint, fuzzy star to the naked eye. In binoculars it is magnificent – a glowing snowball illuminated by the starlight of its ten million stars. Even in binoculars, Omega Centauri is one of the few objects in the night sky that can take on a 3-dimensional illusion that makes it appear as if it’s suspended in front of the stars and dark sky around it. Apart from its beauty, it is extraordinary to know that you are looking at a ball of the first stars to be produced in our galaxy – stars that were already nearly 8 billion years old when our Sun was born.

5.Galaxy: The Milky Way Galaxy

Clearly we can never take a photo of our galaxy, but astronomers reckon that this galaxy – NGC 6744 about 30 million light-years away in the constellation Pavo (the Peacock) – is how the Milky Way might appear from outer space. Image credit Hubble Space Telescope/ESO
What it is…

The Milky Way Galaxy is our home in the universe. It is an enormous barred spiral galaxy around 13.6 billion years old. It is around 110,000 light years across and is stuffed with between 200 and 400 billion stars, thousands, if not millions of which have  planets of their own. The centre itself is a very dense and chaotic place, with stars and gas hurtling around the Milky Way’s supermassive black hole named Sagittarius A* (pronounced Sagittarius A Star), which has the mass more than 4 million times the mass of the sun. Astronomers estimate that its event horizon – the region of space around a black hole from which nothing (not even light) can escape – is around 26 million kilometres. Luckily, we’re in no danger of falling into it as we live out in the galactic suburbs so to speak – the Solar System lies in a spiral arm about 27,000 light years from the centre of the galaxy. 

Where it is…

The Milky Way Galaxy is all around us – every star you can see in the sky lives in our galaxy. Because we live in one of the spiral arms, we’re looking into the plane of our galaxy when we look at that spectacular band of glowing starlight studded with brilliant stars that arches across the sky and which we call the Milky Way. The beautiful frosty glow you see is the starlight of uncounted billions of stars too small or too distant to resolve with our eyes.

Image credit: ESO/S Brunier
What to look for…

The southern Milky Way was made for binocular viewing in dark skies! It is simply magnificent and overwhelmingly rich! There is nothing like lying back and cruising along the length of this sparkling river of billions upon billions of distant stars – through vast frosty star fields studded with stars, over the silvery glow of bright nebulae, past glowing balls of globular clusters, past gloriously bright stars – some beautiful shades of yellow, orange and red – and through dark pools of gas and dust from which the next generation of stars will one day be born.

When to see the Big 5 of the African Sky 

Because the Southern Cross is circumpolar – always above the horizon – at all places south of 35 degrees south latitude, people at mid-southern latitudes can count on seeing the Southern Cross every night of the year, although its ideal position (high in the sky) may be at some awkward hour during some months. The Southern Cross moves like a great big hour hand, circling around the south celestial pole in a clockwise direction throughout the night. However, Sutherland lies at 32.4 degrees south, so the Southern Cross dips below the horizon. Use a planisphere, a planetarium programme (such as “Stellarium”) or an app (such as “Google Sky Map” or “Sky Safari”) to check what time you can see it.

Copyright © Susan Young 2023