Realm of the Crosses

However, I would like to focus on a particular region that owes its prominence to the highly luminous stars contained within. I refer to what I call the Realm of the Three Crosses. For one, these groupings are easy to find in the sky, and two, the bright stars act as superior signposts to nearby treasures.

The deep sky objects mentioned in this article all reside within or close to the crosses. Even the most inexperienced observer should find something interesting here, the beauty of the Three Crosses notwithstanding.

Crux the Southern Cross

We start our journey at alpha Crucis, the brightest member of the Southern Cross. Acrux, as it is officially known, shines with the combined magnitude of 0.8. The 14th brightest star in the sky, a telescope at a power of 100x just separates it into two stars. These very luminous B-type, white stars of magnitude 1.3 and 1.8 shine with the light of 3000 and 1900 Suns respectively. The projected separation is around 500 AU. There is a third bright white star nearby which seems to be a family member. Acrux lies around 370 light years away, and is a member of the great Scorpius-Centaurus Association, a large group of exceptionally bright and energetic stars that share the same proper motion.

The next stop is with brilliant beta, a magnitude 1.3 B-type diamond around 490 light years away. The 20th brightest star, it denotes the eastern arm of the cross. Beta is a pulsating variable, the light fluctuations indiscernible to the eye, shining with the light of 5800 Suns. Within the widely scattered glow of beta's light lies a small, stunning, ruby red carbon star in fine contrast. A large aperture will show the colour superbly.

Gamma Crucis, at magnitude 1.6, is the 28th brightest star in the sky. At a distance of 220 light years, it has a brightness of around 900 times our Sun. This M-type giant has a nearby white A-type optical companion in fine contrast.

The fourth star of the Southern Cross, delta Crucis, marks the western arm, shining at magnitude 2.8. A bright B-type star, it lies around 570 light years away, therefore the computed luminosity is around 1900 Suns.

Epsilon, shining at magnitude 3.6, is a yellow star 125 light years away.

Just southeast of beta Crucis, between it and the dark dust cloud known as the Coal Sack, resides NGC 4755, commonly called the Jewel Box. This is a stunning open cluster of around 50 stars or so, containing some of the most luminous stars in our Galaxy. The brightest stars form an A-shape, with luminosity's of up to 80,000 Suns. The reddish M-type supergiant, kappa Crucis, lies at the centre of the cluster. It has a luminosity of about 16 000 Suns. This stellar playground can be no more than a few million years old, very young on a cosmological scale. A number of fainter clusters pepper the starscape of Crux, mainly within the frame of the cross itself. Many are hard to pull from the rich stellar background haze so care is needed.

The Diamond Cross

The Diamond Cross separates the Southern Cross and the False Cross, its stars forming a fainter but very definite diamond shape. Theta Carinae marks the eastern tip, around 4° south of the expansive eta Carinae Nebula. At a casual glance, theta appears as a bright blob to the naked eye, but have another careful look and theta breaks up into a number of stars. That's right, theta is a cluster of bright stars, a good resolution test for the naked eye. With a telescope, around 30 bright diamonds, spilling out of the field of view, surround magnitude 2.8 theta. Binoculars show this open cluster, IC 2602, in all its glory. Lying at a distance of 700 light years, theta shines with the light of 3800 Suns. This group can't be very old, somewhere between 4 million and 20 million years old.

Omega Carinae is a 3.3 magnitude hot, white B-type, giant star with a luminosity of 200 Suns. Adjacent to omega lies upsilon, a magnitude 2.9 A-type bright giant star with a 6th magnitude companion easily split in small telescopes. The F-type companion lies around 520 AU from the primary.

To the north lies l (el) Carinae, one of the brightest pulsating cepheid variable stars in the sky. This supergiant varies by 0.8 magnitudes every 35.5 days, changing its spectral type from F to K as it pulsates. If the computed distance, 3000 light years, is correct, its luminosity may reach around 12000 times that of our Sun. Just west of this star lies R Carinae, a red giant variable star that varies between magnitude 3.8 and 10.0 every 309 days.

Beta is the brightest of the four stars that delineate the Diamond Cross and appears as a magnitude 1.6 A-type, white giant star of 130 times the brightness of our Sun. The official name of this star, Miaplacidus, literally means 'placid waters'. Nearby, in the bright glow of beta lives a faint, small galaxy, NGC 2822. Large scopes should have little difficulty in pulling this one in with care.

NGC 2808 is an impressive globular cluster, unresolved in smaller telescopes but pretty nonetheless. Globulars are conspicuous by their absence in this neck of the woods so enjoy this one. It lies near the intersection of a line drawn north from beta and west from upsilon.

The False Cross

The False Cross, so often misidentified as the Southern Cross, appears larger and slightly fainter. Two of its stars, iota and epsilon, lie in Carina while the other two, kappa and delta, reside over the border in Vela.

Iota Carinae is an F-type supergiant star with a luminosity of 5200 Suns. This magnitude 2.2 intensely hot star is also known as Aspidiske from the Roman Scutulum or Little Shield. The yellow G-type star of magnitude 5.5 to the west is HD 79698. To the northeast of iota lies NGC 2867, a very small but bright, round, pale bluish planetary nebula.

Epsilon, proper name Avior, shines at apparent magnitude 1.8 and is a K-type bright giant star with a computed luminosity of 1400 Suns. A line from kappa, passing through epsilon, points directly to an easy naked eye haze. Binoculars will show a glorious sight, a large and brilliant open cluster, NGC 2516. A wide-field telescope almost contains this group; the stars scattered far and wide, a bright orange giant star near the centre.

Around 100 stars are seen, ranging from magnitude seven downwards. This cluster has an age of about 110 million years, middle-aged if you like and contains a smattering of yellow and orange stars. There are a number of double stars within this vast stellar city and around 10 should be available to a 20cm telescope. Use 100 times magnification or above and inspect each star closely. You may be pleasantly surprised.

Moving on up to delta Velorum, we find a magnitude 1.9 A-type, white main sequence star. With a luminosity of 70 Suns and a distance of around 75 light years, it appears to share the same proper motion across the sky as Sirius and a number of other bright stars, this group known as the Ursa Major Stream. Delta has a close companion of magnitude 6.0, which will be resolved with high power. Another star of the 10th magnitude lies 69" away and can be resolved into two stars of magnitude 10.5 and 13.0, separated by 6".

To the north and slightly west lies omicron Velorum, a 3.6 magnitude B-type star surrounded by a scattered group of bright young white stars known as IC 2391. Another young cluster, not as impressive as IC 2602, but worth a look anyhow. Nearby, to the east, lies NGC 2669, a large open cluster, little compressed, with around 40 faint stars.

Kappa Velorum, also known as Markab as it was called in the Alfonsine Tables of 1521, shines at magnitude 2.4. This B-type subgiant lies around 500 light-years away and throws out 1900 times more light than the Sun.

To the northeast lie two largish open clusters, NGC 2910 and NGC 2925. Both groups are rich in stars though loosely compressed.

NGC 2899 is a planetary nebula lying midway between kappa and a third magnitude orange, K-type giant to the southeast. This round haze appears brighter toward the centre and the surrounding star field enhances the view.

This completes our brief tour of the Realm of the Crosses. Many other objects lie in the vicinity and are worth a detour to see.

Clear skies and good hunting.

~ CosMos