“I would rather be adorned by beauty of character than jewels. Jewels are the gift of fortune, while character comes from within.” -Plautus
When it comes to astronomy, there’s no doubt that I’ve got a northern hemisphere bias. It’s no surprise, of course; I live here. And while I often write about the skies that we all share, astronomy has a historical bias in favor of the northern hemisphere. You know this, too. Ask most people to name one single thing in the night sky, and you’re most likely going to end up with this.
The Big Dipper is arguably the most prominent, distinct grouping of stars in the night sky, and offers its own unique set of glorious sights for the astronomer. But below a certain latitude, the Big Dipper is invisible at all times of the year.
What’s more, a whole new set of stars, nebulae and more — invisible from the northern hemisphere — are visible from south of the equator. The most prominent grouping in the southern skies, arguably, looks like this instead.
The Southern Cross, also known as Crux, is directly opposite to Cassiopeia in the night sky; both are never simultaneously visible from anyplace on Earth. But this bright, compact constellation contains many of its own secrets. Today, I want to highlight just one of the sights I’ve never yet had the opportunity to chase with my own eyes.
Because the Southern Cross lies directly in the plane of the Milky Way, some of the fainter secrets are a little more difficult to tease out.
Combined with the nearby pointer stars, alpha and beta centauri, the Southern Cross can be used to find the South Celestial Pole. Just draw an imaginary line down the long side of the cross and another perpendicular to the pointer stars; where they intersect, that’s true south.
But that’s not the secret I want to share. Right next door to the cross, the black haze in the Milky Way is known as the Coalsack nebula. The nebula, in fact, forms the head of the most well-known dark mythological feature in the entire night sky: the Giant Emu-in-the-Sky.
But it’s not the emu that I wanted to share with you, either. The left-most star in the cross — Beta Crucis — lies just above the Coalsack nebula in the plane of the Milky Way. But head on down, just a tiny bit, and you’ll see what looks like a star that’s just barely visible to the naked eye under ideal, dark skies.
But that’s not a star, and you’d see that if you brought a telescope or good binoculars with you.
Kappa Crucis (or NGC 4755) is better known, today, as the Jewel Box cluster (sorry to those of you expecting something different from the title; it’s my jewels in a box), ever since the astronomer John Herschel viewed it through a substantial telescope, and described it as
“a casket of variously coloured precious stones.”
The European Southern Observatory, with a modest (2.2-meter) telescope, can clearly demonstrate why.
The actual image is much higher resolution than I can display here, but I can easily crop out for you the most spectacular part of the cluster.
Most of the stars are bright blue, telling us right off the bat that this is an incredibly young cluster of stars, full of very hot, short-lived stars that haven’t run out of fuel yet. Well, one of them has, of course, because that’s what explains the uniquely orange color of the extremely bright star in there: it’s a red supergiant!
And, as I told you earlier in the week, when it comes to resolution, the size of your telescope is everything. How much better can our view get if we jump up to the 8.2-meter VLT: ESO’s Very Large Telescope?
With an age of just 14 million years, the Jewel Box Cluster still contains a great many bright, massive B-stars, some of which will go supernova during the next few million years. The question on my mind, of course, is whether the supergiant, Kappa Crucis, will be the next one in our galaxy to do so?
I hope we’ll be around to find out, but with a star like this, it could happen next week or it could not happen for hundreds of thousands of years. But looking deep inside, as only Hubble can, we can see that behind the bright blue stars and the red supergiant lie the dim, faint stars, down to just 40% the mass of the Sun, vastly outnumbering the bright infants of this cluster.
Four-and-a-half billion years ago, our Sun formed in a region not much different than this, and was not unlike one of the relatively dim, yellow stars you can barely see in the background. At just 0.3% the age of our Sun, this star cluster is in for some spectacular fireworks in the near future, and I can’t wait to see if we get one in our lifetimes.
Regardless of the tools you’ve got, this secret of the southern skies, masquerading as a single, dim star to the naked eye, is just waiting for you to unlock it. Enjoy the Jewel Box Cluster if you can, one of the least-known treasures of the night sky!