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SYFY WIRE Bad Astronomy


By Phil Plait
Rigel (top center) is the brightest star in Orion, but in this very deep exposure there is a lot more to see. Credit: Adam Block /Steward Observatory/University of Arizona

I recently posted a stunning photo of the star Betelgeuse, taken by my friend and astronomer Adam Block. The star, a red supergiant, marks Orion's left shoulder, but is only one of four stars that mark the iconic quadrilateral of the constellation.

Kitty-corner from Betelgeuse, and in general slightly brighter by eye, is Rigel. It's a blue supergiant, a monster of a star that is every bit the equal of its redder companion on the other side of the constellation. It has a mass over 20 times the Sun's, and shines with an energy equal to more than 100,000 Suns, so luminous that even from over 800 light years away it's one of the brightest stars in the sky.

Adam took an image of the area around Rigel, too, a complement to the one of Betelgeuse. Incredibly, despite its terrifying power, Rigel is not the star (so to speak) of the show.

Everything else is.

Rigel (top center) is the brightest star in Orion, but in this very deep exposure there is a lot more to see. Credit: Adam Block /Steward Observatory/University of Arizona

Holy yikes. What a shot! Click here for a much larger version.

Rigel is the brightest star in the photo, to the upper right. But it's hardly alone.

The most obvious structure here is the wispy, ghostly blue glow to the left. Its technical name is IC 2118, but more colloquially it's called the Witch Head Nebula. Take a good look at it and you'll see why; it does look like a cartoon witch facing to the right.

The Witch Head Nebula (left) looks like its eponym, but turn it 90° counterclockwise and it becomes the Running Ghost Nebula. Credit: Adam Block /Steward Observatory/University of Arizona

One thing I love about it is how, if you turn it 90° counterclockwise, it maintains the Halloween theme, becoming the Running Ghost Nebula! See it? The U-shape at the top is its ectoplasmic arms above its head (the bump in the bottom of the U), running/flying to the right, its ghostly tail dragging out behind it to the left. That always makes me smile.

The nebula is actually dust (small grains of rocky and carbon material) lit by Rigel. The nebula is a bit farther away than the star, though its exact distance is difficult to measure. When I first saw a photo of it I figured it was shaped by Rigel as well, the light and winds from the star sculpting it. But I suffered from a bias: Because it looks like a face, and that face seems to be looking toward Rigel, I thought they were associated physically.

However, looking at it like a nebula, it's elongated in a direction perpendicular to Rigel, and the bottom end points toward a huge loose association of extremely luminous and powerful stars called the Orion OB-1 association. Specifically, it may be that the intense stars of Orion's Belt — Alnitak, Alnilam, and Mintaka — are the ones blasting out radiation that has carved the shape of the Witch Head. However, those stars are around 200 light years from the nebula, which is a very long way. I wonder instead if it's the stars in Orion's Dagger (of which the Orion Nebula is a part) may be behind it; they're a bit closer at 125 light years or so to the nebula.

Either way, those stars are at a huge distance from it to command such a degree of shaping. Massive stars are… unsettling.

An incredibly deep and spectacular mosaic of Orion shows huge amounts of gas and dust littering the constellation, distracting form the constellation’s familiar shape. Betelgeuse is the bright reddish star to the lower left. Credit: Rogelio Bernal Andreo

Think of it this way: Rigel is actually four stars in orbit around each other, but are so far from us they look like one by eye. The biggest star is the blue supergiant. It's orbited by a binary system, which itself has a star orbiting them. All three of those companion stars are what we call B stars, which are on their own plenty powerful and bright. But next to the supergiant they're difficult to detect even with big telescopes.

Rigel is in the foreground of all the nebulosity you see here. Behind it is the ridiculously huge Orion Molecular Cloud Complex, over a thousand light years from Earth, which is a sprawling collection of cold gas and dust where stars are born. It's one of the most prolific star-forming factories in our galaxy, and indeed the Orion Nebula is just one small part of it.

The red glow is from gas in the complex energized by the light of the mighty OB-1 stars. The electrons in the atoms jump to higher energy states, and when they cascade back down they emit that characteristic red light. The blow glow, on the other hand, is from that dust, which scatters the blue light from stars, essentially reflecting some of it back to us. Because of these processes, the red gas is called an emission nebula, and the blue dust a reflection nebula.

Note the fingers of gas below the Witch Head, also pointing toward the lower right, the direction to Orion's belt and dagger. That gas is being pummeled by the stars, blown back and eroded like a sandbar in running water. These structures are sometimes called elephant trunks or pillars of creation (after the iconic Eagle nebula), and are where stars are being born. Note also the bright star in the lower right corner: That's Eta Orionis, which is also part of the OB1 association, and is a powerful star as well. Those fingers point pretty much right at it, so it may be playing a role here, too.

Mind you, even from a dark site, looking at Rigel by eye doesn't even hint at the chaos going on around it. I've scanned this area by telescope many times, and besides dozens of stars easily spotted (including Tau Orionis to the lower right of Rigel in the photo, and Beta Eridani, the brighter star left of center), there's no glimmer of the gas and dust strewn everywhere. The reason you see all this structure in the photo is that it's a 48-hour exposure, a four-panel mosaic with each panel in turn comprised of three four-hour exposures in red, green, and blue filters to produce this wide-field natural-color scene.

I'll note that being a massive supergiant, Rigel, like Betelgeuse, will one day explode as a supernova. It's not clear when, though it may not be for another million years. In fact, the stars in Orion's Belt are all going to go that way someday, as will a handful of others in the constellation. Betelgeuse will likely go sooner, but a hundred thousand years is still a long time from now.

Orion is up in the northern hemisphere in late winter months, so it currently sets with the Sun. But come cold weather once again I'll be looking this way, and when I spot Orion, and the blue sapphire marking its left knee, I'll have a much deeper appreciation of what I'm seeing.