AstroWeek Part 3: Orion’s Got It All

OK, so as we saw Tuesday, the Big Dipper has a ton of cool stuff going on. But as cool as it is, Orion is even cooler. Because Orion is full of action.

Tangent: All of us* love action movies. We love things that crash or blow up, and this is top of mind for me right now because Bird Whisperer and I just finished watching the 3^rd Lord of the Rings movie, Return of the King. Over the past few months I’ve been reading the whole series aloud to him, and when we finish a book, we rent the movie. BW’s enjoyed the whole saga, but he’s particularly drawn to the action scenes. We watched the film over Sunday and Monday evenings. Tuesday evening BW reloaded the DVD and watched the “Battler of Pelennor Fields” scene probably 15 times.

*OK, kind of a broad generalization. How about if I qualify “all of us” as “all of us not-quite-fully-mature males”?

Nested Tangent: I’d seen the film before, but it was probably close to a decade ago. This time around I was struck by 2 things:

First, is it just me, or does the film have a way stronger homoerotic undertone than the book? I’m not saying there’s anything wrong with it, but in the film the hobbits seem to be really good friends. Certainly Tolkien emphasized these bonds of friendship in the book, but in the film, a couple of times I swear those hobbits were about to start making out.

Second… second… well, I’ll get to the second thing later, when I remember what it was. This is a long post with a lot of science, and I need to get rolling.

Orion Overview

Orion is an Action Constellation. Even if you don’t know anything about it, it’s the only blazingly obvious constellation that’s doing something. The Big Dipper is cool, but it’s, well, a saucepan. Orion is lone hunter, armed to the teeth, who’s just let an arrow fly- how cool is that? But when you learn about the stars that make up Orion, there’s way more action than you thought.

It’s a big constellation, with well over a dozen big, bright stars. The brightest is Rigel, Orion’s left* foot. It’s a Blue Supergiant that shines 85,000 times as brightly as our sun, which makes it- even at a distance of 860 light years- the 7^th brightest star in the sky.

*”Left” and “right” here is from Orion’s perspective, not yours, assuming he’s facing you.

Only sometimes it isn’t the brightest star in Orion. Sometimes the brightest star is his right shoulder, Betelgeuse, a red giant in the later stages of its lifecycle, which varies in brightness in semi-regular cycles lasting from 6 months to 6 years. Betelgeuse is big enough that its color is apparent with the naked eye. If you’re not convinced stars come in different colors, grab a pair of binoculars and start with Rigel and Betelgeuse; the color difference will be immediately obvious.

But the coolest thing about Betelgeuse is this: it’s gonna blow up. Hold that thought.

It’s Got A Nebula

The favorite target in for new stargazers in Orion is the Orion Nebula. A nebula is an interstellar cloud of dust, gas and/or plasma*. There are several different kinds of nebulae. Some are the remnants of supernova explosions, which we’ll talk about in a moment. The Orion Nebula (pic left) is a different type, called a diffuse nebula, and it’s the kind of nebula in which new stars are born. It’s located right at the middle star of the 3-star “sword”, Theta Orionis. If you’re sharp-eyed on a clear night, away from light pollution (i.e. not in Salt Lake Valley) you may notice this star is just a titch fuzzy. Check it out with binoculars, and it’s downright blurry. This blurriness is the interstellar cloud, lit up by Theta Orionis.

*Plasma in this sense is ionized gas, or gas with (some of) the electrons unbound from the atomic nuclei, as opposed to blood plasma, like they are always pumping into gunshot victims on medical-drama TV shows.

It’s Got New Stars

Theta Orionis is itself a fascinating star. It’s actually 4 stars, which can be distinguished through a telescope or decent binoculars. Theta Orionis-C is the monster of the pack, but A, B, and D are all formidable stars as well, and together they’re called the Trapezium (pic right). They’re all “new” stars, all less than a million years old*. And at least 3 of the 4 have companion stars- some of which have companions, and so on- making this an 11-star system! Given so many stars, so close together, their configuration is gravitationally unstable, and astronomers expect one after another to be ejected from the cluster over the next few million years until C goes supernova and all hell breaks loose.

*Which is like practically nothing in star-time. Really, these stars are like cookies just out of the oven.

It’s Got Brown Dwarfs

More than 10 brown dwarfs (artist’s conception, left) have also been found in the nebula. A brown dwarf is a sorta-almost-star, sometimes called a substar. Too big to be a planet, but too small to achieve stellar ignition, or in other words, to sustain hydrogen fusion in the core, the biggest brown dwarfs (actual photo right, orbiting larger star) are about 75 times the mass of Jupiter. The smallest are generally considered to be around 13-15 times the mass of Jupiter, and this lower limit is set by the inability of the body to achieve any hydrogen fusion. But it’s worth noting that Jupiter, Saturn and Neptune all radiate more energy than they receive from the sun. A big gas giant is a sorta-almost brown dwarf, or maybe a subsubstar.

It’s Got A Molecular Cloud

So there’s a lot going on in the Orion Nebula; it’s like a “stellar nursery.” But even cooler is that the nebula is a just the small, “lit-up” part of a way, way larger interstellar cloud: the Orion Molecular Cloud (OMC). The OMC- about 1,500 light-years away- is a few hundred light-years across.

It’s a shame we can’t see it (it supposedly glows wonderfully in infrared), because it’s practically the biggest thing in the night sky. Look up at Orion, stick your arm straight out, hand open at the constellation. The area covered by your outstretched hand is roughly the area covered by the OMC. That’s a big cloud.

It’s Gonna Have An Open Cluster

Eventually the Orion Nebula will be largely cleared out by star formation and a supernova or two, and the Trapezium and associated other new stars (that haven’t been flung out and away) will be an open cluster of stars. If you want to see what an open cluster looks like, look to the right (West) of Orion, in the direction in which he’s shooting an arrow. Go about 1.5 times the distance from Orion’s foot-to-shoulder distance and just a titch up (North.) You’ll see a tight little cluster of bright stars, probably 7 of them, but maybe- if you’re away from a city- up to 14.

The 7 make a little micro-version of the Big Dipper. This is the Pleiades, an open cluster 440 light years away. It’s ~100 million years old, and expected to stick together for about another 250 million years. The 14 stars BTW are only the ones you can see. The cluster has something like a thousand stars (including many brown dwarfs) mostly packed into a little space only 8 light-years across. Think about that- it’s as if there were 1,000 stars between us and Sirius*.

*There are between 2 and 6, depending on how circuitous your path is.

It’s Got A Red Supergiant

OK, but let’s get back to Betelgeuse and that whole blowing-up thing. It’s at the other end of its life; it’s dying. But it’s actually only several million years older than the newborns of the Trapezium, and this leads to one of the things about stars thats oddly counterintuitive for laypeople: the bigger you are, the faster you burn out. Here on Earth, we tend to think of bigger things as longer-lived. Big plants- like Giant Sequoias- live way longer than say Dandelions or Sunflowers, and big animals- like elephants, whales and people- far outlive gerbils and houseflies. But with stars it’s exactly the opposite.

Our sun is around 4.5 billion years old, and it’s got another 5 billion good years ahead before things get all weird and bad news for anyone on Earth. But Betelgeuse is only 10 million years old, and could go supernova any day. It’s almost certain to blow in the next million years, and in fact it could blow up tomorrow.

Side Note: I should mention that this size-lifespan inverse correlation works the other way as well, meaning that if a star is less than 80% the mass of the Sun, its lifespan is over 13 billion years, leading to the fascinating corollary that in the 13.7 billion year history of the universe, no star of .8 solar masses or less has yet died.

Stars shine because in their core, the immense gravitational pressures push hydrogen atoms together strongly enough that they start fusing into helium. If the star is at least 75X the mass of Jupiter, the reaction is sustained- a continual thermonuclear reaction. A star that’s fusing hydrogen in the core- like ours- is called a main sequence star. Eventually though, the hydrogen in the core is used up, and when this happens, gravitational pressure in the core ignites a secondary reaction, fusing helium into carbon, (3 He atoms fuse into 1 C atom) and even oxygen (4 He fuse to 1 O) Meanwhile, Hydrogen fusion still occurs in the outer “shell” layers of the star. The two different reactions/layers vary and turn on/off sequentially, causing the star to pulsate, or vary in brightness, like we see with Betelgeuse. Well sort of, because with big stars things get more complicated.

5-6 billion years from now, after our Sun has fused up all its hydrogen and helium, it’ll shrink to a super-compact* glowing, gradually cooling ball called a white dwarf. But for big stars, say of 10 times the mass of our sun, something else happens.

*Mass of the sun, but size of the Earth. Density of roughly 1 metric ton per cubic centimeter. No, that’s not a typo.

And It’s Gonna Blow Up

The gravitational pressure in the core of these massive stars is strong enough that yet another fusion reaction sets is, fusing the carbon and oxygen into a mixture of magnesium, oxygen and neon, with other, earlier fusion reactions occurring in the outer shells, like a set of those Russian dolls. But we’re not done- when the carbon is fused up in the core, the magnesium-oxygen-neon mix starts fusing into silicon and sulfur, which in turn undergoes a fifth fusion reaction into iron. And that’s where things stop. Iron doesn’t fuse into anything. And when the core fusion is done, with a burnt-out iron core, that’s when the whole giant thing collapses, like a sub-prime mortgage lender in Southern California.

The core collapse is catastrophic. With no fusion-powered force to counter-balance gravity, the iron breaks down into subatomic particles- protons, neutrons and electrons. The resultant blast tears through the outer shells of the star and blasts them out into interstellar space- a supernova. The left-behind core collapses into a ball of solid, wall-to-wall neutrons, only several miles across, called a neutron star, of almost inconceivable density*.

*1 cubic centimeter weights ~800 million metric tons. Neutron stars only occur if the stellar mass is at least 1.4 times that of the sun; below that degenerate electron pressure supports the structure as a white dwarf. About ~5 times or greater the mass of the sun, not even neutrons can support the structure, and the core collapses into a black hole.

Roughly every 30 to 50 years a star goes supernova in our galaxy. About every 200 years one of these is easily visible from Earth, as a new, super-bright star, lasting for weeks or even months. The effects on nearby stars (and planets) can be significant. One of those effects is a gamma ray burst. If a nearby supernova sprayed Earth with a strong gamma ray burst it would cause a dramatic increase of the amount nitrogen oxide in our atmosphere which could do 2 things. First, it’s could severely damage our ozone layer; it’s estimated that ½ the ozone layer would be destroyed within seconds. Stripped of its UV protection, the planet would likely experience a wave of extinctions and huge biological change. The increased nitrogen oxides, specifically nitrogen dioxide, might also create a sun-blocking layer leading to global cooling and a possible ice age.

The Second-Worst Extinction

In fact, it’s suspected that this actually has happened at least once before. Most readers are probably familiar with the mass extinction that wiped out the dinosaurs 65 million years ago, which is thought to have been caused by an asteroid impact in the Yucatan peninsula. But what you might not know is that that extinction- known as the K-T Event- was only the 5^th and most recent of major extinctions of life on this planet, and it wasn’t even the worst. That title goes to the Permian-Triassic Extinction, way back ~251 million years ago. But the second worst ever- the Ordovician-Silurian extinction- happened still earlier- roughly 440-450 million years ago, and wiped out well over half of all genera in existence at the time. The world was a lot different then- life was still confined to the oceans (drawing right)- but it’s recently been suggested that the extinction patterns shown in the fossil record are consistent with dramatically increased UV exposure, followed by an extremely sever global ice age, one that followed an extended period of worldwide climate stability.

Side Note: This hypothesis, though certainly plausible, is still just a hypothesis. There are a ton of unknowns and variables involved, and in fact there’s some evidence suggesting that this extinction was 2 separate extinction events separated by a few million years. You can check out this paper if you’re interested.

Even if they don’t cook or freeze us, nearby supernovae can affect us in other ways, not all of them bad. Here’s something weird about our region of space: we live in a big bubble.

Orion And The Local Bubble

Interstellar space is filled with very low density dust and gases. But strangely, in the part of the Orion Arm in which our solar system lies, the density of such gases is way, way, way lower than normal. Like 1/10^th the normal density throughout this part of the galaxy. We sit in a super-clear bubble roughly 300 light years across.

This is good news for astronomers- our telescopes can see farther and more clearly than they could otherwise. Even better, the bubble has expanded and busted clear out of the galactic plane, giving us clear shots of all sorts of extra-galactic stuff. The only plausible explanation for this bubble, called- get ready for it- the Local Bubble, is that a supernova blast cleared out the neighborhood roughly 300,000 years ago.

Yeah, yeah, bubble-shmubble. Shouldn’t this just be a tangent or something? What’s it got to do with Orion? Here’s what: astronomers have found the neutron star left over from the blast that created the local bubble. It’s called Geminga, and it’s in the constellation Gemini, just above (North) Orion. But here’s the thing- it’s moving. Fast.

When star-cores collapse, they don’t always do so symmetrically, and if the collapse is asymmetric enough, the neutron star-core-remnant can go shooting off like a bullet across interstellar space. Geminga is moving at 60 miles per second, and 300,000 years ago, it would have originated out of a supernova blast in… Orion.

300,000 years ago, when our ancestors were just on the verge of being called “modern humans”, when Neanderthals were still sitting around campfires from Spain to Syria, Orion let fly an arrow straight up, at 10 times the speed of a bullet, and the twang from that bowshot cleared out everything around us.

Tangent: Speaking of bows and arrows, I remember what the second thing was about Lord of the Rings: Legolas. He’s both mine and BW’s favorite LOTR character by a mile. The guy kicks ass. He’s smart, tough, hyper-coordinated, cool under pressure. His eyesight is way better than any human’s and on top of all that, he’s immortal! In every one of the films there’s an incredibly acrobatic Legolas-centric fight scene that leaves you breathless. And it makes you wonder- why didn’t they just send a group of Elves to get rid of the Ring? Those guys Kick Ass. What’s with all the diversity? Elves are better at everything, and clearly should be running the show.

Know who Legolas reminds me of? Spock, the Vulcan guy on the original Star Trek series. No, not just because they both had pointy ears, but because they were both way, way, way more competent than everyone else around them. Here’s Captain Kirk, following his hunches, getting all emotional and hitting on the alien ladies, and Spock’s just calm, cool, logical and capable. Was I the only one who thought that Spock should be Captain? I always thought that Spock must be thinking like all the time, “Holy crap. How did I wind up working with these losers?”

*The original series was the only one I really liked. Yes, it was sexist and formulaic and excessively Cold-War-allegorical, but I liked it anyway. The Next Generation crew I always found annoying and pontificating, like your overly-self-serious neighbor who always manages to work how much he recycles into almost every conversation. Voyager was better, in spite of the ridiculousness of Seven-Of-Nine’s “uniform”, but every time the captain appeared in a scene I always thought, “Mary Poppins.”

No one knows which star will be our next visible supernova, but Betelgeuse is on just about everyone’s top 5 list*. It’s highly unlikely we’ll be harmed by the blast; it’s a ways off, and gamma ray bursts are most severe along an exploding star’s axis of rotation. Thankfully Betelgeuse’s isn’t pointed our way. It may blow up a few hundred thousand years from now, or it may blow up in your grandchildren’s lifetime. Maybe it’ll blow up today.

*The #1 candidate may be Eta Carinae, which is regrettably only visible from the Southern hemisphere.

Or maybe, just maybe, it blew up 640 years ago, and early tomorrow morning when you look South before sunrise, the star will light up, brighter and brighter.For weeks, maybe months, Betelgeuse will shine more brightly than any star you’ve ever imagined- brighter than Sirius, brighter than Jupiter, brighter than Venus- as bright as a 3/4 –full moon. For weeks, maybe months, the night sky will be completely different, unlike anything you, your parents or your great-great-great-great grandparents have ever seen.

I have 2 big astro-hopes for my lifetime: that astronomers find an Earth-like planet* orbiting another star, and that I live to see a supernova. I’ve got my fingers crossed on Betelgeuse.

*Right size, distance from star and composition of atmosphere.

Several times in this blog I’ve mentioned how wonderful it is that when you check out just about any plant or animal, it turns out to have an amazing story behind it. After a week+ researching the night sky, it seems like almost every star does as well.