Passing shadow

In astronomy, an occultation occurs when one celestial object passes in front of another, temporarily blocking it from sight (from the perspective of an observer on Earth).  Some occultations are entirely unremarkable; the Moon occults thousands of stars every month on its trips around the Earth, and we barely notice it because it's so ordinary.  And technically, solar eclipses are occultations, even though we don't usually think of them that way.

Other occultations, though, are rarer and more interesting.  The rings of Uranus were discovered in 1977 when they occulted the star SAO 158687, making it appear to blink on and off five times.  Pluto occulted stars in 1988, 2002, and 2006, and as it passed in front of the stars the alteration in the stars' light allowed astronomers here on Earth to study the chemistry of Pluto's thin atmosphere, a technique called atmospheric limb sounding.

Of course, because the two objects have to be lined up perfectly, occultations of bright and/or familiar objects are rare occurrences.  Jupiter, for example, is going to occult the planet Saturn -- but not until February 10, 7541.

That's a long time to wait.

Because natural occultations are so uncommon, there's a proposal to create an artificial version of occultation -- and use it to try and find more exoplanets.  A proposed orbiting telescope called BOSS (for Big Occulting Steerable Satellite -- of course it has a cute acronym, because these are astronomers we're talking about) will, if deployed, have a shield that can block the light of stars and allow the much dimmer reflected light of their planets to be detected.  Ordinarily, the glare of even an ordinary star is so bright that it swallows up the signal; shielding it, creating an artificial eclipse, might reveal hidden planetary systems.

The reason this topic comes up is that a loyal reader of Skeptophilia alerted me to an occultation that is going to occur at 1:08 AM, Greenwich Mean Time, tonight.  The bright star Betelgeuse is, for seven minutes, going to be covered by the asteroid 319 Leona -- so for that time, the familiar figure of Orion will appear to be missing his left shoulder.

Unfortunately for me, the occultation won't be visible from upstate New York, but you're in luck if you're in the southern Mediterranean or the southern tip of Florida.  Here's a map of where the occultation will be visible:

The coolest part is that Betelgeuse is so huge -- if it were where the Sun is, its outer edge would be near the orbit of Jupiter, and we'd be inside the star -- that despite its distance of 548 light years, its angular diameter is larger than that of the much closer asteroid that's occulting it.  So through a telescope, it'll be an annular occultation -- the occulting object won't quite be able to cover the entire star up.

Even so, Betelgeuse will for seven minutes dim to near invisibility to the naked eye.

You have to wonder what the ancients would have made of this.  Many cultures believed the heavens to be timeless and changeless, which is why the appearance of comets and events like the supernovae of 1054 and 1604 ("Kepler's supernova") were considered such portents of evil.

Even though we now know better, it still will be a little unnerving to have such a familiar star suddenly appear to vanish from sight.  It's a pity I won't get to see it -- it's a little late to purchase a plane ticket for Spain.  If the path of occultation passed across upstate New York, I'd even get up in the middle of a frigid December night to see one of the brightest stars in the night sky blink off for a few minutes.

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Kablooie

I'm kind of an excitable type.

I think that may be why I went into science.  The rigorous, evidence-basted methods of science were a nice antidote to the fact that my natural state is having my emotions swinging me around by the tail constantly.

Even after years of studying (and teaching) science, and twelve years of writing about it here at Skeptophilia Central, I still have the capacity for going off the deep end sometimes.  Which is what happened when I read a paper (a preprint, actually) from the Monthly Notices of the Royal Astronomical Society called "The Evolutionary Stage of Betelgeuse Inferred from its Pulsation Periods," by Hideyuki Saio (Tohoku University) and Devesh Nandal, Georges Meynet, and Sylvia Ekström (Université de Genève).

The constellation Orion.  Betelgeuse is in the upper left corner of the image.  [Image licensed under the Creative Commons Mouser, Orion 3008 huge, CC BY-SA 3.0]

First, a little background, before I get to the squee-inducing part.

Stars exist in a state of tension between two forces -- the inward pull of gravity and the outward pressure from the heat produced by fusion in the core.  At the very beginning of their lives, stars form from a loose cloud of mostly hydrogen gas that collapses under its own attractive gravitational force.  That collapse increases the pressure and temperature, and -- if the initial cloud was big enough -- eventually they rise high enough to trigger the fusion of hydrogen atoms into helium.  This is a (very) energy-releasing reaction -- physicists call such reactions exothermic -- and that energy pushes outward, balancing the inward pull of gravity.  The star goes into equilibrium.

But there's not an infinite supply of hydrogen.  The hydrogen fuel in the core is eventually exhausted, so fusion slows down.  The temperature drops, as does the outward pressure, so -- for a while -- gravity wins.  The star collapses, heating the core up, until the temperature and pressure become sufficient to fuse the helium "ash" in the core into carbon.  (This process, incidentally, is where the carbon in the organic molecules in our bodies comes from; Carl Sagan was spot-on in saying "We are made from star stuff.")

Helium fusion is also exothermic, so once again, the star goes into equilibrium.  But then the helium runs out, and the collapse resumes until the pressure and temperature are high enough to fuse carbon into oxygen. 

Then oxygen into silicon.  Then silicon into iron.

Two things are important here.  The first is that each of the reactions -- from hydrogen fusing into helium through silicon fusing into iron -- produces less energy than the one before it but requires higher temperatures and pressures to make it happen.  The second is that something happens when you pass that final reaction, which is that any subsequent fusion into heavier elements is an endothermic, or energy-consuming, reaction.

So when the silicon is used up, and the star's core is made mostly of iron, there's pretty much nowhere to go.  The gravitational collapse picks up again, and there is no "next reaction" that might produce energy to balance it.  So the collapse continues until finally there's such a tremendous temperature spike that the entire star goes kablooie.

This is called a supernova, and it releases more energy in a few seconds than the star liberated in the entire rest of its life.  The unimaginable pressures do fuse some of the iron in the core into those heavier elements, despite the energy required, and that's where all the elements on the periodic table with atomic numbers higher than 26 come from, from the gold in our jewelry to the silver in our coinage and the copper in our electrical wires.

With me so far?  Because there's one more thing I haven't told you.

Each stage in a star's life takes much less time than the one before it.

The hydrogen to helium stage lasts millions to billions of years.  (The Sun is in the hydrogen-burning stage, and is estimated to have another five billion years to go.)  Higher-mass stars have higher pressures and temperatures, and consume their fuel at a greater rate, but we're still talking tens to hundreds of millions of years.  Helium-to-carbon lasts maybe a million years; carbon-to-oxygen, we're talking decades.

After that, it's pretty much a ticking time bomb with a very short fuse.

Now for the punch line: the Saio et al. paper suggests that the pulsation periods of the red supergiant star Betelgeuse indicate that it is nearing the end of the carbon burning stage.  So we might actually have a shot at seeing one of the brightest stars in the sky go supernova in our lifetimes.

This paper has even the scientists flipping out.  One of my favorite science vloggers, astronomer Becky Smethurst of Oxford University, did a YouTube video about this paper and you could tell she was barely keeping it together.  Ordinarily, whenever you hear about anything impressive in sciences like astronomy and geology -- such as a supernova or gamma-ray burster, or the Yellowstone Supervolcano erupting or the East African Rift Zone tearing Africa apart -- the scientists will respond with a deep sigh and a monotone "as we've explained many times before, blah blah blah astronomical/geological time scales blah blah blah."

Now, though, the astronomers are actually acting like this is the real deal.  (And in fact, if Saio et al. are right, Betelgeuse has probably already blown itself to smithereens; at six-hundred-odd light years away, we just haven't gotten the memo yet.)

When this happens, it's gonna be spectacular.  A supernova that close will be bright enough to read by at night, most likely for months, and will be easily visible during the day.  The happy news is that it's not close enough to do us any damage; a supernova under twenty-five light years away could be catastrophic, doing nasty stuff like blowing away the atmosphere.  (Fortunately, there are no supernova candidates anywhere near that close to us.)  Betelgeuse will just create some amazing fireworks, as well as permanently changing the contour of the familiar constellation of Orion.

So my opinion is: bring on the supernova.  We could use a little excitement down here.

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Prelude to a cataclysm

Dear Readers:

I'm going to be taking a short break next week.  However, I hope you'll continue to send me ideas for new posts -- I'll be back in the saddle again soon, and I always value your suggestions.

The next Skeptophilia post will be Monday, January 6.  A very Happy New Year to all of you, and I hope this century's Roaring Twenties bring you everything you desire!

cheers,

Gordon

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There'd be nothing like a good supernova to liven things up around here.

Far and away the most spectacular event in the universe, a supernova of a massive star releases more energy in a few seconds than our Sun will release in its entire lifetime.  The colossal explosion is set off by the exhaustion of the fuel in the star's core, a phenomenon that deserves a little more explanation.

Stars are kept in equilibrium by two forces, the outward pressure of the heat produced by fusion in the core, and the inward pull of gravity.  When the star runs out of fuel, the heat diminishes, and gravity wins -- causing a sudden collapse and a phenomenally quick heating of the star's atmosphere.

The result is a supernova, which temporarily outshines everything else in the near vicinity.  Actually, "outshine" is the wrong word; nearby star systems would be flash-fried, and even at a relatively safe distance the high-energy electromagnetic radiation could severely damage a planet's atmosphere.  (Just as a clarification, I'm talking about planets in other star systems; if there were planets in the supernova's system, they'd be instantaneously vaporized.)

The collapsed core of the star then becomes either a neutron star or a black hole, depending on the star's initial mass.  The exploded remnants continue to glow brightly for several months, before finally cooling, fading, and disappearing from the night sky.

As it happens, we've got a good candidate for a supernova not too far away (well, 640 light years away, which isn't exactly next door, but is still close by astronomical standards).  It's called Betelgeuse, and it's the familiar star on Orion's right shoulder.  A red supergiant, the star is about eleven times the mass of the Sun (putting it in the "neutron star" range after it blows itself to smithereens).  However, volume-wise, it's enormous; if you put Betelgeuse where the Sun is, its edge would be somewhere between the orbits of Jupiter and Saturn.

Yes, that's what it sounds like.  If Betelgeuse replaced the Sun, we here on the Earth would be inside the star.

The constellation of Orion; thats's Betelgeuse in the upper left [Image licensed under the Creative Commons Rogelio Bernal Andreo, Orion Head to Toe, CC BY-SA 3.0]

Betelgeuse has long been known as one of the better supernova candidates that are relatively close by.  Asked when it's going to explode, though, astronomers have always played it cagey; could be tomorrow, could be a hundred thousand years from now.  But its recent behavior has made a lot of scientists wonder if the actual date of the explosion might not be closer to the "tomorrow" end of the spectrum than we'd thought.

The star has been exhibiting some odd behavior lately.  It's long been known as a variable star, varying in magnitude between about 0.0 and +0.5 (the bigger the number, the fainter the star).  This means it oscillates between being the fifth brightest star in the night sky and the tenth, with the period of its variation averaging at a little over a year.  But in the last few months, it's defied expectations, dimming to a magnitude of +1.3 and dropping to 23rd place on the list of brightest stars.

Could this herald the beginnings of the collapse that initiates the supernova?  Could be, but the truth is, we don't know.  Supernovae are uncommon events, and nearby ones nearly unheard of -- the last one was "Kepler's Star," the 1604 supernova in the constellation of Ophiuchus.  So what the leadup will look like, we aren't really sure.

What's certain is that this is unprecedented, at least since we've kept detailed records.  It merited a press release from the Villanova University Department of Astronomy three weeks ago, so even the astronomers -- ordinarily the most cautious of scientists -- are admitting that something's up.

Now, we still don't know what's going to happen.  Like I said, we've never been able to observe the events leading up to a supernova before.  But you can bet that the astrophysicists are paying close attention.

And with good reason.  If Betelgeuse went supernova -- no, correction, when Betelgeuse goes supernova -- it's going to be spectacular.  It's estimated it will brighten to a magnitude of -10, which (for reference) is sixteen times the brightness of the full Moon, and over a hundred times the brightness of the planet Venus.  It will be easily visible during the day and will provide enough light to read by at night.  And this won't be a blink-and-you-miss-it occurrence; the supernova will only fade gradually, over a period of eight to nine months, and during that time it will be (other than the Sun itself) the brightest thing in the sky.

And I can say unequivocally that I hope it happens really soon.

It'd be nice to have something happen out there in the universe to take our minds off of how screwed up things are down here, you know?  It'd be a good reminder that there are bigger and more powerful things than we are, and our petty little squabbles are really pretty minuscule by comparison.

So as far as I'm concerned: bring on the supernova.

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