Eta Carinae

No, not Wonder Woman’s sidekick Etta Candy, Eta Carinae!  “Ate a car in a what???” you ask, “what’s this Eta Carinae?”  Eta Carinae is a popular interstellar sightseeing spot, so some of you may already know – if you do, don’t spoil it for everyone, let me tell them about it.

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Eta Carinae is a somewhat mysterious stellar system about 7,500 light years away from us in the Carina constellation that, best we can tell from here, contains two (or possibly more) very large stars that are slowly circling one another.  Like me, you may not have heard of the Carina constellation if you live in the Northern hemisphere, because it’s a southern hemisphere constellation that by the way is supposed to be the rudder on a constellation ship that belongs to Jason of “And The Argonauts” fame.  If you’re in the southern hemisphere and look up without a telescope, Eta Carinae just looks like any other ordinary star, but with a telescope becomes something much, much more fascinating. But before we can really get to that, you need to know how Eta Carinae’s brightness first drew our attention, and before that, you need to have some kind of grasp on how we grade such things.

The ancient Greeks came up with a system of grouping all stars into one of six magnitudes (“Pop pop!”) of brightness, each descending group about half as bright at the previous.  This scale counts “backwards,” so that the lower the number of magnitude, the brighter the stars in it are, meaning a “1st magnitude” star is one of the brightest stars in the night sky, and about twice as bright as a 2nd magnitude star, which is in turn about twice as bright as a 3rd magnitude star, and so on.  A 6th magnitude star is at the bottom of the scale, about one one-hundredth as bright as a 1st magnitude star and so faint that you can barely see it without a telescope or a pair of binoculars, things which the Greeks didn’t have (that we know of).

But we have those things and know now that there are stars out there that are too dim to see without them.  The Greek system was also pretty vague, so modern astronomers embellished it to assign each star an actual number that more tightly defines how bright it is, which we call its “apparent magnitude”.  Rather than spending forever writing out the words “apparent magnitude”, astronomers notate the number as m, and I don’t fault them for doing this one damn bit.  This apparent magnitude m by the way is based on how bright any star looks to us here on earth by the way, not by how much total light that star actually emits – something else we can reliably measure that the ancient Greeks couldn’t.  This means that a small star closer to the earth is going to appear brighter to us and get a higher magnitude (with a lower number) than a behemoth that’s further away; if you want to compare stars by their total light output, you compare their “absolute magnitude” instead.

Modern astronomers apparently wanted to make this “low number equals high brightness” scale even more confusing, so they chose the fifth-brightest star Vega, not the brightest star period, as the gold standard for this scale, because reasons.  This means Vega has a zero apparent magnitude rating, or 0.0 m.  Like the Greek scale, a star assigned the larger number of 1.0 m is only 40% as bright as Vega’s 0.0 m, a 2.0 m star is only 16% as bright, etc.  Also reflecting the Greek system, anything below a 6.0 m requires a device to see; a pair of binoculars lets you see down to about 10.0 m, Pluto is about 15.0 m, and the Hubble Space Telescope can see down as far as 30.0 m.

Speaking of struggling to see a small point of light, I know all this may be seem pointless, but there is a light at the end of this Hubble, I mean tunnel.  You will soon understand why a star’s brightness drew our interest to Eta Carinae; besides, I believe that learning something is never pointless and is always good for you, so even if you don’t need this to understand the Eta Carinae story, tough.  I do promise you there won’t be a test afterwards, though.

To demonstrate this system and maybe help make it clearer, here’s a list of the ten brightest stars in the night sky and their apparent magnitudes.  You can see how stars that are brighter than Vega get a negative number and the stars that are fainter than Vega get larger numbers the further down the list you go.

  1. Sirius:  -1.5 m  (the brightest star in our sky – seriously! – and under ten light years away)
  2. Canopus:  -0.72 m  (interestingly, in the same Carina constellation with Eta Carinae)
  3. Alpha Centauri:  -0.29 m  (a triple star system that’s so bright because it’s the closest stars to ours at just over 4 light years)
  4. Arcturus:   -0.04 m
  5. Vega:  0.0 m
  6. Capella:  0.08 m
  7. Rigel:  0.18 m  (most of the above stars are within say 40 light years of earth; Rigel is huge, but over 836 light years away)
  8. Procyon:  0.34 m (also pretty close to our solar system)
  9. Achernar:  0.46 m
  10. Betelgeuse:  0.45 m  (also huge, but something like 600 light years away)

Betelgeuse, an old favorite of mine, can switch places with Achemar on this list, depending on which source you check, because Betelgeuse’s apparent magnitude changes and has been measured between 0.0 and 1.3 m.  C’mon, Betelgeuse, pick a lane!  Betelgeuse’s apparent magnitude changes like this because it’s a “variable star,” a star that shows different brightness at different times, duh.  Betelgeuse does this because it’s near the end of its life and is blowing off a hell of a lot of its stuff into space, not unlike me at work on a Friday.  These flare-ups can make it go brighter for a bit, but they also mean it’s shrinking, so it can appear dimmer at times – all of which takes us back to Eta Carinae, so let me finally get around to telling you what Eta Carinae’s apparent magnitude is.

It’s variable.  Ha ha! Don’t worry though; this apparent magnitude dog isn’t as shaggy as it first appears to be.

See, Eta Carinae was first mentioned in text in 1677 by comet-famous Edmond Halley who said it was about 3.3 m.  That doesn’t even break into the top hundred of the brightest stars, you need 2.5 m or less for that club.  Eta Carinae may have brightened a tad over the next two hundred years, but in 1827 someone looked up and was surprised to find it had become a “first magnitude” star (that’s somewhere between -1, 0 and +1 m), suddenly on the list of the 25 brightest stars in the night sky.  Ten years after that, Eta Carinae really began powering up for the next eighteen years, ending up shining brighter than Canopus – the second brightest star in the sky at about -0.72 m as noted above.  Just after brushing against Sirius’ all-time brightest rating of -1.5 m, Eta Carinae then began to fade.  By 1886 it was below 6.0 m, then returning to naked eye status after about 1940, and jeez Eta Carinae, wtf?  Right now it’s a 4th magnitude star, placing it somewhere at the bottom of the list for the top five hundred brightest stars.

I can hear you right now, “all these numbering systems and comparisons are just riveting, Megara, great story.”  Just wait, we’re almost there.

Such severe swings in brightness obviously drew people’s attention and lead us to investigate and then uncover what causes Eta Carinae’s variability, once we got the Hubble orbiting the earth and took a really good look.  Turns out that one of the two stars that make up Eta Carinae is a variable star like Betelgeuse that’s also nearing the end of its short life. The two stars we’re pretty sure are in the Eta Carinae stellar system are called “η Carinae A” and “η Carinae B” (η, or “eta” is the seventh letter of the Greek alphabet by the way, which astronomers use to designate the seventh-brightest star in a constellation).

While η Carinae B is “only” about thirty times the mass of our sun and a million times as bright, η Carinae A is just colossal, an Brobdingnagian beast that may have once been as big as 250 times the mass of our sun, but is now down to about 90 times the mass, but still big enough to extend out to Jupiter’s orbit if you replaced our sun with it (again, just like Betelgeuse).  η Carinae A is the troublemaker here, because it’s a “luminous blue variable” which are some of the largest stars known, and also pretty rare (only twenty have been found so far), perhaps because they only exist for about 3 million years before changing into a different star thanks to mass loss, or going supernova and becoming a neutron star or a black hole. We expect η Carinae A to supernova (or even hypernova, which is ten to a hundred times bigger) sometime between now and the next ten thousand years, but in the meantime, η Carinae A shines about five millions times brighter than our sun (there’s that “absolute magnitude” again) – so brightly that it’s eroding itself as the outward pressure from its light and radiation overcome the inward pull of its gravity.   Every star ‘exhales’ to some extent, we call that a “solar wind”, but few disgorge it like η Carinae A does.  Its solar wind is so dense that we can’t really see the stars inside of it, we can’t even tell where η Carinae A’s surface begins or ends – and that’s under normal conditions, before…

η Carinae A erupted back in the 1840s when it was flaring up, belching out about twenty suns worth of glowing matter, far more than its usual prodigious output.  The Great Eruption as we call it created a dense, double-mushroom cloud of gas and dust that’s still expanding about 1.5 million miles an hour and has surrounded the pair of stars inside, which is why we’re not one hundred percent on what’s going on inside.  Now about a light year across, you could effectively lose our solar system from the sun to Pluto in that cloud, roughly 860 times over. We call the cloud the Homunculus Nebula, and that nebula is what makes Eta Carinae so stunning a sight.

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Blah blah blah.  Now, I know all that preceding dribble was off-puttingly boring, but Eta Carinae is seriously much more dramatic than all that might imply, which when you think about it, is saying an awful lot.  You don’t really even need to know any of this stuff to enjoy the terrible beauty of Eta Carinae – a rose by any other number and all that – but understanding the background information transforms it from just a pretty picture into a truly monumental stellar landmark.

EtaCarinae.jpg

Just look at this magnificent bastard and try to comprehend its growing immensity and the secret stars within, one big enough to render ours nearly insignificant dancing with an even larger, deteriorating monster.  The whole thing looks alive, like the beating heart of a cosmic organism. I would say we should have sent a poet, but we don’t need to, the photo came to us and is now ours to spin our own thousand words from, thanks to the Hubble Space Telescope.  We’ve seen nothing else like it in our galaxy, and only five total similar structures, each in another galaxy. Just look at this awe-inspiring sight and meditate on its overwhelming enormity.

[Credit for this photo goes to Nathan Smith (University of California, Berkeley), and The Hubble Space Telescope via NASA.  Starting from the center, you can see the purple-tinted light reflected from the stars inside, the strange jets and expanding lobes of gas laced with filaments of dark dust, and finally some debris from previous eruptions and general out-gassing, made visible here from just a narrow band from the spectrum of red light.  Images taken with red and near-ultraviolet filters were combined to make this photo, with eight exposures needed to capture the huge range of brightness and dimness, because those outer ejecta blobs are 100,000 times fainter than the brilliant central stars. The smallest details you see here are still about ten billion miles across, meaning about the size of our solar system.  This picture was taken and processed on 10 June 1996, so anyone care to work out how much bigger Eta Cainrae’s Homunculus Nebula grown at its 1.5 million miles per hour since then?]

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I know I said you wouldn’t be quizzed afterwards, but if you’d still like to earn some extra credit, why not hold this light-year-sized explosion in your hand via a 3D printer model from NASA?  You can grab the files to do so here.

Also feel free to leave comments pointing out how inept my science and research were, how I’m barely a 6th magnitude writer, and then don’t just take my word for it or go watch some Neil deGrasse Tyson video about it, read up on Eta Carinae your own damn self!  You can’t trust the Avocado to teach you everything there is to know about something!