Category: Astronomy

The faces of Licinia

By Phil Plait | September 6, 2012 6:35 am

In April, 2012, the Dawn spacecraft was a mere 272 kilometers (170 miles) from the surface of the asteroid Vesta when it took this wonderful picture of the crater Licinia:

[And oh yes, you want to click to envirgingoddessenate – it links to a picture of the area around the crater as well.]

Licinia is about 25 km (15 miles) across – too big to fit in Dawn’s field-of-view from that height. But it does show spectacular detail, including what look like landslides into the bowl from the crater rim; you can see them as dark streaks running down the crater wall. Mounds of material at the base of the crater wall indicate bigger landslides, too. Vesta’s gravity is far weaker than Earth’s – it’s about 1/40th what we experience here – but even then, it’s a force that won’t be denied.

While I was inspecting the crater floor, I saw something that made me laugh out loud. The floor is lit by the distant Sun, but a sharp shadow of the crater rim is cast on it as well. The inky black shadow is irregular due to the uneven crater rim. Inset here is a piece of that shadow line. Do you see the dark shadow "face" looking to the left at the top? It jumped right out at me… and then I saw another face just below it, this time bright and looking to the right and slightly up!

Once you see it…

Man. Pareidolia is a force almost as strong as gravity.

Anyway, Dawn’s visit to Vesta has come to an end. It left the asteroid on September 5, and began the long two-and-a-half year voyage to visit Ceres, the largest of the main belt asteroids between Mars and Jupiter. Vesta is a fascinating place, but so is Ceres, and we know very little about it.

That’s all about to change. But then, that’s what exploration is for.


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MORE ABOUT: crater, Dawn, Licinia, Vesta

A little sunset CARMA

By Phil Plait | September 4, 2012 11:00 am

Jeff Frost is a photographer who wanders the world taking pictures of interesting things – abandoned buildings are a favorite of his.

He was in Yosemite, near Bristlecone Pine, in mid-August to try to get some pictures of the Perseid meteors. He got rained out, unfortunately, but apparently he got some good karma at sunset… or rather, great CARMA:

[Click to embiggen.]

CARMA is the Combined Array for Research in Millimeter-wave Astronomy, a collection of dishes that observes objects in the Universe that emit light at millimeter wavelengths – past the infrared region of the electromagnetic spectrum, but not as far as radio waves. Cold objects glow with this kind of light: dust, molecules between the stars, comets, and the leftover radiation from the Big Bang itself.

Jeff got to the array as the Sun was setting, and took this beautiful shot. What’s neat is that they were probably active at that time; millimeter waves go right through our air and aren’t scattered like visible light – and the Sun is pretty dark at those wavelengths – so CARMA can watch the sky day and night. Even through clouds!

In grad school I took a class in radio astronomy, which is turns out is very different than ultraviolet/optical/infrared astronomy. I did OK in the class, but my heart lies in the near-visible spectrum and higher energies. I’ll leave the weird radio stuff to people like NoisyAstronomer.

And I’ll leave you with this lovely time lapse video Jeff made called Flawed Symmetry of Prediction:

Image credit: Jeff Frost, used by permission

CATEGORIZED UNDER: Astronomy, Pretty pictures

Temba, his spiral arms wide

By Phil Plait | September 3, 2012 7:02 am

Lying roughly 50 million light years from Earth is the magnificent spiral galaxy NGC 5033. Although that distance is a soul-crushing 500 quintillion kilometers, it’s actually relatively close by on the cosmic scale. Close enough that a lot of detail can be seen in the galaxy… and it also makes for a stunner of a picture:

[Click to darmokenate.]

This shot was taken by friend-of-the-BA-blog Adam Block using the 0.8 meter Schulman Telescope on Mount Lemmon in Arizona. It’s a whopping 13 hour exposure taken in near-true color.

It’s amazing what you can see in just this picture if you know what to look for. The spiral arms of the galaxy are fairly open, which is common enough, but the outer ones stick out a bit more than you might expect. The nucleus is very small and bright, more so than I’d expect for a typical spiral as well. Both of those things led me to expect this is an active galaxy, and that turns out to be the case.

Every big galaxy – ours included – has a supermassive black hole in the center. The Milky Way’s is 4 million times the mass of our Sun! In some galaxies, like ours, happily, the black hole is just sitting there. But in some there is gas actively falling into the hole. It spirals around and forms a very hot and very large disk, which glows fiercely as the matter is heated to temperatures of millions of degrees. They disk can blast out light from radio waves up to X-rays, and we say that the galaxy is "active".

A quick search of the literature didn’t turn up any measurements for the mass of black hole in NGC 5033, but it does confirm that it’s an active galaxy. Interestingly, the black hole is not located in the exact center of the galaxy! That’s very unusual, and indicates that NGC 5033 recently merged with another galaxy, probably a smaller one. It’s a cannibal! But then, most big galaxies are. It’s how they get big… and you’re living inside a big one, so there you go.

This may explain the wide arms on the galaxy as well; a collision and merger can distort the shape of the galaxy. Also, check out all the pink blobs along the arms: those are sites of furious star formation, the hot energetic massive young stars lighting up the gas around them. That also is common after a big collision.

Finally, one more nifty thing. You can see long ribbons of dark dust festooning the galaxy in the inner region. Dust absorbs light from stars behind it. But see how the dust looks like it’s only on one side of the galaxy, the half in the picture below the center? That’s an illusion, sortof. In reality there’s dust orbiting all around the center. However, there are stars above and below the disk of the galaxy, and the ones between us and the far side fill in the darkness a little bit, so the dust is less apparent. I’ve written about this before, and it does happen in quite a few spirals. Click the links in the Related Posts section below to see more gorgeous galaxies with this feature.

It’s funny how much information you can squeeze from a single picture! You have to be careful and not over-interpret it, and of course a lot of the things I’ve written here wouldn’t have been known without other observations of NGC 5033 using different telescopes and different methods in different types of light.

But even just one picture can tell you a lot. And in my opinion – and I tend to be right about these kinds of things – the wave of beauty that flows over you when looking at this picture is only enhanced by knowing more about the galaxy itself… and is boosted in no small way by the fact that we can know these things.

Image credit: Adam Block/Mount Lemmon SkyCenter/University of Arizona


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CATEGORIZED UNDER: Astronomy, Pretty pictures

Awesomely blemished inverted solar beauty

By Phil Plait | September 2, 2012 7:01 am

Alan Friedman’s photos are no stranger to this blog; I’ve posted a lot of his amazing pictures of the Sun (See Related Posts, below). So many, in fact, that one needs to be surpassingly cool to add to the lineup.

So, yeah:

[Click to ensolarnate.]

Yegads. He took this on July 29, 2012. Because the image is inverted – dark things appear bright, and vice-versa – sunspots are intense white patches, bright plages appear dark, and towering filaments are whitish-gray.

Note how the Sun’s face gets darker toward the center and brighter toward the edge – meaning in reality the center is bright and the edge dimmer. This is called limb-darkening (the opposite of limb-brightening seen in some gas clouds), and occurs because gas around the Sun absorbs its light. We look through more of it near the edge than toward the center, so there’s less light coming from the limb of the Sun.

I’ll note that only the face of the Sun is inverted, though. Everything outside that is normal, so the leaping prominences of gas on the edge are bright, as they should be. That might be a bit confusing, but it does make for a dramatic picture.

And given how volatile our local star, you don’t have to go very far to get drama out of it.

Image credit: Alan Friedman


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Time lapse: When the Moon ate Venus

By Phil Plait | August 31, 2012 6:30 am

On the morning of August 13 – 14 (depending on where you were in the world) the Moon slipped directly in front of Venus in the sky, an event called an occultation. It was cloudy here in Boulder so I missed it, but halfway across the world in Korea, astrophotographer Kwon O Chul had a magnificent view, and made this lovely time lapse video of the event.

Occultations like this are relatively rare. If all the planets and moons orbited the Sun in exactly the same plane – that is, if you looked at the solar system from the side and all the orbits aligned perfectly, like looking at a DVD from the side – we’d see occultations all the time.

But in reality all the orbits are tilted a little bit. Venus circles the Sun in an orbit canted by about 3° compared to Earth’s. The Moon’s orbit is tilted by 5 °. The Moon orbits the Earth once per month or so, but it usually passes by Venus, missing it by a bit because the orbits aren’t aligned. But sometimes, every few years, the dance comes together, and the Moon wil slip directly in front of Venus.

An occultation is an amazing thing to see. I saw a lunar Venus occultation when I was a kid and just starting out as an amateur astronomer. It takes a few seconds for the Moon to cover a planet, so you can watch as the planet dims and then pops out when it gets completely covered. Also, the Moon commonly passes in front of stars, which are so far away and appear so small they just wink out, blip!

You can get a list of upcoming occultations at the International Occultation Timing Association website. If you get a chance to see the Moon occult a star, take it! Binoculars help a lot, and it’s fun to watch the star just suddenly blink out.

Tip o’ the dew shield to Astropixie.

CATEGORIZED UNDER: Astronomy, Cool stuff, Pretty pictures

Kepler finds a planet in a binary star's habitable zone

By Phil Plait | August 29, 2012 11:16 am

Oh, this is too cool: scientists have found a planet orbiting a binary star (a pair of stars in tight orbit around each other) that is at the right distance to have liquid water! Let me be clear: this planet is much bigger than Earth, and is likely to be a gas giant. So it’s not Earth-like, and probably not itself habitable.

But it might have moons…

[Note: this image is artwork based on the science. Click to tatooineneate.]

OK, first: Kepler is an orbiting telescope that has been staring at one spot in the sky for about three years now. It’s looking at about 100,000 stars. If these stars have planets, and the orbits of these planets are seen edge-on, then they will occasionally pass directly between us and their parent star blocking a little bit of the light. This is called a transit, and if the planet is big enough it can block enough light from the star to be detected by Kepler. So far, 77 planets have been confirmed using Kepler, and over 2000 more have been detected and are awaiting confirmation.

The new discovery deals with a binary star called Kepler-47. It’s about 5000 light years away, which is pretty far for a Kepler system – it’s faint at that distance. Still, the observations look very good, and the conclusions convincing to me.

One of the two stars is very Sun-like, about the same size, temperature, and brightness as our home star. The second is fainter, smaller, and cooler. They comprise an eclipsing binary: their orbit is seen edge-on from Earth, so they pass in front of each other as seen by us as they circle each other. Their orbit is pretty tight: they’re only about 13 million kilometers (8 million miles) from each other, and their orbit is just 7.5 days long.

Two planets were actually found orbiting the stars. Kepler-47b is about 3 times the diameter of the Earth. Its mass isn’t known, but it’s likely 7 – 10 times ours. It’s hot: the orbit is just 50 million km (30 million miles) out, closer than Mercury is to the Sun. It takes about 50 days to orbit.

The second planet, Kepler-47c, is the interesting one. It’s even bigger, 4 – 6 times Earth’s diameter, roughly the size of Uranus, and most likely 20 times our mass. Its orbit is almost exactly the same size as Earth’s, coincidentally, taking 300 days to orbit the binary (its year is shorter than ours because the two stars together have more mass, and therefore more gravity, than the Sun).

Taking into account the orbital size and the physical properties of the stars, the scientists have determined that the planet is at the right distance to be in the stars’ habitable zone: the distance where liquid water could exist on a solid body.

As I pointed out, the planet is probably a gas giant. But it could have moons – in fact, given our own solar system configuration, it seems likely. It’s not crazy to think that these moons, should they exist, might be habitable. That’s amazing.

These two new worlds put the roster of confirmed circumbinary planets (that is, planets orbiting binary stars) to six. And we only just started looking a few years ago! Given the number of stars observed and the planets found, and applying a little statistics, it seems entirely possible that there are millions of such planets in our Milky Way galaxy alone.

That’s right: millions of possible Tatooines just waiting to be found! And we may yet find them. Finding gas giant planets is far easier than finding their much smaller moons, but one of the goals of exoplanet astronomy is to improve the technology and the techniques to the point where such moons can be detected as well. It may take bigger telescopes and more time, but there is nothing stopping us except our will to do so.

Think of that: we can detect potential Earths around stars quadrillions of kilometers away! And all we have to do is want it enough.


[P.S. If you want to keep up with exoplanet news, there’s a wonderful iPhone/iPad app called Exoplanet that has info, diagrams, and updates when new planets are found. I use it myself and really like it.]

Image credit: NASA/JPL-Caltech/T. Pyle


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CATEGORIZED UNDER: Astronomy, Cool stuff, Pretty pictures

BAFact Math: The Sun is mind-crushingly brighter than the faintest object ever seen. Seriously.

By Phil Plait | August 29, 2012 10:10 am

[BAFacts are short, tweetable astronomy/space facts that I post every day. On some occasions, they wind up needing a bit of a mathematical explanation. The math is pretty easy, and it adds a lot of coolness, which I’m passing on to you! You’re welcome.]


Today’s BAFact: How much brighter is the Sun than the faintest object ever seen? About Avogadro’s number times brighter.

Yesterday and the day before I wrote about how much brighter the Sun is than the Moon, and how much brighter the Sun is than the faintest star you can see (note that here I mean apparent brightness, that is, how bright it is in the sky, not how luminous it actually is). I have one more thing to add here.

Years ago, I worked on a Hubble Space Telescope camera called STIS – the Space Telescope Imaging Spectrograph. At the time, it was the most sensitive camera ever flown in space, and I was constantly amazed at what we saw using it.

Hubble did a series of observations called the Deep Fields: it stared at one spot in the sky for days, letting light from incredibly faint objects build up so that they could be detected. For the Deep Field South, STIS was used to observe a particular kind of galaxy, a quasar called J2233-606. The total observation time was over 150,000 seconds – nearly two days!

I worked on these images, and was chatting with a friend about them. We were astonished at the number of objects we could see, distant galaxies so faint that they were unnamed, uncategorized, because no one had ever seen them before. Playing with the numbers, we figured that the faintest objects we could see in the observations had a magnitude of about 31.5. That’s incredibly faint.

How faint, exactly?

The faintest star you can see with just your eye has a magnitude of about 6. Using the magnitude equation I wrote about earlier, plugging those numbers in we get

Brightness ratio = 2.512(31.5 – 6)) = 2.51225.5 = 16 billion

Wow.

But we can do better than that. A lot better. After all, the Sun is the brightest object in the sky, of course, with a magnitude of -26.7. Just for grins, how much brighter is the Sun than the faintest objects ever seen?

Brightness ratio = 2.512(31.5 – (-26.7)) = 2.51258.2 = 2 x 1023

Um.

That’s 200,000,000,000,000,000,000,000. 200 sextillion. Holy yikes.

That number is crushing my mind. It’s ridiculous. A sextillion is simply too big a number to grasp. And 200 of them? C’mon!

But hey, wait a sec…

Does the number 2 x 1023 look familiar to you? It does to me: it’s the same order of magnitude (factor of 10) as Avogadro’s number! It’s the number of atoms of an element in a mole of the element, where a mole is the number of atoms in 12 grams of pure carbon-12. I know, it’s an odd unit, but it’s handy in chemistry, and a lot of (geeky) folks have heard of it.

Avogadro’s number is actually about 6 x 1023. So if we could detect stars or galaxies just a hair more than a magnitude fainter, the ratio of the brightness of the Sun to those objects would be Avogadro’s number. Huh.

I’m not sure that helps, but it’s fun in a spectacularly nerdtastic kind of way.

Science, baby. I love this stuff!


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CATEGORIZED UNDER: Astronomy, BAFacts, Cool stuff, Science

Saturn's shadow slices the rings

By Phil Plait | August 29, 2012 6:30 am

There is a whole lot of awesome in a picture of Saturn and its rings just released from the Cassini spacecraft. Check this out:

Mmmmm, ringalicious.

Cassini was about 2 million kilometers (1.2 million miles) from Saturn when it took this picture, so we’re seeing a decently wide-angle view. At the time, the spacecraft was below the plane of the rings, looking north (up, if you like). The Sun is off mostly to the left and up a bit.

The first cool thing is obviously the shadow of the planet itself cast on the rings. It cuts across like a black scythe! As I looked at the picture my eyes and brain kept trying to fill in the missing arc of rings, which was amplified by a slight afterimage as my eyes moved around. It’s a difficult illusion to ignore.

Second, I love how you can see all the different rings in the picture, including the thin, lumpy F-ring outside the main band. The big gap is called the Cassini Division; it’s not really an empty space since there are many faint thin rings inside it. They’re just hard to see here. The Cassini Division is fairly easy to spot even through a small telescope, looking from Earth like someone took a knife to the rings and sliced them.

Third, you can see the tiny moons Janus (below the rings on the left) and Epimetheus (above the rings on the left) as well. I wonder how hard it is to get a picture like this without seeing any moons in it? Saturn has quite the fleet of them.

Fourth, look to the left, just where the inner arc of the rings cuts across Saturn. You can see the planet right through the rings! The rings aren’t solid; they’re composed of gazillions of particles of nearly pure water ice. There are spaces between the particles, so we can partially see through them, like looking through a screened window.

Fifth, and perhaps most cool of all: the part of Saturn we’re seeing here is the night side, entirely unlit by the Sun. The bottom (southern) part of Saturn is only noticeable by its absence! But what’s that glow in the north?

That, my friends, is ringshine! Although this part of Saturn is in nighttime, the Sun is still shining on the rings (wherever you don’t see Saturn’s shadow across them). The ring particles are very bright and shiny. They reflect the sunlight, which then illuminates the northern hemisphere of Saturn. The southern half is still dark because the ice particles tend to reflect light back up, like a mirror. Since the Sun is coming from the north, that’s the way the light gets reflected. I’ll note that most of the light gets reflected away from Saturn, to the upper right in this picture, but enough is reflected back to make the cloud tops glow softly.

This happens on Earth too, when sunlight reflects off the Earth and illuminates the dark part of the Moon. This is called Earthshine, also poetically called "the old Moon in the new Moon’s arms." It’s quite lovely.

And it’s science! Which is lovely, too.

Image credit: NASA/JPL-Caltech/Space Science Institute


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CATEGORIZED UNDER: Astronomy, Pretty pictures

BAFact Math: The Sun is 12 *trillion* times brighter than the faintest star you can see

By Phil Plait | August 28, 2012 10:00 am

[BAFacts are short, tweetable astronomy/space facts that I post every day. On some occasions, they wind up needing a bit of a mathematical explanation. The math is pretty easy, and it adds a lot of coolness, which I’m passing on to you! You’re welcome.]


Today’s BAFact: The Sun is 12 trillion times brighter than the faintest star you can see with your naked eye.

In yesterday’s BAFact, I showed how the Sun is about 400,000 times brighter than the full Moon – and I showed my math. That’s an amazing brightness difference, but while I was writing it I had to wonder: how much brighter is the Sun than the faintest star you can see?

The faintest stars visible to the naked eye have a magnitude of about 6. This depends on lots of stuff, like how dark the sky is, how good your eyesight is, and so on. Some people with excellent vision can see stars down to magnitude 7, and there are reports of a few extraordinary people who can see even fainter. But on a dark night, the average person can just barely see 6th magnitude stars.

Let’s use that number then. All we have to do is plug that into the equation I gave yesterday (and remembering that the Sun has a magnitude of -26.7):

Brightness ratio = 2.512(6 – (-26.7)) = 2.51232.7 = 12 trillion

Yegads! That’s 12,000,000,000,000 times brighter!

Now, to be fair, that’s not really the brightness range your eyes can detect. You can’t look right at the Sun easily or comfortably; it’s simply too bright. So the range of brightness your eye can see is probably smaller.

We can put a lower limit on it easily enough using the Moon. The Moon is the second brightest object in the sky, and we know we can look at that easily enough, so let’s do that math (the Moon’s magnitude is -12.7 when it’s full):

Brightness ratio = 2.512(6 – (-12.7)) = 2.51218.7 = 30 million

Wow. So you can comfortably see objects over a brightness range of 30 million. That’s impressive! The eye is a pretty cool little machine.

As an aside, your eye isn’t linear; it’s logarithmic (in reality, it’s more complicated than this, and I’m simplifying, but close enough). In other words, a star giving off twice as much light doesn’t look twice as bright as another. The way your eye responds to light squeezes down the scale, making it easier to see fainter and brighter objects at the same time.

So how faint do objects get? Ah, that’ll be tomorrow’s BAFact. Stay tuned!


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MORE ABOUT: magnitudes, Moon, star, Sun

BAFact Math: The Sun is 400,000 times brighter than the full Moon

By Phil Plait | August 27, 2012 10:02 am

[BAFacts are short, tweetable astronomy/space facts that I post every day. On some occasions, they wind up needing a bit of a mathematical explanation. The math is pretty easy, and it adds a lot of coolness, which I’m passing on to you! You’re welcome.]


Today’s BAFact: The Sun is 400,000 times brighter than the full Moon in the sky.

If you’ve ever looked at the full Moon through a telescope you know how painfully bright it can be. But you can do it if you squint, or use a mild filter to block some of the light.

On the other hand, if you try the same thing with the Sun (hint: don’t) you’ll end up with a fried retina and an eyeball filled with boiling vitreous humor.

So duh, the Sun is much brighter than the Moon. But how much brighter?

Astronomers use a brightness system called magnitudes. It’s actually been around for thousands of years, first contrived by the Greek astronomer Hipparchus. It’s a little weird: first, it’s not linear. That is, an object twice as bright as another doesn’t have twice the magnitude value. Instead, the system is logarithmic, with a base of 2.512. Blame Hipparchus for that: he figured the brightest stars were 100 times brighter than the dimmest stars, and used a five step system [Update: My mistake, apparently he didn’t know about the factor of 100, that came later.]. The fifth root of 100 = 2.512 (or, if you prefer, 2.5125 = 2.512 x 2.512 x 2.512 x 2.512 x 2.512 = 100), so there you go. I’ll give examples in a sec…

Secondly, the other weird thing about the magnitude system is that it’s backwards. A brighter star will have a lower number. It’s like an award; getting first place is better than third. So a bright star might be first magnitude, and a dimmer one third magnitude.

To figure out how much brighter one star actually is than another, subtract the brighter star’s magnitude from the dimmer one’s, and then take 2.512 to that power. As an example, the star Achernar has a magnitude of roughly 0.5. Hamal, the brightest star in the constellation of Aries, has a magnitude of 2.0. Therefore, Achernar is 2.512(2.0 – 0.5) = 2.5121.5 = 4 times brighter than Hamal. So you can say it’s four times brighter, or 1.5 magnitudes brighter. Same thing.

It’s weird, but actually pretty handy for astronomers. And it doesn’t stop at 0. A really bright object can have a negative magnitude, and the math still works. For example, Sirius, the brightest star in the night sky, has a magnitude of about -1.5 (making it 6 times as bright as Achernar – check my math if you want). Which brings us to the topic at hand…

The Moon is pretty bright, and when it’s full has a magnitude of about -12.7. That’s bright enough to read by! But the Sun is way, way brighter. It’s magnitude is a whopping -26.7. How much brighter is that?

Well, it’s 2.5(-12.7 – (-26.7)) = 2.514 = 400,000.

In other words, the Sun is 400,000 times brighter than the full Moon!

This would explain why you can look at the Moon easily enough with just your eye, but trying that with the Sun is not – wait for it, wait for it – a bright idea.

Image credit: NASA/SDO


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Announcing BAFacts: a daily dose of sciencey fun

CATEGORIZED UNDER: Astronomy, BAFacts, Cool stuff
MORE ABOUT: brightness, magnitudes, Moon, Sun
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