[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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My friend Cara Santa Maria is a scientific researcher and educator. She’s also Senior Science Editor with The Huffington Post, where she does a video show called "Talk Nerdy To Me". She contacted me recently because she wanted to do an episode on solar storms – how they work, and how they can affect us here on Earth. She interviewed me about them, and the episode is online at HuffPo:
[Note: If the video doesn't appear directly above this sentence, refresh your screen.]
The Sun has been a bit feisty lately, spitting out some decent flares and coronal mass ejections. So far, none has been both strong enough and aimed at us to do any damage (there was a fairly powerful CME in July, but it was on the other side of the Sun, directed away from us). And while they can’t hurt us directly due to our protective atmosphere, as I say in the video solar storms can disrupt our power grid and our satellites, creating havoc. The more we study the Sun the better we understand it, and the more likely we’ll be able to protect ourselves should it decide to throw another major hissy fit.
I’ll add that Cara’s good people, and I like her show (she interviewed me for the Venus Transit, too). She’s passionate about science education, and like me she finds real joy and wonder in all fields of science and nature. You can follow her on Twitter.
[Edited to add: the shirt I'm wearing in the video (watch to the very end!) is available at Lardfork's Spreadshirt store.]
The wonderful astrophotographer César Cantú takes amazing pictures of the sky, and his shots of the Sun are truly cool. On Wednesday, August 8, 2012, he took this image of the Sun and a sunspot called Active Region 1524:
The Sun is a 1970s orange shag carpet!
Actually, César used an Hα filter, which blocks almost all the light from the Sun except for a very narrow slice of color where hydrogen emits light, and in fact this is preferentially given off by hydrogen under the sway of magnetic fields on the Sun, so this image accentuates magnetic activity. You can see lots of structure like the sunspots and the plasma flowing along magnetic fields – especially along the Sun’s edge, where they’re called prominences.
The Sun looks amazingly different depending on how you look at it. Far from being a featureless white disk, it actually has detail all the way down to the resolution of our best telescopes. The surface of the Sun is fiendishly complex, and the amount to understand is equally daunting. And, as usual with astronomy, with this complexity comes astonishing beauty.
Image credit: César Cantú, used with permission.
On August 1, astrophotographer Bart Declercq went outside to get some shots of our nearest star. He used a 30 centimeter (12") telescope, two filters, and a pretty nice camera, he took several thousand frames of video of the Sun. Using a technique that allows combining the frames to get the highest resolution image – and then using a further technique called deconvolution to sharpen it a bit more – he was able to create this tremendous picture of a sunspot:
[Click to ensolarnate and get the 3000 x 2000 pixel hi-res version; this picture displayed here is only one part of a much larger image!]
Wow! This sunspot is officially called Active Region 1532, and is still visible on the Sun’s surface. The detail you can see here is amazing; the spot’s umbra (the dark region) is obvious enough, but the amount of small-scale features in the penumbra (the lighter outer region) is incredible. Surrounding the spot are granules; the pebbly-looking structures which are actually huge convection cells on the Sun. Hot plasma (gas where its atoms are stripped of one or more electrons) from inside the Sun rises to the surface, cools, and sinks back down. Granules are the tops of these convection cells.
Sunspots are where the Sun’s fiercely complex magnetic field breaks through the surface, looping outward and back down, beginning and ending in the spots. These loops of magnetism tend to suppress convection. The plasma cools but cannot sink down. The brightness with which the plasma glows depends on its temperature, so the cooler plasma in the spot appears dark against the hotter material around it.
The scale of all this is hard to grasp. A quick measurement on the image indicates that the spot is about 20,000 kilometers (12,000 miles) across… in other words, just the spot’s umbra alone is roughly the same size as the Earth!
It’s easy to forget just how mighty the Sun is, but pictures like this really slam it home. The Sun is a star, 1.4 million kilometers across, 330,000 times as massive as the Earth, and a complex, amazing, and wondrous place.
Image credit: Bart Declercq
Just a reminder: the Sun is awesome:
[You MUST click that to get the fully enGdwarfenated 2200 x 2200 pixel picture. It's stunning.]
I’ve talked about Alan Friedman’s amazing Sun portraits so many times I need not elaborate here; just read the Related Posts below. But man! What a star.
Image credit: Alan Friedman.
Looking back on it, I should’ve realized friend of the BABlog and masterful solar photographer Alan Friedman would send me a jaw-dropping picture of the vastly ginormous sunspot cluster AR1520 that I wrote about yesterday.
And of course he did:
Holy solar retinopathy! [Click to embiggen.]
This huge cluster of spots is just now coming around the edge of the Sun’s disk, having formed on the far side where we can’t see things directly from Earth. It’s showing up just as the also huge Active Region 1515 is moving around to the Sun’s other side. Since our star takes about 25 days to spin once, these new spots will be visible for another week at least. They may grow in size, and they’ll certainly change shape, and it’s a decent bet they’ll blow off an interesting magnetic storm or two. AR 1515 sure did, and this cluster may be even bigger. Size isn’t a guarantee of activity, but it’s correlated.
Image credit: Alan Friedman, used by permission.
[UPDATE: Oh for criminy's sake. I made a mistake here. The big sunspot group I describe below is actually AR 1515 which has been on the Sun for a while now. AR 1520 can be seen in the full Sun pic on the very left, on the Sun's edge! It's still huge, roughly the same size as 1515, and it's just now coming into view. Most everything I said below about the sunspot group is correct, except it's about 1515 and not 1520! I've corrected the details below. Sorry about the confusion, and thanks to zAmboni in the comments for pointing this out!]
The Sun is at it again:
Active Region 1520, a vast sunspot group, is currently blemishing the Sun’s face. Active Region 1515, a vast sunspot group, has been blemishing the Sun’s face for days, and is being joined by Active region 1520, another huge group just now coming around the Sun’s edge.
This Solar Dynamics Observatory image shows AR 1515 on the lower right and AR 1520 on the lower left. A quick measurement shows both are about 200,000 kilometers (120,000 miles) across – fifteen times the width of the Earth! If one end were placed on Earth, it would stretch halfway to the Moon.
Having a hard time grasping that? Let me help: here’s the sunspot cluster AR 1515 zoomed in a bit, with the Earth overlaid pretty much to the same scale:
Holy. Crap. The Sun never does anything small, does it?
In fact, this cluster is so flippin’ big you can see it without a telescope! I just went outside and looked using certified eclipse glasses I got for the last solar eclipse. The sunspot cluster was obvious to my naked eye! It’s even easier to see than Venus was during the transit in June.
[WARNING: Listen folks, never look at the Sun without adequate protection. While looking at the Sun won't necessarily cause permanent or total blindness, it's not a good idea, and you should NEVER look at the Sun through binoculars or a telescope unless they are outfitted properly. If you don't know what you're doing with astronomical equipment, the best bet is don't do it. If you don't have eclipse glasses or the right kind of filter, I suggest using binoculars to project the Sun's image on a piece of paper (noting this can still damage your binoculars). I have notes on viewing something like this safely on my Transit of Venus page. Read that first before trying anything!]
This cluster should be visible over the next ten days or so as the Sun rotates. While AR 1515 will rotate around to the Sun’s far side in a few days, AR 1520 is coming into view now and will be visible for about two weeks. The magnetic field associated with them will be huge as well, so it wouldn’t surprise me if we get some activity in the form of flares from these guys, too. Since I’ll be at Comic Con for several days starting Thursday, I suggest checking SpaceWeather and Universe Today for updates on any solar activity this cluster might unleash. Maybe we’ll get more aurorae!
It’s been a while since we’ve had a big flare from the Sun. Active region 1515 was looking like it might do the trick — over the past week this group of sunspots has been hissing and spitting, but the flares have been relative small. Astronomers rate flares by their X-ray energy: A, B, C, M, and X, where X is the highest. Some of the flares from AR1515 were C class and some M class – moderately strong.
Between July 5 and 6 it put out about a dozen of those smaller flares:
Then, late on July 6, it blew out the first X-class flare of the summer:
This sequence of images from the Solar Dynamics Observatory shows the flare over a bunch of different ultraviolet wavelengths, where flares are most obvious. You can see that it was pretty bright! Here’s a video showing it erupting:
The video again shows the Sun at different wavelengths of UV light. The flickering is due to the software automatically setting the brightness level; when the flare gets bright it sets the image to be dimmer, so the Sun appears to flicker. The long dashed-line spikes are not real; those are due to the way the detector in SDO sees X-ray light, like the spikes you see in bright stars in some telescopic images.
Flares occur when the Sun’s magnetic field gets tangled up. In a sense, the field short-circuits, releasing vasts of built-up energy, and we call that a flare. A big one can release 10% of the entire energy of the Sun! This can emit high-energy light and a huge blast of subatomic particles which cross the inner solar system and slam into us. While we’re safe on the ground, this can damage satellites, cause blackouts, and of course trigger gorgeous aurorae — the northern and southern lights.
This flare was still pretty small even for an X class; we had bigger ones over the past year (see the Related Posts for links to some of those). This particular group of sunspots is heading over the edge of the Sun now as our star rotates, so we probably won’t be seeing it again; sunspots tend not to last that long. But there will be more. We’re still approaching the peak of the sunspot cycle, probably late next year, so expect plenty more — and more powerful — flares to come.
Tip o’ the welder’s goggles to Camilla Corona SDO on Google+. Image credit: NASA/SDO
- HD Footage of last night’s flare
- The Sun lets out a brief flare
- The Sun aims a storm right at Earth: expect aurorae tonight!
- GORGEOUS solar eruption!
- The birth of a sunspot cluster
Today – July 5, 2012 – at about 04:00 UTC (a few hours ago as I write this) the Earth reached aphelion, the point in its elliptical orbit when it’s farthest from the Sun.
According to the US Naval Observatory, we were 1.016675058 Astronomical Units from the Sun at that time. An AU is the average distance from the Earth to the Sun, and is defined as 149,597,870.7 kilometers (92,955,807.2 miles).
That means that at aphelion the center of the Earth was 152,092,424 km (94,505,851 miles) from the center of the Sun.
Over the next six months we’ll slowly approach the Sun again until we reach perihelion – the closest point in the Earth’s orbit to the Sun – on January 2, 2013, at about 05:00 UTC.
When we’re farther from the Sun it appears a little bit smaller in the sky, but you’d never notice. For one thing, staring at the Sun is a bad idea! For another, the change is so slow day by day that it’s impossible to notice anyway. For a third thing, the total change over the course of six months isn’t very big either. Astronomer (and friend of the blog) Anthony Ayiomamitis took two pictures that show this:
These are from aphelion and perihelion in 2005, but the scale is always about the same every year. As you can see, the change in the Sun’s size isn’t terribly big.
So even though you may not notice it, it’s still neat to think that after the past 183 days or so we’ve been steadily moving farther from the Sun, and now we’re on our way back in. And even neater… the Earth has done this over four and half billion times before. So it has some experience here.
Stars are one of the fundamental building blocks of the Universe. Huge, hot, and powerful, they emit energy that can be detected across vast reaches of space. For as long as they live (so to speak) they glow with a fierce luminosity.
And even when they die they can announce their presence in weird and wonderful ways.
Meet U Camelopardalis, just such a dying star about 1400 light years from Earth:
[Click to doomsdaymachinenate.]
U Cam is a red giant, a star that was once like the Sun but has gone much further along its evolutionary path. Our Sun is fusing hydrogen steadily into helium in its core, providing warmth and light for us. U Cam ran out of hydrogen in its core long ago, and began fusing helium into carbon. Then it even ran out of helium as a fuel! The core is now essentially an extremely hot ball of carbon, squeezed by pressure to within an inch of its life. There’s still helium outside the core, and gets so much heat from the core’s radiation that it’s fusing in a thin shell. Think of it like a very hot skin around an orange.
This helium fusion is ridiculously dependent on temperature. Increase the heat just a wee bit and the fusion rate increases madly, generating huge amounts of energy, which get dumped into the outer layers of the star on very short timescales. And by "short" I mean like years. Not millions of years. Just years.
When this happens the star swells immensely and ejects its topmost layer, like a solar wind on cosmic steroids. This event doesn’t last long, maybe a century or so, then it subsides. But that shell of ejected gas expands out from the star, eventually dissipating over millennia.
And that’s where U Cam is right now. Not long ago its core underwent one of these paroxysms, and the star blasted out the shell of material you can see in this Hubble Space Telescope image. Measuring the expansion rate, it looks like this shell was ejected about 700 years ago, and the event only took 50 years to unfold! Because these events don’t last long compared to the life of the star, it’s rare to see them. U Cam is one of the best of only a handful of such stars known.
In the image you can see how thin the shell is, indicating the event happened rather quickly (if it took a long time the shell would be thicker). I’ll note that the total mass of the shell is only about a tenth the mass of the Earth! It’s spread out over so much volume of space that it’s barely more dense than the vacuum surrounding it.