The scale of things

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Distant Stars Look Very Much Like Our Local Neighbors | Phil Plait
Bad Astronomy | Thursday, April 4, 2013, at 8:00 AM


^ Image credit: X-ray: NASA/CXC/Univ.Potsdam/L.Oskinova et al; Optical: NASA/STScI; Infrared: NASA/JPL-Caltech

A gorgeous image of stars being born in a nearby galaxy has revealed that in some ways, they may not be very different from stars in our own local neighborhood.

The picture above is a combination of images from several orbiting observatories sensitive to different kinds of light: X-rays from the Chandra Observatory (colored purple in the picture), visible light using Hubble (colored blue, green, and red), and infrared light from Spitzer (colored red). It shows the magnificent nebula NGC 602, a vast star-forming region in the Small Magellanic Cloud (or SMC), a satellite galaxy to our Milky Way.

The gas cloud itself is a bubble; the bright stars in the center are massive, and their fierce light has eaten away at the material around them, carving out the interior. Denser parts of the gas take longer to erode, so they leave wakes behind them, like sandbars; those are the fingers of material around the edge that point toward the center.

Not all the stars being born are monsters, though. Most are far less massive, and much cooler, like the Sun is. However, there is a big difference: Star in the SMC tend to have lower amounts of heavy elements in them. Astronomers call these “metals”: anything in the periodic table above hydrogen and helium. Compared to the Sun, and most stars in the Milky Way, on average stars in the SMC have less oxygen, carbon, iron, and so on.

The question is, does that make any difference? Do these stars behave differently, or are their other characteristics similar to stars like ours?

That’s why these observations were made. The astronomers found that the young, low-mass stars being born in one part of NGC 602 are bright in X-rays. It’s known that low-metal stars don’t generate much wind—that is, they have no or a far weaker solar wind of subatomic particles blowing from their surfaces. That’s how X-rays in gas clouds like these are usually generated, but that’s not the case here.

What must be causing the X-rays is activity on the star’s themselves. That means they have magnetic activity, much like our Sun’s sunspots, flares, and other solar eruptions. Given that this activity is similar to stars in our own galaxy of comparable age and mass (and also looking at results from the observations in visible and IR light), that implies these stars may be similar in other ways as well. The astronomers behind this study speculate that may include the way the stars form, with disks of swirling material around them.

Our neighbors, ourselves!

Well, kinda. We have to be careful not to extrapolate this too far. The amount of metals in a star does have some consequences. High mass stars can have even greater mass if they are metal poor, because some of those elements are really good at absorbing light. A star with those elements in it gets hotter due to the trapped energy, expands bigger, and at a certain mass will get so energetic that it will tear itself apart (this is called the Eddington Limit, and we do see stars near it that are very eruptive and unhappy). All things being equal, a metal-poor star can get more massive than its metal-rich counterpart.

One interesting aspect of this that surprised me a few years ago was that stars in our own galaxy that are metal poor can still have planets! I’d have thought planet formation would be easier if there were more heavy elements to form silicates and such, like in our own solar system. It may be harder to form planets if a star is deficient in those elements, but it’s not impossible. We do know that metal-rich stars are more likely to form big planets, but it’s not known what happens with planets down to the size of Earth. That’s something I’d be very curious to know about.

Anyway, one of the best ways to learn about ourselves is the compare-and-contrast method, and NGC 602 provides us with a great example of both. Those stars are over 150,000 light years away—1.5 million trillion kilometers—I’ll note. And even born under very different circumstances, distant, foreign stars look very much like the ones we have at home. If there’s a life lesson to be taken from this, feel free to find it.

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Solar Spacecraft Gets a Little Loopy
Universe Today, Nancy Atkinson | April 4, 2013

Twice a year, the Solar Dynamics Observatory performs a 360-degree roll about the axis on which it points toward the Sun. This produces some unique views, but the rolls are necessary to help calibrate the instruments, particularly the Helioseismic and Magnetic Imager (HMI) instrument, which is making precise measurements of the solar limb to study the shape of the Sun. The rolls also help the science teams to know how accurately the images are aligned with solar north.

But take this rolling imagery, add some goofy music and hopefully it adds a smile to your day!


^ A normal view for SDO: This is the peak of a M2.5 class solar flare, which propelled plasma into space on June 7, 2011. Credit: NASA/ Solar Dynamics Observatory.

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Hubble Finds a Special Record-Breaking Supernova | Phil Plait
Bad Astronomy | Thursday, April 4, 2013, at 12:30 PM


^ A deep image of the sky taken by Hubble reveals the most distant Type Ia supernova yet seen (lower left, in the box). Click to chandrasekharenate. Image credit: NASA, ESA, A. Riess (STScI and JHU), and D. Jones and S. Rodney (JHU)

I don’t generally post record-breaking discoveries—most distant, brightest, etc.—because most of the time those are incremental steps. Cool, but not critical to our understanding of the Universe.

But this time we have something honestly important: A supernova detected using Hubble Space Telescope is the most distant of its kind yet seen. In the picture above, the box at the lower left shows the position of the exploding star. The sequence below shows it more clearly:


^ Sequence showing the supernova more clearly: the galaxy (left), the galaxy during the supernova event (middle), and the supernova itself after the galaxy light has been subtracted (right). Click to embiggen. Image credit: NASA, ESA, A. Riess (STScI and JHU), and D. Jones and S. Rodney (JHU)

On the left is an image of the galaxy taken before the star blew up. The middle one shows it when the supernova was near its peak, and on the right is the middle image with the left one subtracted away. That removes the galaxy light, leaving behind just that from the supernova.

The galaxy is at a staggering distance of about 10.5 billion light years away; the Universe was practically a toddler when this star blew up.

Exploding stars have been seen farther away than this, but the key part of this is the kind of supernova it is: It’s called a Type Ia, and it happens when a white dwarf star explodes. Most people think of supernovae coming from massive, hot stars. Those get a lot of press, but the Type Ia’s are important too. We think they all explode with roughly the same amount of energy, making them what’s called a "standard candle", a way of easily measuring their distance. If they are all the same brightness, then ones that look dimmer are farther away, just like a candle a hundred meters away is faint compared to one right in front of your eyes.

In fact, Type Ia supernovae were the kind used to figure out that the Universal expansion is accelerating due to dark energy, one of the biggest scientific discoveries in modern history.

An important step in determining that is finding ever-more-distant Type Ia supernovae, because that gives us a better handle on what the Universe is doing in its farthest reaches. This new find, temporarily named SN UDS10Wil, breaks the previous record by hundreds of millions of light years, greatly expanding (pun intended!) our knowledge of the distant Universe.

The supernova was not found by accident, either. In 2010, astronomers started observing the sky as part of the CANDELS+CLASH Supernova Project, observing and re-observing the same patches of the sky. Supernovae take weeks to brighten and fade, so by looking at a big patch of sky over and over again, and by taking very deep images to see faint objects, catching supernovae is inevitable.


^ Two scenarios for a Type Ia supernova: a white dwarf siphoning material off a normal star (top), and two white dwarfs mergin (bottom). Image credit: NASA/CXC/M.Weiss (adapted a bit by Phil Plait)

The CANDELS+CLASH survey is finding something else that’s interesting, too. There are two ways to make a Type Ia supernova. One is for a white dwarf to draw matter off a normal (Sun-like) companion star, gaining mass until it explodes. The other is for two white dwarfs to merge, causing them to blow up. Given how stars are born, age and die, the first scenario can start occurring not long after the Universe itself formed, just a billion years or so. The second one, though, takes far longer, since the two white dwarfs have to spiral in and merge, a process that takes billions of years.

If you look far away in space, you’re looking back in time, so you can take a survey of what’s happening when. The CANDELS+CLASH project has found a drop-off in Type Ia supernova from roughly seven to 10 billion years ago, making it look like the double-dwarf merger is the more common type going off back then. This jibes with other research done looking in the local Universe as well.

I find that interesting indeed. Both mechanisms occur to create Type Ia supernovae, but knowing which is more common, where, and when, tells us more about the way the Universe works. Supernovae like this are a fantastic tool for understanding the cosmos. Bright, easily seen over billions of light years, occurring everywhere, and happening in such a way that allows us to quantify them (apply numbers and math to them): all of this makes them the Swiss Army Knife of astronomy.

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Cheers to another way of seeing things—again .

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Upside-down Moonset | Phil Plait
Bad Astronomy | Monday, April 8, 2013, at 8:00 AM

Randy Halverson is a fantastic astrophotographer, which is why I follow him on Google+; I’m always amazed at the images he captures of the night sky.

He posted one recently that really threw me for a moment, though. Certainly the beauty is captivating, but on closer examination it revealed a prejudice of mine.

See for yourself how gorgeous it is:


^ The Moon setting into dappled waters...but what’s wrong with this picture? Click to embiggen. Image credit: Randy Halverson, used by permission

That is the Moon setting over the ocean, the water surface reflecting it like a pylon made of light itself. The cloud just below the Moon is dappled in light and shadow, and the symmetry of its curve is lovely to behold.

I was gaping at this, and then the stars caught my attention. I know my way around the sky, and immediately recognized Orion directly above the Moon. But the orientation of the constellation confused me; it was more than just on its side, it was starting to flip over, nearly upside-down to my eye.

My first thought was, “Where the heck did he take this shot?” And then I saw the picture caption—“Orion and Moon set on the Indian Ocean”—and I laughed. Of course! I was seeing this picture through my northern-hemisphere bias.

He took this shot on Mar. 17, 2013 near Kalbarri in Western Australia, about 27° south of the Equator. From that location on our spherical planet, things look different. I’m used to Orion oriented very differently. From home, when Orion’s high over the southern horizon, the legs are down and he stands upright. He moves left to right as I face south.

But in the southern hemisphere it’s all wonky. He rises to the right as you face north, is standing on his head when he’s high above the northern horizon, and moves right to left.

That’s seriously weird to my borealic brain. I was in Australia a few years ago, and the two things that totally freaked me out were seeing Orion upside down, and the crescent Moon facing the wrong way at sunset. When you’re looking at the same objects you always see, but essentially upside-down compared to what you’re used to, things get all swapped and weird. Left is right, up is down… but the stars still rise in the east and set in the west. The whole Earth, at least, spins the same way.

Still, living on a round ball is distressing when you’re reminded of it in this way. But it’s a great reminder that the Universe is the way it is, and it’s our own ossified sensibilities that give us grief about it. Once you shake that off, the depth of its beauty becomes even more profound.

I’ve written about Randy’s work many times; you can see more in these posts:


⋅ The Hunter and the Meteor
⋅ Another Jaw-Dropping Time-Lapse Video: Tempest
⋅ A Meteor’s Lingering Tale
⋅ Milky Way, Meteor, Meterology
⋅ The Milky Way and the Mashed Potatoes Mountain (a personal favorite)

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Some of the Best Images of Earth from Space
Universe Today, Nancy Atkinson | April 5, 2013

This video compilation from the Goddard Space Flight Center takes a look back at the best views of our planet from space in the last year, including true color satellite images, Earth science data visualizations, time lapses from the International Space Station, and computer models.


^ A ‘Blue Marble’ image of the Earth taken from the VIIRS instrument aboard NASA’s most recently launched Earth-observing satellite – Suomi NPP. This composite image uses a number of swaths of the Earth’s surface taken on January 4, 2012. Credit: NASA/NOAA/GSFC/Suomi NPP/VIIRS/Norman Kuring.

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This is really cool. It's an animation with the Earth and the distance to Mars. Check it, yo.

http://www.distancetomars.com/

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82, here’s another trip! It’s the aurora borealis, a virtual reality experience, seen in Sweden on March 17, 2013. See yah when you get back.

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Immerse Yourself in a Virtual Aurora | Phil Plait
Bad Astronomy | Tuesday, April 9, 2013, at 12:54 PM


^ Aurora over Östersund, Sweden on Mar. 17, 2013. Click to enborealate. Image credit: Göran Strand

This is pretty cool: Photographer Göran Strand (who took one of the amazing comet and galaxy pictures I posted recently) went to a town in Sweden called Östersund on Mar. 17, 2013. He set up his camera and took 30 Gb of images of the dancing aurora borealis that night. Not satisfied with just having gorgeous pictures of the experience, he put them together into a virtual video tour that allows you to pan and scan through the sky as the aurorae unfolds in front of you:

[Allegro omitted posting the movie. See it here ]

I had to shrink it to fit the blog here and it doesn’t quite work the way it should, so I strongly urge you to go to his site and play with it yourself in full resolution. I’ve seen a lot of these virtual tours before, but not one that has video that plays as you move around. That’s pretty nifty. It’s fun to watch Orion and Jupiter set as the green glow from high-speed subatomic particles from the Sun slam into our atmosphere 100 kilometers up.

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NGC 3132: The Southern Ring Nebula
Image Credit: Hubble Legacy Archive, ESA, NASA; Processing - Donald Waid

Explanation: It’s the dim star, not the bright one, near the center of NGC 3132 that created this odd but beautiful planetary nebula. Nicknamed the Eight-Burst Nebula and the Southern Ring Nebula, the glowing gas originated in the outer layers of a star like our Sun. In this reprocessed color picture, the hot purplish pool of light seen surrounding this binary system is energized by the hot surface of the faint star. Although photographed to explore unusual symmetries, it’s the asymmetries that help make this planetary nebula so intriguing. Neither the unusual shape of the surrounding cooler shell nor the structure and placements of the cool filamentary dust lanes running across NGC 3132 are well understood.

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New Stars: Blazing and Blue
Universe Today, Nancy Atkinson | March 27, 2013


^ This image from the Wide Field Imager on the MPG/ESO 2.2-metre telescope at ESO’s La Silla Observatory in Chile, shows the bright open star cluster NGC 2547. Between the bright stars, far away in the background of the image, many remote galaxies can be seen, some with clearly spiral shapes. Credit: ESO

A new image from ESO shows a pretty sprinkling of bright blue stars, the star cluster NGC 2547, a group of recently formed stars in the southern constellation of Vela. Even though we recently got a more precise estimate on how old the Universe is from the Planck mission (13.82 billion years), this is a look at some fairly young — new and blue — stars.

But how young are these cosmic youngsters really? ESO scientists say that although the exact ages of these stars remain uncertain, estimates range from 20 to 35 million years old. That doesn’t sound all that young, after all. However, our Sun is 4.6 billion years old and has not yet reached middle age. That means that if you imagine that the Sun as a 40 year-old person, the bright stars in the picture are three-month-old babies.

Most stars do not form in isolation, but in rich clusters with sizes ranging from several tens to several thousands of stars. Clusters are key objects for astronomers studying how stars evolve through their lives. The members of a cluster were all born from the same material at about the same time, making it easier to determine the effects of other stellar properties.

While NGC 2547 contains many hot stars that glow bright blue, also visible are one or two yellow or red stars which have already evolved to become red giants. Open star clusters like this usually only have comparatively short lives, of the order of several hundred million years, before they disintegrate as their component stars drift apart.


^ This picture was created from images forming part of the Digitized Sky Survey 2. It shows the rich region of sky around the young open star cluster NGC 2547 in the southern constellation of Vela (The Sail). Credit: ESO/Digitized Sky Survey 2. Acknowledgement: Davide De Martin

The star cluster NGC 2547 lies in the southern constellation of Vela (The Sail), about 1500 light-years from Earth, and is bright enough to be easily seen using binoculars. It was discovered in 1751 by the French astronomer Nicolas-Louis de Lacaille during an astronomical expedition to the Cape of Good Hope in South Africa, using a tiny telescope of less than two centimeters aperture.

Between the bright stars in this picture you can see plenty of other objects, especially when zooming in. Many are fainter or more distant stars in the Milky Way, but some, appearing as fuzzy extended objects, are galaxies, located millions of light-years beyond the stars in the field of view.

Source: ESO

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The Secret History of the Splinter Galaxy | Phil Plait
Bad Astronomy | Tuesday, April 9, 2013, at 8:00 AM

Spiral galaxies have an interesting property: They’re flat. When you see them face-on, you can take in their full spirally goodness. But if they happen to be oriented edge-on to us, they can look nearly as thin as a knife’s edge.

NGC 5907 is a great example of that. Located roughly 40 million light years away—fairly close by as galaxies go—it is very nearly sideways with respect to us, and so thin it was given the nickname “The Splinter Galaxy”:


^ The edge-on spiral NGC 5907. But it's hiding its dreadful past... (click to galactinate) Image credit: Patrick Hochleitner and Dieter Beer

That shot was not taken using some gigantic professional observatory. It was taken using—and this really is incredible—a 120mm (5”) telescope! Astrophotographers Patrick Hochleitner and Dieter Beer took it, using a DSLR camera and a special astrophotography camera. They combined a total of 129 separate images to make a single shot that was the equivalent of a 23 hour 25 minute exposure: nearly an entire day.

The image is revealing. You can see how flat NGC 5907 is, and also see the dark filigree across its middle that’s composed of countless clouds of interstellar dust. What struck me right away is something that’s missing: a bulge in the middle. Most spiral galaxies have a spherical hub of stars surrounding their cores, but NGC 5907’s is weak at best.

But this galaxy is even more amazing than you can see in this image. Beer and Hochleitner took all their images, added them together without color information, and then inverted them so that black is white and vice-versa. This allows fainter detail to be seen, and allows this otherwise lovely but mundane galaxy to suddenly reveal a deep, faint secret:


^ Deep imaging (and made negative), the secret of NGC 5907 is revealed. Click to embiggen. Image credit: Patrick Hochleitner and Dieter Beer

Look at that! It’s surrounded by a faint ribbon of…something. You can’t tell in this image, but deep imaging by larger telescopes has revealed that this loop-de-loop is actually made of stars. It’s technically called a tidal stream, a river of stars orbiting the central galaxy in a gigantic flow hundreds of thousands of light years long.

What the heck can do that?

< The tidal stream is even more obvious in this deeper image taken with the BlackBird Remote Observatory 0.5 meter telescope. A jaw-dropping color version of this was featured on APOD in 2008. Image credit: David Martınez-Delgado et al.

There are two (related) ways to generate this ribbon of stars. One is cannibalism: a small companion galaxy, a satellite to the big galaxy, is on an orbit that takes it through the disk of the galaxy and out the other side. Repeated passes through the big galaxy strip the gas and dust from the smaller one, and the gravity of the big galaxy rips the stars out over time as well, disrupting it. Over hundreds of millions of years you get this long ribbon wrapped around the galaxy.

Another way is to create a tidal stream is with a major merger: when two galaxies of approximately equal size interact gravitationally and eventually merge together into one bigger galaxy. That can eject a stream of stars that looks quite a bit like this as well.

The difference between the two scenarios—small galaxy versus big one getting eaten—may seem trivial, but in fact predicts different eventual outcomes. For example, a small galaxy should leave a small, dense core of stars that is held together more tightly by gravity, and is therefore harder to rip asunder. No such core is seen here. However, big mergers generally create a lot more chaos, but NGC 5907 looks fairly serene (though, to be fair, a tidal stream can last for over a billion of years, plenty of time for the sound and fury of a merger to quiet down).

< Computer model of stars in the Sagittarius Dwarf stream in the Milky Way. Image credit: Steve Majewski et al.

The bottom line is that we’re not exactly sure which of these two scenarios played out here. And we’d like to know, since we think all big galaxies, including our own Milky Way, grew to their current size by either eating smaller galaxies or merging with big ones. We have plenty of evidence for that, including a tidal stream very similar to NGC 5907’s seen in our own galaxy, caused by the Milky Way eating what's called the Sagittarius dwarf galaxy!

And there’s more: we’re on a collision course ourselves. In a few billion years, the Andromeda galaxy will collide with and merge into the Milky Way. Both galaxies are pretty beefy, among the largest galaxies in the Universe, so when we’re done we’ll be even heftier.

Galaxies are a perfect example of the irony of what looks like stately, serene beauty actually being due to incredibly violent acts (in this case wanton cannibalism and cosmic train wrecks). And you don’t need a huge telescope to peer into the vasty depths of the Universe to see it either; it happens right in our neighborhood, and even in our own house.

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Looking Into The Green Eye Of Planetary Nebula IC 1295
Universe Today, Tammy Plotner | April 10, 2013


^ This intriguing picture from ESO’s Very Large Telescope shows the glowing green planetary nebula IC 1295 surrounding a dim and dying star. It is located about 3300 light-years away in the constellation of Scutum (The Shield). This is the most detailed picture of this object ever taken. Credit: ESO

Located on Cerro Paranal in the Atacama Desert of northern Chile, the ESO’s Very Large Telescope was busy using the FORS instrument (FOcal Reducer Spectrograph) to achieve one of the most detailed observations ever taken off a lonely, green planetary nebula – IC 1295. Exposures taken through three different filters which enhanced blue light, visible green light, and red light were melded together to make this 3300 light year distant object come alive.

Located in the constellation of Scutum, this jewel in the “Shield” is a miniscule star that’s at the end of its life. Much like our Sun will eventually become, this white dwarf star is softly shedding its outer layers, like an unfolding flower in space. It will continue this process for a few tens of thousands of years, before it ends, but until then IC 1295 will remain something of an enigma.

“The range of shapes observed up to today has been reproduced by many theoretical works using arguments such as density enhancements, magnetic fields, and binary central systems. Despite this, no complete agreement between models and properties of a given morphological group has been achieved. One of the main reasons for this is selection criteria and completeness of studied samples.” say researchers at Georgia State University. “The samples are usually limited by available images in few bands such as Ha, [NII] and [OIII]. Of course they are also limited by distance, since the further away the object is, the harder it is to resolve its structure. Even with the modern telescopes, obtaining a truly complete sample is far from being achieved.”

Why is this common deep space object like IC 1295 such a mystery? Blame it on its structure. It is comprised of multiple shells.- gaseous layers which once were the star’s atmosphere. As the star aged, its core became unstable and it erupted in unexpected releases of energy – like expansive blisters breaking open. These waves of gas are then illuminated by the ancient star’s ultraviolet radiation, causing it to glow. Each chemical acts as a pigment, resulting in different colors. In the case of IC 1295, the verdant shades are the product of ionised oxygen.


^ This video sequence starts with a broad panorama of the Milky Way and closes in on the small constellation of Scutum (The Shield), home to many star clusters. The final detailed view shows the strange green planetary nebula IC 1295 in a new image from ESO’s Very Large Telescope. This faint object lies close to the brighter globular star cluster NGC 6712. Credit: ESO/Nick Risinger (skysurvey.org)/Chuck Kimball. Music: movetwo

However, green isn’t the only color you see here. At the heart of this planetary nebula beats a bright, blue-white stellar core. Over the course of billions of years, it will gently cool – becoming a very faint, white dwarf. It’s just all part of the process. Stars similar to the Sun, and up to eight times as large, are all theorized to form planetary nebulae as they extinguish. How long does a planetary nebula last? According to astronomers, it’s a process that could be around 8 to 10 thousand years.

“Athough planetary nebulae (PNe) have been discovered for over 200 years, it was not until 30 years ago that we arrived at a basic understanding of their origin and evolution.” says Sun Kwok of the Institute of Astronomy and Astrophysics. “Even today, with observations covering the entire electromagnetic spectrum from radio to X-ray, there are still many unanswered questions on their structure and morphology.”

Original Story Source: ESO Photo Release.

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* Andrey Avkhimovich is credited for the music track in the video above, but his association with movetwo couldn’t be verified, as of today.

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Finally, some solar action

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Weekend Aurora Alert: The Sun Lets Loose an Earth-Directed CME
Universe Today, Nancy Atkinson | April 11, 2013


^ NASA’s Solar Dynamics Observatory captured this image of an M6.5 class flare at 3:16 EDT on April 11, 2013. This image shows a combination of light in wavelengths of 131 and 171 Angstroms. Credit: NASA/SDO.

The Solar Dynamics Observatory captured this view as the Sun let loose with its biggest solar flare of the year so far. It’s not a real big one — a mid-level flare classified as an M6.5 – but an associated coronal mass ejection is heading towards Earth and could spur some nice auroae by this weekend. Spaceweather.com predicts the expanding cloud (see animation below) will probably deliver a glancing blow to Earth’s magnetic field late on April 12th or more likely April 13th. The NOAA Space Prediction Center forecasts this event to cause moderate (G2) Geomagnetic Storm activity, and predicts geomagnetic activity to start in the mid to latter part (UTC) of April 13. They add that the source region is still potent and well-positioned for more geoeffective activity in the next few days.


^ They came from outer space–and you can have one! Genuine meteorites are now on sale in the Space Weather Store. Own your own meteorite | SUNSET CRESCENT MOON: When the sun goes down tonight, look west into the fading twilight for an exquisitely-slender (3%) crescent Moon. Astronomically speaking, it’s nothing special–just a nice way to end the day. | STRONG SOLAR FLARE: The magnetic field of sunspot AR1719 erupted on April 11th at 0716 UT, producing an M6-class solar flare. NASA’s Solar Dynamics Observatory recorded the explosion’s extreme ultraviolet flash: Coronagraph images from the Solar and Heliospheric Observatory show a CME emerging from the blast site of the M6.5 solar flare. Credit: NASA

See this NASA page for info on solar flares, CMEs, and more.


^ NASA’s Solar Dynamics Observatory captured this image of an M6.5 class flare at 3:16 am EDT on April 11, 2013. This image shows a combination of light in wavelengths of 131 and 171 Angstroms. Credit: NASA/SDO.

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The Streets Are Paved with… Vortices? | Phil Plait
Bad Astronomy | Friday, April 12, 2013, at 8:00 AM

“I told you!” shouted Ford, leaping to his feet. “Eddies in the space-time continuum!”

“And this is his sofa, is it?” asked Arthur.

—Douglas Adams, “Hitchiker’s Guide to the Galaxy”

Life is complex. So is fluid dynamics.

Actually, you’d think fluid motion isn’t all that complicated. Liquids flow downhill, moving around stationary objects, and generally going from point A to point B.

But in reality fluid motion is incredibly complex, one of the most ridiculously overwrought fields in all of mathematical physics. Turbulence, oscillations, viscosity, boundary layers: They all add up to make fluid motion fiercely hard to understand and calculate.

But in complexity there can be beauty. Commander Chris Hadfield, on board the International Space Station, took this picture on Mar. 26, 2013, showing exactly that fluidic gorgeosity:


^ Clouds swirl downwind of Isla Socorro, tracing complex von Kármán vortices. Click to enturbulenate. Image credit: NASA

That is Isla Socorro, a volcanic island located a few hundred kilometers off the west coast of Mexico and the southern tip of Baja California. As wind blows past the island, lovely atmospheric swirls form on the downwind side. These swirls are called von Kármán vortices, and the long chain of them gets the name von Kármán vortex streets. The cloud pattern makes them visible to the eye.

What causes them? In detail the mathematics is quite fierce, but how it works isn’t all that hard to understand in principle.

Imagine you have a cylinder (a pencil, or a bucket, or a concrete pylon) that you place in flowing water. It’s an obstacle, and the water will flow around it.

However, near the cylinder’s surface the water slows, piling up a bit. The water farther from the cylinder is moving faster. This causes eddies (vortices) to form, curls in the water. This kind of motion is a bit unstable, and can cause a slight force, pushing the water perpendicular to the direction of flow. But the water all around the flow pushes back, causing a sort of oscillation, like a pendulum swinging. The result is a series of vortices forming and flowing downstream, one on each side of the obstruction, alternating in pattern.

An animation, in this case, is worth way more than a thousand words:

[Allegro omitted the movie. See it here ]

See how the fluid wiggles like a tadpole tail downstream? Eventually those vortices dissipate, losing coherence due to turbulence and drag. This process from start to finish is called “vortex shedding”, which just sounds intrinsically cool.

Mind you, a fluid is anything that can flow, not just liquids. Air is a fluid, and when you have a steady wind blowing past, say, a Pacific island, you can get vortex shedding leading to von Kármán vortex streets. Here’s another fine example taken from space, showing vortices off Isla Alejandro Selkirk and Isla Robinson Crusoe off the coast of Chile.


^ A pair or parallel von Kármán vortices shed off of Pacific islands. Click to embiggen. Image credit: NASA/Jeff Schmaltz/LANCE MODIS Rapid Response

This phenomenon has everyday effects, too. Along the highway to and from my local airport there are metal warning posts with reflectors on them along the side of the highway. The metal posts are thin sheet metal, and the wind is commonly howling along the plains there. This causes vortex shedding off the posts. This applies a force to the post, pushing it to the side. But the post is flexible, so it bounces back. If the timing of the vortex shedding is in phase with the natural time it takes the post to flex and relax you get a resonance; the vibration is reinforced, and the post will continue to oscillate.

On some days as I drive home the airport I see those posts really whipping back and forth, sometimes vibrating violently enough to rapidly move them many centimeters side-to-side (like the scene in “Close Encounters of the Third Kind” where the rotor wash from the government helicopter makes the sign posts vibrate). This can happen with car antennae and other anchored objects where there is wind blowing past them…like power lines, buildings, and tall towers. Engineers have to deal with this, because if the force gets too strong, the object can be damaged by the vibrations.

And I can’t help but mention this in parting: Isla Socorro, in that first picture at the top of this article, is part of a mostly underwater ridge that runs to the west of Mexico. The name of that particular geological feature?

Mathematicians Ridge!

Math and science are everywhere, even deep beneath—and above—the ocean waves.

Tip o’ the windbreaker to Tino Eberl for identifying the island, to Evan Hadfield (Chris’s son) for some help with the photo, and to Joe Seatter for reminding me I wanted to write about this effect!

[Allegro]

May I ask, ‘Math and science and music are everywhere?’

I found these mathematicians’ quotes here.

_________________

“We can no more come to understand mathematics by examining its final product than we can understand the experience of music through simply looking at a score or an analysis of one; there is an experience that lies underneath and behind the systematic organization of the material.”
~ Edward Rothstein, music critic/composer (p. 38 of his 2006 book Emblems of Mind: The Inner Life of Music and Mathematics)

“You cannot evade quantity. You may fly to poetry and music, and quantity and number will face you in your rhythms and your octaves.”
~ Alfred North Whitehead (1861-1947), English mathematician

“Musical form is close to mathematics—not perhaps to mathematics itself, but certainly to something like mathematical thinking and relationship.”
~ Igor Stravinsky, 20th century Russian composer

[smoking since 1879]

Hi Alegro,

Thanks for your amazing posts

I saw these and thought of you...


A Computational Fluid Dynamics (CFD) simulation of a Von Karman vortex street behind a cylinder. Reynolds number Re = 250. Codes used: Gmsh, dolfyn, and VisIt. (wervelstraat, Wirbelstrasse)


This is the result of solving flow-structure interaction task by the viscous vortex domains method (vvd).
vvd method is a lagrangian method for solving 2D Navier-Stokes equations in case of viscous incompressible fluid.

peace