Surf's up

One thing that never fails to leave me feeling awestruck is when I consider that astronomers figured out the shape and size of the Milky Way Galaxy while residing inside it.

I mean, think about it.  Imagine you're a tiny being (with a telescope) sitting on a raindrop near one edge of a huge cloud, and your task is to try to measure the distances and positions of enough other raindrops to make a good guess about the size and shape of the entire cloud.  That's what the astronomers have accomplished -- enough to state with reasonable confidence that we're in one of the arms of a barred spiral galaxy.

If ever there was an image you need to study in detail, this is it.  Take a look at the original, close up.  The Solar System is in the Orion Arm, directly down from the center of the galaxy.  The thing that blew me away is the circle marked "Naked Eye Limit" -- literally every star you have ever seen without the use of a telescope is in that little circle.  [Image licensed under the Creative Commons Pablo Carlos Budassi, Milky Way map, CC BY-SA 4.0]

What's even more astonishing is that the stars making up the Milky Way (and every other galaxy) are moving.  Not fast enough, on that kind of a size scale, that the map will be inaccurate any time soon; but fast enough to be measurable from here on Earth.  In fact, it was anomalies in galactic rotation curves -- the plot of the orbital speed of stars around the center of the galaxy, as a function of their distance from the center -- that clued in the brilliant astrophysicist Vera Rubin that there was (far) more matter in galaxies than could be seen, leading to the bizarre discovery that there is five times more dark matter (matter that only interacts via gravity) than there is the ordinary matter that makes up you, me, the Earth, the Sun, and the stars.

All of this makes the new study out of the European Space Agency even more incredible.  New data from the Gaia Telescope has found that the entire Milky Way is rippling as it rotates, a little like the fluttering of a Spanish dancer's frilly skirt.  The period of this wave-like motion is on the order of ten thousand light years, and it appears to affect the entire galaxy.

The astrophysicists are still trying to figure out what's causing it.

"What makes this even more compelling is our ability, thanks to Gaia, to also measure the motions of stars within the galactic disc," said lead author Eloisa Poggio, an astronomer at the Istituto Nazionale di Astrofisica (INAF) in Italy.  "The intriguing part is not only the visual appearance of the wave structure in 3D space, but also its wave-like behavior when we analyze the motions of the stars within it."

The discovery hinged on the use of standard candles, something you may be familiar with if you've read any cosmology.  Calculating distances of astronomical objects is tricky, for the same reason that it's difficult to tell how far away a single light is at night.  If the light seems bright, is it intrinsically bright (and perhaps quite distant), or are you looking at something that is dimmer, but close to you?  The only way to calculate astronomical distances is to use the small number of objects for which we know the intrinsic brightness.  The two most common are Cepheid variables, stars for which the oscillation period of luminosity is directly related to their brightness, and type 1a supernovas, which always have about the same peak luminosity.  Between these two, astrophysicists have been able to measure the changing positions of stars as the ripple of the wave passes them.

So the stars in our galaxy are riding the cosmic surf, and at the moment no one knows why.  One possibility is that this is a leftover gravitational effect from a collision with a dwarf galaxy some time in the distant past -- a little like the ripples from dropping a pebble into a pond lasting long after the pebble has come to rest on the bottom.  But the truth is, it will take further study to figure out for sure what's causing the wave.

Me, I find the whole thing staggering.  To think that only a little over a hundred years ago, there were still astronomers arguing (vehemently) that the only galaxy in the universe was the Milky Way, and all of the other galaxies were merely small local nebulae.  The last century has placed us into a universe vaster than the ancients could ever have conceived -- and I have no doubt that the next century will astonish us further, and in ways we never could have imagined.

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News from overhead

Seems like I've been writing a lot about the skies lately.

My general opinion is that what's going on up there is not only interesting, it's useful in taking my mind off the shitshow that's going on down here.  Be that as it may, in the last few weeks we've seen new discoveries about dark energy, a likely nearby supernova candidate, neutron stars, and the possibility of extraterrestrial life in unlikely places.  The astronomers and astrophysicists have been kept on their toes lately by the number of new discoveries and surprising observations -- in fact, in today's post, we'll take a look at not one, but two more pieces of news from above.

In the first, a team at the European Space Agency was working on mapping stellar positions in the "Gould's Belt," a ring of stars surrounding the Milky Way but tilted at about twenty degrees away from the galactic plane.  And what the team found was that within the Gould's Belt, there is a huge structure (from our perspective here on the Earth, it extends across half the sky) that shows regular up-and-down periodicity -- an enormous wave with a wavelength of about six thousand light years.

What could have created this structure is unknown, but waves are usually created when something gravitationally perturbs the pre-existing structure, so astronomers are trying to find something massive enough to cause a pattern change on this scale.  There don't seem to be any good candidates, so the current guess is that (once again) we may be talking about a clump of dark matter.

Whatever the hell that is.

"But this is very speculative at the moment, and other scenarios are as plausible, as an accretion of gas either from the halo of the Milky Way, stretched by the tidal forces of the galaxy (hence its narrowness)," said study lead author João Alves of the University of Vienna.  "Or maybe this is what spiral arms look like up close.  In summary, we have many ideas that we will be testing with future releases of Gaia data but we don't have a favorite scenario at the moment, and that is pretty exciting."

Another odd feature is the the wave is "damped' -- its amplitude decreases along its length.  This suggests that whatever created the disturbance interacted with the stars in the Belt and then passed on -- much like the waves from a rock dropped into a pond decrease in amplitude as they move outward.  It turns out that the Sun was passing through the belt about thirteen million years ago, but if it caused anything untoward here on Earth, it's left few traces.

"There was no obvious mass extinction event thirteen million years ago, so although we were crossing a sort of minefield back then, it did not leave an obvious mark," Alves said.  "Still, with the advent of more sensitive mass spectrometers, it is likely we will find some sort of mark left on the planet."

The second story is about another upcoming stellar explosion, this one more predictable (although less spectacular) than the Betelgeuse supernova about which I wrote two weeks ago.  The star in question is V Sagittae -- which, actually, is a binary system, a white dwarf (a collapsed stellar core) and a larger main-sequence companion.  Because of the white dwarf's gravitational pull, it is siphoning off matter from the surface of its partner, and the interaction is slowing down their rotation around their barycenter, so they're getting closer -- and will ultimately collide.

When that happens, it will cause a colossal explosion which will -- even at our position, 7,800 light years away -- release so much energy that for a short while, V Sagittae will be the brightest star in the sky.  And because we know a good bit about its rotational period and the rate at which the matter is being pulled from the main-sequence star, the astrophysicists also have a good idea of when this will happen: 2083, give or take a few years in either direction.

V Sagittae [Image is in the Public Domain courtesy of NASA/JPL]

So there are people alive today who will see this happen.  Sadly, I'm probably not going to be one of them, because in 2083 I'd be 123 years old.  And even though I fully intend to live forever (so far, so good), I must grudgingly admit that the chances of my making it to 123 aren't that high.

Still, there's always the possibility of some advance in genetic engineering extending our life spans.  I'm not exactly optimistic about the likelihood of this, but hope springs eternal and all that nonsense.  And if I'm not going to get to see Betelgeuse go kablooie, then V Sagittae sounds like a decent second-best.

So that's the news from overhead for today.  It's hard not to be impressed by the strides we're making in figuring out how the universe works.  Even though we've got a lot more questions still to answer -- which, after all, is how science works -- the idea that sitting here, on a little planet around an average star in an average galaxy, we can figure all this out is pretty damned impressive.

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This week's Skeptophilia book of the week is scarily appropriate reading material in today's political climate: Robert Bartholomew and Peter Hassall's wonderful A Colorful History of Popular Delusions.  In this brilliant and engaging book, the authors take a look at the phenomenon of crowd behavior, and how it has led to some of the most irrational behaviors humans are prone to -- fads, mobs, cults, crazes, manias, urban legends, and riots.

Sometimes amusing, sometimes shocking, this book looks at how our evolutionary background as a tribal animal has made us prone all too often to getting caught up in groupthink, where we leave behind logic and reason for the scary territory of making decisions based purely on emotion.  It's unsettling reading, but if you want to understand why humans all too often behave in ways that make the rational ones amongst us want to do repeated headdesks, this book should be on your list.

[Note: if you purchase this book using the image/link below, part of the proceeds goes to support Skeptophilia!]