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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