stickmaker | JOHT 79: Relative Motion

The Joy of High Tech

Rodford Edmiston

Being the occasionally interesting ramblings of a major-league technophile.

Please note that while I am an engineer (BSCE) and do my research, I am not a professional in this field. Do not take anything here as gospel; check the facts I give. If you find a mistake, please let me know about it.

Relative Motion

When someone asks "Which way is the Earth moving?" the proper response is "Relative to what?" Our planet (if you're not from Earth, please just pretend you are for a moment) goes around the Sun. Only it's not that simple. The Earth-Moon _system_ goes around the Sun, while those two bodies orbit each other around a common balance point, or barycenter. The Earth is massive enough in comparison to the Moon that the shared gravitational center is actually on average about 4,671 km (2,902 mi) from the Earth's center, or about 1,707 km (1,061 mi) below the surface. This produces a distinct, though rhythmic wobble in the Earth's path around the Sun.

On a larger scale, our solar system - Sun, Earth and Moon included - is moving through the Milky Way galaxy in an even more complicated manner. We have a pretty good idea of where we are and how we're moving right now. Unfortunately, tracking the movement of our solar system back or in time and space or predicting forward becomes more and more difficult the further you go. Beyond a few hundred thousand years ago or to come we can only say where we were or will be in general. Very general. Most of the information we currently have which allows for such modeling comes not only from bigger and better optical telescopes, but from bigger and better radio telescopes on Earth and infrared telescopes in space. Dust clouds tend to block optical wavelengths, but may let IR and/or radio through. X-ray and gamma ray telescopes - also in space - further expand our view and our understanding.

The Milky Way galaxy moves as a fluid. Imagine a large bowl filled with water in which neutrally-bouyant particles are randomly distributed. Now, stir this fluid mixture around its center until the whole mess is moving at a good clip. You'll see clumping and empty areas as the fluid carries the particles in a way which is mathematically chaotic. (That is, particle paths and current locations are determinate, but incredibly hard to plot due to the complicated situation.) This physical model will be especially accurate if the particles slightly attract each other. (Given surface tension they probably will.) In an actual galaxy or other large cluster of stars this movement is complicated by the presence of dark matter, which has gravity but otherwise rarely interacts with "normal" matter.

Our solar system is hurtling through space while angled nearly perpendicular to the direction of its movement through the Milky Way. That is, the solar plane of the ecliptic - the flat disc marking the orbits of the planets around the Sun - is at a steep angle with respect to the plane formed by the average orbits of the stars in the Milky Way. To gain a bit of perspective, our galaxy is a substantial body. In fact, the more we have learned about our home galaxy in recent decades the more our understanding of its size (part of our visualization of the Cosmic All) has increased. Astronomers now estimate its diameter to span over a hundred thousand light years, making it roughly the same size as the Andromeda galaxy. Which, itself, has had estimates of its size reduced in recent years.

The Solar System currently takes roughly 230 million years to make one circuit around the galaxy. However, its path is not a circle; instead, it moves in and out from the center in an ellipse. It also bobs up and down through the galactic disc, completing a cycle roughly every 60 million years. (Our passage through the galactic plane every 30 million years seems to be followed by a burst of cometary bombardment and accompanying mass extinctions.) During this slow "vertical" oscillation our solar system strays some 200 light years above or below that imaginary plane. This is a substantial fraction of the approximately 1000 light year thickness of the galaxy at our current location in it. We are currently very close to the galactic plane, with most estimates placing us as having crossed it from "below" within the last three million years. Any comets sent hurtling inwards from the Oort Cloud by this crossing could take more than a million years to reach the inner solar system, so we might seeing the tail end of this swarm now. These times are very approximate, however. Some analyses of our solar system's bobbing movement have us still approaching the galactic plane. Data collection and analysis continues.

This sort of three-dimensional movement seems to be typical for stars and clusters of stars in galaxies, though - of course - our own star's specific path is unique. It also isn't set, but changes through each close encounters with other stars or clusters, which is more likely during passage through one of the spiral arms. The spirals, by the way, are byproducts of pressure waves sweeping around the Milky Way. They are analogous to those demonstrations where pith balls are sprinkled on a horizontal speaker and form patterns dependant on the frequency. Like waves on the ocean, the arms can "pick up" stars or clusters and sweep them together, leaving behind volumes of low stellar population. Once the wave moves on, most of the stars will resume their essentially random motions. However, sometimes stars will actually be carried along for a while, like a bit of Styrofoam on the surf is carried towards shore if a wave catches it in the right way.

One candidate for the cause behind the spirals is the Andromeda galaxy. Some astronomers now believe that about ten billion years ago Andromeda and the Milky Way had a close encounter, which drew spirals out in both. (Doc Smith actually mentioned something similar in the Lensman series, attributing the large number of habitable planets in both galaxies to the close encounter causing bursts of star formation (and the accompanying planet formation). However, he was referring to Lundmark's Nebula, not Andromeda. (My thanks to George W. Price for correcting which galaxy Doc actually mentioned.) That pass - the real one, with Andromeda - was close enough that the two bodies are now gravitationally bound. (That bonding may have actually happened before that pass.) The two galaxies have passed maximum separation and are headed back towards each other. They should begin colliding in about four billion years. This won't be as cataclysmic as it sounds. In fact, the bodies will pass through each other, their shapes disrupted by the gravitational interaction between them. Then they will move apart again before repeating the process, until they finally merge in to an elliptical galaxy, about six billion years in the future. Eventually, their core black holes will also combine. However, the individual stars in each agglomeration are so far apart that even when the two galaxies finally merge there will be few additional collisions between stars because of that. Though the gravitational effects will cause a huge burst of star formations as clouds of gas and dust collapse in the aftermath.

Note that, as mentioned above, these dates are very general. In large part because galaxies (despite a key point in the second pilot of the original Star Trek series) don't have distinct boundaries, but simply fade away at their borders.

Stars which form together don't always stay together. The Sun's metallicity is actually higher than that of the average for the stars currently around us. Some astronomers think our solar system was formed much closer to the galactic core, where such metallicity is typical. It may have been one member of a double or multiple star system. We most likely were moved to our current neighborhood over many millions of years by gravitational interactions, proceeding separately from our neighbors then and now. Our system's motion with respect to the galaxy as a whole also does not match that of any of the stars around us. Of course, except for multi-star systems, few of those stars are moving the same way as any of the other stars near them.

Because of all this movement - it's not just us, remember, but all the stars around us as well - accurately tracking the solar system's path back for more than a few million years is impossible. (Well, unless there's some ancient culture out there which is mapping stars just to keep track of them. If so I wish they'd let us know.) For the same reason, beyond the next few million years we can only make general predictions about where we are going. While there are unlikely to be significant changes to any of the general motions described above any time soon (cosmically speaking) a close encounter with a compact globular cluster of a few thousand stars could change all of that. Right now we're nowhere near any such clusters. However, there could be a quiet black hole (one currently without an accretion disc) or neutron star we don't yet see, lurking just a few thousand years in our future which could change our course. Keep your plans flexible. :-)

There are other changes which occur besides movement. Stars age and evolve with time, and some of them eventually explode. White dwarves with close companions can actually explode repeatedly, as material from the other star piles up until it reaches a critical point and creates what's known as a Type Ia Supernova. Our solar system has been within a hundred light years of a supernova of some type within the past few million years, as testified by the amount of short-lived Iron-60 here. This is verified by returned lunar samples: Cosmic rays from a supernovae will plough into the Moon's unprotected surface, leaving trails of damage in surface minerals which are visible under a microscope. These fast moving particles also impact atoms there and in our upper atmosphere to create exotic isotopes such as Krypton-83 and Xenon-126. There are multiple suggestions for missions to sample lunar lava flows of various ages, to see how they have absorbed different contaminants through time.

Remember that bobbing up and down, described above? Once our solar system gets well above or below the galactic plane it is exposed to much higher levels of radiation from outside the galaxy. The periodicity of this has been suggested as yet another cause of repeated extinctions on Earth. Also, as our motion around the Milky Way brings us toward the leading edge - where the galaxy's motion through space causes energetic interactions between its gas and dust shell and the intergalactic medium - ambient radiation will increase. Both effects are pretty minor compared to other - though generally shorter term - variations in the radiation which enters our solar system.

So we see that, as is usually the case, the universe is a complicated place. Our science and technology are constantly improving and thereby improving our understanding of what lies around us, even on the short term scale of decades. Which gives us a lot to look forward in the near term.