When you’re walking through a forest, it’s nearly impossible to know the forest’s boundaries or layout. This same idea applies with our galaxy, yet we know the Milky Way is a spiral galaxy, and we know our Sun orbits the center of the galaxy from about 26,000 light-years out. With no way to propel ourselves outside of the galaxy and look at it from that view, how have astronomers learned so much about the Milky Way? By tracking the positions and movements of stars and gas clouds. And astronomers just received details from the Gaia spacecraft of a billion stars.
To map these stars, astronomers use astrometry — measuring the precise positions of stars. It isn’t the sexiest field of astronomy, but it’s extremely important. Astrometry lets you track the movements of stars and gas clumps, and that provides a way to map and measure the structures in space.
To measure distances nearby, astronomers use the parallax method. For a basic idea of what this is, hold your index finger a few inches from your face and close your right eye. Note the position of your finger compared to the background. Next, keep your index finger in the same physical location but open your right eye and close your left. Again, note the position of your finger against the background. That apparent movement is parallax, and nearby stars do the exact same thing against a background sky. The difference is that instead of using the distance between your two eyes, astronomers use the distance between Earth at opposite points in its orbit around the Sun. The tinier shift they spy, the farther away the star is from us. Using geometry plus a few known values (the amount the star shifts on the sky and the distance Earth traveled over those six months of its orbit), you can calculate the distance to the star. And while Gaia isn’t on Earth, it’s near our planet and so this same idea applies.
Gaia uses two telescopes pointed at different directions and separated by a constant angle. (The telescope mirrors, along with other instruments, are welded to a solid ring to ensure the angle never changes.) Gaia slowly rotates, so that the telescopes continually scan the sky. Each day, the telescopes collect light amounting to 40 gigabytes of data. Over its 5-year mission, Gaia will see the complete sky 70 times, and collect detailed position information for about 1 billion stars during those 70 observations. It will also measure the three-dimensional motions of two million of the brightest stars.
Because everything in the galaxy is moving at all times, the positions and distances are measured with respect to the other stars. That makes for an enormous amount of data that needs to be analyzed. A collaboration of 450 scientists from 160 institutions is up to the task. “We take all the raw telemetry that comes from the mission — all these zeroes and ones that you see coming in from the sky — and we turn them, through our six data processing centers, into data products that the scientists can interpret,” said Anthony Brown, who leads the Gaia Data Processing Analysis Consortium, during the press conference announcing the first data release. It’s one enormous, complex moving machine.
The science within the map
For the past two decades, astronomers have relied on the positions and distances measured by the Hipparcos satellite. With Hipparcus, they could only measure parallax out to a few hundred light-years, but Gaia is far more sensitive. Gaia extends the parallax measurements out to a couple hundred thousand light-years; when mission is complete, we’ll have a 3-D map of the Milky Way out to that distance from us.
This map isn’t just for kicks. Knowing the directions and velocities that stars are moving in, astronomers can then turn back the clock and figure out where those stars came from. Gaia is also collecting data about the stars’ compositions and temperatures. From all of this information, scientists will tease out which stars were born with what other stars, or if a grouping of suns came from a satellite galaxy that crashed into the growing Milky Way. They’ll know more about the evolution of our galaxy.
And on smaller scales, they will reveal the structures within gas clouds and known star clusters. This first data release already uncovered some 400 million stars that hadn’t been individually seen before, said Brown during the press conference. And there’s no doubt that some of the pinpoints of light Gaia sees are not stars but asteroids in the solar system or perhaps something far outside our galaxy.
The distance measurements astronomers are collecting will lead scientists to better calibrate the tools they use to measure distances much farther out than a couple hundred thousand light-years. Any distances greater than the Hipparcos distances had been measured using specific types of brightness-varying stars. The amount of time it takes for such a star cycle from its max brightness to its minimum brightness and back to its max depends on the amount of light it produces. So if you measure the time it takes for that cycle, you can calculate how bright it should be. That gives you a specific-wattage light bulb. (If we know that bulb should be, say, 100 watts, and we see it as fainter than that, we can calculate how far away it is.) With Gaia, astronomers have measured the distance of a few hundred of these types of varying-brightness stars using parallax. And those detections can be used to calibrate the variable-star period-distance relation and serve as a way to double-check that scientists had understood that relation. So far, so good.
This first batch of Gaia data includes brightness changes for 3,000 variable stars, the positions of 1.1 billion stars, motion information for more than 2 million of those 1.1 billion stars, and positions for more than 2,000 active galaxies. The team plans to release another three data sets before the final release, which scheduled for the early 2020s and incorporating all five years of observations.
Using the telescopes that have come in the decades before Gaia, we know that Earth orbits the Sun, and the Sun lies within a disk of stars and gas and dust. The cloudy stream arcing across the sky at night is the Milky Way’s disk. At the center of the disk lies a supermassive black hole, and our galaxy’s spiral arms pinwheel and rotate around that center. From decades of mapping the stars and gas clouds, we have a two-dimensional illustration of our spiral galaxy. But with Gaia, we’re switching from that folded map in your glove box to a spectacular “3D motion picture,” as Brown puts it.