on Gaia

Our knowledge of where things are in space is going to be changed forever. Over the summer, the European Space Agency’s Gaia mission is going to release its first data, and with it, distances to two million stars. Eventually, once the full mission is complete, there will be distances to somewhere between one and three BILLION stars. This is incredibly exciting.

Getting the distances to all those stars is going to make for amazing science, because once you know the distances to stars you know their true brightnesses. Back in 1912, Ejinar Hertzprung and William Norris Russel used distance measurements to show that stars don’t come in any kind of brightness and color you want; if you correct the stars’ luminosities for how far away they are, the red ones are almost always very dim, and the blue ones are almost always very bright. The study of this “HR diagram” and other similar relationships is still driving huge amounts of stellar astrophysics research to this day. And distances are what allows us to identify structures like streams and the arms of the Galaxy.

Right now, the state of the art when it comes to distances comes from Hipparcos, an earlier satellite launched by the European Space Agency. Hipparcos had two major instruments on board: Hipparcos, which measured distances and motions (of the star around the Galaxy; the technical term is “proper motion”) and positions of 118,000 pre-selected stars, and Tycho, which measured the colors and proper motions and positions of 2.5 million stars, which is basically everything down to the brightness limit of its sister, the Hipparcos instrument. Hipparcos’ mission ran from 1989-1993, and the results took until 1997 to process correctly.

Hipparcos (credit: ESA)

The Hipparcos catalog is revolutionary, and it powers most of the 3-D universe simulations you can find today. The Celestia space simulator (although it does come with some additional data for nearby stars); the “Stars” Chrome Experiment, AMNH’s Digital Universe (which also has numerous addons and additional datasets), the iOS Exoplanets app, all the distances given by the Stellarium planetarium software, and Gaia’s own Gaia Sky 3d space simulator (as of April, anyway)…. it’s all Hipparcos. The only software that has more stars is SpaceEngine, which generates fake stars to fill in the rest of the Galaxy realistically. And then there’s the scientific papers built off of the Hipparcos catalog – there are nearly 4,000 citations to the original 1997 catalog and nearly 2,000 citations for a re-processing of the data in 2007*, which increased the precision of the catalog by about a factor of 2.

Prior to the Hipparcos catalog, the biggest compilation was the General Catalog of Trigonometric Parallaxes (often called the Yale Parallax Catalog, because it was maintained at Yale), whose fourth edition in 1995 contained distances to about 8,100 stars. Not only did Hipparcos have 14 times more stars than everything else combined, its distances were generally much more precise.

Hipparcos wasn’t perfect though. The Hipparcos and Tycho instruments had what astronomers consider fairly high brightness limits. For instance, Wolf 359 is too faint to appear in either the Hipparcos or Tycho catalogs, even though it’s the fifth-closest star system (according to the General Catalog of Trigonometric Parallaxes, which has a very good distance measurement). Astronomers, including my PhD adviser, seized on that opportunity to provide distances to really faint things. (If it sounds like I’m gushing, I’m more of a competitor than a friend of Gaia)

Still, even if you combine (and I have) Hipparcos with all the distances published before and after Hipparcos, there’s still only roughly 128,000 stars in the entire universe with actual measured distances, and nearly all of them are within 300 parsecs (1000 light years) of the Sun. That is literally all we have. Everything else is an estimate built off of assumptions. Careful assumptions, but assumptions nevertheless.

Hipparcos (and all the other catalogs mentioned) used the most accurate method of getting distances: Annual trigonometric parallaxes. As the name implies, the method relies on trigonometry, with very few estimates that normally go into distance finding. Our brains actually use parallax all the time. Try this: Close one eye, extend your arm, and cover something (like a doorknob, or a sleeping cat) with your thumb. Now close that eye and open the other one. The thumb isn’t covering the cat any more. That perspective shift is called parallax, and your brain uses it to triangulate the distances to objects. Of course, the distance between your eyes is only about two inches, so your brain has to rely on experience, depth cues, and motion to determine the distances to things more than about 30 feet away.

The idea of using parallaxes to measure the distances to stars goes back all the way to Galileo himself, who realized that if the universe was Sun-centric, the stars should make little circles (ellipses, actually) as the Earth swung around the Sun, January to June. Unfortunately for him, the technology and techniques of 16th Century Italy simply weren’t up to it. In fact, it wasn’t until the 1830s that a stellar parallax was actually measured.

Even now, the measurements are still hard and time-consuming. While stars are making tiny loop-de-loops from the motion of the Earth around the Sun, they’ve also got that proper motion I mentioned earlier, and you have to watch them for at least two years (more is better) to disentangle the two effects.

Once you have the parallax motion (which is generally measured in arcseconds, which are 1/60 of 1/60 of one degree on the sky – for example, the moon is half a degree or 1800 arcseconds across; the finest detail you can see without binoculars is one arcminute or 60 arcseconds) you can convert the parallax into a distance: 1/parallax = distance (in parallax-arcseconds, or parsecs). What that means is a star with a parallax of 0.1 arcseconds is 10 parsecs away, and one with a parallax of 0.01 arcseconds is 100 parsecs away. The more distant the star, the smaller the angle… and the harder it is to measure. And Hipparcos’s problem was harder than that, because unlike most observing programs, it was trying to do all of its stars at once, rather than one at a time.

So what of Gaia? Gaia is both a successor to Hipparcos, but also much more.

The Milky Way Shines on Paranal
Gaia. Image Credit: ESA/ATG; background image ESO/S. Brunier

It has a much better brightness limit. Astronomers quantify brightness as magnitudes, where 0th magnitude stars are the brightest ones in the sky, and 6th magnitude stars are the faintest ones you can see without a telescope. Hipparcos (and Tycho) were limited to 12th magnitude (250 times fainter than what you can see with your eyes); Gaia will reach magnitude 20.3 (500,000 times fainter than you can see). On the other hand, stars brighter than 6th magnitude (as in, every star you can see in the night sky) are all too bright for Gaia to handle. Fortunately, Hipparcos measured most of them.

It will measure far more stars. Where Hipparcos and its 118,000 stars were a 14x improvement over everything that came before, Gaia’s billion stars will be an 8000x improvement. It’ll certainly be more than you can display in a computer. On the other hand, given that the Milky Way is supposed to have over 100 billion stars, Gaia’s still only going to get about 1% of them.

It is also MUCH more precise. Prior to Hipparcos, the measurement precisions people were getting meant that only stars within 50 parsecs (160 light years) of the Sun were really well determined. Hipparcos’s distances are good out to about 100-150 parsecs (about 500 light years). If they accomplish what they promise, Gaia will get good distances to stars 5,000 parsecs (16,300 light years) away. It’ll even do fairly well for objects on the far side of the Galaxy – the black hole at the center is 8,000 parsecs (26,000 light years) away, and I’m sure they’ll get lucky and pull out a few good measurements of stars farther away than that. We should be able to see the sizes and distributions of the galactic arms, and get extremely accurate distances to various clusters and star forming regions (which we already know approximately from estimates). Another way to look at it is that nearby stars will have distances measured to the accuracy of only a few astronomical units. Right now the most accurately determined distance to any star (other than the Sun) is Proxima Centauri, which we know to an accuracy of 107 AU – basically, we could be off by the width of the entire Solar System.

The region of the Galaxy covered by Hipparcos (yellow) and covered well by Gaia (red). Image by Robert Hurt (NASA-JPL/Caltech)

It will also measure radial velocities – where proper motions are the motion ACROSS the sky, radial velocities are the motion towards and away from us. Radial velocities are the final piece that will allow us to completely determine the position and motion of stars. Hipparcos didn’t do anything like that, but Gaia will get radial velocities for the brightest stars it sees.

Gaia’s final catalog is not going to be released until 2022 (assuming it runs from 2013-2018). Computing power may have increased exponentially since the 1990s, but so has the size of the project. The consortium is currently trying to convince the European Space Agency to give them an additional three years, which would increase their precision further.

This summer, however, the consortium is releasing the “Tycho Gaia Astrometric Solution”, which will give us parallaxes to 2 million stars. This is a combination of the first year of Gaia data and the positions and motions from Tycho. With the 25 years between the first (1989) and last (2014) position measurements, they’ll be able to remove the stellar space motions very well, and it’ll get good distances out to about 300 parsecs (1000 light years) for 20x more stars than we currently have distances for now.

It will be a heck of a teaser for the full data coming in five years’ time.



*For the original Hipparcos catalog, I’m counting both the Perryman et al. (1997) Astronomy and Astrophysics paper and the ESA printed catalog; for the 2007 re-reduction I’m counting both the book and the Astronomy and Astrophysics paper that validated it.

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