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An international team of astronomers called the Gaia Collaboration has released a new updated catalog from the astronomical satellite Gaia. The catalog is disconcerting: Lists the positions, distances, colors, luminosities and motions of nearly two billion stars in the Milky Way.
Two. Billions.
This is the third version of the huge observational catalog, with many updates that include more stars and most importantly, better data calibration. This is typical of a survey observatory that releases a lot of data; over time, the telescope and camera become better understood and better calibration becomes available.
How big is this version? Very. It has:
- 1.811.709.771 positions of the stars
- 1,467,744,818 stellar distances and sky movements
- 1,540,770,489 colors of the stars
- 1,614,173 extragalactic sources
Gaia has been operational since January 2014 – 2,323 days – and produced 86,000 gigabytes of data for disk busting. Yes, over 86 terabytes. And it’s still ongoing.
It seems a bit strange, just measuring these basic properties of stars. But when you do it all over the sky, looking deep into the galaxy and beyond, and you do it with exquisite care and accuracy, what you truly end up with is a revolution in astronomy. As I wrote before, many fundamental and important discoveries have already been made: the distance to the Polaris and the Pleiades, solved an old puzzle about the star Albireo, discovered a cluster behind the brightest star in the night sky, found the remains of an old galaxy eaten by the Milky Way, and then we found many streams of stars indicating many old galaxies and clusters that we ate.
With this new catalog all this can be refined and new discoveries can be made.
For example…
This animation shows the movements of more than 40,000 stars within 100 parsecs (326 light years) of the Sun, projecting them into the future 1.6 million years.
The length of the path denotes the speed; some stars are faster than others and others are closer, so they appear to be moving faster. You may notice more stars on the right side than on the left towards the end of the animation; this is due to the motion of the Sun with respect to the stars. The stars chosen for this animation are the closest now, but as the Sun moves in one direction, the stars appear to move in the other. At the end of the animation there is a short display showing the paths representing 400,000 years of movement.
The image above is interesting: it’s not really an image! It is a map of the entire sky (using a Mollweide elliptical projection) in which the positions, colors and brightness of the stars are plotted using Gaia data. You can clearly see the shape of the galaxy – we live in the disk of the Milky Way, so we see it as a thick flat line projecting into the sky – huge clouds of dust, and in the lower right the two Magellanic Clouds, our satellite galaxies.
Compare that to this image, which shows the same information but does not map the brightness of the stars, so it shows the density of stars in the sky instead, with brighter spots showing more stars in an area than darker spots:
In the density map, you can see a straight line coming down from the galactic center: it is made up of stars from the dwarf galaxy of Sagittarius, which is being torn apart by the gravity of the Milky Way. It is not visible in the color image because the stars are faint and the color image favors brighter stars. This is exactly the kind of thing that studying Gaia’s data will show.
Nearby stars tend to be brighter and their motion wider, so the data for them is better. This animation offers a tour of the more than 300,000 stars within 100 parsecs of the Sun:
The focus on the two clusters is important. The common movement of the stars in the cluster can be used to find their distance (called mobile cluster method), and it’s one of many steps in what we call distance scale, where we can measure distances to nearby stars using parallax directly, then use those numbers to get better distances to the farthest stars in the clusters, then use them to look at more distant clusters, etc. Each rung along the way takes us farther and farther, ultimately allowing us to literally measure the most distant objects in the Universe. Gaia is critical to refine distances from nearby objects, which in turn it helps us to measure the entire universe.
So yes, this new version of data is a big deal.
It also produces a rather strange number: the acceleration of the solar system due to the gravity of the galaxy. Acceleration is by definition the change in the speed of an object, and speed is a combination of speed and direction. Calculated as a vector (the centripetal acceleration), the acceleration of the solar system should point to the center of the Milky Way, since the center of mass of the galaxy is there.
[Click through to see the thread by Gaia astrophysicist Paul McMillan.]
How do you measure it? Well, there is a phenomenon called aberration of light. If you move, the light from the objects seems to be coming from a little further than where they would be if you weren’t moving. It is the same thing as the rain that seems to come from in front of you when you walk through it. So it sounds weird, but you’re probably already used to this effect.
Gaia has measured the very precise positions of over a million quasars, extremely distant galaxies. They don’t move at all on their own when viewed from Earth – they’re too far away to see any movement they might have – but the aberration of light is easily measured using them. This aberration changes when the solar system moves around the galaxy by a very small amount … but Gaia has detected it! It is equal to 0.2 nanometers per second per second.
That is phenomenal small. It equates to about 7 kilometers per second in a million years, a very small acceleration. Earth’s gravity accelerates you to 10 meters per second per second (in other words, you are moving 10 meters per second faster every second you fall), so the force upon you from our planet is 50 billion times what you hear from the entire galaxy!
Furthermore, the measured acceleration actually points to the center of the galaxy. Not exactly, because the Great Magellanic Cloud also draws on us and there are other effects, but it’s pretty much exactly where expected, showing how precisely Gaia is working.
There is a lot more here too, but I think you get it. I’ll write more about this in the months to come, no doubt, as astronomers dig through the catalog to see how it affects their designs. This catalog has hundreds of millions of new sources and measurements, and there are sure to be many more things discovered within it.
It is a large galaxy. Oh, and this reminds me: While this database contains nearly 2 billion stars, the galaxy as a whole has more than 100 billion, so this only represents 2% of the Milky Way.
It’s a very great galaxy.
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