There are about 9,000 stars in total visible with the naked eye from a dark sky anywhere on planet Earth. In 1989, a star catalog was compiled to help the Hubble Space Telescope target objects of interest in space. It contained about 20 million stars. Last week, the European Space Agency’s Gaia mission released a slew of data on a phenomenal 1.7 billion stars in the Milky Way!
It goes without saying it’s the most thorough star catalog to date. To put that number in perspective, it would take about 54 years to count to 1.7 billion at the rate of one count a second. Large as it seems, even that number accounts for just a little more than 1% of the stars in the Milky Way galaxy. And remember, we’re just one galaxy among a trillion.
The new data reveal all kinds of details about the make-up of the galaxy’s stars: their precise positions, colors, brightnesses and how they move. All of this will help astronomers better understand how our galaxy came to be and what’s in store for is future.
Astronomers get the distance to a star by measuring its apparent shift in the sky as the Earth orbits the sun called parallax. You can see parallax in action by holding your index finger a short distance in front of your eyes straight out from your nose. Open and close each of your eyes in turn, and you’ll see your finger jump from left to right against the more distant background. If you measure the distance between your eyes and the shifting angle of your finger, you can find out exactly how far away it is using simple trigonometry.
Nice, short video about the Gaia mission and its record-busting parallax measurements
Astronomers apply the same concept to measuring star distances using the diameter of Earth’s orbit — the equivalent of two eyes separated by 180 million miles. They need “eyes” this far apart because stars are so far away they shift only the tiniest fraction of a degree from one side to the other of our orbit.
For that reason and also because of the blurring effects of Earth’s atmosphere, astronomers can only measure distances out to a few hundred light years. Gaia, with its billion-pixel camera situated far from Earth, can do much, much better. Knowing precise distances to stars gives provides an understanding of their true brightnesses, sizes, masses and motion through space. I recently dropped by the Gaia data archive to get an approximate distance for the recent nova outburst of V392 Persei. Yeah, even rookies can dig around in there.
Gaia’s treasure trove of data also includes information about the stars in a dozen nearby galaxies including the Milky Way’s next-door neighbor, the Large Magellanic Cloud (LMC). This image above, which includes newfound knowledge on how the stars move in the galaxy, gives us a swirling imprint of the stars revolving clockwise around the galaxy’s center. We can almost see the LMC turning.
Closer to home, Gaia observed about 14,000 known asteroids and comets to better map their orbits and 500,000 quasars (distant galaxies illuminated by voracious black holes in their centers). More data releases are on the way with a final catalog expected in late 2022. Gaia, the name from the ancient Greek goddess of the Earth, just keeps on giving.