One of the things we don’t do often enough is talk about why some of the lesser-known space telescopes are so awesome. Today, I’m going to take a moment to talk about the European Space Agency’s Gaia mission. 

Launched in 2013, this telescope has a purpose-built design that is like no other telescope. Instead of a standard round mirror, it has a series of curved rectangular mirrors that allow it to accomplish a variety of different tasks simultaneously. Those tasks? Gaia is designed to catalog with great accuracy the positions, motions, and colors of known objects of interest — one billion known objects of interest. That may sound like pretty standard stuff, but the how of it is technological wizardry of the highest level.

Here is what is going on: Gaia is slowly spinning, and as it spins, its mirrors are collecting light coming in from the sides of the telescope, perpendicular to that access of rotation. Yes, the telescope with the rectangular mirror is looking out through the sides of its tube while spinning. This slow spin causes the light it is detecting to drift across its detectors over time. Unlike your standard digital camera that collects all the light from a single point in a single set up pixels and then reads out the image one column at a time after it finishes collecting the light, Gaia constantly reads out the image, one column at a time, while the image drifts across the detector at the same rate that the detector is being read out. This means an object’s exposure time is equal to the amount of time it takes to read out the entire detector. It’s kind of weird, and it totally works, which makes it really wonderful. 

This kind of drift-scan technique also means that they can slowly move the light from entering one kind of detector to another. Initially, the mirrors will send an object’s light onto its sky mapper detector that checks for rapid variations and confirm that what is being seen is an object and not a cosmic ray or other mistake. From the sky mapper, the light path shifts onto the astrometric field detector; this is the highest resolution camera sent into space and is precise enough to measure how the position of stars changes due to planets going around them. 

From here, the light is measured in shades of blue and shades of red, and finally, some of the light is put through a spectrometer that measures the chemistry of the light source and its velocity toward or away from us. This amazing system allows us to actually get the three-dimensional motion of objects as the astrometric system sees their N-S-E-W motions, and the spectra get that last toward and away motion.

And because Gaia is traveling alongside the Earth and  Moon in our orbit around the Sun, its change of perspective allows it to also catch the slight changes in where a star appears that are due to our own change of perspective. 

To understand what I mean, close one eye and block some distant object with your finger. Now switch which eye is open. You’ll see your finger appear to jump, and the amount it jumps will change depending on how far from your face you hold your finger. If you call your left eye June and your right eye December, you can imagine you are seeing how nearby stars appear to move compared to distant galaxies as the telescope moves from one side of the Sun to the other. This means that Gaia has not only the three-dimensional motions of stars but also their positions, and since this is a multi-year mission, it gets to watch as those positions actually change with their motions over time.

This system is my favorite of all astronomical systems.

And we have new and amazing science from this system today. Science that is so much more awesome when you know more about how it was detected.

Now eight years into its mission, Gaia has had the chance to visit the same stars over and over, improving its data and refining our understanding of stars’ positions, motions, compositions, surface gravities, and more, and with one billion stars to study in a large sphere around our position in a galactic arm, this allows researchers to ask super focused questions and get high-quality answers.

A newly published analysis of 250,000 stars found that the outer parts of our galaxy’s disk – essentially its top and bottom surfaces – were older than we knew. This region is called the galactic thick disk.

Models of stellar evolution allow us to know how long certain parts of a star’s life should last, and when we see stars of a given type, we can estimate their age as being the duration of all earlier parts of their life, up to the age of the next stage of their life. This is like looking at a child that has taken its first step and knowing it has completed the learning to hold up its head, learning to roll over, and other prior steps, and should be about twelve months old.

The shorter the duration of any stage, the more accurately we can know the star’s age. And conveniently, the bright sub-giant stage of stellar evolution is just such a stage.

In a new paper from Maosheng Xiang and Hans-Walter Rix that appears in Nature, they use Gaia data, along with data from China’s Large Sky Area Multi-Object Fiber Spectroscopic Telescope. According to Maosheng: With Gaia’s brightness data, we are able to determine the age of a subgiant star to a few percent.

And since subgiants with different masses and different chemistry all have different ages, much like an adult mouse and an adult human are very different ages, they were able to put together a timeline for the formation and evolution of our galaxy.

Roughly 800 million years after the Big Bang, the first stars in our galaxy started forming. Initially made of hydrogen, helium, and only a tenth of the iron of our Sun, each new generation of stars fused these atoms together to build up the atomically diverse galaxy we live in today. In fact, the last stars formed in this region were three times richer in iron than our Sun.

Stars didn’t form at a continuous rate, with the same number of stars forming year after year. Instead, about eleven billion years ago, a massive burst of star formation was triggered when the ill-named Gaia-Sausage-Enceladus galaxy collided with the early Milky Way. This collision shocked clouds of gas into star formation splashed stars into the galaxy’s outer sphere of stars, and this burst was able to use up the thick disk’s gas in just three billion years. Ziang says: The Milky Way has been quite quiet for the last eight billion years.

If it continues to come together successfully, the JWST will have the capacity to be able to see galaxies similar to our own forming in the past. If this happens — can we say “when this happens”, yet? When/if this happens, we’ll bring you those images right here on the Daily Space.

More Information

ESA press release

“A time-resolved picture of our Milky Way’s early formation history,” Maosheng Xiang & Hans-Walter Rix, 2022 March 23, Nature

Here’s the best timeline yet for the Milky Way’s big events (Science News)