Sky-scanning complete for Gaia | Hacker News Reader

Sky-scanning complete for Gaia

esa.int

177 pointssohkamyung67 comments

Direct link to some very very nice images and animations: https://www.esa.int/Science_Exploration/Space_Science/Gaia

Two of my favorites: https://www.esa.int/ESA_Multimedia/Images/2025/The_best_Milk...

https://www.esa.int/ESA_Multimedia/Images/2025/01/The_best_M...

I get how Gaia could make the best edge on image, but how could Gaia (or anything man made) get the the "best" face on image?

All of these are "Artist's Impressions". My best guess is they run a simulation based on the data from the spacecraft and then can pan the camera around as they see fit

From the page:

[Image Description: A model image of what our home galaxy, the Milky Way, might look like edge-on, against a pitch-black backdrop. The Milky Way’s disc appears in the centre of the image, as a thin, dark-brown line spanning from left to right, with the hint of a wave in it. The line appears to be etched into a thin glowing layer of silver sand, that makes it look as if it was drawn with a coloured pencil on coarse paper. The bulge of the galaxy sits like a glowing, see-through pearl in the shape of a sphere in the centre of this brown line.]

That's an AI produced accessibility description so I thought it seemed wrong. But more directly from the article text: This is a new artist’s impression of our galaxy, the Milky Way, based on data from ESA’s Gaia space telescope.

Is it AI produced (if so, do they communicate it somewhere?) or do you believe it is?

"The best Milky Way map, by Gaia (edge-on)"

The "by Gaia" implies the opposite to me. Unless the "artist's impressions" are from someone named Gaia???

"This is a new artist’s impression of our galaxy, the Milky Way, based on data from ESA’s Gaia space telescope."

I'm sure you know of headlines vs details; when it comes down to it, space science relies on marketing to get some funding and interest in it, and using 100% accurate headlines is not good marketing.

The face-on galaxy image is credited to Stefan Payne-Wardenaar (https://stefanpw.myportfolio.com/home), whose Twitter and Bluesky bios say, "I make astronomy visualizations in Blender."

The whole purpose of Gaia is to precisely measure the position of stars (and other objects). Once positions are known, a 3D model can be built. But how are the distances measured? The answer is parallax, essentially triangulation. You look for very small changes of position against the background sky. You use the width of the earth's orbit as the baseline and measure at different times of the year.

It can't. The galaxy is assumed to be roughly symmetrical, and they fill in the missing data with what we can see on our side of the galaxy. It's "best" in the sense that it's the most accurate fiction, I suppose.

Gaia is good to about 13,000 light years: https://en.wikipedia.org/wiki/File:Galaxymap.com,_map_12000_...

The Milky Way is maybe 100,000 light years in diameter. So we're only getting good distance readings on a small fraction, and nothing behind the central bulge of our galaxy. The first won't improve until we send an astrometry telescope way outside the orbit of the Earth, for better baselines, and the second is going to need a telescope sent 10,000 light years out of the galactic ecliptic.

We can infer the general distribution of mass on the other side of the galaxy from observing the trajectory of stars, can't we?

It takes 230 million years for the Sun to make one full orbit around the Milky Way.

huh? how can sun orbit the milky way if it is within the milky way

Around the center of the Milky Way. The sun orbits the center just like the planets orbit the sun.

is there some sort of gravitational body in the middle that makes everything orbit in galaxies? it must be massive right

One of today's lucky ten thousand. Most galaxies have a black hole at the center that mass at least a hundred thousand times more than our sun: https://en.wikipedia.org/wiki/Supermassive_black_hole

does that mean all galaxies will eventually be consumed by the black holes at their center?

That’s one of the theories, called Big Crunch: https://en.wikipedia.org/wiki/Big_Crunch

There are other (more probable) theories about the end of the universe, and if you’re up to it you can read more here: https://en.wikipedia.org/wiki/Ultimate_fate_of_the_universe

I’m in favor of the Big Chill, since I like the concept of entropy as introduced by the second law of thermodynamics: https://en.wikipedia.org/wiki/Heat_death_of_the_universe

sometimes it feels weird reading anything non fiction because we know so little about reality

The Big Crunch has nothing to do with Sagittarius A*, the black hole at the center of our galaxy. No theories actually predict that it will consume all stars in our galaxy.

No. Does this mean that solar systems will eventually be consumed by the stars at their centre, planets falling out of their orbits due to gravity? It does not. Gravity doesn't work like that.

The planets may be consumed, when the star runs out of fuel and swells a lot, and such is the Earth's fate. But that scenario is not one that happens to black holes.

> mass at least a hundred thousand times more than our sun

The sun is 99.86% of the mass of solar system. So if you orbit the centre of mass of the solar system, you orbit the sun, more or less. Give or take a small correction for Jupiter.

But ... there are a lot more than a hundred thousand stars in the milky way. So if I guess right, the ratio of central mass vs the rest would be very different for the Milky way? It's more of a blob.

Even at "The current best estimate of its mass is 4.2 million solar masses" it does not dominate? https://en.wikipedia.org/wiki/Sagittarius_A*

It's not a singular source of gravity at the center though, it's the collection of all mass in the galaxy interacting with each other as well. Like a daisy chain of gravity, which explains why it looks like a spiral instead of an evenly distributed circle.

(I think anyway, I just made it up, I'm not learned in this area, just a HN shitposter)

Right, not a point source and not exactly a blob either; more like swirls. I'm not learned in this area either.

> is there some sort of gravitational body in the middle that makes everything orbit in galaxies?

No. The Sun's orbit is determined by the total mass of stars, gas, and dark matter interior to the orbit. This is mostly due to the stars (we're not far enough out from the center for dark matter to be the dominant component) and is on the order of several tens of billions of solar masses.

(There is a supermassive black hole at the center of our galaxy, but its mass is only about 4 million solar masses, so it's negligibly small compared to the mass of all the stars.)

Orbit around the center. It’s like saying the Earth orbits around the solar system.

But the earth doesn't orbit around the solar system. it orbits around the sun as part of the solar system. the solar system as a unit orbits around the center of the galaxy. if you've ever seen the concept images of the Oort cloud, you could visualize that snowball looking roundish object as a visual for the solar system traveling through the galaxy.

The same way that the Earth can orbit the centre of mass of the solar system and also be within the solar system. We say that the Earth orbits the Sun because that's where 99.86% of the mass of the solar system is located.

The Sun in turn orbits the the centre of mass of the Milky Way. But I don't think that the mass of the Milky way's central supermassive black hole dominates in the same way.

Gaia has a 1.0 × 0.5 m focal plane array on which light from both telescopes is projected. This in turn consists of 106 CCDs of 4500 × 1966 pixels each, for a total of 937.8 megapixels.

Neat.

The really neat part is the instrument precision. It's terrifyingly good and I have no idea how it (really) works.

- "Gaia measures their positions to an accuracy of 24 microarcseconds, comparable to measuring the diameter of a human hair at a distance of 1000 km"

https://www.esa.int/Science_Exploration/Space_Science/Gaia/C...

Isn't that just the distance between pixels and the image projected onto them?

No: it's far weirder and I'm not knowledgeable enough to explain it.

Probably not. The accuracy with which you know the pointing of the telescope probably also plays into it (unless the FOV is large enough to have other stars as a reference?), and you can do subpixel positioning of objects to get more accuracy than full pixel steps.

To nitpick with the grammar in the quote: It's capable of measuring to the accuracy of 120 μm at 1000 km. So it cannot accurately measure the diameter of a human hair (which ranges from around 20 to 200 μm) at that distance, but only to the accuracy of a human hair.

You're right: this precision is hundreds of times below the diffraction limit of even the James Webb telescope. It can't possibly measure the width of an object that finely; rather, only the relative displacement of its centroid position between two points in time. (And it's a seriously confusing physics miracle that that much is possible).

For point source astrometry, there are a few ways to beat the diffraction limit. For instance, some observers will purposely defocus their optics to spread the target photons over a larger number of pixels, which with sufficient SNR lets you gain a better lower bound (from a Cramer Rao perspective) on precision. I think Gaia actually does something similar to this but “blurs” through time, rather than across space, by purposely not perfectly tracking stars so that they drift at sub-pixel rates across the FPA.

Yes, Gaia slowly spins to that the stars move across its CCD array.

It takes about 14 pictures of each star during orbit (which is quite close to Earth’s orbit around the Sun), so approximately once per month, and then compares those to calculate the star’s distance from the parallax.

And Gaia also has a downlink speed of approx 3Mbps. So it will process as much as possible on board and just send down less than 20 pixels per each star imaged. That is why you can not get a direct image out of it.

IIRC Gaia had a performance degradation because of stray light, probably ice on the border of it's aperture[1].

How has that affected this result?

[1] https://blogs.esa.int/gaia/2014/06/16/preliminary-analysis-o...

It was not ice, but fibers from the sun shield. The ice issue was resolved by heating the satellite. The stray light issue affected spectra measurements, but not the astrometric side of the mission

I wonder if it could keep giving us useful data without the precision rotation? Intuitively it seems like we should be able to figure out where it's pointing by star-matching plus dead reckoning based on the last frame.

It's possible...but the point of this instrument is to measure star locations very precisely. It probably has a star tracker for positioning doing what you're suggesting. If you were to use that type of positioning info you could introduce inaccuracies into the measured data eventually.

Also, every mission comes to an end eventually - better to do it in the right way and have the right amount of propellent saved for either a graveyard orbit or de-orbiting. It met the mission timeline and goals.

Yea ok. Still, it seems like it could produce a lot of very useful data if switched to a blind spinning mode.

The current coordinate system is based on extremely distant radio sources. Ground based scopes found some bright galactic sources which GAIA aligned to, and is measuring everything relative to those. And now GAIA is the defining source of the ICRS for optical observations.

when you're trying to take relative measurements of the motion of objects within the field of view, which is how you do these fine position measurements, you don't have a lot of choices for objects with a measured location accuracy similar to the measurements you're taking. You're measuring the precise position of something compared to other things you don't know the precise position of. That requires a very stable platform so you can do comparisons internally. There are lots of options for precisely determining where you're pointing but pretty much all of them involve a loss of precision whenever the platform moves, so if you have to slew to a guidestar and back your accuracy is limited to the measurement of the guidestar and then the error induced when you slewed. You also end up using different instruments for alignment measurements vs. the actual observation (for example because the observation requires a long time at a static position), and there's an imprecision involved in comparing the different instruments because they slightly move relative to each other with thermal effects and so on. When you really get into it, using some instruments will reduce the precision of other instruments because they vibrate the platform or create heat. You have to account for all of this with a complex model.

In practice these highly precise measurements, at least in the domain I'm familiar with, become sensor fusion problems where you take a lot of sources of position info, weight them based on their accuracy, and integrate them over time. The less stable the platform, the more error is induced by the integrating over time. Nothing in that realm is really all-or-nothing, as we're seeing with Hubble as it racks up more and more failures, but the loss of the rotation will mean more error in combining position references which will mean less accurate final observations. They may no longer be that much more accurate than measurements obtained by other means.

I'm not sure if I explained that very coherently, it's a complex field that I used to write software in but, well, I was the person writing the software, not the person figuring out the theory. The general idea is that space-based instruments tend to have a bunch of different factors that go into their final accuracy and that accuracy normally gets worse over time as you run out of fuel and things degrade and ultimately stop functioning. Fortunately since space-based systems cost so much to build and launch, the teams behind them have usually put a lot of thought into how they'll continue to get the best use out of them as they get older. That often means having future plans for different missions that just don't require as much accuracy, which is the case with Gaia---it's ending this "phase" of the mission plan.

Farewell, friend. Hello, LSST.

Has anyone created a 3D map, available via web and with ability to fly through, jump to stellar objects by name, look around, etc?

Hope they have captured an image of Planet Nine somewhere there, and eventually are able to pinpoint it.

I don't think it would have a detection.

Gaia is a a satellite for mapping star positions and any possible planets past Neptune would be very faint.

It's also not a telescope in the traditional sense, it's more like a bar scanner in a supermarket but it's spinning around.

I'm really curious whether it exists and / or if they will ever be able to find it, it's wild to consider they can map the galaxy but can't spot something that's relatively close to us. But, detection relies on reflected sunlight or blotting out stars etc behind it, and given the supposed distance that would be minimal.

That said, if it does exist I'm sure they'll find it eventually.

Uh-huh, just in time it seems.

"Gaia’s fuel tank is now approaching empty"

Well, congrats to all involved to such a supremely successful and important mission. When I went to school, it was said that astronomers were happy if they get the order of a measurement right. No such excuses anymore (at least for some 2 billion "nearby" objects)!

And now to use the data to make the most realistic scifi game. With correct stellar motion during relativistic travel.

Elite Dangerous already has more known star systems (160k) in the Milky Way than you can realistically visit - and the rest (400 billion) filled in with plausibly simulated systems: https://elite-dangerous.fandom.com/wiki/Galaxy#Milky_Way - it even kind-of predicted a star system that was only discovered after the game was released: https://elite-dangerous.fandom.com/wiki/Trappist-1#Impact_of...

I didn’t realize a game actually used an approach like this, really cool. Reminds me of the simulated MMORG world in Neal Stephenson’s README, where they tried to simulate the formation of the planet to get realistic mineral deposits and topology.

REAMDE

Oops, thanks!

I just looked this up, sounds like a fantastic read. I'll add it to my backlog.

Thing is, they can make a predictive model based on known observations, then on this scale they will get it right or right enough plenty of times. I'm here for it. I hope long term (VERY long term) humanity will go to these stars predicted by Elite and name planets after the players that first discovered them, lol.

Now that most stars are mapped, next step: map all the planets.

I really wish they would have identified Gaia as some kind of satellite. Gaia is also a name for Earth itself.

You may want to know: the high-res images which are offered for downloading contain the same image which is shown on the page, that is, the infographic.

Not worth the download, as I thought that it would contain a huge panorama of the sky.

The downlinked data is claimed to be 142TB compressed. I suspect that the huge panorama might be a little big for your computer.

For real data you can use Gaia ESA archive: https://gea.esac.esa.int/archive/

I went to study MSc in Space Science and Technology as a hobby few years ago. In one course (2022) we had an assignment to find Supernovae from recent Gaia data (Python code). Then made sure this is observable by University’s robotic telescope (and compliant with local weather forecast). Next requested the observation from the telescope and if successful, received the pictures next day. Had to analyse the results as well. It surprised me how much data there actually is available in quite open format from ESA missions.

Controlling remote telescope few thousand kilometres away was also a nice experience.

Which university if you don't mind me asking? I'm interested in doing something similar as a hobby.

The Open University in UK (living in Estonia myself): https://www.open.ac.uk/postgraduate/qualifications/f77

The course was S818.

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