Explanation: This grand allsky view of our Milky Way and nearby galaxies is not a photograph. It's a map based on individual measurements for nearly 1.7 billion stars. The astronomically rich data set used to create it, the sky-scanningGaia satellite's second data release, includes remarkably precise determinations of position, brightness, colour, and parallax distance for 1.3 billion stars. Of course, that's about 1 percent of the total number of stars in the Milky Way. Still, the flat plane of our galaxy dominates the view. Home to most Milky Way stars it stretches across the center of Gaia's stellar data map. Voids and rifts along the galactic plane correspond to starlight-obscuring interstellar dust clouds. At lower right are stars of the Large and Small Magellanic Clouds, neighboring galaxies that lie just beyond the Milky Way.
That's a fantastic picture. However, Gaia is giving us amazing parallax and hence distance information. What I really want to see is a 3D map of the Milky Way from it. And definitely one I can play with. (Hmmm ... I sound a lot like a spoiled child.)
Re: APOD: Gaia's Milky Way (2018 Apr 27)
Posted: Fri Apr 27, 2018 6:23 am
by Ann
I am hugely interested in what Gaia will be able to tell us.
However, today's APOD makes me slightly confused. We see a lot of dust structures in our own galaxy, and we can very clearly see the Large and the Small Magellanic Clouds. We can very clearly see the yellow bulge of our galaxy, and we can tell the difference between the yellow inner bulge and inner disk and the non-yellow outer parts of our galaxy.
But where are all the bright stars? There are a handful of obvious bright individual lights in the picture, but they are clearly globular clusters. To the left of the Small Magellanic Cloud at bottom right in the picture is a bright light, which is our galaxy's second brightest globular cluster, 47 Tuc. The brightest of our galactic globulars, Omega Centauri, can be seen in splendid isolation in the right-hand part of the picture, just above the plane of the Milky Way. But again, where are the stars?
The Pipe Nebula and the Antares region.
Photo: Martin P Campbell.
Take a look at the picture at left. The Pipe Nebula is the very black and slightly pipe-like dust structure at left. To the right in the picture is Antares and the Rho Ophiuchi region. The brightest star here is Antares itself, which can be seen right below the very orange patch of yellow nebulositiy in Martin P Campbell's picture. To the right of Antares is another yellowish bright patch, which is globular cluster M4.
In the APOD, we can pick out the Pipe Nebula and the dusty tendrils rising high above the plane of the Milky Way. These lofty tendrils point us to the place where Antares and the Rho Ophiuchi region ought to be. But there is only one bright spot to be seen here in the APOD, and I believe that the one bright spot is globular cluster M4.
I'm pretty certain that Gaia simply can't photograph bright stars, because their light would overload Gaia's sensors (or something). To me, however, the lack of bright stars is confusing, making it hard for me to orient myself in the picture.
In the left-hand part of the APOD, there is only one individual object that I can pick out - or maybe two, because the "one" object is the Double Cluster. It can be seen as an elongated small bright structure just below the plane of the Milky Way relatively far to the left in the picture.
But it seems to me that we should be able to spot the Andromeda Galaxy. Where is it? Is it below Gaia's horizon in this picture?
And where are the Pleiades? The brightest stars here are too bright for Gaia, undoubtedly. But there are fainter stars here too, and maybe they might be tolerable for Gaia? But maybe they are below Gaia's horizon, like Andromeda?
Ann
Re: APOD: Gaia's Milky Way (2018 Apr 27)
Posted: Fri Apr 27, 2018 12:58 pm
by NGC3314
According to the ESA science summary, the standard Gaia scanning mode works for stars of (green) magnitude 3 and fainter, with a special mode available for brighter stars (not clear to me whether they have actually been done by now). I suspect an equally large issue with the all-sky visualization is that stars are not represented as point-spread functions larger than a pixel, so that there is a practical limit to the brightness range shown. A Gaia map of the Andromeda galaxy from DR2 has circulated on social media, but the only more permanent version I can find so far is from the previous data release. It shows only the brightest individual stars, filtering out the unresolved light from fainter ones (and similarly for M33). It migrant be possible to find that spiral pattern by zooming in to the all-sky map, but I haven't seen it there yet.
Re: APOD: Gaia's Milky Way (2018 Apr 27)
Posted: Fri Apr 27, 2018 1:03 pm
by BDanielMayfield
MarkBour wrote: ↑Fri Apr 27, 2018 4:43 am
That's a fantastic picture. However, Gaia is giving us amazing parallax and hence distance information. What I really want to see is a 3D map of the Milky Way from it. And definitely one I can play with. (Hmmm ... I sound a lot like a spoiled child.)
Yes. This is a tantalizing first look into Gaia's data, but a 2D look at a 3D map is like looking at a color TV show on an old Black and White TV. What I'd really love to see is views of our galaxy from high galactic latitudes, way above the disk so we can really see its barred spiral nature.
Bruce
Re: APOD: Gaia's Milky Way (2018 Apr 27)
Posted: Fri Apr 27, 2018 1:43 pm
by Chris Peterson
MarkBour wrote: ↑Fri Apr 27, 2018 4:43 am
That's a fantastic picture. However, Gaia is giving us amazing parallax and hence distance information. What I really want to see is a 3D map of the Milky Way from it. And definitely one I can play with. (Hmmm ... I sound a lot like a spoiled child.)
There will be an interesting 3D data set, and an app to manipulate it graphically in three dimensions is trivial. But it won't be a very good representation of our galaxy as a whole, because too much of it simply isn't visible in the wavelengths Gaia detects, and because the dataset- both in terms of content and accuracy- is very biased towards what is fairly close to us.
Re: APOD: Gaia's Milky Way (2018 Apr 27)
Posted: Fri Apr 27, 2018 1:53 pm
by Chris Peterson
NGC3314 wrote: ↑Fri Apr 27, 2018 12:58 pm
I suspect an equally large issue with the all-sky visualization is that stars are not represented as point-spread functions larger than a pixel, so that there is a practical limit to the brightness range shown.
Even the most basic star charting programs will offer stars readily seen with binoculars, so covering perhaps 12 magnitudes, or more than 5 decades of brightness. And these are displayed on monitors with about 2 decades of dynamic range. So the programs cheat by making brighter stars larger, giving them a diameter greater than a single pixel. That fools our eyes and brains a bit, but it doesn't present a terribly realistic image. But it's similar to the artifacts created with images.
I agree that this data is being presented to give the maximum possible spatial detail, which means representing each star as a point. That means compressing many orders of magnitude in brightness down to just a few hundred hue/brightness values. The result is that we don't visually detect any of the individual stellar patterns we're accustomed to.
Re: APOD: Gaia's Milky Way (2018 Apr 27)
Posted: Fri Apr 27, 2018 2:00 pm
by neufer
Click to play embedded YouTube video.
MarkBour wrote: ↑Fri Apr 27, 2018 4:43 am
That's a fantastic picture. However, Gaia is giving us amazing parallax and hence distance information. What I really want to see is a 3D map of the Milky Way from it. And definitely one I can play with. (Hmmm ... I sound a lot like a spoiled child.)
Hey, fabulous! What most amazes me (though it should not!) are the dust lanes. They are defined so clearly! But of course they are nothing but the ABSENCE of stars in that direction!
Re: APOD: Gaia's Milky Way (2018 Apr 27)
Posted: Fri Apr 27, 2018 3:27 pm
by Dale Helms
I would like to know where we are on this amazing" map", an arrow saying "you are here". I am serious.
Re: APOD: Gaia's Milky Way (2018 Apr 27)
Posted: Fri Apr 27, 2018 3:48 pm
by Chris Peterson
Dale Helms wrote: ↑Fri Apr 27, 2018 3:27 pm
I would like to know where we are on this amazing" map", an arrow saying "you are here". I am serious.
We are where your eyes are. This is our view of the sky from the Earth. We're outside the window framed by this image.
NGC3314 wrote: ↑Fri Apr 27, 2018 12:58 pm
I suspect an equally large issue with the all-sky visualization is that stars are not represented as point-spread functions larger than a pixel, so that there is a practical limit to the brightness range shown.
Even the most basic star charting programs will offer stars readily seen with binoculars, so covering perhaps 12 magnitudes, or more than 5 decades of brightness. And these are displayed on monitors with about 2 decades of dynamic range. So the programs cheat by making brighter stars larger, giving them a diameter greater than a single pixel. That fools our eyes and brains a bit, but it doesn't present a terribly realistic image. But it's similar to the artifacts created with images.
I agree that this data is being presented to give the maximum possible spatial detail, which means representing each star as a point. That means compressing many orders of magnitude in brightness down to just a few hundred hue/brightness values. The result is that we don't visually detect any of the individual stellar patterns we're accustomed to.
Thanks, NGC3314 and Chris. The point-source representation of stars and lack of "bleeding pixels" explain why we don't see any bright stars in Gaia's image.
Ann
Re: APOD: Gaia's Milky Way (2018 Apr 27)
Posted: Fri Apr 27, 2018 5:49 pm
by BDanielMayfield
heehaw wrote: ↑Fri Apr 27, 2018 3:15 pm
Hey, fabulous! What most amazes me (though it should not!) are the dust lanes. They are defined so clearly! But of course they are nothing but the ABSENCE of stars in that direction!
In this case (as in many things), absence of evidence is not evidence of absence.
NGC3314 wrote: ↑Fri Apr 27, 2018 12:58 pm
I suspect an equally large issue with the all-sky visualization is that stars are not represented as point-spread functions larger than a pixel, so that there is a practical limit to the brightness range shown.
Even the most basic star charting programs will offer stars readily seen with binoculars, so covering perhaps 12 magnitudes, or more than 5 decades of brightness. And these are displayed on monitors with about 2 decades of dynamic range. So the programs cheat by making brighter stars larger, giving them a diameter greater than a single pixel. That fools our eyes and brains a bit, but it doesn't present a terribly realistic image. But it's similar to the artifacts created with images.
I agree that this data is being presented to give the maximum possible spatial detail, which means representing each star as a point. That means compressing many orders of magnitude in brightness down to just a few hundred hue/brightness values. The result is that we don't visually detect any of the individual stellar patterns we're accustomed to.
Thanks, NGC3314 and Chris. The point-source representation of stars and lack of "bleeding pixels" explain why we don't see any bright stars in Gaia's image.
Ann
Even more than that, the brightest stars aren't included in the data set at all. I imagine that it would be difficult to measure the precise positions of very bright stars with this instrument, so they where excluded from the data.
Bruce
Re: APOD: Gaia's Milky Way (2018 Apr 27)
Posted: Fri Apr 27, 2018 6:00 pm
by Craig Willford
Dale Helms wrote: ↑Fri Apr 27, 2018 3:27 pm
I would like to know where we are on this amazing" map", an arrow saying "you are here". I am serious.
If I understand the mapping, the center of the galaxy is in the center, the galactic poles, north and south, are at the top and bottom, respectively and half way around, either left or right, or 180 degrees from the center of the galaxy will be represented by the extreme left and right.
Imagine taking this map and stretching it around you in a sphere. Face the center of the galaxy and keep it there while wrapping the left and right around you until they touch behind you. Then also stretch the top and bottom until they are directly above and below you. When you have finished stretching all of it correctly, you will be enveloped in a sphere.
Craig Willford
Re: APOD: Gaia's Milky Way (2018 Apr 27)
Posted: Fri Apr 27, 2018 7:31 pm
by Ann
I think I may have found the Pleiades after all. There is a faint extended "something" that is vaguely "Pleiades-shaped" near bottom left, below a quite dark dust lane.
But I still can't find the Andromeda galaxy. Unless it is the bright star-like object about midway between the Double cluster and the Cocoon nebula.
Ann, since this is described as a map of the Milky Way, I think only this galaxy and its satellites where included, while other members of our local group where intentionally excluded.
Bruce
Re: APOD: Gaia's Milky Way (2018 Apr 27)
Posted: Sat Apr 28, 2018 1:18 am
by bls0326
The "Not a photograph" link https://apod.nasa.gov/apod/image/milkyway_lund_big.gif is a 1950s drawing version of this photograph. It does show quite a few stars and other astronomical info, including a co-ordinate system. Those folks did a good job also.
Brian
Re: APOD: Gaia's Milky Way (2018 Apr 27)
Posted: Sat Apr 28, 2018 4:50 am
by Ann
bls0326 wrote: ↑Sat Apr 28, 2018 1:18 am
The "Not a photograph" link https://apod.nasa.gov/apod/image/milkyway_lund_big.gif is a 1950s drawing version of this photograph. It does show quite a few stars and other astronomical info, including a co-ordinate system. Those folks did a good job also.
Brian
That map was put together by, I think, Knut Lundmark, and it was definitely made at Lund Observatory, just some 15 miles or so north of where I live. And at the Observatory of Malmö, which I visit sometimes, there is a wall-size version of this map.
Ann
Re: APOD: Gaia's Milky Way (2018 Apr 27)
Posted: Sat Apr 28, 2018 5:07 am
by Ann
BDanielMayfield wrote: ↑Sat Apr 28, 2018 1:00 am
Ann, since this is described as a map of the Milky Way, I think only this galaxy and its satellites where included, while other members of our local group where intentionally excluded.
Bruce
You think so?
Obviously Gaia hasn't been commissioned to measure parallaxes in Andromeda. But would the "Gaia people" intentionally blot out the signal from Andromeda?
I just think that Andromeda is bright enough that it ought to show up in the data. But maybe I'm wrong.
I keep thinking that Gaia's map is showing us M33. Our Local Group smallish spiral would be the grainy object at lower left that forms a triangle with The Pleiades and open cluster M34. In the map, the Pleiades would be the bottom left vertex, M34 would be at top and M33 would be the bottom right vertex.
Ann
Re: APOD: Gaia's Milky Way (2018 Apr 27)
Posted: Sat Apr 28, 2018 4:56 pm
by Chris Peterson
Ann wrote: ↑Sat Apr 28, 2018 5:07 am
Obviously Gaia hasn't been commissioned to measure parallaxes in Andromeda. But would the "Gaia people" intentionally blot out the signal from Andromeda?
I just think that Andromeda is bright enough that it ought to show up in the data. But maybe I'm wrong.
I keep thinking that Gaia's map is showing us M33. Our Local Group smallish spiral would be the grainy object at lower left that forms a triangle with The Pleiades and open cluster M34. In the map, the Pleiades would be the bottom left vertex, M34 would be at top and M33 would be the bottom right vertex.
Again, it isn't simply a question of brightness. It's a question of stellar density, as well. Andromeda has too low a surface brightness, and not many resolvable stars. M33 has a higher surface brightness than M31, and does, indeed, show up in this image. As do several other high surface brightness spiral galaxies in Sculptor, Fornax, and maybe a few more.
Ann wrote: ↑Sat Apr 28, 2018 5:07 am
Obviously Gaia hasn't been commissioned to measure parallaxes in Andromeda. But would the "Gaia people" intentionally blot out the signal from Andromeda?
I just think that Andromeda is bright enough that it ought to show up in the data. But maybe I'm wrong.
I keep thinking that Gaia's map is showing us M33. Our Local Group smallish spiral would be the grainy object at lower left that forms a triangle with The Pleiades and open cluster M34. In the map, the Pleiades would be the bottom left vertex, M34 would be at top and M33 would be the bottom right vertex.
Again, it isn't simply a question of brightness. It's a question of stellar density, as well. Andromeda has too low a surface brightness, and not many resolvable stars. M33 has a higher surface brightness than M31, and does, indeed, show up in this image. As do several other high surface brightness spiral galaxies in Sculptor, Fornax, and maybe a few more.
Thanks for confirming the fact that M33 is indeed visible in Gaia's picture, Chris.
Bright stars in relatively faint M33. Bleeding pixels.
Photo: Antonio Fernandez Astrophotography.
But I'm surprised that you said that M33 has a higher surface brightness than M31. I thought it was the other way round. According to Guide, the "mean surface brightness" of M31 is 12.93 ± 0.6 magnitudes per square arc minute, while for M33 it is 13.86 ± 0.5 magnitudes per square arc minute. Guide also said that the "surface brightness at 25% level" of M31 is 14.37 ± 0.8 magnitude, while for M33 is is 14.79 ± 0.7 magnitude. In other words, M33 would have a lower surface brightness than M31.
But I get it when you say that M31 has too few resolvable stars, while M33 does have many stars that are bright enough to be detectable by Gaia. The stellar populations of M31 and M33 are very different.
So the way to understand Gaia's ability to detect M33 and its inability(?) to detect M31 is to realize that Gaia can't photograph a galaxy "all at once", but only "star by star" or "discrete light source by discrete light source". And M31 has too few sufficiently bright discrete light sources in order to show up clearly in Gaia's picture.
Gaia's Milky Way
Mark throwing a tantrum and leaving home.
