APOD: 100 Million Stars in the Andromeda... (2015 Jan 06)

[APOD Robot]

100 Million Stars in the Andromeda Galaxy

Explanation: What stars compose the Andromeda galaxy? To better understand, a group of researchers studied the nearby spiral by composing the largest image ever taken with the Hubble Space Telescope. The result, called the Panchromatic Hubble Andromeda Treasury (PHAT), involved thousands of observations, hundreds of fields, spanned about a third of the galaxy, and resolved over 100 million stars. In the featured composite image, the central part of the galaxy is seen on the far left, while a blue spiral arm is prominent on the right. The brightest stars, scattered over the frame, are actually Milky Way foreground stars. The PHAT data is being analyzed to better understand where and how stars have formed in M31 in contrast to our Milky Way Galaxy, and to identify and characterize Andromeda's stellar clusters and obscuring dust.

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[bystander]

http://spacetelescope.org/news/heic1502/
http://hubblesite.org/news/2015/02

http://asterisk.apod.com/viewtopic.php?p=237447#p237447

[Nitpicker]

That's a lot of stars ... and they're heading our way!

Just downloaded the "bigger" image of 35 million pixels. I suppose the version of the image resolving 100 million stars is many times bigger again. That's okay, my brain is already hurting.

[Ann]

To me this looks like a typical "scientific" image, although likely prettified a little for public use. Its aim, as far as I can understand, is to find out as much as possible about the stellar populations of Andromeda in order to unlock the secrets of of our big sister galaxy's star forming history. But Hubble has done its photography job using only two filters, 475W and 814W. I'm glad that one of the filters is pretty blue, at least. The other is an infrared one.

I can't help being reminded of Gaia's mission in our own galaxy. Gaia is going to map one million stars in the Milky Way and determine their parallaxes and therefore the distances to them. Gaia is equipped with only two filters, a red and a blue one. So while its astrometry is going to be superb, its spectrophotometry will leave something to be desired.

It seems to me that Hubble has done something resembling what Gaia will do, except that the stars it has photographed are in Andromeda rather than in the Milky Way. Hubble has photographed a hundred times more stars than Gaia will measure, but Hubble probably hasn't measured any parallaxes in Andromeda.

Ann

[Nitpicker]

It seems to me that Hubble has done something resembling what Gaia will do, except that the stars it has photographed are in Andromeda rather than in the Milky Way. Hubble has photographed a hundred times more stars than Gaia will measure, but Hubble probably hasn't measured any parallaxes in Andromeda.

Ann

Gaia is cutting-edge in terms of its parallax measuring ability, but even so, it is limited to stars in the order of "tens of thousands of light-years from Earth", or closer, very much within the Milky Way. Nothing in humanity's arsenal can measure such small angles as the parallaxes to stars in Andromeda.

[starsurfer]

The Andromeda Galaxy is amazingly complex! It even contains some Wolf Rayet nebulae! I think there is a project to study its outer tidal tails and halo.

Also the clue for tomorrow's APOD isn't cryptic enough!

[rgendler]

Unfortunately APOD still has the developement site link I originally gave them which requires a username and password. Hopefully they will make the correction early today. The correct links are below http://hubblesite.org/newscenter/archiv ... es/2015/02
and ESA
http://www.spacetelescope.org/news/heic1502/

Below are some links which may be helpful.

My personal web page of PHAT M31 with a selection of image sizes and high res crops.
http://www.robgendlerastropics.com/M31-PHAT-Mosaic.html

A composite image I made filling out the composition and HII regions using the PHAT data
plus Subaru/DSS/NOAO data.
http://www.robgendlerastropics.com/M31- ... -NOAO.html

A short video illustrating the massive task of assembling several hundred ACS frames.
http://www.robgendlerastropics.com/PHAT-Tiling.mp4

[Boomer12k]

Maybe someone over there is doing the same thing with us????


Science is soooo amazing....

:---[===] *

[songwriterz]

"My God, it's full of stars"

Then again, it is a galaxy, so that's kinda what its supposed to be. But still quite beautiful.

[geckzilla]

I'm glad that one of the filters is pretty blue, at least. The other is an infrared one.

It's on the very short end of the infrared wavelengths, so if shorter wavelengths are your favorite, these are both the bluer ones. If you had infrared eyes you would say this is a pretty infrared. Hubble only has one bluer infrared filter and it peaks at 775 nm, only 39nm away from the 814W filter. They're both pretty blues by infrared standards.

[Ron-Astro Pharmacist]

If I had 2 dollars for every star then I just won tonight's Mega Million lottery. Unfortunately the chance is 1 in 3 times that many stars.

[Chris Peterson]

So while its astrometry is going to be superb, its spectrophotometry will leave something to be desired.

Spectrophotometry of stars is a specialized thing. With only rare exceptions, stars are blackbody radiators- their spectrum is determined by their temperature. And as near ideal blackbodies, that spectrum can be fully characterized by just two samples at different wavelengths. That's why we see bichromatic measurements used so often. Adding one (or a few) more filters provides almost no additional information, but is very costly in terms of imaging time.

Actual stellar spectrophotometry is useful for determining finer detail- metallicity, for instance. But it can't typically be done with filters, requiring instead much higher spectral resolution. An entirely different type of instrument.

[PhotoginTN]

Why does the Hubble telescope have a rectangle sensor and not a round one - or at least a hexagon one? A rectangle or square one seems a waste of a good part of the precision-ground mirror.

[Chris Peterson]

Why does the Hubble telescope have a rectangle sensor and not a round one - or at least a hexagon one? A rectangle or square one seems a waste of a good part of the precision-ground mirror.

There are many technological problems with non-quadrilateral sensors. Having an equal number of rows, and an equal number of columns, allows for special rows and columns along the sides used for calibration, and vastly simplifies the readout timing. Currently, the best performance sensors are all CCDs, and the nature of CCDs is such that you pretty much have to have a square or rectangular sensor.

The current HST camera does use square detectors- 4Kx4K UV/vis and 1Kx1K IR. Earlier cameras utilized different sensors and optical paths that were slightly less efficient taking advantage of the full image plane.

[geckzilla]

Why does the Hubble telescope have a rectangle sensor and not a round one - or at least a hexagon one? A rectangle or square one seems a waste of a good part of the precision-ground mirror.

In addition to what Chris mentioned about technical difficulties, it's worth noting that it's also doing other things outside of that rectangle. There are other cameras, some of which operate in parallel, so Hubble is taking two pictures at once of two different pieces of space. There also are some other instruments which operate on their own and take their own little slice of the pie. It's modular so all instruments can be added or swapped out for new ones if you can get up there.

I heard they are just going to let it burn up later but you've still got maybe a decade to design the ultimate sensor which will take up the entire focal plane and plan for a salvaging mission to raise the telescope and replace its instruments if you'd like.

[bystander]

Galaxy seen shuddering from ancient collision
Nature | Ron Cowen | 2015 Jan 05

The Nature and Origin of Substructure in the Outskirts of M31 – II. Detailed Star Formation Histories - Edouard J. Bernard et al

• Monthly Notices of the RAS 446(3) 2789 (2015 Jan 21) DOI: 10.1093/mnras/stu2309
  arXiv.org > astro-ph > arXiv:1406.2247 > 09 Jun 2014 (v1), 30 Oct 2014 (v2)

[BillBixby]

Why does the Hubble telescope have a rectangle sensor and not a round one - or at least a hexagon one? A rectangle or square one seems a waste of a good part of the precision-ground mirror.

Thank you for asking such a great question. The two answer/comments made the discuss area very special in the way i have come to love the APOD.

Bill

[Ann]

So while its astrometry is going to be superb, its spectrophotometry will leave something to be desired.

Spectrophotometry of stars is a specialized thing. With only rare exceptions, stars are blackbody radiators- their spectrum is determined by their temperature. And as near ideal blackbodies, that spectrum can be fully characterized by just two samples at different wavelengths. That's why we see bichromatic measurements used so often. Adding one (or a few) more filters provides almost no additional information, but is very costly in terms of imaging time.

Actual stellar spectrophotometry is useful for determining finer detail- metallicity, for instance. But it can't typically be done with filters, requiring instead much higher spectral resolution. An entirely different type of instrument.

But a problem for Gaia will be that it will be hard to know, using only a red and a blue filter, whether a red-looking star is really a cool star, or if it is a hot star that is dust-reddened.

Ann

[Chris Peterson]

But a problem for Gaia will be that it will be hard to know, using only a red and a blue filter, whether a red-looking star is really a cool star, or if it is a hot star that is dust-reddened.

True, but as survey instrument designed for astrometry, that doesn't really matter. Using additional filters would help, but at the expense of reducing throughput (actually, Gaia doesn't use filters, but prisms, so additional channels could potentially be added- an engineering decision, I'm sure). And only a tiny fraction of stars that Gaia will be examining (well under 1%, I'd think) are significantly reddened by scatter, so in reality the color indexes obtained will be quite good. And most of the stars Gaia will be observing are already cataloged, so there may be other information available. Finally, its spectrometer provides additional information that can be used for determining if reddening is occurring. So all in all, a great deal of high quality information should be available.

[Byork]

The Hubble Space Telescope needs to map the entire disc and field of the Andromeda Galaxy. The Robert Gendler portraits of the Andromeda Galaxy set the standard on how earth-based optics can depict the nearest major spiral galaxy with precision using professional optical gear and sound judgement. Galactic structures depicted in those portraits revealed complex braided patterns in the spiral arms of M31. Apparently more than just stars and dust is involved in the dynamic process shaping spiral integration. The Andromeda Galaxy needs to be the subject of numerous Hubble Space Telescope surveys to chart individual stars and star groups and spiral structure in the major spiral galaxy nearest to Earth. Identification of objects such as individual star clusters and nebulae may be feasible given the resolving power of HST.

[Chris Peterson]

Apparently more than just stars and dust is involved in the dynamic process shaping spiral integration.

Stars are probably not a major factor, except in terms of their total mass. Dust is a minor contributor. Gas is probably the major factor. It's unlikely anything else is involved.

[FloridaMike]

With only rare exceptions, stars are blackbody radiators-...

Chris,

Can you tell us what kind of star would be the exception and not be a blackbody radiator ?

[Chris Peterson]

Can you tell us what kind of star would be the exception and not be a blackbody radiator ?

Carbon stars come immediately to mind, where an unusual atmosphere dramatically alters their color and spectrum.
I was referring generally to the rare cases where some mechanism results in a spectrum that doesn't look like a blackbody, and therefore a bichromatic measurement can't be reliably used to determine temperature. The dust reddening that Ann mentioned would do that even for true blackbody radiators.

[starsurfer]

The Hubble Space Telescope needs to map the entire disc and field of the Andromeda Galaxy. The Robert Gendler portraits of the Andromeda Galaxy set the standard on how earth-based optics can depict the nearest major spiral galaxy with precision using professional optical gear and sound judgement. Galactic structures depicted in those portraits revealed complex braided patterns in the spiral arms of M31. Apparently more than just stars and dust is involved in the dynamic process shaping spiral integration. The Andromeda Galaxy needs to be the subject of numerous Hubble Space Telescope surveys to chart individual stars and star groups and spiral structure in the major spiral galaxy nearest to Earth. Identification of objects such as individual star clusters and nebulae may be feasible given the resolving power of HST.

Many star clusters and nebulae in the Andromeda Galaxy have already been catalogued, some before the launch of the HST. There are many hundreds of clusters known in the Andromeda Galaxy.

I think something that has an effect on the structure of the Andromeda Galaxy is its possible interaction with the nearby galaxy M33. I'm not sure but I think a tidal tail has been found between the two galaxies, I don't have time to research much further at the moment.

[LoneStarG84]

Question: So I downloaded the 348 MB (!) TIFF file and zoomed in. In between the Milky Way foreground stars you can see hundreds of thousands of dots. Are these actually Andromeda's stars or digital noise?