The Deepest Photo Ever Taken 218
Astroturtle writes "Astronomers using the Hubble Space Telescope's powerful new Advanced Camera for Surveys (ACS) have taken the deepest visible-light image ever made of the sky. The 3.5-day (84-hour) exposure captures stars as faint as 31st magnitude, according to Tom M. Brown (Space Telescope Science Institute), who headed the eight-person team that took the picture."
3.5 Day Exposure? (Score:5, Interesting)
It would be interesting to know... (Score:4, Interesting)
Details on the exposure techniques? (Score:5, Interesting)
Derek
All in all... (Score:4, Interesting)
What's up with the points? (Score:4, Interesting)
The biggest example I see is about 3/4s of the way to the right and about 1/5 of the way down on the image, where there is a huge-looking star.
Why four points? Why do we see them even when the star itself is not in the picture (look on the top border for examples, like the one almost directly in the middle)? I guess I would expect that if the light source is too bright the spread would be in a circular formation and simply blur the star, not blur it in just those four directions so much stronger then the rest.
Is it just QM at play? If so, why it is almost always directly up, down, left, and right, instead of random and perhaps even changing over time directions (which probably would get right back to simply looking blurred)? Detector flaws?
Re:Details on the exposure techniques? (Score:5, Interesting)
Interesting. I was actually thinking more along the lines of automatic compensation, but I hadn't even thought about gyroscopes vs. impulse jets. I poked around a little on the hubble site for the instrumentation and flight computer and I found the handbooks for the instruments at this site [stsci.edu]. Appearently, the gyroscopes are used for coarse motion detection and the FGS uses constellational guidance. The manuals actually make a pretty interesting read.
On a side note, a constellational guidance is related to how head mount displays like UNC's HiBall [unc.edu] work.
Derek
The full size TIFF screensaver? (Score:2, Interesting)
I'd like to see it zoom in to the picture, while also changing x/y of the camera on a spline (etc). And each time choose a different starting point, and make it's speed adjustable.
All of those high-res pics are beautiful! maybe i'm 'a gonna dust off the 'gl..
Re:Streaks (Score:3, Interesting)
I'm just curious here, what are they? I thought maybe it could be a bit of space debris that whizzed in front of the camera, but with an exposure of 3.4 days, the streak would go from one side or another.
The streaks are probably something that moved, though some of them seem brighter in the center, which would indicate it was oscillating. I'm not exactly sure. Anything could move any distance in 3.4 days.
And that big bright cluster in the lower bottom, what's that? It looks pretty close galaxy-wise.
The bright cluster is probably a globular cluster, which is a tight grouping of old stars. It's most likely in our galaxy.
It's a neat pic for sure, a little blurry, which makes it less jawdropping than other hubble efforts but makes sense for a 3.4 day exposure.
Scale it down and it's definitely not blurry. At least not the 6116x7014 image.
lower bottom
What the hell?
Big Picture... (Score:5, Interesting)
Re:Very impressed... (Score:5, Interesting)
M 31 is 2.2 million light-years away. This is the galaxy that Hubble originally resolved into stars, thereby settling the Shapley-Curtis debate [nasa.gov] on the true scale of the Universe. However, the stars Hubble saw were the very brightest supergiants in M 31. In this HST image, we see stars 2 magnitudes fainter than the ancient main-sequence turn-off; i.e., stars which are intrinsically fainter than our Sun! This lets us learn a lot about the ages and chemical composition of M 31's halo stars, which turn out to be quite different from the stars in our halo (our halo is entirely composed of ancient, metal-poor stars; M 31's halo contains stars that are only 6 Gyr old, and much more metal-rich than our halo).
I heard Tom Brown give a talk on this work last week; very cool stuff.
The imag is not blurred; thats whats interesting! (Score:3, Interesting)
This Nature [nature.com] article describes how....hmmm I had better quote:
"As a beam of starlight hops towards us through countless Planck times, its speed varies. This would smear the beam out so that different parts arrive at different times and distort our picture of where it came from. The longer the journey, the bigger the smear."
So that means that these deep Hubble photographs should all get more blurry the deeper you look and not razor sharp like we have come to love.
Its a fascinating problem!
How many stars are in the "visible" sky? (Score:3, Interesting)
a = angle subtended by capture (radians) = 2*pi*3.1/(360*60) = .000901
.0000008118
.0000000646
b = area of capture on a sphere of radius 1 =approx= a^2 =
c = fraction of entire sphere = b/(4*pi) =
d = number of visible stars in entire sky = 300000/c = 4,643,000,000,000
So that means almost 5 trillion stars are visible by Hubble in the entire sky. That's a lot of stars to catalog. (Assuming I didn't err like they did in the $97 trillion RIAA calculation... someone pls double check and flame me if appropriate.)
helps to be a professional astronomer... (Score:5, Interesting)
Let's suppose that the picture was taken in the "V" filter. I just happen to have the number of photons per second per meter squared that arrive from a star of 20th magnitude: 86.157. (taken from here [utoronto.ca]).
So the faintest stars in this picture are 31st magnitude? That's 11 mags fainter than 20, which by the handy old formula
mag1-mag2 = -2.5 * log(flux1/flux2)
which means that the 30th magnitude star puts out about 4x10^(-5) times as much flux.
Using the reference star's flux from above, this means that 0.0034299 photons per second per meter squared arrived at Hubble. The exposure was 84 hours, and the area of Hubble is (2.5m)^2*pi, so tada:
The total number of photons in the picture from the faintest star is: 20365.83
Still not too shabby. They probably could have found even fainter stuff.
Re:How many stars are in the "visible" sky? (Score:3, Interesting)
Your mistake is the assumption that this image is representative of the entire sky's stellar density. HST was pointed near the Andromeda Galaxy for this image; almost all of the stars you are seeing are in the Andromeda Galaxy. Most points on the sky will have a much lower density of stars. See, for example, the Hubble Deep Field [hubblesite.org], which was purposely pointed at an "empty" region of sky, and which contains only a handful of stars.
Re:A Galaxy ... (Score:3, Interesting)
Re:How many stars are in the "visible" sky? (Score:3, Interesting)
Re:Details on the exposure techniques? (Score:2, Interesting)