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Re: APOD: A Flight through the Hubble Ultra... (2013 Aug 27)

Posted: Tue Sep 03, 2013 2:07 am
by Mizar
Ann wrote:I once built a super-simple model of the inner solar system. I used cotton balls, 0.02 meters in diameter, for the Earth, Venus and Mars. I used yellow peas for Mercury and the Moon, and a round table-cloth, 2 meters in diameter, for the Sun. I then placed the "planets" at the appropriate distances from the Sun. The Earth was about 200 meters from the Sun.
When I was a kid I did an even cruder scale model, really it was just marking the distances of the planets from the Sun across the floor of the house. My sister harshly criticized me saying something like it was stupid. In retrospect, that was more of a self-reflection. :mrgreen:
DavidLeodis wrote:The whole sky contains 12.7 million times more area than the Ultra Deep Field. To observe the entire sky would take almost 1 million years of uninterrupted observing.
That doesn't seem correct that the whole sky could be as much as 12.7 million times the HUDF area, but it's not like I calculated it. That means the HUDF did an exposure of the area for nearly 29 days straight! That's a lot of valuable Hubble observing time.
BDanielMayfield wrote:Great comment David. So the image this simulation was made from covered only 1/12,700,000th of the sky. There was absolutely nothing special about this location in space. This site was simply just an extremely empty looking patch of sky, with no foreground stars or galaxies in the way. In even large land based telescopes this must look like an untold number of other boring looking dark, empty places with maybe only a few, barely discernable faint fuzzies, if anything at all. But keep Hubble focused on this place for nearly a million seconds and voila, 10,000 galaxies appear! (To make the video Dr. Summer’s team estimated the distances of about 5,000 of these.)
A million seconds would be only about 11.5 days for the exposure time. One of you (or both) doesn't have this right.
BDanielMayfield wrote:So this random sampling probes a volume of space of between about 18.4 and 30 billion light-years out and found about 10,000 galaxies. Across the entire celestial sphere this would indicate the presence of 10,000 x 12.7 million = 127 billion galaxies, not counting any less than 18.4 B ly nor any more than 30 B ly away. If the density of galaxies inside the 18.4 billion light-year radius is about the same as that inside the volume probed by the HUDF survey we would have another 38 billion galaxies, so this survey suggests the presence of at least around 165 billion galaxies in the observable universe.
And you're sure that the HUDF didn't image any galaxies closer than 18.4 billion light years?
BDanielMayfield wrote:
DOUGLAS L. MARTIN wrote:HOW CAN SOMETHING TRAVEL THROUGH 30 BILLION YEARS IN A UNIVERSE ONLY 13.7 BILLION YEARS OLD?
We can see objects that are NOW further than 13.7 billion light-years, but no light has been traveling longer of course. But as light travels through space, space itself is expanding, carrying source and observer apart. The estimated distance of 30 billion light-years is the co-moving distance, accounting for the separation that has occurred during the time the light has taken to get to us.
This is a good explanation of how it's supposed to work. The only thing I'd add is the age of the universe has been slightly upgraded to 13.8 billion years.
geckzilla wrote:If a guy throws a baseball at you while he is 13.7 meters away from you and then you both immediately run away from the ball after it's released, by the time the ball hits you in the back of the head the guy and yourself are already 30 meters apart from one another. The distance between yourself and the guy at the moment the ball was released and the distance between the both of you the moment it hit you in the head are two different measurements.
That's a good analogy, except for the 13.7 meters corresponds to the current time (universe is 13.7 billion years old) and the 30 meters corresponds to the far distant age of the universe. Therefore, the analogy is centered on an explanation for 16.3 billion years from now. :)
Anthony Barreiro wrote:Isn't the observable universe estimated to be 42 billion light years across? So if the universe suddenly stopped expanding, light would take 42 billion years to travel across the universe, right?
Yes, but the observable universe is actually estimated at 93 billion light-years. http://en.wikipedia.org/wiki/Observable_universe

I understand why astronomers are calling the observable universe this large. Essentially it's due to the expansion of space, in a nutshell. But I have to say I object to this definition. You're not actually observing anything up to 46–47 billion light-years away (minus the time/distance for the universe to become transparent). This is only observing something that far away as it is at the current time. Looking out in space is looking back in time. So we are not observing anything any further away than 13.8 billion light-years, since we're observing it as it was in the distant past, and at the distance it was then. We're only assuming that what we're looking at now is currently up to 47 billion light-years away, but we don't know that nor do we receive any data from it. For all we know, some mysterious force has halted its recession from us and is really just out of view at 13.9 billion LY, at the current time. For this reason, I don't think astronomers have any business saying the observable universe is any larger than 27.6 billion LY across.

It reminds me of the no-longer-calling-Pluto-a-planet issue. Illogical. :roll:

Re: APOD: A Flight through the Hubble Ultra... (2013 Aug 27)

Posted: Tue Sep 03, 2013 2:18 am
by geckzilla
Mizar wrote:It reminds me of the no-longer-calling-Pluto-a-planet issue. Illogical. :roll:
The only thing illogical about it is to get emotional over the subject.

Re: APOD: A Flight through the Hubble Ultra... (2013 Aug 27)

Posted: Tue Sep 03, 2013 12:54 pm
by DavidLeodis
In your post of Sep 03 2013 3:07 am Mizar you state "DavidLeodis wrote: The whole sky contains 12.7 million times more area than the Ultra Deep Field. To observe the entire sky would take almost 1 million years of uninterrupted observing.
That doesn't seem correct that the whole sky could be as much as 12.7 million times the HUDF area, but it's not like I calculated it. That means the HUDF did an exposure of the area for nearly 29 days straight! That's a lot of valuable Hubble observing time."

I got the information from the Hubble NewsCenter release STScI-2004-07. That will be found through clicking the second 'HUDF' link in the APOD of 2013 Aug 27. That brings up the APOD of 2004 Sept 29. Clicking the 'Hubble Ultra Deep Field' link in that APOD brings up STScI-2004-07, which has the information that I noted. If, as you suggest, the 12.7 million times is wrong then blame the Hubble people rather than your implication that I calculated it wrong.

Re: APOD: A Flight through the Hubble Ultra... (2013 Aug 27)

Posted: Tue Dec 03, 2013 6:06 am
by Mizar
geckzilla wrote:
Mizar wrote:It reminds me of the no-longer-calling-Pluto-a-planet issue. Illogical. :roll:
The only thing illogical about it is to get emotional over the subject.
Who's getting emotional? Not me. But it did create a lot of emotional controversy for a long time. And no, there were a whole load of illogical aspects to it. But, that would be way digressing.
DavidLeodis wrote:In your post of Sep 03 2013 3:07 am Mizar you state "DavidLeodis wrote: The whole sky contains 12.7 million times more area than the Ultra Deep Field. To observe the entire sky would take almost 1 million years of uninterrupted observing.
That doesn't seem correct that the whole sky could be as much as 12.7 million times the HUDF area, but it's not like I calculated it. That means the HUDF did an exposure of the area for nearly 29 days straight! That's a lot of valuable Hubble observing time."

I got the information from the Hubble NewsCenter release STScI-2004-07. That will be found through clicking the second 'HUDF' link in the APOD of 2013 Aug 27. That brings up the APOD of 2004 Sept 29. Clicking the 'Hubble Ultra Deep Field' link in that APOD brings up STScI-2004-07, which has the information that I noted. If, as you suggest, the 12.7 million times is wrong then blame the Hubble people rather than your implication that I calculated it wrong.
Okay, thanks for the source. But, instead of all that nested browsing, don't you have a direct link? I didn't imply you calculated it wrong, I merely noted the contradiction between what you and BDanielMayfield were saying, and hence that one of you (or both) had to not have it right (the info). I said nothing of your calculations, nor that you were the original source of the info. Where did you get any of that from?

Re: APOD: A Flight through the Hubble Ultra... (2013 Aug 27)

Posted: Tue Dec 03, 2013 1:08 pm
by rstevenson
Ignoring the discussion about who said what, here are some numbers, from the Ask an Astronomer site at Cornell University...
... the sphere of the sky is 41,253 square degrees. This corresponds to 148,510,800 square arcminutes.
According to the Space Telescope Science Institute, the Hubble Ultra Deep Field has an angular size of 11.5 square arcminutes. That means that it would take 12,913,983 Deep Field images to cover the entire sphere of the sky!
So the figure of 12.7 million is a bit off, but close enough for horseshoes.

Rob

Re: APOD: A Flight through the Hubble Ultra... (2013 Aug 27)

Posted: Wed Dec 04, 2013 7:04 am
by Mizar
rstevenson wrote:So the figure of 12.7 million is a bit off, but close enough for horseshoes.
From that, it would appear DavidLeodis was more correct than BDanielMayfield. But when I read STScI-2004-07 I noticed where BDanielMayfield must have seen his info:
Called the Hubble Ultra Deep Field (HUDF), the million-second-long exposure reveals the first galaxies to emerge from the so-called "dark ages"...
So indeed, it's a case of the experts having widely varying figures that affect what the exposure time was. Which I never precluded that possibility. It's a fairly common thing. :P

On that Ask an Astronomer page, interesting to note they got the number of galaxies in the observable universe wrong. They were off by two orders of magnitude, as from every source I've heard the estimate should be that
there are over 100 billion galaxies in the universe.
Extrapolation from number of stars

In any case, it's astounding there's 12.9 million times as much sky to examine as the 3.4 arc-minute square area of the HUDF, and that there's 10,000 galaxies in that small an area. My high power eyepiece on my Mak sees just 18.72 minutes across. The HUDF is only 18% of that width of view and only 3.3% the area, yet there's 10,000 galaxies there. :shock:

Re: APOD: A Flight through the Hubble Ultra... (2013 Aug 27)

Posted: Wed Dec 04, 2013 2:41 pm
by geckzilla
With Frontier Fields, Hubble is going to do more ultra deep field observations so that the galaxy estimate can be more accurate. They're doing parallel observations of nearby sections of sky which should be pretty similar to the previous ultra deep fields while looking at lensed galaxies. They're doing six of these! The first one is already done and the data is available. I should go process it!

Re: APOD: A Flight through the Hubble Ultra... (2013 Aug 27)

Posted: Thu Dec 05, 2013 2:16 am
by geckzilla
And so I spent a large proportion of the rest of the day doing just that...
Note if you download the original PNG file by right clicking the image at Flickr, you will be downloading a 12.3 megabyte file.
http://www.flickr.com/photos/geckzilla/11214379925/

Re: APOD: A Flight through the Hubble Ultra... (2013 Aug 27)

Posted: Thu Dec 05, 2013 2:58 am
by rstevenson
geckzilla wrote:And so I spent a large proportion of the rest of the day doing just that...
Note if you download the original PNG file by right clicking the image at Flickr, you will be downloading a 12.3 megabyte file.
http://www.flickr.com/photos/geckzilla/11214379925/
Thank you, thank you, thank you! :D

Rob

Re: APOD: A Flight through the Hubble Ultra... (2013 Aug 27)

Posted: Thu Dec 05, 2013 3:14 pm
by geckzilla