The Detection Of Galaxies In Deep Space

[current]

 

How is it possible that the light emitted in deep space by galaxies ; or any light emitting object , can only be observed by Hubble Telescope by focusing that particular point in space. What one would consider ; empty space . Since the area focused on is just black ; nothing visable seen ?

 

I get the time exposure .

 

Yet if the light emitted is so distant , 13billion light years away or so ; how could it become detectable ?

[exchemist]

How is it possible that the light emitted in deep space by galaxies ; or any light emitting object , can only be observed by Hubble Telescope by focusing that particular point in space. What one would consider ; empty space . Since the area focused on is just black ; nothing visable seen ?

 

I get the time exposure .

 

Yet if the light emitted is so distant , 13billion light years away or so ; how could it become detectable ?

Because a telescope concentrates light. 

 

The Hubble has a diameter of 2.4m, so it can focus all light rays from an area of pi x 1.2 x 1.2 sq metres down onto an image that is a tiny point.

 

So just as a magnifying glass can concentrate heat from the sun enough to set fire to something, a telescope can concentrate enough light from a very distant object to make it detectable. 

[JMJones0424]

This is a good question.  Essentially, you're asking why we can see something if we stare at it long enough when we couldn't if we just briefly observed it.  The area focused on by the Hubble or other instruments when doing deep surveys is decidedly not empty, as those observations show.  There are two things to consider.  

 

First, you have to take into account the angle of view.  This is the portion of the sky you are looking at.  The smaller the angle, the more defined your observation is at a great distance.  For instance, imagine the difference between observing the pixelation of standard definition TV with 4k HDTV from the same distance to the screen.  If you were to set up a viewing apparatus that could observe only one pixel on an SD TV from a fixed point, then the angle from you to that pixel would be far greater than if you were to focus on one pixel of a 4k HDTV.  A smaller angle of focus allows you to determine greater detail.

 

Second, one must consider exposure length.  If you are focusing on an apparently "empty" region of space for simply a fraction of a second, such as you do when you look at a black portion of the sky, then your receptor does not receive enough photons to register as an observation.  However, if you can extend and add all photons received over a longer period of time, then you can "see" things that were previously "invisible".  The difference is simply a matter of photons received over time. Or, alternatively, the longer you collect photons from an observed area, the more likely you are to receive a definitive picture of that area.

 

I am not sure, but perhaps you are confused by the fact that an observer exists as one point on a sphere whose radius is the distance between the observer and the photon emission source.  Because the surface area of a sphere is 4 pi r squared, as the distance between the emitter and the observer increases, the number of photons received by the observer falls by a factor of the distance squared.  Our sun is a phenomenally bright star to us because of our relative distance to it.  However, if we were 100 times further away, then it would appear to be 10,000 times dimmer because 100 squared is 10,000 and we can imagine the photons emitted by our sun as being evenly spread out upon a sphere with a radius equal to the distance between us and our sun.  As that radius expands, the concentration of the photons falls by a power of two.  Capturing enough photons to distinguish distant galaxies takes time. In some cases it takes weeks of observations of the same tiny point in the sky just to accumulate enough received photons to be able to discern far distant galaxies.

Edited March 7, 2017 by JMJones0424

[OceanBreeze]

Yet if the light emitted is so distant , 13billion light years away or so ; how could it become detectable ?

 

Many people have asked just the obverse of that question:  “Why isn’t the night sky full of stars” known as Olbers' paradox.

 

[JMJones0424]

It is my understanding that Olber's paradox isn't an issue as the age of the universe is finite.  Or, to put it another way, it IS the case that we can observe areas of the universe that are less bright than others.  I suspect that you meant to link to this wikipedia page, which explains the situation far better than I could.

https://en.wikipedia.org/wiki/Olbers%27_paradox

Edited March 7, 2017 by JMJones0424

[OceanBreeze]

It is my understanding that Olber's paradox isn't an issue as the age of the universe is finite.  Or, to put it another way, it IS the case that we can observe areas of the universe that are less bright than others.

 

 

Right. I wasn’t raising it as an issue, just an interesting counter point to the question in the OP.

 

 

 I suspect that you meant to link to this wikipedia page, which explains the situation far better than I could.

https://en.wikipedia.org/wiki/Olbers%27_paradox

 

 

Yes, thanks, that was the page I was linking to; don’t know how it changed.