Unraveling the Mystery of Light From Stars

[Justin Kirk]

I always wondered why we see light from stars. If the light travels outward in all directions from a star then you could imagine the light as a growing sphere and the portion we see is a small area on the surface of this sphere. Well if a star is a million light years away this would mean the surface area is about a million million years large. Does this mean there are photons all along the surface of this sphere? If the light are rays coming from the star, wouldn't there be gaps eventually in the rays since they are all pointing outward?

 

[Matterwave]

The photons going other directions aren't detected by your detector (i.e. your eyes). You only see the light that is directed at you in a line more or less. This is true for all objects and all light. Otherwise, all you'd ever see was a blob, and there'd be no definition on anything.

 

[Justin Kirk]

So would it be safe to say the amount of photons on the surface of the "visible sphere" is infinite or incredibly large?

 

[mgb_phys]

So would it be safe to say the amount of photons on the surface of the "visible sphere" is infinite or incredibly large?

Not infinite but pretty large!
At billions of km away your 5mm diameter eye only captures a very few of them

 

[russ_watters]

Note, stars that we see with the naked eye are all thousands of light years away, not millions. For a telescope collecting light from galaxies that are millions of light years away, it is very possible that individual photons will hit the detector (a ccd plate) at a rate of only a handful of photons per second. Or for an individual pixel (representing an individual or a few stars), the photons may come many seconds apart. Part of the reason our eyes can't see very dim objects is that our detectors are not capable of long exposure type photon collecting. The other reason of course is aperture.

 

[mgb_phys]

The brightest stars in the sky give about 1000 photons/cm^2/s/Angstrom at the top of the atmosphere.
Assuming no atmospheric absorption and an eye pupil diameter of 0.5cm you will have about 10,000 visible photons/s entering your eye.
And that's for a star 25 light years, or 200,000,000,000,000 km away

 

[Justin Kirk]

Ok, so couldn't this mean the universe is infinitely large and that there are an infinite amount of objects but there are not enough photons entering our atmosphere for them to be detected?

 

[mgb_phys]

Ok, so couldn't this mean the universe is infinitely large and that there are an infinite amount of objects but there are not enough photons entering our atmosphere for them to be detected?

Not quite, if the universe was infinitely large and infinitely old, then which ever direction you looked you would be looking straight at the surface of a star, and the photon would have had time to reach you - the entire sky would be like looking at the surface of the sun.
This is called Olber's paradox.

Because the sky at night is mostly dark, th universe can't be infintiely large and infinitely old.

 

[Justin Kirk]

good point. But how do we measure the age of the universe?

 

[russ_watters]

good point. But how do we measure the age of the universe?

Among other things, the redshift of the background radiation.

 

[Justin Kirk]

Ok. What we detect has taken some amount of time to reach Earth. If the oldest light we see is say, 15 billion years, could there exist some object 16 billion light years away? Does the amount of objects we see in the sky increase every year?

 

[mgb_phys]

Ok. What we detect has taken some amount of time to reach Earth.

Yes

If the oldest light we see is say, 15 billion years, could there exist some object 16 billion light years away?

Tricky answer.
We can see objects as they were nearly 15Bn years ago but they were much closer when the light left them AND they are now more than 15bn lyr away. The observable universe = the sphere around us that light from the start of the universe could have reached us = is about 45Bn ly radius.
This odd picture is because the universe is expanding faster than light

Does the amount of objects we see in the sky increase every year?

No (except that our detectors get more sensitive so we see nearby fainter objects)
We can see the microwave background, this radiation was created before any stars, it is the earliest object in the universe that emitted light.
We can't see earlier than this.
Further than this is impossible because of the expansion mentioned above.

 

[Chronos]

Star light does not have a blackbody spectrum. This is a failed explanation for the cmb. We won't see starlight from 16 billion light years ago. Stars did not exist then.

 

[russ_watters]

Star light does not have a blackbody spectrum. This is a failed explanation for the cmb.

What does the spectrum look like if not a black body? Both star light and the CMB are light emitted by hot plasma...how is that not a black body?

 

[mgb_phys]

What does the spectrum look like if not a black body? Both star light and the CMB are light emitted by hot plasma...how is that not a black body?

 

[Chronos]

My error, Russ, star light has a very rough black body spectrum. The cmb has a perfect blackbody specturm. See http://www.astro.ucla.edu/~wright/stars_vs_cmb.html for the rest of the story.

 

[Justin Kirk]

Questions. One, hate to be a noob, but what is "cmb"? Two, I understand the universe is expanding but where from? There is a single point from which all matter was created, no? If so, then shouldn't we be able to detect this point in space? If we know how far away stars are, we know how old they are, couldn't the point be figured out from some trigonometry and geometry?

 

[russ_watters]

Oh, I misread that post. I thought "this is a failed explanation for the CMB" meant that a black body spectrum was a failed explantion for the CMB - you meant that red-shifted starlight was a failed explanation for the CMB.

Worse than the little imperfections in an individual star's black body spectrum, a starlight CMB would be comprised of the spectrums of many stars all radiating a different temperatures!

 

[russ_watters]

Questions. One, hate to be a noob, but what is "cmb"?

Cosmological Background Radiation. The universe glows like being inside a fluorescent bulb...though at a much cooler/lower temperature.

Two, I understand the universe is expanding but where from? There is a single point from which all matter was created, no? If so, then shouldn't we be able to detect this point in space? If we know how far away stars are, we know how old they are, couldn't the point be figured out from some trigonometry and geometry?

No, there is no such center to the universe. The entire universe expanded from a single point, yes, but that means that that point is/was everywhere.