Queston about the darkness of the universe

[phinds]

While listening to a talk on TED by David Deutsch (the talk was "Our Place in the Cosmos" from July 2005), he made a statement which just dumbfounded me. He certainly sounded like a knowledgeable guy in the rest of the talk, but here's what he said. I would appreciate any comments on this.

This is NOT an exact quote or even close, but it IS an exact representation of the meaning of what he said.

A typical place in the universe would be far away from any stars or galaxies and would be, visually, TOTALLY BLACK. You would not see anything. No stars, no galaxies, nothing. In fact, if you were looking in the direction of a typical-distance super-nova at the time when its light arrived at your location it would be so faint that you would not see anything.

HUH ?

 

[BillSaltLake]

In a "typical place" you might be a few 100 mega light years from the nearest galaxy, and its brightest zone is ~0.1 Mly across, which would appear about a minute of arc across. Surface brightness is only a few micro lightwatts/sq meter, which is barely perceivable when looking straight at it (but more perceivable looking off your center of vision, where resolution is lower). These 1-minute-across objects would be almost impossible to see directly, but they would be visible with mid-sized binos with a sufficient exit pupil.

 

[phinds]

OK, thanks. That helps me understand the galaxies (and certainly therefore the stars), but how about the supernova?

 

[BillSaltLake]

A supernova may be as bright as a galaxy. Same power output, spread over a minimum resolvable 1 minute, equals same apparent brightness as a galaxy.

 

[phinds]

I'm really amazed. Thanks for the clear explanation.

 

[Drakkith]

This is similar to looking up at the sky at night and seeing blackness between the visible stars. There is most definitely light there, but our eyes are not sensitive enough to see it. A telescope works by having a mirror collect light from a much greater surface area than our eye and focusing it down to a point about the size of our eye. I have a telescope with a 6 inch mirror on it. It collects around 1000 times more light than my eye, allowing me to see things that are many many times fainter than I could see otherwise. Even the Hubble Space Telescope works pretty much the exact same way.

 

[bcrowell]

This is similar to looking up at the sky at night and seeing blackness between the visible stars. There is most definitely light there, but our eyes are not sensitive enough to see it.

Cool explanation!

 

[phinds]

This is similar to looking up at the sky at night and seeing blackness between the visible stars. There is most definitely light there, but our eyes are not sensitive enough to see it. A telescope works by having a mirror collect light from a much greater surface area than our eye and focusing it down to a point about the size of our eye. I have a telescope with a 6 inch mirror on it. It collects around 1000 times more light than my eye, allowing me to see things that are many many times fainter than I could see otherwise. Even the Hubble Space Telescope works pretty much the exact same way.

Yes, I understand and completely agree w/ what you are saying, but I think you missed the point of my question. You specifically say that to see a dark area, you have to look where there aren't any stars or galaxies BUT my point was that I find it amazing that you could be ANYWHERE in the universe and face total black in all directions ... that is, everywhere you look you are looking at where you can't see anything. If, from the earth, you look away from the band of the milky way, you will be looking on at least some objects that are very far away, and it surprises me to think that all of them are still so close as to support the explanation given by BillSaltLake. I'm not saying that I find that explanation wrong, just that I find it quite amazing. Your own experience seem to support what I'm saying, unless I misunderstand you.

 

[Drakkith]

Phinds, what I mean is that both are the same effect. Between the visible stars in the sky are dmmer stars and galaxies. If you are in the middle of nowhere you might not be able to see anything, not because there isn't anything there to see, but because the light is too faint no matter which direction you look. The only reason the stars in the night sky are visible is because of a combination of their brightness and their distance from us. (Meaning that most are very bright stars that are relatively close to us.) For example, the star Sirius, the brightest star in the northern sky, is very very close to us on a stellar scale, only 8.6 light years away and is 25 times as luminous as the sun. On the other hand, Rigel, the brightest star in the constellation Orion, is less bright to us on earth, yet Rigel is 3400 times as luminous as Sirius is, or 85,000 times that of the sun. It is less bright to us because it is 700-900 light years distant, about 100 times as far as Sirius is.

Out of all the stars visible to the naked eye in the night sky, over 95% of them are MORE luminous than our sun. But get this, 90+% of all stars in our Galaxy are LESS luminous than our sun. This means that only the very brightest or very closest of all stars are visible to us. If ours eyes were able to discern single photons, the night sky would be covered in light. Since we cannot, we only see the brightest and closest stars to us.

Yes, it is amazing when you think of the enormous distances between us and the visible stars, yet when you compare this to the scale of the universe it is almost nothing.

 

[Chalnoth]

Another fun fact: only about 6000 or so stars are distinguishable by the naked eye, while our galaxy includes roughly 400,000,000,000 stars.

 

[phinds]

I think my biggest point of confusion about all this is that I clearly recalled reading that there had been observations of galaxies and supernova from billions of light years off, BUT what I cleverly forgot was that none of that was with the naked eye and in fact some of it was with in the Hubble Deep Field observations which is WAY far from naked eye.

I stand amazed and corrected.