What happens to the energy emitted by stars?

Factual Questions
21

What do you mean by this?

That’s not what scr4 said, and it’s not correct. If we look in any direction, we do see CMBR photons, since they were emitted uniformly from everywhere. But the observable universe is finite, so we do not see light from a galaxy in every direction, however long we look.

The Hubble Ultra Deep Field that scr4 linked to is evidence of that. It looks back almost to the beginning of time, to when the universe was around 5% of its current age, and it shows that there are a whole lot of galaxies in the observable universe. But the image is still mostly dark space. Looking back that little bit further would not fill in all those gaps with light from galaxies, however sensitive the telescope.

22

And even if you’re looking directly at one of those galaxies, you’re still probably not going to hit anything.

23

But if the universe were infinite in time and space, then the whole sky would be as bright as the surface of a star, because any given direction, you see a star. Apparent surface brightness does not change with distance.

24

In fact, when you think about it, the fact that we detect the CMBR is evidence that there cannot be an object in every direction. The first stars were formed when the universe was around 200 million years old, long after the CMBR was emitted. So the surface of last scattering is behind all the stars. If there were no line of sight that were empty of stars (whether or not the stars were detectable) the surface of last scattering would be completely hidden, so we would not detect the CMBR - every CMBR photon would have hit a star.

25

I was going to say the same thing, but then I found this recent paper in Astrophysical Journal that says

This paper is mentioned by Phil Plait here where he says “No matter where you look—up, down, left, right—and no matter how much you magnify the view through a telescope, at some point wherever you’re looking there’s a galaxy.”

26

I remember reading the Plait article, although I never looked at the original paper. As Plait says at the beginning, it doesn’t claim that there is a whole lot of mass that we didn’t know about, just that it’s distributed into a larger number of smaller galaxies.

And I’m skeptical that it can possibly mean that every line of sight hits a galaxy. Plait says:

That’s not a clear statement at all. If a “part” of the sky is covered “in part”, then there’s still dark space in between, presumably.

27

I think Riemann’s contention is that there are microatto-square-seconds-of-arc where we could see all the way to the limit of the “transparent” universe … and I think he’s right in a kinda sorta nitpicky way … perhaps I should qualify my statement that for any given milli-square-second-of-arc, we’ve received at least one photon in the past 4 billion years …

I excluded the Milky Way because of the vast stretches of dust clouds … vis-a-vie how our atmosphere limits the number of photon striking our telescope mirror …

28

ETA:
If a given mass is divided into a larger number of smaller objects, the surface area is greater, so it makes sense that the paper implies more of the sky is covered. I’ll have to read it properly this evening.

And if the sky is completely “covered” in galaxies, what was the error in my reasoning about the fact that the surface of last scattering would be hidden?

29

Well, no - I might have been wrong, but I wasn’t nitpicking. My contention was that most of the sky (by area) gives a clear line of sight to the surface of last scattering, i.e. you don’t hit a galaxy (whether or not detectable in practice). Maybe that paper says I’m wrong about that, I’m not yet sure.

30

I’m not sure, although the paper says “Since most galaxies are extremely faint and cannot be easily observed with today’s technology, we can only detect the presence of these galaxies through the cosmic background light.” Also, I assume a galaxy doesn’t “block” the CMBR like a solid star would.

31

I’m going to win any argument about who’s the most wrong here …

32

On the contrary, I’m now keen to find out if I was wrong - if so I will have learned something really interesting.

33

Yes, that does point to a possible answer. Every galaxy is mostly empty space, of course. So it may be that every line of sight does pass within the “city limits” of a galaxy, even if most lines of sight do not pass through a star within that galaxy.

34

Of course every line of sight from us will pass through at least one galaxy. The real question is, will it pass through at least two?

35

Then I’m wrong about being wrong … HAH … can’t be any more wrong than that …

36

Also note that the expansion of the universe has red-shifted the light, which reduces it’s energy. Although that loss is tiny compared to the inverse square law.

But to demonstrate just how many galaxies there are follow this link to see the Hubble eXtreme Deep Field images.

http://hubblesite.org/image/3098/news_release/2012-37

Some of those galaxies are 13.2 billion years old, not long after the big bang in the big picture.

37

Lots of the usual highly educated and very intelligent answers. I have a not-so-smart one, which might be a little more useful.

It’s fairly simple: when a star shines, it sends it’s energy out in every direction. As each little energy packets (photons or neutrinos or powdered donuts, or whatever the star likes to emit) move outward from the star, they obey the Newtonian law of keeping on in the line they started from, unless something affects them.

If you are right next to the star, even though each packet is going in a different direction, a lot of them will hit you, and you’ll have what we scientifically refer to as “a bad time of it.” But the farther away from the star you are, fewer packets hit you, because they are headed in other directions. Since the closest star to us other than that big orange thingy is four light years (approximately) distant, almost none of the energy coming from it is directly headed for us.

You could do a small experiment if you like, to visualize it more easily. Point a flashlight at a wall, while standing next to it. Then back away, and notice that the area that is lit up gets larger, but it doesn’t get lit up as brightly. That’s because the photons being “sprayed” on the wall by the flashlight, are going in a lot of different directions, and not just straight ahead. Use a laser pointer, and you have to back away much further to see the effect.

In short, aside from running into the bits of junk between us and the distant stars, NOTHING happens to the energy coming towards us. It’s just that the vast majority of the energy of each star ISN’T coming towards us.

By the way, the reason why the “sky” is dark, isn’t because the stars haven’t been shining long enough. It’s dark because most of the light coming from each star, is going somewhere other than here.

The OP really had the answer right at the outset, when he referred to most of the local star energy going elsewhere, and not “hitting” us. The energy from all the other stars, is ALSO not hitting much, because most of spacetime is empty.

38

What you are describing is the Inverse Square Law

But it is important to remember the crazy * wave particle duality*

Photon’s paths are affected by both gravity or the curvature of spacetime and do not follow the Newtonian model.

But they also act as waves, and the inverse square law is easier to think about in that context, think of a pebble tossed in a pond.

(There are other implications like the expansion of the universe, which causes the red shift and thus reducing the perceived energy level by an observer)

This results in some awesome sights due to effects like gravitational lensing

39

This part is incorrect.

It’s true that light spreads, and its brightness goes down as inverse square. However, in any given patch of the sky (say, in every square degree of the sky), the further you go, the more stars there are in that patch of the sky - because that “patch” also expands as you look further. For every light-year you look further, there number of stars in that slice of the patch goes as square of distance, while the light from each star goes down as inverse square. Which means every slice contributes the same amount of light to you. So if the stars extended to infinity, the brightness of that patch of sky will be infinity. Hence Olbers paradox.

40

nitpick: orange is K; the nearest star is a G (yellow)