Why looking further into space = looking at the beginning of the universe?

Question

I mean, if the stars in the clusters you are observing are of particular size and not as bright - does that mean they are running out of fuel, e.g. the cluster is old; or they are just too far away? How you can determine that this cluster is 2b years old, and not just way out of the way?

Answer 1

"...if the stars in the clusters you are observing are of particular size and not as bright - does that mean they are running out of fuel..."

Not as bright as what? Say the cluster is 500 light years away. We'd have to study it for 500 years before we could see any difference in the stars' brightness and/or size. If we observed the cluster tomorrow and noticed that the stars were dimmer and smaller then we'd know that the change had occurred 500 years ago.

"...How you can determine that this cluster is 2b years old, and not just way out of the way?..."

One of the surest ways to get a handle on the stars' ages would be to determine what element(s) are fueling their nuclear fusion. The heavier the element, the older the stars.

Answer 2

When you look far out into the universe, all you're seeing is the light that's finally hitting Earth. Remember that light travels only at the speed of light - it's not instantaneous. So actually, the stuff you're looking at has probably changed, but we won't know that until a few billion years have passed on Earth, when the new batch of light hits. When a supernova from three light years away explodes, we don't see it explode until three years later - because that's how long the light from the explosion takes to get to us.

Also, you're talking about the dimness of stars' lights. The reason we use other gadgets (like radio wave detectors and heat sensors) is for this very reason - judging by light alone, you can't tell if it's a dim star, or just far away. So we measure other forms of radiation coming from that star, to determine that.