Why Do We See Stars As A Single Point Of Light?

[ConfusedChild]

Given:

 

Current accepted Theory is that the Universe as we know it (observe it) started from a "BIG BANG" from a "SINGLE POINT" around 13 Billion earth years ago

 

AND

 

that the Universe continues to expand

 

Further, recent Deep space observations show that the further out in space you look the closer together the galaxies appear to be, supporting the idea of Universe Expansion.

 

My confusion:

 

If light travelling from the far reaches of the Universe is in fact showing us events that occurred in the past, based on the theory of speed of light, then why isn't the sky either completely full of light or showing stars as arcs of light and or exaggerated donuts and or ellipses?

 

Assuming that a single star is moving away from us ... at 1 billion light years, it was 1 billion light years away from us ... if it is moving away from us then it should be closer to us at some other distance (in light years) ... so the closer in time we observe this object, should it not be in a different location and showing us its path in light ... perhaps this is the Red/blue shift described by scientists ... in that the amount of distance moved isn't enough for us to observe?

 

What I can't get my head wrapped around is the possibility that if Light lasts forever in a vacuum, then every point of light should link back to its origin. Understanding that there are other collisions of matter causing Stars to either die or be reborn then each of those should also show their point of origin.

 

Assuming there is only one Universe and one Big Bang and it is deemed to be 13 Billion years old ... then at point 0 when the Big Bang occurred, if the Universe expanded at the speed of light the maximum size the current universe could be is 26 Billion light years in diameter ... the Earth is somewhere in this orb of the Universe, so when we look out one could assume that in one direction there would be a minimum amount of time we could observe < 13 Billion (the Earth's reference point to the closest extent of the Universe) ... in the opposite direction one would expect that we should be able to see back to the beginning of time > 13 Billion ... If we could see back to the beginning of time, then again I don't follow why we are only able to see the stars as single points of light in space.

 

Perhaps what we see is a result of a focal point reference (a snapshot in time) and that if one were able to change the focal length of a telescope at light speed then we could possibly observe what I describe?

[CraigD]

Welcome to hypography, ConfusedChild! :) Please feel free to start a topic in the introductions forum to tell us something about yourself.

 

Your confusion about the appearance of the night sky and its explanation according to the Big Bang theory seems to me of a common sort, which I think I can clear up straightforwardly.

 

First, we don’t see the remnants of the earliest moments of the Big Bang, when the universe was very small, but from a short period about 377,000 years later, when it became possible for light to travel more than sub-atomic distances without interacting with free-traveling electrons, because the electrons combine with protons and heavier nuclei to form atoms, a period known as the recombination epoch. The universe at this time was large – about 94,000,000 light years in diameter – but about 1000 times smaller than its present day 94,000,000,000.

 

The sky is “completely full of light” from this epoch. However, because it has expanded “cosmically” (vs. mechanically, as could be explained using classical, Newtonian physics), this light, which was originally had the spectrum of a black body at around 3000 K, is expansion redshifted by about a factor of 1000, so that it now has the spectrum of a BB at 2.726 K. This is no longer noticeable in the visible light range, but rather in the microwave range, and is known as the cosmic microwave background.

 

The first stars didn’t begin radiating light ‘til about 150,000,000 years after the Big Bang, at which time the universe had nearly its present diameter. They don’t completely fill the night sky, because there’s a finite number of them in sufficiently large volume of space.

 

From the 17th to 19th century, many astronomers believe the universe was infinite in extent, number of stars, and possible age, in which case why the night sky was not completely filled by them was a real paradox, known now as Oblers’ paradox, for which several now obsolete explanations were proposed.

 

If light travelling from the far reaches of the Universe is in fact showing us events that occurred in the past, based on the theory of speed of light, then why isn't the sky either completely full of light or showing stars as arcs of light and or exaggerated donuts and or ellipses?

Why the sky isn’t solid starlight is explained by the modern resolution of Oblers’ paradox.

 

Why we see stars as points (or, with a sufficiently powerful telescope, discs) rather than streaks is because we don’t see car headlights or other compact moving light sources as streaks, unless we record them over a long period (a long “exposure time”, in camera terms). A fixed telescope with a camera does produce streaked images of stars, due to their apparent motion due to the Earth’s rotation. Nearby bodies such a planets and comets sometimes appear as streaks in long exposures of telescopes moved to counteract the rotation of the Earth, though usually such motion is detected by comparing images from separate exposures many days apart (see “blink comparator” for more). As you speculated, the camera exposure time to see distant stars as streaks is much too short – hundreds or thousands of years long exposure times would be needed. They do appear to move – slight changes in the shape of the constellations between diagrams drawn by ancient Greek astronomers and observations by 17th century astronomers (see this NASA webpage for more).

 

Assuming that a single star is moving away from us ... at 1 billion light years, it was 1 billion light years away from us ... if it is moving away from us then it should be closer to us at some other distance (in light years) ... so the closer in time we observe this object, should it not be in a different location and showing us its path in light ... perhaps this is the Red/blue shift described by scientists ... in that the amount of distance moved isn't enough for us to observe?

No. Redshift and blueshift refer to a shift in the frequency of light in the recognizable peaks of the spectra – the graph of light at different frequencies for a single source – of stars and other objects with know spectra. Distant stars and galaxies moving toward us are blueshifted – their spectra are shifted to higher frequencies – those moving away from us, as most are, are redshifted to lower frequencies. Those that are not getting nearer or farther aren’t shifted.

 

Unlike the CMBR described above, starlight is not much redshifted due to the cosmic expansion, because most of this expansion happened before the first stars formed. It’s shifted mostly because the stars are moving away, or more rarely, toward us. Stars in the Andromeda galaxy, for example, is blueshifted, because it’s moving toward us. Nearly all the other outside of our Milky way galaxy are redshifted, because they’re moving away from us. The amount of redshift is fairly precisely proportional to their distance, a relationship known as the Hubble flow.

[Guest MacPhee]

This is a good question, because it's hard to understand how we can see the stars at all.

 

Consider this - the nearest star is supposed to be 25,000,000,000,000 miles away! At that huge distance, how could a star give out enough light, to let us see it from so far away! The light must spread out and get so faint, that we shouldn't be able to see it.

 

Perhaps the stars are closer than we think, and are actually only small objects orbiting the Solar System. Like Asteroids.

[CraigD]

Consider this - the nearest star is supposed to be 25,000,000,000,000 miles away! At that huge distance, how could a star give out enough light, to let us see it from so far away! The light must spread out and get so faint, that we shouldn't be able to see it.

You’re correct, MacPhee, that light spreads out as it travels. The simple geometry of this is that the intensity of light of a star at a distance of a distance 1x, where x is any amount, is exactly 4 times what it is at a distance of 2x, a relationship called an inverse squared law, and usually written something like [imath]I = \frac{I_1}{r^2}[/imath]. So if star identical to the Sun were at the distance of our nearest neighboring star, Proxima Centuri, which is about 269,000 times as distant from the Earth as the Sun, its light would be about 1/72,361,000,000 times as intense as the Sun’s. Proxima Centuri is actually much (about 1/18700 times) dimmer than the Sun, so even at its close distance, is too dim to be seen with the naked eye, but it’s close neighbor Alpha is about 1.5 time as bright as the Sun, and is one of the brightest stars in the sky.

 

What may be leading you to think that objects 72 billion + times dimmer than the Sun shouldn’t be visible, MacPhee, is relying on an intuitive sense of brightness, rather than using math. Our intuitive sense that light sources can’t be seen at great distances comes largely from everyday experience, such as with electric lights. A 100 W electic light at 1 m is pretty bright, yet at a distance of 10,000 m in clear air, its intensity reduced 100,000,000 times, isn’t visible. A star like the Sun, however, is about 4,000,000,000,000,000,000,000,000 (4 x 10²⁴) brighter than a 100 W light. Such big numbers just can’t be imagined intuitively – you’ve got to use arithmetic.