The visible universe

[tballou]

Here is a question I have often pondered - how would the universe appear to us humans from a different perspective than our current one? For example, the Andromeda galaxy appears as a bright shining spiral in photos taken with telescopes, but if we were located half as far away, would it appear any different in our night sky than what we see from earth today? Or would it be like the Orion nebula, where we see a few of the brightest stars and a faint nebulosity only? Our own galaxy appears only as a relatively faint band across the darkest night skies, and we are located inside it - would the Andromeda galaxy appear the same way if we were only 1 million light years, or only 500k ly?

I think the answer to this may relate to the vast scale of galaxies, nebula etc. and their extremely low density of matter - too far away and you can barely see them, too close and they are so spread out you see right thru them, more or less.

[bystander]

The Large and Small Magellanic Clouds are at 157 and 197 kly and we can see them, and they are probably a lot less dense than M31 (Andromeda). If M31 was 5 times closer (500 kly) it would be 5 times larger in our sky, but we could still see it, and what a majestic sight that would be. The Milky Way is a band across our night sky because we are located inside it. We can't see the spiral structure they would see from M31 or M33. IMHO

[Chris Peterson]

Here is a question I have often pondered - how would the universe appear to us humans from a different perspective than our current one? For example, the Andromeda galaxy appears as a bright shining spiral in photos taken with telescopes, but if we were located half as far away, would it appear any different in our night sky than what we see from earth today?

It would appear bigger, but no brighter. It is already a naked eye object that is about the same brightness as the Milky Way, which isn't surprising since it and the Milky Way are pretty much the same kind of thing. As it got closer and closer, it would simply cover more of the sky, with its own light and dark lanes looking similar to the structure we see in the Milky Way from a dark site.

I think the answer to this may relate to the vast scale of galaxies, nebula etc. and their extremely low density of matter - too far away and you can barely see them, too close and they are so spread out you see right thru them, more or less.

It's related to this, but more to the nature of light coming from extended sources. Things can never get brighter than we see them with the naked eye. A telescope makes an object appear closer, but no brighter. If the object actually gets closer, you get more light, but it also covers more area, so the surface brightness doesn't change.

Also, there are almost no extended objects in the Universe that are bright enough to trigger our color vision, so everything looks gray. That doesn't change with magnification or distance, either. So all those space art posters and science fiction shows with ships flying past colorful nebulas are just fantasy. Wherever we ultimately go, we're still just going to see levels of gray with our eyes.

[neufer]

Things can never get brighter than we see them with the naked eye. A telescope makes an object appear closer, but no brighter.

You certainly didn't mean to say that.

Optics limits the brightness of an image to below the original brightness of the object (e.g., a magnifying glass focused sun image never be hotter than the surface of the sun itself) but other than that there few constrants on how bright a telescopic image can be.

[Chris Peterson]

Things can never get brighter than we see them with the naked eye. A telescope makes an object appear closer, but no brighter.

You certainly didn't mean to say that.

Optics limits the brightness of an image to below the original brightness of the object (e.g., a magnifying glass focused sun image never be hotter than the surface of the sun itself) but other than that there few constrants on how bright a telescopic image can be.

I meant exactly that. No imaging system can increase the brightness of an object over its intrinsic brightness. No image through a telescope is ever brighter than what you see looking at the same thing with your naked eye.

[neufer]

Things can never get brighter than we see them with the naked eye. A telescope makes an object appear closer, but no brighter.

You certainly didn't mean to say that.

Optics limits the brightness of an image to below the original brightness of the object (e.g., a magnifying glass focused sun image never be hotter than the surface of the sun itself) but other than that there few constrants on how bright a telescopic image can be.

I meant exactly that. No imaging system can increase the brightness of an object over its intrinsic brightness. No image through a telescope is ever brighter than what you see looking at the same thing with your naked eye.

Quite right!

Extended astronomical objects that are resolvable by the naked eye (e.g., the moon, Jupiter's disk, comets, M31, zodiacal light, the sky itself)
cannot be made brighter by using a telescope as the increase in photons is solely due to the engagement of more photo receptors in the eye. It is only unresolved astronomical objects like stars that are made brighter (e.g., the brightness of an Airy disk increases as {objective diameter divided by wavelength}).