Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: jeffkatyspanky on 10/11/2015 01:14:34

Title: Why isn't the night sky white with stars?
Post by: jeffkatyspanky on 10/11/2015 01:14:34

If there are close to an infinite number of stars in the universe (are there?) then my line of sight should end on the surface of a star no matter which direction I look. But there appears to be a lot of space between them. Is that because they are too dim and actually if they all had the same luminosity the night sky would be white? Or is it because there's gas or matter between me and them? But if that's true, wouldn't that matter heat up and glow?

I vaguely remember this being discussed in an intro Astronomy class years (35) ago but I don't remember the answer, nor can I find my old Astronomy book.

Steve

Title: Re: Why isn't the night sky white with stars?
Post by: chiralSPO on 10/11/2015 02:03:42

Yes, this is Olbers' paradox. ( https://en.wikipedia.org/wiki/Olbers'_paradox )

My Understanding of the solution is that while there may be an infinite number of stars, light travels at a finite speed, and space has only been transparent for a finite amount of time, so only the light from a finite number of stars has reached us.

Additionally, the fact that the universe is expanding (assuming this continues) means that the light from some (most) stars will never reach us.

Title: Re: Why isn't the night sky white with stars?
Post by: RD on 10/11/2015 03:49:45

Only part of the universe is "observable" ... https://en.wikipedia.org/wiki/Observable_universe#The_Universe_versus_the_observable_universe (https://en.wikipedia.org/wiki/Observable_universe#The_Universe_versus_the_observable_universe)

Title: Re: Why isn't the night sky white with stars?
Post by: evan_au on 10/11/2015 10:19:25

Even if you only count stars within 13 billion light-years: Their light has reached us, but light from more distant galaxies is red-shifted, which stretches its wavelength, and reduces its received power.

In particular, the light from the Cosmic Microwave Background Radiation (http://en.wikipedia.org/wiki/Cosmic_microwave_background#Relationship_to_the_Big_Bang) is reaching us from all directions. It was emitted when the universe cooled below about 3000°C. The sky does not radiate a temperature of 3000C today because this radiation has been red-shifted so that it has an effective temperature of 2.7K, so it emits no visible light. It is now only visible in the microwave frequency band.

Many stars in the plane of the Milky Way are obscured from our view by dust clouds. But they are only able to remain cold and dark beacuse of the expansion of the universe.