24 people. A clear Friday Night - a thing which has almost been an oxymoron this year. The viewing wasn't great, with a thin layer of high clouds but at least we could see things including a few faint fuzzies. Venus was both brilliant and very definitely "half moon" shaped at about 43% illuminated. We are so used to having the Moon brightest at full moon that it is odd that Venus is far from its brightest at "full Venus" but rather between then and "new Venus". This week we reached the point in the combination of the Earth and Venus orbits with the greatest elongation (angular distance of Venus from the Sun) but we won't reach maximal brightness for another 30 days or so. Venus changes its size more than any other planet. At its closest, its angular width is more than 7 times its angular width at its most distant. It is the combination of an advantageous angle and proximity which produces the largest illuminated surface of Venus - the period when Venus is at its brightest.

In all the years that I have chatted with folks about astronomy, no one has ever asked me a question which someone asked last night. A gentleman asked "Why is the sky dark at night?" This apparently naive question is actually a very subtle question that plagued astronomers for a very long time. It became named Olber's Paradox after the German astronomer Heinrich Wilhelm Olbers who asked the question in its modern form in 1823. This question had also been asked and not solved by such great astronomers as Johannes Kepler (in a letter written in 1620) and Edmund Halley during the 1700s.

Here is how the Paradox goes. If we assume that the Universe is either infinite in distant or in age, then no matter where we look, we should eventually "bump into" a star in that direction. Every star we see is bright and on average for a given angular dimension they are all about as bright as the Sun. In other words, if we allowed a pinprick of light from the Sun to be compared to any similar sized star's tiny pinprick in size, the light from both stars would be about the same. Oh, to be sure some stars are dimmer than the Sun and others brighter but the Sun is rather representative. So if the Universe is infinite, then the whole sky should shine about as brightly as the surface of the Sun.

Of course it doesn't - and a good think or we'd all be in an oven nearly 6100 degrees Kelvin - hot enough to vaporize every element. The resolution is quite simple from the viewpoint of modern astronomy. We now know that the Universe while huge is not infinite. It is something like 150,000 billion billion miles across and has been around for a bit more than 13 billion years. The assumption that the Universe is infinite is wrong both in its age and its size.

Yet at a very odd way, Olber's Paradox comes back to "bite" us. When the Universe was very young, it was extremely hot and murky. As it expanded and the murk thinned enough suddenly it was ablaze with light. This happened at around 300,000 years after the Big Bang. The Universe expanded and the wavelengths of this light were stretched and stretched yet again becoming ever longer and lower frequency. The "color" of the light shifted from x-ray and gamma ray frequencies, down through the ultra-violet, the visible frequencies, the infrared and microwave frequencies until they have become a background fairly deep in the microwave and radio frequencies.

Olber's light really is there but it is no longer at a frequency that our eye can detect but it is easily "seen" by large radio telescopes. I'm afraid that my explanation was a bit of a disappointment to the gentleman who asked the question. He wanted the answer to be the distant stars were being obscured by large numbers of black holes. Alas for this explanation, this cannot be. First of all, if a cloud of black holes this dense existed, the gravity of these holes would be cumulatively so great that the Universe could never expand. The other real problem with this assumption is that while the actual black hole is indeed very dark, the area around the black hole is brilliantly illuminated when the black hole captures matter. In fact, black holes that capture stars in large numbers are the brightest objects in the Universe - the quasars.

We had a troop of girl scouts who were camping at Burlingame State Park nearby. We had a number of families. We saw not only Venus, but Saturn, Jupiter, M87, and the Sombrero (M104). We did a fair amount of star identification and answered lots of questions the kids had.

-Les Coleman

Author:

Leslie Coleman

Entry Date:

Jun 15, 2007

Published Under:

Leslie Coleman's Log