By Bill Pellerin

Houston Astronomical Society

GuideStar Editor

At our annual Astronomy Day event, I’ll often point to Vega and say, “That star is 25 light-years away and this means that the light takes 25 years to reach us.” But, how do we know that Vega is 25 light years away? How did we determine this?

This is an interesting subject. Even the distance to the Sun wasn’t well determined until the 1960’s when radar ranging was used to measure the distance. The process is easy, send a radio pulse to an object, and see how long it takes to get there and bounce back. If you know the speed of the radio pulse, and we do, it’s easy to calculate the distance using the formula we learned in high school — d=r*t. That is, the distance is equal to the rate (speed) multiplied by the time. Since the radio pulse has to travel to the object and back the calculation for this is d=r*t/2.

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By Bill Pellerin

Houston Astronomical Society

GuideStar Editor

Last month, I wrote an article about the way we categorize nebulae (clouds in the sky). There are emission, reflection, and nebulae, which tells you how they are visible to us. The other way to categorize nebulae is as pre-stellar (star forming) and post-stellar (a remnant of a burned out star). I recently viewed the Wonders of the Universe video series by Brian Cox (highly recommended) and one of his subjects is the lifecycle of the universe and how objects in space fit into that story. That is, if we look at how the universe began, how it got to where it is today, and the future of the universe we get another perspective on nebulae.

So, let’s go all the way back to the beginning of the universe, the Big Bang. Various light elements came into being as the result of the Big Bang, but the heavier elements did not. The Big Bang, while extremely hot was also of short duration. In other words, there was not enough time for heavier elements to form as a result of the Big Bang. This formation of elements as a result of the Big Bang is called ‘Big Bang nucleosynthesis’.

What the universe began with, and what were the building blocks of the future universe, were (75%) hydrogen, (25%) helium, and small amounts of lithium and beryllium. That’s it. No carbon, oxygen, or anything else existed yet existed. The universe had to wait for these elements to be formed.

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