FirstLight Astronomy Club

33°29.6'N / 117°06.8'W / 1190 ft.

Answering Questions from Readers

Those of us steeped in the universe of astronomy often hear questions - honest questions from well-meaning people - that need answering if for no other reason than to satisfy a person's curiosity. Here are just a couple I've come across...

1) "We all know stars twinkle, but someone once told me that each eye gets its own twinkle. Is that true?"

Why, yes, as a matter of fact that is true. The light from stars travels literally trillions of miles through space to reach our eyes. It really isn't until it reaches our atmosphere that its path gets knocked on its head. As soon as it reaches our skies its path bends or refracts, similar to what happens when light passes though air and then through glass, slightly distorting the object behind the glass.

But the star's light doesn't bend nicely and smoothly in one direction. As it passes through tiny pockets of warmer and cooler air it can take quite the jagged little path. Close one eye and look at a bright star and you will see the flicker as the star's photons take their separate paths causing the star to jump around or twinkle.

But the changes in the path are subtle and discrete, meaning that the photons that hit one of your eyes take slightly different paths than the photons hitting your other eye. Each eye get its own set of photons with unique, erratic paths.

You can see this if you go outside and look at a bright star. Cross your eyes slightly so you see the star doubled. Then notice that the twinkle of one of the stars is not the same as the other.

Try not to let the neighbors see you do this. Standing outside, staring up at the skies with your eyes crossed can fire up the neighborhood rumor mill.

2) "There is no gravity on the moon, is there?"

This is a belief held by more people than you think. The modern human mind seems to think that once we leave Earth, gravity has little, if any, effect.

But regular readers here know that anything that has mass has gravity. The Moon has mass, therefore it has gravity. Now the Moon certainly does not have as much gravity as we - just one-sixth of what we have on our surface - but it does have gravity.

I asked a student of mine who believed the moon had no gravity how she thought the astronauts walked around without it. "Well," she said, "they must have had lead in their space boots," not seeing that in order for lead to weigh one down one needs gravity.

Do you have any questions? Feel free to write me with them! Until next time, clear skies!

Redshift and Age

One of the latest bits of news to make it out of the astronomical camp is the discovery of the farthest known galaxy. Although the jury is still out on whether or not it truly is the most distant galaxy seen, it has been reported nonetheless that this galaxy is at a redshift of 10.3. That is equivalent to 13.2 billion light years away.

Now that is far away to be sure, taking us all the way back to the first pages of this book we call the universe. But "redshift 10.3"? What on earth does that mean? It all has to do with our expanding universe. A non-math, over-simplistic reply goes something like this:

Recall that since we cannot travel out into the cosmos, astronomers rely on light to reveal to them the great mysteries of the universe. In the 1920's, Edwin Hubble figured out that this universe was expanding just by looking at the light of galaxies.

He knew this because the tell-tale wavelengths of light from certain elements in the spectra of galaxies weren't in their normal places. They had been shifted towards the red end of the spectrum. They were "red-shifted," so to speak. But why?

There are a couple different causes for redshifts but it turns out that the best explanation of this mystery was a "cosmological redshift," a redshift caused by an expanding universe. As the fabric of space expands it carries the galaxies with it and also stretches the waves of light racing through it, shifting them towards the longer-wavelengthed "red" end of the spectrum. This expansion makes wavelengths of light longer than when they first left their original stars.

Now the light from nearby galaxies - only a billion or so light years away - hasn't been traveling for long and thus has not been stretched - or redshifted - too much. The light from far away galaxies, however, whose light has traveled eons to reach us, has been stretched out much more and their redshifts are considerable.

Astronomers, through some math beyond the scope of this article, assign numbers to how much the distant galaxies' spectra are redshifted. And since how much something is redshifted is a reflection of how far away it is, the redshift number can tell us the distance to far away places. Small redshifts mean nearby, big redshifts mean far away. Got that?

So when astronomers make the claim that the newly found galaxy is at "redshift 10.3," which is huge, a little astromath translates that movement into a distance of 13.2 billion light years away. And since it took light that long to get here, we are seeing the galaxy as it was 13.2 billion years ago, right after the Creation Event itself.
Temecula Valley High School / Temecula, CA · Some images © Gemini Observatory/AURA Contact Me