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Saturn and Regulus Part 1 - Brightness
Observing >
Those of us even slightly familiar with the night skies look up at this time of the year and welcome our old friend, Leo the Lion. It is one of those more easily spotted cats in the heavens, distinguished from the rest of the constellations by what seems to be a gigantic backwards question mark. Go out tonight, face south, look almost directly up and there it is.
But some may question the presence of a new mark on the lion, just at the bottom of the question mark, a mere finger width left from the bright bottom star. The stranger is bright enough to disrupt the familiarity of what we normally see there. It is no new star, but the planet Saturn, right there above our heads in all its golden glory.
Of course I would encourage you to take a look at the Ringed One in the next weeks. It is always a crowd-pleaser no matter how many times you look at it.
But I thought this might be a good opportunity to toss in a brief look into the relative brightnesses of heavenly objects.
First, if you go out tonight and take a look, you will notice the brightest star in Leo's big question mark is that same one right next to Saturn. That is Regulus. It is a monster star, more than four times bigger than our own sun. And it pours out over 200 times the energy of our star.
It is an impressively big star, to be sure, yet it is dimmer than Saturn, a mere planet in our solar system that pours out essentially nothing as far as energy is concerned. On a brightness scale we say Regulus has an "apparent magnitude" of about 1.3. Saturn's apparent magnitude is about 0.5, which, in the crazy world of magnitudes, makes it nearly twice as bright as Regulus.
So why is Regulus dimmer than Saturn if it is so much more violent and inherently bright? The obvious answer is that it is farther away - obvious maybe, but in astronomy critical as well.
Regulus is about 78 light years away. This is not a great distance in a universe that is over 13 billion light years in all directions. But it is, nevertheless, about 480 trillion miles away. Take a star that far and even the big ones can get pretty small and dim pretty quickly.
This distance dimming thing is an essential tool in astronomy for this reason: If we know how bright a star should be, and we look up and see how bright it appears to our eyes, we can estimate how far away it is.
Here's an analogy. You know how much light pours out of a 100-watt light bulb at arm's length. If your friend were holding that lit 100-watt bulb somewhere down the street, we could use some simple instruments to measure how bright it appears to be, and with some uncomplicated math, we can then estimate how far away he would have to be for the bulb to be only that bright. We are using brightness to estimate distance.
That is a wonderful tool in a discipline where it is impossible to stretch out tape measures even hundreds of miles, let alone hundreds of trillions.
Saturn is brighter because it is so very close to us, just over 72 light minutes away, about 800 million miles. But even though it is next door, it is not all that bright because, as is true for all planets, it does not give off its own light, but reflects the light of the sun.
Next time here we will continue to look at Saturn and Regulus and see why one twinkles and the other does not - distance plays another starring role - and why it is sort of important to see Saturn this year and not procrastinate until next.
Until next time, clear skies!
Posted by Mark Ritter at 2008.04.20 10:22 AM | Comments (0)
