FirstLight Astronomy Club

33°29.6'N / 117°06.8'W / 1190 ft.

Saturn and Regulus Part 1 - Brightness

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!

Big Boom by Boötes

An explosion bigger than anyone has seen in recorded history happened a couple weeks ago and few seem to have noticed. There was March Madness, Democratic infighting, and Paris Hilton doing more nothing, but only a few humans on Earth were aware of what happened in the sky in the constellation Bootes.

It was there that a special satellite named Swift noticed just the faintest flash of light. But what a flash of light! If you had been out in the darkness away from the city lights and looked exactly in the right place at exactly the right time you too would have seen it - just barely.

How exciting, huh?! No? Well, actually it was! Let me tell you some more of the facts and then maybe you will feel sorry that you, too, weren't able to witness this extraordinary event.

Once astronomers got a quick bead on this transient flash, they were able to conclude that it was an elusive gamma ray burst (GRB). These phenomena are the most energetic group of explosions we know about, even bigger than the heavens' conventional weapon, the supernova. There are several possible models for what these critters might be, but the latest ideas make them out to be superduper supernovae.

The more common supernovae we've all heard about are the explosive deaths of stars. They go kaboom, we go "wow," life goes on.

But these gamma ray bursts seem to be supernovae on steroids, focusing all their explosive power and energy out two narrow poles instead of all over the place. If we happen to be in the sights of one of the poleblasts we get a face full of energy right before our face vaporizes. And there is enough energy in those bursts to wipe out life on earth if one of them occurs in our galaxy and is aimed our way.

Now back to our Big Blast from a couple weeks ago and why it was so astonishingly colossal.

When astronomers looked at the data, they discovered that this particular GRB was a ways away. Not thousands of light years away, not millions, but billions of light years away. About 7.5 billion to be precise.

To put things in a time perspective, this means the star went blooey billions of years before our own star and its planets (including Earth) were even born. And its light just reached us now.

To put things in an energy perspective, let's use our star for comparison. You can imagine that if we moved our blindingly bright sun farther away from us it would get dimmer. Move it about 40 light years away and it would be so dim we couldn't see it naked eye. That's just 40 little baby light years, about 240 trillion miles.

And yet we could see this bad boy from 7.5 billion light years away. Imagine how incredibly bright it had to be to be seen halfway across the known universe!

This is what amazed astronomers - but not many other humans - when GRB 080319B appeared in our skies. The amount of energy needed to be that bright from that far away was more than 2 million times greater than the already mind-numbing amount of energy from the brightest recorded supernova. That is inconceivable.

This is why astronomers, professional and amateur, would have been in heaven just to catch a faint glimpse of the light with their own eyes. It was an historical moment in the Realm of Astronomy.

But alas! That GRB is dead and gone. All we can hope for is another big one. But, please, not too big or too close, too energetic or too focused. We would like to live to tell the tale.

Until next time, clear skies!
Temecula Valley High School / Temecula, CA · Some images © Gemini Observatory/AURA Contact Me