FirstLight Astronomy Club

33°29.6'N / 117°06.8'W / 1190 ft.

A Reading of the First Magnitude

You are sitting in your backyard after sunset watching the evening sky begin its fade to black. Even before the sun fully sets you notice a small bright light gradually appearing in the west. Of course that's Venus. As the sky darkens a gleaming spot in the southwestern skies seems to brighten, as well. That would be Jupiter. Then slowly over the next half hour or so, stars seem to just show up from out of nowhere.

If you are away from the city lights and the Moon is not out at the moment, the sky will seem to fill with stars over the next hours.

If you didn't before, you now get an idea of how varied the brightness of stars can be: from celestial bodies so bright you can see them just as the sun sets, to stars so dim you can only see them after the sky goes pitch and your eyes have adapted for the darkness.

Of course the brighter stars have always played a major role in the construction of constellations, and in the mythologies and oral traditions of many people groups. But it wasn't until just a couple thousand years ago that some people tried to take a more scientific look at all stars, bright to dim.

Over two millennia ago, Hipparchus, one of the great astronomers of the ancient days, categorized the stars into a brightness scale. The brightest stars in the sky were classified magnitude 1. The dimmest, nearly imperceptible stars were magnitude 6. All the rest of the stars fell somewhere in between.

We still use basically the same scale today, but not without some ado.

For example, what is disturbing for modern minds is that the brightest stars up there have the lowest magnitudes. So a star of magnitude 5 is considerably dimmer than a magnitude 2. That's not a big problem, but being counterintuitive it can be a little annoying.

More exasperating, though, is what happened as our knowledge of the skies grew.

What about objects in the sky that were brighter than the brightest stars - like Venus? What magnitude should they be assigned? Well, inconveniently astronomers agreed to take those brighter objects down to zero and then lower, into the negative numbers! So, for example, Venus, the brightest object in the sky besides the sun and moon, has a magnitude of -4.4!

But why stop there? The Moon, when full, has a magnitude of less than -12. And the brightest star in the sky, the sun, is magnitude -27! Yikes!

When telescopes were invented and could open their bigger and more sensitive eyes to the heavens, they could pick up light from objects our tiny eyes could never see. The scope was like a battering ram that broke open the door on the other side of Hipparchus' scale. With their help we can now see objects with magnitudes near 30! To give you an idea of how dim that is, the Hubble Space Telescope had to keep its shutter open for more than a day just to pick up enough light from those distant objects to get an image!

This whole crazy magnitude scale, like that embarrassing uncle at family gatherings, is here to stay, but once you get used to it, it isn't that bad.

Sadly, today, because of light pollution we are losing the higher magnitude stars to the bright lights of malls and casinos. For those living near cities today, it is a good night if one can see down to magnitude 3 stars. To the disappointment of many, the starry hosts of yesteryear are becoming the starry handful for today's generation.

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