Star Bright

Hubble catches sight of stars so dim it's like seeing the glow of a single cigarette on the moon.

By Bob Berman|Sunday, June 1, 1997

Nothing in astronomy is more basic than the classification of star brightness. The ancient Greeks gave us our modern method, which unfortunately is an illogical, reverse-order system. For reasons unknown, the Greeks allocated lower numbers to brighter stars and reserved higher numbers for dimmer stars. Sixth magnitude, at the top of the ancient Greek scale, represents the faintest objects seen on a perfect moonless night far from city lights. At the other extreme lies the brilliance of first- or even zero-magnitude stars like familiar Arcturus.

Over the years, we’ve tailored the ancient system to suit our modern tastes: we now quantify star brightness precisely, so that five magnitudes represent a 100-fold change. Put another way, one magnitude is the fifth root of 100, or 2.512. (Thus a star of magnitude 3 is 2.512 times brighter than a star of magnitude 4.)

June is ideal for viewing this system splayed across the sky. Start by following the Big Dipper’s two leftmost stars down to the North Star, at the end of the Little Dipper’s handle. This month the handle curves upward into the Little Dipper’s bowl, a quadrangle containing only one medium-bright star named Kochab. Kochab, like the North Star, is a second-magnitude star. The bowl’s next brightest star, Gamma, is third magnitude. The next star, Zeta, is fourth magnitude, and the dimmest, Eta, is fifth. All but one hit their respective magnitudes to within eight- hundredths of a point; together, they represent the core of the so-called polar sequence and serve as standards for the comparison of star brightness. This month the polar sequence, which never sets as seen from the United States and Canada, stands highest and most prominent.

Glance from Kochab to Gamma, or Zeta to Eta, and so on, and you’ll see what a 2.5-fold brightness change looks like. The rest of the June sky displays only two zero-magnitude stars (Arcturus and Vega, both high up), five first-magnitude stars, and more than a hundred seconds and thirds. Away from city lights the heavens are awash with 2,000 fifth- and sixth-class stars. The pattern leaps out: the fainter you go, the more stars you see.

That’s why using a simple pair of binoculars suddenly increases the number of stars seen from 2,500 to 20,000. The profusion of newcomers-- including even ninth-magnitude stars--is especially dramatic when sweeping binoculars along the eastern or southeastern midnight sky this month, as the Milky Way is rising.

Typical amateur telescopes carry the observer to the thirteenth magnitude; look through telescopes such as the Palomar and you’ll see stars of magnitude 20. Until recently, the dimmest thing ever perceived, with the use of lengthy electronic image-intensifying exposures on giant telescopes, was magnitude 29. But in early 1996 the Hubble Space Telescope hit the jackpot. For ten days Hubble focused on a single spot in the handle of the Big Dipper and snapped more than 300 pictures. When combined, the images revealed never-before-seen stars and galaxies--some of them thirtieth magnitude. Astounding: A thirtieth-magnitude star is 10 billion times dimmer than the faintest star of the Little Dipper. Imagine that you could spot the glow of a single cigarette--on the moon.

This feat proves that a moderate-size telescope above Earth’s atmosphere can outperform much larger instruments when it comes to plumbing the magnitude system’s murky basement. That unfathomably dim region is home to a universe of unexplored treasures. Yet the journey really begins in a most accessible way--with a glance at the Little Dipper.

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