ANN ARBOR—Michigan sky watchers sometimes ask, ” How many stars can I see at night?” The best answer is, ” It depends.”

According to University of Michigan astronomer Richard Teske, several factors determine how many stars will be visible when you go outside after dark—including your location, the time of year, the weather and the health of your eyes.

” If everything is favorable, you may be able to see 1,500 stars; if conditions are bad, fewer than a hundred stars may greet you,” Teske said. ” You can estimate how many stars you will see by determining the brightness of the dimmest stars visible. Amateur astronomers call this the ‘magnitude limit’ of your sky.”

Astronomers—amateur and professional alike—quote the brightnesses of stars in terms of their ” magnitudes,” which are given as numbers. ” Magnitudes have been used for 2,000 years, because they provide a compact and useful scale of brightness, and because nobody has yet devised something better,” Teske explained.

Using this scale, the average of the 20 brightest stars visible in the northern sky is given a magnitude of 1. Somewhat fainter stars are assigned magnitude 2. Still fainter ones have magnitude 3 and so on. Decimals are allowed. A star intermediate in brightness between magnitudes 2 and 3 might be accorded a magnitude of 2.4. The bigger the number, the fainter the star.

If all is well, the faintest stars visible to your naked eye are about magnitude 6, according to Teske. ” This is the limit you can see from a dark location with a transparent atmosphere overhead, after spending at least 15 minutes in the dark getting your eyes adapted to dim light. If you are a smoker, the adaptation time should be increased to 30 minutes,” Teske said. ” With a good location and careful dark adaptation, you will see about 1,500 stars filling the night sky. Under extremely favorable mountain top conditions, observers with excellent eyesight might get a magnificent view of about 2,500 stars.”

You can determine the magnitude limit of your sky with some help from stars in the Little Dipper, according to Teske. This constellation can be seen any time, because it is always ” up” in the sky, circling around the North Pole of the heavens all night.

This month’s star chart (see attached) is a guide to magnitudes of stars in the Little Dipper. The chart shows the constellation with the North Star in its handle and gives magnitudes of the stars there. ” For example, if you can barely pick out the 5.5 magnitude star just inside the bottom corner of the bowl of the Dipper, then your site’s limiting magnitude is 5.5 and about 800 stars are visible overall,” Teske said. ” If you can see any of the 6th magnitude stars represented by the smallest dots on the star chart, then l,500 stars are visible.”

If street lights or shopping center lights illuminate the night sky, or if the weather is hazy, it may be difficult to see faint stars. Then the visible number drops dramatically because most of the stars in the sky are faint. When observing conditions are only slightly bad, your magnitude limit can change from magnitude 6 to magnitude 5 and the number of visible stars dwindles from 1,500 to only a little more than 400. The time of year also may play a role in how many stars you see. There are more bright stars in the winter evening sky—fewer during late spring and early summer. If conditions in your area allow you to see only the brightest stars, winter will provide your best viewing opportunity.