Star Points for July, 1998; by Curtis Roelle Star Light Star Bright As far as bright evening planets go we are out of luck in July. So, this month we tackle a nuts and bolts issue by discussing the stellar magnitude scale, or how star brightness is represented. When we gaze into the sky on a clear dark night far away from city lights it is possible for our dark-adapted unaided eyes to view several thousand individual stars. What immediately becomes apparent is that some stars are much brighter or fainter than others. How do astronomers describe the apparent brightness of a given star? The solution involved the development of a scale for measuring stellar brightness, or luminosity. The original form of the scale was developed by the Greek astronomer Hipparchus of Nicaea (ca. 190 - ca. 120 BC). He compiled a catalog of stars visible to the naked eye and divided them into six groups according to their brightness, or magnitude. The brightest stars were designated magnitude 1, or 1st magnitude; the faintest were 6th magnitude. Thus, the smaller the magnitude the brighter the star. Hipparchus' magnitudes were fairly subjective and eventually a more uniform and unambiguous definition of the scale was needed. In the currently used scheme a difference of five magnitudes amounts to a difference of 100 times in brightness. Since the difference between 1st and 6th magnitude is five magnitudes, a magnitude 1 star is 100 times brighter than a star of magnitude 6. The brightness factor between adjacent magnitudes is 2.512. Thus, a 3rd magnitude star is 2.512 times brighter than a 4th magnitude star, which is 2.512 times brighter than a 5th magnitude star. The 3rd magnitude star in turn is 6.31 (or 2.512 x 2.512) times brighter than the fainter 5th magnitude star. The important point to remember is this: The smaller the magnitude the brighter the object. Negative magnitudes are sometimes used for very bright objects. The sun is magnitude -27, Venus at greatest brilliance is magnitude -4.5. Vega, the brightest star in the Summer sky is magnitude 0. As you can see magnitudes may also be expressed as decimal numbers. For more information on magnitudes see the NASA Space Academy's web page at http://astro-2.msfc.nasa.gov/academy/UNIVERSE/mag.html. Another interesting NASA web page is "Liftoff to Space Exploration" featuring space news, information on satellites, comets, the space academy, and plenty of stuff especially for kids. Its URL is http://liftoff.msfc.nasa.gov/. You may wonder why some stars appear brighter than others. Are the brighter ones truly bright or are they simply closer to us than the fainter ones? The answer is both. For instance a close street light appears brighter than a distant one and a lamp glows more brightly than a faint firefly viewed at the same distance. The point is that our magnitude scale described above measures only the "apparent" brightness of a star, which has no bearing on the true or absolute brightness of the star or its distance. With a telescope or binoculars you can view may times the number of stars that you can see with your unaided eye because the telescope's light gathering power allows you to see fainter stars. If you are a telescope owner you might be asking, what is the faintest magnitude I can see when using my telescope? In order to calculate the "Limiting Visual Magnitude" (LVM) of your telescope you will need a calculator that supports the logarithm function (usually on a key labeled "LOG"). LVM is a function of the telescope "aperture." Aperture is simply the diameter of the main lens or mirror. LVM = 9.7 + 5 x log(D), where D is the aperture measured in inches. Alternatively, if you prefer measuring D in millimeters (mm), then replace 9.7 with 2.7 in the equation above. For example, the LVM of a 60 mm refracting telescope is 2.7 + 5 x log(60), or mag. 11.5. A 4" reflecting telescope has an LVM of 9.7 + 5 x log(4), or mag. 12.7. For amateur astrophysicists a large assortment of formulas for telescopic properties such as LVM may be found at http://www.moonstar.com/FileLib/space/scope.txt. Correction: The URL for the Maryland Iridium page given last month should have been http://scivis.nps.navy.mil/~cmcholar/iridmd.htm. Please e-mail your questions and comments to crux2000@yahoo.com.