Marian Physics! Apparent Magnitude. Flat Prairie Publishing

Transcription

1 Marian Physics! Apparent Flat Prairie Publishing

2 Apparent

3 Assignment Apparent Apparent The scale used in astronomy to measure the brightness of stars is steeped in history and perhaps not a whole lot of common sense. It began with the Greek astronomer by the name of Hipparchus. In the 3 century B.C. he cataloged many of the visible stars. He started by calling the brightest objects 1, the second brightest 2 and so on. He ended at 6 magnitude as the dimmest star an average person can see with your eye on a dark clear night. Due to light pollution in most major cities you can only see to magnitude 3 stars. You need to get away from the city lights to see magnitude six stars. So, using this system a higher number means a dimmer star. Hipparchus did not classify the moon, the planets or stars that he could not see from his location on earth. When we added the brighter planets to the system as well as some of the stars he could not see we wound up with negative magnitudes. So objects with negative magnitudes are brighter that objects with positive magnitude and the higher the positive number the dimmer the star. Yes. I know, this sounds like a scale a sociologistwould make up. The apparent magnitude of some of the common objects in the sky. 2

4 Sun -27 Full moon -13 Venus -4 Vega 0 Polaris 2 This gives the ration of brightness of the two stars, where Lm and Ln represents the magnitude o the stars of magnitude m and n. An equivalent formula is: In 1856 N.R. Pogson refined the scale and found the difference from magnitude 1 to magnitude 6 was 100 units of measured brightness. This made each magnitude change times each time you went up or down the scale. The difference between a 1 magnitude star and a 2 magnitude star is times. The difference between a 1 magnitude star and a 3 magnitude star is times or 6.3 times. An example may help. The fictional star Frozzbozz has an observed magnitude of 0.37 and fictional star Pota has a magnitude of Compare their brightness. You are trying to find the ratio of their brightness... in other words times brighter is Frozzbozz than Pota. The general formula is: ` So we conclude that Frozzbozz is 9.04 times brighter than Pota. Problems: (do on engineering paper) 3

5 1. The apparent magnitude of the sun is and that of the star Sirius is -1.44, How many times brighter is the sun when compared to Sirius? Is the sun actually brighter than Sirius? 2. Photometric comparisons show Mizar has an apparent magnitude of 2.4 and is 4.37 times brighter than a star called Alcor. What is the apparent magnitude of Alcor? 3. How much brighter is Alnair, apparent magnitude of than Proxima Centauri with a magnitude of 11.09? 4. What is the ratio in brightness of Sirius, with apparent magnitude -1.44, to Polaris, with apparent magnitude 1.97? Sidebar: Now you may notice that many stars are named using letters from the Greek alphabet; α, β, γ δ and ε (alpha, beta, gamma, delta, epsilon being the first five) followed by the standard three-letter abbreviation for each constellation (Cru for Crucis or Crux and Cen for Centaurus). So, alpha Centauri is the brightest star in the constellation Centauri. Beta Centauri is the second brightest star in Centauri etc. An exception to this rule is alpha Orionis or Betelgeuse. It is in fact fainter than β Ori, Rigel by a small amount. This is an interesting historical case solved by the realization that Betelgeuse has dimmed slightly in brightness since it was named under the Bayer system. Some of the bright stars also have their own specific name. Sirius (α Scorpius) is one such examples while α Centauri is also called Rigel Kentaurus. Most stars in fact do not have a specific name or Bayer classification. These days astronomers have compiled vast catalogs of stars, some with over 10 million objects, so most stars only have a catalog number. Of course, to make things complicated astronomers cannot agree on a single catalog. There are about 30 different catalogs of stars out there, so a star may have a name, a Bayer name and 30 different catalog numbers. The brightest star is labelled α, the next β and so on. This system is called the Bayer system, after Johann Bayer who introduced it in The brightest star in a constellation is assigned the letter α, the next β and so on. 4