Sunday, November 3, 2013


Even before going out and taking your first serious look at the night sky, you undoubtedly know that some stars are brighter than others. In astronomical jargon, the brightness of a star is known as magnitude. The magnitude scale is unusual in that it works in both positive and negative numbers. On the scale of brightness, the lower the number, the brighter the object. 

There are two kinds of magnitudes, apparent and absolute. The absolute magnitude is the actual brightness of a star. Stellar distances greatly vary. Some small stars that give off a relatively small amount of light are close and appear bright while some giant stars are very far away but appear dim. A comparison can be made with light bulbs. A nightlight at five feet away will look brighter than a 100 watt bulb at 300 feet away. The same is true of the stars. But for observational purposes, the magnitude to be concerned is the apparent magnitude, which simply refers to how bright the star appears to be in the night sky. The Sun, undoubtedly the brightest object in the sky, blazes away at an apparent magnitude of -27 while, for most people, a magnitude of +5 to +6 is the naked eye limit on the dim side of the spectrum. Needless to say, a combination of good eyes and dark sky can produce the ability to see even dimmer stars.

The magnitude scale is not an arithmatic scale because stellar brightness does not increase or decrease by a factor of one. A difference of one stellar magnitude translates to about a 2.5 change in brightness. For example, a zero magnitude star is about 2.5 times brighter than a first magnitude star. To compare brightness of stars, just multiply 2.5 to the power of magnitude difference. For example, to find the magnitude difference between a third and zero magnitude star, multiply 2.5 x 2.5 x 2.5 (2.5 to the third power) for an answer of 15.6, which means that the zero magnitude star is about 16 times brighter than the third magnitude star.
After brightness, the next thing to look at with stars is their color, which is a direct giveaway to a star's temperature. When it comes to stellar classifications by color/temperature, there are 7 classes that matter to the visual astronomer: O, B, A, F, G, K, M. A common way to remember this is by the saying 'Oh(O), be(B) a(A) fine(F) girl/guy(G), kiss(K) me(M)!' For students, the saying 'Oh boy, an 'F' grade kills me!' will also work equally well. When it comes to what the classifications mean, here's what we get:

(Hottest, usually largest)
O: Blue, hottest stars
B: Blue-white
A: White
F: Yellow-white
G: Yellow (our Sun is a 'G' star)
K: Orange
M: Red (coolest stars we can see without optical aid)

(Coolest, usually smallest save red giants)

Generally speaking, most of the stars in the sky fall between the A and G classification. Want proof of this? Just go out and look up. Now, it should be said that the stars do not look like Christmas lights in the sky, the colors are far more subtle. Still, though, by looking around the night sky, one can see that, while not obvious, the stellar colors mentioned above are, without doubt, very present. Obviously, in a telescope, the colors become very apparent. 

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