CN+Milestone+Map+-+Magnitude+and+Comparison+Stars

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=Explain what the magnitude is. Explain what a comparison star is and how to use it. Use the comparison star to find Magnitude Estimates.=

A **comparison star** is a star that is used to help in determining the brightness of a variable star.
 * Magnitude** "is the expression of brightness, the light that we can see." (__Observing Variable Stars__)

When you observe variable stars you will relate their respective brightness by comparing their magnitude with comparison stars. "The scale of magnitudes may seem confusing at first, because the larger the number, the fainter the star. The highest star that usually is visible to the naked eye is at 6th magnitude." ([|AAVSO]) "Comparing the brightness of stars is one of the fundamentals to variable star observing. You will always be comparing the brightness of one star, the variable, to another, the comparison star, so as to make an estimate of the variable’s magnitude." (__Observing Variable Stars__)

Magnitudes are at their brightest the lower the number is. Stars can have a negative magnitude, but few do. There is a difference between apparent magnitude and absolute magnitude. Apparent magnitude is the apparent brightness of a star observed from the Earth. It is the measure of the star's //flux// received by us. Apparent magnitudes are written with the lowercase letter “m.” "The absolute magnitude is defined as how bright a star would appear if it were exactly 10 parsecs (about 33 light years) away from Earth. Absolute magnitudes are seen with the capital letter “M.”" ([|Liftoff])

Magnitudes of Selected Objects



A star sequence is a list of stars used to judge the brightness of a variable star. They are the comparison stars and they should be selected carefully.

A comparison star,
 * should not be variable
 * you should have a number of stars, some that are a little bit brighter and some that are a bit fainter than the span of the variable stars brightness
 * if possible, select comparison stars that have a similar spectral type as the variable star
 * they should also be relatively close to your variable star, too

"On AAVSO charts, the comparison stars are designated with numbers which indicate their magnitude to tenths. The decimal point is omitted to avoid confusion with the dots which represent stars. Thus 84 and 90 indicate two stars whose magnitudes are 8.4 and 9.0, respectively ... It is important for the observer to keep a record of which comparison stars are used when making an estimate of a variable's brightness." ([|AAVSO])

"To estimate magnitudes of variable stars, you will need to interpolate. Interpolation is the process of estimating a value between two known values. Near the variable star you will be observing are two or more comparison stars of known magnitude. Knowing the values of the magnitudes of the comparison stars and the magnitude range of the variable star itself, you can interpolate or estimate the magnitude of the variable star as it changes over time." ([|AAVSO HOA])

Observing Variable Stars by Gerry A. Good http://www.geocities.com/leemaisler/VariableStar.html http://hoa.aavso.org/estimating.html http://chandra.harvard.edu/edu/formal/variable_stars/mag.html http://liftoff.msfc.nasa.gov/academy/universe/MAG.HTML http://www.aavso.org/observing/charts/howtouse.shtml
 * References:**