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The Brightness of Stars

The Brightness of Stars. The Simple Answer to: How Bright?. Quantifying the brightness of stars started with Hipparchus (2 nd C. BC) and his magnitude scale He designated the brightest star he could see as a “1” magnitude and the dimmest a “6” magnitude

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The Brightness of Stars

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  1. The Brightness of Stars

  2. The Simple Answer to: How Bright? • Quantifying the brightness of stars started with Hipparchus (2nd C. BC) and his magnitude scale • He designated the brightest star he could see as a “1” magnitude and the dimmest a “6” magnitude • Astronomers still labor under a more quantified version of this system • One tragic consequence is that objects brighter than the brightest star have negative magnitudes!

  3. However… • We will have to account for: • Filtering • Distance • Reddening • Extinction • But first things first…

  4. Apparent vs Absolute • The Apparent Magnitude of a star is how bright it appears to the naked eye, disregarding any interfering factors • On our Hipparchian scale, the Sun would have an apparent magnitude of -26, the Moon -11, and Venus -3 • The Absolute Magnitude is how bright a star (or other object) would appear at a distance of 10 parsecs ~ 32.6LY • The Sun’s absolute magnitude is 4.83

  5. The Difference • Consider a 100W light bulb; 100W is its intrinsic brightness • It emits 100W of light no matter how far away it is; at the specified distance of 10 parsecs it would have some (very tiny) absolute magnitude • However, since a 100W bulb in your face seems much brighter than a 100W bulb 10 parsecs away, its apparent magnitude would depend on how close or far away it is

  6. Rule of Thumb • You can’t add magnitudes, absolute or apparent, directly because they are calculated with base ten logarithms • A difference of 1 magnitude means a factor of 2.512 in brightness • So, if you ask how bright two 3-magnitude stars are together, it’s not 6, it’s not 5.048, it’s 2.25* • Don’t worry, you won’t have to calculate the summed brightness of multiple stars, but you do have to know that you can’t just add magnitudes • And you must realize that smaller numbers, even negative numbers, mean brighter objects *2m = m-2.512log(2) = 2.247 ~ 2.25

  7. Intrinsic Brightness: Blackbody Radiation • Stars emit light because they are hot! • Their color is determined by their temperature • Consequently, their brightness is dependent on their temperature (among other things)

  8. Adjustable Wein Curves (if connected)

  9. Stars that are cool, ~3500K, will be reddish; stars that are hot, ~10,000K, will be white • White light is a combination of all colors, so a hot star will appear brighter than a red star, all other things being equal, because not all light from a star is visible to the human eye • This fact obscures a star’s intrinsic brightness

  10. Filters • Astronomers use filters to see how bright a star is in a certain color range • The filters are simply colored glass that goes over the mirror or lens of a telescope • Astronomers say Vega has an MV of 0, which means Vega has an absolute magnitude of 0 in the V (for visible--no filters) color band See how the red filter lets very little green and practically no blue through?

  11. Intrinsic Brightness: Size • The surface area of a star is another factor in the brightness of a star • Two stars of the same temperature will have different magnitudes, depending on their size • A red supergiant can emit vastly more light than a red dwarf

  12. Apparent Brightness: Distance • The light received from a star is dependent on the inverse square of its distance from us. • Knowing this helps astronomers find its distance using a method known as standard candles

  13. Standard candles works this way: say you know the intrinsic brightness of a star and its magnitude; • If you see an identical star but with a different magnitude, you can use the inverse square law to find the distance

  14. Reddening • One of several “seeing” problems • The dust in the disk of the galaxy absorbs the blue component of a stars light, making it seem redder than it is.

  15. Extinction • Another “seeing” problem • Anything in the light path from a star, nebula, or galaxy absorbs or scatters light • This attenuation is called extinction

  16. Summary • The brightness of a star or other celestial object is quantified by its magnitude • Factors that determine the light output of a star: • Temperature  color • Size • Factors that determine its perceived brightness: • Color • Distance • Reddening • Extinction

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