1 / 18

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

niesha
Download Presentation

The Brightness of Stars

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  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

More Related