Apparent brightness and inverse square law light

Info icon This preview shows pages 1–3. Sign up to view the full content.

View Full Document Right Arrow Icon
Apparent brightness and inverse square law Light appears fainter with increasing distance If we increase our distance from the light source by 2, the light energy is spread out over four times the area (area of sphere = 4pid^2) Flux = luminosity / 4pid^2 To know a star’s luminosity we must measure its apparent brightness (flux) and know its distance. Then, Luminosity = Flux x 4pid^2 The Magnitude Scale
Image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full Document Right Arrow Icon
2 nd century BC, Hipparchus ranked all visible stars – brightest = magnitude 1 faintest = magnitude 6 To our eyes, a change of one magnitude = a factor of 2.5 in flux Hence, the magnitudes scale is logarithmic A change of 5 magnitudes means the flux 100 x greater Our eyes use logarithmic Apparent magnitude – star’s apparent brightness when seen from its actual distance Absolute magnitude – apparent magnitude of a star as measured from a distance of 10 pc Sun’s apparent magnitude (if seen from a distance of 10 pc) is 4.8 This is then the absolute magnitude of the Sun Starlight: Who Cares? We do! Primary source of “life energy” on Earth Many living things convert sunlight to energy Most other living things eat them (or eat things that eat them, or . . .) Also, heat/temperature Living things want liquid phase Need the right star/distance combination for this Also, want STABLE temperatures for long time (i.e. millions, or better yet, BILLIONS of years) H – red, turquoise, violet He – red, yellow, aquamarine, violet Stellar Temperature: Color You don’t have to get the entire spectrum of a star to determine its temperature Measure flux at blue (B) and yellow (“visual”=V) wavelengths Get temperature by comparing B-V color to theoretical blackbody curve Stellar Temperature: Spectra 7 stars with same chemical composition
Image of page 2
Image of page 3
This is the end of the preview. Sign up to access the rest of the document.

{[ snackBarMessage ]}

What students are saying

  • Left Quote Icon

    As a current student on this bumpy collegiate pathway, I stumbled upon Course Hero, where I can find study resources for nearly all my courses, get online help from tutors 24/7, and even share my old projects, papers, and lecture notes with other students.

    Student Picture

    Kiran Temple University Fox School of Business ‘17, Course Hero Intern

  • Left Quote Icon

    I cannot even describe how much Course Hero helped me this summer. It’s truly become something I can always rely on and help me. In the end, I was not only able to survive summer classes, but I was able to thrive thanks to Course Hero.

    Student Picture

    Dana University of Pennsylvania ‘17, Course Hero Intern

  • Left Quote Icon

    The ability to access any university’s resources through Course Hero proved invaluable in my case. I was behind on Tulane coursework and actually used UCLA’s materials to help me move forward and get everything together on time.

    Student Picture

    Jill Tulane University ‘16, Course Hero Intern