1 / 13

Colors and Magnitudes

Colors and Magnitudes. PHYS390 (Astrophysics) Professor Lee Carkner Lecture 2. Answers. On the summer solstice, what RA is on the meridian at midnight? Sun’s RA is 6 hr, so when RA 6 is on the other side of the Earth, RA 18 (6+12) is overhead

moses-nixon
Download Presentation

Colors and Magnitudes

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. Colors and Magnitudes PHYS390 (Astrophysics) Professor Lee Carkner Lecture 2

  2. Answers • On the summer solstice, what RA is on the meridian at midnight? • Sun’s RA is 6 hr, so when RA 6 is on the other side of the Earth, RA 18 (6+12) is overhead • On what date will a star with an RA of 15 hr be on the meridian at midnight? • Want sun to have RA of 15-12 = 3 hr, which is half way between Mar 20 and Jun 21 or ~May 4

  3. Flux and Luminosity • Photometry • Flux • W/m2 • Luminosity • W • From inverse square law F = L/4pr2 • Sometimes use units of Lsun = 3.839 X 1026 W

  4. Magnitude • Eye has semi-log response, so a 1 magnitude difference is a brightness difference of about 2.5 • apparent bolometric magnitude = m • apparent = • bolometric = a • Smaller m, brighter star • Flux = easy, magnitude = hard

  5. Magnitude and Flux • If m1-m2 = 100 then F2/F1 = 100 m1-m2 = -2.5 log (F1/F2) • m (apparent magnitude) • M (absolute magnitude) • M is equal to the apparent magnitude the star would have if it were at 10 pc m-M = -2.5 log [(L/4pd2)/(L/4p102)] m-M = 5 log (d/10pc) • m-M is called the distance modulus • n.b., sometimes distance is “r” and sometimes “d”

  6. Colors • Can’t detect all wavelengths at once • Examples: UBVRI = apparent magnitude in ultraviolet, blue, visible (green), red, and infrared • We write apparent magnitude in a filter band with a capital letter (e.g., V or B)

  7. Bolometric Correction • e.g., B-V, U-V • The smaller the color index, the more important the wavelengths of the first filter are • low U-B: • low B-V: • We can also apply the bolometric correction (BC) to get the bolometric magnitude • Where BC is constant for a specific spectral type • BC tells us what fraction of the total energy distribution V is

  8. Apparent and Absolute • Apparent magnitude • mbol (for bolometric) • Absolute magnitude • Mbol (for bolometric) • Note also that the color index is a the same for apparent or absolute magnitudes • e.g., B-V = MB-MV

  9. Spectral Type Information • Stars are classified by spectral type • Tells us temperature • Absolute magnitudes (MU , MB, MV, MR, MI, Mbol) • Color indices (B-V, U-B)

  10. Color-Color Diagram • The color index tells us something about the shape of a star’s spectral energy distribution • Negative B-V = • Positive B-V = • A star’s color index tells us its temperature B V

  11. Normalizing the Scale • We can also relate the magnitude to the flux integrated over some wavelength range and a constant C • C is a constant chosen to normalize the magnitude scale to standard stars mbol = -2.5 log (∫ Fl dl) + Cbol • Where the integral is now the total flux

  12. Flux Comparisons • Note that our magnitude scale relates two magnitudes to two fluxes m1-m2 = -2.5 log (F1/F2) • e.g., we could input absolute magnitudes and the flux at 10 pc M-Msun = -2.5 log [(L/4p102) / (Lsun/4p102)] M = Msun - 2.5 log (L / Lsun)

  13. Next Time • Read: 3.3-3.5 • Homework: 3.4, 3.5, 3.8, 3.9a-3.9d

More Related