PHYS 205 Analyzing Starlight PHYS 205 Apparent brightness

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PHYS 205 Analyzing Starlight

PHYS 205 Analyzing Starlight

PHYS 205 Apparent brightness 2 nd century BC Hipparchus devised 6 categories of brightness.

PHYS 205 Apparent brightness 2 nd century BC Hipparchus devised 6 categories of brightness. In 1856 Pogson discovered that there is a 1: 100 ratio in brightness between magnitude 1 and 6 mathematical tools are possible. m 1 -m 2 = 2. 5 log (I 2/I 1) m 1 and m 2 are visual magnitudes, I 1 and I 2 are brightness.

PHYS 205 Example Vega is 10 times brighter than a magnitude 1 star I

PHYS 205 Example Vega is 10 times brighter than a magnitude 1 star I 2/I 1 = 10. m 1 = 1 2. 5 log (I 2/I 1) = 2. 5 1 - m 2 = 2. 5 m 2 = -1. 5 Using the same calculations we can find that Sun : -26. 5 Full Moon : -12. 5 Venus : -4. 0 Mars : -2. 0

PHYS 205 Inverse Square Law Sun is very bright, because it is very near

PHYS 205 Inverse Square Law Sun is very bright, because it is very near to us, but is the Sun really a “bright” star. The amount of light we receive from a star decreases with distance from the star.

PHYS 205 Absolute Magnitude If two pieces of information is known, we can find

PHYS 205 Absolute Magnitude If two pieces of information is known, we can find the absolute magnitude, M, of a star: 1. Apparent magnitude, m 2. Distance from us. 3. Example: 4. Take the Sun, 1 AU = 1 / 200, 000 parsecs away from us. 5. At 10 parsecs the Sun will be (2, 000)2 times less bright. 6. log(2, 0002) = 31. 5 magnitudes dimmer 7. -26. 5 (apparent) + 31. 5 = 5 (absolute) 8. We define the absolute magnitude as the magnitude of a star as if it were 10 pc away from us.

PHYS 205 Distance modulus m –M : distance modulus Example: We have a table

PHYS 205 Distance modulus m –M : distance modulus Example: We have a table in our hands with distance moduli and we need to find the actual distances to the stars. How do we proceed? ? Distance modulus = 10 means 10(10/2. 5) = 10, 000 times dimmer than the apparent magnitude (10, 000) = 1002 (inverse square law) 10 pc x 1000 pc away

PHYS 205 20 Brightest Stars Common Luminosity Name Distance Spectral Proper Motion R. A.

PHYS 205 20 Brightest Stars Common Luminosity Name Distance Spectral Proper Motion R. A. Declination Solar Units LY Type arcsec / year hours min deg min Sirius Canopus Alpha Centauri Arcturus Vega Capella Rigel Procyon Betelgeuse Achernar Beta Centauri 40 1500 2 100 50 200 80, 000 9 100, 000 500 9300 9 98 4 36 26 46 815 11 500 65 300 A 1 V F 01 G 2 V K 2 III A 0 V G 5 III B 8 Ia F 5 IV-V M 2 Iab B 3 V B 1 III 1. 33 0. 02 3. 68 2. 28 0. 34 0. 44 0 1. 25 0. 03 0. 1 0. 04 06 45. 1 06 24. 0 14 39. 6 14 15. 7 18 36. 9 05 16. 7 05 12. 1 07 39. 3 05 55. 2 01 37. 7 14 03. 8 -16 43 -52 42 -60 50 +19 11 +38 47 +46 00 -08 12 +05 13 +07 24 -57 14 -60 22 Altair 10 17 A 7 IV-V 0. 66 19 50. 8 +08 52 Aldeberan 200 20 K 5 III 0. 2 04 35. 9 +16 31 Spica 6000 260 B 1 V 0. 05 13 25. 2 -11 10 Antares 10, 000 390 M 1 Ib 0. 03 16 29. 4 -26 26 Pollux 60 39 K 0 III 0. 62 07 45. 3 +28 02 Fomalhaut 50 23 A 3 V 0. 37 22 57. 6 -29 37 Deneb 80, 000 1400 A 2 Ia 0 20 41. 4 +45 17 Beta Crucis 10, 000 490 B 0. 5 IV 0. 05 12 47. 7 -59 41 Regulus 150 85 B 7 V 0. 25 10 08. 3 +11 58

PHYS 205 Color and Temperature

PHYS 205 Color and Temperature

PHYS 205 Wien’s Law: 1/T The higher the temperature The lower is the wavelengths

PHYS 205 Wien’s Law: 1/T The higher the temperature The lower is the wavelengths The “bluer” the star.

PHYS 205 Temperature Dependence Question: Where does the temperature dependence of the spectra come

PHYS 205 Temperature Dependence Question: Where does the temperature dependence of the spectra come from? Answer: Stars are made up of different elements at different temperatures and each element will have a different strength of absorption spectrum. Take hydrogen; at high temperatures H is ionized, hence no H-lines in the absorption spectrum. At low T, H is not excited enough because there are not enough collisions.

PHYS 205 Color Index To categorize the stars correctly, we pass the light through

PHYS 205 Color Index To categorize the stars correctly, we pass the light through filters. B is a blue filter, V is a visible filter. Hot stars have a negative B-V color index. Colder stars have a positive B-V color index.

PHYS 205 Spectral Types We now know that we can find the temperature of

PHYS 205 Spectral Types We now know that we can find the temperature of a star from its color. To categorize the “main sequence” stars we have divided the colors into seven spectral classes: Color Class solar masses solar diameters Temperature -----------------------------------------bluest O 20 – 100 12 - 25 40, 000 bluish B 4 - 12 18, 000 blue-white A 1. 5 - 4 10, 000 white F 1. 05 - 1. 5 1. 1 - 1. 5 7, 000 yellow-white G 0. 8 - 1. 05 0. 85 - 1. 1 5, 500 orange K 0. 5 - 0. 8 0. 6 - 0. 85 4, 000 red M 0. 08 - 0. 5 0. 1 - 0. 6 3, 000 Also each spectral class is divided into 10: Sun G 2

PHYS 205 What do we learn? Temperature and Pressure: ionization of different atoms to

PHYS 205 What do we learn? Temperature and Pressure: ionization of different atoms to different levels. Chemical Composition: Presence and strength of absorption lines of various elements in comparison with the properties of the same elements under laboratory conditions gives us the composition of elements of a star. Radial velocity: We can measure a star’s radial velocity by the shift of the absorption lines using Doppler shift. Rotation speed: Broadens the absorption lines, the broader the lines, the higher the rotation speed. Magnetic field: With strong magnetic fields, the spectral lines are split into two or more components.