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Stellar Magnitude, Distance, and Motion Apparent Magnitude How bright a star appears; the "what you see is what you get" magnitude Convolution of the true brightness and the effect of distance on the observed brightness Every 5th magnitude is 100 times brighter than the one before o A 1st magnitude star is 100 times brighter than a 6th magnitude star o Makes it easy to compare star brightness ratios Apparent Visual Magnitudes Object Sirius (brightest star) Venus (at brightest) Full Moon The Sun Faintest naked eye stars Faintest star visible from Earth telescopes Faintest star visible from Hubble Space Telescope Apparent Visual Magnitude -1.5 -4.4 -12.6 -26.8 6-7 ~25 ~? Absolute Magnitude Actual star brightness The apparent magnitude that a star would have if it were (in our imagination) placed at a distance of 10 parsecs (which is 32.6 light years) from the Earth Used to describe luminosity - The amount of energy a star gives off each second The 20 Brightest Stars in the Sky Common Name Luminosity Solar Units Distance Spectral LY Type Proper Motion arcsec / year R. A. hours min Declination deg min Sirius 40 9 A1V 1.33 06 45.1 -16 43 Canopus 1500 98 F01 0.02 06 24.0 -52 42 Alpha Centauri 2 4 G2V 3.68 14 39.6 -60 50 Arcturus 100 36 K2III 2.28 14 15.7 +19 11 Vega 50 26 A0V 0.34 18 36.9 +38 47 Capella 200 46 G5III 0.44 05 16.7 +46 00 Rigel 80,000 815 B8Ia 0.00 05 12.1 -08 12 Procyon 9 11 F5IV-V 1.25 07 39.3 +05 13 Betelgeuse 100,000 500 M2Iab 0.03 05 55.2 +07 24 Achernar 500 65 B3V 0.10 01 37.7 -57 14 Beta Centauri 9300 300 B1III 0.04 14 03.8 -60 22 Altair 10 17 A7IV-V 0.66 19 50.8 +08 52 Aldeberan 200 20 K5III 0.20 04 35.9 +16 31 Spica 6000 260 B1V 0.05 13 25.2 -11 10 Antares 10,000 390 M1Ib 0.03 16 29.4 -26 26 Pollux 60 39 K0III 0.62 07 45.3 +28 02 Fomalhaut 50 23 A3V 0.37 22 57.6 -29 37 Deneb 80,000 1400 A2Ia 0.00 20 41.4 +45 17 Beta Crucis 10,000 490 B0.5IV 0.05 12 47.7 -59 41 Regulus 150 85 B7V 0.25 10 08.3 +11 58 Here is a list of the 314 stars brighter than apparent magnitude 3.55 in both hemispheres. H-R Diagram Hertzsprung-Russell diagram A way to compare star temperatures & spectral types with their absolute magnitude & luminosity Most stars fall along a patterned sequence - main sequence stars Size indicated by dwarf (like our Sun), giant, and supergiant Stellar Distances Easiest way to tell distance = sight a star at different locations and see how far the star moves relative to a distant background = parallax Combining the distance of 1 A.U. (our distance from the Sun), with the angle (in arc seconds) an object makes with the Earth and the Sun, we define distance in Parsecs: o A star that is 1 parsec from the Sun has a parallax of one arc second o d (parsecs) = (1/p)(seconds of arc) 1 A.U. 1 Parsec 1 Arc sec Example: If a star has a parallax angle of ¼ (or 0.25) arc seconds, how many parsecs is it away from the Sun? Some common distances o Light Year: the distance that light travels in one year (9.46 x 10^17 cm) o Parsec (pc): 3.26 light years (or 3.086 x 10^18 cm).; also kiloparsec (kpc) = 1000 parsecs and megaparsec (Mpc) = 1,000,000 parsecs o Astronomical Unit (AU): the average separation of the earth and the sun (1.496 x 10^13 cm) Some representative distances o The Solar System is about 80 Astronomical Units in diameter. o The nearest star (other than the sun) is 4.3 light years away. o Our Galaxy (the Milky Way) is about 100,000 light years in diameter. o Diameter of local cluster of galaxies: about 1 Megaparsec. o Distance to M87 in the Virgo cluster: 50 million light years. o Distance to most distant object seen in the universe: about 18 billion light years (18 x 10^9 light years). Stellar Motion Proper motion - the apparent change of position of a star on the celestial sphere o Denoted by the Greek symbol "mu" o Is a velocity in units of seconds of arc per year o Proper motion is not large. The star with the largest proper motion is called Barnard's Star. It moves 10.3 seconds of arc per year. o Since the moon subtends about 1/2 of a degree (which is 1/2 x 60 x 60 = 1800 seconds of arc) on the celestial sphere, it takes Barnard's star about 1800/10.3 ~ 180 years to change its position by the angular diameter of the moon. All other stars have smaller proper motions. The actual motion of stars involves a path in three space dimensions, so the proper motion is just the projection of this true motion on the celestial sphere. This true velocity of the star is called the space velocity o Tangential velocity is responsible for the proper motion To determine this generally requires that we know the distance to the star o Radial velocity is the motion of stars away from Earth (outward component) Creates a Doppler shift of the spectral lines that can be used to determine it directly The full space velocity of a star follows from Pythagoras' Theorem if both the tangential and radial velocities are known. Typical values for the space velocities of stars are 20-100 km/s.