Stellar Magnitude, Distance, and Motion
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 |
|[pic] |
|Object |Apparent Visual |
| |Magnitude |
|[pic] |
|Sirius (brightest star) |-1.5 |
|Venus (at brightest) |-4.4 |
|Full Moon |-12.6 |
|The Sun |-26.8 |
|Faintest naked eye stars |6-7 |
|Faintest star visible from |~25 |
|Earth telescopes | |
|Faintest star visible from |~? |
|Hubble Space Telescope | |
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 |
|[pic] |
|Common |Luminosity |Distance |Spectral |Proper Motion |R. A. |Declination |
|Name |Solar Units |LY |Type |arcsec / year |hours min |deg min |
|[pic] |
|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)
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.
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