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.

-----------------------

[pic]

................
................

In order to avoid copyright disputes, this page is only a partial summary.

Google Online Preview   Download