Star Notes Everything scientist know about a star they determined by looking at a dot. .

Star Chart Coordinates Right Ascension Count to the left 24 hours of RA hours, minutes, seconds Declination Count up & down 90 degrees of Dec + & -, not N & S

Apparent Magnitude In the magnitude system, m = 0 is a very bright star and m = 6 is at the naked eye limit. As m increases, brightness decreases. A change m = +5 corresponds to a factor of 100 decrease in brightness

Naming a Star First word is the Greek letter of the alphabet Second word is the name of the constellation Alpha Centauri Alpha Scorpii Delta Tauri

What can we know about a star? Its surface temperature the Sun is a yellow star (the solar spectrum peaks in the yellow part of the electromagnetic spectrum) the Sun’s photosphere has a temperature of 5800 K white and blue stars are hotter than the Sun orange and red stars are cooler than the Sun Infrared are warm Its brightness intensity = L/4D2 L is (luminosity) proportional to R2T4 (R is radius of star; T is temperature)

Stellar Parallax In astronomy, we measure position in angles. The arc second (1” = 1/3600) is the standard unit for measuring small changes in position. The parallax, p, is a star’s apparent shift (measured in ”) as a result of the Earth’s orbit around the Sun. The distance to the star, in parsecs (pc) is Its distance distance (parsec) = 1/parallax (in arcsec) d = 1/p 1 pcs = 3.26 light years

Temperature and Color Different spectral type stars have different surface temperatures: their spectra peak at different wavelengths, making them different colors. The intensity ratio in the blue and visible optical bands is a measure of a star’s “color”

The spectral sequence spectral type O B A F G K M mnemonic Oh Be A Fine Girl Kiss Me temperature 40000 20000 8000 6500 5500 4500 3000 color       

Star: Betelgeuse Spectral Type: M2 I Parallax: 0.00763″ Distance: 131 pc Apparent Magnitude: 0.41 Luminosity: 38,000 L☼ Star: Procyon Spectral Type: F5 IV-V Parallax: 0.28593″ Distance: 3.50 pc Apparent Magnitude: 0.37 Luminosity: 7.4 L☼ Star: Rigel Spectral Type: B8 I Parallax: 0.00422″ Distance: 237 pc Apparent Magnitude: 0.14 Luminosity: 70,000 L☼ Star: Sirius Spectral Type: A1 V Parallax: 0.37922″ Distance: 2.64 pc Apparent Magnitude: -1.46 Luminosity: 26 L☼

Absolute Magnitude The intensity depends on luminosity and distance. I = L / 4d2 The apparent magnitude a star would have if it were at a distance of 10 pc is called the absolute magnitude, M. The difference between the apparent magnitude m and absolute magnitude M is called the distance modulus: m - M = 5 log d - 5 This way, when we know the apparent magnitude m and the parallax p, we can calculate a star’s absolute magnitude, M.

Relative Luminosity Luminosity of Star = R2 x T4 How much brighter is a star that is twice as hot and three times bigger?

Relative Luminosity Luminosity of Star = R2 x T4 How much brighter is a star that is twice as hot and three times bigger? (3 x 3) X (2 x 2 x 2 x 2) 9 X 16 144 times as much light given off

Star Chart