Star Notes

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

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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

• 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

Apparent Magnitude

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/4D2

• 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) isIts 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 Mmnemonic Oh Be A Fine Girl Kiss Metemperature 40000 20000 8000 6500 5500 4500 3000

color

Star: ProcyonSpectral Type: F5 IV-VParallax: 0.28593″Distance: 3.50 pcApparent Magnitude: 0.37Luminosity: 7.4 L☼

Star: SiriusSpectral Type: A1 VParallax: 0.37922″Distance: 2.64 pcApparent Magnitude: -1.46Luminosity: 26 L☼

Star: BetelgeuseSpectral Type: M2 IParallax: 0.00763″Distance: 131 pcApparent Magnitude: 0.41Luminosity: 38,000 L☼

Star: RigelSpectral Type: B8 IParallax: 0.00422″Distance: 237 pcApparent Magnitude: 0.14Luminosity: 70,000 L☼

Absolute Magnitude

• The intensity depends on luminosity and distance.

I = L / 4d2

• 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