Most of a Stars features are interrelated and thus can be used to determine other features

• Apparent brightness
• Distance
• Luminosity

Luminosity

• The luminosity of a star is its total energy output into all space over all wavelengths.
• The Sun is equal to 1. So if a star had a luminosity of 10 it would be 10 times more luminescent than our sun.
• If you know a star’s distance its luminosity can be determined by its brightness

Photometry

• Photometry is the measurement of a star’s apparent brightness.
• Inverse-Square law
• Relates apparent brightness and luminosity.
• The apparent brightness is inversely proportional to the square of the observers distance from the source
• Formula b= L/4?d2
• b=apparent brightness of a stars light in W/m2
• L=stars luminosity in W
• d= distance to star in meters

Determining luminosity from apparent brightness

• L/L? = (d/d?)2 * (b/b?)
• L/L? is the ratio of the stars luminosity to the Sun’s luminosity
• d/d? is the ratio of the stars distance to the Earth-Sun distance
• b/b? ratio of the star’s apparent brightness to the Sun’s app brightness
  

Need to know 2 things to determine a star’s luminosity
Apparent brightness and distance to the star

Magnitude Scale

Absolute Magnitude: A measure of a star’s true energy output (luminosity).

• This is what the star would look like (apparent brightness) as seen from 10 pc away
• Can be used to determine brightness of a star

Apparent (m) and Absolute (M) Magnitude

• A star’s distance can be determined by knowing the star’s M and m.
• The M or the m can by determined by knowing the distance and either the m or M.
• m-M= 5 log d – 5
• m = star’s apparent magnitude
• M = stars absolute magnitude
• d = distance from the star in parsecs

Stars Color

• The surface temperature determines the color of the star.
• Thinking about flames which color do you think equals a hotter star.
• Blue, red, white, yellow????
• Wein’s law lambda max = 0.0029/Temp
• Law can be reworked to determine the temperature from the wavelength of the light emitted

Hertsprung/Russell diagram

• All of the above information is interrelated and can be plotted interdependently of each other.
• 2 Astronomers independently came up with a graph that represents stars lumiosity, temperature, magnitude.
• Danish astronomer – Ejnar Hertzsprung
• American astronomer – Henry Norris Russell (2 years later
• The Hertzsprung-Russell diagram is a statistical plot of luminosity and spectra
  

Spectrum

• Stars spectrum were analysed and a classification system was developed.
• Keep in mind that temperature plays a large part in determining the color and spectra.

Questions

• Compared to a star in the middle of the diagram, a star in the lower left part of the Hertzsprung-Russell diagram is?
• What is the size of a star in the upper right part of the Hertzsprung-Russell diagram compared to one in the middle of the diagram?

Spectral Classification

• Spectral classification of a star into the lettered categories, O, B, A, F, G, K, M,
• is carried out by examining the relative depths of absorption lines from various neutral and ionized atoms in a stellar spectrum.

Using the Hertzsprung-Russell diagram determine which type of star has the following characteristics: surface temperature of 40,000 K and luminosity 100,000 times that of the Sun.

• The spectral type of a star is most directly related to (and determines uniquely) its surface temperature.
• In order of decreasing temperature, the complete spectral sequence for stars and brown dwarfs, as determined by relative spectral absorption line strengths, is OBAFGKMLT.
• In the spectral sequence of star types, each category has been divided into ten intervals. The Sun is classified as G2
• The spectral class of the Sun is G2 and the star Enif is K2. From this information, we know that Enif is cooler than the sun

A metal to an astronomer is any element heavier than hydrogen or helium. In terms of spectrums absorption lines of He II would indicate very high surface temperature

Astronomers can find the luminosity class (I, II, III, IV, or V) of a star using the star's spectrum because the absorption lines in the spectrum are affected by the density and pressure of the star's atmosphere.

Odds-n-Ends
Spectroscopic parallax is the distance to a star measured using the spectral-luminosity class of the star and the inverse square law
interstellar dust makes the star look redder than it really is.