Lecture 15: Stellar Properties
Astronomy 101/103
Terry Herter, Cornell University
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Lecture
Topics
  • The Hertzsprung-Russell Diagram
  • Luminosity Classes of Stars
  • Binary Stars

Luminosity
vs.
Color of Stars
  • In 1911, Ejnar Hertzsprung investigated the relationship between luminosity and colors of stars in within clusters.
  • In 1913, Henry Norris Russell did a similar study of nearby stars.
  • Both found that the color (temperature, spectral type) was related to the luminosity.


Schematic
Hertzsprung-Russell
Diagram


Observational Effects
  • An H-R diagram of the brightest stars will preferentially show luminous stars since we can see them farther away.
  • An H-R diagram of the nearest stars show many M type stars since M stars are very numerous.


Absolute
Magnitudes

 

Notes on H-R Diagram

  • There are different regions
    • main sequence, giant, supergiant, etc.
  • Most stars lie along the main-sequence.
  • For a given spectral class (e.g. K), there can be more than one luminosity.
    • i.e. main-sequence, giant or supergiant
  • On the main sequence, there are many more K and M stars than O and B stars.


Luminosity
Classes

  • Ia : Brightest Supergiants
  • Ib : Less luminous supergiants
  • II : Bright giants
  • III : Giants
  • IV : Subgiants
  • V : Main-sequence stars

Luminosity Classes


Luminosity

  • Total energy per second radiated from a star of radius R.
  • The luminosity, L, is given by:

  • So supergiants must be big!!


How Big
are
Supergiants?
  • Betelgeuse: M2 Iab (supergiant)
    • L ~ 40,000 Lsun, T ~ 3,500 K
  • Sun: G2 V (main-sequence)
    • T ~ 5,000 K

 

H-R Diagram Showing Radii of Stars


Spectorscopic
Parallax
  • From a star's spectrum, we can determine its spectral and luminosity class.
  • Given the star's apparent brightness (observed flux), we can then estimate its distance.
  • This distance determination technique is called spectroscopic parallax.

Spectroscopic Parallax Example

  • Observe a G2 Ia star (supergiant) which has
    • mv = 10 (apparent magnitude)
  • The absolute magnitude (from the H-R diagram) is Mv = -5.
  • How far away is the star?
  • but mv - Mv = -5 + 5*log10(d)
    => log10(d) = 20/5 = 4
    => d = 10,000 pc


Setellar
Properties

  • What do we know about stars?
    • temperature
    • luminosity
    • radius
    • composition
  • All we need now is the mass.


Stellar
Masses

  • The mass of a star is very important in determining its properties.
  • The mass and composition are all you need to know about a star!
    • determine the temperature, radius, and luminosity of a star over its lifetime.
  • But, how do we "weigh" a star?
  • Binary stars
    • pairs of stars that orbit each other
    • used to determine masses of stars


Binary Stars


Types of
Binaries
  • Visual Binary
    • Stars are separated in a telescope.
  • Spectroscopic Binary
    • See two sets of spectral lines Doppler shifted due to orbital motion.
  • Eclipsing Binary (rare)
    • Stars cross in front of one another.


Binary Stars
Importance

  • 75 % of all stars are "binary stars"
  • Studies give:
    • Stellar Masses (Visual & Spectroscopic)
    • Stellar Radii (Eclipsing)
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