All the Stars, How do we know what they are?

Visualizing the Universe, one pixel at a time.

"Stars—-all the Stars —we didn't know at all. We didn't know anything. We thought six stars in a universe is something… "
-- Isaac Asimov, "Nightfall"
"Twinkle,. Twinkle little star…

Stellar Evolution --How Do We Know?

The Electromagnetic Spectrum
can be described in terms of a stream of photons, each traveling in a wave-like pattern, moving at the speed of light and carrying some amount of energy. The only difference between radio waves, visible light, and gamma-rays is the energy of the photons. Radio waves have photons with low energies, microwaves have a little more energy than radio waves, infrared has still more, then visible, ultraviolet, X-rays, and gamma-rays.


Here's a really good NASA explanation of the Electromagnetic Spectrum


Some things to note

Radiation is Energy emitted in the form of waves (light) or particles (photons).

In many cases, people may say a photon "sometimes acts like a wave, and sometimes acts like a particle". This is slightly misleading, because a photon always acts like both.

Cosmic Rays
(Are not part of the Electromagnetic Spectrum)
In astrophysics, cosmic rays are radiation consisting of energetic particles originating beyond the Earth that impinge on the Earth's atmosphere. Cosmic rays are composed mainly of bare nuclei, roughly 87% protons, 12% alpha particles (helium nuclei) and most of the rest being made up of heavier atomic nuclei. Electrons, gamma rays, and very high-energy neutrinos also make up a much smaller fraction of the cosmic radiation. They originate in energetic events from beyond the solar system and travel very close to the speed of light.

Visible Light --- The Solar Spectrum

Types of Spectra

ROY G BIV, White light component colors, Red, Orange Yellow Green Blue, Indigo and Violet. Prism

Three kinds of spectra

Spectra Source

Three kinds of spectra

Types of Spectra

Spectrum of the Sun!

Solar spectrum

Stellar Spectra

Stellar Spectra

The Classification of Stars

AnnieJumpCannon 185px-Leavitt_aavso

The HR-Diagram

The Horizontal Axis = Spectral Type = Temperature

Each spectral class is subdivided into 10 parts

O0 ,   O1 , O2, ...  O9

B0 ,   B1 , B2, ...  B9


Our Sun is Spectral Type G2

Surface Temperature 5,500 ˚ K (11,000 ˚ F)

Color = Yellow

The spectral classes have been expanded to to include

W, O, B A, F. G, K, M, R, N, S and L, T, Y

For a more robust explanation of the types of stars included in the above "classes" Read Wiki's Stellar Classification

The Vertical Axis = Absolute Magnitude or Luminosity

The Sun's Luminosity is = 1

Luminosity is the total brightness of a star (or galaxy). Luminosity is the total amount of energy that a star radiates each second relative to the sun.

The Sun's absolute magnitude = +4.56

Absolute magnitude is a measure of the inherent brightness of a celestial object. This scale is defined as the apparent magnitude a star would have if it were seen from a standard distance of 32.6 light-years (10 parsecs). The lower the number, the brighter the object. Negative numbers indicate extreme brightness.

Parallax is the apparent change in the position of a star that is caused only by the motion of the Earth as it orbits the Sun. Friedrich Wilhelm Bessel first detected the parallax "motion" of a star in 1838 observing the star 61 Cygni (this was definitive proof that the Earth orbits the Sun, and not the other way around).

A parsec is a unit of distance that is equal to 3.26 light-years. It is the distance at which a star would have a parallax of 1 second of arc

Yerkes Classification of stars (Includes Luminosity)

  • O    Hypergiants
  • Ia    Most luminous Supergiants
  • Ib    Less luminous Supergiants
  • II     Luminous Giants
  • III    Normal giants (Red Giants)
  • IV    Subgiants
  • V     Main sequence stars (Dwarfs)
  • VI   Sub Dwarfs
  • VII   White Dwarfs

The HR Diagram


These pictures will link you to the Space Telescope Institute and detailed information about the pictures
Star Formation globules Eagle

Evolution of a 1 solar Mass Star called the Sun

1solarmassHR-Diagram track

  • Nebula
  • T-Tauri
  • Main Sequence
  • Red Giant
  • Nova
  • White Dwarf (C-N-O)

Evolution of a 5 - 8 solar mass star


  • Carbon Star explodes (fusion stops at C-N-O)
  • No remnant

Evolution of a 8 - 80 solar mass star (Super Giant)

Star StructureCore view

  • Shell detonation
  • blows off envelope
  • leaves collapsed core
  • Either a neutron star, or a Black Hole
  • Fe Fusion adsorbs energy
  • The Star collapses
  • Super Novas
  • Some heavy elements beyond Fe (Iron) Form during the Super Nova

Types of Super Novae

  • Type I
    • Type Ia White Dwarf Binary
      • Type Ib No helium in spectra
      • Type Ic Helium in spectrum
        • Ib and Ic are Giant stars that lost their outer envelope (really belong to Type II)
  • Type II
    • Type IIP Reaches a "plateau" in its light curve (Smaller star?)
    • Type IIL Displays a "linear" decrease in its light curve (linear in magnitude versus time)

End Points

White Dwarfs

Neutron Stars

Black Holes
Radius 1000 - 10000 Km 10 - 100 Km 0
Density 106 to 108 gm/cm3 1014 gm/cm3 infinite
Initial mass M < 5 solar masses M > 8 Solar Masses ?
M (remnant) M < 1.4 SolarM
M > 0.7 SolarM

White Dwarf
M < 3.2 SolarM

M > 3.2 Solar M
Example v838 crabmovie blackhole

Examples of End Points

Dwarf Stars become Planetary Nebulae Ghost Nebula
M-1  The Crab Nebula
A Neutron Star




Special Cases

movies were produced on the UKAFF computer.

Next -- Cosmology