A&B's Astronomy Lab, Spring 2002, No. 1

Spectral Classification of Stars

How do blackbodies radiate?
Why can we assume stars as blackbodies?
What's the difference between blackbody spectra and stellar spectra
What can we learn from stellar spectra?

Blackbody Radiation & Plank's Law

All objects have internal energy manifested by the microscopic motions of particles. If the object is in thermal equilibrium, it can be characterized by a single quantity, it's temperature. An object in thermal equilibrium emits energy at all wavelengths resulting in a continuous spectrum, and we call this thermal radiation. A black object or blackbody absorbs all light which hits it. This blackbody also emits thermal radiation. e.g. photons (e.g. a glowing poker just out of the fire). The amount of energy emitted (per unit area) depends only on the temperature of the blackbody.

In 1900 Max Planck characterized the light coming from a blackbody. The equation that predicts the radiation of a blackbody at different temperatures is known as Planck's Law. Note that the peak shifts with temperature. The peak emission from the blackbody moves to shorter wavelengths as the temperature increases (Wien's law). The hotter the blackbody the more energy emitted per unit area at all wavelengths. Bigger objects emit more radiation The wavelength of the maximum emission of a blackbody is given by:

Kirchhoff's laws & Spectral Lines

Let's recall three laws that govern the spectrum we see from objects (known as Kirchhoff's laws).

1. A hot solid, liquid or gas at high pressure has a continuous spectrum.
2. A gas at low pressure and high temperature will produce emission lines.
3. A gas at low pressure in front of a hot continuum causes absorption lines.

Stars as Blackbodies
A star is a self-luminous sphere of gas. Almost all stars show a "continuum" spectum with "absorption" lines (can you make connection this concept to Kirchoff's third law?).

Despite having absorption lines, the spectrum of a star is close to that of a blackbody. What we see is produced by the hot surface called the photosphere. If stars are similar to blackbodies, then the spectrum will be close to Planck's law and we can estimate the temperature.

Spectral Classification of Stars
In the late 19th century astronomers catagorized stars according to the strength of the hydrogen absorption lines in the spectrum. They labels these A, B, ... from strongest to weakest.

Unfortunately, this was the wrong way to do it! Annie Jump Cannon arranged the spectra of stars in a sequence which corresponds to their temperatures (She classified over 500,000 stars in her career!)

While the differences in spectra might seem to indicate different chemical compositions, in almost all instances, it actually reflects different surface temperatures. With some exceptions, material on the surface of stars is "primitive": there is no significant chemical or nuclear processing of the gaseous outer envelope of a star once it has formed. Fusion at the core of the star results in fundamental compositional changes, but material does not generally mix between the visible surface of the star and its core. Ordered from highest temperature to lowest, the seven main stellar types are O, B, A, F, G, K, and M. O, B, and A type stars are often referred to as early spectral types, while cool stars (G, K, and M) are known as late type stars. The nomenclature is rooted in long-obsolete ideas about stellar evolution, but the terminology remains. The spectral characteristics of these types are summarized below:

Type Color Approximate Surface Temperature Main Characteristics Examples

O Blue > 25,000 K Singly ionized helium lines either in emission or absorption. Strong ultraviolet continuum. 10 Lacertra

B Blue 11,000 - 25,000 Neutral helium lines in absorption. Rigel
Spica

A Blue 7,500 - 11,000 Hydrogen lines at maximum strength for A0 stars, decreasing thereafter. Sirius
Vega

F Blue to White 6,000 - 7,500 Metallic lines become noticeable. Canopus
Procyon

G White to Yellow 5,000 - 6,000 Solar-type spectra. Absorption lines of neutral metallic atoms and ions (e.g. once-ionized calcium) grow in strength. Sun
Capella

K Orange to Red 3,500 - 5,000 Metallic lines dominate. Weak blue continuum. Arcturus
Aldebaran

M Red < 3,500 Molecular bands of titanium oxide noticeable. Betelgeuse
Antares

The temperature of the stellar photosphere determines the rate and severity of collisions between molecules, atoms and ions which in turn determines:
1. the molecular equilibrium - if the star is too hot, fragile molecular bonds will be broken apart. Most molecules such as TiO are seen only in spectra of the coolest stars ( T = 3000 -- 4000K). Strong molecules such as CH and CN can be seen in somewhat hotter stars like the sun.
2. the ionization equilibrium - the hotter the temperature, the higher the ionization state of the atoms in the stellar atmosphere will be. Atoms are ionized (or partially ionized) when they lose or gain an electron. In cool stars most atoms will be neutral. At higher Temperature, easily ionized atoms such as Na, Ca, etc, will be ionized; above T = 10,000K hydrogen becomes ionized and above about 15,000K helium becomes ionized.
3. the number of atoms in excited states. At low T, almost no H atoms are in the n=2 orbit, capable of absorbing "Balmer" photons, but as T increases the n=2 population increases and we see the hydrogen features, reaching a maximum in stars with T = 10,000.
Among the other things that we may determine from the absorption spectrum are: density, chemical composition, magnetic field strength, and radial velocity. These are all secondary effects compared to temperature.

Blackbody Radiation & Plank's Law
All objects have internal energy manifested by the microscopic motions of particles. If the object is in thermal equilibrium, it can be characterized by a single quantity, it's temperature. An object in thermal equilibrium emits energy at all wavelengths resulting in a continuous spectrum, and we call this thermal radiation. A black object or blackbody absorbs all light which hits it. This blackbody also emits thermal radiation. e.g. photons (e.g. a glowing poker just out of the fire). The amount of energy emitted (per unit area) depends only on the temperature of the blackbody.
In 1900 Max Planck characterized the light coming from a blackbody. The equation that predicts the radiation of a blackbody at different temperatures is known as Planck's Law. Note that the peak shifts with temperature. The peak emission from the blackbody moves to shorter wavelengths as the temperature increases (Wien's law). The hotter the blackbody the more energy emitted per unit area at all wavelengths. Bigger objects emit more radiation The wavelength of the maximum emission of a blackbody is given by:

Type	Color	Approximate Surface Temperature	Main Characteristics	Examples
O	Blue	> 25,000 K	Singly ionized helium lines either in emission or absorption. Strong ultraviolet continuum.	10 Lacertra
B	Blue	11,000 - 25,000	Neutral helium lines in absorption.	Rigel Spica
A	Blue	7,500 - 11,000	Hydrogen lines at maximum strength for A0 stars, decreasing thereafter.	Sirius Vega
F	Blue to White	6,000 - 7,500	Metallic lines become noticeable.	Canopus Procyon
G	White to Yellow	5,000 - 6,000	Solar-type spectra. Absorption lines of neutral metallic atoms and ions (e.g. once-ionized calcium) grow in strength.	Sun Capella
K	Orange to Red	3,500 - 5,000	Metallic lines dominate. Weak blue continuum.	Arcturus Aldebaran
M	Red	< 3,500	Molecular bands of titanium oxide noticeable.	Betelgeuse Antares