Colors, the Spectrum, and its Uses

Colors, the Spectrum, and its Uses

How the color spectrum applies to stars:

When you look at a rainbow, you are seeing light being broken down into the different colors in the color spectrum. The same thing happens when light enters a prism. Light that hasn't been broken down into different colors is called white light.

By looking at the different colors that make up white light, scientists can tell a lot about the material that the light is being emitted from. A special device, called a spectrometer, is used to do this. Every element in the periodic table, when it is heated enough to be vaporized, emits its own 'fingerprint' of colors when it is observed with a spectrometer. Here are a few examples:

Helium	Argon
Hydrogen	Neon
Magnesium	Nitrogen
Oxygen	Calcium

These diagrams of what you'd see with a spectrometer are approximate, but as you can tell, they are very different. This is very useful because spectrometers can be used on stars to see what they're made of. They can also tell us if stars are moving towards us or away from us, and (in the Sun's case) can tell which direction it is rotating.

Above is a representation of the spectrum of the sun. There are several dark lines, which are called 'absorption' lines. These lines occur when light emitted in the hotter inner parts of a star are absorbed by materials (atoms) on the outside of the star that are cooler. By comparing the placement of the dark lines with the lines produced by different elements, scientists can figure out what elements are in the star.

    A and B - Oxygen
    C - Hydrogen
    D1 and D2 - Sodium
    E - Iron
    F - Hydrogen
    G - Iron and Calcium
    H - Calcium

Stars were first being photographed in 1885 at the Harvard College Observatory, and Deaf American astronomer Annie Cannon did most of the work. The research led to the idea that stars could be arranged according to the intensities of certain absorption lines in their spectrums. The intensities of hydrogen lines were probably the most important. By doing this, scientists could get clues about the ages and stages of development of stars. Here is the Hertzsprung-Russell diagram that illustrates this:

Stars are organized into different groups, or classes, represented by letters. On the above diagram, each set of stars in a group has similar compositions and the groups are arranged from the hottest to the coolest. The surface temperatures of stars range from 22,200 degrees Celsius down to 1,760 degrees. The average temperature inside a star is about 20 million degrees!
While most stars are located on the broad, wavy line in the middle of the graph (called the Main Sequence), some stars are located above or below that line. The stars that are above the line of stars are called Red Giants, and are much larger and brighter than other stars of the same temperature. Stars that are below the line are fainter and much smaller, and are classified as white dwarfs or neutron stars.

Class O - This group is made up of extremely hot stars - Has lines of hydrogen, helium, oxygen, and nitrogen.

Class B - This group of stars are slightly cooler than class O stars, and an example is the star e Oroinis - Has hydrogen and helium lines.

Class A - This group is made up of 'hydrogen' stars, and a typical star of this group is Sirius also known as the dog star - Absorption lines of hydrogen.

Class F - An example of this group of stars is the delta Aquilae star - Has strong lines of hydrogen and calcium.

Class G - This group of stars are commonly referred to as 'solar' stars because our sun belongs in this group - It has weaker calcium lines as the F group, but strong hydrogen lines. Iron is present in these stars.

Class K - An example of this group is the star Arcturus - It has strong calcium lines and lines that indicate other metals.

Class M - The star Orionis is an example of this group of stars - It has bands indicating the presence of metallic-oxide molecules, especially titanium oxide.

There are other uses for star's spectrums also. Absorption lines, mentioned above, can be used to find the temperature and pressure of a star's atmosphere. It can also be used to find out how abundant certain elements are in a star.

The luminosity of a star can be found by summing the intensities of the colors of the star's spectrum. By using the luminosity of the star and its surface temperature, it's radius (size) can be found.