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Space Facts

Spectral Type and Luminosity Class

How are stars classified?

When a star first forms, the mass it collects through gravity determines the star's color and brightness. Low mass stars are cooler, dimmer, and redder. Highly massive stars are hotter, brighter, and bluer. Mass determines how hot the nuclear fire within a star burns.

As an analogy, imagine the heating elements of a toaster. When electricity starts to pulse through the coils, those elements glow a deep brownish red. Very quickly, they warm to bright orange. If electricity were to continue increasing in strength, those heating elements would soon become bright yellow, then white. If the heat doesn't melt the elements, the increasing temperature would move the color toward a brilliant white, tinged with blue and violet.

The color of a star is directly related to the temperature of its surface. As the temperature at the surface of any object increases, the peak frequency of energy output increases along the spectrum from red toward the blue and violet.

Stars are nuclear furnaces. At their cores, sustained chain reactions of hydrogen fusion persist until the hydrogen fuel there is depleted. Depending on the initial mass of the star, this may take millions, billions, or even trillions of years to occur.

One of the chief byproducts of this fusion, besides heat and light, is helium. Once the core starts to run out of its primary fuel, gravity takes over, further compressing the star. Compressing gasses produce more heat, and the additional energy builds until it is hot enough to ignite the helium. Helium fusion produces far more energy than hydrogen. This causes the star to expand, first toward sub-giant stage, and later toward that of a red giant. Much more massive stars than our own achieve giant status while their surfaces are still quite blue, and later those stars become super-giants like Betelgeuse and Antares.

The traditional color scale is labelled with the letter sequence, O - B - A - F - G - K - M, from the blue to the red ends of the spectrum. Within each letter range, the scale is further divided numerically. For instance, within the "G" range, G0 and G1 are bright yellowish stars, while G9 is a deeper, orangish yellow. Our own sun is a class G2 star.

What is Luminosity Class?

Luminosity class labels a star's intrinsic brightness, and roughly places it within the star's lifetime. Each star spends the bulk of its life in the "main sequence." Here it is relatively stable in its light output, while it burns its hydrogen fuel. This is the "dwarf" stage, marked by the luminosity class roman numeral "V."

Luminosity class "IV" is a bit more complicated. Some class IV stars are pre-main sequence. These stars have not yet fully compressed to ignite their nuclear furnaces. The glow from the heat of compression makes the infant star appear to be a sub-giant. Later, the star shrinks, its surface becoming brighter (and bluer), and enters the main sequence. Much later, when its main fuel has been spent, the star balloons out to become, once again, a sub-giant — this time at the beginning of old age.

In star catalogs, labelling of spectral types may follow many varying conventions, some of them containing much more than color and luminosity class. Thus, a detailed discussion of spectral types is beyond the scope of this brief article.

This is an oversimplified view of the subject. The changes in a star as it grows old are far more complicated. Those changes depend, in part, on the original chemistry of the star — whether pure hydrogen, or hydrogen laced with heavier elements. The initial mass also plays a big part in determining the path a star takes in its development. A more massive star ends its life with an explosion that is galactic in energy output. It is from those explosions, and the complicated nuclear processes leading up to them, that we get all of the elements heavier than iron.