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Spectral Classification

The spectrum of a star reveals dark lines, called absorption lines.

Absorption lines are produced by cooler thin gas in the upper layers of a star absorbing certain colors of light produced by hotter, denser lower layers.

Each element has a unique pattern of absorption lines; a spectrum is like a fingerprint.

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A star's spectrum can also reveal the star's radial speed and whether it is moving toward or away from us (via Doppler shift).

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Stars are divided into 7 spectral classes. Each spectral class is divided into 10 parts (the lower the number, the hotter the temperature). A0 is hotter than A1, etc.

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Distribution of Spectral Classes among Milky Way stars. We observe the following distribution of stars in our neighborhood of the Milky Way based on spectral types. Small, cool K and M class stars are the most common. Large, very hot stars are rare.

Each spectral class is divided into 10 parts (the lower the number, the hotter the temperature: A0 is hotter than A1, etc.

Spectral class O is exception (sub-divided into O4 - O9).

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Spectral Classes in the night sky

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The H-R Diagram

In 1913, astronomers Hertzsprung and Russell independently recognized a relationship between stellar luminosity and temperature.

They plotted this relationship on the now-famous “Hertzsprung-Russell”(H-R) Diagram.

The H-R diagram can tell us the following information about a star:

• temperature
• luminosity
• radius
• age
• mass
• total lifetime
• and how the star will die

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More than 90% of the stars plotted on an H-R diagram fall along a band called the “Main Sequence”. The position of a star on the Main Sequence depends on the star’s mass. The more massive a star is, the hotter and brighter it is. Mass increases from spectral class M (0.08 Msun) up the main sequence to spectral class O (50-100 Msun).

For any star on the Main Sequence, we only need to know one property (mass, surface temperature, luminosity, or radius) to be able to determine the remaining properties.

More than 90% of the stars plotted on an H-R diagram fall along a band called the “Main Sequence”.

The position of a star on the Main Sequence depends on the star’s mass. The more massive a star is, the hotter and brighter it is.

Mass increases from spectral class M (0.08 Msun) up the main sequence to spectral class O (50-100 M_sun).

Path to the Main Sequence

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The time between molecular cloud and settling onto the Main Sequence depends on the mass of the protostar

Stars with too little mass do not have enough gravitational compression in their cores to produce the required high temperatures and densities needed for nuclear fusion. The lowest Main Sequence star mass is about 0.08 solar masses or about 80 Jupiter masses. Stars less massive than this do not undergo fusion and are called brown dwarfs.

Stars with too much mass have so much radiation pressure inside pushing outward on the upper layers, that the star is unstable. The observed mass limit of a star is about 100 solar masses.

When a molecular cloud fragments into a cluster of protostars of differing masses, the evolving stars will reach the main sequence at different times depending on their mass.

The more massive stars begin burning hydrogen first, and in beadlike progression the others arrive along the zero-age main sequence from upper left down to the lower right in the diagram.

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The path to the Main Sequence depends on the pass of the protostar.