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The Main Sequence

The distinctive band of stars that appear on the HR diagram

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The Main Sequence

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The main sequence is the name for a continuous and distinctive band of stars that appear on a plot of stellar colour versus brightness.

These colour-magnitude plots are known as Hertzsprung-Russell diagram after their co-developers, Ejnar Hertzsprung and Henry Norris Russell. Stars on this band are known as main-sequence stars or dwarf stars.

After a star has formed, it generates energy at the hot, dense core region through the nuclear fusion of hydrogen atoms into helium. During this stage of the stars lifetime, it is located along the main sequence at a position determined primarily by its mass, but also based upon its chemical composition and other factors. In general, the more massive the star the shorter its lifespan on the main sequence. After the hydrogen fuel at the core has been consumed, the star evolves away from the main sequence.

The main sequence is visible as a prominent diagonal band that runs from the upper left to the lower right.

The life span of a star is linked to its mass (in other words the amount of Hydrogen fuel). A star such as Betelgeuse is about 20 times more massive than the Sun and about 14,000 times brighter. Because it is bigger and brighter it burns fuel at a rate 14,000 times faster than our the Sun. Our Sun will still be shining as bright as ever when Betelgeuse has used all its fuel. In fact our Sun will live approx 7000 times longer than Betelgeuse.

Main sequence stars have been extensively studied through stellar models, allowing their formation and evolutionary history to be relatively well understood. The position of the star on the main sequence provides information about its physical properties.

The temperature of a star can be approximately determined by treating it as an idealized energy radiator known as a black body. In this case, the luminosity L and radius R are related to the temperature T by the Stefan-Boltzmann Law:

L=4 pi sigma R^2 T^3
Equation 26 - Stefan-Boltzmann Law

The colour index, or B - V, measures the difference in this energy emission by means of filters that capture the stars magnitude in blue (B) and green-yellow (V) light. (By measuring the difference between these values, this eliminates the need to correct the magnitudes for distance.) Thus the position of a star on the HR diagram can be used to estimate its radius and temperature.

A star remains near its initial position on the main sequence until a significant amount of hydrogen in the core has been consumed, then begins to evolve into a more luminous star. (On the HR diagram, the evolving star moves up and to the right of the main sequence.) Thus the main sequence represents the primary hydrogen-burning stage of a stars lifetime.

Last updated on: Tuesday 20th June 2017

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