Main sequence: Difference between revisions

Revision as of 07:18, 22 March 2024

Revision as of 08:19, 22 March 2024

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==Luminosity-color variation==

[[File: The Sun in white light.jpg|thumb|upright=1.0|The [[Sun]] is the most familiar example of a main-sequence star]]

As non-fusing helium ash accumulates in the core of a main-sequence star, the reduction in the abundance of hydrogen per unit mass results in a gradual lowering of the fusion rate within that mass. Since it is fusion-supplied power that maintains the outflowpressure of fusion-suppliedthe energycore thatand supports the higher layers of the star, the core isgradually gets compressed,. producingThis higherbrings temperatureshydrogen-rich andmaterial pressures.into Botha factorsshell increasearound the ratehelium-rich ofcore fusionat thusa movingdepth where the equilibriumpressure towardsis asufficient smaller,for denser,fusion hotterto coreoccur. producingThe morehigh energypower whoseoutput increasedfrom this outflowshell pushes the higher layers of the star further out. ThusThis there iscauses a steadygradual increase in the luminosityradius and radiusconsequently luminosity of the star over time.<ref name=clayton83/> For example, the luminosity of the early Sun was only about 70% of its current value.<ref name=sp74/> As a star ages thisit luminosity increasethus changes its position on the HR diagram. This effectevolution resultsis reflected in a broadening of the main sequence band becausewhich contains stars areat observedvarious at randomevolutionary stages in their lifetime. That is, the main sequence band develops a thickness on the HR diagram; it is not simply a narrow line.<ref name=padmanabhan01/>

Other factors that broaden the main sequence band on the HR diagram include uncertainty in the distance to stars and the presence of unresolved [[binary star]]s that can alter the observed stellar parameters. However, even perfect observation would show a fuzzy main sequence because mass is not the only parameter that affects a star's color and luminosity. Variations in chemical composition caused by the initial abundances, the star's [[stellar evolution|evolutionary status]],<ref name=apj128_3/> interaction with a [[binary star|close companion]],<ref name=tayler94/> [[stellar rotation|rapid rotation]],<ref name=mnras113/> or a [[stellar magnetic field|magnetic field]] can all slightly change a main-sequence star's HR diagram position, to name just a few factors. As an example, there are [[metallicity|metal-poor stars]] (with a very low abundance of elements with higher atomic numbers than helium) that lie just below the main sequence and are known as [[subdwarf]]s. These stars are fusing hydrogen in their cores and so they mark the lower edge of the main sequence fuzziness caused by variance in chemical composition.<ref name=cwcs13/>