Main sequence: Difference between revisions

Revision as of 01:38, 23 June 2025

Revision as of 17:00, 25 June 2025

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[[File:Evolutionary track 1m.svg|thumb|left|Evolutionary track of a star like the sun]]

When a main-sequence star has consumed the hydrogen at its core, the loss of energy generation causes its gravitational collapse to resume and the star evolves off the main sequence. The path which the star follows across the HR diagram is called an evolutionary track.<ref name="Iben2012">{{cite book |author=Icko Iben |title=Stellar Evolution Physics |url=https://books.google.com/books?id=IU357EiecWwC&pg=PA1481 |date=29 November 2012 |publisher=Cambridge University Press |isbn=978-1-107-01657-6 |pages=1481–}}</ref> The start of the main sequence track is known as the zero age main sequence (ZAMS), while the end point where the core hydrogen is consumed is known as the terminal age main sequence (TAMS).<ref>{{cite journal

| title=Spectroscopic evolution of massive stars near the main sequence at low metallicity

| last1=Martins | first1=F. | last2=Palacios | first2=A.

| journal=Astronomy & Astrophysics

| volume=645 | at=id. A67 | date=January 2021

| doi=10.1051/0004-6361/202039337 | arxiv=2010.13430

| bibcode=2021A&A...645A..67M }}</ref>

[[File: Open cluster HR diagram ages.gif|right|thumb|upright=1.2|[[Hertzsprung–Russell diagram|H–R diagram]] for two open clusters: [[NGC 188]] (blue) is older and shows a lower turn off from the main sequence than [[Messier 67|M67]] (yellow). The dots outside the two sequences are mostly foreground and background stars with no relation to the clusters.]]

Stars with less than {{solar mass|0.23}}<ref name=romp69>{{cite journal |author1 last1=Adams, | first1=Fred C. |author2 last2=Laughlin, | first2=Gregory |title=A Dying Universe: The Long Term Fate and Evolution of Astrophysical Objects |journal=Reviews of Modern Physics |date=April 1997 |volume=69 |issue=2 |pages=337–372 |doi=10.1103/RevModPhys.69.337 |bibcode=1997RvMP...69..337A |arxiv=astro-ph/9701131 |s2cid=12173790}}</ref> are predicted to directly become [[white dwarf]]s when energy generation by nuclear fusion of hydrogen at their core comes to a halt, but stars in this mass range have main-sequence lifetimes longer than the current age of the universe, so no stars are old enough for this to have occurred.

In stars more massive than {{solar mass|0.23}}, the hydrogen surrounding the helium core reaches sufficient temperature and pressure to undergo fusion, forming a hydrogen-burning shell and causing the outer layers of the star to expand and cool. The stage as these stars move away from the main sequence is known as the [[subgiant branch]]; it is relatively brief and appears as a [[Hertzsprung gap|gap]] in the evolutionary track since few stars are observed at that point.