<P> In April, 2018, astronomers reported the detection of the most distant "ordinary" (i.e., main sequence) star, named Icarus (formally, MACS J1149 Lensed Star 1), at 9 billion light - years away from Earth . </P> <P> When a protostar is formed from the collapse of a giant molecular cloud of gas and dust in the local interstellar medium, the initial composition is homogeneous throughout, consisting of about 70% hydrogen, 28% helium and trace amounts of other elements, by mass . The initial mass of the star depends on the local conditions within the cloud . (The mass distribution of newly formed stars is described empirically by the initial mass function .) During the initial collapse, this pre-main - sequence star generates energy through gravitational contraction . Upon reaching a suitable density, energy generation is begun at the core using an exothermic nuclear fusion process that converts hydrogen into helium . </P> <P> When nuclear fusion of hydrogen becomes the dominant energy production process and the excess energy gained from gravitational contraction has been lost, the star lies along a curve on the Hertzsprung--Russell diagram (or HR diagram) called the standard main sequence . Astronomers will sometimes refer to this stage as "zero age main sequence", or ZAMS . The ZAMS curve can be calculated using computer models of stellar properties at the point when stars begin hydrogen fusion . From this point, the brightness and surface temperature of stars typically increase with age . </P> <P> 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 star's lifetime . </P>

A zero-age main-sequence star is one that can
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