In article ,
Yousuf Khan writes:
http://news.nationalgeographic.com/n...onomy-science/

The link above has a link to a preprint of the paper.

According to existing stellar evolution models, sunlike stars—those
that are similar in size and chemical composition to our sun—swell to
become so-called red giants in their final stage of life, before
losing their atmospheres in a spectacular bubble of gas and dust.

But now astronomers have found that not all sunlike stars follow the
same rules when it comes to the end of their life cycles, and that
some can skip the AGB phase altogether.

The above press release text is literally correct but could be
misleading because of the abrupt switch from "red giant" (RG) to
"asymptotic giant branch" (AGB). The RG phase is an early stage of
post-main-sequence evolution where the star is "burning" hydrogen in
a shell (the core having been depleted of hydrogen). The AGB phase
is a later, much briefer phase where the core is also depleted of
helium, and the star is burning hydrogen in an outer shell and helium
in an inner shell.

What the new work shows is that some stars -- more precisely some
stellar populations -- skip the AGB phase. The reason is unknown,
but it may have to do with helium abundance or details of mass loss
in the RG phase. All stars (in the relevant mass range) go through
the RG phase; they don't evolve directly from the main sequence to
white dwarf, despite what the press release might suggest.

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