. Horizontal-branch stars are sometimes referred to as "helium main-sequence stars." In what sense is this true?

Short Answer

Expert verified
The term 'helium main-sequence stars' refers to the horizontal-branch stars because they are in a stable, longest phase of their lifecycle, similar to main sequence stars, but instead of burning hydrogen, they are burning helium in their core.

Step by step solution

01

Understand Main Sequence Stars

Understand that a Main Sequence Star is a star that is in the longest stage of its life cycle where it is burning hydrogen in its core. Typical examples of such stars are our own Sun, Alpha Centauri A and Sirius A.
02

Know about Horizontal-branch Stars

Horizontal-branch stars are a stage of a star's life-cycle, that occurs after a red giant has used up most of its core hydrogen and starts burning helium in its core.
03

Draw the Comparison

With the comparison 'helium main-sequence stars' what is meant is that horizontal-branch stars are similar to main-sequence stars in the sense that they are going through the most extended, most stable phase of their lifecycle. However, instead of burning hydrogen, as main-sequence stars do, they are burning helium in the core.

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