The text says a star does not change its mass very much during the course of its main-sequence lifetime. while it is on the main sequence, a star converts about 10% of the hydrogen initially present into helium (remember it’s only the core of the star that is hot enough for fusion). look in earlier chapters to find out what percentage of the hydrogen mass involved in fusion is lost because it is converted to energy. by how much does the mass of the whole star change as a result of fusion? were we correct to say that the mass of a star does not change significantly while it is on the main sequence?