Binary Mass Transfer and Stellar Evolution

The simple picture that we have painted concerning stellar evolution is for isolated stars. Then essentially a single parameter, mass, determines their fate. The situation becomes more complicated if the star in question has a close companion in a binary system. In that case, mass transfer between the stars can lead to very different histories than we would expect for an isolated star.

Phi Persei
An example is phi Persei, which is visible as a 4th magnitude star just north of the Andromeda Galaxy (M31), about 720 light years away in the constellation Perseus. Detailed observations indicate that it is a binary with a period of 126 days and an average separation of about 1 AU. It contains a very hot star of about a solar mass and a much more massive hot, blue companion that is spinning rapidly with a large disk of gas around it. Theory coupled with the observations suggests the history for this binary that is illustrated in the following figure.

Observations suggest that the now massive blue star is spinning so fast that matter travels at 450 km/s at its equator, and that this causes it to lose hydrogen into a ring that is about 8 times larger than the star itself.

Thus, in this system the transfer of mass between close binary companions has led to a completely different evolution than would have been expected for isolated stars. Further, there may be additional parts to the story, since the now massive blue star will exhaust its newly acquired fuel within about 10 million years. As it leaves the main sequence, it may transfer mass back to the now small companion (which could produce a type Ia supernova; see later in this chapter) or perhaps engulf the companion as it swells.

The Algol Paradox
Actually, we have already encountered a situation closely related to the one described above, though we might not have recognized it at the time. In the Algol binary system studied in Chapter 19 (coincidentally, also in the constellation Perseus), the primary star is a blue B8 main sequence star of 3.7 solar masses and the secondary star is a red K2 subgiant of about 0.8 solar masses.

Based on even the qualitative discussion to this point, this should sound an alarm bell. We have stated clearly that mass is the primary deciding factor in how fast a star evolves. How then can the 3.7 solar mass star in the Algol system still be on the main sequence, while the much less massive companion has already left the main sequence and begun evolving into the giant region (assuming, as is logical for a binary, that the two stars formed at the same time)? Similar features are observed in various other binary systems, and the general situation of a less massive star in a binary appearing to be more evolved than a more massive one is sometimes called the Algol paradox.

Solution of the Algol Paradox
The Algol paradox is only apparent, and the solution is mass transfer again. Consider the following figure, and let us refer to the present B8 star in Algol as Star B and the present K2 star as Star A in the following discussion.

When the Algol system formed, we believe that Star A was initially the more massive star and Star B was less massive (opposite the present situation). Star A, being more massive, evolved faster. When it left the main sequence it expanded to fill its Roche lobe and began to rapidly pour matter onto Star B. As this mass transfer proceeded, Star B soon became more massive than star A. Eventually, the mass transfer from Star A to Star B slowed to a trickle (the present apparent situation), leaving us with Star B as the more massive star. But Star B is still on the main sequence, despite its age and mass, because it was not born with this mass but received it as a gift later in its life. Algol appears to be a paradox only if we assume (apparently incorrectly) that the present more massive star has always been the more massive.

Blue Stragglers Again

For completeness, let us also recall an issue that appeared in our earlier discussion of evolution in star clusters (Chapter 19). In the HR diagram for evolved star clusters, one sometimes finds main sequence stars that lie above the turnoff point. These are called blue stragglers. Such stars appear to be far too young to exist in an old cluster. The likely explanation for blue stragglers has already been mentioned in Chapter 19. Blue stragglers may be an extreme case of mass transfer where the members of a binary merge into a single star and begin evolving as a single more massive star. In this explanation, the blue stragglers are in a sense younger than the cluster in which they live, because they were formed long after the birth of the cluster in a binary star merger.