Evolution of Close Binary Systems
• There are many multiple star systems in the
Galaxy
• For the vast majority, the separation of the
stars is large enough that one star doesn’t
affect the evolution of the other(s)
The Algol Mystery
• Algol is a double-lined eclipsing binary
system with a period of about 3 days
(very short). The two stars are:
– Star A: B8, 3.4 M main-sequence star
– Star B: G5, 0.8 M subgiant star
• What is wrong with this picture?
The Algol Mystery
• The more massive star (A) should have
left the main sequence and started up the
RGB before the less massive star (B).
• What is going on here?
• The key is the short-period orbit.
The Algol Mystery
• Originally the system contained Star A at
1.2 M and Star B at 3.0 M
• Between the two stars is a point where
the gravitational forces of the two stars
balance. This is called a Lagrange point.
The Algol Mystery
• As Star B evolves and expands as it heads
up the RGB
• When its radius equals the distance of the
L1 point (called the Roche Radius) the
material in Star B’s envelope feels a
stronger attraction to Star A and there is
mass transferred from B to A.
The Algol Mystery
• In the case of Algol, Star B transferred
2.2 M of material to Star A
Star A: 1.2 M => 3.4 M
Star B: 3.0 M => 0.8 M
Mass Transfer Binaries
• Think about the continued evolution of
Algol and you have the explanation for
novae.
• If the original primary transfers most of
its mass to the original secondary, you are
left with a massive main-sequence star
and a White Dwarf.
• When the original secondary starts to
evolve up the RGB, it transfers some
material back onto the White Dwarf.
Nova: white dwarf re-ignition in a binary system
• A Nova is a faint star suddenly brightens by a factor of 104
to 108 over a few days or hours
• It reaches a peak luminosity of about 105 L
• A nova is different from a supernova (luminosity of 109 L)
• Material from an ordinary star in a close binary can fall
onto the surface of the companion white dwarf
• Because of strong gravity, the transferred hydrogen mass is
compressed into a dense layer covering the while surface
• When the temperature reaches about 107 K, hydrogen
fusion ignites through the surface layer, producing the
sudden increase in luminosity
Novae
• Nova Persei became
one of the brightest
stars in the sky in
1901. There is an
expanding shell from
the explosion. The
velocity of the
material is about
2000 km/sec
Novae
• Nova Cyg (1992)
illuminated a cloud
of nearby hydrogen
gas
• The expanding
shell of the nova
could be seen a
few years later
with HST
Novae
• Nova Cyg in 1994.
• Most nova are recurrent
• Every year there are 20 -
30 novae observed in
the Milky Way. “Naked
eye” nova occur more
likely one per decade.
Light curves of Novae