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Variable and Binary Stars
Not so Unusual
All stars are variable to some extent
Some over periods of days or weeks
Others over centuries or millennia
Often many modes of variability
The Sun is variable +/- 0.07%
By a small majority, all stars are binary or trinary
– 53% of star systems are multiple
– 75% of O-type stars seem to have a companion
– If Jupiter had been ~100 times more massive, the
Sun would have a companion star
When an Astrophysicist talks about variable stars, s/he
usually means a dramatically variable star
Some are variable even to the naked eye, once you
know what you are looking for
– Mira (o Ceti): T ~ 1 year
AKA “Wonderful” in Latin
– Algol (b Persei): T ~ 1 week
AKA “The Demon” in Arabic
– Suggests ancient peoples noticed these changes
Intrinsic variability due to instability in the Hydrostatic
Extrinsic variability due to shape, bright spots, rotation,
(some) Categories of Variables
RR Lyrae
T Tauri
Explosive variables (novae, discussed
Eclipsing binaries
– A combination of our two subjects!
– Also ellipsoidal binaries
The first Cepheid
variable, Delta
Cephei, is close
enough for a parallax
Population I, giant
yellow stars
– F6-K2
– Young, metal rich
– 103,104 Lsun
T ~ days to months
Henrietta Leavitt and
Cepheid Variables
One of Edward
Pickering’s “harem”
“a straight line can
readily be drawn ...
showing that there is
a simple relation
between the
brightness of the
variables and their
Rosetta Stone
Since Delta Cephei is close enough for a
parallax measurement, and since the magnitude
variation a la Leavitt gives a good measure of
intrinsic brightness, distance to other Cepheids
can be obtained
Used to find the distance to the Small Magellenic
Cloud, changing ideas about the scale of the
– hmmmmmm
RR Lyrae
Common in globular clusters
– Spectral type F2-A2, yellow or white giant
– Population II (as one would expect in a globular cluster)
Periods of a day down to several hours
Amplitude of 0.3-2 magnitudes
Also a reliable distance indicator
T Tauri
Very young low mass
– > F, G, K, M
– >10 Myr
– > 3 Msun
Still undergoing
gravitational contraction
Intermediate stage
between a protostar and
a main sequence star.
Binary Systems and Variability
If two stars coalesce out of a globule near
enough to each other but with sufficient relative
velocity, they will orbit about their common
AKA Barycenter
Beta, not part of the system
Alpha Centauri A
Proxima Centauri, currently
the closest star to the Sun
star, not a
binary but
on its
way. By
11,700 it’ll
be the
Rho Ophiuchi Trinary System
Binary stars
Mizar, an eclipsing binary
Spectral Red-Shift
The shift in the spectrum
can show the speeds in a
binary pair also
Also useful in finding
extra-solar planets
A new device called a
laser comb produces a
series of spectral lines at
precisely spaced
frequencies, allowing
measuring star motions
60 times more precisely
Light Curve, one more time
Piper Reid, Dripping Springs
High School, Austin TX
Link to her paper
Contributions to a Nova
This binary system has a giant
and a dwarf star
The dotted line is the Roche
If the giant expands during the
course of its demise beyond
the Roche lobe, the dwarf can
steal material and go nova
Blue Stragglers: O-types
merging gently
But That Is Just One Possible
Fate of a Star