Download Binary Stars (Professor Powerpoint)

Over half of the stars in the sky have stellar
companions, bound together by gravity and in orbit
around each other.
Types of Binaries
Visual Binaries
Optical Binaries- are chance superpositions,
where two stars appear close together but do not
actually orbit one another. (Like Mizar & Alcor)
Physical Binaries- where one star orbits
another, and each star can be seen in the
telescope.
OPTICAL DOUBLES
• Not a true binary system
• Stars only appear close together in the sky
• Mizar & Alcor in the Big Dipper
While Alcor and Mizar are Optical Double stars
and only appear to be near each other, Mizar is
actually a Physical Binary star.
Orbits and Masses of Visual Binaries
The primary importance of binaries is that they
allow us to measure stellar parameters
(especially mass). The center of mass is the
location where a fulcrum would be placed to
balance the stars on a seesaw.
Masses of Binary stars
Newton’s Modification of Kepler’s Law
P must be in years, a in AU
M in solar mass, where Sun = 1
Types of Physical Binaries
Eclipsing Binary –(If the angle is good ) two
stars that regularly eclipse one another
causing a periodic variation in brightness.
Spectroscopic Binary - two stars that are
found to orbit one another through
observations of the Doppler effect in their
spectral lines .
At least half of the stars in the sky are
binaries. Eclipsing Binary stars are also
referred to as Extrinsic Variable Stars.
Eclipsing Binaries
Sometimes the orbital plane is lined up so
that the stars pass in front of each other as
seen from the Earth. Each eclipse will cause
the total light from the system to decrease.
The amount of the decrease
will depend on how much of
each star is covered up.
The period is from one
large dip to the next and
with the period you can
get the mass of both.
Spectroscopic Binaries
Some binaries are too close together to be resolved,
you may still be able to detect the binary through
the Doppler shift (in one or both stars). They must
be relatively close to each other (short orbital
period).
If you can see both stars’ spectrums, you may
be able to use Doppler shifts to measure the
radial velocities of both stars.
This gives you the mass ratio, regardless of the
viewing angle (e.g. nearly face-on, nearly edgeon, etc.). This is usually useful information.