Download AST121 Introduction to Astronomy

Binary stars and clusters
Chapter 11
• Properties of stars
Review
– apparent brightness (apparent magnitude)
• measure energy/area/second
– luminosity (absolute magnitude)
• calculate distance, use inverse-square law
– distance to the star
• parallax
– temperature
• blackbody curve (color, wavelength of max. energy)
– spectral class (OBAFGKM)
• absorption spectrum, temperature
– radius
• for two stars of same T, star of larger radius has greater luminosity
• for two stars of same luminosity, star of smaller T has greater radius
H-R Diagram
What else would we like to
know?
•
•
•
•
•
mass?
part of a binary (or triple) system?
part of a cluster?
age?
evolution?
Why do we want to know the
mass of stars?
• the total mass of the universe has
implications for its ultimate fate (the Big
Crunch?)
• mass is related to the age of the star
• mass is the single most important
characteristic that determines the life and
fate of a star
How can we calculate the mass
of a star?
• The same way we calculate the mass of the Sun,
the mass of Jupiter, the mass of ...
• Kepler’s Third Law (corrected by Newton of
course)
• But we need to analyze something orbiting the
star!
• Use a binary system (i.e. system of two stars
orbiting each other)
• simulation
Important points to consider
• Both stars orbit about the center of mass of
the system.
• We must consider how we are viewing the
binary stars (top view, side view, at an
angle?)
• How can we analyze the orbit?
Mass related to luminosity
• For binary stars, that we
can reliably measure their
masses and luminosities,
graph luminosity vs. mass
• HUGE changes in
luminosity correspond to
small changes in mass -power relationship!
• L ~ M4
• this is only true for main
sequence stars
Mass-Luminosity diagram
How do we find binary stars?
• visual binaries
– viewed with a telescope
• spectroscopic binaries
– stars are too close together to be resolved with a telescope
– measure period shifting of two sets of aborption lines in a single
spectrum
• astrometric binaries
– stars are too close together to be resolved with a telescope
– one set of absorption lines are too faint to be seen
– analyze doppler shift in the aborption lines of one star, thus
measuring its “wobble” and then calculate the properties of the
other star.
– similar to how we discover extrasolar planets
• eclipsing binaries
– one star eclipses the other causes a change in its brightness
Star Clusters
• Most stars in the Milky Way are in multiple
systems but far from other stars.
• Some stars group together to form clusters
• Types of clusters: open and globular
Open Clusters
•
•
•
•
•
Example: Pleides
~100-1000 stars
~10-100 ly across
few stars/ly3
HR diagram tells us
something about
stellar evolution--more
massive stars are red
giants
Notice that the more
luminous stars in the
Pleiades are off the main
sequence.
These are red giants.
Evidently, the higher
mass stars have evolved
into red giants.
This is a typical
characteristic of open
clusters.
What does this
tell us about the
age of a cluster?
The more stars
that are on the
main sequence,
the younger the
cluster.
The more stars
that are off the
main sequence,
the older the
cluster.
Globular Clusters
• more dense
– 1000-1,000,000 stars
– ~100 stars/ly3
– up to 1 million stars
jammed into a sphere
of diameter ~ 100 ly
HR diagrams look
similar for all globular
clusters.
They are similar in age,
probably formed early
in the universe (11-14
billion y).
The horizontal branch is
“signature” of globular
clusters.
The horizontal branch
may indicate fewer
heavy metals in the star,
as would be the case for
very old stars.