Ch 1113: Core Concepts

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Ch 11-13 Core Concepts
Luminosity L depends on both star radius R and
temperature T Can get T from spectra (colors and
absorption liens) Brightness is different from
luminosity. Get L from brightness and
distance Get Distance from parallax Star in 3
main families MS, giants, white dwarfs HR
diagram (prior class) Get mass from binary stars
(this class) L of a star comes from nuclear
fusion making H, He, O, etc Evolve all stars
are born and die Stars spend 90 life turning H
into He MS stars Low mass stars evolve from MS
to giants to white dwarfs High mass stars end as
supernovae High mass stars live shortest time Ch
11 Star Clusters and their HR Diagrams
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Binary Stars
Most stars have one or more companions, bound
together by gravity and permanently in orbit
around each other. Sun is unusual. binary,
triple, or quadruple star systems.
We classify binaries by their appearance, not
intrinsic differences Visual Binaries can see
both stars using some telescope. Spectroscopic
Binaries spectra shows a mixture of the lines
expected for two stars. Eclipsing Binaries
brightness changes as one passes in front of the
other
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Resolving Visual Binary Stars
To resolve a visual binary the stars should be
near enough to the sun that the angle of
separation is resolution of your telescope 1
arcsecond (using conventional telescope), 0.01
arcsec using adaptive optics on a 10m telescope,
or around 0.001 arcsec using an interferometer
with a 100m.
Many of these apparently single stars will be
unresolved binaries.
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Binary Stars always have similar Proper Motions
and Radial Velocities
Which of these is a real binary, rather than a
chance alignment of stars at very different
distances? The stars in real binaries have
similar same proper motions and radial velocities.
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Stars orbit their center of Mass
Binaries allow us to measure stellar parameters
(especially mass). Each star is on an elliptical
orbit, centered on the center of mass, or balance
point, of the pair.
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Orbits and Masses of Binaries
Binaries allow us to measure stellar parameters,
and especially mass. Lab 12 covers this topic in
detail. You can see some binaries moving in
applets at end of this web site http//instruct1.
cit.cornell.edu/courses/astro101/java/binary/binar
y.htm
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Famous Visual Binary Star in Big Dipper
Alcor is 12 arcmin below its fainter binary
partner Mizar, just resolved in this photo. Used
in ancient time to test eyesight.
Telescopes show that Mizar is itself a 14 arcsec
binary.
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Sirius is a Binary
Sirius, brightest star in the sky, found along a
line from the 3 stars in the belt of Orion, is a
binary with a very faint white dwarf companion.
Albireo
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A Visual Binary for Summer and Fall
The primary (brighter, K type) star is yellow and
the secondary (fainter, B type) blue and 34
arcsec away easy for binoculars. Without
binoculars the binary is unresolved, and the
second brightest star in Cygnus. Its second name
is beta-Cyg, where beta (ß) is the second letter
in Greek alphabet
beta-Cyg Albireo
How do we know which star is larger?
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Answer
Both stars are at the same distance from us.
The difference due to their orbit is too small
to notice. The brighter star, the yellow one
from the photo, is more luminous. LR2T4 The
blue star is hotter, larger T, yet it has smaller
L. The yellow star must have much later radius R.
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Spectroscopic Binaries
Spectroscopic binaries are often too close to
resolve in images. However, the spectrum shows
light from two different types of star. The
absorption lines can be double, showing two
Doppler shifts, one for each star. The absorption
lines move as the stars go around their orbits.
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Spectroscopic Binaries the tilt problem
Two views of the same circular orbit. We
underestimate the star masses in all cases,
except when orbit is completely edge on.
Why is blue star not at center of orbit of red
star?
Nearly Edge on orbit. We are again in front of
the page, and we now see the velocities of the
stars changing a lot as they orbit. The red star
is either coming to or going away from us, and
the blue star is moving in the opposite direction.
Face on orbit. We are in front of the page, and
we see the circular orbit. We do not see any
Doppler effect since none of the orbital motion
is towards or away from us.
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Eclipsing Binaries
Edge on orbits can give eclipses.
Change in total brightness (both stars together)
depends on surface areas and temperatures. You
can see applets of eclipsing binaries http//home
.achilles.net/jtalbot/glossary/binary.html and
http//www.astro.ubc.ca/scharein/a311/Sim.html
Eclipsing Binary Stars
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Eclipsing Binaries give full information
If you see eclipses, the orbits are very nearly
edge on. You can then find the size of the
system, including the masses.
The shape and timing of the eclipses gives the
shape and size of the stars.
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The Mass-Luminosity Diagram
Main sequence stars
If we plot mass of a star against its luminosity
see linear correlation for MS stars but not
for others.
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Mass on the MS
For MS stars only, mass increases with T and L.