Download Stars

Stars
• Composed of
~98% H and He
• Fusion in the core
supports the star
• Full spectrum of
masses
Key Properties
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Apparent Brightness
Luminosity
Temperature / Color
Mass
Evolutionary State
Brightness
• Absolute brightness
– Luminosity
– Power emitted by star
into space
– Only depends on star
– Lsun = 4 X 1026 Watts
• Apparent brightness
– How bright star appears
in the night sky
– Power per unit area
– Depends on star’s
brightness and distance
Inverse square law for light
• Apparent brightness
measured in watts per
square meter
• Drops off as square of
distance
L
B
2
4d
Measuring Distance
• Stellar Parallax
– Caused by motion of
Earth in its yearly orbit
– d = 1/p
where p is in arcsecs and
d is in parsecs
– 1 parsec = 3.26 lyrs
Magnitudes
• Logarithmic
• Large values are dim
objects
• Small values are bright
objects
 F1 
m1  m2  2.5 log  
 F2 
Magnitudes
Absolute Magnitudes
• A bright a star would appear
if it were 10 pc away
Apparent Magnitudes
• How objects appear from
here on Earth
• Does not depend on
distance
• Depends on distance
 d 

m  M  5 log 
 10 pc 
• We can only see objects
with m≤6
Color and Temperature
• Color is the difference
between intensity in
two filters
• B-V color is a good
proxy for temperature
• Color is independent of
distance
Spectral Type
• Spectral types are
subdivided for
intermediate
temperatures
• Values run from 0-9
• Smaller numbers are
hotter
• Larger numbers are
cooler
• Eg. B1 is hotter than B7
Spectral Types
• Order was alphabetical
depending on strength
of Hydrogen line
– Williamina Flemming
• Revised to follow a
more natural order
– Annie Cannon
Measuring Stellar Masses
Using Binary Systems
Visual Binaries
Eclipsing Binaries
Spectroscopic Binaries
HR Diagram
• Main Sequence
• Giants
• Supergiants
• White Dwarfs
HR Diagram
• Luminosity
class gives size
and luminosity
information
Main Sequence
• Mass is the most
important property for
a star on the MS
• Stars spend 90% of
their lives here, burning
H in their cores
• MS lifetime depends on
mass
Main Sequence
• More massive stars
live much shorter
lives
– Burn fuel very
quickly to support
such a large star
• Less massive stars
live longer
– Less fuel, but burn it
more slowly
Life After the Main
Sequence
• When stars run out of H in their
cores, they evolve off the MS
• Giants and Supergiants expand
to extremely large sizes
– Temperatures are very low
– Luminosity is very high
• White dwarfs are small and hot
– Have no nuclear fusion
– Heated by collapse of gas
L  4r  T
2
4
Star Clusters
• All stars in the cluster formed about
the same distance from Earth
• All stars in the cluster formed at
about the same time
• Very useful in understanding stellar
formation and evolution
– Can use them as clocks
• Most of what we know about stars
comes from studying clusters
Open Clusters
• Only a few million
years old
• Contain lots of
luminous blue stars
• Contain several
thousand stars
• ~30 lyrs across
Globular Clusters
• Often several billion
years old
– Some of the oldest
objects in the galaxy
– Contains mostly
smaller stars
• Around 105-106
stars concentrated
in a relatively small
volume
• 50-150 lyrs across
Age of Cluster
• Main Sequence Turnoff
(MSTO) – more massive
stars have evolved off of
the Main Sequence
• MSTO gives age of
cluster
– Lifetime of cluster same
as MS lifetime of stars at
the MSTO
MSTO
• Young clusters still have
their massive stars on
the MS
• Old clusters are missing
the massive blue stars
on the MS