Download Stars II

A105
Stars and Galaxies
Today’s APOD
 Homework due today
 Remote observing this weekend
Announcements…
• Remote Obs Oct. 14 @ 10 PM and
Oct. 16 @ 7 AM
• Take the Swain West elevator to the
3rd floor and follow the signs to SW
403
Hertzsprung Russell Diagram
-10
-5
Absolute Magnitude
The HR
Diagram
Giants and
Supergiants
0
5
Main
Sequence
10
White
Dwarf
15
20
30000
25000
20000
15000
10000
Temperature (K)
5000
0
Luminosity
An H-R
diagram
plots the
luminosity
and
temperature
of stars
Temperature
Most stars
fall
somewhere on
the main
sequence of
the H-R
diagram
WHY
WHY
WHY
???
High-mass stars
Low-mass stars
Mass
measurements
of mainsequence stars
show that the
hot, blue stars
are much more
massive than
the cool, red
ones
Binary Stars
We can determine the
masses of stars from
their motions in binary
star systems
A Visual Binary - Mizar
Mizar is the middle
star in the handle
of the Big Dipper
88 light years from
Earth
Both stars are themselves binaries!!
Visual Binary
We can directly observe the orbital motions
of these stars
Eclipsing Binary
We can measure periodic eclipses
Playing with Binary Stars!
• Binary Simulations
http://www.astro.ubc.ca/~scharein/a311/Sim.html
Spectroscopic Binary
We determine the orbit by measuring Doppler shifts
Main-sequence
stars are fusing
hydrogen into
helium in their
cores like the Sun
massive mainsequence stars
are hot (blue)
and luminous
Less massive
stars are cooler
(yellow or red)
and fainter
High-mass stars
Low-mass stars
The mass of a
normal,
hydrogenburning star
determines its
luminosity and
temperature!
Mass & Lifetime
Sun’s life expectancy: 10 billion years
Life expectancy of 10 MSun star:
10 times as much fuel, uses it 104 times as fast
10 million years ~ 10 billion years x 10 / 104
Life expectancy of 0.1 MSun star:
0.1 times as much fuel, uses it 0.01 times as fast
100 billion years ~ 10 billion years x 0.1 / 0.01
Main-Sequence Star Summary
High Mass:
High Luminosity
Short-Lived
Large Radius
Blue
Low Mass:
Low Luminosity
Long-Lived
Small Radius
Red
Off the Main Sequence
• Stellar properties depend on both mass and
age: those that have finished fusing H to He in
their cores are no longer on the main sequence
• All stars become larger and redder (and cooler)
after exhausting their core hydrogen: giants and
supergiants
• Most stars eventually end up small and hot after
fusion has ceased: white dwarfs
Star Clusters
Star
Clusters
Goals:
– Understand how we learn about stellar evolution
from the properties of stars in clusters
– Understand how we can determine the distances
of star clusters
– Understand how we can determine the ages of
star clusters
“Globular" Clusters and “Open" Clusters
Globular Clusters
Open Clusters
•104-106 stars
•old!
•compact balls of stars
•high star density
•10-104 stars
•generally young
•loose
•low star density
Properties of Stars in Clusters
• Formed at the
same time
• Stars are the same
age
• All stars have the
same composition
• The stars are held
in a group by their
common gravity
Constructing an HR Diagram
Apparent Magnitude
0
5
10
15
-0.5
0
0.5
B-V Color
1
1.5
2
What’s this B-V color?
• Astronomers measure the brightness of stars in
different colors
– Brightness measured in blue light is called “B” (for
“Blue”)
– Brightness measured in yellow light is called “V” (for
“Visual)
• Astronomers quantify the “color” of a star by
using the difference in brightness between the
brightness in the B and V spectral regions
• The B-V color is related to the slope of the
spectrum
The slope of the spectrum is different at
different temperatures
Cluster HR Diagrams
Hotter stars are brighter
in blue light than in yellow
light, and have low values
of B-V color, and are
found on the left side of
the diagram.
Cooler stars are brighter
in yellow light than in blue
light, have larger values of
B-V color, and are found
on the right side of the
diagram.
hotter
cooler
Distances to Star Clusters
The Sun has a “B-V”
color of about 0.6.
Stars like the SUN
hotter
cooler
What would the
apparent magnitude of
the Sun be at the
distance of the cluster
Messier 6?
Stars in Messier 6 with
B-V colors of 0.6 have
similar mass and
luminosity to the Sun
Ages of Star Clusters
The “bluest” stars
left on the main
sequence of the
cluster tell us the
cluster’s age.
As the cluster ages,
the bluest stars run
out of hydrogen for
fusion and lose their
“shine”
hotter
cooler
Jewelbox
The HR diagrams of
clusters of different
ages look very
different
5
10
M 67
0
15
-0.5
0
0.5
B-V Color
1
1.5
2
Apparent Magnitude
Apparent Magnitude
0
5
10
15
-0.5
0
0.5
B-V Color
1
1.5
2
Main Sequence Turnoffs of Star Clusters
Here we see a
series of HR
diagrams for
sequentially
older star
clusters that
have been
superimposed
Burbidge and Sandage 1958, Astrophysical Journal
Ages of Star Clusters
Cluster
Turnoff Color
Age
NGC 752
0.35
1.1 billion years
M 67
0.45
2.5 billion years
Hyades
0.15
800 million years
Pleiades
-0.15
100 million years
M 34
-0.10
180 million years
Thinking Question: Why has a cluster
Jewelbox
-0.25
16 million years
with a turnoff
color of B-V=1.0
never
been discovered?
 Stellar Evolution – 60, 61, 62,
64
 News Quiz on Tuesday
 Homework Due EACH THURS.