The Suns of
Other Worlds
What stars might be suitable planet
hosts?
How does the evolution stars affect
the origin and evolution of life?
IU Astrobiology Workshop, June 2006
Caty Pilachowski, IU Astronomy
The
Sun
Today
Can our knowledge
of the Sun and stars
guide us where to
look for planets?
Image credit: Solar Orbiting Heliospheric Observatory/MDI
www.spaceweather.com
About
Stars
 Basic Properties of Stars
 temperature
 diameter
 brightness
 The Hertzsprung-Russell Diagram
Familiar Stars
The Nearest Stars
1000 ly
A little farther out
Properties of
Stars
We can’t see the stars’
diameters through a telescope.
Stars are so far away that we
see them just as points of light.
If we know a star’s temperature and its luminosity,
we can calculate its diameter.
How do we determine a star’s
temperature?
Luminosity depends on….
TEMPERATURE the hotter a star is,
the brighter it is.
DIAMETER –
the bigger a star is,
the brighter it is.
Stars range in size from about the diameter
of Jupiter to hundreds of times the Sun’s diameter
The Nearest and the Brightest
Exploring the Solar Neighborhood
– What types of stars do we see in the sky?
Exploring the HR Diagram:
– How do our familiar stars fit into a
Hertzsprung-Russell diagram?
– What about the nearest stars?
The
Brightest
Stars in
the Sky
(no need to copy
these down!)
Star
Distance
(LY)
Temperature
(K)
Absolute
Magnitude
Sun
0.000015
5800
4.8
9
9600
1.4
232
7600
-2.5
Alpha Cen A
4
5800
4.4
Arcturus
37
4700
0.2
Vega
25
9900
0.6
Capella
42
5700
0.4
Rigel
773
11000
-8.1
Procyon
11
6600
2.6
Achernar
144
22000
-1.3
Betelgeuse
427
3300
-7.2
Hadar
335
25000
-4.4
Acrux
321
26000
-4.6
Altair
17
8100
2.3
Aldebaran
65
4100
-0.3
Antares
604
3300
-5.2
Spica
263
2600
-3.2
Pollux
34
4900
0.7
Sirius
Canopus
-10
-5
Absolute Magnitude
An HR Diagram for the
Solar Neighborhood
Plot Absolute Magnitude vs. Temperature
0
5
10
15
20
30000
25000
20000
15000
10000
Temperature (K)
5000
0
Distance
(LY)
Temperature
Absolute
Magnitude
Prox Cen
4
2800
15.53
Alp Cen A
4
5800
4.4
Alp Cen B
4
4900
5.72
Barnard’s
6
2800
13.23
Wolf 359
7.5
2700
16.57
Lal 21185
8
3300
10.46
Sirius A
9
9900
1.45
Sirius B
9
12000
11.34
Luyten 726-8A
9
2700
15.42
UV Ceti
9
2600
15.38
Ross 154
10
3000
13.14
Star
The Nearest Stars
Hertzsprung Russell Diagram - Brightest Stars
-10
-5
Absolute Magnitude
Plot Absolute
Magnitude
vs.
Temperature
0
5
10
15
20
30000
25000
20000
15000
10000
Temperature (K)
5000
0
-10
-5
Absolute Magnitude
Adding
the
Nearest
Stars to
the HR
Diagram
Hertzsprung Russell Diagram
0
5
10
15
20
30000
25000
20000
15000
10000
Temperature (K)
5000
0
The HR
Diagram
Hertzsprung Russell Diagram
-10
Giants and
Supergiants
The most
common stars
in the Solar
Neighborhood
are dim and
cool
Absolute Magnitude
-5
0
5
Main
Sequence
10
White
Dwarf
15
20
30000
25000
20000
15000
10000
Temperature (K)
5000
0
Summarizing
Stellar
Properties
Luminosity:
10-4 - 106 LSun
Temperature:
3,000 K - 50,000 K
Mass:
0.08 - 100 MSun
The Sun is an
ordinary main
sequence star
Only certain sizes and colors are allowed
• Most stars fall on the “main sequence”
• Main sequence stars are fusing hydrogen into helium in
their cores
The mass of a
main sequence
star determines
its luminosity and
temperature
Main-sequence stars like the Sun are fusing
hydrogen into helium in their cores
•Massive main-sequence stars are hot (blue)
and luminous
•Less massive stars are cooler (yellow or red)
and fainter
What are
the
typical
masses of
newborn
stars?
Observations show that star formation makes
many more low-mass stars than high-mass
stars
Why does
the Sun
Shine?
• Nuclear fusion reactions
• Hydrogen fuses into helium
• Mass converted to energy
Nuclear Potential Energy (core)
Luminosity
~ 10 billion years
How does
nuclear
fusion
occur in
the Sun?
• The core’s extreme temperature
and density are just right for
nuclear fusion of hydrogen to
helium through the protonproton chain
• Gravitational equilibrium acts as
a thermostat to regulate the
core temperature because
fusion rate is very sensitive to
temperature
How do we know nuclear reactions are going
on in the Sun?
•Neutrinos created during
fusion fly directly out of
the Sun
•These neutrinos can be
detected on Earth
Balancing Gravity
Gravitational
contraction:
Provided energy that
heated core as Sun was
forming
Contraction stopped
when fusion began
Gravitational equilibrium:
Energy provided by fusion maintains the pressure
Solar Thermostat – STABILITY!
Decline in core temperature
causes fusion rate to drop, so
core contracts and heats up
Rise in core temperature
causes fusion rate to rise, so
core expands and cools down
Stellar Mass and Fusion
• The mass of a main sequence star determines
its core pressure and temperature
• Stars of higher mass have higher core
temperature and more rapid fusion, making
those stars both more luminous and shorterlived
• Stars of lower mass have cooler cores and
slower fusion rates, giving them smaller
luminosities and longer lifetimes
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 Lifetimes
High Mass:
High Luminosity
Large Radius
Blue
Short-Lived
Low Mass:
Low Luminosity
Small Radius
Red
Long-Lived
Explaining
the HR
Diagram
Energy
Gravity
Energy
Transport
During hydrogen burning, basic physics forces a star to
lie on the main sequence. A star’s position on the MS
depends on its mass.
Star Clusters
Stellar Evolution in Action
Stars in clusters tell us about stellar evolution
Star clusters tell us the ages of stars
Star Clusters and Stellar Lives
• Our knowledge of the
life stories of stars
comes from comparing
mathematical models of
stars with observations
• Star clusters are
particularly useful
because they contain
stars of different mass
that were born about
the same time
Constructing a Star Cluster HR Diagram
Apparent Magnitude
0
5
10
15
-0.5
0
0.5
B-V Color
1
We measure the brightness and temperature
of each star in the cluster.
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
The HR diagrams of
clusters of
different ages look
very different
Jewelbox
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
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
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
We can determine ages from
the “color” of the main sequence
“turnoff”
The
Jewels of
the Night
Image from the Cerro
Tololo Inter-American
Observatory's 0.9meter telescope
 NGC 4755 is an open star cluster in the southern
constellation Crux
 It is popularly known as the Jewel Box because an
early catalog described it as a "superb piece of
jewelry“
 Distance ~7500 light years
How Old Are the Jewels?
• Create a color-magnitude
diagram of the Jewelbox
and estimate its age
The End of Solar-type Stars
Main
Sequence
Red
Giant
Planetary
Nebula
White
Dwarf
When the carbon core reaches a density that
is high enough, the star blows the rest of its
hydrogen into space.
The hot, dense, bare core is exposed!
Surface temperatures as hot as 100,000 degrees
The hot core heats the expelled gas and makes it glow
Planetary Nebulae
• Fusion ends with
a pulse that
ejects the H and
He into space as
a planetary
nebula
• The core left
behind becomes
a “white dwarf”
Earth’s
Fate
•
Sun’s radius will grow to near
current radius of Earth’s orbit
Earth’s
Fate
•
Sun’s luminosity will rise to 1,000
times its current level—too hot for
life on Earth
What about Massive Stars?
• Massive stars continue
to generate energy by
nuclear reactions until
they have converted all
the hydrogen and helium
in their cores into iron.
• Once the core is iron, no
more energy can be
generated
• The core collapses and
the star explodes
Iron builds up
in core until
degeneracy
pressure can
no longer
resist gravity
Core then
suddenly
collapses,
creating
supernova
explosion
Is life on Earth safe from harm
caused by supernovae?
Earth is safe at the present time
because there are no massive stars
within 50 light years of the Sun.
But other types of supernovae are
possible…
What
stars
might be
good
hosts for
life?
Low mass stars
– long-lived
– stable
More massive stars
– short lives
– often variable