Properties of Stars
“All men have the stars,” he answered,
“but they are not the same things for
different people. For some, who are
travelers, the stars are guides. For others
they are no more than little lights in the
sky. For others, who are scholars, they
are problems. For my businessman they
were wealth. But all these stars are
silent. You – you alone – will have the
stars as no one else has them.”
Antoine de Saint-Exupery (1900 – 1944)
from The Little Prince
WHAT DO YOU THINK?
1.
How near to us is the closest star other than the Sun?
2.
What colors are stars, and why do they have these
colors?
3.
How luminous is the Sun compared with other stars?
4.
Are brighter stars hotter than dimmer stars?
5.
Compared to the Sun, what sizes are other stars?
6.
Are most stars similar to the Sun, one star with
planets, or in multiple-star groups?
A Snapshot of the Heavens
• How can we learn about the lives of stars,
which last tens of millions to hundreds of
billions of years?
• we will never observe a particular star evolve
from birth to death
• so how can we study stellar evolution?
How can we study the
“Life Cycles” of stars?
• Key: All stars were NOT born at the same time.
• stars we see today are at different stages in their lives
• we observe only a brief moment in any one star’s life
• by studying large numbers of stars, we get a “snapshot” of
one moment in the history of the stellar community
• we can draw conclusions just like we would with human
census data…we do stellar demographics!
A Snapshot of the Heavens
• What two basic physical properties do
astronomers use to classify stars?
•
What does that classification tell us?
Classification of Stars
• Stars were originally classified based on:
• their brightness
• their location in the sky
• This classification is still reflected in names of the
brightest stars…those we can see with our eyes:
Classification of Stars
Order of brightness
within a constellation
Latin Genitive of
the constellation
 Orionis
 Geminorum
Classification of Stars
• The old classification scheme told us little about a
star’s true (physical) nature.
• a star could be very bright because is was very close to
us; not because it was truly bright
• two stars in the same constellation might not be close to
each other; one could be much farther away
Classification of Stars
• In 20th Century, astronomers developed a more
appropriate classification system based on:
• a star’s luminosity
• a star’s surface temperature
 These properties turn out to depend on:
 a star’s mass and
 its stage in life
 measuring these=> reconstruct stellar life cycles
WHAT DO YOU THINK?
1.
How near to us is the closest star other than the Sun?
2.
What colors are stars, and why do they have these
colors?
3.
How luminous is the Sun compared with other stars?
4.
Are brighter stars hotter than dimmer stars?
5.
Compared to the Sun, what sizes are other stars?
6.
Are most stars similar to the Sun, one star with
planets, or in multiple-star groups?
WHAT DO YOU THINK?
DISTANCE
What colors are stars? TEMPERATURE
How luminous are stars? LUMINOSITY
Are brighter stars hotter? TEMP vs. LUMIN.
What sizes are stars?
SIZE
Single or Multiple?
ORBITS  MASS
1. How near are stars?
2.
3.
4.
5.
6.
Step 1: Distance!
DISTANCE
What colors are stars? TEMPERATURE
How luminous are stars? LUMINOSITY
Are brighter stars hotter? TEMP vs. LUMIN.
What sizes are stars?
SIZE
Single or Multiple?
ORBITS  MASS
1. How near are stars?
2.
3.
4.
5.
6.
PARALLAX
•Determines distance based
on Earth’s Orbit around Sun.
•SMALL angular shift!
•Even closest stars (4.3 light
years) show shift less than
1/4000th of a degree!
PARALLAX
•Example
http://www.solstation.com/stars/61cyg
ni2.htm
•Animation
http://ircamera.as.arizona.edu/NatSci1
02/NatSci102/lectures/otherstars.htm
•Current telescopes can
measure angles as small as
1/400,000th of a degree
(400+ light years)
Step 2: TEMPERATURES!
DISTANCE
What colors are stars? TEMPERATURE
How luminous are stars? LUMINOSITY
Are brighter stars hotter? TEMP vs. LUMIN.
What sizes are stars?
SIZE
Single or Multiple?
ORBITS  MASS
1. How near are stars?
2.
3.
4.
5.
6.
COLORS of stars
lead to Surface Temperatures!
Pickering’s “Harem” of women
computers at Harvard College
Annie Cannon
Spectral Type Classification System
O B A F G K M (L)
Oh Be A Fine Girl/Guy, Kiss Me!
50,000 K
3,000 K
Temperature
Starlight Follows 2 Key Radiation Laws
Peak of radiation is at shorter wavelengths for hotter stars
MUCH more intensity at every wavelength for hotter stars
Step 3: BRIGHTNESS!
DISTANCE
What colors are stars? TEMPERATURE
How luminous are stars? LUMINOSITY
Are brighter stars hotter? TEMP vs. LUMIN.
What sizes are stars?
SIZE
Single or Multiple?
ORBITS  MASS
1. How near are stars?
2.
3.
4.
5.
6.
How LUMINOUS are stars?
Bright because they are nearby…
or…
Bright because they are really, truly bright?
Sirius is bright because it
is CLOSE!
Rigel is bright because it
is REALLY bright!
Sirius is bright because it
is CLOSE!
Apparent Magnitudes
Ancient method for measuring stellar brightness from
Greek astronomer Hipparchus (c. 190 – 120 B.C.)
This scale runs backwards:
The bigger the number, the fainter the star
Brightest stars are #1, next brightest are #2, etc.
Inverse Square Law for Light
(how distance relates to brightnes)
Absolute Magnitudes
•Takes into account distance of star!
•Treats all stars as if they were same
distance away!
•True measure of star’s real brightness.
Relating Measurable Quantities
Measure distances of nearby stars
Deduce how bright they really are
Determine surface temperatures from
spectra
HOW do temperatures and brightness
relate to one another?
Step 4: HR Diagram!
DISTANCE
What colors are stars? TEMPERATURE
How luminous are stars? LUMINOSITY
Are brighter stars hotter? TEMP vs. LUMIN.
What sizes are stars?
SIZE
Single or Multiple?
ORBITS  MASS
1. How near are stars?
2.
3.
4.
5.
6.
Sun is much, much dimmer
than most “bright” stars
Our Star! The Sun
Sun is brighter than most
“nearby” stars
The Hertzsprung-Russell
(HR) Diagram
BRIGHT
HOT
COOL
FAINT
90% of all stars lie on
the main sequence!
WHY??
Hypotheses to Explore
Stars are born like children, cool and
small, and then heat up and grow brighter
over time.
Stars are like candles, hot and bright, and
then cool off and get dimmer over time.
Stars are born with different temps and
brightnesses, and change little over 90%
of their lives.
Hypotheses to Explore
IF…. Stars are born like children, cool and
small, and then heat up and grow brighter
over time.
THEN… new clusters of stars should all
be hot O stars, and old clusters only M
stars.
Hypotheses to Explore
IF…Stars are like candles, hot and bright,
and then cool off and get dimmer over
time.
THEN… new clusters should show only M
stars, and older ones should show O stars.
So… DO the EXPERIMENT!
The Pleiades
Hypotheses to Explore
 IF…. Stars are born like children, cool and small,
and then heat up and grow brighter over time.
 IF…Stars are like candles, hot and bright, and
then cool off and get dimmer over time.
“New” clusters like the Pleiades show all
kinds of stars!
Step 5: Size!
DISTANCE
What colors are stars? TEMPERATURE
How luminous are stars? LUMINOSITY
Are brighter stars hotter? TEMP vs. LUMIN.
What sizes are stars?
SIZE
Single or Multiple?
ORBITS  MASS
1. How near are stars?
2.
3.
4.
5.
6.
Diameter (Size) of Stars
Calculated from known values:
Luminosity
Temperature
Laws of Physics
Stefan-Boltzmann Law:
Luminosity ~ Surface Area x T4
PingPong balls, volleyballs, and Stars…
HUGE
Bigger
tiny!
Smaller
Determining Size with SPECTRA!
Larger stars can be less dense at edges
Less dense gas changes absorption lines
Luminosity
Classes of Stars
•Based on
Spectral Line
Shapes & Density
•Tied to physical
SIZE
Step 6: Mass!
DISTANCE
What colors are stars? TEMPERATURE
How luminous are stars? LUMINOSITY
Are brighter stars hotter? TEMP vs. LUMIN.
What sizes are stars?
SIZE
Single or Multiple?
ORBITS  MASS
1. How near are stars?
2.
3.
4.
5.
6.
Masses from BINARY STARS
Masses from BINARY STARS
•Doppler Shift of Spectra Lines tells us orbital velocities
•Velocities get us Orbital Sizes
•Orbits get us Mass
Eclipsing BINARY STARS
Mass from Light Curve
Know periods from the light curve…
Get orbit distances from Kepler’s Laws
Get relative masses from Law of Gravity!
Match mass to main sequence star type
FIND: O-stars are more massive than M
stars…
Mass-Luminosity
Relationship
for
Main Sequence
stars
HR Diagram
Mass-Luminosity
Relationship
for
Main Sequence
stars
So… what is
going on?
• Why are only
SOME stars in the
supergiant
regions?
•Why are 10% in
the “giant” region
•What are the tiny
ones in the “white
dwarf” region?
Clusters of Stars as Key Tests
Look at large populations of stars
Open clusters in Milky Way’s “disk”
Globular Clusters around galaxy
Assume all stars *about* same age
Assume all stars *about* same distance
Open Clusters
•
•
•
•
100’s of stars
106 - 109 years old
irregular shapes
gas or nebulosity is
sometimes seen
Pleiades (8 x 107 yrs)
Globular Clusters
105 stars
8 to 15 billion years
old (1010 yrs)
spherical shape
NO gas or
nebulosity
M 80 (1.2 x 1010 yrs)
Pleiades H-R
Diagram
Globular Cluster
H-R Diagram
Palomar 3
Summary of Key Ideas
Magnitude Scales
 Determining stellar distances from Earth is the first step
to understanding the nature of the stars. Distances to the
nearer stars can be determined by stellar parallax, which
is the apparent shift of a star’s location against the
background stars while Earth moves along its orbit
around the Sun. The distances to more remote stars are
determined using spectroscopic parallax.
 The apparent magnitude of a star, denoted m, is a
measure of how bright the star appears to Earth-based
observers. The absolute magnitude of a star, denoted M,
is a measure of the star’s true brightness and is directly
related to the star’s energy output, or luminosity.
Magnitude Scales
 The absolute magnitude of a star is the apparent
magnitude it would have if viewed from a
distance of 10 pc. Absolute magnitudes can be
calculated from the star’s apparent magnitude
and distance.
 The luminosity of a star is the amount of energy
emitted by it each second.
The Temperatures of Stars
 Stellar temperatures can be determined from
stars’ colors or stellar spectra.
 Stars are classified into spectral types (O, B, A,
F, G, K, and M) based on their spectra or,
equivalently, their surface temperatures.
Types of Stars
 The Hertzsprung-Russell (H-R) diagram is a graph on
which luminosities of stars are plotted against their
spectral types (or, equivalently, their absolute
magnitudes are plotted against surface temperatures).
 The H-R diagram reveals the existence of four major
groupings of stars: main-sequence stars, giants,
supergiants, and white dwarfs.
 The mass-luminosity relation expresses a direct
correlation between a main-sequence star’s mass and
the total energy it emits.
 Distances to stars can be determined using their spectral
types and luminosity classes.
Stellar Masses
 Binary stars are surprisingly common. Those that can be
resolved into two distinct star images (even if it takes a
telescope to do this) are called visual binaries.
 The masses of the two stars in a binary system can be
computed from measurements of the orbital period and
orbital dimensions of the system.
 Some binaries can be detected and analyzed, even
though the system may be so distant (or the two stars so
close together) that the two star images cannot be
resolved with a telescope.
Stellar Masses
 A spectroscopic binary is a system detected from the
periodic shift of its spectral lines. This shift is caused by
the Doppler effect as the orbits of the stars carry them
alternately toward and away from Earth.
 An eclipsing binary is a system whose orbits are viewed
nearly edge-on from Earth, so that one star periodically
eclipses the other. Detailed information about the stars in
an eclipsing binary can be obtained by studying its light
curve.
 Mass transfer occurs between binary stars that are close
together.
Key Terms
absolute magnitude
apparent magnitude
binary star
center of mass
close binary
eclipsing binary
giant star
Hertzsprung-Russell
(H-R) diagram
initial mass function
inverse-square law
light curve
luminosity
luminosity class
main sequence
main-sequence star
mass-luminosity relation
OBAFGKM sequence
optical double
photometry
radial-velocity curve
red giant
spectral types
spectroscopic binary
spectroscopic parallax
stellar evolution
stellar parallax
stellar spectroscopy
supergiant
visual binary
white dwarf
WHAT DID YOU THINK?
How near to us is the closest star other
than the Sun?
Proxima Centauri is about 25 trillion mi (40
trillion km) away. Light from there will take
about 4 years to reach Earth.
WHAT DID YOU THINK?
How luminous is the Sun compared with
other stars?
The most luminous stars are about a
million times brighter, and the least
luminous stars are about a hundred
thousand times dimmer than the Sun.
WHAT DID YOU THINK?
What colors are stars, and why do they
have these colors?
Stars are found in a wide range of colors,
from red through violet as well as white.
They have these colors because they
have different temperatures.
WHAT DID YOU THINK?
Are brighter stars hotter than dimmer
stars?
Not necessarily. Many brighter stars, such
as red giants, are cooler but larger, than
hotter, dimmer stars, such as white dwarfs.
WHAT DID YOU THINK?
Compared to the Sun, what sizes are
other stars?
Stars range from more than 1000 times
the Sun’s diameter to less than 1/100 the
Sun’s diameter.
WHAT DID YOU THINK?
Are most stars isolated from other stars,
as the Sun is?
No. In the vicinity of the Sun, one-third of
the stars are found in pairs or larger
groups.