Download Lecture 13: The Stars –

Lecture 13:
The Stars –
The Nature of Stars
•Classification of Stars
•Birth and Death of Stars
Quick Review…
A star’s color
reveals its
surface
temperature.
What color is
this star?
Green
Add absorption
lines… present for
all stars!
Sun’s Blackbody Curve
• Wien's Law tells us that
objects of different
temperature emit spectra
that peak at different
wavelengths.
• Hotter objects emit most of
their radiation at shorter
wavelengths; hence they
will appear to be
bluer/whiter .
• Cooler objects emit most of
their radiation at longer
wavelengths; hence they
will appear to be redder.
• Furthermore, at any
wavelength, a hotter object
radiates more (is more
luminous) than a cooler one.
Spectra of Stars
Women Computers (1890)
Diversity Leads to Revolution
• Annie Jump Cannon
• Antonia Maury
Both above contributed to
identifying Stellar
Classification Systems
• Meghnad Saha
Pressure and Temperatures of
Stellar Atmospheres
• Cecilia Payne-Gaposchkin:
First to show Sun is mainly
composed of ________.
Her work
cataloging stars
was integral to
the
development of
our
contemporary
classification
system
Annie Jump Cannon (1863-1941)
Classification Scheme
A
B
C
D
E
.
.
.
S
Where “A” stars had the
most Hydrogen and “S”
stars had the least
Hydrogen!
Antonia Maury
(1866-1952)
Spectral classes might make more sense
if arranged by temperature
Absorption Line
Spectra of Stars
Wavelength
increases to right
O
B
A
Temperature
increases as go up
F
Mnemonic:
G
Oh Be a Fine Girl
(Guy) Kiss Me
K
M
A Revolution
• Most astronomers believed that the
differences in spectral lines were due
to subtle differences in chemical
abundance.
• Indian physicist Meghnad Saha offered
another explanation, which was
confirmed at Harvard by Cannon and
Maury’s work.
Pressure and
Temperatures of
Stellar Atmospheres
Meghnad Saha (1893-1956)
Theory of thermal ionization of atoms
First to
show that
the Sun &
stars are
mainly
composed
of
hydrogen,
contradicting
accepted
wisdom at
the time.
Cecelia Payne-Gaposchkin (1900-1979)
First PhD in Astronomy from Harvard/Radcliffe
Together Saha and PayneGaposchkin
• Gave theoretical explanation for Cannon’s
classification scheme.
• Showed that the differences in spectra (absorption
lines) are due to temperature and thermal
ionization of atoms not abundance of elements
• Provided a convincing argument that stars are
mostly made of hydrogen.
Stars are classified by their spectra as
O, B, A, F, G, K, and M spectral types
What does this give us?
A new way to classify stars:
• color
• peak wavelength of the black body curve
• spectral class
all of which are indicators of a star’s
temperature
Summary of Spectral Classes
(What type is our Sun?)
O
B
A
F
G
K
M
hotter than 25,000 K
11,000 - 25,000 K
7500 - 11,000 K
6000 - 7500 K
5000 - 6000 K
3500 - 5000 K
cooler than 3500 K
Stars are classified by their spectra as
O, B, A, F, G, K, and M spectral types
•
•
•
•
OBAFGKM
hottest to coolest
bluish to reddish
Each spectral type is now broken up into 10s
temperature sub-ranges. 0 (hottest) to 9 (coolest)
• An important sequence to remember:
–
–
–
–
Oh Be a Fine Guy (or Girl), Kiss Me
Overseas Broadcast - A Flash: Godzilla Kills Mothra
Over-Budget Adult Films Give Knights Merriment
One Boring Afternoon, Frank Grew Killer Marijuana
Looking for correlations:
Height vs. IQ ?
Height
Height
A
B
Height vs. Weight ?
QUESTIONS:
• Are more luminous stars always
larger?
• What combinations of temperature
and luminosity are possible?
THE H-R DIAGRAM
• done independently by Enjar
Hertzsprung and Henry Norris Russell
• graph of luminosity (or absolute
magnitude, M) versus temperature
(or spectral class)
The HertzsprungRussell (H-R) diagram
identifies a definite
relationship between
temperature and
absolute magnitude
HR DIAGRAM
absolute magnitude vs
temperature
or
luminosity vs
spectral type
The HertzsprungRussell (H-R)
diagram identifies a
definite relationship
between temperature
and absolute
magnitude
HR DIAGRAM
absolute magnitude vs
temperature
or
luminosity vs
spectral type
MAIN SEQUENCE
• Goes from top left (hot and
bright) to bottom right (cool
and dim).
• 90% of the stars are in the Main
Sequence stage of their lives
• Includes our Sun.
• Main Sequence
stars are found
in a band from
the upper left to
the lower right
RED GIANTS
• Really Big, Not Very Hot but
VERY BRIGHT!
• Betelgeuse: 3500 K , 100,000
times more luminous than the sun
• radius must be 1000x that of Sun!
• Red Giant and
Supergiant stars are
found above and to
the right of the
Main Sequence
stars
WHITE DWARFS
• Very Small, Very Hot but
Not Very Bright
• Sirius B: 27,000 K, but gives
off 1000 times less light than
the Sun
• 100 times smaller than the Sun
One of the nearest globular clusters in the sky.
• Tiny White Dwarf
stars are found in the
lower left corner of
the HR diagram
Determining the Sizes of Stars from an HR Diagram
• The Smallest stars are the
tiny White Dwarf stars
and are found in the lower
left corner of the HR
diagram
• Main sequence stars span
a range of sizes from the
small found in the lower
right to the large found in
the upper left
• The largest stars are the
Giant and Supergiant stars
which are found in the
upper right corner
Determining the Sizes of Stars from an HR Diagram
• The Smallest stars are the
tiny White Dwarf stars
and are found in the lower
left corner of the HR
diagram
• Main sequence stars span
a range of sizes from the
small found in the lower
right to the large found in
the upper left
• The largest stars are the
Giant and Supergiant stars
which are found in the
upper right corner
Spectral Type
O5
10,000
B5
A5
G5
K5
M5
Red Giants
a
d
1,000
-5
b
100
0
10
Main
Sequence
1
e
5
.1
.01
.001
f
10
c
White Dwarfs
.0001
15
20,000
10,000
Temperature (K)
5,000
Absolute magnitude
Luminosity (solar units)
F5
How does the size of a star near the top left of the
H-R diagram compare with a star of the same
luminosity near the top right of the H-R diagram?
A.
Spectral Type
They are the same size.
O5
10,000
D.
The star near the top right is
larger.
There is insufficient
information to determine
this.
A5
G5
K5
M5
Red Giants
a
d
1,000
-5
b
100
0
10
Main
Sequence
1
e
5
.1
.01
.001
f
10
c
White Dwarfs
.0001
15
20,000
10,000
Temperature (K)
0/0
F5
5,000
Absolute magnitude
C.
The star near the top left is
larger.
Luminosity (solar units)
B.
B5
The star Rigel is about 100,000 times brighter than
the Sun and belongs to spectral type B8. The star
Sirius B is about 3000 times dimmer than the Sun
and assume that it also belongs to spectral type B8.
Which star has the greatest surface temperature?
A. Rigel
B. Sirius B
C. They have the same
temperature.
D. There is insufficient
information to determine
this.
0/0
The HertzsprungRussell (H-R)
diagram identifies a
definite relationship
between temperature
and absolute
magnitude
HR DIAGRAM
absolute magnitude vs
temperature
or
luminosity vs
spectral type
Which is hotter a B3 or an A7?
Which is hotter a B0 or a B9?
Spectral Type
O5
10,000
B5
A5
G5
K5
M5
Red Giants
a
d
1,000
-5
b
100
0
10
Main
Sequence
1
e
5
.1
.01
.001
f
10
c
White Dwarfs
.0001
15
20,000
10,000
Temperature (K)
5,000
Absolute magnitude
Luminosity (solar units)
F5
B9
B0
Spectral Type
O5
10,000
B5
A5
G5
K5
M5
Red Giants
a
d
1,000
-5
b
100
0
10
Main
Sequence
1
e
5
.1
.01
.001
f
10
c
White Dwarfs
.0001
15
20,000
10,000
Temperature (K)
5,000
Absolute magnitude
Luminosity (solar units)
F5
What about the Masses of Stars
on the H-R Diagram?
• Main Sequence stars range from 0.1M to
~100M • The masses of Main Sequence stars increase
with increasing luminosity, size and temperature
• Main Sequence stars increase in mass from the
lower right to the upper left of the H-R Diagram
There is a
relationship
between mass
and luminosity
for Main
Sequence stars
Bigger (more massive)
is brighter and hotter!
There is a
relationship
between mass
and luminosity
for Main
Sequence stars
the numbers shown
are masses in terms
of the Sun’s mass
Bigger (more massive)
is brighter and hotter!
There is not a simple
relationship for the Mass of
Non-Main Sequence stars:
• Giants and Supergiants: range
from M to about 20M
• White Dwarfs: approximately
M or less
Average Densities:
• SUN: about density of water
• GIANTS: One thousand times less
dense than AIR!
• DWARFS: about 1 million times
the Sun’s density
– one teaspoon: 5 tons!!!
Extrasolar Planets (Exoplanets)
One of the latest discovered Exoplanets:
(reported September 29, 2010)
Gliese 581g is the first planet found to lie
squarely in its star’s habitable zone, where
the conditions are right for liquid water.
The new planet is about three times the
mass of Earth, which indicates it is probably
rocky and has enough surface gravity to
sustain a stable atmosphere.
Gliese 581g – Habitable Exoplanet?
http://www.wired.com/wiredscience/2010/09/
real-habitable-exoplanet/
It orbits its star once every 36.6 Earth days
at a distance of just 13 million miles (1
Astronomical Unit = 92955 887.6 miles)
Is this exoplanet closer to its sun than we are
to our Sun, or farther away? Whose LAW
tells you?
The discovery is based on 11 years of
observations using the HIRES spectrometer at
the Keck Telescope in Hawaii, combined with
data from the HARPS (High-Accuracy Radialvelocity Planet Searcher) instrument at the
European Southern Observatory in La Silla,
Chile.
The surface of a planet that close to our sun would be
scorching hot. But because the star Gliese 581 is only about
1 percent as bright as the sun, temperatures on the new
planet should be much more comfortable. Taking into account
the presence of an atmosphere and how much starlight the
planet probably reflects, astronomers calculated the average
temperature ranges from minus 24 degrees to 10 degrees
above zero Fahrenheit.
Gravity dictates that such a close-in planet would keep
the same side facing the star at all times, the same
way the moon always shows the same face to Earth.
That means the planet has a blazing-hot daytime side,
a frigid nighttime side, and a band of eternal sunrise or
sunset where water — and perhaps life — could
subsist comfortably. Any life on this exotic world
would be confined to this perpetual twilight zone,
Vogt says, but there’s room for a lot of diversity.
Extrasolar planets
or Exoplanets
G. Marcy and P. Butler
HOW MANY
EXOPLANETS
FOUND TO DATE?
Over 500!!
Planet detection and methods:
1. Doppler Shift
2. Pulsar Timing
3. Direct Imaging
4. Gravitational Lensing
5. Transit of planets in front of Star/Sun
(method used by the K _ _ _ _ _ telescope)
Doppler shifts used to measure the radial
velocity of the star's movement
Radial Velocity Curve
(positive when moving away from observer)
1) What is radial velocity and how does
increasing it affect the Doppler shift?
2) When is radial velocity defined to be positive?
3) What is the period of an orbit and how does
the period of a star and its exoplanet compare?
4) What point do the star and its exoplanet orbit
about?
5) Which direction does the exoplanet orbit if the
Star orbits clockwise?
A
Orbit of star
Radial Velocity
20 m/s
Earth
10 m/s
D
B
Time
-10 m/s
- 20 m/s
Orbit of planet
C
Given the location marked on the star's radial
velocity curve, at what location in the planet's
orbit (1-4) would you expect the planet to be?
Given the location marked with the dot on the star’s radial
velocity curve, at what location (A, B, C or D) would you
expect the planet to be located at this time?
Transit of an Exoplanet
across a sun.
Demonstration!
(note condition for viewing this)
KEPLER TELESCOPE uses this method!