Download Main Sequence stars

The Nature of Stars
MRS 2
• Test 3 Thursday April 24
• Part 1 Due 1 week from Today (4/17)
• Read
http://www.planetary.org/blogs/emily-lakdawalla/2013/12091832-curiosity-results-at-agu-age-
dating.html
• Read
• Answer questions on website
• If you do not understand/have questions
contact us!!!
http://www.sciencemag.org/content/343/6169/1247166.abstract
Pollux has an apparent magnitude of 1.1 and an absolute
magnitude of 1.1. Epsilon Eridani has an apparent
magnitude of 3.72 and an absolute magnitude of 6.1. From
which of these stars do we receive more light?
a) Pollux
b) Epsilon Eridani
Imagine that you are viewing a star that has an apparent
magnitude of 0.2 and is located about 100 parsecs away
from us. Which of the following is most likely the star’s
absolute magnitude?
a)
-4.8
b)
0.1
c)
0.2
d)
0.3
e)
5.2
A star’s color reveals its surface temperature
What color is
this star?
Diversity Leads to Revolution
• Annie Jump Cannon
• Meghnad Saha
• Cecilia Payne-Gaposchkin
Women Computers (1890)
Annie Jump Cannon (1863-1941)
O
B
A
F
G
K
M
A Revolution
• Most astronomers believed that the
differences in spectral classes (O-M)
were due to differences in chemical
abundance.
• Indian physicist Meghnad Saha offered
another explanation, which was
confirmed at Harvard by Cannon
Meghnad Saha (1893-1956)
Theory of thermal ionization of atoms
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, and spectral class all of
which are indicators of a star’s
temperature
Summary of Spectral Classes
Stars are classified by their spectra as
O, B, A, F, G, K, and M spectral types
•
•
•
•
OBAFGKM
hottest to coolest
bluish to reddish
An important sequence to remember:
– Oh Be a Fine Guy (or Girl), Kiss Me
For thousands of nearby
stars we can find:
• the total luminosity
• the temperature (color or spectral
type)
• the size (radius)
• the distance
CAN WE FIND ANY RHYME, REASON,
OR RELATIONSHIPS?
Looking for correlations:
Height vs. IQ ?
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) 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
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
• Burning Hydrogen to Helium in
core
• 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
• 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
Tutorial: H-R Diagram (p.117)
• Work with a partner!
• Read the instructions and questions carefully.
• Discuss the concepts and your answers with one
another. Take time to understand it now!!!!
• Come to a consensus answer you both agree on.
• If you get stuck or are not sure of your answer, ask
another group.
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?
1. They are the same size.
2. The star near the top left is
larger.
3. The star near the top right
is larger.
4. There is insufficient
information to determine
this.
0/0
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 also belongs to spectral type B8. Which star has
the greatest surface temperature?
1. Rigel
2. Sirius B
3. They have the same
temperature.
4. There is insufficient
information to determine
this.
0/0
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 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!!!
The evolution of stars is determined by a
constant battle between gravity and pressure
gravity pulls things together
pressure pushes things apart
40
Stars condense from clouds of gas and
dust (the interstellar medium) that exist
throughout the disk of the galaxy
Interstellar medium
Gas = Hydrogen
Dust = Carbon and
Silicon
Pillars of Creation
Eagle Nebulae
Becoming a Star Step 1 – Cloud collapses
• Why do these clouds of gas and dust collapse?
– One idea is that a shockwave from the explosion at the death of a star
known as a supernova cause the gas and dust cloud to become unstable
and start to collapse
Becoming a Star Step 1 – Cloud collapses
• As the cloud collapses, the
center becomes very very hot
and very very dense -
Becoming a Star Step 2 – Fusion
• As the gas cloud collapses due to gravitational
forces, the core becomes hotter and the density
inside the core increases
• Eventually, the temperature and density reach a
point where nuclear fusion can occur
Fusion is the combining together of
light atoms, into heavier atoms
For all Main Sequence stars, the temperature and density in their cores are
so great that Hydrogen atoms combine to make Helium atoms and release
energy – a process known as thermonuclear fusion
4H  He + energy
The Main Sequence is defined by stars converting hydrogen to
helium in their core
Becoming a Star Step 3 – Balance
All Main Sequence stars are in
hydrostatic equilibrium
• Fusion produces radiation (light) that creates
an outward pressure
• During hydrostatic equilibrium there is a
balance between the gravitational collapse of
the star pushing inward and the outward
pressure produced by photons from nuclear
fusion in the core.
It’s a matter of balance.
• This balance is called hydrostatic equilibrium
• gravity (
) wants to collapse the star, but pressure
(
) pushes outward against the collapsing material
Fusion: 4H  He + energy(light)
• All Main Sequence stars are in hydrostatic
equilibrium because nuclear fusion of
hydrogen is producing enough outward
pressure to balance gravitational collapse.
It takes a few million years to get there but - stars spend
most of their life time as a Main Sequence star
STELLAR LIFETIMES
• Which will have a greater core temperature
and density – a high mass star or a low mass
star?
• Which will then have a greater fusion rate?
• Which will use up its fuel more quickly?
• What is the fuel?
STELLAR LIFETIMES
• Consider a main sequence star with 10 times the
mass of the Sun
• It will
– have higher temps and pressures at the core
– have greater fusion rates - consumes fuel at 1000
times the rate of the sun
– be 1000 times as bright and last 1/100 as long
• “Burn bright, die young.”
LIFETIMES
• Bright O-type stars live very short
lives (about 10 million years)
• Very small stars live a long time (100
billions of years)
• Our SUN: will live a total of about
10 billion years (half used up)
The more massive a star, the faster
it goes through its main sequence
phase
Tutorial: Star Formation and
Lifetimes (p.119)
• Work with a partner!
• Read the instructions and questions carefully.
• Discuss the concepts and your answers with one
another. Take time to understand it now!!!!
• Come to a consensus answer you both agree on.
• If you get stuck or are not sure of your answer, ask
another group.
Stars spend most of their life cycles on
the Main Sequence
• Main Sequence stars are in hydrostatic equilibrium
because nuclear fusion is turning hydrogen into helium
and producing enough outward pressure to balance
gravitational collapse.
• 90% of all stars are found on the Main Sequence
• 90% of the whole life of all stars is spent on the Main
Sequence
• BUT – What happens when the hydrogen runs out?