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Chapter 9
Life and Evolution of Stars
Outline
I.
II.
III.
IV.
V.
VI.
Masses of Stars: Binary Stars
Variable Stars
Spectral Types of Stars
H-R Diagram
The Source of Stellar Energy
Life Story of a Star
Video Trailer:
Birth of Stars
I. Masses of Stars:
Binary Stars
1. Binary Stars
More than 50 % of all
stars in our Milky Way
are not single stars, but
belong to binaries:
Pairs or multiple
systems of stars which
orbit their common
center of mass.
If we can measure and
understand their orbital
motion, we can
estimate the stellar
masses.
The Center of Mass (Active_Figure_12)
center of mass =
balance point of the
system
Both masses equal
=> center of mass is
in the middle, rA = rB
The more unequal the
masses are, the more
it shifts toward the
more massive star.
Types of Binaries:
1. Visual Binaries
2. Spectroscopic Binaries
3. Eclipsing Binaries
Visual Binaries
The ideal case:
Both stars can be
seen directly, and
their separation and
relative motion can
be followed directly.
(Video_Visual_Binaries)
Sirius A and Siruis B (white dwarf)
Spectroscopic Binaries (Video_Spectr_Binaries)
Usually, binary separation “a”
can not be measured directly
because the stars are too
close to each other.
A limit on the separation
and thus the masses can
be inferred in the most
common case:
Spectroscopic
Binaries
Spectroscopic Binaries (2)
The approaching star produces
blue shifted lines; the receding
star produces red shifted lines
in the spectrum.
Doppler shift  Measurement
of radial velocities
 Estimate
of separation “a”
 Estimate
of masses
Spectroscopic Binaries (3)
Typical sequence of spectra from a
spectroscopic binary system
Time
Eclipsing Binaries (Animation)
Usually, the inclination
angle of binary systems is
unknown  uncertainty in
mass estimates
Special case:
Eclipsing Binaries
Here, we know that
we are looking at the
system edge-on!
Eclipsing Binaries (2)
Peculiar “double-dip” light curve
Example: VW Cephei
Eclipsing Binaries (3)
Example:
Algol in the
constellation
of Perseus
From the light
curve of Algol, we
can infer that the
system contains
two stars of very
different surface
temperature,
orbiting in a
slightly inclined
plane.
The Light Curve of Algol
II. Variable Stars
Video Trailer: Variable Stars
Chi Cygni expands and dims, and then contracts
and brightens over 408 days
Variable Stars
• A variable star is a star that has lost its hydrostatic
equilibrium. The brightness and size of a variable
star change with time as it evolves.
• Two Types of Variable Stars:
– Pulsating stars: stars that appear to pulsate and change
brightness. Examples are:
Cepheid variables – RR Lyrae – Neutron stars
(1 to 60 days - About 1 day - A couple of seconds)
– Exploding stars: stars that show extreme brightness
variability. Examples are:
Nova – Supernova – T Tauri
Nova outburst
(Active_Figure_27)
III. Spectral Types of Stars
Spectral Classification of Stars (1)
Temperature
Different types of stars show different
characteristic sets of absorption lines.
Spectral Classification of Stars (2)
Mnemonics to
remember the
spectral
sequence:
Oh
Oh
Only
Be
Boy,
Bad
A
An
Astronomers
Fine
F
Forget
Girl/Guy
Grade
Generally
Kiss
Kills
Known
Me
Me
Mnemonics
Stellar Spectra
F
G
K
M
Surface temperature
O
B
A
The Composition of Stars
From the relative strength of absorption lines (carefully
accounting for their temperature dependence), one can
infer the composition of stars.
IV. H-R Diagram
Organizing the Family of Stars:
The Hertzsprung-Russell Diagram
We know:
Stars have different temperatures,
different luminosities, and different sizes.
Absolute mag.
or
Luminosity
To bring some order into that zoo of different
types of stars: organize them in a diagram of
Luminosity
versus
Temperature (or spectral type)
Hertzsprung-Russell Diagram
Spectral type: O
Temperature
B
A
F
G
K
M
The Hertzsprung-Russell Diagram (Simulation)
The Hertzsprung-Russell Diagram (2)
Same
temperature,
but much
brighter than
MS stars
Lα
2
R
x
where,
L = Luminosity of star
R = Radius of star
T = surface temperature of the star.
4
T
,
The Brightest Stars
The open star cluster M39
The brightest stars are either blue (=> unusually hot)
or red (=> unusually cold). (Is this a contradiction?)
The Radii of Stars in the
Hertzsprung-Russell Diagram
Betelgeuse
Rigel
Polaris
Sun
The Relative Sizes of Stars in
the HR Diagram
V. The Source of Stellar
Energy
Energy of Stars
• All stars are considered as huge balls of gases
where nuclear fusion in their cores produces
most of their energies.
• It is possible to calculate an approximate star’s
lifetime by determining its mass (tlife ~ 1/M2.5)
• Cold (red ones) stars have longer lifetime than
hot stars:
– O star: ~ 1 million years
– G star (Sun): ~ 10 billion years
– M star : ~ 5,000 billion years
• First stage: all stars start fusing hydrogen (H) to make
helium (He)
• This stage is considered to be the longest stage in a star’s
lifetime ( 90% of its total lifespan)
• Second stage: Fusing of helium (He) to make carbon (C)
• The life of some stars (like our Sun) stops after this stage,
but others will continue processing heavier and heavier
elements than carbon in their cores.
• For the massive stars (more than 8 solar masses), iron will
be the last element that a star can form in its core.
• Stars start their lifetime with a light element core (H) and
end up with a heavy element core.
Simulation
VI. Life Story of a Star
Life Story of a Star
• Stars are born inside huge interstellar clouds following three
stages:
– Giant molecular cloud
– Dense cores
– Protostar  T Tauri star
• Stars are divided into two main groups:
– Stars with masses less than 8 solar mass
– Stars with masses larger than 8 solar mass
•
Stars with mass less than 8 solar mass
–
–
–
–
–
–
–
Giant molecular cloud
Dense core
T-Tauri star
Main-sequence star: fusing H to make He
Giant star: fusing He to make C
Planetary Nebulae
White Dwarf (with mass less than 1.4
solar mass)
A T-tauri stage of a star: fast stellar winds
While on the main sequence, a star is in “hydrostatic equilibrium”:
inward pressure due to gravity balances the outward pressure due to heat.
Simulation
Do we see white dwarfs?
A special binary system: a white dwarf and a regular star
Outcome: a nova (or Supernova type Ia)
March 1935
Nova
Herculis
1934
May 1935
•
Stars with mass larger than 8 solar mass
– Giant molecular cloud
– Dense core
– T-Tauri star
– Main-sequence star: fusing H to make He
– Supergiant star: fusing He to make C, O, Ne,
Mg, Si, ..Fe
– Supernova explosion
– Neutron star (with mass less than 3 solar
mass), black hole (with different masses..)
Betelgeuse: a supergiant star
Do we see neutron stars?
Neutron star: size no bigger than a city (10-15 km)
‫النبّاض‬
‫اإلشعاعي‬
Pulsar
(Video1)
Pulsar: Lighthouse Model
‫‪Crab Pulsar‬‬
‫نباض السرطان‬
‫‪30‬نبضة في الثانية‬
Do we see black holes? M87
Another special binary system: a black hole and a regular star
Life Story of a Star: Summary (Simulation)
Properties of WD, NS, and BH
(Simulation)
• White dwarfs (WD):
– Size: Earth’s size
– Mass: less than 1.4 solar mass (Chandrasekhar limit)
• Neutron stars (NS):
– Size: 10 to 15 km
– Mass: less than 3 solar mass
• Black holes (BH):
– Size: depends on the mass (3 - ….. Km) (Simulation)
– Mass: 1 – ……. Solar mass