life and evolution of stars chapter 9. outline i.masses of stars: binary stars ii.variable stars...
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Life and Evolution of StarsChapter 9
Outline
I. Masses of Stars: Binary Stars
II. Variable Stars
III. Spectral Types of Stars
IV. H-R Diagram
V. The Source of Stellar Energy
VI. 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
systemBoth 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.
1. Visual Binaries
2. Spectroscopic Binaries
3. Eclipsing Binaries
Types of Binaries:
Visual Binaries (Video_Visual_Binaries)
The ideal case:
Both stars can be seen directly, and
their separation and relative motion can be followed directly.
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)T
ime
Typical sequence of spectra from a spectroscopic binary system
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)
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.
Example:
Algol in the constellation of Perseus
The Light Curve of Algol
II. Variable Stars
Video Trailer: Variable StarsChi 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)
Tem
pera
ture
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
OB
A
F
GKM
Surface tem
perature
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.
To bring some order into that zoo of different types of stars: organize them in a diagram of
Luminosity versus Temperature (or spectral type)
Lum
inos
ity
Temperature
Spectral type: O B A F G K M
Hertzsprung-Russell Diagram
orA
bsol
ute
mag
.
The Hertzsprung-Russell Diagram (Simulation)
Most stars are found along the
Main Sequence
The Hertzsprung-Russell Diagram (2)
Stars spend most of their
active life time on the M
ain
Sequence (MS).
Same temperature,
but much brighter than
MS stars
L α R2 x T4 ,where,
L = Luminosity of star
R = Radius of star
T = surface temperature of the star.
The Brightest StarsThe 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
10,000 times the
sun’s radius
100 times the
sun’s radius
As large as the sun
Rigel Betelgeuse
Sun
Polaris
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)
NovaHerculis
1934
March 1935 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)
Pulsar: Lighthouse Model
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Pulsar(Video1)
Crab Pulsarالسرطان نّباض
الثانية 30 في نّبضة
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