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Colors of Stars
Stars come in many different colors.
The color tells us the star’s temperature according to Wien’s Law.
Bluer means hotter!
Masses of Stars
• Mass is the single most important property of any star.• at each stage of a star’s life, mass determines…
• what its luminosity will be• what its spectral type will be
• The mass of a star can only be measured directly by …• observing the effect which gravity from another object has on
the star• This is most easily done for two stars which orbit one
another…a binary star!
The orbit of a binary star system depends on strength of gravity
Binary Stars(two stars which orbit one another)
• Optical doubles• two unrelated stars which are in the same area
of the sky
• Visual binaries• a binary which is spatially resolved, i.e.
two stars are seen (e.g. Sirius)
Binary Stars
• Spectroscopic binaries• a binary which is spatially unresolved, i.e only
one star is seen; the existence of the second star is inferred from the Doppler shift of lines.
Binary Stars
• Spectroscopic binaries• a binary which is spatially unresolved, i.e only
one star is seen; the existence of the second star is inferred from the Doppler shift of lines.
Binary Stars
• Spectroscopic binaries• a binary which is spatially unresolved, i.e only
one star is seen; the existence of the second star is inferred from the Doppler shift of lines.
SO...
For a few thousand stars we can now find:� the distance� the total luminosity� the temperature (color or
spectral type)� the radius CAN WE FIND ANY
RHYME, REASON, OR RELATIONSHIPS?
Looking for correlations:
Height vs. IQ ?
Height vs. Weight ?
Height Height
A B
B – V (Temperature
or spectral type)
L
HOTCOOL
BRIGHT
FAINT
• A very useful diagram for understanding stars
• We plot two major properties of stars:
• Temperature (x) vs. Luminosity (y)
• Spectral Type (x) vs. Absolute Magnitude (y)
• Stars tend to group into certain areas
The Hertzsprung-Russell Diagram
Normal hydrogen-burning stars reside on the main sequence of the H-R diagram
Low-Mass Stars
High-Mass Stars
The Main Sequence (MS)
90% of all stars lie on the main sequence!
Normal hydrogen-burning stars reside on the main sequence of the H-R diagram
Low-Mass Stars
High-Mass Stars
The Main Sequence (MS)
90% of all stars lie on the main sequence!
Stars with low temperature and high luminosity must have large radius
GIANTS
SUPERGIANTS
Temperature
Lum
inos
ity
H-R diagram depicts:
Temperature
Color
Spectral Type
Luminosity
Radius
*Mass
*Lifespan
*Age
Temperature
Lum
inos
ity
Which star is the hottest?
A
BC
D
Temperature
Lum
inos
ity
Which star is the hottest?
A
BC
DA
Temperature
Lum
inos
ity
Which star is the most luminous?
A
BC
D
Temperature
Lum
inos
ity
Which star is the most luminous?
A
BC
DC
Temperature
Lum
inos
ity
Which star is a main-sequence star?
A
BC
D
Temperature
Lum
inos
ity
Which star is a main-sequence star?
D
A
BC
D
Temperature
Lum
inos
ity
Which star has the largest radius?
A
BC
D
Temperature
Lum
inos
ity
Which star has the largest radius?
C
A
BC
D
Temperature
Lum
inos
ity
Which star is most like our Sun?
A
B
C
D
Temperature
Lum
inos
ity
Which star is most like our Sun?
A
B
C
D
B
Temperature
Lum
inos
ity
Which of these stars will have changed the least 10 billion years from now?
A
B
C
D
Temperature
Lum
inos
ity
Which of these stars will have changed the least 10 billion years from now?
C
A
B
C
D
Temperature
Lum
inos
ity
Which of these stars can be no more than 10 million years old?
A
B
C
D
Temperature
Lum
inos
ity
Which of these stars can be no more than 10 million years old?
A
A
B
C
D
Regions of the H-R Diagram
RED GIANTS• Cool but VERY BRIGHT!• Betelgeuse: 3500 K (10% as
bright/unit area as Sun) but 100,000 times as luminous--must have 1 million times the area
• radius must be 1000x that of Sun!
ÿAuaKaÿelo
WHITE DWARFS• Hot but not very luminous• Sirius B: 3% as luminous as
Sun but same temp. as Spica (10,000x)--Sirius B must be 1/600 the radius of Spica
• Also much smaller than Sun
Stellar Masses on the H-R Diagram
Mass–Luminosity Relation
• All main sequence stars fuse H into He in their cores.
• Luminosity depends directly on mass because:• more mass means more weight from the star’s
outer layers• nuclear fusion rates must be higher in order to
maintain gravitational equilibrium
Mass-Luminosity Relation
L � m3.5for main sequence stars only
We use binary stars to measure directly the masses of stars of every type. We find a
• As one moves to the upper-left of the main sequence:• stars become more massive • stars become even much more luminous• stars become fewer in number
Lifetime on the Main Sequence
How long will it be before MS stars run out of fuel? i.e. Hydrogen?
How much fuel is there? M
How fast is it consumed? L � M3.5
How long before it is used up?
t = M/L = M/M3.5 = M-2.5 = 1/M2.5
Lifetime on the Main Sequence
• Our Sun will last 1010 years on the Main Sequence • MS Lifetime � = 1010 yrs / M2.5
So for example:
B2 dwarf (10 M) lasts 3.2 x 107 yrF0 dwarf (2 M) lasts 1.8 x 109 yr
M0 dwarf (.5 M) lasts 5.6 x 1010 yr
But the Universe is ~1.37 x 1010 yr old!
Every M dwarf that was ever created is still on the main sequence!!
Stellar Evolution
• Stars are like people in that they are born, grow up, mature, and die.
• A star’s mass determines what life path it will take.• We will divide all stars into three groups:
– Low Mass (0.08 M < M < 2 M)– Intermediate Mass (2 M < M < 8 M)– High Mass (M > 8 M)
• The H-R Diagram makes a useful roadmap for following stellar evolution.
Stellar Evolution• The life of any star is a battle between two forces:
– Gravity vs. Pressure
• Gravity always wants to collapse the star.• Pressure holds up the star.
– the type of star is defined by what provides the pressure
• Newton’s Law of Gravity:– the amount of gravitational force depends on the mass– gravitational potential energy is turned into heat as a star
collapses
hydrostatic equilibrium
The Stellar Womb
• Stars are born deep in molecular clouds.– cold (10 – 30 K) dense nebulae– so cold that H2 can exist
• A cold cloud can fragment– gravity overcomes thermal
pressure in dense regions– these regions (cores) become
more dense and compact
dark molecular cloud in Scorpius
Stellar Gestation
• Something happens to perturb a molecular cloud and make it begin to fragment
• As a core of gas collapses, it heats up– it radiates infrared from its surface
– protostar• The protostar collapses until its
core reaches 107 K in temperature – The proton – proton chain fusion
reaction begins and …. infrared image of Orion
A Star is Born!
MakaliÿiEagle NebulaHubble Space Telescope
Movie. Click to play.
Star Formation• As the protostar collapses, angular momentum is conserved
– the protostar rotates faster– matter falling in to the protostar flattens into a (protostellar) disk– a planetary system could form from this disk
Direct Evidence of Disks & Jets
a disk forms
Star Formation
• As the protostar heats up, enough thermal energy is radiated away from surface to allow collapse to continue.– energy is transported to surface first via convection– as core gets even hotter, transport via radiation takes over
• The protostar must rid itself of angular momentum, or it will tear itself apart– magnetic fields drag on the protostellar disk– fragmentation into binaries
• Fusion reactions begin when core reaches 107 K
Stages of Star Formation on the H-R Diagram
Arrival on the Main Sequence
• The mass of the protostar determines:– how long the protostar
phase will last– where the new-born star
will land on the MS i.e., what spectral type the star will have while on the main sequence
When a protostar ceases to accumulate mass, it, becomes a pre-main-sequence star.
It’s life path is forever determined by its initial mass
Missing the Main Sequence
• If the protostar has a mass < 0.08 M:– It does not contain enough gravitational energy
to reach a core temperature of 107 K– No fusion reactions occur– The star is stillborn!
• We call these objects Brown Dwarfs.• They are very faint, emit infrared, and have
cores made of Hydrogen– degenerate cores
Detection by
Michael Liu (IfA)
January, 2002
“Brown Dwarf”
orbiting a star
at same
distance as
Saturn in Solar
system
Life on the Main Sequence
The internal structure is different for MS stars of different masses.
The more massive a star, the faster it goes through its main
sequence phase
When core hydrogen fusion ceases, a main-sequence star becomes a giant
• The star can no longer support its weight• The enormous weight from the outer layers
compresses hydrogen in the layers just outside the core enough to initiate shell hydrogen fusion.
• This extra internal heat causes the outer layers to expand into a giant star.
Leaving the Main Sequence• The core begins to collapse
– H shell heats up and H fusion begins there– there is less gravity from above to balance this pressure– so the outer layers of the star expand– the star is now in the subgiant phase of its life
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