Download Star Formation, HR Diagram, and the Main Sequence (Professor

Star Formation
Protostars form in cold dark nebulae
Inter Stellar Matter with a high enough density, and a
low enough temperature for proto-stars to form.
Giant Molecular cloud (GMC) in Orion
•About 1000
GMCs are
known in our
galaxy
• These
clouds lie in
the spiral
arms of the
galaxy, where
the dust &
gases are.
Size of cloud – large, Compression area - small
Size: r ~ 50 pc
Mass: > 100,000 Msun
Temp.: a few 0K
Interstellar clouds of mostly molecular hydrogen H
Warmer GMCs resist forming stars, kinetic energy
opposes the force of gravity to collapse the gas.
A cooler gas is needed, and the GMC must be
disturbed to induce it to collapse.
•Star formation is triggered when a
sufficiently massive pocket of gas is
squeezed by some external event, such as a
shock wave
Sources of Shock Waves:
(1). Since massive stars die young,
Supernovae explosions happen near sites of
recent star birth.
(2) Previous star formation can trigger further
star formation. (Stellar winds)
(3) Spiral arms rotating can cause shock
waves.
When you compress a gas it heats up. When a gas
expands it cools
As a proto-star evolves, it shrinks, its density
increases and it temperature rises.
Proto-stars are physically larger than the mainsequence stars that they will become.
What types of stars are formed?
OB – Few
AFG – More
KM – Many, Many
Observations of star formation:
Evaporating gaseous globules
(“EGGs”): Newly forming stars
exposed by the ionizing
radiation from nearby massive
stars
The Birth of Stars
near the stars
The collapsing
protostar eventually
heats up, and blows
away its cocoon.
T Tauri Stars
All proto-stars will eject gas before they reach
main sequence but the cooler stars G,K, and M, do
so more vigorously and are called T Tauri Stars..
Below is a T Tauri star with an accretion disk, and a
jet of hot gas.
•Low-mass stars that eject gas before becoming
main sequence stars, may lose as much as 40% of
its mass.
Some
young
disks & jets
revealed
Low-Mass Proto-stars
Collapse is slower for lower masses:
•1 Msun takes ~30 Myr
•0.2 Msun takes ~1 Billion years
When core temperature ~ 10 Million K:
•Ignite core P-P chain fusion
•Stellar wind blows away the cocoon
•Settles slowly onto the Main Sequence
•Some of the clump material settles into a
rotating disk, from which planets might form
Actual Protoplanetary Disks
• Four
protoplanetary
disks in the Orion
Nebula, 1500 light
years away.
• The disks are
99% gas and 1%
dust.
• The dust shows as a
dark silhouette
against the glowing
gas of the nebula.
To Reach Main Sequence
As the core heats up, H fusion runs faster:
Core temperature rises
Core pressure rises
Collapse begins to slow down
If the core temperature reaches at least 10 million
deg K, the proto-Star becomes a Star
Finally:
•Pressure=Gravity & collapse stops.
•Star reaches the Zero-Age Main Sequence
•(ZAMS).
Meanwhile, back in the GMC, things are still
happening
Meanwhile the original stars are growing
Star Form in Clusters
Stars do not form isolated, but in large
groups, called Open Star Clusters .
Our own Sun is part of an open cluster than includes
other nearby stars such as Alpha Centauri and
Barnard's star.
Gravitational
interactions between the
stars and other objects
will cause these clusters
to eventually disperse
over time
Hertzsprung-Russell
Diagram:
Hertzsprung-Russell Diagram:
In 1905, Danish astronomer Einar
Hertzsprung, and independently
American astronomer Henry Norris
Russell, noticed that the luminosity
of stars decreased from spectral
type O to M.
To bring some order into the different
types of stars: they organize them in a
diagram, the H-R diagram
H-R Diagram Basics
Each star is represented by a dot.The position of
each dot on the diagram corresponds to the star's
luminosity and its temperature
Luminosity (Lsun)
The vertical position represents the
star's luminosity.
106
104
The horizontal position represents
the star's surface temperature.
102
1
10-2
10-4
40,000
20,000
10,000
5,000
Temperature (K)
2,500
Notice that the plot is not completely random, so
there is some sort of relationship. H–R Diagram
or L-T Diagram
Luminosity (Lsun)
106
104
102
1
10-2
10-4
40,000
20,000
10,000
5,000
Temperature (K)
2,500
H–R Diagram
Supergiants
Luminosity (Lsun)
106
104
102
Giants
1
10-2
10-4
40,000
White Dwarfs
20,000
10,000
5,000
Temperature (K)
2,500
Color Version of H-R Diagram
BRIGHT
Cool
Stars
get
larger
Stars get hotter
Luminosity
classes
Class Ia,b : Supergiant
Class II: Bright giant
Class III: Giant
Class IV: Sub-giant
Class V:MS
The Sun is a G2 V star
Mass-Luminosity relation
Radii on the Main Sequence L = 4πR2σT4
If you know L & T, you can calculate R
There is a unique mass & radius for each star
along the main sequence
R  20 R M  30 M
R  5 R M  7 M
R  1 R M  1 M(sun!)
R  0.3 R M  0.2 M
In the last few years, two new groups were added
to the OBAFGKM classification, they are L & T.
These stars have been found due to greatly
improved infrared detectors aboard satellites.
Both L & T are Brown Dwarfs.
They are visible in the red, and infrared regions ,
Classification of Stars
Ia
Ib
II
III
IV
V
Bright supergiant
Supergiant
Bright giant
Giant
Subgiant
Main sequence star
Classification:Spectral Class
• Alternate way of describing
temperature: SPECTRAL CLASS
•
•
•
•
•
•
•
O = 40,000 K
B = 20,000 K
A = 10,000 K
F = 7500 K
G = 5500 K
K = 4500 K
M = 3000 K
• The spectral classes
OBAFGKM began
as a method of
classifying stars
according to the
appearance of the
absorption lines in
their spectra.
Random Sample of Stars
If you took a random sample of 1,000,000
stars from our galaxy. In this sample, you
will find, on the average:
•
•
•
•
900,000 main sequence stars
95,000 white dwarfs
4000 giants
1 supergiant
Main
Sequence
•Pre-main sequence evolutionary tracks
Most everything about a star's life depends
on its (MASS).
Life Tracks for Different Masses
• Highermass stars
form faster
• Lower-mass
stars form
more slowly
Luminosity
Stars more
massive
than
150MSun
would blow
apart ****
Temperature Increases
Stars less
massive
than
0.08MSun
can’t
sustain
fusion
Main sequence
• Zero-age main sequence (ZAMS):
ZAMS, phase at which star first gets
all its energy from H burning (star no
longer contracts).
• Main sequence (MS): phase of core
hydrogen burning, this is the longest
stage in stellar life.
• A star spends 90% of their life on the
MS
•
Main Sequence Lifetimes
(predicted)
Mass
(suns)
25
15
3
1.5
1.0
0.75
0.50
Surface temp Luminosity
(K)
(suns)
35,000
80,000
30,000
10,000
11,000
60
7,000
5
6,000
1
5,000
0.5
4,000
0.03
Lifetime
(years)
3 million
15 million
500 million
3 billion
10 billion
15 billion
200 billion
Normal gas
• Pressure is the force exerted by atoms in a gas
• Temperature is how fast atoms in a gas move
• Hotter
atoms move faster
higher pressure
• Cooler
• atoms move slower
lower pressure
Pressure balances gravity, keeps stars from collapsing
Core-Envelope Structure
Outer layers press down on the inner layers.
The deeper you go, the greater the pressure.
Gas Law : Greater pressure = hotter, denser gas
The star develops a
Core-Envelope
structure:
A hot, dense,
compact central
CORE
surrounded by a
cooler, lower
density, extended
ENVELOPE
Where fusion
takes place
Supplies gravity
to the core
Stars on the Main Sequence, are in equilibrium.
Gravity pulling inward wants to contract the star.
Pressure pushing outwards from fusion wants to
make the star
expand.
When there is a balance between the two, we have a
condition of Hydrostatic Equilibrium.
In this condition, the star neither expands, nor contracts.
Thermodynamics says : Heat always flows
from hotter regions into cooler regions.
In a star, heat flows from the hot core, out through
the cooler envelope, to the surface where it is
radiated away as light
Radiation
Energy is carried by photons.
which leave the core, hit atoms or
electrons and get scattered.
They slowly stagger to the surface.
Takes ~1 Million
years for a photon
to reach the surface.
Convection
Energy carried from hotter
regions to cooler regions above by
bulk buoyant motions of the gas.
Everyday examples of convection
are boiling water.
Main-Sequence Stars and Fusion
Energy is generated by fusion of 4 1H into 1 4He.
There are two nuclear reaction paths by which a
star might accomplish this fusion:
1. Proton-Proton Chain: Low mass stars
Relies on proton-proton reactions
Efficient at low core Temperatures (TC<18M K)
4 x 1H  1 x 4He + energy.
Fuse 4 protons (1H) into 1 4He nucleus.
This reaction produces the following by-products:
Gamma-ray photons, 2 positrons , and 2 neutrinos that
leave the Sun.
2. CNO Cycle: High mass stars
Efficient at high core
Temperatures(TC>18MK)
In stars that are hotter than 18 million
degrees Kelvin, protons are fused into 1
Helium nucleus via a multi-step nuclear
reaction , where Carbon is the catalyst.
More massive star will have the shorter life time
•O & B burn fuel like a bus!
•M burn fuel like a compact car!
Every M dwarf ever created is still on the main sequence!!
Main Sequence Lifetimes
Spectral Type
Mass
(Solar masses)
Main sequence lifetime (million
years)
O5
B0
A0
40
16
3.3
1
10
500
F0
1.7
2700
2.7 BY
G0
K0
1.1
0.8
9000
14 000
9 BY
14 BY
M0
0.4
200 000
200BY
Largest Star known: LBV 1806-20 Pistol Star
150-200 solar mass Temp 12,300 K Discovered 1995
Radius 500 time sun’s
Distance 45,000 ly
This star will eject gases into space, and by the
time it becomes a main-sequence star, its mass
may be 10 solar masses.
Coolest White Dwarf SDDSS-J1403 Mass 0.6 solar mass
Temperature 4,300 K
WD Radius 0.01 times Sun
Distance 145 ly
Hottest Star White Dwarf Central star of NGC 2440
Temperature 211,000 K
Radius 0.028 times Sun
Mass 0.6 solar mass
Distance 7,100 ly
Doppler Motion(vr)
(Radial Motion)
Actual Motion
Line of Sight
(vt).
Proper Motion
(Tangential Motion)
v
Radial Velocity
The radial velocity of a star is how fast it is moving directly towards
or away from us. (Doppler Effect)
Earth
Radial velocities are measured using the Doppler Shift of
the star's spectrum:
•Star moving towards Earth: Blueshift
•Star moving away from Earth: Redshift
•Star moving across our line of sight: No Shift
In all cases, the Radial Velocity is Independent of Distance.
Tangential Velocity
Over a period of time, a star will have moved across the
sky a distance. Divide that distance by the time and get the
Velocity and also measure the Proper Motion Angle..
Tangential Velocity (vt).
where:
m = Proper Motion in arcsec/yr
d = Distance in parsecs
The formula above gives vt in km/sec.
Each of these velocities forms the legs of a right triangle
with the true space velocity (v) as the hypotenuse.
We can then use the Pythagorean Theorem to derive the
True Space Velocity (v):
v  v  (4.74 m d )
2
2
Thanks to the following for allowing me to
use information from their web site :
Nick Stobel
Bill Keel
Richard Pogge
John Pratt
NASA
W.H.Freeman & Company