Inter Stellar Matter with a high enough density, and a low enough temperature for proto-stars to form.

Protostars form in cold dark nebulae Star Formation

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

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.

Size: r ~ 50 pc

Mass: > 100,000 Msun

Temp.: a few 0K

Interstellar clouds of mostly molecular hydrogen H

•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.

As a proto-star evolves, it shrinks, its density increases and it temperature rises. Proto-stars are physically larger than the main-sequence stars that they will become.

What types of stars are formed?

OB – Few

AFG – More

KM – Many, Many

When you compress a gas it heats up. When a gas expands it cools

Evaporating gaseous globules (“EGGs”): Newly forming stars

exposed by the ionizing radiation from nearby massive

stars

Observations of star formation:

The Birth of Stars

near the stars

The collapsingprotostar eventually heats up, and blows away its cocoon.

T Tauri Stars

Below is a T Tauri star with an accretion disk, and a jet of hot gas.

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..

Some young disks & jets revealed

•Low-mass stars that eject gas before becoming main sequence stars, may lose as much as 40% of its mass.

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 .

Collapse is slower for lower masses:

•1 Msun takes ~30 Myr

•0.2 Msun takes ~1 Billion years

Low-Mass Proto-stars

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.

Finally: •Pressure=Gravity & collapse stops. •Star reaches the Zero-Age Main Sequence•(ZAMS).

To Reach Main Sequence As the core heats up, H fusion runs faster:

Core temperature rises Core pressure rises Collapse begins to slow downIf the core temperature reaches at least 10 million

deg K, the proto-Star becomes a Star

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

40,000 20,000 10,000 5,000 2,500

106

104

102

1

102

104

Temperature (K)

Lu

min

osi

ty (

Ls

un)

Each star is represented by a dot.The position of each dot on the diagram corresponds to the star's luminosity and its temperature

The vertical position represents the star's luminosity.

The horizontal position represents the star's surface temperature.

H-R Diagram Basics

H–R Diagramor L-T Diagram

40,000 20,000 10,000 5,000 2,500

106

104

102

1

102

104

Temperature (K)

Lu

min

osi

ty (

Lsu

n)

Notice that the plot is not completely random, so there is some sort of relationship.

H–R Diagram

40,000 20,000 10,000 5,000 2,500

106

104

102

1

102

104

Temperature (K)

Lu

min

osi

ty (

Ls

un)

White Dwarfs

Giants

Supergiants

Main Sequence

Cool

BRIGHT

Color Version of H-R Diagram

Stars get hotter

Stars get larger

Class Ia,b : SupergiantClass II: Bright giantClass III: GiantClass IV: Sub-giantClass V:MSThe Sun is a G2 V star

Luminosity classes

Mass-Luminosity relation

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

There is a unique mass & radius for each star along the main sequence

Radii on the Main Sequence L = 4πR2σT4

If you know L & T, you can calculate R

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 Bright supergiantIb SupergiantII Bright giantIII GiantIV SubgiantV 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.

• 900,000 main sequence stars

• 95,000 white dwarfs

• 4000 giants

• 1 supergiant

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:

Main Sequence

•Pre-main sequence evolutionary tracks

Most everything about a star's life depends on its (MASS).

Life Tracks for Different Masses

• Higher-mass stars form faster

• Lower-mass stars form more slowly

Temperature Increases

Lu

min

os

ity

Stars more massive than 150MSun would blow apart ****

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)

Surface temp (K)

Luminosity (suns)

Lifetime (years)

25 35,000 80,000 3 million

15 30,000 10,000 15 million

3 11,000 60 500 million

1.5 7,000 5 3 billion

1.0 6,000 1 10 billion

0.75 5,000 0.5 15 billion

0.50 4,000 0.03 200 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

Where fusion takes place

Supplies gravity to the core

The star develops a Core-Envelope structure:

A hot, dense, compact central CORE surrounded by a cooler, lower density, extended ENVELOPE

When there is a balance between the two, we have a condition of Hydrostatic Equilibrium. In this condition, the star neither expands, nor contracts.

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.

Thermodynamics says : Heat always flows from hotter regions into cooler regions.

In a star, heat flows from the hot core, out throughthe 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

1. Proton-Proton Chain: Low mass stars

Relies on proton-proton reactions

Efficient at low core Temperatures (TC<18M K)

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:

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 thatleave 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.

Main Sequence LifetimesSpectral Type Mass

(Solar masses)Main sequence lifetime (million

years)

O5 40 1

B0 16 10

A0 3.3 500

F0 1.7 2700 2.7 BY

G0 1.1 9000 9 BY

K0 0.8 14 000 14 BY

M0 0.4 200 000 200BY

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!!

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

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 Mass 0.6 solar mass Radius 0.028 times Sun Distance 7,100 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.

Line of Sight

Doppler Motion

(Radial Motion)

Proper Motion

(Tangential Motion)

(vt).

(vr)

vActual Motion

Radial Velocity

The radial velocity of a star is how fast it is moving directly towards or away from us. (Doppler Effect)

                                                                          

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.

Earth

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 theVelocity and also measure the Proper Motion Angle..

Tangential Velocity (vt).

where:

= 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):

2 2(4.74 )v v d

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