On the Main Sequence

What holds a star up while it is on the MS?

Parts of a MS star

How does energy get out?

Radiation & Convection May take a million years to reach the surface.

Brown Dwarfs : Stars with core mass < .08 Msun (failed stars).

Brown Dwarfs do not get hot enough to fuse H, but they do fuse Deuterium for a very short time. Deuterium is an isotope of H, with a neutron. About 1,000 Brown Dwarfs have been found. They radiate in the infrared wavelength.

The smallest stars

Red Dwarfs : Stars with a core mass of .08 to 0.4 solar mass

Coolest and dimmest of all MS stars. They remain on MS hundreds of billions of years. When all the H is converted to He fusion ceases, they cool down, moving down and to the right in the H-R diagram.

Red Dwarfs are very low mass stars with no more than 40% of the mass of the Sun and represent the majority of the stars.

. They have relatively low temperatures in their

cores; red dwarfs transport energy from the core to the surface by convection.

A low-mass main-sequence star of spectral classes M and L. Red dwarf stars range from about 0.6 solar mass at class M0 down to 0.08 solar mass in cool M

Death of Low Mass Star

0.5 - 1.4 White Dwarf

Core Mass Final State

Thermodynamics

When Fusion stops, core shrinks & temperature of core rises.

When the Envelope expands its temperature cools down.

Main Sequence Phase

Energy Source: H fusion in the core

Using P-P cycle H fuses to He

Slowly builds up an inert He core

Evolution of Low-Mass Stars

0.5 - 1.4 Msun

He Fusing

Envelope

When the H in the core is almost completely converted into He, H fusion stops in core. The left over H is pushed out into a shell ring around the He core. The core collapses & heats up.

Outer layer expands and cool forming a

Red Giant

The increasing temp will cause the H shell to fuse forming He that will join the core

(On the H_R Diagram)

•                                                                                        

•The star gets brighter and redder, climbs up the Giant Branch. (Takes 1 Byr)

At the top of the Giant Branch, the star’s envelope is about the size of Venus’ orbit

• The core will contract until it gets hot enough to fuse the He in the core into Carbon & Oxygen.

When the fusion begins, the burning occurs rapidly because of the H shell burning and the He burning in the core. This is called the “Helium Flash”. Some of the outer layers are blown outward causing the star to loose mass.

The star gets hotter, and moves onto the

Horizontal Branch

•C-O core collapses and heats up •He burning shell outside the C-O core •H burning shell outside the He burning shell

 The core never gets hot enough to fuse the Carbon & Oxygen                                     Outside: Envelope swells &cools because of H & He burningClimbs the Asymptotic Giant Branch

Once the He fuses and forms C & O, the core contracts.

Climbs the Giant Branch again, slightly to the left , and higher, becoming a super red giant. .

                                                                                        

With weight of envelope taken off, core never reaches Carbon fusion temp of 600 Million K

Outer envelope gets slowly ejected . This is a non-violent ejection; a series of puffs or burps.Expanding envelope forms a ring nebula around the contracting C-O core.

Core and Envelope separate, takes ~100,000 yr

C-O core continues to contract:

A Planetary Nebula forms

                                                                                   

Hot C-O core is exposed, moves to the left Becomes a White Dwarf

Planetary Nebulae

Fig. 13.16cButterfly Nebula

Outer shellsof red supergiant“puffed off”

Ejection not explosive

Nebula shell expands

Hot dwarf left behind

Cools down to form a WD

Planetary Nebula

The nebula is ionized, and heated by the. Ultra-violet radiation from the hot star

•After ~ 50,000 years, the nebula spreads so far that the nebulosity simply fades from view.

White Dwarfs have a mass that is less than 1.4 MoThey will shine for a long time but no fusion is taking place.

Contraction of the core is stopped by electron degeneracy. The electrons repel each other as they are pressed closer together and a White Dwarf forms.

•One teaspoon weighs about 5 tons.

Electron energy levels

• Only two electrons (one up, one down) can go into each energy level.

• In a degenerate gas, all low energy levels are filled.

White Dwarf’s mass < than the White Dwarf’s mass < than the Chandrasekhar massChandrasekhar mass (1.4 Solar Mass)(1.4 Solar Mass)

Radius (a little smaller than Earth!)

Temp. – anywhere from 100,000 to 2500 K.

White dwarfs shine by leftover heat, no fusion.

WD will cool off and fade away slowly, becoming a "Black Dwarf“.

Takes ~10 Tyr to cool off , so none exists yet.

White Dwarfs are planetary in size, but have a stellar mass

Sirius B

Sirius B Temp. 25,000 KSize: 92% Earth's diameterMass: 1.2 solar masses

Sirius B

The most famous W.D. is Sirius’ companion .

The mass of a star, in the size of a planet.

White Dwarfs are so small, that they can only be seen if close-by, or in a binary systems.

A lone white dwarf is a cooling corpse but a white dwarf in a binary system can be revived

There is more !! A White Dwarf in a binary system…

White DwarfEvolving (dying) star

Roche Lobes

Evolving (dying) starWhite Dwarf

Accretion Disk

Roche Lobe filled

Evolving (dying) star

I

II

III

Type 1a

super NOVA!!

W.D. can take on material but, if the W.D. exceeds 1.4 solar masses (Chandrasekar limit)powerful explosions take place and they could happen more than once. The star will get down below 1.4 solar mass.

Since the Type 1a supernova is always a white dwarf they can be used to judge very great distances (using the inverse square law). Type Ia: No hydrogen lines in the spectrum

Type II: Hydrogen lines in the spectrum

There is a further subdivision of I into Ia, Ib, Ic

Low Mass StarsSun

Core forms a White Dwarf

White Dwarf becomes a Black Dwarf (dead star)

If the White Dwarf is a binary star, a Supernova type 1a can form, if its mass becomes greater than 1 ¼ solar masses

Envelope separates from core and forms a planetary nebulaRed Giant

Becomes Red Giant when H is almost gone

Only H , He in shells, C & O in core left C & O do not fuse

Orbit out to almost Venus

Becomes a Red Super Giant

Red Super Giant

Of course you know the relationship is just going to end in a Type 1a supernovae...but I suppose its better to have transferred mass and exploded than to have never transferred mass at all...

Wanted

Crab NebulaSupernovaRemnantStellar Graveyard

High Mass Stars

1.4 < M < 3.0 Neutron Star

Final Core Mass Final State

massive stars evolve more rapidly due to rapid nuclear burning, and massive stars produce heavier elements

Massive stars have the same internal changes as we saw in low mass stars ,

Evolution of Massive Stars

except :

Evolution of High-Mass Stars High-Mass Stars

O & B Stars core mass >1.4 and <3 Msun •Burn Hot •Live Fast •Die Young

Main Sequence Phase:

•Burn H to He in core using the CNO cycle

•Build up a He core, like low-mass stars

•But this lasts for only ~ 10 Myr

Red Supergiant Phase

After H core exhaustion: •Inert He core contracts & heats up the H burning in a shell .• Envelope expands due to the burning H shell and cools•Envelope ~ size of orbit of Jupiter

                                           

Moves horizontally across the H-R diagram, becoming a Red Supergiant star

                                                                                   

Takes about 1 Myr to cross the H-R diagram.

Core Temperature reaches 170 Million K

Helium Flash : Helium ignites

This Helium flash is not as explosive as the one for low mass stars.

Helium Fusion produces C & O in core:

Star heats up and becomes a Yellow Supergiant.

Star becomes a Yellow Supergiant.

Yellow

When He exhausted in core

•Inert C-O core collapses & heats up the H & He burning in shells. Star expands and becomes a Red Supergiant again

                                                                                   

C-O Core collapses until: Tcore> 600 MillionK • Carbon in the Core ignites.

C fuses to form : Ne , and O

•Core at the end of • Carbon Burning •Phase:

1. Hydrogen burning: 10 Myr2. Helium burning: 1 Myr3. Carbon burning: 1000 years4. Neon burning: ~10 years5. Oxygen burning: ~1 year6. Silicon burning: ~1 day Finally builds up an inert Iron core

Nuclear burning continues past HeliumThings happen fast!

End of the line!!

Massive star at the end of Silicon Burning: Onion Skin of nested nuclear burning shells

                                                                                                   

Protons & electrons form neutrons & neutrinos. Collapse is final.

At the start of Iron Core collapse:

•Radius ~ 6000 km (~Rearth)

•Density ~ 108 g/cc

A second later!! , the properties are:

•Radius ~50 km

•Density ~1014 g/cc

•Collapse Speed ~0.25 c !

Supernova explosionNeutron degeneracy pressure halts the collapse

Material falling inwards rebounds.

Outer layers of the atmosphere, including shells, are blown off in a violent explosion called a supernova.

The star will outshine all the other stars in the galaxy combined.

The ejected material often attain speeds of 100,000 km/sec.

Elements heavier than Lead are produced in the explosion and ejected into space. Stars do recycle.

Close to 150 supernova remnants have been detected in the Milky Way.

There are smaller numbers of massive stars and so smaller amount of explosions.

The Famous Supernova

Before

At maximum

type II Supernova

SN 1987A

Supernova remnants

Cas A in x-rays (Chandra)

Vela

SN1998bu

Remnant of SN386, with central pulsar (Chandra)

Cygnus Loop (HST): green=H, red=S+, blue=O++

The rings of

SN 1987A

are from previous

mass loss

1a is binary with a White DwarfType II : Hydrogen lines in the spectrum

Supernova explosion

1, The iron core collapses

2. Neutrons stop the collapse

3. The rebound of the core sends shock waves causing an explosion that blows the outer atmosphere into space as a super nova

• The Crab Nebula.

• A supernova that, according to the Chinese, exploded in 1054.

• Despite a distance of ~ 7,000 light-years, the supernova was brighter than Venus for weeks before fading from view after nearly two years.

•Even today, the nebula

• is still expanding at

• more than 3 million

•miles per hour.

Structure of a Neutron Star

•Diameter~ 12 km in diameter

•Mass -about 1.4 times that of our Sun.

•One teaspoonful of material would weigh a billion tons! Rotation Rate: 1 to 100 rotations/sec

The magnetic axis is miss-aligned with the rotation axis of the neutron star .The star's rotation sweeps the beams outward as it rotates.If we are in the sight path, will see regular, sharp pulses of light (optical, radio, X-ray.)

Lighthouse Model:

field generates a

Spinning magnetic

a strong electric field.

Pulsars: emitted sharp, 1 millisecond-long pulses every second at an extremely repeatable rate.

A typical pulsar signal, received with a radio telescope

The connection between pulsars and neutron stars was the discovery of a pulsar in the crab nebula.

Iron

Proto-stars (born in cool gas GMC)

Main Sequence Stars (H Fusion)

Core Mass (CM) > 1.4 MO

Red Super Giant

Red Super Giant

Yellow Super Giant

Supernova (Type II)

Neutron Star Black Hole

CM > 1.4 & < 3 CM > 3

Black Dwarf

Red Giant

Red Super Giant

Planetary Nebula

White Dwarf

Binary can produce Type ia supernova

Brown

Dwarf

CM<0,08

Core Mass (CM) 0.5- 1.4 MO

Red Dwarf 0.08 - 0 .5 MO

CM 0.5 – 1.4 MO

White Dwarf

Neutron Star

High

Mass

Very High M

ass

Black Hole

Supernova

Massive Stars

Outer layers of the atmosphere, including shells, are blown off in a violent explosion called a supernova

Red Supergiant

Red Supergiant

Massive star

Becomes Yellow Supergiant when He exhausted

Orbit size of Jupiter

Becomes Red Supergiant

Yellow Supergiant

Becomes a Red Supergiant when H exhausted

Black Holes

We know of no mechanism to halt the collapse of a compact object with mass > 3 Msun.

It will collapse into a single point – a singularity:

=> Becoming a Black Hole!

Honeycutt H

Has He

Caused C

No Ne

Oxford O

Student Si

Injury (Iron) Fe

To memorize this sequence, use this :

Massive stars form the following:H, He, C, Ne, O, Si, Fe . Iron will not fuse. Low mass stars form only H, He, C, O

Thanks to the following for allowing me to use information from their web site :

Nick Stobel

Bill Keel

Richard Pogge

NASA