In H II regions along spiral arms HII regions

M51-HST

NGC 3079

HST

Stars form in nebulae

Orion A - NOAO

Star Formation: Main Steps

1. Gas cloud collapse

2. Main Sequence stage (H fusing or burning)

3. Red Giant or supergiant phase (He fusion)

4. Ejection: Planetary nebula or Supernova

5. Core remnant stage:• white dwarf• neutron star/pulsar or• black hole

Initial gas cloud collapse phaseGiant Molecular Cloud

105~ 106s M

cold gas & dustProtostar

Conversion of GPE into KE & heat

2000~3000K

IR & microwaves emitted

T. Tauri stage

star-like. Strong jets along rotational axes. Satr starts fusion

Protostar & T Tauri track

Large Magellanic Cloud

6 degrees, 160,000 ly

Small Magellanic Cloud

5.6 degrees, 240,000 ly

Starbirth in nearby galaxies

30 Doradus (Tarantula nebula)

Tarantula nebula in the LMC (HST)

Barnard 86,

a Bok globule

NGC 6520,

an open cluster

ORION

The Orion nebula - our nearest stellar nursery

Visible light is absorbed by dust & gas

IR light travels through the dust & gas, allowing us to view star birth

Proplyds (protoplanetary disks)

these are possible precursors to solar systems

M16 - a stellar nursery

T Tauri (CFHT)

T Tauri type stars

T Tauri type stars

Material ejected along rotational axes

AFGL 2591: A Massive Star Acts UpYoung star AFGL 2591 is putting on a show. The massive star is expelling outer layers of dust-laced gas as gravity pulls inner material toward the surface. AFGL 2591 is estimated to be about one million years old -- much younger than our own Sun's 5 billion-year age -- and has created a nebula over 500 times the diameter of our Solar System in just the past 10,000 years. The above image in infrared light is one of the first from the new NIRI instrument mounted on one of the largest ground-based optical telescopes in the world: Gemini North. Sharp details are discernable that are blocked by opaque dust invisible-light images. Close inspection of the image reveals at least four expanding rings, indicating an episodic origin to the mysterious activity. AFGL

2591 lies about 3000 light years away toward the constellation of Cygnus.

Stars form in clusters

The Pleiades, an open cluster

The Jewel box open cluster

M. Bessell (MSSSO)

Mass determines a star’s MS luminosity!!!

Lifetimes of main-sequence starsM

-/M

€Teff (K) L

-/L

€MS life(106 yrs)

25 35 000 80 000 3

15 30 000 10 000 15

3 11 000 60 500

1.5 7000 5 3000

1.0 6000 1 10 000

0.75 5000 0.5 15 000

0.50 4000 0.03 200 000

Formation of stars & planets

Stellar Evolution and the HR Diagram

• Our Sun as a star

• Nuclear fusion and energy transport in the sun

• Stages of stellar evolution for low and intermediate mass stars

• The Hertzsprung Russell diagram

Light Travel from the Sun

The speed of light is c = 3x108 ms-1. A photon leaving the surface of the sun reaches the earth after a time T = distance/c = 8 minutes.

How does the Sun burn?

The Sun must be at least as old as the earth (4.6 billion years).It has a luminosity of L = 3.9 x 1026 Joules s-1.

Its mass composition is H: 74% He: 24% rest: 2%

What produces the Sun’s energy?

SOHO image of the solar chromosphere in ultraviolet light.

Some Solar Values

Value NotesDistance to thesun

150,million km 1 astronomical unit

Radius 700,000 km 109 x Rearth

Mass 2 x1030 kg 300,000 Mearth

Mean density 1.4 x 103 kg m-3

(1.4 g cm-3)0.25 <ñearth>

SurfaceTemperature

6000 K strongest emission atyellow wavelengths

Solar Constant 1.5 kWatts /m2

1/2o

A star is a balancing act between:

P,T

Gravity acting inwards

The internal pressure gradients must counteract thegravitational force G. (What happens otherwise?)

This is a fundamental requirement for all stars.

P: PressureT: Temperature

acting outwards

Hydrostatic equilibrium

Solar Energy Source: Some early ideas:

Normal chemical reactions - such as the combustion of coal

Large numbers of meteorite impacts (10,000 years)

Slow gravitational collapse (20 million years)

In the 1930’s a major breakthrough in astronomy was the understanding that the energy source in stars is from :

Nuclear fusion reactions at high temperatures and pressures.

Nuclear Fusion in the Sun

Core temperature = 1.5 x 107 KCore radius = 0.25 Rsurface

Core mass = 10% total stellar mass

The sun’s energy is generated in the core by nuclear fusion reactions which convert Hydrogen to Helium:

4 1H 1 4He + energy (photons and neutrinos)Energy released = mc2

= 3.85 x 1026 J/s

Fusion: Hydrogen Helium

This Proton-Proton chain is the energy source of stars like our Sun

CNO cycle

Uses C-12 as nuclear catalyst to convert 4 protons into He-4

Dominates in more massive (hence hotter) MS stars

Comparison of PP & CNO

CNO contributes only ~1% Sun’s energy

What mass of hydrogen is converted to helium?

Mass s-1 = luminosity / c2: 4 x 109 kg s-1

How long can the sun survive by burning hydrogen?

Hydrogen burning lifetime = H mass available in core Rate of conversion

This gives a timescale of approximately 1010 years, ie 10 billion years.

Our Sun is roughly half-way through its hydrogen burningphase.

Energy transport from the core to the visible surface of the Sun

2

1. Core region: R < 0.25 Rsun

Nuclear fusion zone

2. Radiative region: 0.25 < R < 0.75Rsun

photons diffuse through hot gas.

3. Convective Region: 0.75 < R < Rsun

Energy transported by bulk gas motions.

4. Photosphere - the visible surface of the sun. Thickness ~ 500 km. T = 6000K

Energy from the sun’s interior is released as photons (‘particle of light’) and as

neutrinos (zero or very low mass particles).

Stellar EvolutionStars form with masses between 0.1 and 100 times the mass of the sun.

For most of their lifetimes they burn by the nuclear fusionof hydrogen to helium. These are the ‘Main Sequence’ stars.

Low mass stars convert hydrogen more slowly and spend longer in this phase. They are also cooler and smaller in size.

Main sequence lifetime ~ 1010 years for Mstar = Msun

Main sequence lifetime ~ 106 yrs for Mstar = 30 Msun

What happens when the core hydrogen runs out??

Later StagesAs the core is used up the stellar core contracts under gravity. This raises the central gas density, pressure and temperature.

At a temperature of ~ 2 x 108 K the stellar core ignites Helium in the triple-alpha reaction: 3 4He 12C + (gamma ray).

To balance the pressure gradients across the star the outer layers expand greatly and cool down.

The star is now a luminous Red Giant.

Red Giant Stars

Core helium burning

Outer hydrogen atmosphere

By the time a star has become a Red Giant, its radius has become about 150 times larger than in the core-hydrogen burning stage.

The Red Giant stars are very luminous: L = 4R2 Teff 4

The surface temperatures are typically ~ 3000 K (reddish)

R

Red Giant stars have dense compact cores and much lower

density expanded atmospheres.

Triple alpha (helium) flash

Fuses He into C, releasing energy. Red Giant phase

For stars of one solar mass, the Red Giant phase lasts for approximately 107 years.

After the helium core burning phase ends, the stellar energy is supplied by nuclear fusion in two layers around the core. In this ‘double-shell’ burning stage the star is known as an Asymptotic Giant Branch (AGB) star.

The AGB stars have extremely strong STELLAR WINDS. The stellar winds remove most of the stellar atmospheres which are blown outwards into the interstellar medium.

The mass-loss rates of AGB stars are typically 1018 kg s-1. This is a billion times higher than for the sun.

Asymptotic Giant Branch Stars

Nucleosynthesis in stars:

Mass is the key factor!

• Low mass stars; convert hydrogen into helium HHe

• Stars like our Sun; hydrogen into helium, then helium to carbon and oxygen

• High mass stars (>5xSun); HHe, He C,O, Ca, Fe, Ni, Cr, Cu & others! Then SUPERNOVA heavier elements

Very high core temperature

~ 4 x 109 KCan fuse up to iron

Fusion products in MS & Red GiantsFuel Products Min

Ignitiontemp (K)

M-/M

€needed

Fusionduration in25M

€ star

H He 4 x 106 0.1 7 x 106 yrs

He C, O 120 x 106 0.4 0.5 x 106 yrs

C Ne, Na, Mg,O

600 x 106 4 600 yrs

Ne O, Mg 1.2 x 109 8 1 yr

O Si, S, P 1.5 x 109 8 0.5 yr

Si Ni to Fe 2.7 x 109 8 1 day

The death of stars

Once all fusion has occurred and outer layers expelled the final remnant of the star depends on the mass of the remaining core:

1. If mass < 1.4 M white dwarf

2. For mass 1.4 M < M < 3.0 M neutron star (pulsar)

3. If M > 3.0 M black hole!

Planetary NebulaeAt the end of the AGB phase the stars have lost so muchmatter that their dense central cores become visible. Nuclear burning now occurs in an outer hydrogen layer.

Ultraviolet photons from the core sweeps up and ionises some of the stellar wind into a shell around the core.

The swept up shell is seen as a PLANETARY NEBULA.

Planetary nebulae can have very exotic and beautiful shapes.

NGC 6369 IC 3568

Two examples of ‘circular’ planetary nebulae - HST images

For many examples of P. Nebulae - see the HST web pages

The bipolar planetary nebula M2-9

Credit: V. Bujarrabal (OAN, Spain), WFPC2, HST, ESA, NASA

As the million-kilometer per hour gas rams into the surrounding interstellar gas, a supersonic shock front forms whereionized hydrogen and nitrogen glow blue.

The Making of the Rotten Egg Nebula 1.4 light yrs

Fast expanding gas clouds mark the end for a central star in the Rotten Egg Nebula. The once-normal star has run out of nuclear fuel, causing the central regions to contract into a white dwarf. Some of the liberated energy causes the outer envelope of the star to expand. The result is a proto- planetary nebula.

The Cat’s Eye Nebula

The death of a star - formation of a planetary nebula

White DwarfsAt the end of the planetary nebula stage, the outer hydrogenis largely depleted.

The star is left with an extremely hot (30,000K), compact (Rstar = Rearth), and dense core (star = 106earth).

The star is now a WHITE DWARF.

Very little nuclear fusion occurs in White Dwarf stars.

The stars support themselves against gravitational collapse by electron degeneracy pressure. This is where electrons are forced into high energy states.

White Dwarfs cool very slowly and gradually fade into darkness.

The Evolution of Solar Mass Stars

Star Type Energy supply Timescale(years)

Main Sequence Core hydrogenburns to Helium

1010

Red Giant Core Heliumburns to Carbon

107

AsymptoticGiant Branch

Double-shellburning

106

PlanetaryNebula

Hydrogen-shellburning

105

White Dwarf little fusion –electron pressure

1010

What about more massive stars?

Eta carinae

Their fate?

They go out in a bang as a supernova:

these exploding stars outshine an entire galaxy for a few weeks

Massive stars live fast & die young!!!

(and leave spectacular remnants)

Crab nebula - a supernova remnant

The precursor star went supernova in AD 1054, recorded by Chinese astronomers

VLT

Core of Crab nebula

HST

The centre of the Crab nebula contains a rapidly rotating neutron star - a pulsar.

It contains about twice the mass of the Sun but is only10 km across, spinning dozens of times a second!

The Cygnus Loop supernova remnant

This expanding cloud carries heavy elements out into space.

You and I are made up of elements formed and transported in this way! The accompanying shockwave can trigger new bouts of star formation as it passes through other nebulae.

Even more massive stars end up as black holes!

The Hertzsprung Russell Diagram

The HR diagram was first plotted by Hertzsprung (1911) and Russell (1913). It is used to study the evolution and properties of stars.

The HR diagram is a plot of :

Stellar Luminosity or Absolute Magnitude (y-axis)

against

Stellar (surface) Temperature or colour (x-axis).

Giants

Supergiants

Main Sequence

White dwarfs

Hertzsprung Russell Diagram for Nearby Stars

To plot the HR diagram we need to know the individual stellar distances - or use a group of stars in a star clusterwhich are known to be at the same DISTANCE.

HR Diagram for close & well known stars

Schematic view of the evolutionary path of a one solar mass star.

Lum

inos

ity

(sol

ar u

nits

)

Effective Temperature (K)

RedBlue

1

103

10-3

6000 300020000

Sun-like star

Planetary nebulae

Red Giant

White Dwarf track

Asymptotic Giant Branch

Main Sequence

HR sequence for cluster of stars: I

II

III

IV

HR diagram for the globular cluster M5 - plotted as V magnitude against B-V colour.

The globular clusters contain old (population II), highly evolved stars.

This cluster shows well-defined giant and horizontal branches.

B - V

B-V

V

As a cluster ages the ‘turn-off’ point moves further down the Main Sequence. This can be used to determine the age of a stellar cluster.

Final mass after mass-loss

Our Sun

Some resources used for this talk

Resource NotesCambridgeEncyclopaedia ofAstronomy

Publishers: Jonathon CapeEditor: Simon Mitton

http://sohowww.nascom.nasa.gov

Solar and Heliospheric Observatory withTeacher links

http://oposite.stsci.edu/pubinfo

Hubble Space Telescope PublicInformation

http://www.dur.ac.uk/~dph0irs/gcCm/gcCm_top.html

Good description and plots of the HRdiagram for M5. Notes for(undergraduate?) students on the cluster 47Tuc.