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BEGINNERS ASTRONOMY Martin Crow Crayford Manor House Astronomical Society THE STARS

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The Stars. Beginners Astronomy. Martin Crow Crayford Manor House Astronomical Society. Last time. We looked at the properties of light and how it carries information from the stars. Martin Crow Crayford Manor House Astronomical Society. This week. The formation of stars. - PowerPoint PPT Presentation

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Page 1: Beginners Astronomy

BEGINNERS ASTRONOMY

Martin Crow Crayford Manor House Astronomical Society

THE STARS

Page 2: Beginners Astronomy

Martin Crow Crayford Manor House Astronomical Society

Last time

We looked at the properties of light and how it carries information from the stars.

Page 3: Beginners Astronomy

Martin Crow Crayford Manor House Astronomical Society

This week

The types of stars and their properties.

Variable stars

Measuring the distances to the stars

The formation of stars

Page 4: Beginners Astronomy

Martin Crow Crayford Manor House Astronomical Society

What is a Star?

A star is a body that creates and emits its own light and does this by usingnuclear fusion to turn lighter elements into heavier ones and releasing hugeamounts of energy in the process.

So how do stars form?

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Martin Crow Crayford Manor House Astronomical Society

Stars form from cold Giant Molecular Clouds.

Star formation is triggered by the shock waves from exploding Supernova, UV radiation from newlyformed stars or the gravitational interaction of colliding galaxies causing gravitational collapse.

GMCs are many tens of light years in size and have masses Between 10⁴ - 10⁶ solar masses.

They have an average density of 100 – 1000 particles per cubic centimetre. This is similar to the very best vacuums created in the laboratory. The average density in the solar vicinity is one particle per cubic centimetre.

In the Milky Way it is estimated that there are 6000 molecular Clouds each containing 10⁵ solar masses and average temperature of 10K.

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Martin Crow Crayford Manor House Astronomical Society

M51 HST

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Martin Crow Crayford Manor House Astronomical Society

Antennae Galaxies

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Martin Crow Crayford Manor House Astronomical Society

Localised areas start to collapse due to uneven density.

As these areas collapse they start to rotate forming a flattened disk of materialcomposed chiefly of Hydrogen and small amounts of dust particles.

A core is forming in the middle with material falling in on it.

As the clouds density around it increases it become optically opaque and the collapse is slowed right down.The core heats up to around 2000 K at which point the Hydrogen is ionised.

This process absorbs energy and allows the collapse to continue.

When the gas becomes hot enough a state of hydrostatic equilibrium is reached andthe collapse is halted. This is now a Proto-star also known as a T Tauri star.

Material continues to accrete via the circumstellar disk of material and bipolar flowsof material are produced

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Martin Crow Crayford Manor House Astronomical Society

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Martin Crow Crayford Manor House Astronomical Society

Bipolar flows

As mass is added to the proto-star continued contraction occurs raising the core temperature until fusion can start. It is now a star and joins the ‘Main Sequence’.

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Martin Crow Crayford Manor House Astronomical Society

The UV radiation now starts to clear away material revealing the new star.

Julian Tworek

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Martin Crow Crayford Manor House Astronomical Society

Double cluster – Caldwell 14

Stars form in clusters

Age = 5.6 x 10⁶ and 3.2 x 10⁶ years.

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Martin Crow Crayford Manor House Astronomical Society

Age = 75 – 150 x 10⁶ years

Image by Julian Tworek

Page 14: Beginners Astronomy

Martin Crow Crayford Manor House Astronomical Society

The largest stars form in the middle of the clusters.

These stars are likely to explode as supernova and so trigger more star birtharound them.

Many stars will form as binary or multiple systems.

Mizar

Page 15: Beginners Astronomy

Martin Crow Crayford Manor House Astronomical Society

The variability of Algol is due to it being a binary system.

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Martin Crow Crayford Manor House Astronomical Society

Stars come in a range of sizes:

The largest is 100 times the mass of our Sun. Stars of up to 200 -300Solar masses formed in the early universe due to low metallicity.

The smallest is around 8% the mass of the Sun. Anything below thisand fusion will not start.

Anything below this mass is called a Brown Dwarf.

Element Solarmasses

Hydrogen 0.01

Helium 0.4Carbon 5Neon 8

Page 17: Beginners Astronomy

Martin Crow Crayford Manor House Astronomical Society

Stars do not all live to the same age.

Really massive stars have lives measured in tens of millions of years.

Dwarf stars have lives measured in many billions of years.

This is because as the mass increases the luminosity goes up by the forth power.

Mass / Luminosityrelationship

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Martin Crow Crayford Manor House Astronomical Society

Fuelmaterial

Temperature(million kelvins)

Density(kg/cm3)

Burn duration(τ in years)

H 37 0.0045 8.1 millionHe 188 0.97 1.2 millionC 870 170 976Ne 1,570 3,100 0.6O 1,980 5,550 1.25S/Si 3,340 33,400 0.0315

The table below shows the amount of time required for a star of 20 solar masses to consume all of its nuclear fuel. As an O-class main sequence star, it would be 8 times the solar radius and 62,000 times the Sun's luminosity

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Martin Crow Crayford Manor House Astronomical Society

Page 20: Beginners Astronomy

Martin Crow Crayford Manor House Astronomical Society

So how stars live their lives depends largely on their mass.

Low mass stars (0.8 M₀-0.5 M₀) live long lives due to them being entirely convective.

This means that all of the Hydrogen in the star can be used as fuel.

When all of the fuel is used up they will cool and contract to form a white dwarf.The universe is not yet old enough for this to have happened!!!!

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Martin Crow Crayford Manor House Astronomical Society

Mid mass stars (0.5 M₀ – 5.0 M₀) fuse Hydrogen in a core with heat being carried to the surface by convective currents.

When the Hydrogen fuel in the core is used up the star will collapse and trigger Helium fusing into Carbon, Nitrogen and Oxygen (CNO cycle).This causes the stars outer layers to swell up turning them into Red giants.

Eventually the outer layers are thrown off forming a planetary nebular and Leaving the exposed core as a white dwarf.

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Martin Crow Crayford Manor House Astronomical Society

M57 HST

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Martin Crow Crayford Manor House Astronomical Society

High mass – massive stars (>5.0 M₀). These stars have a convective core with energycarried radiatively to the surface

Due the very high temperatures in the core of these stars the CNO cycle is the dominant energy producer.

Their inner structure takes on an onion like appearance due to successive cycles of hydrostatic instability as fuel is consumed.

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Martin Crow Crayford Manor House Astronomical Society

Once the star has fused silicon into iron no more energy can be released and thestar undergoes a core collapse resulting in a supernova explosion.

Although a supernova can out shine an entire galaxy of 100 x 10⁹ suns this is only1% of the output, 99% goes unseen in the form of neutrinos.

SN2012A

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Martin Crow Crayford Manor House Astronomical Society

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Martin Crow Crayford Manor House Astronomical Society

The remnant, depending on the mass, will take the form of a rapidly spinning neutron star or a black hole.

M1 the Crab Nebula

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Martin Crow Crayford Manor House Astronomical Society

Imaged by Chandra X-ray telescope

The Crab pulsar.

30 times per second

174 times per second

642 times per second

Some more neutron stars

Neutron stars.So called because the core collapse is halted by neutron touching neutron.

They are incredibly dense 3 x 10¹⁷ kg/m³and a radius of around 12 km.During the core collapse angular momentumis conserved and so they rotate rapidly andstrong magnetic fields generate radio beams.

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Martin Crow Crayford Manor House Astronomical Society

Black holes

Black holes form when the mass of collapsing matter is greater than even neutron degeneracy pressure can resist and a singularity is formed.Black holes are so massive that the escape velocity is greater than the speed of light. Therefore, no signal can escape.

This is why we cannot see a black hole directly, only its effects on its surroundings.

A point called the ‘event horizon’ exists around the black hole. Anything crossing this is lost from the universe.

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Martin Crow Crayford Manor House Astronomical Society

A binary system where a small compact object draws matter from a companion gives us a chance to measure its mass giving an indication of what it might be.

Some effects

If a black hole passes in front of a background objectGravitational lensing could give it away.

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Sagittarius A Evidence for a 4 million solar mass black hole at the centre of our galaxy.

Martin Crow Crayford Manor House Astronomical Society

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Martin Crow Crayford Manor House Astronomical Society

Stars have a spectral classification which is derived from their spectra.

ClassTemperature(Kelvin)

Conventional colour

Apparent colour

Mass(solar masses)

Radius(solar radii)

Luminosity(bolometric)

Hydrogenlines

Fraction of allmain sequence stars

O ≥ 33,000 K blue blue ≥ 16 M☉ ≥ 6.6 R☉≥ 30,000 L☉

Weak ~0.00003%

B 10,000–33,000 K

blue to blue white

blue white

2.1–16 M☉

1.8–6.6 R☉

25–30,000 L☉

Medium 0.13%

A 7,500–10,000 K white

white to blue white

1.4–2.1 M☉

1.4–1.8 R☉

5–25 L☉ Strong 0.6%

F 6,000–7,500 K

yellowish white white 1.04–1.4

M☉

1.15–1.4 R☉

1.5–5 L☉ Medium 3%

G 5,200–6,000 K yellow yellowish

white0.8–1.04 M☉

0.96–1.15 R☉

0.6–1.5 L☉ Weak 7.6%

K 3,700–5,200 K orange yellow

orange0.45–0.8 M☉

0.7–0.96 R☉

0.08–0.6 L☉

Very weak 12.1%

M ≤ 3,700 K red orange red

≤ 0.45 M☉

≤ 0.7 R☉ ≤ 0.08 L☉Very weak 76.45%

O Be A Fine Girl Kiss Me

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Martin Crow Crayford Manor House Astronomical Society

The Hertzsprung – Russell Diagram

Stars remain on the MainSequence for much of their lives.

The diagram is a plot of temperatureagainst luminosity.To do this you also need to know the stars distances so that the starsabsolute brightness can be calculated.

A star’s position on the diagramchanges when it stops burningHydrogen.

All stars will fit on this plot. The position is determined by howevolved they are.

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Martin Crow Crayford Manor House Astronomical Society

Simplified illustration of the evolution of a star with the mass of the Sun.The star forms from a collapsing cloud of gas (1),and then undergoes a contraction period as a protostar (2),before joining the main sequence (3).Once the Hydrogen at the core is consumed it expands into a red giant (4),then sheds its envelope into a planetary nebula and degenerates into a white dwarf (5).

The Sun’s evolution as plotted on the H – R diagram

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Martin Crow Crayford Manor House Astronomical Society

Measuring the distance of stars.

There are a number of methods for determining stellar distance.

Parallax.

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Martin Crow Crayford Manor House Astronomical Society

This plot has been derived fromplotting two different star clusters.

By looking at a cluster you knowthat all of the stars in that cluster are all at a similar distance.Comparing the stars measuredbrightness with the H – R diagramcan give a rough distance to the cluster.

Comparing a cluster population to the H-R diagram.

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Martin Crow Crayford Manor House Astronomical Society

Cepheid variable stars

Cepheids are stars that pulsate and have a mass – luminosity relationship.

Delta Cephei is the prototype, first discovered by John Goodricke in 1784.

These type of stars pulsate causing their brightness to vary. The period of this variation is dependent on the stars mass.

If you can measure the period then you know the mass. If you know the massyou know how bright it should be.

Using the inverse square law a distance can be calculated.

There are complications with this method:An accurate initial distance is needed for calibration.

There are different sub types of Cepheid.

Unknown amounts of light are absorbed by interstellar dust.

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Martin Crow Crayford Manor House Astronomical Society

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Martin Crow Crayford Manor House Astronomical Society

The Cepheids and other pulsating stars exist on the ‘instability strip’ on theH – R diagram.

The instability strip

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Martin Crow Crayford Manor House Astronomical Society

Type 1a supernova.

A type 1a supernova is produced when a white dwarf star receives matter from an orbiting companion star .

If the mass of accreted matter on thesurface of the white dwarf exceeds theChandrasekhar limit of 1.4 solar massesit will catastrophically collapse to producea supernova releasing 10⁴⁴ Joules of energy.

Because they always produce the same amount of energy they can be used as a standard candle.

Because of their brightness they can be seen in distant galaxies enabling a distance to be measured.

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Martin Crow Crayford Manor House Astronomical Society

Page 41: Beginners Astronomy

Martin Crow Crayford Manor House Astronomical Society