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
The types of stars and their properties.
Measuring the distances to the stars
Variable stars
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 using
nuclear fusion to turn lighter elements into heavier ones and releasing huge
amounts of energy in the process.
So how do stars form?
Martin Crow
Crayford Manor House Astronomical Society
Stars form from cold Giant Molecular Clouds.
In the Milky Way it is estimated that there are 6000 molecular
Clouds each containing 10⁵ solar masses and average
temperature of 10K.
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.
Star formation is triggered by the
shock waves from exploding
Supernova, UV radiation from newly
formed stars or the gravitational
interaction of colliding galaxies
causing gravitational collapse.
Martin Crow
Crayford Manor House Astronomical Society
M51
Martin Crow
Crayford Manor House Astronomical Society
HST
Antennae Galaxies
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 material
composed 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 and
the 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 flows
of material are produced
Martin Crow
Crayford Manor House Astronomical Society
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’.
Martin Crow
Crayford Manor House Astronomical Society
The UV radiation now starts to clear away material revealing the new star.
Julian Tworek
Martin Crow
Crayford Manor House Astronomical Society
Stars form in clusters
Double cluster – Caldwell 14
Age = 5.6 x 10⁶ and 3.2 x 10⁶ years.
Martin Crow
Crayford Manor House Astronomical Society
Image by Julian Tworek
Age = 75 – 150 x 10⁶ years
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 birth
around them.
Many stars will form as binary or multiple systems.
Mizar
Martin Crow
Crayford Manor House Astronomical Society
The variability of Algol is due to it being a binary system.
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 -300
Solar masses formed in the early universe due to low metallicity.
The smallest is around 8% the mass of the Sun. Anything below this
and fusion will not start.
Anything below this mass is called a Brown Dwarf.
Element
Martin Crow
Solar
masses
Hydrogen
0.01
Helium
0.4
Carbon
5
Neon
8
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 / Luminosity
relationship
Martin Crow
Crayford Manor House Astronomical Society
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
Fuel
material
Temperature
(million
kelvins)
Density
(kg/cm3)
Burn duration
(τ in years)
H
37
0.0045
8.1 million
He
188
0.97
1.2 million
C
870
170
976
Ne
1,570
3,100
0.6
O
1,980
5,550
1.25
S/Si
3,340
33,400
0.0315
Martin Crow
Crayford Manor House Astronomical Society
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!!!!
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.
Martin Crow
Crayford Manor House Astronomical Society
M57 HST
Martin Crow
Crayford Manor House Astronomical Society
High mass – massive stars (>5.0 M₀). These stars have a convective core with energy
carried 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.
Martin Crow
Crayford Manor House Astronomical Society
Once the star has fused silicon into iron no more energy can be released and the
star 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 only
1% of the output, 99% goes unseen in
the form of neutrinos.
SN2012A
Martin Crow
Crayford Manor House Astronomical Society
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
Martin Crow
Crayford Manor House Astronomical Society
Neutron stars.
So called because the core collapse is halted by neutron touching neutron.
They are incredibly dense 3 x 10¹⁷ kg/m³
and a radius of around 12 km.
During the core collapse angular momentum
is conserved and so they rotate rapidly and
strong magnetic fields generate radio beams.
The Crab pulsar.
30 times per second
Some more neutron stars
174 times per second
Imaged by Chandra X-ray telescope
642 times per second
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.
A point called the ‘event horizon’ exists around the black hole. Anything crossing this
is lost from the universe.
This is why we cannot see a black hole directly, only its effects on its surroundings.
Martin Crow
Crayford Manor House Astronomical Society
Some effects
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.
If a black hole passes in front of a background object
Gravitational lensing could give it away.
Martin Crow
Crayford Manor House Astronomical Society
Sagittarius A
Evidence for a 4 million solar mass black hole at the centre of our galaxy.
Martin Crow
Crayford Manor House Astronomical Society
Stars have a spectral classification which is derived from their spectra.
O Be A Fine Girl Kiss Me
Class
Temperature
(Kelvin)
Conventional Apparent
colour
colour
Mass
(solar
masses)
Radius
Luminosity Hydrogen
(solar radii) (bolometric) lines
Fraction of
all
main
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
white
0.8–1.04
M☉
0.96–1.15
R☉
Weak
7.6%
K
3,700–5,200 K orange
yellow
orange
0.45–0.8
M☉
0.7–0.96 R☉ 0.08–0.6 L☉ Very weak 12.1%
M
≤ 3,700 K
orange red ≤ 0.45 M☉
Martin Crow
red
≤ 0.7 R☉
0.6–1.5 L☉
≤ 0.08 L☉
Crayford Manor House Astronomical Society
Very weak 76.45%
The Hertzsprung – Russell Diagram
All stars will fit on this plot. The
position is determined by how
evolved they are.
The diagram is a plot of temperature
against luminosity.
To do this you also need to know
the stars distances so that the stars
absolute brightness can be
calculated.
Stars remain on the Main
Sequence for much of their lives.
A star’s position on the diagram
changes when it stops burning
Hydrogen.
Martin Crow
Crayford Manor House Astronomical Society
The Sun’s evolution as plotted on the H – R diagram
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).
Martin Crow
Crayford Manor House Astronomical Society
Measuring the distance of stars.
There are a number of methods for determining stellar distance.
Parallax.
Martin Crow
Crayford Manor House Astronomical Society
Comparing a cluster population to the H-R diagram.
This plot has been derived from
plotting two different star clusters.
By looking at a cluster you know
that all of the stars in that cluster
are all at a similar distance.
Comparing the stars measured
brightness with the H – R diagram
can give a rough distance to the
cluster.
Martin Crow
Crayford Manor House Astronomical Society
Cepheid variable stars
Delta Cephei is the prototype, first discovered by John Goodricke in 1784.
Cepheids are stars that pulsate and have a mass – luminosity relationship.
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 mass
you 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.
Martin Crow
Crayford Manor House Astronomical Society
Martin Crow
Crayford Manor House Astronomical Society
The Cepheids and other pulsating stars exist on the ‘instability strip’ on the
H – R diagram.
The instability strip
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 the
surface of the white dwarf exceeds the
Chandrasekhar limit of 1.4 solar masses
it will catastrophically collapse to produce
a 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.
Martin Crow
Crayford Manor House Astronomical Society
Martin Crow
Crayford Manor House Astronomical Society
Martin Crow
Crayford Manor House Astronomical Society