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Star
s
What Are Stars Made Of?
Planets – Solids (Rock, Ice) or Gas
Stars are made from the fourth state
of matter, plasma.
Heat up a gas such as the hydrogen
/ helium mixture in the Sun, and it's
molecules will break apart into
individual atoms. Heat up the gas
further and the atoms will separate
into positively charged ions and
negatively charged electrons. This is
the plasma state.
The Sun – A Summary
Sunspots are cool regions on the Sun's photosphere
The frequency of sunspots
appearing on the Sun's
surface follows an 11 year
cycle. But occasionally
maxima are separated by 9 or
14 years.
In 1645 there began a 70 year
spell of persistent sunspot
minima – the Maunder
minimum that was associated
with a mini ice-age on the
Earth. Sunspots can affect the
Earth's climate.
Sunspot maxima are
associated with an increase in
the solar magnetic field
strength. At the end of each
solar cycle the magnetic field
briefly disappears before
reappearing with the north
and sole poles reversed.
Sunspots always appear in pairs near to the equator at maxima, but
appear closer to the poles at minima. They rotate around the Sun,
following the Sun's rotation. The Sun's equator rotates faster than its
poles.
In 1908 George Hale discovered
that sunspots were regions of
intense magnetic field. We now
know that they areas of the sun's
surface where magnetic field loops
emerge – leading to sunspot pairs.
Magnetic fields are created by the
motion of the charged particles in
the sun's plasma.
The solar wind continually bombards the Earth with charged particles
from the Sun's plasma, that give rise to the Aurora. But at solar
maxima large groups of sunspots at the equator can generate
enormous flares that eject charged particles away from the Sun. If
the flare is directed towards the Earth it can cause massive damage
to electrical systems, orbiting satellites, and astronauts.
Stars are very massive. The Sun, a typical star, has a mass of
2.0×1030 kg
that's
2 000 000 000 000 000 000 000 000 000 000 kg!
Therefore stars have an extremely large gravitational attraction
that keeps their plasma held together. As gravity acts equally in all
directions the plasma that forms the star is moulded into a sphere.
But there must be some force keeping the star from collapsing in
on itself. Because stars are so massive, their cores are very hot
and at very high pressure. When the temperature exceeds a few
million degrees fusion occurs inside the core, that generates a lot
of heat that radiates outwards from the centre of the star. This
radiation pressure keeps the star from collapsing.
Nuclear Fusion – Burning Hydrogen
Ani!
In the core of our Sun and stars like it
hydrogen atoms (the lightest element) are
fused together to form helium. Without
nuclear fusion to keep them hot stars would
cool down only after a few million years.
Einstein showed us that matter can be
converted into energy at atomic scales. Mass
from the hydrogen is converted into gamma
rays!
The hot plasma inside the Sun emits radiation (i.e. light) that passes
through the 1.4 million km wide solar interior. After 10 million years
this reprocessed radiations passes through the Sun's outer layers –
the photosphere, where it has now cooled to just 5700 K.
T (Kelvin) = T (Celsius) + 273
0K = -273 C (Absolute Zero)
The light emitted by a hot plasma always has a
characteristic spectrum – it's intensity (or
brightness) is different at different energies (or
frequencies, wavelengths, colours).
Stars are almost blackbodies (i.e. they emit and
absorb light equally) and so have a
characteristic blackbody spectrum dependent
on their surface temperature.
Spectra
Analysing spectra gave rise to the subject of Astrophysics. Previously
astronomy was just about categorising objects based on their
appearance (morphology). But by studying their spectra, we can use
physical models to understand their nature. Many of these astronomical
objects just appear as dots in the sky, but by studying their spectra we
can determine their shape, temperature, and a whole load of other useful
information.
The first simple way of measuring spectra is by observing the colour of
an object – a redder object emits more low-energy (long-wavelength)
photons, and a bluer object emits more high-energy (short-wavelength)
pgotons.
Intensity
Intensity
Red
Blue
Energy
Red
Blue
Energy
Telescope Spectrometers: Dispersion Gratings and Prisms
But measuring the spectra accurately, determining exactly how the light
intensity depends on wavelength, gives far more information.
Fraunhofer Lines
Using a spectrometer Fraunhofer observed dark lines in the spectrum of light from
the sun.
Cool elements in the sun's photosphere absorb some of the light before it escapes
from the sun. Elements only absorb at certain wavelengths – black lines in the
spectrum : absorption lines or Fraunhofer lines.
Each element has a
characteristic pattern
of lines in the
spectrum -> discovery
of Helium in the sun.
Ani!
How lines affect the blackbody spectrum
What else can spectral lines tell us?
Due to the Doppler shift effect, sometimes the position of lines of a given
element will in the spectrum will appear shifted in wavelength.
Ani!
This implies that the absorbing material is moving either towards or
away from us. Spectral observations of the sun reveal the shift in line
wavelength away from the rest wavelength is greater at the equator
than at the poles. Therefore the equator is moving more rapidly than
the poles.
Spectroscopic Binary Star Systems
Ani!
Close binary star systems cannot be resolved
into two separate stars, however due to
Kepler's third law they must be orbiting each
other quickly.
If they are inclined so that they orbit roughly in
our line of sight, then one of the stars will be
moving towards us, the other away from us.
Therefore the spectral lines of one star will be shifted to longer wavelengths and
the spectral lines of another star will be shifted to shorter wavelengths.
By measuring the change in this shift over time we can see the orbital period,
and can calculate the separation, and masses of the two stars.
Stars – Spectral Types
Since the development of spectrometry the spectra of many stars have
been measured, and from these measurements stars have been
grouped into different types.
The HertzsprungRussell Diagram
In 1912 Hertzsprung and
Russell determined the
temperature and brightness
of all of the stars within a given
star cluster. Since all of these
stars will lie at roughly the
same distance away their
relative absolute brightnesses
are the same as their relative
apparent brightnesses.
So they could plot the absolute
brightnesses of the stars
against their temperature.
They found most stars lie along
a line called the Main
Sequence
Mass-Luminosity Law
Main sequence stars obey a Mass-Luminosity law
The luminosity of a star, L, increases as the mass increases by M3.5
L  M3.5
OB type stars
are up to 100 solar masses
M type stars
are around 0.1 solar masses
Mass
Radius
Stellar Lifetimes
When a star runs out of nuclear fuel
to burn, it will no longer be able to
maintain its balance against the
force of gravity, and will collapse.
We can calculate the lifetime of a
star as being the amount nuclear
fuel it contains divided by the rate at
which it burns nuclear fuel. This is
roughly equal to the star's mass
divided by its luminosity.
Therefore, T  M / L
using the mass luminosity relation:
T  M / M3.5
or T  1 / M2.5
Therefore, bigger, brighter main
sequence stars die younger
Ani!
Stellar Evolution
Stars are formed in nebulae, such
as the Orion Nebula, when a cloud
of hydrogen gas collapses under its
own gravity
When the centre of this protostar
becomes hot enough it will ignite the
nuclear fusion process, and the star
becomes a main sequence star with
a temperature/spectral type
dependent upon its final mass.
The ignition of new star was seen
for the first time this year in the
Orion Nebula.
Stars spend most of their lives on the main sequence, when their
hydrogen fuel runs out, the core of the star collapses, and the outer
envelope expands and cools. As the core collapses it heats up and
eventually becomes hot enough to ignite the fusion of helium atoms,
which enables the star to maintain an equilibrium. At this stage it
has grown so large that it is now a cool red giant star like Betelgeuse
(Alpha Orionis).
What happens next, after the helium fuel is exhausted depends upon the
star's mass.
In solar mass stars the core collapses further until it forms a dense white
dwarf star (the size of the Earth). The outer layers of the red giant star are
blown away by the intense heat of the core collapse, and form a planetary
nebula.
In more massive stars further core collapse leads to the fusion of heavier
and heavier elements – in these stars all of the metals were formed.
Eventually the star runs out of efficient fusion reactions (once iron is formed),
and the core collapses in a violent explosion – a supernova. Leaving behind
an extremely dense core, containing the mass of the sun within a 20 km
radius – the size of a small asteroid!
These neutron stars rotate very rapidly due to the conservation of angular
momentum, and some that emit radio waves are known as pulsars for their
rapidly pulsed radio emission.