Stars

1
Stars
2
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.
3
The Sun A Summary
Sunspots are cool regions on the Sun's photosphere
4
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.
5
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.
6
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.
7
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.
8
Stars are very massive. The Sun, a typical star,
has a mass of 2.01030 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.
9
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!
10
(No Transcript)
11
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.
12
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
Blue
Red
Energy
Energy
13
Telescope Spectrometers Dispersion Gratings and
Prisms
But measuring the spectra accurately,
determining exactly how the light intensity
depends on wavelength, gives far more information.
14
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.
15
Each element has a characteristic pattern of
lines in the spectrum -gt discovery of Helium in
the sun.
Ani!
16
How lines affect the blackbody spectrum
17
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.
18
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.
19
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.
20
The Hertzsprung-Russell 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
21
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
22
OB type stars are up to 100 solar masses M type
stars are around 0.1 solar masses
Mass
Radius
23
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!
24
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.
25
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).
26
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.
27
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.
28
(No Transcript)
29
(No Transcript)
30
(No Transcript)