Download File - YEAR 11 EBSS PHYSICS DETAILED STUDIES

Unit 1 Physics
Detailed Study 3.2
Chapter 11: Astrophysics
Section 11.3
We know the stars by their light
Analysing starlight
The way we know stars, if by their ‘light’
This light refers to the whole electromagnetic spectrum, this includes the
whole visible spectrum, Inferred (IR), x-ray, gamma rays and UV.
To observe these different forms of light, a variety of telescopes are required,
such as the radio telescope in Parkes, which is used to pick up microwave and
radio radiation.
Some radiation, such as x-rays and some UV cannot penetrate the Earths
atmosphere, so to get a clear picture, we sometimes need to put telescopes
out into space.
Section 11.3
We know the stars by their light
Analysing starlight
Though a picture of a star can tell us a fair
bit about a star, we learn more about it from
the spectrum it produces.
A hot object will produce light of a
continuous rainbow spectrum, unless it
interacts with an outside medium, such as
dust or gas.
In this interaction, the gas cloud can absorb
some light, this shows up as dark bands in
the spectrum.
In the absorption process, atoms of the gas
are move into an excited state and then back
to ground state. In the process, light is
emitted in all directions. This light appears as
bright lines on a dark background.
Section 11.3
We know the stars by their light
Analysing starlight
These spectra of emission and absorption are characteristic of the each of the
elements.
For example, Sodium emits two particular yellow spectral lines. If these lines
appear in the spectrum of a star, we can say that sodium present, whether it is
as part of a compound or as a gas.
These lines, known as Fraunhofer lines, lead to the discovery of Helium in the
sun before its discovery here on Earth.
As we know, we can tell a lot from the light emitted from a star, for example
its temperature.
Section 11.3
We know the stars by their light
Analysing starlight
The temperature of a star as we learnt earlier could be determined by the amount light
emitted from ranges, for example, hotter stars emit more in the Blue and UV ranges
then the visible spectrum.
However, as we have just learnt, if this light were to encounter a cloud of dust or gas,
light may be absorbed, leading to incorrect readings.
Fortunately, the dark bands mentioned earlier can help in the determination of
Temperature.
Initially, stars were categorised according to the presence or absence of certain lines
associated with hydrogen, labelled from A-O
This was later changed to a system that involved placing stars with similar spectra
adjacent to each other in a smooth pattern. The letters were then changed to
OBAFGKM, which was later broken up further by adding 0-9 to each category.
Section 11.3
We know the stars by their light
Analysing starlight
While not known at the time, this categorisation corresponded to the
different temperatures, from O0 (hottest) to M9 (coolest).
The reason for the changes between the classes was to do with some atoms
becoming ionised at various temperature and at cooler temperature the light
may not have sufficient energy to excite the atoms to create spectral lines.
This meant the temperature of a star could be determined without worrying
about losses as light travelled through gasses or dust clouds.
We can also determine the size of a star by its spectrum. To do this we need
to know its Luminosity, the amount of energy given off by each unit area, and
an accurate surface temperature.
Section 11.3
We know the stars by their light
Types of stars- the Hertzsprung-Russell
diagram
Once this information was gathered, the
natural thing to do was plot the information to
look for a relationship between known
quantities. (Surface Temperature, on the x-axis
increasing to the left and Luminosity, on the yaxis with brightest at the top)
There was a noticeable pattern produced, this
is known as the Hertzsprung-Russell, or H-R
diagram.
Most of the time, hot stars are bright, though
this is not always the case.
Stars on the main pattern/line going from top
left to bottom right are known as Main
Sequence stars. This includes the sun.
Section 11.3
We know the stars by their light
Types of stars- the Hertzsprung-Russell
diagram
It is possible to have hot, dim stars, these
types of stars are known as White dwarfs
(located mostly in the bottom left of the H-R
diagram), and are not visible to the naked eye
as they are quite small.
It is also possible to have a cool, bright star.
These stars are known as Giants or Supergiants,
depending on the size.
Giants and Supergiants only make up ~1% of
the stars in the sky, dwarfs only make up ~9%
and main sequence stars make up ~90% of the
stars in the sky.
Section 11.3
We know the stars by their light
Placing stars on the H-R diagram
Placing stars on the H-R diagram is relatively easy when within parallax range
as we can find their surface temperature and luminosity easily.
However, when outside of parallax range, it get a little bit trickier.
To do this we need to look at the spectrum of the light received from the star.
The spectral lines can tell astrophysicists about not only the temperature, but
can be interpreted in a way to give the luminosity of the star also.
One final piece of information needed was to determine the mass of a given
star. After some careful observation, a relationship was established between
mass an luminosity.
L is proportional to m3, so a star twice as heavy as the sun will be eight times
as bright.
Section 11.3
We know the stars by their light
Interpreting the H-R diagram – stellar evolution
For a long time, it was thought that Stars were permanent features of the
night sky, however this is not the case. Understanding the Nuclear process
going on inside the star lead to the understanding that these stars would not
last forever… Billions of years, yes, but not forever.
A star may have enough fuel to last it billions of years, however models have
shown that a stars life may not be as long.
The hydrogen-hydrogen fusion process produces helium, as the
concentration of helium builds up in the core, the reaction zone increases by
~10%, this causes some instability in the sun, causing the sun to expand from a
main sequence star to a giant.
Section 11.3
We know the stars by their light
Interpreting the H-R diagram – stellar evolution
This model indicated to astrophysicists that stars are born on the main
sequence and eventually move to the giant phase.
To test this theory star clusters (stars thought to be born around the same
time, from the same dust cloud) were examined.
In this examination, it was discovered that the heavier, brighter, bluer stars
were moving to the giant phase, before the lighter stars, this is due to the fact
that the heavier stars burn through their fuel faster then lighter stars.
Looking closely at the H-R diagrams of young and old clusters shows that
stars spend most of their lives in the main sequence, before expanding to giant
fairly rapidly.
Section 11.3
We know the stars by their light
Interpreting the H-R diagram – stellar
evolution
Stars start their life as a mass of gas and dust
collapsing on itself under immense gravitation
attraction. This collapse generates loads of
heat, which eventually causes enough heat to
cause fusion to begin.
Once fusion has begun, the star stabilises and
becomes a main sequence star.
Once expanded to the giant phase, there is
enough heat to cause heavier elements to fuse
together, which stabilises the star.
As the fuel beings to run out, the star begins to
contract, increasing the temperature, however,
decreasing the brightness, thus the star moves
towards the white dwarf region, where they
slowly fade away as a black dwarf.
Section 11.3
We know the stars by their light
Interpreting the H-R diagram – stellar evolution
Some stars, however, are destined to go out with a bang.
In stars around 4 times the mass of the sun, after the hydrogen-burning phase, begin
to contract, causing heat ~600 million degrees, causing new fusion reactions, eventually
leading to the burning of silicon to create iron.
At this point, fusion does not produce energy, so the star beings to collapse, increasing
the temperature to increase further.
As the star continues to collapse, the temperature increase further to billions of
degrees in a fraction of a second. It finally collapses to a point where it can collapse no
further, but the rest of the star keeps pushing on the core, causing a huge ‘bounce
back’.
This bounce back causes the star to explode, releasing 1046 J of energy, leaving behind
a neutron star.