CCD - Charged Couple Devices
Advantages Over Film:
1) Higher quantum efficency ~70% (number of bits of info recorded / number of phtons x 100)
2) More linear response (output is more directly proportional to light intensity received)
3) No need to replace
4) Can be used repeatedly
5) Faster Operation
Disadvantages
1) Smaller field of view
2) Need to be kept cool (about 120k)
3) Not portable
4) Resolution is not as good
Made from slices of silicon that store electons from photons and build an image (in pixels) from this.
Pixels are from 5-50 micrometers, and pixels where larger quantities of light land (higher light intensity) have more electons.
A voltage is applies to pixels so that the computer can "read" the image.
Film
Advantages Over CCD:
1) Cheap
2) Good resolution
3) Permanent record
4) Easily transported
Disadvantages
1) Not as efficent
2) Not as linear
3) Have to be replaced
4) Slower to use ( up to 2 hours while CCD is about 1 minute) Because of this, also more expensive because telescope time is expensive.
5) Grain Size is 0.1 - 1 micrometer (large grains are sensitive to light but produce a grainy image while small grains are less sensitive but produce a sharper image)
Benefits of observing from above the Earth's atmosphere:
1) Light Pollution reflects off gaseous atmosphere (known as scattering)
2) Pollution reflects and absorbs light
3) Light passing thrhough atmosphere experiences different refractive indeces
4) Diffraction limits resolving power of telescopes
5) Some wavelengths are absorbed by atmosphere
There are 2 "windows" in atmosphere. They let radio and optical waves through.
where d is diameter of particle in atmosphere, and w is wavelength.
d>w particles act like mirrors
d=w scattering is proportional to 1/w (blue more scattered than red)
d<w amount of scattering is (1/(w^4)) called Rayleigh scattering.
Telescopes:
Hubble Space Telescope
IRAS - For infrared
COBE - infrared and micrwave
ROSAT - X-Ray
Can someone please say a bit more about these?
Information from wavelengths of different stars:
By observing different stars we find that:
1) The spectrum given off can tell us the chemical composition of the gases which make up the star
2) The star emits a range of wavelengths and as the temperature of a star increases i) the light intensity of the radiation increases at all wavelengths ii) the spectrum of radiation shifts towards higher frequencies.
(Wien's law)
These observations lead to Wien's displacement law.
This states that for a star there is a simple connection between the temperature of the star (k) and the wavelength (λmax - lander max) which is the maximum intensitywithin the spectrum.
Note: If a frequency is given Velocity/Frequency = Wavelength
Wien found:
λmax x T = Constant = 2.898 x10^-3 (m.K)
Luminosity of stars:
Luminosity is the total power radiated by a star.
Total power depends on temperature and surface area.
Stefan-Boltzman found that the luminosity is directily proportional to the temperature (k) to the power of 4.
L ɑ T^4
L= σ(T^4) x surface area of sun
Where σ is a constant called the Stefan constant and has a value of σ - ((5.67).(10^-8))W(k^-4)(m^-2)
Measuring Distances:
For large distances:
1 Light year = 9.46 x 10^15 m (Speed of light is 3 x 10^8 m/s)
1 AU - the average distance between the earth and the sun. (may not be in syllabus)
Parallax Method:
Only suitable for relatively close stars
ok...so E is earth at opposite times in the year (6 months apart), S is Sun and T is the star you are looking at...
Imagine an angle at E(T)S..call it d
because the angle is small, (less than 1 degree) ET can be assumed to be the same as ST.
So: tan d =~ sin d = 1AU/D.
Method for annual parallax method:
1. From a position on earth line up the nearby star with a more distant star observed from earth.
2. Exactly 6 months later repeat the process in number 1, by varying the angle.
3. Measure the distance between the angle of 1 and 2.
4.Knowing the average diameter from the earth's orbit radius, the Distance
D = 1AU/tan d (not sure here...why isn't it 0.5 tan d?)
Hertzprung - Russel diagram ( Having trouble uploading the picture).
For a star LANDERmax is proportional to 1/Temperature (from Wien's law whcih is LANDERmax.T= constant.
The further the star is away fro earth, the lower its intensity (I).
Intensity is directly proportional to luminosity
Intensity on earth can be measured, and then Luminosity can be calculated by using the following equation:
I = L/(4piD^2), where D is the distance from earth, adn can be calculated using the parallax method.
(correct me if I'm wrong because I am a little confused..) Using this value of L, the star can now be put on the HR diagram.
The HR diagram can also be used to measue distances ( for nearby or far away stars?):
Measure the temperature T by using Wien's law, then use the HR diagram to estimate a luminosity. Then the luminosity can be used to calculate the distance. [I = L/(4piD^2)]
Cepheid Variables are stars that pulsate. They have used up their main supply of hydrogen.
Cepheid variables go from being very dim, to being very bright in a time range of anything between about 1-50 days.
The period of a Cepheid is related to its luminosity:
Astonomers have found that over a period of pulsations that the time period of pulsation depends on Luminosity. (Almost a direct proportionality based on a relative luminosity)
So astronomers measure the time period T and use it to find the luminosity. then they can use I = L/(4piD^2), to work out the distance.
There are also RR Lyrae variables, but their period is less than 1 day, and they do not follow the same pattern.
Stars...
Stars are born as interstellar gas clouds collapse under their own gravitational contraction.
They are made up of Hydgrogen a small amount of Helium and about 1-2% of heavier elements.
If the mass of the cloud is large enough (100x that of the sun supposedly?) Hydogen nuclei all move towards the centre, and their gravitaional potential energy is converted into kinetic energy. This causes the protostar to heat up and begin fusion where it then becomes a star and settles into a main sequence star. Pressure outwards (radioactive and caused by heat I think) matches pressure inwards of gravity.
Fusion - p-p chain is where Hydrogen is converted to Helium (I think). But for edexcel details of p-p chain are not neccessary.
Main Sequence stars:
Large stars have large gravitational forces acting on them. They therefore have a large inwards pull. They reach higher temperatures faster, and have higher luminosity. Top left of the H-R diagram. SHORT LIFETIMES
Small stars: have small gravitational forces, so lower temps and luminosity. Bottom right of H-R. LONG LIFETIMES
What happens after...just gets messy and complicated in my opinion, but I'll try...
Ms = Mass relative to sun.
White Dwarves
Ms<0.4 Hydrogen is burnt slowly for a very long time. When hydorgen is finshed the star contracts into a white dwarf.
Red Giants
The stars between 0.4 and 8 Ms become Red Giants when most of the H is converted to He. This is because the outwards pressure is no longer balanced with the inwards pressure. This causes the core to heat up, and increases energy flow to gases around the outside of the star. The outside gases begin fusion, which makes the outward forces unbalanced again, meaning that the sun begins to expand (by about ~100x) to become a Red Giant or a Super Giant. (Red giants then go on to become white dwarfs)
Now more and more fusion of heavier and heavier elements occurs at the core (in layers around the centre).
When the fusion reaches Iron, the red giant cannot fuse anymore (because the star has run out of lots of energy, and Iron is very hard to fuse)
Supernovae
Supernovae are created when supergiants collapse..something like becuase the mass is so large the electrons have enough energy to combine with protons. The core collapses, and everything blows up etc. so the shock wave forms a brigh super nova. ( This is for Ms>8 for the original star)
Neutron Stars
If Ms of core of a superovae is >1.4, the central core of neutrons will form a neutron star. They are highly dense (10^15 x normal matter). Are about 20km in diameter and have a mass of 10^30kg.
AS the neutron star rotates it it emits high frquency radio signals. (Is this a pulsar?)
Black Holes
If the core remnants of a supernovae are greater than 2.5Ms a black hole is formed. The supernova collapses to an infinite density called a singularity.
Can be dected by noticing strong X-ray sources near stars
By gravitational lensing
By doppler shifts from stars near by.
Here are some bits and pieces that I missed out...
Intensity is different from luminosity in that it only reguards the luminosity reaching earth.
I get the impression it is useful to know the range of waves in the EM spectrum, and where visible light etc. lie in that...that was probbaly gcse stuff anyway.
10^-12 ----> 10^4
Gamma ----> Radio
Visible Light
700x10^-9 -----> 400x10^-9
R O Y G B I
It would be good of people added bits I missed out, and some corrections/answers...
Thanks very much, Swordsikan, that is really a summary of everything we need to know! My notes are just like this. This is a retake for me, I don't know what happened the first time I took this exam- the questions were just awkward and unusual...
Measuring Distances:
For large distances:
1 Light year = 9.46 x 10^15 m (Speed of light is 3 x 10^8 m/s)
1 AU - the average distance between the earth and the sun. (may not be in syllabus)
Parallax Method:
Only suitable for relatively close stars
ok...so E is earth at opposite times in the year (6 months apart), S is Sun and T is the star you are looking at...
Imagine an angle at E(T)S..call it d
because the angle is small, (less than 1 degree) ET can be assumed to be the same as ST.
So: tan d =~ sin d = 1AU/D.
Method for annual parallax method:
1. From a position on earth line up the nearby star with a more distant star observed from earth.
2. Exactly 6 months later repeat the process in number 1, by varying the angle.
3. Measure the distance between the angle of 1 and 2.
4.Knowing the average diameter from the earth's orbit radius, the Distance
D = 1AU/tan d (not sure here...why isn't it tan d?)
LOL, i like ur diagrams, thats a clever way of doing them
your sin d= tan d thing doesnt make sense and isnt needed, but apart from that, thats exactly how it thought u do the parallax method, but then I looked at one of the mark schemes for the astropaper and they did it completely differently, u hav to draw on parallel rays coming from a distant object, then do the difference between that angle and the angle SET.
Has anyone else done this paper? (Jan 2002), and could someone please explain it
ok, I'll have a look for that question. I am not really sure why my teacher said tan d =~ sin d...but it wasn't just my teacher, it was the whole school..so there must be some truth to it for some reason...
Yeah..I'm really confused now, maybe there are many ways of doing this. I think another way is as you mentioned to compare the movement of a star you see to a star much further away, which does not appear to move (because it is far away)...
Physics discussion, revision and homework help.
Useful Resources:
Edexcel Astrophysics (unit 3 Notes)
