Astrophysics Flashcards
Principle Axis of a lense
An imaginary line that passes through the centre of a lens and through the centres of curvature of the faces of the lens
Focal Point of a lense
The point at which rays parallel to the principal axis of the lens are brought to a focus
Focal Length
The focal length of a lens is the point at which rays parallel to the principal axis of the lens are brought to a focus
How does an astronomical telescope ray diagram look
Spherical Abberation ray diagram
Rays from a distant object are not brought to a focus at a single point
Chromatic abberation ray diagram
Different colours of light are refracted by different amounts
What are the advantages of a refracting telescope over a reflecting telescope
- The lenses in a refracting telescope are held in place by a metal tube. So little maintenance is required. The mirror in a reflecting telescope s exposed to the air and might need recoating
- The mirrors in a small reflector can get out of alignment if the telescope gets knocked. So sometimes the mirrors just need adjustment. The strong constructuon of the refracting telescope makes such misalignment less likely
The second mirror in a relecting telescope has the disadvantage of blocking some of the light from entering the primary mirror
What are the advantages of a reflecting telescope over a refracting telescope
- A good astronomical telescope requires a diameter of about 15cm or more, so that sufficient light is gathered. It is difficult to make a high quality lens of diamater 15cm, easier to make a concave mirror of that size
- Reflecting mirror has no chromatic abberation, because light is reflected over a metal surface without passing through glass
- Sphericak aberration can be reduced more easily in a relecting telescope by making the concave mirror parabolic shape
Explain whether sources can be resolved when two sources, emitting wavelength λ, have angular seperation θ are viewed through an aperture of diameter D
- If θ > λ/D - the sources can be resolved
- If θ = λ/D - the sources can just be resolved
- If θ < λ/D - the sources cannot be resolved
What is a CCD
- charge coupled devices
- a CCD is a slice of silicon that stors electrons freed by the energy of incoming photons
- Charge on the electrons builds up an image as a pattern of pixels
- CCDs have a very high quantum effciiency.
What is quantum efficiency
- Means a high percentage of photons that strike the CCD produce charge carriers, that are then detected
Quantum Efficiency = number of electrons produced per second/ number of photons absorbed per second
Give the relative quantum efficiencies
Eye - 1-4%
Film - 4-10%
CCD - 70-90%
Brightness
The brightness of a star is a measure of how much visible light from the star reaches our eyes
Luminosity
The luminosity of a star is the energy it emits per second
Apparent Magnitude
A star’s apparent magnitude is a measure of its brightness as it appears in the sky
Parralax
Nearby objects apear to move relative to far-away objects when viewed from a different angle
Aborption spectrum
This spectrum is seen as a series of dark lines in a continous spectrum, when some elements absorb speicfic wavelenths of light.
Name the absorption spectrum spectral classes
- O
- B
- A
- F
- G
- K
- M
O stars
Colour: blue
Temperature: 25,000-50,000K
size: very large and massive
Spectra: helium and ultraviolet light
Example: Zeta
B stars
Colour: blue
Temperature: 11,000 - 25,000
size: large and massive
Spectra: helium and hydrogen
Example: Rigal
A stars
Colour: blue-white
Temperature: 7500-11000K
size: moderate size and very luminous
Spectra: Strong hydrogen lines, ionized metals
Example: Sirius
F stars
Colour: white
Temperature: 6000-7500K
size: 1.2-1.6 times bigger than the sun
Spectra: weak hydrogen lines, strong calcium + other ionised metals
Example: Canopus
G stars
Colour: yellow-white
Temperature: 5000-6000K
size: 0.8-1.1 times bigger than the sun
Spectra: weak hydrogen lines, neutral ionized metals
Example: Capella
K stars
Colour: Orange
Temperature: 3500-5000K
size: smaller and cooler than the sun
Spectra: faint hydrogen lines, strong neutral metallic lines
Example: Alpha Tauri
M stars
Colour: red
Temperature: <3500K
size: half the size of sun
Spectra: Neutral atoms, titanium oxide
Example: Antares + Betelgeuse
Explain the Hertzsprung-Russel diagram
Lifecycle of a star
Nebula
- stars are formed in a Nebula
- A Nebula is a very large cloud of gas and dust in space
Protostars
- Gravity makes dense region of gas more compact
- soon take on definite shape and are called proto stars
- Once the core of the protostar reaches a certain temperature, nunclear fusion begins and the protostar ignites to become a star
- A star is a ball of plasma undergoing nuclear fusion and gives off large amounts of energy in the form of electromagnetic radiation
- As long as there is a nuclear reaction taking place, the internal forces will balance the external forces
Main sequence stars
- A star in which hydrogen burning takes place
- This thermonuclear fusion of hydrogen nuclei into helium nuclei
Why are more massive stars more luminous than smaller stars
Gravitational forces that thend to collapse a star increase with mass
So for the star to be in equilibirum, it means the outward pressure from the core must be larger
Therefore, the nuclear reactions must run at a higher rate generating more power, which leads to the star having a higher luminosity
Red Giant
When the hydrogen in a main sequence star is consumed, fusion stops and the forces suddenly become unbalanced
Mass and gravity cause the remaining gas to collapse on the core
The collapsing outer-layers cause the core to heat up and the fusion of helium into carbon begins
The forces regain balance and the outer shell expands rapidly
A planetary nebula is created when this Red Giant blows off its outer layers after it has run out of carbon to burn. These outer layers of gas expand into space, forming a nebula which is often the shape of a ring or bubble.
Red Supergiant
If the mass of a star is 3 times that of our sun or greater, than the Red Giant will become a Red Supergiant
When the massive Red Giant fuses all of the helium into carbon, fusion stops and the outer layers collapse on the core
Supernova
The Core of the supergiant will then collapse in less than a second, combine protons and electrons into neutrons (creating a neutron star core)
Other matter rebounds off the core and fusion again starts in the outer star layers causing a massive explosion called a supernova
In a supernova, a massive shockwave is produced that blows away the outer galaxies of the star
White Dwarf
Planetary Nebula breaks down to its core
A white dwarf is very hot when it forms, but because it has no source of energy, it will gradually cool as it radiates its energy.
This means that its radiation, which initially has a high color temperature, will lessen and redden with time.
As it cools, the light it gives off will fade and form a Black Dwarf
Neutron star
Under the intense gravitational forces in a Supernova, the core collapses under the pull of gravity
The core rises to temperatures as high as 100 billion Kelvin
In such high temperatures and pressures, proyons and electrons can combine, in a reverse beta decay, to form neutrons and neutrinos
In this way the centre of the core turns into a ball of neutrons wich will become a neutron star
Black Holes
If the mass of the surviving star is greater than 3 solar masses than a black hole forms
A black hole is a core so dense and massive that it wil generate so much gravity not even light can escape it
What are the three ways of detecting exo-planets
Variation in Doppler shift
Planetary transits
Direct imaging
Variation in Doppler Shift
A giant planet and a star orbit around a common centre of gravity
There will be times when the star will be moving towards the Earth, and times when the star will be moving away from the earth
So there will be small changes to the spectrum of the star, which will be seen as small redshift and small blueshift
Planetary transits
If an exoplanet crosses in front of a star’s surface, then the brightness we see will drop by a small amount
Therefore a light intensity vs time graph would show a suddent drop in intensity
It allows rough estimations to be made of the planet’s radius and thus mass
Direct Imaging
Light from the star has been digitally removed to enhance view of planets
Hubble Space telescope
Can observe visible, ultra-violet and infared rays
Chandra x ray telescope
Used to observe high energy objects such as neutron stars and supernovae
Spitzer
Infared telescope in orbit around the sun at the same distance as the earth but a long way away - originally cooled by liquid helium
What can magnification also equate to
hi/ho
(Height of image/height of object)
What is a standard candle
A class of objects whose distances can be computed by comparing their observed brightness with their known luminosity.
What is a black body
Emmitters and absorbers of radiation
How does temperature effect wavelength
When yo heat something up from absolute zero it will give off shorter wavelengths as the temperature increases
What is the evidence for the Big Bang theory
- In the early universe protons and neutrons produced
- protons are stable and neutrons decay into protons
- Neutrons combine with protons to form Helium and Deutrium
- These are stable, so no further decay occurs
- Hence by comparing the number of free protons to the number of helium atoms we can test models for the formation of the universe
- Big Bang fits this very well - the universe is made up of H and He
CMBR
- Cosmic Microwave Background Radiation
- Red shifted background radiation left behind after the big bang - radiation stretched that it is observed in the microwave region - hence cosmic microwave background radiation
State two advantages of a larger diameter telescope
- larger resolving power - resolving power proportional to D
- Power is proportional D^2, more power detected by larger diameter telescope
What is a Quasar
A massive and extremely remote celestial object, emitting exceptionally large amounts of energy
They are distant objects powered by black holes a billion times as massive as our sun.
Define parsec
parsec : distance to an object subtending 1 sec of arc to Earth’s orbit
To prove a parsec convert it into radians then covert it into light years
Define Hubbles Constant
Gives the ratio of the (recessional) velocity (of galaxies) to distance from Earth
Define red shift
increase in wavelength (of em radiation) due to relative recessive velocity between observer and source
Give another equation for magnitude
Angle subtented by image at eye/ angle subtended by object at unaided eye
Radio telescopes
situated on the ground as it detects radio waves which are not absorbed by the atmosphere
Large mirrors so collecting power very high
Ultraviolet telescopes
Careful consideration required for the position of an ultraviolet telescope because the majority of ultraviolet is absorbed by the atmosphere
Better resolving power than radio wave telescope as ultraviolet has a shorter wavlength than radio waves
X ray telescopes
X-ray telescopes are usually situated in space because the atmophere prevents the majority of X-radiation reaching the Earth’s surface.
X rays are very penetrating and not easily reflected off metal surfaces
Cepheid variable
A bright star whose intensity caries over a matter of days. The period of the variation of intensity is linked directly to the absolute magnitude of the star
Explain how a CCD operates
The image formed on the CCD is created by incident photons. These photons cause electrons to be released.
The electrons are trapped in (“potential wells” in the CCD)
The number of electrons liberated (in each pixel) is proportional to the intensity of the light/number of photons falling (on each pixel). or so that the pattern of the charge built up is related to the image.
State was is meant by the Rayleigh criterion
Two objects will just be resolved when the first minimum/edg of the airy disc in the diffraction pattern of one image
Coincides with central maximum/centre of the airy disc of the other.
How long does a supernova last
10-400 days
wavelength x axis for light curve
100-2500 nm
Y axis for hertspirng diagram
+15 to -10
Conterversies from measurements of typa 1a
Measurements of supernovae do not agree with predictions (from Hubble’s Law)
So Universe must be expanding at increasing rate/accelerating
(Controversial as) no known energy source for expansion or reference to dark energy
