Astrophysics Flashcards
Chromatic aberration
Glass refracts different colours by different amounts. The principle focus for each colour will be slightly different positions and blurs the overall image.
Advantages of reflectors
No chromatic aberration,no spherical aberration,no distortion,better resolving power, more light gets through as lens absorbs more light.
Spherical aberration and how to fix
Images are blurry or unfocused due to rounded mirrors , parabolic shape ensures parallel rays converge at the same point
What’s in a cassegrain system?
Concave primary mirror, convex secondary mirror, principle focus, eyepiece lens
Reflecting telescopes
Uses mirrors to reflect the light into a lens
Important factors of a refracting telescope
Lens quality, lens diameter, angular magnification, normal adjustment
Disadvantages of refractors
-Glass can have impurities and bubbles of air, absorbing some light. Large lenses are very difficult and expensive to make.
-Large lenses are heavy and can only be supported by the edges so their shape can distort.
-To get good magnification the objective lens needs a long focal length making the telescope very long.
-Spherical aberration can occur
Collecting power
How much light it can take in so a lens with a larger surface area lens has more collecting power
Airy disc
1st order maxima from object diffracting through telescope
CCds
Charge-coupled device: uses the photoelectric effect. They are an array of light sensitive pixels which become charged when they are exposed to lights by the photoelectric effect. Uses Quantum Efficiency= efficiency for detecting photons.
Quantum efficiency of CCD and human eye t
CCD~80%
Human eye~5%
Apparent magnitude scale
Rated scale for stars 1-6, 1 brightest, 6 dimmest
Luminosity
Total energy emitted per second [watts]
Intensity
Power received from star per unit area at earth. This is the effective brightness
Parallax
The greater the angle, the nearer the object is to you. When moving things in the foreground seem to move faster than objects in the distance
What is the Rayleigh Criterion
Two objects will be resolved when the first minimum/edge of airy disc in diffraction pattern of one image coincides with the central maxima of airy disc of other.
Define the parsec
Distance 1AU stubtends an angle of 1/3600th degree
Standard candle
Object whose absolute magnitude is known and the apparent magnitude can be measured
Controversies of measurements of type 1a Supernovae in 1999
Measurements of the supernova don’t agree with predictions from bubbles law. So the universe must be expanding at increasing rate. Controversial as no known energy source for expansion or dark energy is referred to.
Difficulties with detecting earth like planets orbiting sunlike stars
Transit method- dips in brightness as planet crosses in front of star from our point of view. Alignment must be correct for planets to eclipse so many can’t be observed.
Radial velocity method- periodic shift in spectra of star due to stars movement around common centre of mass with planet. Earth like planet mass much less than mass of sunlike star so effect slight. Earth like planet could be detected with highly sensitive spectrometers.
What is evidence for the Big Bang
Expansion of universe, cosmic background radiation, the relative abundance of hydrogen and helium, olbers paradox .
Olbers paradox
If the universe was infinite and stars were evenly spread, everywhere we look would intercept light from a star so the sky would appear uniformly bright. As we don’t see a bright sky, there must be variation from the Big Bang
How does a binary star system work
A high mass star and smaller mass star will both orbit a Bari centre, one instantaneously moves towards you, the other away.
How are protostars formed
Cold hydrogen gas and dust clump together under gravity. The irregular clumps rotate and angular momentum spins them inwards to form a dense centre surrounded by a circumstellar disc. When it’s hot enough elements fuse and produce a strong stellar wind blowing away surroundings.
How is a main sequence star formed
The inward force of gravity and outward force due to fusion of hydrogen nuclei are in equilibrium so the star is stable. The greater the mass of the star, the shorter it’s main sequence period because it uses its fuel quicker
How are neutron stars formed
Mass of core >1.4
The core gets so dense it overcomes electron degeneracy pressure. Electrons are squashed onto the atomic nuclei and combine with protons to become neutrons. A density is reached when the repulsive force of the neutrons is sufficient to stop collapse of the stellar core.
Pulsar
As neutron stars rotate, two beams of radio waves are emitted from the poles which can sometimes be observed from earth, known as pulsars.
Singularity
There is no known force in nature that can stop the collapse of cores greater than 3 solar masses. The collapse continues until the core contracts to an infinitely dense point known as singularity.
Temp, colour and absorption line of O class stars
25000-50000K, blue, hydrogen and helium
Temp, colour and absorption line of B class stars
11000-25000K, blue, hydrogen and helium
Temp, colour and absorption line of
A class stars
7500-11000K, blue/white, Hydrogen
Temp, colour and absorption line of F class stars
6000-7500K, white, ionised metals (anything heavier than helium)
Temp,colour and absorption line of G class stars
5000-6000K, yellow/white, ionised and neutral metals
Temp, colour and absorption line of K class stars
3500-5000K, orange, neutral metals
Temp, colour and absorption line of M class stars
Less than 3500K, red, neutral metals, titanium oxide
Why no green stars?
Light from stars is emitted at a range of wavelengths so there is mixing of colours. Those stars with a max wavelength in the green region will appear to be white. Blue and red colours mix to be green.
