9: Astrophysics Flashcards
In a ray diagram, what is the axis of symmetry called?
-The principal axis
What is the principal focus?
-The point on at which parallel light rays will converge after hitting the lens
What is the focal length?
-The distance between the the centre of the lens and the focal point
What is the equation for magnification?
-M=f0/fe
-Where the angles subtended are less than ten degrees
How does a refracting telescope work?
-Uses two converging lenses
-The objective lens collects light from the object to create an image
-The eyepiece lens magnifies this image to produce a virtual enlarged image
How does a Cassegrain telescope work?
-Uses two mirrors
-A concave primary mirror collects light and focuses the light onto a convex secondary mirror
-The convex mirror reflects the light onto an eyepiece lens
What is chromatic aberration?
-When the lens refracts different colours of light by different amounts
-Light with longer wavelengths converges further along the principal axis
-Solved by using an achromatic doublet
What is spherical aberration?
-Curvature of lens causes light to focus in different places
-Causes image blurring and solved by using a parabolic mirror
-Light that hits the lens closer to the centre focuses further on the principal axis
What are the advantages of reflecting telescopes?
-Very little chromatic aberration
-Easier to increase size of objective mirrors
-Can use parabolic mirrors to stop spherical aberration
-They are lighter
-So will get clearer images than with a refracting telescope
-Although the secondary mirror may block some of the light
How does the size of the objective lens affect what images can be observed?
-The larger the diameter the more light that can be collected
-As collecting power is proportional to the diameter squared
-So for a larger diameter you can see fainter objects
What is the Rayleigh Criterion?
-Two objects will be resolved if the centre of diffraction of one image coincides with the first minima of the other image
-θ ≈ λ / D
How does a single dish radio telescope work?
-Has a large parabolic dish to focus radiation on a receiver
-Placed in an isolated location
-Can be used to observe galaxies, stars and black holes
Why are radio telescope larger than optical telescopes?
-Radio waves have a much larger wavelength
-So in order to have the same resolving power the telescope must have a much larger diameter
How does an infrared telescope work?
-Has a large concave mirror to focus light onto a detector
-Must be cooled with cryogenic fluids to avoid interference
-Must be placed in space as otherwise the infrared light is blocked by the atmosphere
-Can be used to observe cooler regions in space
How does an ultraviolet telescope work?
-Uses a Cassegrain configuration to focus radiation onto solid state devices
-Must be placed in space as otherwise UV light is blocked by the atmosphere
-Can be used to observe interstellar mediums and star formation regions
How does an x-ray telescope work?
-Uses hyperbolic and parabolic mirrors to focus radiation onto a CCD
-Must be placed in space otherwise x-rays are blocked by the atmosphere
-Can be used to observe high-energy events such as galaxies, black holes and neutron stars
How does a gamma telescope work?
-Radiation passes through a detector made up of layers and pixels
-Placed in space as otherwise gamma rays are blocked by the atmosphere
-Can be used to observe gamma ray bursts, quasars, black holes and solar flares
How does the quantum efficiency of a CCD compare to the human eye?
CCD has a QE of about 80% compared to 5% for the human eye
How does the resolution of a CCD compare to the human eye?
-CCDs have a higher resolution so are better at capturing fine detail
How does the convenience of a CCD compare to the human eye?
-CCDs are more convenient for accessing data remotely
-It is easier to analyse data on CCDs via computers
-CCDs have a wider spectral range
What is apparent magnitude(m)?
-How bright a star appears from earth
What is absolute magnitude(M)?
-How bright the star appears 10 parsecs from the Earth
What equation links apparent and absolute magnitude?
-m-M=5log(d/10)
What is the Hipparcos scale?
-Scale invented by Greek astronomer Hipparcos where brightness is defined in terms of apparent magnitude
-The brighter a star the lower the apparent magnitude
What is a parsec?
-The distance to an object that subtends an angle of one arcsecond to the line running from the centre of the Earth to the centre of the Sun
What is a light year?
-The distance that light travels in one year
What is Stefan’s law?
-The power output of a star is directly proportional to its surface area and temperature
-P = σAT^4
-σ is the Stefan constant and equals 5.67x10^-8
What is Wien’s displacement law?
-The peak wavelength of a star’s emissions is inversely proportional to its temperature
-The peak wavelengths multiplied by the temperature is equal to a constant
λmax*T = 2.9 × 10−3
What is a black body?
-A body that absorbs all radiation incident on it
What is the path that our Sun takes?
-Protostar
-Main sequence star
-Red giant
-White dwarf
What are supernovae?
-The explosion of a star causing a sudden and rapid increase in absolute magnitude
How do type Ia supernovae form?
-A white dwarf core accumulates too much mass and explodes
How do type II supernovae form?
-A single star collapses rapidly under its own gravity and its outer layers are ejected
What is a black hole?
-When the core of a star reaches a mass larger than three solar masses it collapses and forms a black hole
-No light can escaper from a black hole
-Produces an event horizon the boundary of which is where the escape velocity is equal to the speed of light
How is the radius of a black hole calculated?
-This is the Schwarzschild radius
-Rs=(2GM)/c^2
What is the Doppler effect?
-When the observed wavelength and frequency of a wave changes as it moves towards or away from the observer
-Only occurs when the velocity of the wave is much smaller than the speed of light
What is red shift?
-When an object is moving away from the observer and its wavelength is shifted towards the red end of the spectrum
What is blue shift?
-When an object is moving towards the observer and its wavelength is shifted towards the blue end of the spectrum
What is Hubble’s law?
-The velocity of a receding galaxy is proportional to its distance form the earth
-v=H0*d
What are quasars?
-Have extremely high power outputs
-Found to be powerful radio sources but can emit all wavelengths of electromagnetic radiation
What are exoplanets?
-Planets that are not in our solar system
-Difficult to observe as their light is often obscured by the stars they orbit
How are exoplanets detected?
-Radial Velocity Method: A star and a planet will orbit their common centre of mass, this means the star will have tiny variations in its distance from Earth, shown by tiny red and blue shifts in its spectrum.
-Transit method: As a planet moves between the star it orbits and the Earth, the star’s brightness appears to decrease slightly.
-We can detect this and use it to calculate the diameter of the planet.
-There is a low chance of this orbit being in the right place for to measure this, so it is mostly only useful for detecting planets with small orbits
What is the Big Bang Theory?
-The belief that billion of years ago the universe exploded from an extremely hot and dense point and is still expanding
What evidence is there for the Big Bang Theory?
-CMBR (Cosmological Microwave Background Radiation) is the heat signature left behind from the big bang.
-The EM radiation released in the explosion shifted from extremely high energy waves into the microwave region as the universe expanded
-There was nuclear fusion of hydrogen into helium which explains the large abundance of helium in today’s universe
Information that can’t be put on flashcards
-Ray diagrams
-Aberration (chromatic and spherical)
-Normal adjustment diagram
-Cassegrain configuration diagram
-Black body curves
-Table for spectral classes
-H-R diagram
-Light curve for supernovae
-Estimating the age of the universe by Hubble’s law
-Light curve for a binary star system
What is a standard candle?
-A standard candle is an astronomical object that has a known absolute magnitude so astronomers can calculate the distance using m - M = 5log(d/10)
Why can type Ia supernovae be used as standard candles?
-. All Type Ia supernovae explosions have the same peak absolute magnitude (approximately -19.3) as they all have the same critical mass (thus have consistent light curves) so they can be used as standard candles.
-Type II supernovae are not as predictable, so they cannot be used as standard candles.
How are quasars formed?
-A quasar is a nucleus of an active galaxy
-A supermassive black hole surrounded by a disc of matter. As matter falls into the black hole, jets of radiation are emitted from the poles of the quasar
