Astrophysics Flashcards
For a single lense to make an image appear to be diminished, where should the object be placed?
Beyond 2f
For a single lense to make an image appear the same size, where should the object be placed?
At 2f
The a single lens to make an image appear magnified, where should the object be placed?
Between f and 2f
For a single lens to make an image appear as a virtual object, where should the object be placed?
Below f
How does a CCD work?
1- a CCD is a silicon chip, divided into picture elements (pixels)
2- each pixel is associated with a potential well in the silicon
3- photons are incident on the CCD, and the photons within the semi-conductor cause the electrons to be released
4- the number of electrons liberated, is proportional to the intensity of the light
5- the electrons are trapped in the potential wells, and an electron pattern is built up that is identical to the image formed on the CCD
6- when the exposure is complete, the charged is processed to form an image
What is meant by quantum efficiency, and what are the values for a CCD vs the human eye?
Number of photons detected/ number of photons incident
CCD: 80% or more
Human eye: 1%
What is chromatic abrasion, and which type of telescope does it effect?
A colour distortion that creates an outline of unwanted colour along the edges of objects in photos, that effects refracting telescopes. (Blue first then red)
What is spherical abrasion, and what type of telescope does it effect?
It occurs when the mirror isn’t a perfect parabola, and it effects both types of telescope
What is the Rayleigh criterion?
2 objects will just be resolved when the first minimum in the diffraction pattern of one image coincides with the central maximum of the other
How can you improve resolving power?
Increase the size of the dish, or look at smaller wavelengths
State 3 optical advantages of reflecting telescopes over refracting telescopes
- mirrors don’t refract light, so they don’t suffer chromatic abrasion
- mirrors can be larger, and therefore have greater resolving power
- mirrors can be larger, and therefore have smaller minimum angular resolution
What is the maximum size of imperfections in a telescope dish can be?
Wavelength/ 20
What is 1 parsec?
The distance at which 1 AU subs tends an angle of 1 arc second
What is 1 arc second in degrees?
1/3600
What is meant by apparent magnitude?
The brightness of a star as seen from earth
What is meant by absolute magnitude?
The brightness of a star as seen from 10 pc away
What is a standard candle?
Objects who’s absolute magnitude is known
What is a black body?
And object that absorbed and emits all wavelengths of light
What are hydrogen balmer lines?
Spectral lines corresponding to the visible wavelengths absorbed when electrons move from n=2 to higher energy levels
Give a brief explanation of the key stages of the evolution of a sun like star
- dust and gas is drawn together via gravity to form a protostar
- protostars continue to contract and heat up until the core is hot enough to fuse hydrogen into helium (core hydrogen burning)
- hydrogen in the core runs out, core contracts and heats up. Outer layers expand and cool, star becomes red giant
- shell starts fusing hydrogen into helium (shell hydrogen burning)
Hydrogen runs out, the core contracts and heats up. Helium now fuses into carbon and oxygen (core helium burning) - helium in the core runs out, core contracts and heats up
- shell starts fusing helium (shell helium burning)
- for a sun like star, once fuel has run out, it contracts into a hot dense core, throwing off outer layers
- it has become a white dwarf
For what mass of a star will the core contract to a white dwarf/ neutron star/ black hole?
1-1.4 solar masses: white dwarf
1.4-3 solar masses: neutron star
>3 solar masses: black hole
What is a supernova?
A star whose luminosity increases enormously due to it exploding
What is a neutron star?
A star with the density of nuclear matter
What is a black hole?
An object with an escape velocity greater than the speed of light
What is the schwarzchild radius?
The point at which the escape velocity is equal to the speed of light
What is the Doppler effect?
The apparent change in frequency of a wave as a result of relative motion between the source and the observer
What is the cosmological principle?
On a large scale, the universe is homogenous and isotropic
What are the 3 pieces of evidence for the Big Bang?
1- red shift of galaxies
2- cosmic microwave background radiation
3- relative abundance of helium and hydrogen
