Astronomy Flashcards
What is the quantum efficiency of a CCD?
The percentage of photons incident on the chip that initiate a detectable response.
What is a common quantum efficiency of a CCD?
80%
What is the advantage of higher sensitivities found in CCDs?
Exposure times are reduced.
What is the dynamic range of a CCD?
Approx 100,000 which corresponds to 14.5 magnitudes.
Give five advantages of using CCDs over photographic film.
- High sensitivity
- Larger dynamic range
- Linear exposure
- Broad range of wavelengths can be detected
- Can be interfaced directly with computers allowing complete automation
How does a CCD work?
- A CCD is a semiconductor chip sensitive to light divided into discrete rectangular areas (pixels).
- Each pixel is made up of 3 electrodes with the central electrode having a potential of 10V and the two outer electrodes 2V.
- This forms a depletion layer at the PN junction with a potential well beneath the central electrode.
- Lights photons cause electrons to be released into the potential well.
- The amount of charge collected is directly proportional to the number of photons falling on each pixel.
- The potential of each electrode is changed in order to move the electrons along the CCD causing them to leave as a current pulse.
How can noise be reduced in images?
Cooling the CCD or photographic film.
How is a colour image produced from a CCD?
Combining images from a red, green and blue filter
What are the two “windows” in the atmosphere know as?
Optical and radio windows
What types of EM radiation can pass through the atmosphere?
UV, IR, visible light, radio
What causes long wavelength UV to be absorbed in the atmosphere?
Ozone
What causes light and IR and visible light to be absorbed in the atmosphere?
O2, H2O, CO2
Where can UV astronomy take place?
Above the Earth’s atmosphere
What can IR astronomy be used for?
- Finding exoplanets
- Observing nebulae
- Observing after-effects of gamma ray bursts
What can UV astronomy be used for?
- Star formation rate
- Analysing quasars
What can X-ray astronomy be used for?
- Finding black holes
- Observing stellar remnants
What is the strongest source of radio waves found on Earth?
The Sun
What is the window of wavelengths for radio waves?
1cm to 15m
What are the three main mechanisms of celestial radio emission?
- Thermal
- Synchrotron radiation
- Hydrogen emission
What is a black body?
An object that absorbs all radiation and can therefore emit at all possible wavelengths.
How can a telescope be made bigger without increasing the size of the dish?
Having multiple telescopes placed over a large distance which produces a telescope effectively of the size of the distance between them.
How is a radio map produced?
Placing the telescope at an angle then allowing the Earth to rotate then changing the angle.
When can wire mesh be used for a telescope?
When the wavelength of the radiation being measured is over 20x the mesh size.
What is the annual parallax of a star?
Half the angle through which the direction of the star shifts as the Earth moves from one side of orbit to the other.
How is the distance from Earth in parsecs calculated?
1 / annual parallax in arc seconds
What is the greatest distance trigonometric parallax can be used for?
100pc
What is an arc second in degrees?
1/3600
What is a black body?
A body that absorbs all radiation that falls on it so is able to emit at all wavelengths.
What is black body radiation?
The spectrum of EM waves emitted.
What does the spectral distribution of a black body depend on?
The surface temperature of the body.
What does the area under a wavelength vs. intensity graph give?
Total energy radiated per unit time per unit surface area or power per unit area
What does E stand for in E=σT^4?
Power radiated per square metre
How is the total power emitted by a star calculated?
P = AσT^4
What is the apparent magnitude of a star?
A measure of brightness as seen from Earth.
What ratio of apparent luminosity does a difference of magnitude 2.5 give?
10
What ratio of apparent luminosity does a difference of magnitude 5 give?
100
What is the absolute magnitude of a star?
A measure of the brightness of the star if the star was moved to 10pc from Earth.
What causes a continuous emission spectra?
Multiple compounds and molecules emitting EM waves.
What causes discrete emission spectra?
Excited electrons returning to lower energy levels causing energy to be released at discrete wavelengths.
What causes absorption lines in emission spectra?
Discrete wavelengths of light travelling in one direction being absorbed by a compound then emitted isotropically.
What is the order of spectral classes of stars?
OBAFGKM
What is the origin of prominent absorption lines in cooler stars?
Molecules
What is the origin of prominent absorption lines in type A-G stars?
Large elements since most molecules have been broken down by the increased temperature.
What is the life cycle of a star of one solar mass?
Dust cloud - Main sequence yellow star - red giant - white dwarf
What is the life cycle of a star of ten solar masses?
Dust cloud - blue/white main sequence - red supergiant - supernova - neutron star
What is the life cycle of a star of 30-50 solar masses?
Dust cloud - blue/white main sequence - red supergiant - supernova - black hole
