# Astronomy Revision Questions

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• Created by: Lizzy Day
• Created on: 26-06-16 13:35
What is parallax?
A measure of how much a nearby object appears to move in relation to the fixed background due to the observers motion. Measured as an angle of parallax.
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What is the equation for the distance to an object involving the angle of parallax?
For small angles, tan0 = 0, where 0 is in radians. d= r/0
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What is a parsec?
The distance of an object from Earth if its angle of parallax is equal to 1 arcsecond = 1/3600
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What is one parsec equal to?
1 pc = 3.08 x 10^16 m
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What is the astronomical unit? Au?
1 astronomical unit is defined as the average distance between the Earth and the Sun. 1 AU = 1.5 x 10^11
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What is a light year?
The distance that electromagnetic waves travel through a vacuum in one year. 1 light year = 9.46 x 10^15 m . 1 parsec = 3.26 light years
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How can you work out the angle subtended by an object in the sky?
As long as the angle is small: 0 = r/d
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Why don't the radius and distance have units?
r/d is just a ratio, so the two quantities just need to be in the same unit.
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What is luminosity?
The total amount of energy emitted by an object each second (in the form of em radiation). Also known as power output.
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What is intensity?
The intensity, I of an object that we observe is the power received from it per unit area at Earth. This is the effective brightness of an object.
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What is apparent magnitude?
A measure of the brightness of an object as observed from Earth.
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What was the ancient Greek scaling system for brightness?
The brightest stars were given an apparent magnitude of 1 and the dimmest stars an apparent magnitude of 6, with other levels catering for the stars in between. In c.19, the scale was refined using a strict logarithmic scale.
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What is the difference in magnitude between a magnitude 1 and 6 star?
A magnitude 1 star has an intensity 1000x greater than a magnitude 6 star.
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How can you calculate the brightness ratio between two stars?
I2/I1 = (1001/5)^m1-m2 = 2.5 ^m1-m2
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What is absolute magnitude?
What an objects apparent magnitude would be if it were 10 parsecs away from earth.
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What is the relationship between apparent and absolute magnitude?
m-M = 5log (d/10)
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What are standard candles?
Objects in space that you can calculate the brightness of directly without knowing their distance
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What is a black body?
A black body is a body that absorbs all em radiation of all wavelengths and can emit all wavelengths of em radiation.
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What is black body radiation?
As black bodies emit all wavelengths of em radiation, they emit a continuous spectrum of em radiation.
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What is Wien's displacement law?
All black body spectra have a peak intensity at a peak wavelength, /\max. The higher the surface temp, the shorter the wavelength. /\maxT = 0.0029mk
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What is Stefan's law?
P = oAT^4
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How can you work out the intensity of a star?
I = P/4pid^2
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What kind of law does intensity follow?
The inverse square law. Intensity is the power of radiation per square metre, so as the radiation spreads out, the intensity decreases.
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How do electrons exist in atoms?
Electrons in an atom can only exist in certain well-defined energy levels. Each level has a number. n=1 is the lowest energy level an electron can be in.(a.k.a ground state)
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How is an atom excited?
An atom is said to be excited when one or more of its electrons is in an energy level higher than the ground state.
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How can an electron move energy level?
Electrons can move energy levels by absorbing a photon. The energy of each photon absorbed can only take a certain allowed value.
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What is the equation for the energy of a photon?
E= hf = hc//\
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What happens if you split the light from a star?
You get a spectrum. Stars are approximately black bodies, so they emit a continuous spectrum of electromagnetic radiation.
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Why do you get absorption lines in this continuous spectrum?
You get absorption lines in the spectrum when radiation from the star passes through a cooler gas.
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Why does this happen in cool gases?
At low temps, most of the electrons in the gas atoms will be in their ground states. Photons of particular wavelengths are absorbed by the electrons to excite them to higher energy levels.
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What are hydrogen balmer absorption lines?
The wavelengths corresponding to the visible part of hydrogen's line absorption spectrum are caused by the electrons in atomic hydrogen moving between n=2 and higher energy levels. This leads to a series of lines called the Balmer series.
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What does the intensity of the balmer lines depend on?
If the temperature is too high, the majority of the electrons will reach n=3, which means there won't be many balmer transitions. So, the intensity of the lines depends on the temp of the star.
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What is escape velocity?
The escape velocity is the velocity that an object would need to travel at to have enough Ek to escape a gravitational field.
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What is the event horizon?
The boundary of the region around the infinitely dense point in which the escape velocity is bigger than the speed of light. Light has just enough Ek to overcome the gravitational pull.
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What is the Schwarzschild radius?
The event horizon of a black hole. It is thought of as the radius of the black hole. Inside it, everything, including light, can do nothing but travel further into the black hole.
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What is the equation for the Swarzschild radius?
Rs = 2GM/C^2
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What is the doppler effect?
The change in the frequency and wavelength of a wave for a source moving towards or away from an observer.
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Why does the frequency of a moving object change?
The frequency and wavelength change as the waves bunch together in front of the source and stretch out behind it. The amount of this depends on the objects velocity.
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What is red-shift?
The shift in wavelength and frequency of a source moving away from us towards (or beyond) the red end of the em spectrum
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What is blue-shift?
When a light source moves towards us, the opposite to red-shift happens and light undergoes a blue shift. The light from the star looks bluer than it actually is.
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What does the amount of 'shift' depend on?
The amount of shift depends on how fast the star is moving away from (or towards ) us. The higher the velocity, the more the waves are shifted.
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What is recessional velocity?
The speed at which an object is receding from Earth.
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What is the equation for red shift?
Z = v/c
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How do you know if the red-shift is positive or negative?
If the source is moving away, V>0, so the red-shift is positive. If the source is moving towards the observer, v
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How can you observe red-shift on a spectrum?
You can work out the red-shift of a star by looking at the wavelength or frequency of known absorption lines in the observed spectrum and comparing them to the wavelengths or frequencies they should be.
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How can you calculate red-shift from frequency?
Z = ^f/f
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How can you calculate red-shift from wavelength?
Z = ^/\//\
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What is cosmological red-shift?
The galaxies aren't actually moving through space away from us. Space itself is expanding and the light waves are being stretched along with it.
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What is the cosmological principle?
The idea that no part of the universe is any more special than another. The universe is homogeneous and isotropic.
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What did Hubble realise?
He realised that the universe is expanding. He used type 1a supernovae as standard candles to calculate the distances to galaxies, as well as measuring their red-shift.
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What is Hubble's law?
v = HD
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What is the Big Bang theory?
The universe started off very hot and dense (infinitely hot, dense point) but has been expanding ever since.
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How can you estimate the age of the universe?
If the universe has been expanding at the same rate its whole life, T = distance/velocity.= 1/H
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What is dark energy?
A type of energy that fills the whole space, and might explain the accelerating expansion of the univserse.
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How are astronomers using the idea of dark energy?
In the late 90s, evidence was found that the expansion is now accelerating. Astronomers are trying to explain this using dark energy.
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What does the Big Bang predict about radiation?
It predicts that lots of em radiation was produced in the very early universe. This radiation should still be observed today.
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What is CMBR?
As the universe has expanded, the wavelengths of this cosmic microwave background radiation have been stretched and are now in the microwave region - so it is known as CMBR
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What satellites are having a detailed look at radiation?
The cosmic background explorer (COBE) and Wilkinson microwave Anistropy probe (WMAP) satellites have been sent into pace to have a detailed look at the radiation. The CMBR has a perfect black body spectrum, with a temp of 2.73K
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What are some of the properties of the CMBR?
The CMBR is largely isotropic and homogeneous, which agrees with the cosmological principle. There are tiny fluctuations in temp due to energy density variations in the early universe.
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What does the Big Bang say about H and He?
The BB explains the large abundance of He in the universe.The early universe had been very hot, so at some point it fused with hydrogen. Together with the theory of synthesis of heavier elements, the relative abundances of all elements are accounted.
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What are spectroscopic binary stars?
About half the stars we observe are actually two stars that orbit each other. Many of them are too far away from us to be resolved with telescopes, but the lines in their spectra show a binary star system.
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What is an eclipsing binary system?
One whose orbital plane lies almost in our line of sight, so the stars eclipse each other as they orbit.
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How can orbital period be calculated?
By observing how the absorption lines in the spectrum change with time. The separation between the lines goes from zero up to the maximum, then back again.
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How does apparent magnitude change for binary system?
If you plot a graph of apparent magnitude against time for an eclipsing binary system, you'll get a graph, with fluctuating magnitude. As the stars eclipse, the apparent magnitude drops as light is blocked out.
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What are quasars?
Quasars (quasi-stellar objects) were discovered in the late 1950s and were first thought to be stars in our galaxy.
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What was different about quasars?
The stars produced a continuous spectrum that was nothing like a black body radiation curve, and instead of absorption lines there were emission lines of elements that astronomers had not seen before
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What was the realisation about quasars?
Maarten Schmidt realised that these lines were simply the balmer series of hygroden, but red-shifted enormously. This suggests they're a huge distance away- the most distant objects seen.
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What are active galactic nuclei?
A quasar is a very powerful galactic nucleus centred around a huge black hole more than 10^6 times the mass of the sun. Almost all galaxies are thought to have 'supermassive' black holes at their centres. Those that do are active galactic nuclei.
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## Other cards in this set

### Card 2

#### Front

What is the equation for the distance to an object involving the angle of parallax?

#### Back

For small angles, tan0 = 0, where 0 is in radians. d= r/0

### Card 3

#### Front

What is a parsec?

### Card 4

#### Front

What is one parsec equal to?

### Card 5

#### Front

What is the astronomical unit? Au?