12.1 Flashcards
What is the speed of light?
3x10^8 ms-1
What is light travel time?
The distance light can travel within a given time period
What is one light second?
3x10^8 m
(300,000km)
How far light can travel in one second
What is one light year?
10^16 m
The distance light can travel in one year
What unit of measurement is used for distances that are a small fraction of one light year?
Astronomical units
What is an astronomical unit?
A unit of measurement, equal to the distance from the centre of the Earth to the centre of the sun
For what objects are astronomical units often used?
Objects within the solar system
For what objects can radar ranging be used?
why?
Objects within the solar system
Because any further and the returning radar signal would be too weak to detect
What is radar ranging?
A radar pulse is bounced off an object and the delay is used to work out the distance of the object from Earth, using distance = speed x time
In radar ranging, what do you have to remember to do to the delay time before calculations?
Divide by two to find just the out time not out and back time
Describe how radar ranging can be used to find the relative velocity of an object (such as an asteroid)
1.Send out a pulse to be reflected and measure the time it takes to return. Find the distance from speed x time
2.Repeat the measurement after a MEASURED period of time
3.Work out the change in distance with the method above
4.Use speed = distance/time
with change in distance and time between pulses to work out the relative velocity
When using speed = distance/time to work out the relative velocity using radar ranging, what is used for distance and time?
distance = change in distance away between pulses time = time between pulses
At what speed does radar travel?
3x10^8 ms-1
it is an EM wave so always travels at the speed of light
For what objects is the Doppler effect used to measure?
Objects too far away for radar ranging
What is the Doppler effect?
A perceived change in frequency of sound emitted by a source moving relative to an observer
What is pitch?
frequency
In Doppler shift, what happens when objects move towards each other?
Pitch is higher
In Doppler shift, what happens when objects move away from each other?
pitch is lower
How does the Doppler effect happen?
NON CONCENTRIC WAVEFRONTS
The wavefronts are centred on where the source was when they were created, but the source moves on
The wavefront are no longer centred normally
Wavelengths are shorter in front of the source and longer behind the source
In Doppler shift where are the wavelengths shorter?
In front of the source
In Doppler shift, where are the wavelengths longer?
Behind the source
What is the equation linking wavespeed, wavelength and frequency?
wavespeed = wavelength x frequency
What can be used for ‘one way radar’
The Doppler effect
What can Doppler shift be used to measure and what can it not be used to measure?
It can measure relative velocity but not distance
How can Doppler shift be used for ‘one way radar’?
Radiation of known wavelength from distant sources is detected. The relative velocity is worked out from the difference between observed wavelength and actual source wavelength
What is the equation for change in wavelength in the Doppler effect?
Δλ/λ = v/c
change in wavelength/source wavelength = source velocity/speed of light
How can the wavelength of a source be known?
Different atoms emit light at a certain wavelength - lines in their spectrum
How is the change in wavelength (after Doppler shift from a distant object) calculated?
The actual spectrum is compared with the received spectrum on Earth
What is luminosity?
Absolute brightness
What is Parallax?
The perceived shift in direction of nearby stars against a backdrop of distant stars, when viewed from different positions
How is parallax used to measure how distant a star is?
Measurements of the stars angle from the sun are taken at opposite ‘ends’ of the Earth’s orbit, 6 months apart
If the angle and distance from sun to Earth is known, then the distance can be worked out using trigonometry
What is the trig formula used for the parallax method?
d=A/tanϴ
What are standard candles?
Objects of a known, fixed brightness (not stars)
You can compare how bright something is known to be to how bright it looks and use the inverse square law to find the distance
What is the inverse square law for brightness?
intensity ∝ 1/distance^2
How is the colour of stars related to their brightness?
blue stars are hotter and brighter than red stars
What are Cepheid variable stars?
Stars which are brighter for a longer period of variation in brightness
They are all roughly the same distance away, so can be used as reference points
What is special about blue supergiant stars?
They are the brightest thing in any galaxy and are all about the same brightness
What does it mean when a galaxy is rotating faster?
It is more massive, so will be brighter
What is special about type IA supernovae?
They are always the same brightness because stars will explode at the same critical mass
What can measurements of relative velocities using Doppler shift be used to work out?
the mass of objects being orbited
What is the gravitational inverse square law?
g = GM/r^2
What is G?
The gravitational constant
How can Doppler shift be used to work out the mass of an object being orbited?
- Find the speed of an orbiting object using doppler shift measurements. One measurement at each ‘end’ of the orbit, one velocity away from and one towards us
- The time taken for this reverse in velocity is half the total time taken for an orbit
- The diameter traveled is 2πr, so r=(2 x time) /2π
- Gravitiational field = (v^2)xr
- Mass,M = gR^2/G
When working out the mass of an object being orbited, what is acceleration?
g=(v^2)/r
What is one parsec?
The distance to an object of parallax angle one second of an arc
(1/3600°)
What is one arc second in degrees?
1/3600°
What are the assumptions made when using radar ranging to measure distances?
- The speed of light is always constant
- The moment of reflection is exactly half way through the time delay
What is Malmquist bias?
Only far away bright stars are seen but nearby average stars are seen. Distances of stars is underestimated.
