Classification of Stars Part 2 Flashcards
What is the doppler effect?
The apparent difference between the frequency at which sound or light waves leave a source and that at which they reach an observer, caused by relative motion of the wave source to the oserver.
What happens when a wave source is travelling in the same direction as the waves?
When a wave source is travelling in the same direction as the waves, it causes the waves to compress together in front of the source therefore the frequency observed appears to increase.
What happens when a wave source is travelling in the opposite direction to the waves?
When a wave source is travelling in the opposite direction to the waves, makes the waves stretch out out behind the source, therefore the frequency observed appears to decrease.
For soundwaves:
If frequency increases, what happens to the pitch?
If frequency increases pitch increases.
Due to the doppler effect what happens to the line spectrum of a distant object when they move away from the Earth?
When a distant object is moving away from the Earth, they are moving in the opposite direction to the wave, so frequency decreases/wavelength increases, therefore the line spectrum is red-shifted.
(Red has longer wavlength than blue on visible light spectrum)
Due to the doppler effect what happens to the line spectrum of a distant object when they move towards the Earth?
When a distant object is moving towards the Earth, they are moving in the same direction as the wave, so frequency increases/wavelength decreases, therefore the line spectrum is blue-shifted.
What does the amount of red or blue shift depend?
The speed at which an object is moving away from or towards us.
The greater the velocity of an object….
…the greater the waves are red/blue-shifted.
Define recessional velocity?
The velocity at which an object is moving a way from us.
How is recessional velocity linked to the doppler effect?
When an object is moving away from us, the wavelength increases, so the object appears red-shifted to the observer.
What is the equation to calclate red-shift?
z = v/c, where v «_space;c (does not work for velocities close to c)
z = red shift (no units) v = recessional velocity (m/s) c = speed of light in a vaccuum (m/s)
How to calculate blue shift?
If v is moving away from us (recessional velocity), v > 0, so v is positive, therefore z is positive = red shifted.
If v is moving towards us, v < 0, so v is negative, therefore z is negative = blue shifted.
*IMPORTANT
How else can you calculate red shift?
z = Δf/f = -Δλ/λ
z = red shift (no units) λ = wavelength (m) f = frequency (Hz) Δf = difference in emitted frequency and observed frequency (emitted - observed) Δλ = difference in emitted wavelength and observed wavelength (emitted - observed)
If frequency increases/wavelength decreases, blue shifted = negative z. If frequency decreases/wavelength increases, red shifted = positive z.
- NOTICE HOW WAVELENGTH EQUATION IS NEGATIVE!!
- I dont really see how both of these allow you to calculate red shift.
How is the red shift evidence for an expanding universe?
- All distant galaxies are red-shifted. They have interpreted this as the expansion of the universe/space. (not the galazies moving away from us in space).
- To distinguish this from red shift produced by moving sources through space, this is known as cosmological red shift.
Hubble was the first person to realise the universe is expanding. What was Hubble’s law?
Hubble’s law states that a galaxy’s recessional velocity is directly proportional to its distance from the Earth. It essentially states that the universe is expanding from a common starting point.
From Hubble’s law we can see that he further the galaxy…?
- The further the galaxy, the greater the red shift.
Which equation summarises Hubble’s Law?
v = Hd
v = Recessional velocity (km/s) H = Hubble's constant (65 kms⁻¹Mpc⁻¹) d = Distance (Mpc)
*If you combine z =v/c with v=Hd, remember the v in z=v/c, is in m/s wheras in V=Hd, it is given in km/s
If you plot a graph of recessional velocity against distance, what type of graph would be produced
A straight line graph though the origin (directly proprtional) with a gradient = H.
How can we calculate age of the universe?
v = Hd can be rearranged to H = v/d, if we flipped v/d to produced d/v, we have made the equation to say distance/speed and we know that this also equals time. If we flip v/d to d/v, we also have to flip H to give 1/H. Therefore 1/H = d/v = t, age of the universe. However if we are using these equations to calculate time in seconds, H, d and v have to be SI units. So H has to be converted from kms⁻¹Mpc⁻¹ to ms⁻¹m⁻¹ (SI units), so 65kms⁻¹Mpc⁻¹ –> (65x10³)/3.08x10²² = 2.1x10⁻¹⁸ = H, t = 1/2.1x10⁻¹⁸ms⁻¹m⁻¹ which gives a value in seconds equal to approx. 15 billion.
*Dividing by 3.08x10²² covnerts Mpc to m.
