ch13 Flashcards
how do we measure large distances in space
light years
what is 1 light year
distance travelled by light in a vacuum in a year
what is the distance and speed of a light year
10^16 m
10^16 m s^-1
Why is the size of the observable universe limited?
to see something light from the object must reach us. Light has a finite speed, so it takes time for that to happen.
universe has finite age, so we can only see so far
what is one astronomical unit, AU
the mean distance between the sun and the Earth
what type of scale of magnitude do we use for large ranges
why is this useful
logarithmic scales, they allow us to cover a large range of distance (each increment is 10 times bigger than the last)
use of logarithmic scales
the HR diagram which is a plot of temperature of stars against their luminosity (power output)
what is the HR diagram used for
why do you use the log scale
to find the ages of stars
makes it easy to see the difference in brightness between dimmest and brightest
Why are images of astronomical objects often displayed using a logarithmic scale of brightness?
So that the difference between the brightest and dimmest parts of the image is less intense
why do astronomers use clusters (groups of stars) to work out ages
it’s impossible to tell how old a single star is, so multiple are used that are assumed to have formed at the same time
how are clusters used with HR diagrams
clusters are plotted on a HR diagram to work out their stage of life
modern way of measuring distances
radar
radio detection and ranging
how does radar work, how fast does the pulse travel
pulse is transmitted and detected by the same place after it has reflected off object
the pulse travels at the speed of the light
the delay in sending and receiving tells you the total time of travel
this is twice the distance to the asteroid
what equation is used when using radar to work out distance to body
distance = speed of light * time / 2
d = ct / 2
divided by 2 because the total distance is to the object and back
how can you use radar to measure the velocity of an object relative to the earth
you send out 2 pulses with a known time interval between them, to give 2 different measurements of the object’s distance
the difference between the distances shows how far the object has moved in a certain time
velocity = change in distance / change in time
what are 2 assumptions made when using radar to calculate distance between us and an object
1: the speed of the signal is the same both ways
2: the time taken for the pulse to reach the object is the same time it took to get back to us
why is radar not suitable beyond solar system
time between emission and detection is too long
return signal too weak
second method of measuring distances:
LIDAR light detection and ranging
laser light emitted and reflected off object
return signal is very weak but can be distinguished from background noise
why don’t we use radar or lidar for larger distances, e.g. beyond the solar system
it would take too long for the signal to come back too us and even if it did, it would not be strong enough to detect
what is parallax
parallax is the apparent shift in position of a nearby object against the background of further objects when the observer moves
for this think about holding your thumb out and closing eyes one by one
parallax is for very near stars
the angular shift is the baseline but this is often very small compared to the distance to nearby stars
what is a standard candle, how is it used to measure distances to nearby stars
standard candle is a star with a know absolute luminosity (the true brightness of a star)
you can study variation in brightness of certain stars to get absolute luminosity and compare to apparent brightness to find out distance
what is red shift (doppler effect)
whats its significance
the perceived increase in wavelength when a source of waves is going away from an observer
when light moves away from you, wavelength becomes longer, longer wavelengths shifts light towards red end of spectrum
red shift gives us the clearest evidence that the universe is expanding
how does amount of red shift increase with how far away a galaxy is, and what does this provide evidence for
the further away a galaxy is, the more red-shifted the light is. this provides evidence for the big bang theory
how do you calculate the change in wavelength for a star red shifting
Δλ / λ = v / c
when v << c
how do stars produce line spectra,
what is this spectra called
how can this be used to see red shift
atoms in distant stars absorb light (radiation) at particular wavelengths, different atoms absorb different wavelenghts, producing a spectrum spectral lines.
this is the stellar spectrum
can be compared to known lab spectrum to see if there’s shift, if moved to the left, its red shifted
how does wavelength change if a star is moving away from the earth, and what shift do they show
if star is receding, the wavelengths of its spectral lines increase, and so they show red shift
how does wavelength change if a star is moving towards the earth
the wavelengths of its spectral lines decrease, blue shifting
What is assumed when using radar or Doppler shift to calculate velocities/distances?
That the object, that you are looking at, is travelling much slower than the speed of light
Is the universe expanding uniformally?
Yes
What is a more accurate measurement of an object’s speed than radar?
Doppler shifts
what is cosmological red shift
galaxies aren’t actually moving away, space itself is expanding, waves are being stretched with it
this is not the same as redshift, which is for objects that are moving
what was Hubble’s law
the further away a galaxy is, the faster its moving away, that velocity is its recessional velocity
what was the equation for Hubble’s law
speed of recession = Hubble’s constant * distance
v = H0 * d (distance)
v = recessional velocity in km s-1
d = distance Mpc
H0 = hubble’s constant km s-1 Mpc-1
how do you get Hubble time
convert hubble constant from km s-1 Mpc-1 to s-1
you do this by turning the km into metres, and dividing by the given value of 1 Megaparsec
1 / Hubble constant
1 / h0
what is the hot big bang theory
what is the clearest evidence for it
universe started off very hot and dense and has since expanded and cooled
cosmic microwave bbackground radiation is the clearest evidence for HBB
(red shift is also good evidence)
origins of CMBR
and how its changed
originated as gamma rays produced in the big bang
but as the universe cooled and expanded, the waves lost their energy and their frequency decreased and wavelengths stretched so now the radiation is in the microwave region
penzias and wilson were setting up a microwave antenna but it was detecting noise in the signal at microwave wavelength
What can space-time wordlines be used for?
show objects moving through space and time
To find the distance to an object from the time interval between sending and receiving a radar pulse
describe space time diagram
time on y, dist on x (same units, seconds and light seconds)
the line is called worldline
line has a grad of 1 (as 1 lightsecond in 1 second)
line reflected at object
Describe how you would draw a radar pulse on a space-time diagram. How do you work out the distance to the object you are sending the pulse to?
Start the line showing the pulse at t=0.
The gradient of the line should be 1 (as light travels 1 light-second in 1 second).
The radar pulse changes direction half way between leaving and arriving back, so the line should too.
The point at which it changes direction is the distance of the object that reflected it
When does time dilation happen?
When something moves close to the speed of light
einsten’s first postulate
physics works in the same way for all observers regardless of their relative motion
when there’s relative velocity between 2 bodies, its impossible to tell which is stationary and which is moving
einstein’s second postulate
speed of light is constant, every moving or stationary observer perceives SoL to be 3 * 108
time dilation
when does this take place
when 2 observers with different relative velocites record a different time taken for an event to occur
this takes place near the SoL
decribe time dilation using a light clock
ray of light bounces between 2 mirrors, for a stationary observer, relative to light clock, light only has to travel up and down
if clock is moving relative to observer, light will seem to travel along hypotenuse, bouncing diagonally as mirrors move, c is constant but distance increases so time increases. each second is ‘stretched’ or dilated
the greater the relative velocity the greater the effect
time dilation equation
when is it ignorable
ignore when v << c, both times very close
t = γ * wristwatch time
t = time by moving observer relative to clock
γ = relativistic factor (1 / (square root 1 - v² / c²))
wristwatch time = stationary observer
