ch13

Question 1

how do we measure large distances in space

Answer 1

light years

Question 2

what is 1 light year

Answer 2

distance travelled by light in a vacuum in a year

Question 3

what is the distance and speed of a light year

Answer 3

10^16 m
10^16 m s^-1

Question 4

Why is the size of the observable universe limited?

Answer 4

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

Question 5

what is one astronomical unit, AU

Answer 5

the mean distance between the sun and the Earth

Question 6

what type of scale of magnitude do we use for large ranges

why is this useful

Answer 6

logarithmic scales, they allow us to cover a large range of distance (each increment is 10 times bigger than the last)

Question 7

use of logarithmic scales

Answer 7

the HR diagram which is a plot of temperature of stars against their luminosity (power output)

Question 8

what is the HR diagram used for

why do you use the log scale

Answer 8

to find the ages of stars

makes it easy to see the difference in brightness between dimmest and brightest

Question 9

Why are images of astronomical objects often displayed using a logarithmic scale of brightness?

Answer 9

So that the difference between the brightest and dimmest parts of the image is less intense

Question 10

why do astronomers use clusters (groups of stars) to work out ages

Answer 10

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

Question 11

how are clusters used with HR diagrams

Answer 11

clusters are plotted on a HR diagram to work out their stage of life

Question 12

modern way of measuring distances

Answer 12

radar
radio detection and ranging

Question 13

how does radar work, how fast does the pulse travel

Answer 13

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

Question 14

what equation is used when using radar to work out distance to body

Answer 14

distance = speed of light * time / 2
d = ct / 2
divided by 2 because the total distance is to the object and back

Question 15

how can you use radar to measure the velocity of an object relative to the earth

Answer 15

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

Question 16

what are 2 assumptions made when using radar to calculate distance between us and an object

Answer 16

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

Question 17

why is radar not suitable beyond solar system

Answer 17

time between emission and detection is too long

return signal too weak

Question 18

second method of measuring distances:

Answer 18

LIDAR light detection and ranging

laser light emitted and reflected off object

return signal is very weak but can be distinguished from background noise

Question 19

why don’t we use radar or lidar for larger distances, e.g. beyond the solar system

Answer 19

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

Question 20

what is parallax

Answer 20

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

Question 21

what is a standard candle, how is it used to measure distances to nearby stars

Answer 21

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

Question 22

what is red shift (doppler effect)

whats its significance

Answer 22

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

Question 23

how does amount of red shift increase with how far away a galaxy is, and what does this provide evidence for

Answer 23

the further away a galaxy is, the more red-shifted the light is. this provides evidence for the big bang theory

Question 24

how do you calculate the change in wavelength for a star red shifting

Answer 24

Δλ / λ = v / c

when v << c

Question 25

how do stars produce line spectra,

what is this spectra called

how can this be used to see red shift

Answer 25

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

Question 26

how does wavelength change if a star is moving away from the earth, and what shift do they show

Answer 26

if star is receding, the wavelengths of its spectral lines increase, and so they show red shift

Question 27

how does wavelength change if a star is moving towards the earth

Answer 27

the wavelengths of its spectral lines decrease, blue shifting

Question 28

What is assumed when using radar or Doppler shift to calculate velocities/distances?

Answer 28

That the object, that you are looking at, is travelling much slower than the speed of light

Question 29

Is the universe expanding uniformally?

Answer 29

Yes

Question 30

What is a more accurate measurement of an object’s speed than radar?

Answer 30

Doppler shifts

Question 31

what is cosmological red shift

Answer 31

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

Question 32

what was Hubble’s law

Answer 32

the further away a galaxy is, the faster its moving away, that velocity is its recessional velocity

Question 33

what was the equation for Hubble’s law

Answer 33

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

Question 34

how do you get Hubble time

Answer 34

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

Question 35

what is the hot big bang theory

what is the clearest evidence for it

Answer 35

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)

Question 36

origins of CMBR
and how its changed

Answer 36

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

Question 37

What can space-time wordlines be used for?

Answer 37

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

Question 38

describe space time diagram

Answer 38

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

Question 39

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?

Answer 39

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

Question 40

When does time dilation happen?

Answer 40

When something moves close to the speed of light

Question 41

einsten’s first postulate

Answer 41

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

Question 42

einstein’s second postulate

Answer 42

speed of light is constant, every moving or stationary observer perceives SoL to be 3 * 10⁸

Question 43

time dilation

when does this take place

Answer 43

when 2 observers with different relative velocites record a different time taken for an event to occur

this takes place near the SoL

Question 44

decribe time dilation using a light clock

Answer 44

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

Question 45

time dilation equation

when is it ignorable

Answer 45

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