# 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