Pp Measuring Distances In Space

Measuring distances in Space

Luminosity = Power of a star (Watts) Intensity = How closely concentrated light is (Watts m -2 ) Light year = distance travelled by light in 1 year Astronomical unit (Au) = Distance from Earth to the Sun Parsec = Length of the side of triangle when the base is 1 Au and the angle is 1 second of arc ( 1/3600 degree) 1 Arcsecond

1 Au 1 Parsec

1 Parsec = 3.26 light year

1 light year = 63 241 Au

Erastosthanes measured the size of Earth using triangulation A = d 360 circumference of Earth A

d A Stick with no shadow – 12 o’clock Centre of Earth

Second stick at known distance Measure shadow angle - 12 o’clock

A similar method can be used to find the distance to objects Tan A = D / d

so

D = d . Tan A

A d

90 0 D

Centre of Earth

The method shown is simplified and the angle is very small so difficult to measure accurately

Triangulation can be used to measure distances to near objects but the angles are very small …..

…. and quickly become too small to measure accurately

It helps if the base length is made as big a possible

The biggest base has measurements 6 months apart

2 Au

But space distances are very large so ... … another method is needed

Moon Earth to scale T = 28 days

Astronomers discovered Cepheid variable stars which get brighter and dimmer

Their absolute magnitude (brightness) changes regularly – typically 5 to 30 days

Brighter = bigger minus number

Cepheid variables with longer periods have greater luminosity

By measuring the period we can calculate the luminosity

The law for the way that intensity changes with distance is known from experiments on Earth Intensity = Luminosity / radius 2

1

4 9

By measuring the intensity and by knowing the luminosity of a star we can work out it’s distance

The data is not very accurate leading to significant but measurable uncertainty in the answers obtained

Both the distance to distant galaxies and the speed that they are moving away from us could now be measured

Hubble’s original graph

The speed at which the Universe is expanding can now be found and also the age of the Universe (1/gradient)

Improved methods now tell us that the Universe is very close to 13.7 billion years old (13 700 million years)

Earth has been around for about 4.2 billion years