Hubble's law

Question 1

What are the alternative distance units that astronomers have defined to cut down the magnitude of the numbers involved?

Answer 1

The light year, the distance light travels in a year, about 10^16m

Astronomical unit (AU) which is the radius of the Earth’s orbit around the sun. 1.5*10^11m

And parsecs

Question 2

What method do astronomers use to measure distances to relatively close stars?

Answer 2

Trigonometric parallax.

Question 3

What is the main idea behind trigonometric parallax?

Answer 3

As the Earth moves around the sun, a relatively close star will appear to move across the background of more distant stars

It’s done by measuring the angle to a star and observe how that changes as the position of the Earth changes.

Question 4

Up to what distance is the trigonometric parallax method accurate to?

Answer 4

650 light years.

Question 5

How many seconds are in a degree?

Answer 5

3600, 60 minutes in a degree, 60 seconds in a minute.

Question 6

What is a parsec?

Answer 6

Abbreviation for ‘parallax second’

The distance a star must be from the sun for the angle ‘Earth-star-sun’ to be 1 second.

Question 7

What is 1pc in meters?

Answer 7

`3.09*10^16m

Question 8

What’s a light year in meters?

Answer 8

3(10^8)365246060=9.4610^15m

Question 9

How do you convert seconds to degrees?

Answer 9

multiply by 1/3600

Question 10

How is the distance to a star determined by its brightness?

Answer 10

The Stefan-Boltzman law only deals with the power output of a star at its surface.

The energy we receive from it depends upon its distance from us.

So if we know the power output of a star, it’s apparent brightness on Earth would give away its distance.

Question 11

What does the inverse square law mean?

Answer 11

The energy emitted by a star will spread out in all directions of a surface of an ever increasing sphere.

Question 12

What is the equation for the ‘radiant energy flux?’

Answer 12

At a distance d from a star,

R.E.F(F)=L/(4pid^2)

Where L is the luminosity of the star in watts.

REF in Wm^-2 (simply means the brightness)

Question 13

What are standard candles?

Answer 13

Some stars including some variable stars and supernovae, they have properties that mean their luminosity can be determined quite separately from other measurements.

Question 14

What are variable stars?

Answer 14

Stars in which brightness changes in a repeating cycle.

The period of this cycle is proportional to the luminosity of the star.

Question 15

What is the least reliable standard candle methodology?

Answer 15

Looking at a star’s spectrum, using its peak wavelength to find temperature from Wien’s law.

Determining if its a main sequence star from the width of its spectral lines

If it is, finding its place on the main sequence of the H-R diagram and reading the luminosity from the y-axis.

Question 16

What causes stars and galaxies to appear in a different colour because they are moving away from the Earth?

Answer 16

The doppler effect, the light we receive from them has red shifted as it has a longer wavelength than when it was first originally emitted.

This suggests that galaxies are moving away from us. The further they are away, the faster they are receding.

Question 17

How does the amount of red shift that a galaxy exhibits, z, allow us to calculate how fast it is moving?

Answer 17

By using the measurements of either wavelength or frequency changes.

z=change in lamda/lamda
=change in frequency/frequency
=speed of galaxy/speed of light

Question 18

What is Hubble’s law?

Answer 18

The recession velocity of a galaxy is directly proportional to its distance away from us.

A value for Hubble’s constant considered to be accurate within 5% is 71km(s^-1)M(pc^-1)

Question 19

How does the amount of red shift that a galaxy exhibits, z, allow us to calculate how fast it is moving?

Answer 19

By using the measurements of either wavelength or frequency changes.

z=change in lamda/lamda
=change in frequency/frequency
=speed of galaxy/speed of light

Question 20

What is Hubble’s law?

Answer 20

The recession velocity of a galaxy is directly proportional to its distance away from us.

A value for Hubble’s constant considered to be accurate within 5% is 71km(s^-1)M(pc^-1)

Question 21

How is the age of the universe determined?

Answer 21

As all objects are moving away, we can imagine that they came from a single point.

Mash up S=vt and v=Hs to get 13.7 billion years.

Be careful of units though!

Question 22

On what basis would the expansion of the universe change?

Answer 22

The mass of the universe and the density of matter.

Question 23

How have astronomers come up with the idea of dark matter?

Answer 23

By measuring the rotational speed of the stars in the galaxies, it’s found that the mass suggested by luminosity calculations aren’t enough to create the centripetal force necessary to keep the galaxies spinning.

The mass of the stars are generally only about 10% of that needed.

This means that galaxies must contain a lot of mass that does not emit light.

Although interstellar gasses and black holes are good candidates for dark matter, scientists are convinced that its of another unknown form

Question 24

What are the flat, closed and open universe theories?

Answer 24

flat: expands to a constant size
closed: stops expanding and collapses to a constant size.
open: expands forever

Question 25

How have astronomers come up with the idea of dark matter?

Answer 25

By measuring the rotational speed of the stars in the galaxies, it’s found that the mass suggested by luminosity calculations aren’t enough to create the centripetal force necessary to keep the galaxies spinning.

The mass of the stars are generally only about 10% of that needed.

This means that galaxies must contain a lot of mass that does not emit light.