1. 1) What is our galaxy called? Milky Way
2) How many stars are there in our milky way? 100,000 million
3) What is a light year? Distance light travels in one year
4) How many light years is our galaxy from one side to the other? 100,000
5) How many galaxies are there in the universe? ~100 million
6) How far away is the furthest galaxy? 13,000 million light years

2. MODELLING THE UNIVERSE The Structure of the Universe
07/08/2018
LO:
Be able to describe the principle components of the Universe
Be able to describe the structure of the Solar System
Be able to define distances measured in AU, pc and ly
Be able to state the approximate values of these units in metres in standard form

3. Can you grasp the scale?

4. Let’s draw THE SOLAR SYSTEM TO SCALE!

5. Planets are to scale, distances are not!

6. Distance from the Sun (km)
Diameter km
Distance from the Sun (km)
Sun
1,400,000
Mercury
4,800
58,000,000
(58x106)
Venus
13,000
108,000,000
(108x106)
Earth
149,000,000
(149x106)
Mars
7,000
228,000,000
(228x106)
Jupiter
144,000
778,000,000
(778x106)
Saturn
121,000
1,430,000,000
(1430x106)
Uranus
53,000
2,870,000,000
(2870x106)
Neptune
50,000
4,500,000,000
(4500x106)
Pluto
2,300
5,900,000,000
(5900x106)
Nearest Star
40,000,000,000,000
(40x1012)
Edge of Milky Way
1,000,000,000,000,000,000
(1x1018)
Nearest Galaxy
20,000,000,000,000,000,000
(20x1018)
Farthest observable object
50,000,000,000,000,000,000,000
(50x1021)

7. What are we looking at? How do they occur?
Next mainland UK full solar eclipse – 23rd September 2090
Next UK full lunar eclipse – 25th May 2013

8. Venus Transit 8th June 2004
Next transit – th June 2012
Then – December 2117!

9. How was our moon formed?
Planetary satellite – A body orbiting a planet E.g. a Moon

10. Are there planets outside of our solar system?
Yes

16. Some Milky Way facts... Diameter 100 000 light years Thickness
Number of stars
100–400 billion
Oldest known star
13.2 billion years
Mass
5.8 × 1011 M☉
Sun's distance to galactic center
light years
Sun's galactic rotation period
250 million years

17. Our closest galaxy - Andromeda

18. New Units Astronomical Units (AU)
The mean distance from the centre of the Earth to the centre of the Sun
1 AU = x 1011m
Why does it have to be the mean distance?
Earth’s orbit is elliptical. Its distance from the Sun varies from x 1011m to 1.52 x 1011m

19. Light Years (ly)
The distance light will travel through a vacuum in one year
Earth year = days
c = x108 ms-1
1 ly = x 1015 m

20. Parsecs (pc)
This unit is based on a method of triangulation and relies on taking measurements of angles from the Earth’s surface to where the star appears in the sky. a b A B c

23. In radians: θ = Arc Length/Radius For very small angles: θ = 2AU / x
x = 2AU / θ θ x
Sun
2AU

24. 1pc = 2.063x105 AU 1pc = 3.086 x 1016 m The Parsec:
You need to be able to describe this diagram!
The Parsec:
One parsec is the distance from a base length of 1AU when the angle is one second of arc (1/3600o or 4.848x10-6 Rad).
Now θ = 1AU/1pc
What is 1pc in AU (use θ in Radians)?
What is 1pc in metres? θ
1 parsec
1pc = 2.063x105 AU
1pc = x 1016 m
1AU

25. From θ = 1AU/1pc Parallax = 1 Distance You need to remember this!
(in Seconds of an arc)
(in Parsecs)
You need to remember this!

26. So a parsec is the distance that gives a stellar parallax of 1 second of an arc
A distant star has a parallax of 0.4 seconds of arc. Calculate the distance from Earth to Tau Ceti in Parsecs, metres and light years.
Distance in pc = = 2.5pc
0.4
So 7.72x1016m and 8.15ly

27. What is the parallax of a star whose distance is 25 parsecs?
Parallax = 1/d = 1/25 = 0.04 seconds of an arc.

28. Early scientists thought the universe must be infinite or else it would collapse due to gravitational forces.
Olbers’ paradox:
The universe is infinite
Stars spread approx evenly so ‘n’ stars per unit volume
For a thin shell the volume is 4πr2t (surface area x thickness). This contains 4πr2tn stars.
For another thin shell twice the distance away the volume is 4π(2r)2t. This contains 16πr2tn stars.

29. Olbers’ paradox continued:
The brightness of a star is inversely proportional to its distance squared.
Brightness at the Earth due to shell A = k4πr2tn/r = k4πtn where k is a constant.
Brightness at the Earth due to shell B = k16πr2tn/(2r) = k4πtn.
So every shell, regardless of distances produces the same brightness at Earth.
There are infinite shells therefore Earth is infinitely bright because of starlight. Obviously not true so the universe cannot be infinite.

30. Olbers’ Paradox in words:
“With an infinite number of stars in an infinite universe, it does not matter which direction you look in – you will always see a star along the line of sight. Therefore, the night sky will be as bright as the day sky”

31. Homework: Due Mon 28th Mar
1) Learn the three main values and units of astronomical distances.
2) Research a planet. Bring 5 key facts to next lesson.