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

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

Can you grasp the scale?

Let’s draw THE SOLAR SYSTEM TO SCALE!

Planets are to scale, distances are not!

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)

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

Venus Transit 8th June 2004 Next transit – 6th June 2012 Then – December 2117!

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

Are there planets outside of our solar system? Yes

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 25 000 light years Sun's galactic rotation period 250 million years

Our closest galaxy - Andromeda

New Units Astronomical Units (AU) The mean distance from the centre of the Earth to the centre of the Sun 1 AU = 1.496 x 1011m Why does it have to be the mean distance? Earth’s orbit is elliptical. Its distance from the Sun varies from 1.471 x 1011m to 1.52 x 1011m

Light Years (ly) The distance light will travel through a vacuum in one year Earth year = 365.25 days c = 2.9979x108 ms-1 1 ly = 9.461 x 1015 m

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

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

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 = 3.086 x 1016 m 1AU

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

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 = 1 = 2.5pc 0.4 So 7.72x1016m and 8.15ly

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

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.

Olbers’ paradox continued: The brightness of a star is inversely proportional to its distance squared. Brightness at the Earth due to shell A = k4πr2tn/r2 = k4πtn where k is a constant. Brightness at the Earth due to shell B = k16πr2tn/(2r)2 = 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.

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”

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.