IB Physics: Option E1 – Introduction to the Universe Introduction to the Universe Outline the general structure of the solar system. The solar system consists of the sun, the eight planets, dwarf planets (E.g. Pluto) and their respective moons. This may come as a surprise, butwe live on planet Earth. Or at least I hope so. Description of planets Name ofplanet Approximateradius/km Approximate distance to the Sun/kmx10^8 Mercury 2440 58
Venus 6050 107.5 Earth 6400 150 Mars 3400 228 Jupiter 71400 778 Saturn 60000 1427 Uranus 25900 2870 Neptune 24750 4497 The planets orbit the sun elliptically – this is different from a circular orbit in that the distance between the orbiting planet and the sun is notalways constant. Below is an example of this happening in action:
As the distance is not always constant, the potential energy isconstantly changing. As the planet moves further away from the sun,the PE (Potential Energy) increases. As the PE increases, naturally,because of ‘Conservation of Energy’, KE (Kinetic Energy) decreases, andas KE = 1/2 mv2 (see unit 2 for more detailed explanation), velocity willdecrease. Therefore, we can conclude that the further the orbiting planet from the sun, the slower the planet orbits the sun. This is also supported byKepler’s laws, but these laws are not covered in this unit. Another object we should be familiar with is the moon – many planets have smaller planets which orbit the planet, which we call moons. Some other terms you’ll want to be comfortable with : Asteroid These are small rocky bodies which orbit the sun between the planesMars and Jupiter – we call this area, the Asteroid Belt.
Comet This is a celestial object consisting of dust and ice, which orbits aroundthe sun with its “tails” pointing away from the sun. Stars This is deﬁnitely a term that you might be familiar with – however, wemust give a more scientiﬁc treatment to the deﬁnition of a star. A star is a massive sphere of plasma held together by gravity – plasma basically means gas where the particles are traveling at such highspeeds that the electrons are all pulled out. Galaxy A galaxy is a large collection of stars which are held intact by gravity –the galaxy we live in is called the Milky Way. Below is an image of the milky way – prepare to be amazed.
Distinguish between a stellar cluster and a constellation. If you’ve done any past-papers before, you’ll realize that this question comes up surprisingly often. Stellar Cluster is a group of stars which are bound together by a gravitational force. Below is an example of a cluster Above is an example of the beehive cluster, and this was ﬁrst observedby Galileo in 1609. Constellation – A group of stars which forms a pattern named after an apparent form or a mythological ﬁgure. These, unlike, stellar clusters, donot need to be gravitationally bounded together. Below is an example of a constellation
Above is an image of “Emu in the sky”, a constellation deﬁned by darkclouds. What does this look like to you? Deﬁne the light year. The light year is a measure of distance, not time, despite the word “year”. For those who refer light year as a period of duration (time),that’s wrong. Light year is the distance light travels in a vacuum in theperiod of a year. As we know light travels at 300,000 km/ second,common sense tells us that light travels quite a distance in one year. A light year is equivalent to approximately 10 trillion kilometers. We use light year to measure distances for objects/places that areextremely far away – for example, it will take 4.3 light years to travelfrom earth to the nearest star, Proxima Centauri. Basically, this shows that the distance between earth and Proxima centauri is 4.3 x 10 trillion = 43 trillion kilometers. That’s quite a distance. Two more units you might consider learning are Astronomical unit (Au)and Parsec (pc). Here are the approximate conversions: 1 AU = 1.5 x 10 11 m 1 pc = 3.26 light years (This is basically light years on steroids)
All in all, just remember that light year is a measure of distance, and nottime.