The following notes were taken primarily from Physics for IB by Chris Hamper and Physics Course Companion by Tim Kirk Are they real? E.1.1 Outline the general structure of the solar system. Students should know that the planets orbit the Sun in ellipses and moons orbit planets. (Details of Kepler’s laws are not required.) Students should also know the names of the planets, their approximate comparative sizes and comparative distances from the Sun, the nature of comets, and the nature and position of the asteroid belt. http://solarsystem.nasa.gov/planets Tons of interesting stuff about our solar system Aug. 2006 the International Astronomical Union declared the official definition of a planet: A “planet” is a celestial body that: a) is in orbit around the Sun b) has sufficient mass for its self-gravity to overcome rigid body forces so that is assumes a hydrostatic equilibrium (nearly round) shape c) has cleared the neighborhood around its orbit. 8 Planets – name them…. Elliptical orbits – to have a circular orbit an object must have a very specific velocity. Any variations create and elliptical or hyperbolic shape Moons Period - 27.3 days Which planets have them???? Asteroids belt between Mars and Jupiter size – dust to hundreds kilometeres. Comets Similar to asteroids but made up of loose particles of ice and rock. Tail is blown off by solar winds and melted by radiation. Some orbit, others only pass the sun once Planetoids??? Pluto…. Relative size video. http://www.wimp.com/starsize/ E.1.3 Define the light year. Light year (ly) – the distance that a beam of light will travel in one year. How far is that? (3 x 108m/s = c) Used to measure distances outside of our solar system Other important units. Astronomical unit (AU) – the average distance between the Sun and Earth 1AU = 1.5 x 1011m Used to measure distances inside our solar system Parsec (pc) – 1parsec = 3.26 ly Defined by making a triangle between the Earth, the Sun and a distant object. If the angle at the distant object is 1 arcsec then it would be 1 parsec away. (more later) E.1.2 Distinguish between a stellar cluster and a constellation E.1.4 Compare the relative distances between stars within a galaxy and between galaxies, in terms of order of magnitude. Stars are not evenly distributed. Stellar cluster – small groups of stars that gravitationally interact with one another. Physically close to each other Closest star, besides the sun is Proxima Centauri - 4.25ly Galaxy – a very large number of stars bound together by gravity Trillions of stars 103 – 105 light years across Each star is approx. 1 ly apart Andromeda is about 2.5x106ly away Galaxy cluster – small group of galaxies that gravitationally interact with one another There are about 20 other galaxies we are clustered with. Supercluster – bigger than a cluster The simplest explanation is that if all the gas is made into stars before the gas has time to form a disk, then you get an elliptical galaxy. if the gas has time to stabalize into a disk before it is all used up, then you get a spiral galaxy. Or perhaps some of the elliptical galaxies are made from merging of other types of galaxies. Observations of distant galaxies indicates that spiral galaxies were more common in the past than they are today. So maybe yesterday's spirals are todays ellipticals. This is an active research area. One problem is that if most of the mass in galaxies is unaccounted for, we have a hard time understanding the dynamics of galaxy formation. Constellations – groups of stars that are “linked” visually Ancient civilizations played “connect the dots” Located in the same general direction from Earth Not necessarily close to each other 88 total Different ones are visible at different times during the year. E.1.5 Describe the apparent motion of the stars/constellations over a period of a night and over a period of a year, and explain these observations in terms of the rotation and revolution of the Earth. This is the basic background for stellar parallax. Other observations, for example, seasons and the motion of planets, are not expected Why do the stars move through the night sky? Because the rotation of the Earth It also matters where you are located on Earth Ex. North Pole http://www.yorku.ca/ns1745b/figs-ch1.html This rotation takes 23h and 56min every time. The effect is that it seams that the stars position at 12:00 changes each night. This means that the Earth rotates 360º in 23h and 56min. Which means 4min it will rotate 1º. Which means it only takes 360 DAYS for the constellations to make one compete rotation. The Sun doesn’t make the same path through sky every day. For us, the summers are high in the sky, winters are low on the horizon. This is because the axis of rotation for the Earth and the axis in which we orbit around the sun aren’t the same angle. The Earth is not a perfect sphere. This means that depending on it’s location in it’s orbit, it will feel more or less pull from the Sun This pulls on the Earths axis of rotation and makes it wooble. This is technically called presession. This means that the “North Star” won’t always be the north star. Period – 26,000 years The word planet comes from the Greek word for wanderer. Planets will shift back and forth in the night sky relative to the constellation background. Apparent east/west motion comes from the Earth’s orbit around the sun. See Diagram on board Apparent north/south motion come from the other planet’s orbital plane being at a different angle from ours.