PHYS Limitations of the Copernican Model a) No better than Ptolemy’s model for predictions b) More complex use of circles than Ptolemy c) Still no physical forces involved; used idea of natural uniform motion Tycho Brahe ( ) & Johannes Kepler ( ) Brahe made very accurate observations to one minute of arc (1’ - there are 60 minutes in a degree and 360 degrees in a full circle) Kepler was a mathematician who used Brahe’s data to develop a heliocentric model. He used the concept of a force between the Sun and planets but did not quantify it.

PHYS Kepler’s Model a) Heliocentric b) Planets move in ELLIPSES not circles, with the Sun at one focus Figs. Z3.15 & K2-20 c) The speed of a planet varies during its orbit - Law of Equal Areas Figs. Z3.16 & K2-23 d) P 2 is proportional to a 3 - if we know a planet’s orbital period (P) we can predict its average distance from the Sun (a) Proposed a “magnetic” force of attraction between Sun and planets but did not investigate its form. Now we know it as gravity. Newton’s contribution: Universal Law of Gravitation Gravitational force F acts on masses m and M separated by a distance r.

PHYS Giordano Bruno and Life In 1584 Bruno proposed that the stars were distant suns scattered throughout an infinite Universe, and that around these suns circled planets. Not based on observations; this was a philosophy. He said that the Earth was not unique and that the planets around other stars may have intelligent life upon them. Denounced by the Church as a heretic, he was imprisoned by the Inquisition in Venice for 8 years and burnt alive on February 17th A.D

PHYS Astrophysics - an Observational Science We can observe but not alter distant systems. Stars, planets, gas clouds etc. emit electromagnetic radiation: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, gamma rays. Figs. Z5.7 & K4-3 Only visible light and some radio waves pass easily through Earth’s atmosphere. We must analyse all characteristics of the e.m. radiation we receive to discover how the Universe works. To first find out how e.m. radiation is emitted, we must look at the structure of atoms. Zeilik Ch. 5, Kuhn & Koupelis Ch. 4 short wavelength, high frequency long wavelength, low frequency

PHYS Matter is made up of atoms Atoms are made up of: protons (p + ), positive electrostatic charge neutrons (n), no electrostatic charge electrons (e - ), negative electrostatic charge Atoms are electrically neutral i.e. number of protons = number of electrons p + and n are about 2000 times heavier than e - { By the way… Like charges repel, unlike attract Force between charges q and Q is Compare this with the expression for gravitational attraction both forces depend on distance squared r  r }

PHYS Elements contain one type of atom The atoms of different elements contain different numbers of p + and hence e - The atoms of different isotopes of an element contain the same number of p + but a different number of neutrons (n). Hydrogen (H) 1p and 1e Deuterium 1p and 1e and 1n (isotope of H) Helium 3 - 2p and 2e and 1n - unstable Helium 4 - 2p and 2e and 2n -stable Lithium 6 - 3p and 3e and 3n - stable Hydrogen, helium and a trace of lithium made in first 5 minutes of the hot Big Bang - most of the other 89 stable elements must have been produced later. e e e e e e p p pp p p n n nn

PHYS Ions and Molecules Ions are atoms that have had one or more electrons ripped off by: a) a light ray (photon, packet of e.m. radiation) b) a collision with another atom or electron The atom is then said to be ionised. Some astronomical shorthand Hydrogen H H I Region H + H II Region 1e - lost Molecules consist of two or more atoms bonded together by sharing some of their electrons e.g. molecular hydrogen H-H or H 2 and water H 2 O O H H pp e e

PHYS Quantum Theory of the Atom a) electrons orbit the nucleus only at fixed distances Figs. Z5.9 & K4-13 b) electrons can be “kicked out” of a nearer orbit and forced into an outer orbit by a light beam or by atoms colliding c) e.m. radiation is emitted when an electron moves from an outer orbit to a nearer orbit d) e.m. radiation, including visible light, has a characteristic frequency which depends upon the energy lost by the electron e) energy lost (E) = frequency (f ) times a constant called Planck’s Constant (h)