Physics Observing The Universe revision
Movement of celestial bodies • The sun appears to travel east-west across the sky once every 24hours.
Sidereal Day • The star appear to move across the sky in a slightly shorter time(23h 56min). This is called a sidereal day. Sidereal day is rotation to face original direction. A solar day the rotation goes all the way back to face the Sun.
The moon • The moon appears to travel east to west across the sky once every 24hrs 49mins. One complete cycle takes 28days. • Why does the Moon take longer to cross the sky than the Sun? • Because it orbits the Earth in the same direction as the Earth rotates. So by the time the Earth rotates enough for a static object to have gotten all the way to the opposite horizon, the Moon hasn't quite gotten there yet because it was moving with the Earth's rotation a little.
Retrograde motion • At certain points in the orbits of planets, other planets (for example mars) appear to move backwards or from west to east across the sky. This is because earth is moving faster than that planet and so overtakes it.
Eclipses • Eclipses do not happen very often as the Sun and moon do not align very often. • The moon’s orbit is tilted relative to the plane of the Earth’s orbit. • Usually Earth, Sun and the Moon are not in line so no eclipse occurs.
S Pegasus Orion Scorpius W E Autumn equinox
W Pegasus Orion S Leo E Winter solstice
E W Scorpius Orion Spring equinox Leo S
E Pegasus Scorpius S Leo W Summer solstice
Focusing parallel light • Stars are so far away that light arriving from them is parallel.
Power of a lens • Calculate the power of a lens: • Power (dioptre)= 1/focal length (m) The more powerful a convex lens, the more curved the surface.
Forming an image of an extended object Real image F F Object Ray 1. Arrives parallel to the Principal Axis – then passes through F. Ray 2. Passes through the optical centre – undeviated. Ray 3. Passes through F first – then emerges parallel to the Principal Axis. Any other rays will be refracted to pass through the same image point. Note that the top of the image is now below the Principal Axis.
Magnification= Focal length of the objective lens Focal length of the eyepiece lens Remember- the more powerful a lens the shorter the focal length. Simple telescopes are made of two converging lenses of different powers. The more powerful lens acts as the eyepiece.
Concave mirrors Modern telescopes have very large mirrors to: Collect light/radiation Produce a more defined/brighter/sharper image See faint sources Reduce diffraction Most astronomical telescopes have concave mirrors, not convex lenses as their objectives.
What are the objects we see in the night sky and how far away the are?
Parallax • The parallax angle is half the angle moved in a 6 month period. α
Parallax • Parallax makes some stars seem to move relative to others over the course of a year. • The smaller the parallax angle is the further away a star is.
Parsecs • A parsec (pc) is the distance to a star with a parallax angle of one arc second.
ArcSeconds An hour can be broken into 60 divisions called minutes A degree can be broken into 60 divisions called minutes. They are written as ‘ eg 20’ Each minute can be broken into divisions called seconds. They are written ‘’ eg 20’’ Each minute can be broken into 60 divisions called seconds So as a fraction of a degree, 1 second is 1/3600 of a degree meaning there are 3600 seconds in a degree. So as a fraction of an hour, 1 second is 1/3600 of an hour meaning there are 3600 seconds in an hour.
Star distance • A parsec is similar in magnitude to a light-year • 1 parsec = 3.1 x 1013 km • 1 light-year = 9.5 x 1012 km • Typical interstellar distances are a few parsecs.
Luminosity • Luminosity (intrinsic brightness) of a star depends on its temperature and its size. • Temperature: a hotter star radiates more energy every second from each square metre of its surface. • Size: a bigger star has more surface that radiates energy. • Observed Brightness- depends on the stars distance from the earth, as well as the stars luminosity. Dust or gas between Earth and the star may absorb some of its light.
Cepheid Variables Measuring the distance to a star in a distant galaxy: Look for a cepheid variable in the galaxy of interest. Measure its observed brightness and its period of variation. From the period, determine luminosity. Knowing both the luminosity and the intensity of its light at the telescope, calculate the distance of the star. • These are stars that pulse in brightness. They have a period related to their brightness.
The great debate Shapley Curtis He challenged Shapley’s claim about the universe. Curtis was studying ‘spiral nebulae’ rather than globular clusters. He felt that they were distant objects- galaxies on their own. Was proved correct by Hubble’s discovery of the Andromeda galaxy. • Measured distance to nebulae. Observed they form a spherical cloud with a centre far from the solar system. • Guessed the nebula was a cluster of stars and they formed a sphere around the Milky Way galaxy. (Globular clusters) • He claimed milky way was the entire galaxy.
Hubble Hubble used the data from cepheids to determine the distances to galaxies. He discovered that all galaxies appeared to be moving away from us. There spectrum has been redshifted. The more distant the galaxy the faster the rate of recession.
Hubble’s Constant Speed of recession = Hubble Constant X distance The first time Hubble estimated the constant he found it to be 500km/s. With more reliable data from the HST the current excepted value is 72 ± 8 km/s-1Mpc-1
Moving Galaxies • The fact that galaxies are moving lead to two important ideas: • The universe itself may be expanding, and may have been much smaller in the past. • The universe may have started by exploding outwards from a single point- the big bang.
flat Universe closed Universe open Universe
Alpha scattering-gold foil experiment • Start with a metal foil. Use gold, because it can be rolled out very thin- thickness of a few atoms. • Direct the source of alpha radiation at the gold foil. Do this in a vacuum as alpha is easily absorbed. • Watch for flashes of light as alpha particles strike the detecting material around the outside of the chamber. • Count the flashes at different angles, to see how much the alpha radiation is deflected.
Interpretation of results • Most alpha particles passed straight through the gold foil, deflected by no more than a few degrees. • A small fraction of the alpha particles were actually reflected back towards the direction from which they had come. • Must be something positive repelling the alpha particles.
What are stars? All hot objects(including stars) emit a continuous range of electromagnetic radiation, whose luminosity and peak frequency increases with temperature.
+ Energy levels A hydrogen atom has: 1 proton in the nucleus 1 electron (in the first shell) The atom does have other shells too … but they are all empty … most of the time. Why does the electron normally occupy the innermost shell? The innermost shell has the lowest energy. The electron drops down through the shells, losing energy as it goes, until it has the lowest possible energy. Each shell represents a specific level of electron energy. In Physics we refer to the shells as ENERGY LEVELS.
-0.9 -1.5 + -3.4 -13.6 Ground state I’m very excited! Taking a closer look at the first 4 energy levels.. • An electron can absorb energy and jump to a higher energy level. This is called EXCITATION. The electron is excited! • But then it will fall back down to a lower energy level, giving out energy as it does so. • The electron can return to the Ground state in a number of ways. How many? I’m excited now! Excitation and de-excitation I’m even more excited!
-0.9 -1.5 -3.4 -13.6 Ground state 4 3 • Each jump between energy levels produces an amount of energy determined by the difference in energies between the 2 levels. • Level 4 to 1 Energy change = -0.9 - (-13.6) = 12.7 units • Level 4 to 3 Energy change = -0.9 - (-1.5) = 0.6 units • Level 3 to 1 Energy change = -1.5 - (-13.6) = 12.1 units In each case the energy is emitted as photons of light. 2 1
Types of spectrum Most very hot objects will emit a continuous spectrum. Hot gases emit only those colours which correspond to the energy released by de-excitation. A line EMISSION spectrum But a cold gas would absorb exactly the same colours because they have just the right energy to jump up to higher energy levels (excitation). A line ABSORPTION spectrum
Comparing the 3 types of spectrum Note that, for a given gas, the emission and absorption spectra are reverse versions each other.
But something odd was noticed in the spectra of distant stars The whole spectrum seems to be shifted towards the red end: a RED SHIFT. Why?