Astronomy Picture of the Day
Review: Kepler's Laws 1. Planets travel aound the sun in elliptical orbits with the sun at one focus of the ellipse. 2. A line connecting the Sun and a planet sweeps out equal areas in equal times. 3. The square of a planet's orbital period is proportional to the cube of its semi-major axis. (P 2 α a 3 )
Review: Newton's Laws of Motion 1. Every object continues in a state of rest or a state of uniform motion in a straight line unless acted on by a force. (Inertia) 2. F = m*a 3. For every action there is an equal and opposite reaction. Newton's Law of Gravity : F = G m 1 m 2 R 2
Orbit of Earth around Sun
Gravity and Orbits Throwing an object fast enough will put the object into orbit. (Neglecting air resistance) Moon is continually “falling” towards the Earth in its orbit (Gravity vs. inertia)
Correction to Kepler’s Third Law Earth and sun actually rotate about their common center of mass Corresponds to a point inside sun Used to detect extrasolar planets
Question The force of gravity between two objects decreases rapidly with the: A)product of the two masses. B)elipticity of the orbit. C) separation distance. D)orbital velocity.
Question The acceleration experienced by a falling object due soley to Earth's gravity (free fall) _____ the mass of the object. A)increases with B) decreases with C) is independent of
That one was complicated, because we need to remember: F g = G m m E R 2 AND a = FgmFgm So, a = G m m E m R 2 = G m E R 2
UNITS!!! Units are IMPORTANT in Astronomy (and science in general) – without the proper units a calculation is meaningless. Two Systems of Units: Metric English gram ounce meter yard degree Celsius degree Farenheight kilogram pound
Chapter 2: EM Waves What is a wave? What are the main properties of waves? What two things do all waves transport?
Waves are a type of disturbance that can propagate or travel. Waves carry information and energy. Properties of a wave wavelength ( ) crest amplitude (A) velocity (v) trough is a distance, so its units are m, cm, or mm, etc. Period (T): time between crest (or trough) passages Frequency (f): rate of passage of crests (or troughs), f v = f 1T1T (units: Hertz or cycles/sec) Equilibrium position
Waves bend when they pass through material of different densities. swimming pool air water prism air glass Refraction
All radiation (including visible light) travels as Electromagnetic waves. That is, waves of electric and magnetic fields travelling together. What are some examples of objects with magnetic fields: What are some examples of objects with electric fields:
All radiation (including visible light) travels as Electromagnetic waves. That is, waves of electric and magnetic fields travelling together. What are some examples of objects with magnetic fields: a bar magnet the Earth the Sun What are some examples of objects with electric fields: } "charged" particles that make up atoms. Anything with too many or to few electrons! Electrical appliances Lightning Protons electrons
Electric Force - opposites attract, likes repel Oscillating charges radiate All objects have temperatures greater than absolute zero - random thermal motion All objects radiate! Why don’t we see the radiation coming from many ordinary objects? Dog whistle analogy
Electromagnetic Radiation (How we get information about the cosmos) What are some examples of electromagnetic radiation and what are the different ways in which we might experience them or make use of them?
Electromagnetic Radiation (How we get information about the cosmos) What are some examples of electromagnetic radiation and what are the different ways in which we might experience them or make use of them? Light(see) Infrared(heat) Ultraviolet(sunburn) Microwaves(cooking, communication) AM radio(communication) FM radio(communication) TV signals(communication) Cell phone signals(communication) X-rays(medical applications)
ff c = 1 nm = m, 1 Angstrom = m The Electromagnetic Spectrum
Spectroscopy and Atoms How do you make a spectrum?
When you bend light, bending angle depends on wavelength, or color. Refraction of light (Prism demo)
What does the spectrum of an astronomical object's radiation look like? Many objects (e.g. stars) have roughly a "Black-body" spectrum: Brightness Frequency also known as the Planck spectrum or Planck curve.
Questions How is temperature related to the amount of energy radiated? How is temperature related to the color of the object? (Blackbody Demo)
"cold" dust "hot" stars "cool" star Sun frequency increases, wavelength decreases The wavelength of peak emission tells us the temperature of the object!
Types of Spectra 1. "Continuous" spectrum - a luminous solid or liquid, or a sufficiently dense gas, emits light of all wavelengths and produces a continuous spectrum 2. "Emission" spectrum - a low-density, hot gas emits light whose spectrum consists of a series of bright emission lines that are characteristic of the composition of the gas. 1."Absorption” Spectrum - a cool, thin gas absorbs certain wavelengths from a continuous spectrum, leaving dark absorption lines in their place, superimposed on the continuous spectrum.
Pattern of lines is a fingerprint of the element
For a given element, emission and absorption lines occur at the same wavelengths. Helium discovered in Sun’s spectrum before being found on Earth! Sodium emission and absorption spectra
The Particle Nature of Light Light interacts with matter as individual packets of energy, called photons. c photon energy is proportional to frequency: E f (or E 1 1 example: ultraviolet photons are more harmful than visible photons.
The Nature of Atoms The Bohr model of the Hydrogen atom: _ + proton electron "ground state" _ + an "excited state" Text Website Video Clips
When an atom absorbs a photon, it moves to a higher energy state briefly When it jumps back to lower energy state, it emits photon(s) in a random direction, conserving the total energy of the system! (video clips)
Other elements Helium Carbon neutron proton Each element has its own allowed energy levels yielding a unique spectral fingerprint.