The Rosette Nebula At its center lies an open cluster of bright young stars that formed about four million years ago from the nebular material. Winds from these stars are clearing a hole in the nebula's center. Ultraviolet light from the hot cluster stars causes the nebula to glow. The Rosette Nebula spans about 100 light-years across and lies about 5000 light-years away.

Homework #3 is due tomorrow, 5:00 pm. Exam 1: Wednesday, Feb. 16 Review session: 7:00-9:00 pm, Tuesday Feb. 15 Swain West 007

The “Science” of Astronomy

In the broadest sense, Science refers to any system of knowledge which attempts to model objective reality.

In the broadest sense, Science refers to any system of knowledge which attempts to model objective reality. a. Uninfluenced by emotions or personal prejudices. b. Based on observable phenomena; presented factually

“Scientific Thinking” a natural part of human behavior. We draw conclusions based on our experiences. Progress is made through “trial and error.”

Theory The Nature of Science Science seeks explanations for observed phenomena that rely solely on natural causes. Science progresses through the creation and testing of models of nature that explain the observations as simply as possible. A scientific model must make testable predictions that could force us to revise or abandon the model. Theory -- a model which survives repeated testing

The word “Theory” has a very different meaning in science than in everyday parlance

A model is only referred to as a theory when it has successfully survived testing to such an extent that it is generally accepted as a valid representation of nature. Even then, testing continues…

Theories Must be Testable Must be continuously tested Should be simple

science, nonscience & pseudoscience “Testable” - science, nonscience & pseudoscience Testing separates science from nonscience: e.g., religion is not accessible to experimental verification Testing invalidates pseudosciences: e.g., astrology fails testing.

Testing of the “Two World” Models: Geocentric vs Heliocentric

Contemporary with Kepler was the “founder of experimental science” Galileo Galilei (1564-1642), the “founder of experimental science” First person known to point a telescope at the sky He wanted to connect the physics understood on earth with objects in the heaven His work got him in trouble with the Church and led to his house arrest for many years.

(Some of) Galileo’s Observations Galileo saw craters and shadows cast by the mountains on the Moon (Moon had a landscape; it was a “place”, not a perfect heavenly body)

(Some of) Galileo’s Observations Galileo saw craters and shadows cast by the mountains on the Moon (Moon had a landscape; it was a “place”, not a perfect heavenly body) Sunspots (sun not “perfect”)

(Some of) Galileo’s Observations Galileo saw craters and shadows cast by the mountains on the Moon (Moon had a landscape; it was a “place”, not a perfect heavenly body) Sunspots (sun not “perfect”) Rotation of sun

(Some of) Galileo’s Observations Galileo saw craters and shadows cast by the mountains on the Moon (Moon had a landscape; it was a “place”, not a perfect heavenly body) Sunspots (sun not “perfect”) Rotation of sun Moons of Jupiter (Heavenly bodies existed which did not orbit the earth)

(Some of) Galileo’s Observations Galileo saw craters and shadows cast by the mountains on the Moon (Moon had a landscape; it was a “place”, not a perfect heavenly body) Sunspots (sun not “perfect”) Rotation of sun Moons of Jupiter (Heavenly bodies existed which did not orbit the earth) Phases of Venus: the two models of the Universe made two very different predictions.

Phases of Venus

Galileo observed all phases! Galileo’s observation of the phases of Venus was the final evidence that buried the geocentric model. Geocentric Heliocentric No gibbous or full phases! All phases are seen! Galileo observed all phases!

The structure of the universe had been totally changed. With Galileo’s observations, the revolution begun by Copernicus was nearly complete… The structure of the universe had been totally changed. The motions of the planets were understood, at least from a geometrical perspective. Earth was no longer a “special” place in the universe. The crowning achievement was yet to come - discovering the laws of nature and that naturally led to the newly determined structure.

Sir Isaac Newton (1642-1727) Invented calculus to understand and predict motion and forces Invented the reflecting telescope Connected gravity and planetary forces: explained and modified Kepler’s Laws

Universal Law of Gravitation Between every two objects there is an attractive force, the magnitude of which is directly proportional to the mass of each object and inversely proportional to the square of the distance between the centers of the objects. G: the Universal Gravitational Constant = 6.674 × 10-11 m3 kg-1 s-2) .

Orbital Paths from Law of Gravitation Extending Kepler’s Law #1, Newton found that ellipses were not the only orbital paths. All orbits are “conic sections” , i.e., the intersection of a plane and the surface of a cone ellipse (bound) parabola (unbound) hyperbola (unbound) Orbital motion takes place around the center of mass

The Center of Mass In Kepler's Laws, the Sun is fixed at a point in space (a focus of an ellipse) and the planet revolves around it. Why is the Sun privileged? Kepler had mystical ideas about the Sun, endowing it with almost god-like qualities that justified its special place. Newton demonstrated that the the Sun does not occupy a privileged postion and in the process he modified Kepler's 3rd Law.

The center of mass is familiar to anyone who has ever played on a see-saw. The fulcrum point at which the see-saw will exactly balance two people sitting on either end is the center of mass for the two persons. m1d1 = m2d2

Recall Kepler’s 3rd law: P2 / a3 = constant Newton realized that in the planet-Sun system the planet does not orbit around a stationary Sun (a planet exerts as much gravitational force on the Sun as the Sun does on a planet). Instead, Newton proposed that both the planet and the Sun orbited around the common center of mass for the planet-Sun system. This led Newton to modify Kepler's 3rd Law. Fg = Gm1m2/d2 Recall Kepler’s 3rd law: P2 / a3 = constant

Newton’s Version of Kepler’s Third Law P2 = 42 a3 / G (m1 + m2) If you can measure the orbital period of two objects (P) and the distance between them (a), then you can calculate the sum of the masses of both objects (m1 + m2).

We will return to Newton and discuss his “Laws of Motion” after we learn some simple background physics