Introduction to Astrophysics

Aristotle, Kepler, Brahe, Newton and the other guys

Bibliography

The History of Modern Astronomy http://csep10.phys.utk.edu/astr161/

Cavendish Experiment http://en.wikipedia.org/wiki/Cavendish_experiment

Brief History of the Universe http://filer.case.edu/~sjr16/advanced/cosmos_history.html

Electroweak Theory http://www.answers.com/topic/electroweak-force

Newton’s Law of Universal Gravitation http://www.slideshare.net/tufdaawg/universal-gravitation, www.nd.edu/~dhoward1/30389s06%20Fox%20Gravitation.ppt, bohr.winthrop.edu/faculty/mahes/link_to_webpages/courses/phys201/lecture4.2.ppt

Newton’s Secret http://www.pbs.org/wgbh/nova/elegant/media2/3012_q_03.html

What we know now

Four types of forces Sun-centred solar system Many other solar systems, galaxies,

nebulae Big Bang theory Theories of special and general relativity

But how did we get here?

 Types of Forces:

Fundamental forces:•Gravitational force•Strong nuclear force•Weak nuclear force-----|•Electromagnetic force--|—Electroweak force

Non-fundamental forces: Pushing, Pulling, Friction, Tension, etc….

These are related to the electromagnetic force. They arise from the interactions between the electrically charged particles that comprise atoms and molecules.

Fundamental Forces

Fundamental Force Example Particles Affected

Relative Strength

Strong nuclear Nuclear fission & fusion

Nuclear 1

Electromagnetic Static Electricity, chemical reactions, friction

Charged 10-2

Weak nuclear Radioactivity Nuclear 10-15

Gravitational Your weight All 10-38

Unification of Fundamental Forces (?)http://hyperphysics.phy-astr.gsu.edu/hbase/astro/imgast/smbrk.gif

Motion in the Heavens and Motion in the Heavens and on Earthon Earth

We know how objects move on Earth. We can describe and even calculate projectile motion.

Early humans could not do that, but they did notice that the motions of stars were quite different. Stars moved in regular paths. Planets moved in more complicated paths.

Many ancient cultures had explanations for their observations about the sky, often combining their observations with religious rites and ideas.

The Celestial Sphere

Pre 1500: Succession of giant spheres with all bodies in sky stuck on them

The heavens are “perfect” The Earth is at the centre of the universe Aristotle & Ptolemy:

http://csep10.phys.utk.edu/astr161/lect/retrograde/aristotle.html

Problems: Planets have varying brightness and retrograde motion

Tycho BraheAge 14

•observed an eclipse of the sun•August 21, 1560.

The date of the event was off by two days as predicted in all of the books of the time, so Brahe decided to become an astronomer to make accurate observations and predictions.

Became royal astronomer of Denmark Given the island of Hven (now Swedish) for

observatory, Uraniborg Lost nose in a duel, wore gold or silver

prosthesis Designed extremely accurate astronomical

instruments Made incredibly accurate observations, later

used to formulate several important theories

Brahe’s Interesting life story…

Brahe’s story (cont’d)

In 1597, after falling out of favor with his sponsor, Brahe moved to Prague.

Became the astronomer to the court of Emperor Rudolph of Bohemia.

Johannes Kepler became one of his assistants.

New ideas: Copernicus’ sun-centred universe (!)

Scientific community and public divided

Brahe: Aristotelian, geocentric (Earth-centred) model

Kepler: interested in Copernican, heliocentric model

Brahe and Kepler

Brahe’s conclusions

Brahe made excellent observations of parallax for stars, and could find none. He

thus concluded that either “the earth was motionless at the center of the Universe, or the stars were so far away that their parallax was too small to measure.”

Brahe’s conclusions

Proposed intermediate model of solar system, between the Ptolemaic and Copernican models (geocentric).

Was widely accepted for a time, although ultimately proved incorrect

Thus, Brahe's ideas about his data were not always correct, but the quality of the observations themselves was central to the development of modern astronomy.

Kepler was convinced that geometry and mathematics could be used to explain the number, distance, and motion of the planets.

He used Brahe’s data to formulate his three laws, which apply to ALL planets, satellites, heavenly bodies and are used to this day.

Kepler’s laws

1. The paths of the planets are ellipses with the sun at one focus.

Kepler’s laws of planetary motion

This was a very big deal at the time!

2. An imaginary line from the sun to a planet sweeps out equal areas in equal time intervals.

Kepler’s laws of planetary motion

Thus, the planets move faster when closer to the sun.

… also kind of a big deal

Kepler’s Second law: another view

3. The square of the ratio of the periods of any two planets revolving about the sun is equal to the cube of the ratio of their average distances from the sun.

Kepler’s laws of planetary motion

(Hunh?)

On your formula sheet,

kT

r

2

3

Kepler’s constant

Note that the first two laws apply to each planet, moon, or satellite individually.

The third law, however, relates the motion of several satellites about a single body.