Introduction to Astronomy

• Announcements

– HW #1 DUE Wednesday 06/18/2008

– “History of Astronomy” powerpoint is on the class website

Physics Department Observatory (PDO)

• Open Monday & Wednesday @ 10:00pm (weather permitting)

• PDO leader– Julia Wickstrom

• juliagw@cc.usu.edu

There is also a “real” mapon the class website

PDO• Observation requirements

– You will observe and document four (4) distinct astronomical objects of your choice

– You will fill out an Observation Log for each object you observe

• Name of object• Position in sky (RA-DEC, alt-az, whatever)• Verbal description of object and its structure(s)• Sketch of the object and its structure(s)

– Color sketch, if you can make out any colors

Gravity & Motion• Gravity holds the universe together…it is

the glue that binds us to the Earth, the Earth to the Sun, the Sun to the Milky Way, the Milky Way to the Local Group, and the Local Group to every other galaxy in the Universe.

• It may ultimately cause everything to collapse back to a single point, in the reverse of the Big Bang, called the “Big Crunch”

Mass & Inertia

• “inertia” = tendency to maintain the status quo

• Object at rest stays at rest• Object in motion remains in motion, unless acted

upon by some force– Sit still…you don’t spontaneously start moving east @ 25

mph, do you?– Slam on brakes, bag of groceries in passenger seat

spills over (bag wants to remain in motion)– Round a corner at speed and you lean to the opposite

side (“centrifugal force”, you want to continue in straight-line motion while car “curves” around you…)

• Kepler’s empirical laws go beyond computing planetary orbits, they are applicable to spacecraft orbiting the Earth, to binary stars rotating about each other to galaxies in their orbits….but why do they but why do they work?work?

• Corollary– If a body is not moving in a straight line at

constant speed, a force must be acting upon the body

– Importance for Astronomy• Planets, stars, move in elliptical orbits• NOT straight lines• Therefore some force must be acting on them

– GRAVITY !!!

• Ball-on-a-string

Comparison between projectile motion and orbital motion

The faster a projectile is fired, the longer it travels before hitting the ground,but if fired fast enough, as the projectile falls it matches the curvature of theEarth, and attains orbit. The projectile is still falling, but the surface of theEarth is falling away from the projectile as well…

Newton’s Laws

• 1st Law– Principle of Inertia

• “A body in straight-line motion at constant velocity remains in such a state, unless acted upon by some force. A body at rest remains at rest, unless acted upon by a force.”

• There must therefore be a force that acts continuously on the planets to keep them moving in their elliptical orbits.

Newton’s Laws• 2nd Law

– Principle of Forces

• “A force applied to an object produces a change in its state of motion. The object accelerates in the direction of the force.”

• Applying a force produces an acceleration

• F = ma

Newton’s Laws

• 3rd Law– Principle of Action & Reaction

• “Two interacting bodies exert equal, but oppositely-directed forces on each other. The net force in a closed system is always zero.”

• What is a “closed system”?

equal & oppositeforces

Newton’s Law of Gravity

• Every mass exerts an attractive force on every other mass. The strength of the force is directly proportional to the product of the masses divided by their separation

• F = (Constant)(Mass#1)(Mass#2) (Distance between centers)2

• Fgravity = GM1M2/r2

• M1 or M2 increase, Fgravity increase

– More massive objects have greater gravitational pull…makes sense.

• r increase, Fgravity decrease

– The closer you are, the harder gravity pulls on you…makes sense.

BUT NEVER ZERO!!! JUST VERY SMALL…Exerting small force on a proton on the other

side of the galaxy…

F1

F3

F2

Force of gravity onspiral galaxy = F1 + F2 + F3

Gravity

• Gravity becomes tool for “measuring” mass of astronomical bodies– Equate centripetal force to gravity force

– Orbital velocity, • M=mass of central body (star, presumably)• R=distance between star and planet

• Can measure R and vorbital, then solve for M– Easy way to measure stellar masses, but requires an

orbiting body…

RGMvorbital

Gravity

• Note: speed doesn’t depend on MASS or SIZE of orbiting body (planet)– Bodies at same distance orbit with same

speed, always– Like bodies on Earth falling at same rate

(hammer vs. marble)– Hammer vs. Feather (feather has more

surface area per unit mass = more drag…but on moon they fall the same, because no atmosphere)

Surface Gravity

• “mass” describes how much material an object contains

• “surface gravity” describes how much that object weighs on the surface of (e.g.) Earth– Explains irregular asteroid shapes, lack of

lunar atmosphere– g = GM/R2 (M=mass of body, R=radius of

body)– gEarth = 9.805 m/s2

– gEarth / gMoon = 6

Escape Velocity

• How fast must an object be thrown upwards to completely escape the pull of Earth’s gravity?– Escape velocity, Vesc=SQRT(2GM/R)

– For Earth, Vesc = 11.2 km/s = 6.72 miles/sec = 24,192 mph !!!

• R = radius of central body• M = mass of central body• Important for shuttle/rocket launches, atmospheric

retention/escape

A rocket moving at less than the escape velocity will fall back to Earth

A rocket meeting orexceeding escape velocitycan travel beyond the grip of Earth’s gravity

Why do we launch the Space Shuttle to the East?

rotation of Earth gives a little boost, helping the rocket achieve escape velocity quicker…

Ideal Gas Law:

atmospheremolecules TKv

constant

Earth’s high temperature whenit was first formed is responsiblefor depleting the firstHydrogen-Helium atmosphere.

As things cooled down, moleculescouldn’t escape into space, so the Earth starts retainingatmospheric gases…

Newton’s Thoughts on Orbits

• If gravity pulls object toward each other, why doesn’t the Moon fall to the Earth?– Answer: if you drop a ball vertically, it falls

straight down. If you throw it, it travels some horizontal distance before hitting the ground

• If you throw it harder, it travels farther…• Eventually, if thrown hard enough, it matches the

curvature of the Earth and goes into orbit!

The 4 Fundamental Forces of Nature

• Electromagnetic force is tremendously stronger than gravitational force

• Felec/Fgrav = 1043

– Electromagnetism is 10,000,000,000,000,000,000,000,000,000,000,000,000,000,000 times stronger than gravity!

• Small charge on plastic comb picks up bits of paper…small electrical force overwhelms gravitational pull of ENTIRE EARTH!

NEXT TIME:

• Project Details

• Light & Atoms– Specfically how they interact, and their

relation to the electromagnetic force.

– Remember that Astronomy is the science of analyzing light from distant objects, so we need to know about what kinds of information these little packets of energy can carry…