General Astronomy

General Astronomy Introduction

Introduction • Administrative Matters • Syllabus • Best guess at this time • NOT cast in granite • General Information • Text • Exams and Quizzes • Labs • Observatory • Class Evaluation • Web Access • http://www.stockton.edu/~sowersj/gnm2225 • Syllabus • General Information • Lectures (Downloadable)

Astronomy as a Physical Science Astronomy is an observational science. • It is difficult to experiment with the Universe It is the 'Mother of Physics' Astronomy's knowledge base has been accumulating since the first cave person noticed the lights in the night sky. Most of our knowledge is recent however – within the last 100 years.

Astronomical Jargon The speed of light, c, is 186,000 miles/second. A Lightyear (LY) is the DISTANCE traveled by light in 1 year. 186000 mi/sec X number of seconds per year = 186000 mi/sec x 60 sec/min x 60 min/hr x 24 hr/day x 365.25 day/yr Or about, Hint – This WILL be on your exam(s) So, how many miles is that? Let's find out… • 1 Ly = 6,000,000,000,000 miles

The Time Machine Light takes, for example, 8.5 minutes to travel the distance from the Sun to the Earth. Another way to state the distance between Earth and Sun is, therefore, to say it is 8.5 lightminutes. Note the 'time machine' effect. We don't see the Sun as it is "now". We see it as it was 8.5 minutes ago.

Distances So here are some distances to try to make this more clear.

Astronomical Jargon A lightyear is too big a measurement to use within our Solar System. A better 'ruler' for these small distances is the Astronomical Unit, or AU An AU is the average distance from the Earth to the Sun. 1 AU = 93,000,000 Miles = 8.5 Lightminutes = 150 Million Kilometers = 0.0000162 Lightyears

Observations What can we actually see when we look at the stars? Position (relative to other stars) Brightness (relative to other stars) Color There is no other information directly available!

Position Note the relative positions in the asterism shown This is a small portion of the constellation Ursa Major

Observation: Position It's difficult to get an absolute position – after all where should we measure from? The best bet is to get a relative position. That is measure the position of stars relative to each other. The best way to do this is to measure their angular separations.

Angular Measurement Very often what we measure is the angle between two objects Angular difference The angle is measured in either seconds of arc, or in radians (Planets, etc, may need bigger measurements) For example, the angular diameter of the Sun is about 30.5' or 30' 30"

Angular Measurements

Distance One parameter, not on our list of directly observable items, is distance. This is very important, but it's hard to measure. After all, sending someone out with a measuring tape is not a really good way of handling the problem.

A quick experiment Hold your arm out full length, close one eye and position your thumb on a figure on the blackboard. Quickly switch your eyes, closing one an opening the other. Did your thumb appear to "move?" This phenomenon is called parallax

Observation: Distance An important distance measurement is parallax. We can infer distance from parallax using the slight apparent shifts in relative position * * * * * * * * * * * * * * *

Parallax D has a value of 1 parallax-second when the angle is seen to shift by 1 second of arc. D would be 1 parsec. The angle is so small that there is really no measureable difference between D and that between the star and earth D * 1" 1AU One parsec is about 3.26 lightyears

Observation: Brightness Are the stars as bright as they appear in a dark night sky? Of course not. They are much, much brighter, but they are very far away. Brightness varies inversely with the square of the distance. That means a 100 watt light bulb will look ¼ as bright if it's distance is doubled. Since we don't always know how far away a star is, measuring the apparant brightness (just what we see) is an important first step.

Relative Brightness But, is it brighter: Because it is closer to us? Because there is dust and gas in between us and it which is dimming the light? Because it is simply a brighter star? Clearly, this star is much brighter than the others

Apparent Magnitude • Brightness as estimated by the 'eye' • The scale is ordinal, that is, we assign a number from 1 to 6 • At least originally, now we use decimal numbers (including negatives; the Sun's apparent magnitude is –26.5) • 1 is bright; 6 is dim (the dimmest that the human eye can make out on a very dark, clear night). • The scale is not linear, in fact each magnitude change is 2 ½ times dimmer than the one before. • A 2nd magnitude star is 2 ½ times dimmer than a 1st magnitude star; a 3rd is 2 ½ times dimmer than a 2nd • A 6th magnitude star is therefore 100 times dimmer than a 1st magnitude star. • "Yes, Virginia. There is a Santa Claus logarithmic scale."

Apparent Magnitude The night sky as seen from Stockton College

Apparent Magnitude The night sky showing stars to 6th magnitude

Apparent Magnitude The night sky showing stars to 3rd magnitude

Apparent Magnitude The night sky showing stars to 2nd magnitude

Apparent Magnitude The night sky showing stars to 1st magnitude

Apparent Magnitude S Vega 0.03 z Lyrae 4.34 d Lyrae 4.22 Sheliak 3.5 Sulifat 3.35

Absolute Magnitude • Suppose we want to compare star's actual brightness. To do this, we have to know how far away they are. • Suppose all the stars were at the same distance – then their magnitudes would give us this information. • Assuming we know the distances to the stars, we can calculate just how bright they would be at any distance. For comparison purposes, we decide to use a fixed distance of 10 parsecs. • The magnitude measured at a distance of 10pc is known as the absolute magnitude. • For example, the Sun from that distance is a 5th magnitude star – just barely visible on a dark, clear night.

Ly M m pc Absolute Magnitude Notice that Vega was very bright because it is close. The much dimmer Sheliak is 35 times farther away and intrinsically a much, much brighter star How much brighter? By nearly 50 times

Color This one has a red tint This is white

Cosmic Overview • Astronomy uses a wide range of numbers to describe its observations • From the radius of a 'classical' electron which is about 3x10-18 kilometers • To an AU = 9.3x107 miles • A Lightyear = 6x1012 miles • To the farthest known object 3x1024 miles • As you can see, scientific notation is a must – there are just too many zeros, both before and after the decimal point without it.

Cosmic Overview We will work our way outwards… • From the Solar System • To the Milkyway galaxy • To other galaxies • To stranger objects in the cosmos • To the Universe

An Observational Science • As noted before Astronomy is an observational science • Most "hard" sciences (Chemistry, biology, geology, physics) are experimental sciences • Each has a strong theoretical component, but their final 'proof' is in the experiment • We cannot experiment in Astronomy • While some professor's egos make them think they can collide galaxies together, turn the stars off and on, and create Universes – they really can't (Though it is wise for the undergraduate not to explain this to these individuals) • We do have a rich observational sample however

An Observational Science • Keep in mind that in addition to many, varied objects to observe, the Astronomical 'Time Machine' is also operational • Due to the finite speed of light, the farther away an object is, the farther back in time we are viewing it • Much of the modern ideas in astronomy have been developed during the 20th century • Better equipment • Ideas from other disciplines (math, chem, physics)

Astronomy and Humans Humans tend to regard as typical those things they perceive through everyday experience and cultural knowledge. For example, our current culture – in general – regards the Earth as round and moving about the Sun. This would not have been easily accepted by an individual living before 1543 AD. Let’s look at some factors which will bias our view of the Universe: This portion of the lecture was adapted from notes written by Dr. Michael Skrutskie of the University of Virginia

Astronomy and Humans Conditions on Earth are not typical of the rest of the Universe • Earth is a place where matter is relatively dense • A cubic centimeter of air contains about 1019 atoms • In intergalactic space, a volume of space about the size of a football stadium contains a single atom. • Earth is about 300 degrees above absolute zero; the Universe is largely about 3 degrees above absolute zero. • Earth orbits a single star – most star systems are multiple systems.

Astronomy and Humans Human senses – vision in particular – provide an extremely limited perspective • The ‘light’ we can see is a tiny fraction of the entire electromagnetic spectrum • Radio, Infrared, Ultraviolet, X-ray and Gamma ray light fill the Universe, but cannot be seen directly by the human eye.

Astronomy and Humans • The human perception of Time is also very limited • The brief span of a human lifetime provides only a ‘snapshot’ of the Universe. • Most cosmic phenomenon do not change appreciably over a lifetime • Even a long lifetime of 100 years is insignificant compared to the lifetime of the Sun (about 10 billion years) • Astronomers must reconstruct the workings and evolution of the Universe from this short snapshot. • This is similar to reconstructing the politics of the Earth from a one-second glimpse of events. • Fortunately the ‘astronomical time machine’ allows us to look back and see varying stages of evolution.

Astronomy and Humans Limited Comprehension of Large Numbers • We can visualize quantities of a dozen or even a few hundred, but what is the difference between a billion and a trillion? • Scientific Notation makes this manageable, but it still doesn’t give it meaning

Comprehending a Billion • A billion seconds ago it was 1973. • A billion minutes ago Rome ruled the known world. • A billion hours ago our ancestors were living in the Stone Age. • A billion days ago 'Lucy' was living in Africa • A billion dollars ago was only 2 hours and 10 minutes, at the rate our government is spending it. If you spent $10,000 per day it would take almost 274 years to spend 1 billion dollars

Observe Reason Experiment Theorize Predict The 'Game' of Science How do we go about playing the great 'game' of science? There are several 'rules' or methods. The one that you know (since about 4th grade) is the Scientific Method If you recall it went something like: Theorize  Hypothesis  Experiment  Verify  Law We've got to be a bit more precise (especially since we cannot experiment).

The 'Game' of Science The following example is from Richard Feynman, "What do we mean when we claim to 'understand' the Universe? We may imagine the enormously complicated situation of changing things we call the physical universe is a chess game played by the gods; we are not permitted to play, but we can watch. Our problem is that we are left to puzzle out the rules of the game for ourselves as best we can by watching the play. We have to limit ourselves to trying to find out the rules – using them to play is beyond our capability (We may not be able to predict the next move even if we know all the rules – our minds are far too limited). So we say if we know the rules, we understand."

How can we tell which rules are right? There are three basic ways: • Simplify • Nature has arranged (or we set up an experiment) where the situation is so simple with so few parts, we can predict the outcome if the rule is correct. • Check rules in terms of less specific ones • For example, we hypothesize that a bishop must move on a diagonal. We can check our idea by observing that a given bishop is always on a red square even if we cannot see it move. (Occasionally Nature permits pawn promotion to a bishop) • Approximation • We can't always tell why a particular piece moves, but perhaps we can generalize to the approximation that protecting the king is a guiding principle.

Reasoning Paradigms • Deductive Reasoning Hypothesis  Observation  Hypothesis … • Inductive Reasoning Observation  Models

Example†: Searching for a rule Let's assume a mother has a young 'Dennis the Menace' type son. He has a set of indestructible blocks – they cannot be destroyed or broken. Every day she places him in his playroom with the blocks. She has observed that there are always 28 blocks. One day, however… 27 Under the rug 26 2 out the window 30 Visiting playmate had some blocks 25 Toy Box ??? He won't let her open the toy box. Mom waits until all the blocks are visible, then weighs The toybox. Then, the next time: Number Blocks = Number Seen + (Weight of Box – Weight of Empty Box)/Weight of a block You've just introduced mathematics into science †Again due to Richard Feynman:

Example†: Searching for a rule Ok, one more… 23 Dirty Aquarium??? Measure the height of the water when all blocks are visible. Measure the height when only one block is missing. (Or compute the volume of a block). Then you can add the following to your "block equation" Blocks under water = (Height of water – Standard Height)/Height caused by 1 block

So what's the rule? How about… There are always the same number of blocks We've just developed a Conservation Law As Dennis gets more ingenious, Mom must come up with equally clever additions to her 'equation'

General Astronomy