general astronomy introduction. administrative matters –syllabus best guess at this time not cast...

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)

http://www.stockton.edu/~sowersj/gnm2225

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

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/yrOr about,

1 Ly = 6,000,000,000,000 miles

The speed of light, c, is 186,000 miles/second.

A Lightyear (LY) is the DISTANCE traveled by light in 1 year.

So, how many miles is that? Let's find out…Hint – This WILL be on your exam(s)

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.

DistancesSo here are some distances to try to make this more clear.

Earth to Sun 93 million miles 8.5 lightminutes

Earth to Moon 238,857 miles 1.25 lightseconds

Atlantic City to Los Angeles

2,443 miles 0.013 lightseconds

Earth to Pluto 2.7 billion miles 6.69 lighthours

Earth to nearest Star 4 lightyears

Earth to nearest large galaxy

2 million lightyears

Earth to end of observable Universe

13.2 billion lightyears

Astronomical JargonA 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!

PositionNote 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 MeasurementVery 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: DistanceAn important distance measurement is parallax.We can infer distance from parallax using the

slight apparent shifts in relative position

*

*

*

*

** * * * *

Parallax

*1AU

1"

D

D has a value of 1 parallax-second when the angle is seen to shift by1 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

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

Clearly, this star is much brighter than the others

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?

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


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

Apparent Magnitude

Vega 0.03

Sulifat 3.35

Sheliak 3.5

d Lyrae 4.22

z Lyrae 4.34

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.

Absolute Magnitude

Vega 0.03 25.3 7.8 0.58Zeta 4.34 153.6 47.1 0.97delta 4.22 898.5 275.6 -2.98Sulafat 3.25 634.6 194.7 -3.20Sheliak 3.52 881.5 270.4 -3.64

m Ly pc M

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 starHow much brighter? By nearly 50 times

ColorThis 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 OverviewWe 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 HumansConditions 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.

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 HumansThe 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.

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

The 'Game' of ScienceHow 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).

ObserveReason

Experiment

TheorizePredict

The 'Game' of ScienceThe 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:1) 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.

2) Check rules in terms of less specific onesFor 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)

3) ApproximationWe 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 ReasoningHypothesis Observation Hypothesis …

• Inductive ReasoningObservation 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

†Again due to Richard Feynman:


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



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



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…

2627 Under the rug

2 out the window




2627 Under the rug

2 out the window


30



26Under the rug2 out the window

30 Visiting playmate had some blocks

25






25

He won't let her open the toy box.

Mom waits until all the blocks are visible, then weighsThe 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

Toy Box ???






25 Toy Box

23






25 Toy Box

23 Dirty Aquarium???






25 Toy Box

23 Dirty Aquarium???

With piranha!




2726

Under the rug2 out the window


25 Toy Box

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 introduction. administrative matters –syllabus best guess at this time not cast...

Documents