General Astronomy - PowerPoint Presentation

Slide1

General Astronomy

IntroductionSlide2

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)Slide3

The most incomprehensible thing about the world is that it is at all comprehensible.

Albert Einstein

This course has a huge amount of material to cover (the entire UNIVERSE) so it is

a mistake to

wait and cram before an exam. Assimilate the material on a daily (OK … a weekly) basis and you will have time to absorb it all. It is not easy, but it is

comprehensible

. Be warned, it is “drinking from a firehose!”Slide4

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.Slide5

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)Slide6

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.Slide7

Distances

So 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 Pluto2.7 billion miles6.69 lighthours

Earth to nearest Star4 lightyearsEarth to nearest large galaxy2 million lightyears

Earth to end of observable Universe13.2 billion lightyearsSlide8

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 LightyearsSlide9

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

!Slide10

Position

Note the relative positions in the

asterism

shown

This is a small portion of the

constellation

Ursa MajorSlide11

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.Slide12

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"Slide13

Angular MeasurementsSlide14

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.Slide15

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 Slide16

*

*

*

*

*

Observation: Distance

An important distance measurement is parallax.

We can

infer

distance from parallax using the slight apparent shifts in relative position

*

*

*

*

*

*

*

*

*

*Slide17

Parallax

*

1AU

1"

D

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

One parsec is about 3.26 lightyearsSlide18

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.Slide19

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?Slide20

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 1

st

magnitude star; a 3

rd

is 2 ½ times dimmer than a 2

ndA 6th magnitude star is therefore 100 times dimmer than a 1st magnitude star."Yes, Virginia. There is a Santa Claus logarithmic scale."Slide21

Apparent Magnitude

The night sky as seen from Stockton CollegeSlide22

Apparent Magnitude

The night sky showing stars to 6

th

magnitudeSlide23

Apparent Magnitude

The night sky showing stars to 5

th

magnitudeSlide24

Apparent Magnitude

The night sky showing stars to 4

th

magnitudeSlide25

Apparent Magnitude

The night sky showing stars to 3rd magnitudeSlide26

Apparent Magnitude

The night sky showing stars to 2nd magnitudeSlide27

Apparent Magnitude

The night sky showing stars to 1st magnitudeSlide28

Apparent Magnitude

The night sky showing stars to 15

th

magnitudeSlide29

S

Apparent Magnitude

Vega 0.03

Sulifat 3.35

Sheliak 3.5

d

Lyrae 4.22

z

Lyrae 4.34Slide30

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 5

th

magnitude star – just barely visible on a dark, clear night.Slide31

Absolute Magnitude

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 star

How much brighter? By nearly 50 timesSlide32

Color

This one has a red tint

This is whiteSlide33

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 milesA Lightyear = 6x10

12

miles

To the farthest known object 3x10

24

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.Slide34

Cosmic Overview

We will work our way outwards…

From the Solar System

To the Milkyway galaxy

To other galaxies

To stranger objects in the cosmosTo the Universe Slide35

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 experimentWe cannot experiment in AstronomyWhile 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 howeverSlide36

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 20

th

centuryBetter equipment

Ideas from other disciplines (math, chem, physics)Slide37

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

Slide38

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 10

19

atomsIn 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.Slide39

Human senses – vision in particular – provide an extremely limited perspectiveThe ‘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 HumansSlide40

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.Slide41

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

Astronomy and HumansSlide42

Comprehending a Billion

A billion seconds ago it was 1986.

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 dollarsSlide43

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 4

th

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).

Observe

ReasonExperiment

TheorizePredictSlide44

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."Slide45

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.Slide46

Reasoning Paradigms

Deductive Reasoning

Hypothesis

 Observation  Hypothesis …

Inductive Reasoning

Observation  ModelsSlide47

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:Slide48

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

†

Again due to Richard Feynman:Slide49

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

†

Again due to Richard Feynman:Slide50

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…

26

27

Under the rug

2 out the window

†

Again due to Richard Feynman:Slide51

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…

26

27

Under the rug

2 out the window

†

Again due to Richard Feynman:

30Slide52

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

26

Under the rug

2 out the window

30

Visiting playmate had some blocks

25

†

Again due to Richard Feynman:Slide53

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

26

Under the rug

2 out the window

30

Visiting playmate had some blocks

25

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

Toy Box ???

†

Again due to Richard Feynman:Slide54

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

26

Under the rug

2 out the window

30

Visiting playmate had some blocks

25

Toy Box

23

†

Again due to Richard Feynman:Slide55

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

26

Under the rug

2 out the window

30

Visiting playmate had some blocks

25

Toy Box

23

Dirty Aquarium???

†

Again due to Richard Feynman:Slide56

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

26

Under the rug

2 out the window

30

Visiting playmate had some blocks

25

Toy Box

23

Dirty Aquarium???

†

Again due to Richard Feynman:

With piranha!Slide57

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…

Example

†

: Searching for a rule

27

26

Under the rug

2 out the window

30

Visiting playmate had some blocks

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

†

Again due to Richard Feynman:Slide58

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'