General Astronomy
Introduction
Introduction
• Administrative Matters
– Syllabus
• Best guess at this time
• NOT cast in granite
– General Information
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•
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•
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.
Hint – This WILL be on your exam(s)
So, how many miles is that? Let's find out…
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,
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.
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 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
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
The night sky showing stars to 5th magnitude
Apparent Magnitude
The night sky showing stars to 4th 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
The night sky showing stars to 15th magnitude
Apparent Magnitude
Vega
0.03
z Lyrae 4.34
d Lyrae 4.22
S
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.
Absolute Magnitude
m
Vega
Zeta
delta
Sulafat
Sheliak
0.03
4.34
4.22
3.25
3.52
Ly
pc
M
25.3
153.6
898.5
634.6
881.5
7.8
47.1
275.6
194.7
270.4
0.58
0.97
-2.98
-3.20
-3.64
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
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).
Observe
Reason
Experiment
Theorize
Predict
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:
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 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)
3) 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
†Again
due to Richard Feynman:
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
Under the rug
due to Richard Feynman:
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
†Again
Under the rug
due to Richard Feynman:
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
†Again
Under the rug
2 out the window
due to Richard Feynman:
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
30
†Again
Under the rug
2 out the window
due to Richard Feynman:
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
30
Under the rug
2 out the window
Visiting playmate had some blocks
25
†Again
due to Richard Feynman:
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
30
Under the rug
2 out the window
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
†Again
due to Richard Feynman:
mathematics into science
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
30
Under the rug
2 out the window
Visiting playmate had some blocks
25
Toy Box
23
†Again
due to Richard Feynman:
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
30
Under the rug
2 out the window
Visiting playmate had some blocks
25
Toy Box
23
Dirty Aquarium???
†Again
due to Richard Feynman:
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
30
Under the rug
2 out the window
Visiting playmate had some blocks
25
Toy Box
23
Dirty Aquarium???
†Again
due to Richard Feynman:
With piranha!
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
30
Under the rug
2 out the window
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:
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'