Astronomy 101
Sept. 11, 2008
Announcements:
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Student contract due by COB tomorrow (you can leave it in my mailbox)
First real OWL homework up tomorrow; due Sept. 24 before midnight
Probably best to work the OWL tutorial before doing the first real
homework
Do you PRS? Maybe not yet…
yet…
New Theme for next several
lectures: Observing the Sky
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The nature of the Milky Way
The celestial sphere, the local sky, and finding stuff up there
The Moon
Eclipses
Discovery-driven vs.
hypothesis-driven science
0 OBSERVATIONS
1 Question
2 Hypothesis
– a tentative explanation
3 Prediction
4 Test
5 Result
– confirm, reject, or modify
Discovery-driven vs.
hypothesis-driven science
• Discovery-driven: observations are
made first, and then ideas are
developed to understand the
observations
• The theoretical ideas in this situation
can be postdictions rather than
predictions.
Discovery-driven vs.
hypothesis-driven science
• Discovery-driven: observations are made
first, and then ideas are developed to
understand the observations
Example: Edwin Hubble’s discovery of the
expansion of the universe.
• Hypothesis-driven: an idea is developed
first, and then experiments/observations are
made to test the idea
Example: Albert Einstein’s (blown!) prediction
of the expansion of the universe.
The funkiest bit of all: the expansion of
the Universe!
• Mostly all galaxies
appear
to be moving
Discovery-driven
science!
away from us.
• The farther away they
(Luck-driven science?!)
are, the faster they
are moving.
– Just like raisins in a
raisin cake; they all
move apart from each
other as the dough
(space itself) expands.
Hallmarks of Good Science
• Science seeks explanations for observed
phenomena that rely solely on natural
causes.
• Science progresses through the creation
and testing of models of nature that explain
the observations as simply as possible.
! Occam’s Razor
• A scientific model must make testable
predictions that could force us to revise or
abandon the model.
Theory -- a model which survives repeated testing
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About Occam’s
razor…
Scientific Models?
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Seek the simplest possible model.
Why?
Nothing requires nature to be simple!
However, by beginning with the simplest
model, we can gain insight and efficiently
make progress.
• Too many ideas and variables → difficult to
figure out what’s really happening
Scientific Models?
Example: models of the atom
The Greek philosopher Democritus hypothesized that matter
is made of tiny, fundamental particles that he dubbed “atoms”
(the Greek word for indivisible). But, he also thought that
elements came in the forms of fire, water, earth, and air…
The modern model of the atom is
rather different:
“Model” vs. “Theory”: a model
Is just a working idea; it may or may not be
correct. A theory has withstood repeated
testing and has more evidence supporting its
validity. Models and theories evolve. This model
atom is better, but it’s not fully correct either.
The Milky Way
• Our Galaxy is shaped like a disk.
• Our solar system is in that disk.
• When we look at the Milky Way in the sky, we are
looking along that disk, i.e., at the disk edge-on
This hypothesis was put forth by Immanuel Kant (philosopher)
and Thomas Wright (astronomer) in the 1700s, with some support
from Galileo’s observations in 1610.
Today, there is abundant evidence supporting this picture.
Democritus. Sound familiar?
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Democritus → democracy
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Many ancient cultures believed that the Milky Way is some
sort of liquid flowing through the sky
Democritus hypothesized that the Milky Way is composed of
many distant stars.
Aristotle disagreed…
disagreed…
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Galileo: first to observe the Milky Way with
a telescope; described as “innumerable faint stars”
(1609)
The Milky Way: a disk-like object
Left side
Right side
From this picture, we
can conclude that the
inner galaxy is on
1. The right side
2. The middle
3. The left side
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The Milky Way: a disk-like object
The current model of our galaxy…
But what are those dark
patches and streaks?
Answer: dust and gas
in space
Also, why is it bulged in one direction?
Answer: because the disk is bulged in that direction.
Historical evolution of the Milky Way
model
• 500 BC: Democritus
• 1609: Galileo
• 1780s: William & Caroline Herschel map the MW
with a telescope
Scientific model: a concept/representation/tool used
to think about how something works.
Assessing the shape of the Milky Way by counting
stars:
Now suppose that we
Suppose that we lived in the
lived at the edge of that
middle of a spherical galaxy:
spherical object:
Textbook reading: Unit 70
In this case, we’d expect to
We’d observe many stars
count roughly the same
to the left and very few
number of stars looking in
looking to the right or
any direction.
straight up or down.
This is the technique used by the Herschels.
Historical evolution of the
Milky Way model Herschel’s sketch of the
Historical evolution of the
Milky Way model
• 500 BC: Democritus
• 1609: Galileo
• 1780s: William &
Caroline map the MW
with a telescope
• 1920s: Kapteyn used
modern techniques &
understanding of star
brightnesses to remap
the Milky Way:
• Simultaneously (1920s),
Harlow Shapley used the
globular clusters to
argue that we are not in
the center but rather are
at the edge of the Milky
Way:
• Black patch: Herschel’s
map
• White dots: globular
clusters
• Yellow dot: the Sun
Note: a parsec is a unit of
distance = 3.3 light years
Milky Way (1784):
2500 parsecs
100 kpc
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Historical evolution of the Milky Way
model: why did Herschel/Kepteyn get it
Example of an astronomical
wrong?
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The Herschels could only detect stars in the immediate vicinity
of the Sun; beyond this region, dust made the stars to dim for
their early telescopes to detect.
dust bunny, er, horse…
Answer: interstellar dust.
Dust fills the interstellar space of the MW
and dims the light of stars.
However, there is much more dust in the
disk.
Globular clusters are in the halo and thus
are less affected by dust.
Shapley used globulars and thus got the shape/location
right, but he failed to correct for dust and ended up with
the wrong size.…
Kepteyn only used stars in a similar small region
because he required “proper motion” to estimate
the stellar distances, and proper motions can only
be measured for relatively nearby stars.
Constellations.
The Problem with
Constellations
PRS question. True or false:
Constellations are important in
modern astronomy.
1. TRUE
No particular significance; just regions in the sky
For example, the “Big Dipper” is a familiar constellaion:
2. FALSE
Constellations: review
from previous lecture
Angular separation
We can measure true distances, but it
is often quite difficult to do. More often,
astronomers measure angles.
A Constellation is just a region of the sky
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Most official constellation names come from antiquity. Some southern
hemisphere constellations were named by European explorers in the 17th
& 18th centuries.
The patterns of stars have no physical significance! Stars that appear
close together may lie at very different distances.
Constellations were useful
in prehistoric times to keep
track of the year, e.g., when
to plant or harvest the crops.
Modern astronomers
mainly use constellations
as landmarks, but their
usefulness is pretty limited.
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