Transcript 8Sept103_2014

Astronomy 103
Lecturer: Prof. Alex Lazarian
Office Location: 6289 Chamberlin Hall
Email:
[email protected]
Website: http://www.astro.wisc.edu/~lazarian/ast103_2014/
Office Hours: Mon. 11am-1pm, Wed. 11am-1pm. Meetings at a different time can also be arranged.
Course Reading: Pathways to Astronomy Schneider & Arny 4nd edition
Exams:
training exam -- 0% (Oct. 6)
1st exam – 25% (Oct. 16)
2nd exam—25% (Nov. 10)
final exam—30% (Dec. 15)
total for homework, quizzes, presentations -- 20%
0th
Main Ideas to be covered:
*Subject of astronomy (Units 1--4)
*Observations of planets and stars (Units 5--12)
*Laws of gravity, dark matter (Units 14-20)
*Light, atoms, spectra (Units 21--30)
*Stars: Sun as an example (Units 49--50)
*Learning about stars (Units 52--55)
*Birth and evolution of stars (Units 56—63, 69)
*When the fuel begins to run out (Units 64--66)
*Supernova, neutron stars and black holes (Units 67--68)
*Galaxies and quasars (Units 74--77)
*Our galaxy- Milky Way (Units 70--73)
*The expanding Universe (Units 78--82)
*Life in the Universe (Units 83--84)
*Review of the course
The Solar System
•
•
•
•
Planets, asteroids, comets and dust all held together by the Sun’s gravity
Everything goes around the Sun on elliptical paths called orbits
All orbits lie in the same plane, like peas rolling around on a dinner plate
Too big to describe using meters – we need something more convenient
A convenient measure –
the Astronomical Unit
• It is convenient to measure planetary
distances using the Astronomical
Unit, or AU
• 1 AU = average distance between
the Earth and the Sun
• 1 AU ~ 150 million km
Some planetary distances:
• Mercury: 0.4 AU
• Mars: 1.5 AU
• Saturn: 10 AU
• Pluto: 40 AU
The Milky Way Galaxy
•
The Milky Way galaxy contains billions of
stars, being born, aging and dying with a
whisper or a bang.
•
•
•
•
Where do stars come from?
How do they age?
How and why do they die?
This process is called Stellar Evolution!
A New Measure of Distance
• Stars in the Milky Way are
very far apart
– Nearest star is 40 trillion km
away – too large to imagine!
– How about hundreds of
thousands of AU? No, still too
big.
– Light travels 10 trillion km in
one year, so we’ll use the light
year (ly) as an easy-to-imagine
measure of distance
– It takes light 4.1 years to travel
from Proxima Centauri to
Earth, so the distance to this
star is 4.1 ly.
Getting to Know the Neighborhood
• The Universe is “clumpy” –
galaxies tend to pull together by
gravity
Central region of the Virgo Cluster
– Our immediate neighborhood is
called the Local Group, a cluster of
around 3 dozen galaxies (3 million
light years across
– The Local Group is part of the
Virgo Cluster, a large (collection of
smaller clusters and groups of
galaxies
– Superclusters: collection of larger
clusters
– The Universe – simply everything!
A Sense of Scale I
A Sense of Scale II
Outward to the
Universe!
The Metric System
English Units (Distance)
12 “lines” = 1 inch
12 inches = 1 foot
3 feet
= 1 yard
5.5 yards = 1 rod
4 rods
= 1 chain
10 chains = 1 furlong
8 furlongs = 1 mile
3 miles
= 1 league
Metric Units
10 millimeters
= 1 centimeter
100 cm
= 1 meter
1000 m
= 1 kilometer
1000 milligrams = 1 gram
1000 g
= 1 kilogram
• Metric Units are
internationally accepted
Units. Used in Astronomy.
Scientific Notation
• 0.0000001 meters is difficult to
write – too many zeros!
• Can use scientific notation to
simplify it.
• 0.0000001 m = 110-7
• Move the decimal place seven
places to the left to determine the
“power” of ten
– Moving it to the left makes the
“power” negative
– Moving it to the right makes the
“power” positive
Commonly used prefixes
Number
Scientific
Notation
Prefix
Abbreviation
1,000,000,000
1  109
giga
G
1,000,000
1  106
mega
M
1,000
1  103
kilo
k
1  10-
centi
c
1  10-
milli
m
1  10-
micro

1  10-
nano
n
0.01
0.001
0.000001
0.000000001
2
3
6
9
Special Units
• The Light Year (ly)
– Distance light travels in
1 year
– Equivalent to a “lookback time”, as the light
we see from a star left a
long time ago
– Example: Proxima
Centauri is 4.1 ly away,
so the light we see from
it today left the star 4.1
years ago!
• The Parsec (pc)
– “PARallax SECond”
– Distance to a body whose
parallax motion covers 1
second of arc (See Unit 5!)
– 1 pc = 3.26 ly
A Sense of Scale
The Scientific Method
• The Scientific Method is the procedure
scientists use to construct their ideas
about how the Universe works.
– Start with a hypothesis – a testable idea
of how something works
– Test the hypothesis!
– If the test fails, modify or abandon the
hypothesis, and retest.
• Hypotheses that pass rigorous testing
become Laws (more mathematical) or
Theories (described using both words
and equations).
• A Model is a complex
description of physical
phenomena incorporating
many laws and/or theories
– Ex: The Celestial Sphere
– Ex: Universal Gravitation
The Nature of Matter
• Protons (positively charged) and neutrons (uncharged) make
up the nucleus at the center of an atom. Electrons (negatively
charged particles) are found relatively far from the nucleus
– If we enlarged the nucleus to be the size of a grape, the electrons
would orbit at a distance of slightly less than a football field!
– Most “solid” matter, then, is made up of mostly empty space!
Fundamental Forces in Nature
• Gravitational Force
– Force between massive
bodies
– Infinite in range, but
weakens with distance
• Electromagnetic Force
– Force between charged
bodies
– Infinite in range, but
weakens with distance
• Strong Force
– Force that holds atomic
nuclei together
– Very short range – 10-15
meters!
• Weak Force
– Force responsible for
radioactive decay
– Very short range – 10-18
meters!
Elementary Particles
• Smallest particles
known are quarks,
which make up
protons and
neutrons
– Up quarks
– Down quarks
The description of the universe and its
contents using elementary particles is
called The Standard Model.
• “Up” and “Down”
are just labels
• Other kinds of
quarks have labels
like “strange” and
“charm”, and again
are just labels.