ASTRO 101
Principles of Astronomy
Instructor: Jerome A. Orosz
(rhymes with
“boris”)
Contact:
• Telephone: 594-7118
• E-mail: orosz@sciences.sdsu.edu
• WWW:
http://mintaka.sdsu.edu/faculty/orosz/web/
• Office: Physics 241, hours T TH 3:30-5:00
Homework
• Homework due February 19: Question 4 from
Chapter 3 (What are the three main functions
of a telescope?)
• Write down the answer on a sheet of paper and
hand it in before the end of class on February
19.
Homework
• Go to a planetarium show in PA 209:
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Tuesday, January 29
Wednesday, January 30
Friday, February 1
Monday, February 4
Tuesday, February 5
Wednesday, February 6
Thursday, February 7
Friday, February 8
1:00 -- 2:00 PM
2:00 -- 3:00 PM
1:00 -- 2:00 PM
11:00 AM -- 12:00 PM
5:30 PM -- 6:30 PM
2:00 -- 3:00 PM
5:30 -- 6:30 PM
2:30 -- 3:30 PM
• Get 10 points extra credit for homework part of grade.
• Sign up for a session outside PA 209.
• Hand in a sheet of paper with your name and the date and time of
the session.
Isaac Newton (1642-1727)
http://www-history.mcs.st-andrews.ac.uk/history/PictDisplay/Newton.html
Isaac Newton (1642-1727)
• Isaac Newton was born the year Galileo
died.
• He was professor of mathematics at
Cambridge University in England. (Steven
Hawking currently hold’s Newton’s Chair
at Cambridge).
• He was later the Master of the Mint in
London, where first proposed the use of
grooved edges on coins to prevent shaving.
Isaac Newton (1642-1727)
• Newton was perhaps the greatest scientist of
all time, making substantial contributions to
physics, mathematics (he invented calculus
as a college student), optics, and chemistry.
Isaac Newton (1642-1727)
• Newton was perhaps the greatest scientist of
all time, making substantial contributions to
physics, mathematics (he invented calculus
as a college student), optics, and chemistry.
• His laws of motion and of gravity could
explain Kepler’s Laws of planetary motion.
Newton’s Laws of Motion
1.
2.
3.
A body in motion tends to stay in motion in a straight
line unless acted upon by an external force.
The force on an object is the mass times the acceleration
(F=ma).
For every action, there is an equal and opposite reaction.
(For example, a rocket is propelled by expelling hot gas
from its thrusters).
What is Gravity?
• Gravity is a force between all matter in the
Universe.
• It is difficult to say what gravity is.
However, we can describe how it works.
What is Gravity?
• The gravitational force between larger
bodies is greater than it is between smaller
bodies, for a fixed distance.
What is Gravity?
• As two bodies move further apart, the
gravitational force decreases. The range of
the force is infinite, although it is very small at
very large distances.
Newton’s Laws
• Using Newton’s Laws, we can…
 Derive Kepler’s Three Laws.
 Measure the mass of the Sun, the Moon, and
the Planets.
 Measure the masses of distant stars in binary
systems.
Laws of Physics
• The models of Aristotle and Ptolomy were
based mainly on beliefs (i.e. that motion should
be on perfect circles, etc.).
• Starting with Newton, we had a physical
model of how the planets moved: the laws of
motion and gravity as observed on Earth give a
model for how the planets move.
• All modern models in Astronomy are based on
the laws of Physics.
Newton’s Laws and Orbits
• Newton realized that
since the Moon’s
path is curved (i.e. it
is accelerating), there
must be a force
acting on it.
Newton’s Laws and Orbits
• If you shoot a
cannonball
horizontally, it follows
a curved path to the
ground. The faster you
launch it, the further it
goes.
Newton’s Laws and Orbits
• If you shoot a
cannonball
horizontally, it follows
a curved path to the
ground. The faster you
launch it, the further it
goes.
• If it goes really far, the
Earth curves from
under it…
Newton’s Laws and Orbits
• Why doesn’t the Moon fall down on the Earth?
Newton’s Laws and Orbits
• Why doesn’t the Moon fall down on the Earth?
• Because of angular momentum…
Angular Momentum
• Angular momentum is a measure of the spin
of an object. It depends on the mass that is
spinning, on the distance from the rotation
axis, and on the rate of spin.
• I = (mass).(radius).(spin rate)
Angular Momentum
• Angular momentum is a measure of the spin
of an object. It depends on the mass that is
spinning, on the distance from the rotation
axis, and on the rate of spin.
• For a given distance to the rotation axis,
more mass means more angular momentum.
Angular Momentum
• Angular momentum is a measure of the spin
of an object. It depends on the mass that is
spinning, on the distance from the rotation
axis, and on the rate of spin.
• For a given distance to the rotation axis,
more mass means more angular momentum.
• For a given mass, a larger distance means
more angular momentum.
Angular Momentum
• Angular momentum is a measure of the spin
of an object. It depends on the mass that is
spinning, on the distance from the rotation
axis, and on the rate of spin.
• For a fixed mass and distance, a higher rate
of spin means a larger angular momentum.
Angular Momentum
• Angular momentum is a measure of the spin
of an object. It depends on the mass that is
spinning, on the distance from the rotation
axis, and on the rate of spin.
• I = (mass).(radius).(spin rate)
• The angular momentum in a system stays
fixed, unless acted on by an outside force.
Conservation of Angular Momentum
• An ice skater demonstrates
the conservation of angular
momentum:
Conservation of Angular Momentum
• An ice skater demonstrates
the conservation of angular
momentum:
• Arms held in: high rate of
spin.
• Arms extended: low rate of
spin.
• I = (mass).(radius).(spin rate)
(angular momentum and
mass are fixed here)
Angular Momentum
• Angular momentum is a measure of the spin of
an object. It depends on the mass that is
spinning, on the distance from the rotation axis,
and on the rate of spin.
• I = (mass).(radius).(spin rate)
• The angular momentum in a system stays fixed,
unless acted on by an outside force.
• An orbiting body will not move towards the
other body unless there is an external force.
Weight and Mass
Weight and Mass
• In Physics, we distinguish between weight
and mass:
Weight and Mass
• In Physics, we distinguish between weight
and mass:
 Weight is a force due to gravity.
Weight and Mass
• In Physics, we distinguish between weight
and mass:
 Weight is a force due to gravity.
 Mass is a measure of the amount of matter in
an object.
Weight and Mass
• In Physics, we distinguish between weight
and mass:
 Weight is a force due to gravity.
 Mass is a measure of the amount of matter in
an object.
 The units of weight are pounds in the British
system or newtons in the metric system.
Weight and Mass
• In Physics, we distinguish between weight
and mass:
 Weight is a force due to gravity.
 Mass is a measure of the amount of matter in
an object.
 The units of weight are pounds in the British
system or newtons in the metric system.
 The units of mass are stones in the British
system or kilograms in the metric system.
Weight and Mass
• Your weight depends where you are (e.g.
on the Earth, on the Moon, in outer space,
etc.).
• Your mass is the same no matter where you
are.
• In most cases on Earth, we can use the
terms weight and mass interchangeably.
Weight and Mass
• The mass is used in Newton’s Gravity
formula:
Next:
• Chapter 3: Light and Telescopes
Coming Up:
• The 4 forces of Nature
• Energy and the conservation of energy
• The nature of light
– Waves and bundles of energy
– Different types of light
• Telescopes and detectors
The 4 “Forces” of Nature
•
There are 4 “fundamental forces” in nature:
1.
2.
3.
4.
Gravity: relative strength = 1, range = infinite.
Electromagnetic: rel. str. = 1036, range = infinite.
“Weak” nuclear: rel. str. = 1025, range = 10-10 meter.
“Strong” nuclear: rel. str. = 1038, range = 10-15
meter.
The 4 “Forces” of Nature
•
There are 4 “fundamental forces” in nature:
1.
2.
3.
4.
•
Gravity: relative strength = 1, range = infinite.
Electromagnetic: rel. str. = 1036, range = infinite.
“Weak” nuclear: rel. str. = 1025, range = 10-10 meter.
“Strong” nuclear: rel. str. = 1038, range = 10-15
meter.
Gravity is an attractive force between all matter
in the Universe. The more mass something has,
the larger the net gravitational force is.
The 4 “Forces” of Nature
•
There are 4 “fundamental forces” in nature:
1.
2.
3.
4.
•
Gravity: relative strength = 1, range = infinite.
Electromagnetic: rel. str. = 1036, range = infinite.
“Weak” nuclear: rel. str. = 1025, range = 10-10 meter.
“Strong” nuclear: rel. str. = 1038, range = 10-15
meter.
The electromagnetic force can be repulsive (+,+
or -,-) or attractive (+,-). Normal chemical
reactions are governed by this force.
The 4 “Forces” of Nature
•
There are 4 “fundamental forces” in nature:
1.
2.
3.
4.
•
•
Gravity: relative strength = 1, range = infinite.
Electromagnetic: rel. str. = 1036, range = infinite.
“Weak” nuclear: rel. str. = 1025, range = 10-10 meter.
“Strong” nuclear: rel. str. = 1038, range = 10-15
meter.
The weak force governs certain radioactive
decay reactions.
The strong force holds atomic nuclei together.
The 4 “Forces” of Nature
•
There are 4 “fundamental forces” in nature:
1.
2.
3.
4.
•
Gravity: relative strength = 1, range = infinite.
Electromagnetic: rel. str. = 1036, range = infinite.
“Weak” nuclear: rel. str. = 1025, range = 10-10 meter.
“Strong” nuclear: rel. str. = 1038, range = 10-15
meter.
Gravity is the most important force over large
scales since positive and negative charges tend
to cancel.
A Thought Experiment
• How does your vision work?
– Do your eyes send out a “scanning” signal?
– Do your eyes receive information from outside?
• How can you tell?
What is Energy?
What is light, and what can it
tell us?
Energy is the ability to do “work.”
“Work” is done when something is
moved.
Forms of energy
• Energy of motion (e.g. moving bodies):
 For a given velocity, a more massive object has
more energy.
 For a given mass, a faster moving body has
more energy.
• Potential energy:
 Chemical energy.
 Nuclear energy.
 Gravitational energy.
Forms of energy
• Thermal (or heat) energy.
• Electromagnetic energy.
Forms of energy
• Thermal (or heat) energy.
• Electromagnetic energy.
• Mass, as in E=mc2.
The conservation of energy:
The conservation of energy:
Energy is neither created nor
destroyed, but may be changed in
form.
Energy changing form:
• Potential energy in gasoline turns into
energy of motion of a car, along with heat
and noise.
• The energy of motion of a falling body
creates an impact crater.
• Matter in turned into energy at the center of
the Sun.
Coming Up:
• The 4 forces of Nature
• Energy and the conservation of energy
• The nature of light
– Waves and bundles of energy
– Different types of light
• Telescopes and detectors
Light is a form of energy.
Light is a form of energy.
Why is this important?
Light is a form of energy.
Why is this important?
With very few exceptions, the
only way we have to study
objects in Astronomy is via the
light they emit.
What is the nature of light?
What is the nature of light?
Light can be thought of as a
wave in an electric field
or
as discrete particles of energy…
What is the nature of light?
Image from Nick Strobel’s Astronomy Notes (http://www.astronomynotes.com)
Light can be thought of as a wave. The wavelength
(usually denoted with a l) is the distance from crest to
crest.
What is the nature of light?
Image from Nick Strobel’s Astronomy Notes (http://www.astronomynotes.com)
Light can be thought of as a wave. The frequency
(usually denoted with n) is the number of crests that pass
a given point each second.
What is the nature of light?
Light can be thought of as a wave. The frequency
(usually denoted with n) is the number of crests that pass
a given point each second.
What is the nature of light?
The velocity of the wave is the wavelength times
the frequency:
The velocity of light in vacuum is constant for
all wavelengths, regardless of the relative
velocities of the observer and the light source.
What is the nature of light?
The velocity of light is not infinite.
What is the nature of light?
Although the velocity of light is large, it is not
infinite.
c = 300,000 km/sec
or
c = 186,000 miles/sec
What is the nature of light?
Although the velocity of light is large, it is not
infinite.
c = 300,000 km/sec
or
c = 186,000 miles/sec
Ordinary matter cannot travel faster than the
speed of light.
What is the nature of light?
Image from Nick Strobel’s Astronomy Notes (http://www.astronomynotes.com)
The above animation shows waves with different
wavelengths moving with the same speed. Their
frequencies are different.
What is the nature of light?
Light can be thought of as a
wave in an electric field
or
as discrete particles of energy…
What is the nature of light?
Light can also behave like discrete particles called
photons. The energy of a photon depends
on the frequency (or equivalently the
wavelength):
The value of h is constant for all situations.
What is the nature of light?
Photons of higher energy have higher frequencies
and shorter wavelengths, since
What is the nature of light?
Image from Nick Strobel’s Astronomy Notes (http://www.astronomynotes.com)
The above animation shows waves with different
wavelengths moving with the same speed. Their
frequencies are different.
Intensity vs. Energy
• A photon’s energy
depends on the
frequency.
• The intensity of a source
refers to the number of
waves or photons from
that source.
Image from Nick Strobel’s Astronomy Notes (http://www.astronomynotes.com)
Different “types” of light.
What light can tell us.
Visible light
• White light is made up of different colors
Visible light
• Different colors correspond to different
frequencies (or wavelengths).
• The colors of the rainbow are ROY G BIV:
red orange yellow green blue indigo violet.
Visible light
• In the visible,
 red has the longest wavelength, the smallest
frequency, and the lowest energy.
 violet has the shortest wavelength, the highest
frequency, and the highest energy.
The Electromagnetic Spectrum
• Visible light is only a tiny
fraction of the
Electromagnetic Spectrum.
• For example, there is invisible
radiation with wavelengths
longer than red light that heats
the thermometer.
The Electromagnetic Spectrum
• As we go to wavelengths slightly longer
than visible (i.e. smaller frequencies and
lower energies), we find infrared radiation,
which is basically perceived as heat.
The Electromagnetic Spectrum
• As we go to wavelengths slightly longer
than visible (i.e. smaller frequencies and
lower energies), we find infrared radiation,
which is basically perceived as heat.
• As we go to longer wavelengths still, we
find microwave radiation, which is often
used to pop popcorn.
The Electromagnetic Spectrum
• At the longest wavelengths, corresponding
to the smallest frequencies and the lowest
energies, we have radio waves, including
AM/FM, shortwave, TV, etc.
The Electromagnetic Spectrum
• Visible light is only a tiny fraction of the
Electromagnetic Spectrum.
• If we go to shorter wavelengths (higher
frequencies and energies), we find
ultraviolet light. With higher energies, UV
photons can damage skin cells.
The Electromagnetic Spectrum
• As we go even shorter in wavelength
(higher in frequency and energy), we get Xrays. With their high energies, X-rays can
be used to image our insides.
The Electromagnetic Spectrum
• As we go even shorter in wavelength
(higher in frequency and energy), we get Xrays. With their high energies, X-rays can
be used to image our insides.
• As the shortest wavelengths and the highest
energies, we have gamma rays. Gamma
rays are sometimes used to sterilize food.
The Electromagnetic Spectrum
• Visible light is only a tiny
fraction of the
Electromagnetic Spectrum.
The Electromagnetic Spectrum
• Gamma rays, X-rays, UV light, visible light,
infrared radiation, microwaves, and radio waves
are all different manifestations of
electromagnetic energy.
• The range in wavelengths typically encountered
span a factor of 1014.
• All forms of electromagnetic radiation travel
with the same velocity.
• The Earth’s atmosphere is transparent to visible
light, some infrared, and the radio. It is opaque to
UV, X-rays, and gamma rays.
Coming Up:
• The 4 forces of Nature
• Energy and the conservation of energy
• The nature of light
– Waves and bundles of energy
– Different types of light
• Telescopes and detectors