ASTRO 101

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ASTRO 101

• Principles of Astronomy

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Instructor Jerome A. Orosz
(rhymes with boris)Contact

• Telephone [phone]
• E-mail orosz_at_sciences.sdsu.edu
• WWW http//mintaka.sdsu.edu/faculty/orosz/web/
• Office Physics 241, hours T TH 330-500

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Text Perspectives on Astronomy First
Editionby Michael A. Seeds Dana Milbank.
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Astronomy Help Room Hours

• Monday 1200-1300, 1700-1800
• Tuesday 1700-1800
• Wednesday 1200-1400, 1700-1800
• Thursday 1400-1800, 1700-1800
• Friday 900-1000, 1200-1400
• Help room is located in PA 215

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Homework

• Assigned question due September 24 Question 3,
  Chapter 4 (Why do nocturnal animals usually have
  large pupils in their eyes? How is that related
  to the design of astronomical telescopes?)

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Looking Ahead

• This week Classes 7 and 8
• Tuesday, September 29 In-class review
• Thursday, October 1 Exam 1
• Extra review session at a time TBD.

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Questions from Before

• What is gravity?
• It is an attractive force between all matter in
  the Universe
• Why do we usually have two high tides per day?
• The tides arise from the differences in the
  gravitational pull of the moon on different parts
  of the oceans. The Earth is pulled away from the
  water on the side opposite of the Moon.

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Questions for Today

• What is energy?
• What is light?
• Why do different lamps have different colors?
• What is the difference between red and blue
  light?
• Why is argon different from Helium?

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Next

• Tides and Orbits
• Telescopes (Chapter 4)

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Newtons Laws of Motion

• 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 (Fma).
• For every action, there is an equal and opposite
  reaction. (For example, a rocket is propelled by
  expelling hot gas from its thrusters).

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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.

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What is Gravity?

• The gravitational force between larger bodies is
  greater than it is between smaller bodies, for a
  fixed distance.

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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.

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Newtons Laws

• Using Newtons Laws, we can
• Derive Keplers Three Laws.
• Measure the mass of the Sun, the Moon, and the
  Planets.
• Measure the masses of distant stars in binary
  systems.

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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.

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Newtons Laws of Motion

• 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 (Fma).
• For every action, there is an equal and opposite
  reaction. (For example, a rocket is propelled by
  expelling hot gas from its thrusters).

What can we conclude about the moon from these?
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Newtons Laws and Orbits

• Newton realized that since the Moons path is
  curved (i.e. it is accelerating), there must be a
  force acting on it.

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Newtons 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.

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Newtons 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

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Newtons Laws and Orbits

• Newton showed mathematically that the expected
  shape for a closed orbit is an ellipse (i.e. he
  explained the origin of Keplers first law).

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Newtons Laws and Orbits

• A geosynchronous satellite has an orbital period
  around the Earth of 24 hours (23 hours and 56
  minutes actually), which is the rotation period
  of the Earth.
• The net effect is that the satellite is always
  above the same spot.

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Newtons Laws and Tides

• If the tides are caused by the Moon pulling on
  the oceans, then why is there usually two high
  tides per day?

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Newtons Laws and Tides

• If the tides are caused by the Moon pulling on
  the oceans, then why is there usually two high
  tides per day?
• Actually tides are caused by differences in the
  gravitational forces.

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Newtons Laws and Tides

• Spring tides are when the Sun and Moon are
  roughly aligned (e.g. new and full moon). The
  tides tend to be higher at these times.
• Local conditions can also effect the height of
  the tides.

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Weight and Mass
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Weight and Mass

• In Physics, we distinguish between weight and
  mass

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Weight and Mass

• In Physics, we distinguish between weight and
  mass
• Weight is a force due to gravity.

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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.

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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.

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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.

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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.

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Weight and Mass

• The mass is used in Newtons Gravity formula

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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
• The spectrum
• Definition
• Emission and absorption
• How light interacts with matter

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The 4 Forces of Nature

• There are 4 fundamental forces in nature
• 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.

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The 4 Forces of Nature

• There are 4 fundamental forces in nature
• 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.

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The 4 Forces of Nature

• There are 4 fundamental forces in nature
• 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.

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The 4 Forces of Nature

• There are 4 fundamental forces in nature
• 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.

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The 4 Forces of Nature

• There are 4 fundamental forces in nature
• 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.

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Next

• Light and Telescopes (Chapter 4 and parts of
  Chapter 5)

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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?

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What is Energy?What is light, and what can it
tell us?
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Energy is the ability to do work.Work is
done when something is moved.
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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.

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Forms of energy

• Thermal (or heat) energy.
• Electromagnetic energy.

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Forms of energy

• Thermal (or heat) energy.
• Electromagnetic energy.
• Mass, as in Emc2.

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The conservation of energy
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The conservation of energyEnergy is neither
created nor destroyed, but may be changed in form.
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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.

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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
• The spectrum
• Definition
• Emission and absorption
• How light interacts with matter

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Light is a form of energy.
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Light is a form of energy.Why is this important?
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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.
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What is the nature of light?
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What is the nature of light?Light can be
thought of as awave in an electric fieldoras
discrete particles of energy
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What is the nature of light?
Image from Nick Strobels 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.
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What is the nature of light?
Image from Nick Strobels 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.
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What is the nature of light?

• More animations and tools
• http//micro.magnet.fsu.edu/primer/java/electromag
  netic/index.html
• http//www.phy.ntnu.edu.tw/ntnujava/viewtopic.php?
  t52

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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.
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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
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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.
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What is the nature of light?
Image from Nick Strobels Astronomy Notes
(http//www.astronomynotes.com)
The above animation shows waves with different
wavelengths moving with the same speed. Their
frequencies are different.
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What is the nature of light?Light can be
thought of as awave in an electric fieldoras
discrete particles of energy
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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.
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What is the nature of light?
Photons of higher energy have higher frequencies
and shorter wavelengths, since
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What is the nature of light?
Image from Nick Strobels Astronomy Notes
(http//www.astronomynotes.com)
The above animation shows waves with different
wavelengths moving with the same speed. Their
frequencies are different.
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Intensity vs. Energy

• A photons energy depends on the frequency.
• The intensity of a source refers to the number of
  waves or photons from that source.

Image from Nick Strobels Astronomy Notes
(http//www.astronomynotes.com)
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Different types of light.What light can tell
us.
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Visible light

• Most people can perceive color.
• Different colors correspond to different
  frequencies (or wavelengths).
• The colors of the rainbow are ROY G BIV red
  orange yellow green blue indigo violet.

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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.

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The Electromagnetic Spectrum

• Visible light is only a tiny fraction of the
  Electromagnetic Spectrum.

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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.

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The Electromagnetic Spectrum

• As we go even shorter in wavelength (higher in
  frequency and energy), we get X-rays. With their
  high energies, X-rays can be used to image our
  insides.

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The Electromagnetic Spectrum

• As we go even shorter in wavelength (higher in
  frequency and energy), we get X-rays. 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.

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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.

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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.

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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.

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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.

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• The Earths atmosphere is transparent to visible
  light, some infrared, and the radio. It is
  opaque to UV, X-rays, and gamma rays.

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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
• The spectrum
• Definition
• Emission and absorption
• How light interacts with matter

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The spectrum

• Definition and types
• Continuous
• Discrete
• The spectrum and its uses
• Temperature
• Chemical composition
• Velocity

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The spectrum

• A graph of the intensity of light vs. the color
  (e.g. the wavelength, frequency, or energy) is
  called a spectrum.
• A spectrum is probably the single most useful
  diagnostic tool available in Astronomy.

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The spectrum

• A spectrum can tell us about the temperature and
  composition of an astronomical object.
• There are two types of spectra of concern here
• Continuous spectra (the intensity varies smoothly
  from one wavelength to the next).
• Line spectra (there are discrete jumps in the
  intensity from one wavelength to the next).

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The spectrum

• Continuous spectrum.
• Discrete or line spectra.

Images from Nick Strobel (http//www.astronomynote
s.com)
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Thermal Spectra

• The most common type of continuous spectrum is a
  thermal spectrum.
• Any dense body will emit a thermal spectrum of
  radiation when its temperature is above absolute
  zero
• The color depends only the temperature
• The total intensity depends on the temperature
  and the size of the body.
• This type of radiation is often called black
  body radiation.

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Black body radiation

• Sample spectra from black bodies of different
  temperatures. Note that the area under the curves
  is largest for the hottest temperature.
• There is always a well-defined peak, which
  crudely defines the color. The peak is at bluer
  wavelengths for hotter temperatures.

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Black body radiation

• Sample spectra from black bodies of different
  temperatures. Note that the area under the curves
  is largest for the hottest temperature.
• There is always a well-defined peak, which
  crudely defines the color. The peak is at bluer
  wavelengths for hotter temperatures.

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Important points

• The luminosity (energy loss per unit time) of a
  black body is proportional the surface area times
  the temperature to the 4th power

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Important points

• The luminosity (energy loss per unit time) of a
  black body is proportional the surface area times
  the temperature to the 4th power
• Hotter objects have higher intensities (for a
  given area), and larger objects have higher
  intensities.

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Important points

• The peak of the spectrum is inversely
  proportional to the temperature (hotter objects
  are bluer)

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Important points

• The peak of the spectrum is inversely
  proportional to the temperature (hotter objects
  are bluer)
• Hotter objects are bluer than cooler objects.

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How light interacts with matter andthe line
spectrum.