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 energy? The ability to do work
• What is light? Waves in an electric field or
  bundles of energy
• Why do different lamps have different colors?
  They have different gasses
• What is the difference between red and blue
  light? The wavelength, or energies of the
  photons, or the frequencies
• Why is argon different from Helium? Argon has
  more protons in the nucleus

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Questions for Today

• How do we measure velocities of things in the
  sky?
• What is a telescope used for?

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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
• Telescopes and detectors

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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.
• Mass, as in Emc2.

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

• 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 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.
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What are Things Made of?

• Among other things, chemistry is the study of
  matter and its composition.

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What are Things Made of?

• Among other things, chemistry is the study of
  matter and its composition.
• Most substances around us can be divided
  chemically into simpler things
• Water --gt hydrogen and oxygen
• Table salt --gt sodium and chlorine

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What are Things Made of?

• Among other things, chemistry is the study of
  matter and its composition.
• Most substances around us can be divided
  chemically into simpler things
• Water --gt hydrogen and oxygen
• Table salt --gt sodium and chlorine
• At some point, certain things dont chemically
  break down into different parts. These are
  called elements.

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What are Things Made of?

• At some point, certain things dont chemically
  break down into different parts. These are
  called elements.
• Examples of elements hydrogen, helium, carbon,
  oxygen, gold, silver, mercury, uranium,

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What are Things Made of?

• At some point, certain things dont chemically
  break down into different parts. These are
  called elements.
• Examples of elements hydrogen, helium, carbon,
  oxygen, gold, silver, mercury, uranium,
• There are 92 stable and common elements.

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What are Things Made of?

• At some point, certain things dont chemically
  break down into different parts. These are
  called elements.
• Suppose you took a sample of an element and
  physically divided the sample into two, and took
  one of the halves and divided it into two, and so
  on. Can you go on forever dividing by two?

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What are Things Made of?

• At some point, certain things dont chemically
  break down into different parts. These are
  called elements.
• Suppose you took a sample of an element and
  physically divided the sample into two, and took
  one of the halves and divided it into two, and so
  on. Can you go on forever dividing by two?
• No, at some point you reach individual atoms. An
  atom cannot be split into parts without changing
  it.

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How Light Interacts with Matter.

• Atoms are the basic blocks of matter.
• They consist of heavy particles (called protons
  and neutrons) in the nucleus, surrounded by
  lighter particles called electrons.

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How Light Interacts with Matter.

• Atoms are the basic blocks of matter.
• They consist of heavy particles (called protons
  and neutrons) in the nucleus, surrounded by
  lighter particles called electrons.
• The nucleus is very tiny.

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How Light Interacts with Matter.

• An electron will interact with a photon.
• An electron that absorbs a photon will gain
  energy.
• An electron that loses energy must emit a photon.
• The total energy (electron plus photon) remains
  constant during this process.

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How Light Interacts with Matter.

• Electrons bound to atoms have discrete energies
  (i.e. not all energies are allowed).
• Thus, only photons of certain energy can interact
  with the electrons in a given atom.

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How Light Interacts with Matter.

• Electrons bound to atoms have discrete energies
  (i.e. not all energies are allowed).
• Thus, only photons of certain energy can interact
  with the electrons in a given atom.

Image from Nick Strobel (http//www.astronomynotes
.com)
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How Light Interacts with Matter.

• Electrons bound to atoms have discrete energies
  (i.e. not all energies are allowed).
• Each element has its own unique pattern of
  energies.

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How Light Interacts with Matter.

• Electrons bound to atoms have discrete energies
  (i.e. not all energies are allowed).
• Each element has its own unique pattern of
  energies, hence its own distinct line spectrum.

Image from Nick Strobel (http//www.astronomynotes
.com)
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How Light Interacts with Matter.

• An electron in free space can have any energy.
• It can absorb a photon of any energy
• It can lose any amount of energy (?E) by emitting
  a photon with energy equal to ?E
• An electron in an atom can only have very
  specific values of its energy E1, E2, E3, EN)
• The electron can absorb a photon with an energy
  equal to (E1-E2), (E1-E3), (E2-E3), and jump to
  a higher level
• The electron can lose an amount of energy equal
  to a change between levels (E1-E2), (E1-E3),
  (E2-E3), and move down to a lower level

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How Light Interacts with Matter.

• An electron in an atom can only have very
  specific values of its energy E1, E2, E3, EN)
• The electron can absorb a photon with an energy
  equal to (E1-E2), (E1-E3), (E2-E3), and jump to
  a higher level
• The electron can lose an amount of energy equal
  to a change between levels (E1-E2), (E1-E3),
  (E2-E3), and move down to a lower level
• Since each element has its own unique sequence of
  energy levels (E1, E2, E3, EN), the differences
  between the levels are also unique, giving rise
  to a unique line spectrum

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Emission spectraandabsorption spectra.
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Emission and Absorption

• If you view a hot gas against a dark background,
  you see emission lines (wavelengths at which
  there is an abrupt spike in the brightness).

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Emission and Absorption

• If you view a continuous spectrum through cool
  gas, you see absorption lines (wavelengths where
  there is little light).

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Emission and Absorption
Image from Nick Strobel (http//www.astronomynotes
.com)
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The spectrum

• View a hot, dense source, get a continuous
  spectrum.
• View that hot source through cool gas, get an
  absorption spectrum.
• View that gas against a dark background, get
  emission spectrum.

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Tying things together

• The spectrum of a star is approximately a black
  body spectrum.
• Hotter stars are bluer, cooler stars are redder.
• For a given temperature, larger stars give off
  more energy than smaller stars.

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• In the constellation of Orion, the reddish star
  Betelgeuse is a relatively cool star. The blue
  star Rigel is relatively hot.

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Tying things together

• The spectrum of a star is approximately a black
  body spectrum.
• Hotter stars are bluer, cooler stars are redder.
• For a given temperature, larger stars give off
  more energy than smaller stars.
• However, a closer look reveals details in the
  spectra

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The Line Spectrum

• Upon closer examination, the spectra of real
  stars show fine detail.
• Dark regions where there is relatively little
  light are called lines.

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The Line Spectrum

• Today, we rarely photograph spectra, but rather
  plot the intensity vs the wavelength.
• The lines where there is relatively little
  light show up as dips in the curves.

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The Line Spectrum

• Today, we rarely photograph spectra, but rather
  plot the intensity vs the wavelength.
• The lines where there is relatively little
  light show up as dips in the curves.
• These dips tell us about what elements are
  present in the star!

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Atomic Fingerprints

• Hydrogen has a specific line spectrum.
• Each atom has its own specific line spectrum.

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Atomic Fingerprints

• These stars have absorption lines with the
  wavelengths corresponding to hydrogen!

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Atomic Fingerprints.

• One can also look at the spectra of other objects
  besides stars, for example clouds of hot gas.
• This cloud of gas looks red since its spectrum is
  a line spectrum from hydrogen gas.