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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Astronomy Help Room Hours

• Monday 1200-1300, 1700-1800
• Tuesday 1200-1600
• Wednesday 1300-1400, 1700-1800
• Thursday 1200-1300, 1700-1800
• Friday 1100-1300
• Help room is located in PA 215
• Office hours extended drop by any time

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Coming Up

• Chapter 12 (Formation of the Solar System)
• Homework due April 30 Question 4, Chapter 12
  (According to the solar nebula theory, why is the
  Suns equator nearly in the plane of the Earths
  orbit?

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Questions for Today

• How old is the Sun?
• How did the Solar System form?

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Next

• Solar System Planets
• The Origin of the Solar System

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Quick Concept Review

• Some useful concepts
• Density
• Albedo
• Gravity

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Density and Albedo
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Density and Albedo

• The concepts of density and albedo are useful in
  planetary studies.

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Density and Albedo

• The concepts of density and albedo are useful in
  planetary studies.
• Density mass/volume
• The density of water is 1 gram per cubic cm.
• The density of rock is 3 grams per cubic cm.
• The density of lead is 8 grams per cubic cm.
• The density of an object can give an indication
  of its composition.

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Density and Albedo

• The concepts of density and albedo are useful in
  planetary studies.
• Albedo of incident light that is reflected.
• A perfect mirror has an albedo of 100
• A black surface has an albedo of 0.
• The albedo of an object is an indication of the
  surface composition.

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The Planets

• Why solar system planets are special

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The Planets

• Why solar system planets are special
• Planets are resolved when seen through telescopes
  (i.e. you can see the disk, surface features,
  etc.).

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The Planets

• Why solar system planets are special
• Planets are resolved when seen through telescopes
  (i.e. you can see the disk, surface features,
  etc.).
• You can also send spacecraft to visit them.

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The Planets

• Why solar system planets are special
• Planets are resolved when seen through telescopes
  (i.e. you can see the disk, surface features,
  etc.).
• You can also send spacecraft to visit them.
• Stars always appear pointlike, even in the
  largest telescopes.

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The Planets

• Why solar system planets are special
• Planets are resolved when seen through telescopes
  (i.e. you can see the disk, surface features,
  etc.).
• You can also send spacecraft to visit them.
• Stars always appear pointlike, even in the
  largest telescopes. Also, they are so far away
  that we cannot send probes to study them.

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The Solar System
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The Solar System

• The Solar System refers to the Sun and the
  surrounding planets, asteroids, comets, etc.

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The Solar System

• The Solar System refers to the Sun and the
  surrounding planets, asteroids, comets, etc.
• Do not confuse solar system with galaxy
• The solar system is the local collection of
  planets around the Sun.
• A galaxy is a vast collection of stars, typically
  a hundred thousand light years across.

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The Solar System Census

• There were 5 planets known since antiquity
• Mercury
• Venus
• Mars
• Jupiter
• Saturn

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The Solar System Census

• There were 5 planets known since antiquity
• Mercury
• Venus
• Mars
• Jupiter
• Saturn
• Since the 1600s (Kepler, Galileo, Newton), the
  Earth was considered a planet as well.

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New Members

• Uranus discovered in 1781 by William Herschel.

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New Members

• Uranus discovered in 1781 by William Herschel.
• Neptune discovered in 1846 by Johann Galle
  (based on the predictions of John C. Adams and
  Urbain Leverrier).

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New Members

• Uranus discovered in 1781 by William Herschel.
• Neptune discovered in 1846 by Johann Galle
  (based on the predictions of John C. Adams and
  Urbain Leverrier).
• Pluto discovered in 1930 by Clyde Tombaugh.

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New Members

• Uranus discovered in 1781 by William Herschel.
• Neptune discovered in 1846 by Johann Galle
  (based on the predictions of John C. Adams and
  Urbain Leverrier).
• Pluto discovered in 1930 by Clyde Tombaugh.
• Asteroids thousands, starting in 1801.

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New Members

• Uranus discovered in 1781 by William Herschel.
• Neptune discovered in 1846 by Johann Galle
  (based on the predictions of John C. Adams and
  Urbain Leverrier).
• Pluto discovered in 1930 by Clyde Tombaugh.
• Asteroids thousands, starting in 1801.
• Kuiper Belt Objects Dozens, starting in the
  1980s.

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Pluto Demoted!

• The definition of a planet was changed
  recently
• Planets The eight worlds from Mercury to
  Neptune.
• Dwarf Planets Pluto and any other round object
  that "has not cleared the neighborhood around its
  orbit, and is not a satellite."
• Small Solar System Bodies All other objects
  orbiting the Sun.
• http//www.space.com/scienceastronomy/060824_plane
  t_definition.html

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The Solar System

• The planets orbit more or less in the same plane
  in space. Note the orbit of Pluto.
• This view is a nearly edge-on view.

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The Solar System

• The Solar System refers to the Sun and the
  surrounding planets, asteroids, comets, etc.
• The scale of things
• It takes light about 11 hours to travel across
  the Solar system.

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The Solar System

• The Solar System refers to the Sun and the
  surrounding planets, asteroids, comets, etc.
• The scale of things
• It takes light about 11 hours to travel across
  the Solar system. This is 0.001265 years.

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The Solar System

• The Solar System refers to the Sun and the
  surrounding planets, asteroids, comets, etc.
• The scale of things
• It takes light about 11 hours to travel across
  the Solar system. This is 0.001265 years.
• It takes light about 4.3 years to travel from the
  Sun to the nearest star.

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The Solar System

• The Solar System refers to the Sun and the
  surrounding planets, asteroids, comets, etc.
• The scale of things
• It takes light about 11 hours to travel across
  the Solar system. This is 0.001265 years.
• It takes light about 4.3 years to travel from the
  Sun to the nearest star.
• It takes light about 25,000 years to travel from
  the Sun to the center of the Galaxy.

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Scale Model Solar System

• Most illustrations of the solar system are not to
  scale.

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Scale Model Solar System

• Most illustrations of the solar system are not to
  scale.
• Usually, the size of the planets shown is too
  large.

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Scale Model Solar System

• Build your own scale model of the solar system
• http//www.exploratorium.edu/ronh/solar_system/
• http//www.umpi.maine.edu/info/nmms/solar/index.ht
  m

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Scale Model Solar System

• Build your own scale model of the solar system
• http//www.exploratorium.edu/ronh/solar_system/
• http//www.umpi.maine.edu/info/nmms/solar/index.ht
  m
• Conclusion the solar system is pretty empty.

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Scale Model Solar System

• Most depictions of asteroids in the movies are
  wrong

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The Scale Model Solar System

• Most depictions of asteroid fields are also not
  to scale. Image from the official Star Wars pages

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The Scale Model Solar System

• Most depictions of asteroid fields are also not
  to scale. Image from Star Trek Voyager.

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The Formation of the Solar System

• A good theory of the formation of the solar
  system should be able to explain the properties
  of our solar system.

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The Formation of the Solar System

• A good theory of the formation of the solar
  system should be able to explain the properties
  of our solar system.
• One should be able to apply this theory to other
  planetary systems around other stars.

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Basic Properties of our Solar System

• The planets orbit in nearly the same plane, and
  in the same direction. The planetary orbits are
  nearly circular.

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Basic Properties of our Solar System

• The planets orbit in nearly the same plane, and
  in the same direction. The planetary orbits are
  nearly circular.
• The rotation axis of the Sun is nearly
  perpendicular to this plane, as are the axes of
  most of the planets.

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Basic Properties of our Solar System

• The rotation axis of the Sun is nearly
  perpendicular to this plane, as are the axes of
  most of the planets.
• The composition of the planets varies as the
  distance from the Sun the rocky bodies are
  close, whereas the gaseous bodies are further out.

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Two Types of Planets

• Planets come in two types
• Small and rocky.
• Large and gaseous.
• Or
• Terrestrial
• Jovian

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Two Types of Planets

• Planets come in two types
• Small and rocky.
• Large and gaseous.
• Or
• Terrestrial
• Jovian

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The Terrestrial Planets

• The terrestrial planets are Mercury, Venus, Earth
  (and Moon), and Mars.
• Their densities range from about 3 grams/cc to
  5.5 grams/cc, indicating their composition is a
  combination of metals and rocky material.

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The Terrestrial Planets

• The terrestrial planets are Mercury, Venus, Earth
  (and Moon), and Mars.

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The Giant Planets

• The giant planets are Jupiter, Saturn, Uranus,
  and Neptune.

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The Giant Planets

• The radii are between about 4 and 11 times that
  of Earth.
• The masses are between 14 and 318 times that of
  Earth.

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The Giant Planets

• The radii are between about 4 and 11 times that
  of Earth.
• The masses are between 14 and 318 times that of
  Earth.
• However, the densities are between 0.7 and 1.8
  grams/cc, and the albedos are high.

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The Giant Planets

• The radii are between about 4 and 11 times that
  of Earth.
• The masses are between 14 and 318 times that of
  Earth.
• However, the densities are between 0.7 and 1.8
  grams/cc, and the albedos are high.
• The planets are composed of light elements,
  mostly hydrogen and helium.

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The Gas Giants

• The composition of the giant planets, especially
  Jupiter, is close to that of the Sun.
• The internal structures of these planets is
  completely different from that of the Earth. In
  particular, there is no hard surface.
• These planets are relatively far from the Sun
  (more than 5 times the Earth-Sun distance), so
  heating by the Sun is not a big factor.

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How Old is the Solar System?
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Meteors Finding the Age of the Solar System
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Meteors

• There are many small chunks of matter orbiting
  the Sun.
• A piece that is in space is a meteoroid.
• A piece that burns up in the Earths atmosphere
  is a meteor (a bright streak of light).
• A piece that lands on Earth is a meteorite.

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Meteors

• Many meteor showers are associated with comets.

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Dust from Comets

• The dust tail contains small particles evaporated
  from the comet.
• These particles remain in orbit about the Sun.
• If the Earth passes through the dust cloud,
  then several meteors may be seen.

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Meteor Showers

• During periods of high meteor activity, most of
  the events appear to come from one spot on the
  sky.
• This point is roughly where the comets tail was.

Dust particles enter the atmosphere and burn up,
causing a streak of light.

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Rocks from Space

• Some early cultures were aware that rocks
  sometimes fell from the sky. These items had
  great religious value, e.g. the Black Stone of
  Kaaba.
• Enlightened scientists in the 18th and 19th
  centuries declared that stones cannot possibly
  fall from space. It was all primitive
  superstition.

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Rocks from Space

• Thomas Jefferson said It is easier to believe
  that two Yankee professors Profs. Silliman and
  Kingsley of Yale would lie than that stones
  would fall from the sky.
• Jefferson was wrong stones do fall from the sky.

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Rocks from Space

• Evidence that rocks fall from space
• There have been eyewitness accounts of impacts.
• In many cases, the mineral composition of samples
  indicates the material cannot be native to Earth.
• Most older samples are iron, most fresh samples
  are stony material.

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Rocks from Space
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Where to Find Meteorites

• Antarctica is one of the best places to find
  meteorites on Earth, owing to the high contrast
  (black rocks on white snow).

http//www-curator.jsc.nasa.gov/curator/antmet/pro
gram.htm
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Where to Find Meteorites

• Over time, meteorites tend to get concentrated in
  certain areas because of large-scale ice flows.

http//www-curator.jsc.nasa.gov/curator/antmet/pro
gram.htm
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Meteorites

• Most older samples are iron.
• Iron is dense and not easily weathered.
• Most fresh samples are composed of stony
  materials.
• This material is easily weathered and does not
  last long on the Earths surface.

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Rocks from Space

• Why is are meteorites useful?
• They are material samples from outside the Earth
  that can be analyzed in the laboratory.
• We can measure the age of the solar system by
  studying meteorites.

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Radioactive Decay

• A chemical element is uniquely determined by the
  number of protons its nucleus has. For example,
  hydrogen has 1 proton, carbon has 6 protons, etc.
• Different isotopes of the same element differ
  only in their number of neutrons. For example 12C
  has 6 protons and 6 neutrons and 14C has 8
  neutrons and 6 protons.

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Radioactive Decay

• Different isotopes of the same element differ
  only in their number of neutrons. For example 12C
  has 6 protons and 6 neutrons and 14C has 8
  neutrons and 6 protons.
• A radioactive isotope is an isotope prone to
  spontaneous change.

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Radioactive Decay

• Different isotopes of the same element differ
  only in their number of neutrons. For example 12C
  has 6 protons and 6 neutrons and 14C has 8
  neutrons and 6 protons.
• A radioactive isotope is an isotope prone to
  spontaneous change.
• 14C changes into 14N
• 40K changes into 40Ar

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Radioactive Decay

• A radioactive isotope is an isotope prone to
  spontaneous change.
• 14C changes into 14N
• 40K changes into 40Ar
• The decay rate for a given isotope is fixed and
  can be measured in the laboratory. The rate is
  usually given as a half life, which is the
  amount of time required for half of a given
  sample to decay.

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Radioactive Decay

• The decay rate for a given isotope is fixed and
  can be measured in the laboratory. The rate is
  usually given as a half life, which is the
  amount of time required for half of a given
  sample to decay.
• The half life can be as short as a fraction of a
  second or as long as billions of years.

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Radioactive Decay

• For a given atom, there is a certain probability
  that it will decay.
• For a large collection of atoms, a
  well-determined half life emerges from the
  statistics of a large number of events.

Image from Nick Strobel (http//www.astronomynotes
.com)
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Radioactive Decay

• Example the half life of 40K is 1.25 billion
  years. Suppose we start with 1 kg.
• In 1.25 billion years, we have 1/2 kg of 40K and
  1/2 kg of 40Ar.

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Radioactive Decay

• Example the half life of 40K is 1.25 billion
  years. Suppose we start with 1 kg.
• In 1.25 billion years, we have 1/2 kg of 40K and
  1/2 kg of 40Ar.
• In 2.50 billion years, we have 1/4 kg of 40K and
  3/4 kg of 40Ar.

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Radioactive Decay

• Example the half life of 40K is 1.25 billion
  years. Suppose we start with 1 kg.
• In 1.25 billion years, we have 1/2 kg of 40K and
  1/2 kg of 40Ar.
• In 2.50 billion years, we have 1/4 kg of 40K and
  3/4 kg of 40Ar.
• In 3.75 billion years, we have 1/8 kg of 40K and
  7/8 kg of 40Ar.

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Radioactive Decay

• My measuring the relative amounts of the
  radioactive parent isotope to the resulting
  daughter isotope in a rock, one can measure the
  amount of time since the rock sample solidified.

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Radioactive Decay

• My measuring the relative amounts of the
  radioactive parent isotope to the resulting
  daughter isotope in a rock, one can measure the
  amount of time since the rock sample solidified.
• In practice one looks at many parent/daughter
  combinations, and also looks at stable isotopes
  of the parent and/or daughter.

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Radioactive Decay

• The oldest rocks on the Earth were solidified
  about 4 billion years ago.

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Radioactive Decay

• The oldest rocks on the Earth were solidified
  about 4 billion years ago.
• The oldest rocks from the Moon were solidified
  4.4 billion years ago.

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Radioactive Decay

• The oldest rocks on the Earth were solidified
  about 4 billion years ago.
• The oldest rocks from the Moon were solidified
  4.4 billion years ago.
• The oldest meteorites solidified 4.55 billion
  years ago.

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Radioactive Decay

• The oldest rocks on the Earth were solidified
  about 4 billion years ago.
• The oldest rocks from the Moon were solidified
  4.4 billion years ago.
• The oldest meteorites solidified 4.55 billion
  years ago. The Sun and the solar system are about
  4.6 billion years old.

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Rocks from Space

• Why is are meteorites useful?
• They are material samples from outside the Earth
  that can be analyzed in the laboratory.
• We can measure the age of the solar system by
  studying meteorites.

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Notes of Radioactivity

• Three types of radioactivity are known
• ? rays, which are helium nuclei (e.g. two
  protons and two neutrons together). These
  particles have positive charge.
• ? rays, which are either electrons (negative
  charge) or antielectrons (positive charge).
• ? rays, which are high energy photons.
• All three types carry energy, hence radioactivity
  can heat a sample from inside.

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Notes of Radioactivity

• The health effects
• ? rays can cause burns to skin tissue, but
  only if the source is very close. A piece of
  paper can stop ? particles.
• ? rays can damage cell structure, more
  penetrating than ? rays.
• ? rays very damaging to cell structure, more
  penetrating than ? rays.

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Next The Formation of the Solar System
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Basic Properties of our Solar System

• The composition of the planets varies as the
  distance from the Sun the rocky bodies are
  close, whereas the gaseous bodies are further
  out.
• Meteorites have peculiar chemical compositions,
  and the orbits of comets have a different
  distribution than the planets.

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Condensation Theory

• The leading theory of the formation of the solar
  system states that the Sun and the Solar System
  condensed out of a much larger cloud of gas and
  dust.

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Condensation Theory

• Interstellar clouds of gas and dust are common in
  the Galaxy.
• This is an HST image of the Eagle Nebula.

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Condensation Theory

• Interstellar clouds of gas and dust are common in
  the Galaxy.
• This is a ground based image of a cloud in Cygnus.

Image from http//www.ras.ucalgary.ca/CGPS/
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Gravity and Angular Momentum

• Two important concepts to consider in the
  formation theory

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Gravity and Angular Momentum

• Two important concepts to consider in the
  formation theory
• Gravity pulls things together

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Gravity and Angular Momentum

• Two important concepts to consider in the
  formation theory
• Gravity pulls things together
• Angular momentum a measure of the spin of an
  object or a collection of objects.

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Gravity

• There are giant clouds of gas and dust in the
  galaxy. They are roughly in equilibrium, where
  gas pressure balances gravity.

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Gravity

• There are giant clouds of gas and dust in the
  galaxy. They are roughly in equilibrium, where
  gas pressure balances gravity.
• Sometimes, an external disturbance can cause
  parts of the cloud to move closer together. In
  this case, the gravitational force may be
  stronger than the pressure force.

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Gravity

• Sometimes, an external disturbance can cause
  parts of the cloud to move closer together. In
  this case, the gravitational force may be
  stronger than the pressure force.
• As more matter is pulled in, the gravitational
  force increases, resulting in a runaway collapse.

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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.
• The angular momentum in a system stays fixed,
  unless acted on by an outside force.

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Conservation of Angular Momentum

• If an interstellar has some net rotation, then it
  cannot collapse to a point. Instead, the cloud
  collapsed into a disk that is perpendicular to
  the rotation axis.

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Condensation Theory

• An interstellar cloud collapsed to a disk.
  Friction in the disk drives matter inward and
  outward (conserving angular momentum).
• Planets eventually form in the disk.

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Condensation Theory
Image from Nick Strobels Astronomy Notes
(http//www.astromynotes.com)
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Condensation Theory

• An interstellar cloud collapsed to a disk.
  Friction in the disk drives matter inward and
  outward (conserving angular momentum).

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Condensation Theory

• An interstellar cloud collapsed to a disk.
  Friction in the disk drives matter inward and
  outward (conserving angular momentum).
• Eventually, there enough mass is at the central
  object to form the protosun.

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Condensation Theory

• Eventually, there enough mass is at the central
  object to form the protosun.
• Heat from the protosun drives away the lighter
  gasses nearby.

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Condensation Theory

• Heat from the protosun drives away the lighter
  gasses nearby.
• The terrestrial planets condense out the rocky
  material that is left over.

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Planetary Development

• In the protoplanetary disk, small density
  perturbations can lead to a runaway growth
• A slightly higher density gives rise to a
  stronger gravitational force.
• The higher force leads to the attraction of
  material.
• The addition of more material leads to a stronger
  gravitational force.
• An so on

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Planetary Development

• In the protoplanetary disk, small density
  perturbations can lead to a runaway growth
• A slightly higher density gives rise to a
  stronger gravitational force.
• The higher force leads to the attraction of
  material.
• The addition of more material leads to a stronger
  gravitational force.
• An so on
• The process stops when there is no more material
  in the protoplanetary disk.

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Planetary Development

• There are 4 main stages of development

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Planetary Development

• There are 4 main stages of development
• Differentiation. As the body grows it heats up.
  Eventually the proto-Earth became hot enough to
  melt the rocky material. Then heavy elements
  tend to sink towards the center, whereas the
  lighter elements tend to migrate away from the
  center.

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Planetary Development

• There are 4 main stages of development
• Differentiation. As the body grows it heats up.
  Eventually the proto-Earth became hot enough to
  melt the rocky material. Then heavy elements
  tend to sink towards the center, whereas the
  lighter elements tend to migrate away from the
  center.
• Bombardment. The newly-formed surface is heavily
  cratered.

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Planetary Development

• There are 4 main stages of development
• Differentiation. As the body grows it heats up.
  Eventually the proto-Earth became hot enough to
  melt the rocky material. Then heavy elements
  tend to sink towards the center, whereas the
  lighter elements tend to migrate away from the
  center.
• Bombardment. The newly-formed surface is heavily
  cratered.
• Flooding. Molten rock from volcanoes and also
  water fill low lying areas.

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Planetary Development

• There are 4 main stages of development
• Differentiation.
• Bombardment. The newly-formed surface is heavily
  cratered.
• Flooding. Molten rock from volcanoes and also
  water fill low lying areas.
• Slow surface evolution via geological processes
  and weathering. This requires the presence of an
  atmosphere!

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Condensation Theory

• Heat from the protosun drives away the lighter
  gasses nearby.
• The terrestrial planets condense out the rocky
  material that is left over.

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Condensation Theory

• The terrestrial planets condense out the rocky
  material that is left over.
• In the outer solar system, the rocky cores can
  capture gas and grow to large sizes (e.g. the
  Jovian planets).

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Condensation Theory

• In the outer solar system, the rocky cores can
  capture gas and grow to large sizes (e.g. the
  Jovian planets).
• Gravitational interactions between the young
  planets eventually cleans out the solar system
  up to about Neptune.

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Condensation Theory

• Gravitational interactions between the young
  planets eventually cleans out the solar system
  up to about Neptune.
• Comets and Kuiper-belt objects are left in the
  outer solar system.

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Condensation Theory

• This theory predicts that the present day Solar
  System should be nearly planar, with all rotation
  in the same direction. This is what is observed.

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Condensation Theory

• One expects to have fluff in the outer reaches
  of the solar system, far away from the largest
  bodies. This is what is observed.

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Condensation Theory

• This theory predicts that the inner planets
  should be rocky, and the outer planets should be
  gaseous. This is what is observed.

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Condensation Theory

• One expects to have fluff in the outer reaches
  of the solar system, far away from the largest
  bodies. This is what is observed.

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Condensation Theory

• Overall, the condensation theory does a
  reasonably good job in explaining how the solar
  system came to be.

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Condensation Theory

• Overall, the condensation theory does a
  reasonably good job in explaining how the solar
  system came to be.
• Can it be applied elsewhere?
• Young star systems.
• Extrasolar planets.

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NEXT

• Planets around other stars