8.5 The Cosmic Engine

Michael Burton, 
School of Physics, UNSW, 
November 1999.   

Contextual Outline:

The universe began with a singularity in space-time and thereafter has seen a continual movement of energy impelling, creating and organising matter. As part of this ongoing process the Sun formed over 4 x 109 years ago from a cloud of gas and dust whose collapse was triggered by a supernova explosion. The condensing gas and dust that formed the Sun contained all its original elements plus the elements formed and inserted during the supernova explosion.

Our solar system is powered by the energy from that original event. More energy is released by nuclear reactions in the core of the Sun and driven outwards as electromagnetic and particle energy. Energy driving the Earth's atmospheric circulation and ocean currents is derived directly and indirectly from this source combined with the energy radiated from the Earth’s core and surface.

The balance between incoming solar radiation and outgoing long-wave radiation defines the Earth’s radiation budget or balance. For the Earth’s surface temperature to remain constant the energy added to the Earth’s surface and atmosphere must be, in the long-term, exactly balanced by emissions. The transfer of energy to maintain thermal equilibrium over the globe relates to the laws of thermodynamics and the mechanisms of heat transfer.

Assumed Knowledge:

  • From Sciences Stage 4-5 syllabus.
http://www.boardofstudies.nsw.edu.au/syllabus_sc/index.html#s

5.6.6a Identify that energy may be released from the nucleii of atoms.

This is the nuclear binding energy, and may be released in two ways: (a) fusion from the combination of light elements into heavier ones (for instance hydrogen into helium); (b) fission from the splitting of heavy elements into lighter ones (such as uranium being split into barium and krypton when bombarded by neutrons).
 

5.7.1a Describe the features and location of protons, neutrons and electrons in the atom.

Protons and neutrons are found in the atomic nucleus, a region of size a few x 10-15 m in extent, and are held together by the nuclear binding energy. Electrons orbit the nucleus, typically 10-10 m away (100,000 times the nuclear radius!), bound with the protons in the nucleus by the forces of electromagnetism. Protons are positively charged, electrons negatively charged and neutrons electrically neutral. Protons and neutrons have similar masses (the neutron is slightly heavier), and are roughly 2000 times more massive than the electron.
 

5.9.1a Discuss current scientific thinking about the origin of the universe

The universe began around 13 billion years ago in a hot, dense, compact state, and has been expanding and cooling ever since. Known as the ‘Big Bang’, its echo is visible through the ‘cosmic microwave background radiation’, thermal radiation at a temperature of just 3 Kelvin that comes uniformly (at least to one part in 100,000) from all directions in space. It is the relic from a time 300,000 years after the Big Bang when the electrons and protons combined to form hydrogen atoms, leaving the radiation (which was then at a temperature of 3,000 K) to travel freely through space. It has been doing so ever since, for over 13 billion years now!
 

5.9.1c Describe some of the difficulties in obtaining information about the universe. 

Our knowledge of the universe comes mostly from the radiation arriving on the Earth from distant astronomical objects. Only a small fraction of that radiation makes it unimpeded through the atmosphere, that in the visible and radio parts of the spectrum. The Earth’s atmosphere attenuates most of the rest, letting some infrared and millimetre wavelength radiation through, but absorbing entirely the UV, X-ray and gamma ray radiation. To observe these parts of the spectrum astronomers have to take their telescopes to remote locations, such as high, dry mountain tops or the Antarctic plateau, high-flying airplanes and balloons and even into space with satellite observatories. Furthermore, the technology to detect radiation outside the visible band has till recently been far inferior to the visible, making sensitive measurements of it even more difficult.
 

5.9.3a Relate some major features of the universe to theories about the formation of the universe.

The cosmic microwave background radiation (CMBR, see 5.9.1a above) indicates that the Universe began in a hot 'Big Bang'. A small ‘dipole’ anisotropy (one direction in space is 3 milli-Kelvin hotter than the other) indicates the Earth, and indeed our entire Galaxy, are moving in one direction in space at a speed of around 600 km/s, probably being pulled by the gravitational field exerted by a large concentration of galaxy clusters (we call this the ‘Great Attractor’ and it is the largest structure we know of in the Universe). Even smaller fluctuations in the CMBR, of level 30 micro-Kelvin, are the seeds from which the Galaxies formed, slight over-densities in the matter distribution which became the nucleation points for the formation of galaxy clusters and super-clusters. The expansion of the Universe, which is evident through Hubble’s observation that galaxies are moving away from us with a speed that is proportional to their distance from us, also indicates that the Universe began in a 'Big Bang'.
 

5.9.3b Describe some changes that are likely to take place during the life of a star. 

Stars are controlled thermo-nuclear bombs, whose life is dominated by the fusion of hydrogen into helium in their cores. The energy this produces heats the star sufficiently so that the internal pressure balances its weight (ie the force of gravity, which is continually trying to make the star collapse). While this balance exists the star leads a relatively sedentary life on what is called the 'Main Sequence'. However, as the hydrogen in the core becomes exhausted the star has to start fusing helium into carbon in order to keep producing enough heat to provide pressure support against collapse. This change in energy production signals the start of a series of changes in the stars appearance and size, all related to the state of the fusion reaction sequence going on in the core. The star puffs up its atmosphere, and expands by about 100 times in radius, becoming a red giant. In expanding the temperature cools, to around 3,000 K, for which the light emitted is predominantly red (hence the name). The envelope will eventually be blown off, producing an expanding shell of material around the star, known as a 'planetary nebula' (the name is a bad one - it has nothing to do with planets!). The end fate of the star itself depends on its mass, but it may simply fade to become a ‘white dwarf’ (a hot, compact sphere about the size of the Earth), or collapse further to a ‘neutron star’ (of size about 10 km) or even entirely as a ‘black hole’. The later two events may be triggered in a ‘supernova’ explosion.
 

Syllabus

  • Ours is just one star in the galaxy and ours is just one galaxy in the universe.
The Universe as Humans see it

http://www.geocities.com/Athens/Atlantis/7119/UniPicture.htm
 

A short synopsis of our changing view of the Universe, including a brief overview of the ideas of the Greeks (Aristotle and Ptolemy) and the Renaissance astronomers of Europe (Copernicus, Kepler, Newton), followed by a more in-depth view of 20th century cosmology, including the expanding universe, the steady-state universe and the Big Bang.
 

Planets, Stars, and Galaxies: A Grand Tour of the Universe

http://cfa-www.harvard.edu/seuforum/teachers/L3/fraknoi.htm
 

This article by Andrew Fraknoi and Sherwood Harrington of the Astronomical Society of the Pacific is a great background piece. Take a trip from Earth to the farthest reaches of the Cosmos! Extracts from it are given below:

THE SOLAR SYSTEM

To begin at home, our Earth is a member of the family of planets and moons known as the solar system. Orbiting our star, the Sun, are nine planets and their more than 40 satellites, each a unique world with its own special characteristics. Assorted cosmic debris - in the form of comets, asteroids, and smaller chunks called meteoroids - also share our system with us.

THE MILKY WAY GALAXY

Beyond the solar system, there is a vast expanse of space, with an occasional grain of dust or elemental atom floating in the dark emptiness. The nearest other star system - Alpha Centauri, best seen from the Earth's southern latitudes - is so far away that Voyager, the fastest spacecraft our species has built, would take about 100,000 years to reach it. Even beams of light, which travel at a phenomenal one billion kilometres an hour, take a little over four years to make the journey between these two systems.

BEYOND THE MILKY WAY

Beyond our own Galaxy lie even larger and emptier regions of what we call "intergalactic space". These areas are so unpopulated that on average you might run across only a single atom in every cubic metre of space. Accompanying our Milky Way are several small "satellite" galaxies, the largest of which are the Magellanic Clouds. They are 150,000 to 200,000 light years away and give astronomers (in the Southern Hemisphere, where the clouds are visible) an excellent opportunity to study another system of stars that has evolved more or less separately from our own.

THE LARGEST OF SCALES

Very recently, astronomers have discovered that even the grand groups of galaxies are not randomly distributed. Galaxy groups seem strung out in vast rounded filaments, separated by enormous voids with relatively few galaxies. This structure, which we are just beginning to glimpse, may hold important clues to the unimaginably violent processes that created the universe.

(From Planets, Stars, and Galaxies: A Grand Tour of the Universe by Andrew Fraknoi and Sherwood Harrington)
 
 

Galaxies Galore

http://amazing-space.stsci.edu/galaxies-galore
 
 

A web-based learning module designed to allow students to use their observational skills, recognize patterns, and learn how galaxies are classified. Students will learn the parts of galaxies and will be able to identify the three main types: spiral, elliptical, and irregular. They will become acquainted with the structure of their home galaxy, the Milky Way. In the assessment activities students test their memory and apply their understanding to new scientific images. Students complete these objectives by undertaking a variety of web-based activities.
 
 

Cosmic Survey: What are your ideas about the Universe?

http://cfa-www.harvard.edu/seuforum/teachers/L3/survey/survey.htm
 
 

Use this pilot lesson plan to launch your pupils on a discussion of size, scale and history of the Universe.
 
 

  • The first minutes of the universe released energy, which changed to matter, forming stars and galaxies.
At a particular instant roughly 13 billion years ago, all the matter and energy we can observe, concentrated in a region smaller than a 5c piece, began to expand and cool at an incredibly rapid rate. By the time the temperature had dropped to 100 million times that of the sun's core, the forces of nature assumed their present properties, and the elementary particles known as quarks roamed freely in a sea of energy. When the universe had expanded an additional 1,000 times, all the matter we can measure filled a region the size of the solar system.

At that time, the free quarks became confined in neutrons and protons. After the universe had grown by another factor of 1,000, protons and neutrons combined to form atomic nuclei, including most of the helium and deuterium present today. All of this occurred within the first minute of the expansion. Conditions were still too hot, however, for atomic nuclei to capture electrons. Neutral atoms appeared in abundance only after the expansion had continued for 300,000 years and the universe was 1,000 times smaller than it is now. The neutral atoms then began to coalesce into gas clouds, which later evolved into stars. By the time the universe had expanded to one fifth its present size, the stars had formed groups recognisable as young galaxies. When the universe was half its present size, nuclear reactions in stars had produced most of the heavy elements from which terrestrial planets were made. Our solar system is relatively young: it formed five billion years ago, when the universe was two-thirds its present size. Over time the formation of stars has consumed the supply of gas in galaxies, and hence the population of stars is waning. Fifteen billion years from now stars like our sun will be relatively rare, making the universe a far less hospitable place for observers like ourselves.

From The Evolution of the Universe by P. James E. Peebles, David N. Schramm, Edwin L. Turner and Richard G. Kron, Scientific American Special Edition, 'The Magnificent Cosmos', March 1998. URL http://www.scientificamerican.com/specialissues/0398cosmos/0398peebles.html
 
 

Timeline for the Universe

http://origins.jpl.nasa.gov/poster/poster.html
 
 

A tutorial describing the entire history of the Universe, from the Big Bang to the advent of life on Earth. The journey passes through the formation of galaxies, the formation of molecular clouds within those galaxies, the formation of stars within those clouds, the evolution of those stars and the subsequent production of the elements through nucleosynthesis, stellar death and the chemical enrichment of the interstellar medium through supernovae, the formation of planetary systems from the interstellar medium and in particular the formation of Jupiter-like and Earth-like planets, ending with the chemistry of life and how it may be started.
 
 

Hubble's Law for the Expanding Universe

http://zebu.uoregon.edu/nsf/hub.html
 
 

This Java applet that allows students to use spectra and galaxy dimensions to simulate Hubble's law, that the speed of recession of a distant galaxy is proportional to its distance from us. Velocities are measured from the redshift of galaxy spectra and distances from their angular size. A good exercise for the more advanced students, but it will take some describing. Note this applet takes some time to load!
 
 

The Expanding Universe

http://btc.montana.edu/ceres/html/uni1.html
 
 

This lesson plan is designed to help students gain a deeper understanding of cosmology. Students develop authentic models and gather evidence supporting the Big Bang theory. This lesson uses observation, interactive media and scientific models. There are 6 activities as part of it:

  1. Explore and define the Hubble Law.
  2. Create a model of the expanding universe (by blowing up a balloon).
  3. Analyse and explain what happens when using different measuring devices.
  4. Answer summary questions to better understand the Hubble Law.
  5. Create an electronic report that describes a Hubble Space Telescope cosmological finding and explain how it relates to the balloon activity.
  6. Classroom de-briefing.
  • Stars have a limited life span and may explode to form supernovas.
The basic difference between a star and a planet is that a star emits light produced in its interior by nuclear ‘burning’, whereas a planet only shines by reflected light. The Sun is our own special star yet, as stars go, it is a very average one. There are stars far brighter, fainter, hotter and cooler than the Sun. Basically, however, all the stars we can see in the sky are objects similar to the Sun. They are great balls of gas held together by their own gravity. The force of gravity is continually trying to force the Sun towards its centre and if there were not some other force counteracting it the Sun would collapse. The necessary outward pressure is produced by the radiation from the nuclear energy generation in the Sun's interior.

Stars form from condensations within huge interstellar gas clouds. These contract due to their own gravitational pull. The star settles down to a long period of stability while the hydrogen at its centre is converted into helium with the release of an enormous amount of energy. This stage is called the main-sequence stage, a reference to the classical Hertzsprung-Russell (HR) diagram, which relates the stellar temperature (or colour) to the luminosity (or 'magnitude'). Most stars lie in a well-defined band in the HR diagram and the only parameter that determines where they lie is the star's mass.

The more massive a star is the quicker it 'burns' up its hydrogen and hence the brighter, bigger and hotter it is. The rapid conversion of hydrogen into helium also means that the hydrogen gets used up far sooner for the massive stars than for the lighter ones. For a star like the Sun the main-sequence stage lasts about 10 billion years whereas a star 10 times as massive will be 10,000 times as bright but will only last for 100 million years. On the other hand, stars a tenth the mass of the Sun have a lifetime far greater than the current age of the Universe!

Stars do not all evolve in the same way. Once again it is the star's mass that determines how they change. Stars similar in mass to the Sun 'burn' hydrogen into helium in their core during the main-sequence phase but eventually there is no hydrogen left there to provide the necessary radiation pressure to balance gravity. The core of the star thus contracts until it is hot enough for helium to be converted into carbon. The hydrogen in a shell continues to ‘burn’ into helium but the outer layers of the star have to expand. This makes the star appear brighter and cooler and it becomes a 'red giant'.

During the red giant phase a star often loses much of its outer layers, blown away by the radiation coming from below. In the more massive stars the carbon may be ‘burnt’ to even heavier elements but eventually all energy generation will fizzle out and the star will collapse to what is called a ‘white dwarf’, if its mass is less than five times that of the Sun.

There are very few stars with masses greater than this but their evolution ends in a spectacular fashion. They go through their evolutionary stages very quickly compared to the Sun. They expand enormously, becoming red 'supergiants'. During this stage many different chemical elements will be produced in the star and the central temperature will approach 100 million Kelvin.

For elements of lower atomic number than iron the addition of more nucleons to the nucleus releases binding energy and so yields a small contribution to the balance inside the star between gravity and radiation. However, to add nucleons to an iron nucleus requires energy and so once the core of the star has been converted to iron no more energy can be extracted. The star's core then has no resistance to the force of gravity and it contracts rapidly. The protons and electrons combine to produce a core composed of neutrons and a vast amount of gravitational energy is released. This energy is sufficient to blow away all the outer parts of the star in a violent explosion and the star becomes a supernova. The light of this explosion shines as bright as an entire galaxy for a few days. During this phase all the elements with atomic weights greater than iron are formed in the expanding shell and are blown out into interstellar space. The central core of neutrons is left as a neutron star, which could be a pulsar, or even collapse to a black hole.

What is remarkable about this process is that the first stars were composed almost entirely of hydrogen and helium, without oxygen, nitrogen, iron, or any of the other elements that are necessary for life. These were all produced inside massive stars and then spread throughout space by such supernovae events. We are made up of material that has been processed at least once, and probably several times, inside stars-we are 'stardust'.

Adapted from Royal Greenwich Observatory Information Leaflet 'What is a Star?', which is obtainable following the "Astronomy Fact Files" link at http://www.rog.nmm.ac.uk/
 
 

Stars and Galaxies

http://www.telescope.org/btl/sg.html
 
 

From the Bradford Robotic Telescope Observatory site. Contains a series of images, MPEG movies and audio clips on stars, galaxies, their origin and evolution. However there is limited text. A CD is available with further information. The following topics are included:

  • Origin of the Universe
  • Formation of Stars
  • Life Cycle of the Stars
  • Energy of Stars
  • The Sun
  • Nebulae
  • Galaxies
  • Some Remarkable Sights
Inverse Square Law

http://jersey.uoregon.edu/vlab/InverseSquare/index.html
 
 

A Java applet allowing students to generate data and graphs to study the inverse square law. By clicking at different distances images containing a variety of light sources (‘stars’) of differing luminosity a plot of the inverse square law is built up (I proportional to  r-2).
 

  • The solar system formed after a supernova explosion.
A cloud of interstellar gas (the "solar nebula") is disturbed and collapses under its own gravity. The disturbance could have been caused, for example, by the shock wave from a nearby supernova. As the cloud collapses, it heats up and compresses in the centre, forming a protostar. Most of the gas flows inward and adds mass to the protostar. However the cloud is rotating and centrifugal forces (and the conservation of angular momentum) prevent all the gas from reaching the protostar. Instead, it forms an "accretion disk" around it. The gas cools off enough for the metal, rock and (far enough from the forming star) ice to condense out into tiny particles of dust. The dust particles collide with each other and form into larger particles. This goes on until the particles get to the size of boulders or small asteroids. Larger particles then are big enough to have a non-trivial gravity and their growth accelerates, forming proto-planets. After about 1 million years the nebula has cooled and the star generates a strong wind, which sweeps away all of the remaining gas. If a proto-planet was large enough its gravity would pull in the nebular gas, and it would become a gas giant (eg Jupiter). If not, it would remain a rocky or icy body (eg Earth). Eventually, after ten to a hundred million years, we end up with ten or so planets, in stable orbits. These planets and their surfaces may be heavily modified by the last, big collision they experience, and they are scarred by impact craters from collisions with smaller bodies.

Planetary structure is determined by chemical differentiation whilst the planet is still molten. Dense elements sink to the core of the planet and light elements rise to the surface. After the planet cools and solidifies this internal structure remain frozen.

Our solar system consists of the Sun; the nine planets, sixty-six satellites of the planets, a large number of small bodies (the comets and asteroids), and the interplanetary medium. The inner solar system contains the Sun, Mercury, Venus, Earth and Mars and the outer solar system contains Jupiter, Saturn, Uranus, Neptune and Pluto.

The orbits of the planets are ellipses with the Sun at one focus, though all except Mercury and Pluto are very nearly circular. The orbits are all more or less in the same plane (called the ecliptic), and are in the same sense. All but Venus and Uranus also rotate in the same sense as the orbit.
 

Supernova Chemistry

http://imagine.gsfc.nasa.gov/docs/teachers/lessons/supernova/supernova_cover.html
 

This lesson gives the student an opportunity to identify elements by using spectroscopy and relate this activity to astrophysics. Students will observe visible spectra of known elements and identify an unknown element or combination of elements by their visible spectra. Requires purchase of a Science Kit (purchasable over the internet from the States) containing spectroscopes and spectrum tubes.
 

The Nine Planets

http://seds.lpl.arizona.edu/billa/tnp/nineplanets.html
 

The Nine Planets is an overview of the history, mythology, and current scientific knowledge of each of the planets and moons in our solar system. Each page has text and images, some have sounds and movies, and most provide references to additional related information.
 

Kepler’s Third Law

http://javalab.uoregon.edu/dcaley/kepler/Kepler.html
 

This Java applet allows students to model Kepler's Third Law (T2 proportional to R3) by examining the orbits of two planets (eg Earth and Mars) and then varying their orbital distances and recording the orbital periods.
 

  • The Sun is a typical star, emitting radiation and particles that influence the Earth.
The Sun is an ordinary star, one of more than 100 billion stars in our galaxy. It is by far the largest object in the solar system, containing more than 99.8% of its mass (Jupiter contains most of the rest). The Sun is, at present, about 75% hydrogen and 25% helium by mass; everything else ("metals") amounts to only 0.1%. This changes slowly over time as the Sun converts hydrogen to helium in its core. Conditions in the Sun's core are extreme. The temperature is 15 million Kelvin and the density 150 times that of water. The Sun's energy output (4 x1026 Watts) is produced by nuclear fusion reactions in the core. Each second about 700 million tons of hydrogen are converted to about 695 million tons of helium and 5 million tons of energy in the form of gamma rays. As the gamma-rays travels outwards, their energy is continuously absorbed and re-emitted at lower and lower temperatures so that by the time they reach the surface, they are primarily visible light. The surface of the Sun, called the photosphere, is at a temperature of about 6,000 K. Sunspots are "cool" regions, only 4,000 K (they look dark only by comparison with the surrounding regions). Sunspots can be very large, as much as 50,000 km in diameter. Sunspots are caused by interactions with the Sun's magnetic field. In addition to heat and light, the Sun also emits a low density stream of charged particles (mostly electrons and protons) known as the solar wind, which travels throughout the solar system at about 450 km/sec. The solar wind can have dramatic effects on the Earth ranging from power line surges, to radio interference, to beautiful aurora.

The Sun's output is not entirely constant. Nor is the amount of sunspot activity. There was a period of very low sunspot activity in the latter half of the 17th century called the Maunder Minimum. It coincided with an abnormally cold period in northern Europe sometimes known as the Little Ice Age. Since the formation of the solar system the Sun's output has increased by about 40%. It is now about 4.5 billion years old and will continue to radiate "peacefully" for another 5 billion years or so (although its luminosity will approximately double in that time). But eventually it will run out of hydrogen fuel and be forced into radical changes, which will result in the total destruction of the Earth (and probably the creation of a planetary nebula).
 

Electromagnetic Radiation on Trial

http://cse.ssl.berkeley.edu/lessons/indiv/nellie/new_nelli_summary.html
 

A web-based exercise in which students are introduced to the properties of electromagnetic radiation in a variety of ways. For example, students create a "Quipu" - a method used by the ancient Incas. Students also are encouraged to use an electronic bulletin board to communicate with each other, posting insights, ideas, evidence and questions on electromagnetic radiation. As part of this activity, students also put the different types of the electromagnetic radiation "on trial", selecting the judge, prosecutor, defence counsel, and jury, and learning about electromagnetic energy by arguing the pros and cons of each wavelength.
 

The Light Tour

http://cse.ssl.berkeley.edu/light/light_tour.html
 

A self-tutorial that introduces the electromagnetic spectrum through the concepts of wavelength and amplitude. Students learn to associate various wavelength ranges with different spectral bands, and can explore images from space astronomy missions in each band.
 

Sunspots

http://cse.ssl.berkeley.edu/segway/lessons/sunspots/
 

This lesson covers solar science, ancient and modern, features an interactive research exercise in which students attempt to correlate the areas of sunspots with those of x-ray active regions. Self-guided sections on history and modern study include researcher interviews.
 

The Sun Song

http://www.astrocappella.com/
 

The Sun Song, from Astro-Capella. A lesson on the Sun, taught by singing. The song is accompanied by background information on the Sun, images and an activity examining convection inside a bowl of miso soup!

  • The conditions at the surface of the Earth are influenced by the interactions between physical phenomena generated by both the Sun and the Earth.
The Earth is divided into several layers which have distinct chemical and seismic properties:
  1. Crust 0-40 km
  2. Mantle 40-2,700 km
  3. Core 2,700-6,400 km
The crust varies considerably in thickness; it is thinner under the oceans, thicker under the continents. The inner core and crust are solid, the outer core and mantle layers are plastic or semi-fluid. The core is probably composed mostly of iron though some lighter elements may be present too. Temperatures there may be as high as 7,500 K, hotter than the surface of the Sun. The Earth's crust is divided into several separate, solid, 'tectonic' plates, which float around independently on top of the hot mantle below.

The Earth's surface is very young. In the relatively short (by astronomical standards) period of 500 million years or so, erosion and tectonic processes destroy and recreate most of the surface and thereby eliminate almost all traces of earlier geologic surface history (such as impact craters). Thus, the very early history of the Earth has mostly been erased. The Earth is about 4.6 billion years old, but the oldest known rocks are about 4 billion years old and rocks older than 3 billion years are rare. The oldest fossils of living organisms are about 3.5 billion years old. There is no record of the critical period when life was first getting started.

Earth has a modest magnetic field produced by electric currents in the core. The interaction of the solar wind, the Earth's magnetic field and the Earth's upper atmosphere causes the auroras. The Earth's magnetic field and its interaction with the solar wind also produce the Van Allen radiation belts, a pair of doughnut shaped rings of ionized gas (or plasma) trapped in orbit around the Earth, extending from 7,500 km to 40,000 km in altitude.
 

Facts, figures and images about the Earth:

http://pds.jpl.nasa.gov/planets/welcome/earth.htm
 

From the JPL planets site. Some great pictures on the Earth and Moon.


 
 

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