8.5 The Cosmic Engine
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.
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 universeThe 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.
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
Planets, Stars, and Galaxies: A Grand Tour of the Universe
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)
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?
Use this pilot lesson plan to launch your
pupils on a discussion of size, scale and history of the Universe.
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
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
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
The Expanding Universe
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:
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
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:
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
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.
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
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
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'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
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
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.
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
The Sun Song
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 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:
From the JPL planets site. Some great pictures on the Earth and Moon.
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