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A Brief History of Astronomy
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[The Month's Sky]
[History of Astronomy]
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A Brief History of Astronomy
From Creation Myths to Big Bang Cosmology |
Support this web site
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The length of the day, the month and the year is known. The five naked eye planets are known.
The planets are known to move against the background of the stars which appear fixed to a crystal sphere. The word planet means "wanderer".
The Pythagoreans think that the motions of the planets are mathematically related to musical sounds and number. These ideas are called "The Music of the Spheres".
He talks about the four elements (earth, fire air and water) which he says are only found on Earth. These elements each have their own tendencies: earth is heavy and falls, fire is light and rises. Motion is in straight lines. The heavier the object, the faster it falls.
A fifth element, the Aether, is only present in the objects of the sky. Its natural motion is circular so celestial objects travel around the Earth in perfect circles. Aristotle assumes that light travels infinitely fast.
The Earth and the heavens are, therefore, subject to different natural laws. Things on Earth are corrupted and subject to change while the heavens are incorruptible and unchanging.
His measurement is within 1% of the correct value.
Aristarchus accurately measures of the distance to the Moon using trigonometry applied to Lunar eclipses. He correctly shows that the moon is 25% as large as the Earth.
He makes the first attempt to find the distance to the Sun. His theory is good but the measurements are difficult and his figure (19 times further than the Moon - 5% of the correct value) is too low. Even so, the Sun is shown to be larger than the Earth.
Aristarchus even suggests that the Earth goes around the larger Sun. This idea does not take root because of lack of evidence and will not become accepted for 1800 years.
He thinks that he had observed positional changes amongst the so called "fixed stars" but he is unsure. He creates a very accurate map of the 1000 or so brightest stars. This map will play an important role in astronomical history 1800 years later.
During his research he discovers that there are two types of year. The Tropical Year and the Sidereal Year differ by 20 minutes. This causes the position of the Celestial Pole to move in a circle taking 26,000 years to complete one cycle. This phenomenon is called the Precession of the Equinoxes.
Poseidonius also measures the distance between the Earth and the Sun to an accuracy of 43%.
He also popularises astrology.
The cosmology is based on Earth being the centre of the Universe with the Sun, Moon, planets, and stars (all set on crystal spheres) revolving around the Earth in a series of circles. The planets, Mercury and Venus always lie close to the line joining the Earth and the Sun.
The system is cumbersome but could be used to predict the motions of the planets to naked eye accuracy. Tables are created that predict the positions of the planets in the future. He republishes the star map of Hipparchus and names the (48) classical constellations with the names they are still known by in the West.
Ptolemy writes that the sphere of the stars is 200 times further away than the Moon.
The book also contains a summary of geographical knowledge with estimates of latitudes and longitudes for places in Europe. These would not be improved for 800 years.
The book is one of the few to survive the chaos of the European Dark Ages. After the fall of the Roman Empire, the book would be translated into Arabic in the Islamic world, and, later, into Latin and will play a part in Europe's Renaissance
Al-Battani also updates the figures for the Precession of the Equinoxes (54.5'' per year) and the tilt of the Earth's axis (23° 35').
His observations show that the Earth's distance to the Sun varies, putting a doubt on the idea of perfect circular orbits.
This is the first written mention of the object that would later be known as the Andromeda Galaxy.
His surveying techniques using triangulation allow him to measure the radius of the Earth as 6339.6 km, a value that would not be improved for 500 years.
He suggests that the velocity of light is immense compared to that of sound. He theorises that the appearance of the Milky Way is due to it being made up of countless stars, an assertion that would not be verified until the invention of the telescope 500 years later.
The (Christian) Catholic Church adopts Aristotle's cosmology. In the coming centuries, disagreement with this cosmology would become a heresy.
The model enlarges Aristotle's ideas of the corrupt Earth and the perfect heavens. The most corrupt part of the Universe is Hell which is situated in the centre of the Earth. Both Earth and Hell are imperfect and both are subject to change, corruption and decay. Man's Sin causes the corruption of the Earth. Above the Earth is the atmosphere. This is less subject to change but changes enough to produce the weather. Aurora, meteors and comets are also considered to be atmospheric phenomena.
The Moon being further from the Earth, changes less. It changes its phases and has a blotchy appearance but has the perfect circular motion of a celestial object. The Sun and planets come next. They don't change and also move in circles around the Earth. Most distant is the crystal sphere containing the stars. The stars are unchanging and eternal. God (the most perfect part of the Universe) is on the outside of this final crystal sphere. All heavenly motion is in circles (a perfect shape) or 'circles within circles'.
Dante, would later write about descending the nine circles to Hell and ascending the celestial spheres to God.
Based on Biblical chronologies, the Earth and the Universe were considered to be only a few thousand years old.
His opponents counter with several arguments. The Earth could not carry the Moon around with it if it was moving. Winds would blow us off if the Earth was rotating. The stars should show a parallax (i.e they would change relative position as our vantage point changed).
Copernicus has few answers but suggests that the stars fail to show a parallax because they are very distant. The book is later banned by the Catholic Church until the late 20th Century.
The new Sun-centred system is less cumbersome than Ptolemy's and can be used for predicting the positions and movements of the planets.
Brahe finds no parallax indicating that it is a real stellar object and not something close to the Earth. The star fades after a couple of years. This is an indication that the starry heavens do change.
He also studies a comet and shows that it is moving in an elongated orbit amongst the planets. This indicates that comets are not atmospheric phenomena and that there are no crystal spheres holding the planets since objects can move freely between the planets. It also shows that not all heavenly motion is circular.
This is the first observational evidence that Aristotle and Ptolemy's ideas may be flawed. Brahe disagrees with Copernicus, however, and writes that the planets do indeed go around the Sun but that the Sun (carrying all the planets) orbits the Earth. This half-way idea is not taken seriously. He measures the year to an accuracy of one second. This helps promote the introduction of the Gregorian Calendar (now the international standard) in 1582.
Tycho Brahe is the last of the European naked-eye astronomers. His detailed and accurate observations of the motion of Mars would lead to a better understanding of planetary orbits after his death.
He is eventually burnt at the stake for heresy!
On the first night, Galileo sees stars that are invisible to the naked eye. If these stars were being seen for the first time, the 'ancients' could not have known everything! The Milky Way is seen to be a vast collection of stars too numerous to be seen individually.
Observing the Sun, he sees sunspots, imperfections in the 'perfect' Sun. He watches them move across the Sun as it rotates; the first time a celestial body has been observed to rotate on its axis. This leads to the thought that if the large Sun could rotate, why not the smaller Earth.
He sees Venus go through a complete cycle of phases. The planet appears to change its shape like a miniature Moon from full to half to crescent. This could only happen if it was moving around the Sun. If Venus was always between the Sun and the Earth (as Ptolemy thought) it would only exhibit a crescent phase at all times.
Through the telescope, the Moon appears to have mountains and plains. This showed it to be a world, no different to the Earth.
Looking at Jupiter Galileo discovers its four large moons, resembling little stars. This proves that not everything is moving directly around the Earth. It is also an indication that it is possible for a body to carry its moons with it as it moves around the Sun. If Jupiter could carry four moons why could the Earth not carry its single moon.
Galileo's observations support the Sun-centred Universe of Copernicus and he advocates this system in all his writings. Unlike most academics of the time who only write in Latin, Galileo writes his books in the local language so that they could be read by everyone. The Catholic Church is angered and forces him to deny that the Earth is moving around the Sun.
Although not the first to perform experiments, Galileo makes it fashionable and disproves some of Aristotle's assertions. By dropping cannon balls from a tower (actually, the Leaning Tower of Pisa) he proves that heavy objects fall to the Earth at the same rate as light objects. He shows that falling bodies accelerate as they fall to Earth. More interestingly, moving bodies could be subject to two separate forces acting independently. This explains how objects could be carried on the Earth even if it was moving.
He also shows that the period of a pendulum swing is constant for a given length. This will eventually lead to accurate timepieces.
He attempts to measure the speed of light but fails due to lack of accurate equipment. Galileo's physics experiments set the stage for Newton's work.
In addition, Kepler shows that the closer a planet is to the Sun, the faster it moves. This is the same effect that causes ballet dancers to rotate faster when they bring their arms in. The planets are seen to be following mechanical laws similar to those on the Earth. This is a further blow to the ancient idea of one law for the Earth, another for celestial objects.
Kepler discovers a simple mathematical relationship between the period of a planet to orbit the Sun and its distance from the Sun. The square of the period is proportional to the cube of the distance. This provides a scale for the Solar System. If any single distance in the Solar System could be measured it would be possible to calculate all the others. Saturn, the furthest planet, is shown to be 10 times further from the Sun than the Earth.
Kepler suggests that the Sun somehow pulls the planets around it. He correctly predicts the passage of the planets Mercury and Venus in front of the Sun. These are called transits and they would later help in accurately determining the distance from the Earth to the Sun.
He proves that the Moon's orbit around the Earth is an ellipse and suggests that the irregularities in the orbit were due somehow to the Sun. He also suggests that Jupiter and Saturn affect each other's orbits.
He discovers a new type of object, the Orion Nebula. This is a fuzzy cloud-like nebulous object amongst the stars. Huygens guesses the distance to the brightest star, Sirius by assuming it is the same luminosity as the Sun. He calculates the distance as being over 25,000 times the distance between the Earth and Sun. This is a very large distance but is actually only one twentieth of the correct distance.
He shows mathematically that two bodies that attract each other gravitationally will orbit each other in an elliptical path (explaining Kepler's results). The more massive body will appear to move less while the less massive body will appear to move more. By studying these motions it is possible to show that the Sun is far more massive than all the planets since they all appear to move around it. The theory allows the motions of the Moon and planets to be calculated from first principles. Most planetary orbits are shown to be almost circular apart from that of Mercury, which is strongly elliptical. Newton also confirmed that the planets affect each other's paths as they orbit the Sun.
His equations of gravity show that all objects should fall to the Earth with the same acceleration, as Galileo found. Newton extends the experimental results of Galileo into his three laws of motion. These explain why we do not feel the rotation of the Earth on its axis or its motion around the Sun. They also explain why the planets did not need to be pushed around the Sun and remove the need for planetary 'crystal spheres'. According to Newton, the period of a pendulum can be used to measure the force of gravity on the surface of the Earth.
Newton also explains the tides. They are caused mainly by the Moon (and to a lesser extent, the Sun). His equations explain why there were two tides every day. The Moon is also shown to be responsible for the Precession of the Equinoxes, discovered by Hipparchus.
For the first time, the laws in the heavens are shown to be the same as the laws on the Earth.
The motions of the Solar System (the Sun and planets) are now understood in detail. The stars, however, are still considered to be lights set on a distant crystal sphere beyond the planets.
Apart from his astronomical discoveries, Newton does important work on optics and mathematics.
With the publication of Newton's work, the Age of Reason is considered to have begun.
His discovery of four moons around Saturn destroys Huygens' view of Solar System perfection.
Using the best distance measurements available, Roemer calculates the speed of light. His figure is 75% of the correct value, an excellent value for the times. Aristotle's idea of an infinite speed for light is shown to be wrong. The fact that light has a finite (though very large) speed means that the further we look into space, the further back in time we can see.
Assuming that stars were moving at the same rate as planets, it is possible to make an estimate of stellar distances. At the estimated distances, the stars had to be sun-like in their real brilliance (luminosity). This is the first hint that the Sun is an ordinary star rather than the light at the centre of the Universe.
Halley also works out the orbit of the comet that bears his name. It is a highly elliptical orbit. Up to then, comets were thought to come and go at random. Halley shows that even comets follow Newton's laws of gravity.
They do, but not in the way expected. Bradley discovers a phenomenon called the Aberration of Light. This is the first direct proof that the Earth is in motion but does not yield stellar distances. It is caused by the fact that light has a finite speed. Bradley's observations give a value for the speed of light which is close to the correct value.
Bradley also measures the diameter of Jupiter and finds that it is much larger than the Earth. Not only is the Earth not the centre of the Solar System but it isn't even the largest of the planets.
He thinks that the Milky Way is an "Island Universe" of stars arranged as a flat disk, and that some of the nebulous objects in the sky may be other similar systems outside the Milky Way. This idea would not be accepted for 170 years.
He attempts to measure stellar parallax by looking at stars that are close together in the sky. He assumes that one star may be closer than the other so that the parallax movement will be easier to observe and measure. In many cases, he finds movement but this is independent of the Earth's motion around the Sun. The stars are actually in orbit around each other. These are called Binary Stars. This demonstrates that the stars are not fixed to a crystal sphere and that Newton's law of gravity also operates amongst the stars.
Herschel also discovers many stars that change their brightness. These are called Variable Stars. Stars can no longer thought of as unchanging and uninteresting.
By counting stars, measuring their motions and applying statistics, Herschel makes the first estimate of the size of the region occupied by the stars. This region is now called the Galaxy. The observations indicate that the Solar System is a tiny speck within the Galaxy. It is apparently situated close to the galactic centre because the Milky Way appears symmetrical in the sky. Herschel's estimate of the diameter of the Galaxy is enormous (9000 Light Years) but is actually less than 10% of the true value.
The Sun is shown to have a motion of its own relative to other stars. This motion is towards the constellation of Hercules.
Herschel and others continue to speculate about the existence of other galaxies.
Friedrich Bessel measures the parallax of a faint star called 61 Cygni. It had been chosen because it has a large Proper Motion and is therefore assumed to be nearby.
Even the nearest star is over 270,000 times further away than the Sun!
The stars are so far away that they must be Sun-like in luminosity to be visible from the Earth. The Sun is thus shown to be an ordinary star seen from close up. Copernicus had been correct when he stated that the stars were too distant for a parallax to be easily visible.
The orbit of the planet Mercury is found to have an anomaly which Newton's laws of gravity cannot explain. This has to wait for Einstein sixty years later.
Foucault also measures the speed of light in the laboratory to a high level of accuracy.
He finds that the Sun and stars are mainly made of Hydrogen. He measures the Doppler Effect of the star, Sirius and finds that it is moving away from us. Comets are shown to contain glowing carbon compounds. Many Nebulae produce spectra that show that they are glowing gases rather than stars. He uses photography to obtain spectra of very faint objects.
Aristotle's 2100 year old idea that the heavens are made of a different element (the Aether) is finally proved to be wrong.
Michelson and Morley consider that the experiment has failed because it could not be reconciled with the physics of the day. It turns out to be the most glorious failed experiment in the history of science, eventually laying the groundwork for Einstein's Theory of Relativity.
Red stars are coolest. Orange stars are hotter; then come yellow stars; hotter still are white stars. Blue stars are the hottest. Very cool stars give out their energy in the infra-red. The very hottest stars shine mainly in the ultra-violet.
The Sun's surface temperature is shown to be around 6,000°C. Some stars are hotter than the Sun.
Theoretically, Wien's energy pattern could not be explained by the physics of the day. The explanation would have to await the development of Quantum Theory.
These stars provide a yardstick for measuring distant objects in the Universe. The period gives the luminosity; the luminosity can be compared with the apparent brightness of the star as seen from the Earth; this gives the distance to the star. If the star is part of a group, cluster or nebula, the distance to that object is known.
This new tool allows Ejnar Hertzsprung and Henry Russell to measure the distance to nearby Cepheid variables thus providing the scale to Leavitt's cosmic yardstick.
Hertzsprung and Russell go on to find a relationship between the colour and luminosity of stars. Blue (hot) stars tend to be luminous, yellow (medium) stars tend to be less luminous, red (cool) stars tend to be faint. More than 90% of stars fit this classification and are called Main Sequence stars. Some red stars are too luminous for their colour. These are called Red Giants because they are very large. Some white stars are too dim for their colour. These are small and very dense stars called White Dwarfs.
A graph of these results is known as the Hertzsprung-Russell (or H-R) diagram. Special stars (like Cepheid variables) occupy distinct zones in the H-R diagram. The diagram is very important in the study of stellar structure and provides a foundation for ideas about stellar evolution.
Russell studies the spectrum of the Sun to determine its chemical composition. The Sun is 90% Hydrogen, 9% Helium and 1% everything else. Most stars have a similar composition.
Niels Bohr applies Planck's quantum ideas to atoms helping to explain why and how atomic spectra form.
He brings Planck's ideas of quanta into prominence by using them to explain a previously mysterious effect when light shines on metals (the Photoelectric Effect).
He explains a strange movement of small particles in a liquid (called Brownian Motion) by proving mathematically that it must be due to the random motions of atoms and molecules. This is the first direct proof of atomic theory and allows the size of these small particles to be determined.
His Special Theory of Relativity explains why the Michelson and Morley experiment had apparently failed. Absolute motion cannot be measured: all motion is relative. This leads to the idea that the velocity of light is the maximum speed that any material body can have. No information can travel faster than light. When we look into distant space we are looking at the past! A further development leads to the famous equation
which shows that matter is a concentrated form of energy. This would allow future scientists to explain the source of the energy of the stars. Time and space turn out to be changeable and dependent on the position and motion of the observer. This defies common sense but would be found to be in accord with observation.
Einstein's General Theory of Relativity changes the way humans look at gravity. Newton had envisaged gravity as a force between all matter. Einstein sees matter as distorting the very fabric of space, causing it to curve. This curvature of space causes matter to move in non-linear paths. Under most conditions, the differences between the two theories of gravity are minimal. However, Einstein's theory explains the anomalies in the orbit of Mercury found by Leverrier sixty years earlier.
General Relativity also predicts that light would be bent by a gravitational field. This would be proved during a total eclipse of the sun a few years later. Another prediction is that a strong gravitational field would give a spectral red shift separate from that produced by the Doppler shift. This is proved when the spectrum of a very dense White Dwarf star is examined. The star is a companion of Sirius so the Doppler effect could be accounted for since the two stars move together.
The General Theory of Relativity gives an overall view of the entire Universe indicating that it is not static. Einstein thinks that the Universe is static and disregards this part of his equations. He would soon be proved wrong.
If this idea is correct, the Sun's planetary system could be unique since stellar encounters are very rare. Stars are too far apart to interact with others very frequently.
He assumes that the centre of these clusters is the centre of the Galaxy. If so, then that centre is 50,000 Light Years away from the Solar System. Not only is the Earth not the centre of the Solar System; the Solar System is nowhere near the centre of the Galaxy. Shapley points out that the Milky Way looks symmetrical from the Earth because of the existence of dark nebulae (interstellar clouds) blocking out distant stars.
Shapley's measurements to the centre of the Galaxy turn out to be an over-estimate, however. This is the first time that the size of the Universe is over-estimated. The currently accepted figure is 30,000 Light Years. In 1930 Robert Trumpler would show that interstellar dust dims the Globular Clusters making them look further than they actually were.
He estimates the interior temperature of the Sun to be in the millions of degrees. This is so hot that Jeans' idea of planetary formation would not work.
Eddington discoveres the Mass-Luminosity Law for stars. More massive stars are more luminous. His studies allow him to explain how Cepheid stars vary in brightness by pulsating.
The centre of the Galaxy is confirmed to be 30,000 Light Years from the Sun's position. The Sun requires 200 million years to orbit the Galactic centre. The Galaxy has enough matter to make 100 thousand million stars like the Sun.
The Andromeda spiral is in fact a galaxy outside our own and is now called the Andromeda Galaxy.
Our Galaxy, with its thousands of millions of stars, is not unique.
More galaxies are quickly found; there are billions now known. The Universe is far, far larger than previously thought. Hubble finds that there are three types of galaxies: spiral, elliptical and irregular. From its overall properties our Galaxy appeared to be a spiral.
Vesto Slipher had previously measured the velocities of many nebulae by taking photographs of their spectra.
Hubble analyses the velocities of the ones now recognised as galaxies. He finds that the overwhelming majority of galaxies are moving away from us. Their spectra show a Red Shift. He shows that there is a simple mathematical relationship between the distance of the galaxy and its velocity away from us. This relationship is now called Hubble's Law.
Hubble's Law provides another yardstick with which to measure distance. The Red Shift of a galaxy can be measured from its spectrum. This gives its velocity of recession from us. Hubble's Law provides the distance.
The simplest way to explain these observations is to assume that the Universe is expanding. Einstein's General Theory of Relativity had already predicted that the Universe would not stable if it was static. Hubble's work shows that the Universe is, indeed, not static.
Modern Cosmology (the study of the overall structure of the Universe) can be said to have begun with Hubble's work.
Lemaître suggests that all the matter in the Universe was once contained in a very dense "cosmic egg". This object exploded and the matter was spread out through space. We see the effects of this explosion when we observe the galaxies moving away from each other.
Gamow predicts that the echo of the explosion should be detectable as radiation with a temperature of about 5 degrees above Absolute Zero. This radiation should permeate throughout the Universe. It would not be detected for over 30 years.
Using Hubble's Law and working backwards, they estimate that the age of the Universe is 2 thousand million years. This figure is smaller than the age of the Earth as calculated by geologists.
Alexander Friedman uses Einstein's equations of General Relativity to work out that there are two possible ends to the Big Bang Universe.
If the amount of matter in the Universe is above a certain critical level, then the expansion of the Universe would eventually slow down and stop. The Universe would then contract with all the galaxies and stars moving towards each other until they were back in a small area. This is known as the Big Crunch.
If the amount of matter in the Universe is below the critical level, then the expansion would continue forever. Eventually the Universe would expand so much that galaxies would not be visible to each other. Cold, dark, and isolated embers would be all that was left of the galaxies.
To distinguish between these two scenarios requires a knowledge of how quickly the Universe is expanding compared to how much matter it contains. This problem would not be solved for 70 years.
This marks the birth of Radio Astronomy.
He develops a new theory of planetary formation that is a normal part of stellar evolution rather than the rare stellar encounter of Jeans' model.
Walter Baade studies the stars in the Andromeda Galaxy. He discovers that there are two populations, each with different ages and chemical compositions. The Cepheids of each population have a slightly different Period-Luminosity Law. This discovery corrects the distances to the galaxies as measured by Hubble.
The distance to the Andromeda Galaxy is tripled to over 2 million Light Years.
These changes increase the age of the Universe to 6 thousand million years. This is longer than the geologists' estimate of the age of the Earth.
Nobody can suggest how this new matter arises. For the idea to work a few hundred atoms would need to be created per cubic kilometre every year.
William Morgan studies the distribution of luminous hot blue stars in our galactic neighbourhood. He finds that they are arranged in parallel lines which mark out our Galaxy's spiral arms. The arm that includes the Sun is called the Local Arm. Away from the centre is the Perseus Arm. Closer to the centre is the Sagittarius Arm.
These observations are later confirmed by studying the distribution and motions of glowing nebulae. Using optical techniques, observations can only be made to a distance of about 10,000 Light Years. This is only one third of the distance to the centre of the Galaxy. The Galaxy contains dust and gas which block out light from the very distant stars.
Hendrik van de Hulst uses radio telescopes to map the positions of clouds of Hydrogen. This allows the Galaxy to be mapped over a larger area. He finds another spiral arm outside the Perseus. Radio waves travel through gas and dust better than light does.
Quasars are mysterious objects: highly luminous and very small. The nearest Quasar (called 3C273) is at a distance of 2 thousand million Light Years. This is over 800 times further than the Andromeda Galaxy. It shines with the luminosity of 100 normal galaxies! Its brightness varies in periods of about a month so it must be small compared to a galaxy. 3C273 has been estimated to have a diameter of over 750,000 million kilometres. This is a million times smaller than our Galaxy or 4800 times the distance between the Sun and the Earth.
No Quasars are found in the regions of space near our Galaxy. They are now considered to be very young and active galaxies.
Because light takes time to travel across space, Quasars show that the early Universe was different in the past. The Universe is therefore changing in time; it is an evolving Universe. This contradicts the Steady State Theory.
Arno Penzias and Robert Wilson discover a Universal Background Radiation coming from all directions equally. This is an effect of the Big Bang predicted by Gamow. The heat produced during the explosion should have cooled down to a temperature of a few degrees above Absolute Zero.
The new radiation indicates a temperature around 3 degrees above Absolute Zero. This phenomenon cannot be explained by the Steady State Theory.
The Big Bang Theory is now accepted by most scientists. Speculation begins about how the Universe will end. Will it expand forever or will it eventually contract back to nothingness? This depends on the amount of matter in the Universe.
At the centre of a Black Hole, there would be an object with an infinite density and zero size. This is called a Singularity. As bizarre as they sound, Singularities are not precluded by the General Theory of Relativity.
Stephen Hawking shows that if the Theory of Relativity is correct, then the Universe would have begun as a Singularity rather than as Lemaître's "cosmic egg". At the time of the Big Bang, the Singularity would have exploded and the Universe would have come into being. Space, time, and energy would have been created and would expanded together. The original state would have had an extremely high temperature. As the temperature dropped, matter would form out of the energy and eventually, stars and galaxies would have formed out of the matter.
The abundances of various isotopes of certain elements within galaxies also agree with theoretical predictions for the Big Bang.
The luminosity of a spiral galaxy is related to the properties of a particular radio emission in its spectrum.
The apparent light smoothness of elliptical galaxies is related to their distance.
Distant galaxies that give off X-rays affect the Universal Background Radiation lying between them and us in a way dependent on the distance.
Distant Quasars passing close to a large galactic mass may have their light bent. This produces double or multiple images of the Quasar. There is a relation between the angle of the bending, the time between light variation of the Quasar to be repeated in the duplicate images, and the distance to the Quasar.
The idea is that the early expansion of the Universe was very rapid for a short while before settling down to the rate seen today. These theories explain several points in the Big Bang Theory. However, there is no observational evidence for them.
Our galaxy is a member of a group (the Local Group) consisting of about 20 galaxies in a region that is 5 million Light Years in diameter. Our galaxy, The Andomeda Galaxy and a third (called M33) are all large spirals. The Andromeda galaxy is the dominant member of the group with 400 thousand million stars. Our galaxy and M33 contain about 100 thousand million stars.
The spirals have a number of satellite galaxies. The Andromeda Galaxy has two elliptical companions. Our Galaxy has five companions. Two are irregular galaxies called the Magellanic Clouds. These are visible in the Southern Hemisphere and resemble detached portions of the Milky Way. Three are small almost-spherical ellipticals hidden behind the Galactic centre. The rest of the galaxies of the group are small.
The Local Group is on the edge of a cloud of galaxies called the Coma-Sculptor Cloud. This is about 25 million Light Years across. This cloud is part of the Virgo Supercluster. This supercluster contains over 1000 galaxies that are mainly elliptical. The centre of the Virgo Supercluster is 60 million Light Years away from our Galaxy. Our Galaxy appears to be moving towards the centre of the Virgo Supercluster at a speed of 600 kilometers per second.
It is found that spiral and elliptical galaxies are generally stable and unchanging. Irregular galaxies are active and changing. Even when the Universe was only 30% of its current age, galaxies had already formed. It appears that star formation was more active when the Universe was only 50% of its current age.
Carlos Frenk simulates the early history of the Universe on a supercomputer to try and reproduce the wrinkled structure of the Universe discovered by Smoot. The results only work if the expansion of the Universe increases with time.
The Universe resembles fractals produced by mathematical Chaos Theory. This has led to speculation that this structure may have been caused by random quantum fluctuations during the very early phase of the Universe.
Saul Perlmutter and his team complete a study of Supernovae (exploding stars) in other galaxies. The luminosity of these stars can be calculated by studying the way their brightness fades. The study looks at stars out to a distance of 7 thousand million Light Years. The results indicate that the expansion of the Universe is increasing.
Brian Schmidt confirms that the expansion of the Universe was 15% greater when the Universe was half its current age. There is speculation of a repulsive force present on the large scale. This leads to ideas about the existance of dark energy.
New ideas, called M Theory, may explain the origin of the Big Bang as the collision of 11 dimensional spaces.
© 2002, 2006 KryssTal
This essay is dedicated to Patrick Moore and Isaac Asimov.
History of Science A large collection of resources looking at the history of astronomy, physics, chemistry and mathematics.