Visions of the Cosmos
Our Star the Sun
Since time immemorial ,people worshipped the great golden star that rose every morning with unerring precision above the eastern horizon. For countless millennia, its resplendent disc was held in awe by all the peoples of the civilisations of the ancient world and men and women everywhere worshipped the Sun. Together with water they thought of it as the source of life. They blessed it and, at times, when the weather becomes too hot, feared it. It is only during the last two centuries that numberless scientists have deeply studied it and begun to understand it. Even now however it holds many mysteries. The people of medieval Europe believed that the Sun went round the Earth. The great contribution of Copernicus, the Polish mathematician and astronomer was to recognise the rising and setting of the Sun as an illusion and to realise that the Earth went round the Sun.
The early years of the seventeenth century were marked by a large increase in scientific investigations and not long after the tragic judicial murder of Giordano Bruno, Galileo used the telescope to magnify the light of the Moon, the planets and the distant stars. His great contributions to the knowledge of the day were the discovery of the four moons of Jupiter and the realisation that the stars were other suns. In his day it was believed by many people that the world was made of four elements earth, water, air and fire and that the 'heavenly bodies' were composed of a different element which they named 'quintessence'. Even the great discoveries of Copernicus, Tyco Brahe, Galileo, Casssini, Huygens and other great scientists of the day did not contribute to the knowledge of the CHEMISTRY of the Sun, the Moon and the stars. It was left to another science which developed much later in the nineteenth and twentieth centuries to tell us that the matter that made the Earth was essentially the same as that which made the rest of the Universe. This science was SPECTROSCOPY.
Spectroscopy - The Language of the Stars
Somewhere, over the rainbow, way up
There's a land that I heard of Once in a lullaby.
Somewhere, over the rainbow, skies are blue.
And the dreams that you dare to dream
Really do come true.
People had known about the existence of rainbows since time immemorial and they used to make up fairy stories about the great bands of colour that stretched across the sky after a rainstorm. Now we know that although there is not a pot of gold at the end of the rainbow, there is indeed a treasure far more wonderful and beyond the dreams of avarice and its name is the science of spectroscopy. The knowledge of the spectrum has contributed immensely to the magic world we now inhabit thanks to the discoveries of Newton, Fraunhöfer, Bunsen, Kirchoff, Ångstrőm (Angstrom) and countless more clever men and women of the last two hundred years.
The first experiment in spectroscopy in relatively modern times actually happened quite early on when, in 1665, Isaac Newton carried out his famous experiments on light.
Newton obtained a prism, and set it up so
that a spot of sunlight fell onto it and projected a
beautiful spectrum onto a white screen. He then proved
that that the prism was not colouring the light. He put a
screen in the way of his spectrum, and cut a slit cut in it,
and only let the green light go through.. Then he put a
second prism in the way of the green light. If it was the
prism that was colouring the light, the green would have
come out a different colour. The pure green light remained
green, unaffected by the second prism.
In another Experiment, after getting a spectrum with his prism, he placed another prism upside-down in the way of the light spectrum after passing the first prism. The band of colours combined again into white sunlight.
In these experiments, Newton had proved that white light was made up of colours mixed together, and the prism merely separated them - he was the first person to understand the rainbow.
It was not until the beginning of the eighteenth century that further important progress was made in the study of the spectrum and the true science of spectroscopy was established. I 1802 William Hyde Wollaston, an English scientist at Cambridge produced a spectrum in which he noticed a few dark lines. It was however another decade before Josef von Fraunhöfer began his observations on sunlight and established spectroscopy as an exact science. In 1814 he invented the spectroscope and discovered 574 lines in the Solar Spectrum. He improved the instruments used by former workers and was rewarded by a solar spectrum of a far more detailed resolution. He labelled the eight most distinct lines from A to H. He also noted that the sodium lines corresponded exactly in wavelength to the D absorption lines in the solar spectrum.
Finally he invented diffraction grating and in doing so transformed spectroscopy from a qualitative art to a quantitative science by demonstrating how one could measure the wavelength of light accurately. He found out that the spectra of Sirius and other first-magnitude stars differed from each other and from the Sun, thus founding stellar spectroscopy
The original spectroscope used by Josef Fraunhöfer in his discovery of the ‘Fraunhöfer Lines’ and a copy of the spectrum obtained are housed in the Munich Museum. Credit for photographs Műnchen Museum
The Solar Spectrum showing the Fraunhöfer lines
It was almost four decades before Robert Bunsen and Gustav Kirchoff of the University of Heidelberg published the results of their work on spectral analysis. They found that on heating compounds of lithium, sodium, potassium, calcium, strontium and barium they obtained a number of distinctive spectral lines. They also discovered two new chemical elements rubidium which gave a ruby red spectral line and caesium which gave a beautiful blue line. These lines were EMISSION SPECTRA. They were of identical wavelengths to some of the Fraunhöfer absorption lines proving that these chemical elements occurred in the outer layers of the Sun.
With the aid of the spectroscope the coded messages carried across the vast abyss of interstellar space by the light emitted from the stars were deciphered and the new science of spectroscopy led to a knowledge of the chemical composition of the stars.
It shown once and for all that the stars were made of the same type of matter found on Earth. One of the most spectacular triumphs of early astrochemistry was the discovery of the element HELIUM in the atmosphere of the Sun before it was detected on the Earth. The element was given its name from the Greek word 'Helios' meaning the Sun.
In 1868 Anders Ångstrőm (Angstrom) of the University of Uppsala in Sweden produced a monumental work on the lines of the solar spectrum. He listed the wavelengths of over a thousand lines to six significant figures in units of 10-10 metre. Thus 1 nano-metre = 10 Angstrom units
Credit for diagram Joe Leeson Jet Propulsion Laboratory NASA
Our star the Sun is so huge that its immensity lies beyond words and defies our comprehension. Even our Earth seems enormous by comparison with our ordinary daily lives. Although our Sun is only one of many trillions of similar stars scattered in the stupendous Universe, it is vast beyond our most fantastic dreams. Something of its enormous size can perhaps be dimly grasped by looking at the diagram that compares our star with the size of the Earth, the planet Jupiter and the orbits of their Moons.
The deepest layer of the Sun that is visible is the photosphere. It is only about 500 kilometers thick. It has a temperature of around 5,840 degrees K. It emits a continuous spectrum. However some of the atoms in an outer layer called the chromosphere absorb light of very specific wavelengths giving rise to dark lines in the spectrum usually referred to as the Fraunhöfer lines after the man who discovered them. The temperature of the chromosphere goes up to about 50,000 degrees K. The Sun is surrounded by a sphere of extremely hot gases called the Corona - the temperatures range between 800,000 to 3,000,000 degrees K.
The Sun is a ball of gas, so it does not have a well-defined surface. When we speak of the surface of the Sun, we normally mean the photosphere. The term gas can have two meanings - gases on the Earth such as hydrogen, oxygen, carbon dioxide and water vapour are molecular gases. Because of the high temperature the Sun is strictly speaking a PLASMA since the gases are ionised into atomic nuclei and free electrons
It is a dense enough gas that you cannot see through it. It emits a continuous spectrum.
Beyond the photosphere lies another zone the chromosphere which is hotter the the photosphere and causes the absorption spectra of the chemical elements giving the Fraunhofer lines. It also produces a red colour (hence its name) due to the ionisation of hydrogen known as hydrogen alpha
The Sun is surrounded by a sphere of extremely hot gases called the Corona - the temperatures range between 800,000 to 3,000,000 degrees K. The Corona only becomes visible during an eclipse. It can be studied however by using a telescope with a opaque sphere the same diameter as the main globe of the Sun. This produces and 'artificial eclipse' and blocks out the brilliant light of the Sun and allows this last outer outer layer - the corona - to be studied
Sunspots are colder zones between 4,300 to 5,680 degrees and only appear to be dark because they are fer less bright than the surrounding areas. They are caused by magnetic activity.
The Sun is a violent and terrible place - in yet from a distance it is our friend and the source of most energy needed for life on our planet
The illustrations shown below by courtesy of the European Space Agency and NASA were taken from the Solar Space Observatory SOHO
|EIT 304Å image of a huge,
handle-shaped prominence taken on Sept. 14, 1999 -- Prominences are
huge clouds of relatively cool dense plasma suspended in the Sun's
hot, thin corona. At times, they can erupt, escaping the Sun's
atmosphere. Emission in this spectral line shows the upper
chromosphere at a temperature of about 60,000 degrees K. Every
feature in the image traces magnetic field structure. The hottest
areas appear almost white, while the darker red areas indicate
|An extensive erupting prominence
taken on 15 May, 2001 Prominences are huge clouds of
relatively cool dense plasma suspended in the Sun's hot, thin
corona. At times, they can erupt, escaping the Sun's atmosphere.
Emission in this spectral line of EIT 304Å shows the upper
chromosphere at a temperature of about 60,000 degrees K. The hottest
areas appear almost white, while the darker red areas indicate
Huge sunspot group On 30 March 2001, the sunspot area within the group spanned an area more than 13 times the entire surface of the Earth! It was the source of numerous flares and coronal mass ejections, including the largest flare recorded in 25 years on 2 April 2001. Caused by intense magnetic fields emerging from the interior, a sunspot appears to be dark only when contrasted against the rest of the solar surface, because it is slightly cooler than the unmarked region.
This LASCO C2 image shows a very large coronal mass ejection (CME) blasting off into space on 2 December 2002. It presents the classic shape of a CME: a large bulbous front with a second, more compact, inner core of hot plasma. This material erupts away from the Sun at speeds of one to two million kilometres per hour. Note that this photograph was taking using a coronoscope which creates a sort of artificial eclipse by blotting out the main disc of the Sun
Solar & Heliospheric Observatory SOHO
The Solar & Heliospheric Observatory generally referred to as SOHO is the name given to a Space Observatory specially designed to study the Sun. It is a wonderful example of international collaboration between ESA and NASA to study the Sun from its deep core to the outer corona and the solar wind. SOHO was launched on December 2, 1995. The SOHO spacecraft was built in Europe by an industry team led by prime contractor Matra Marconi Space (now EADS Astrium) under overall management by ESA. The twelve scientific instruments on board SOHO were provided by European and American scientists.
Credit for diagram ESA/NASA SOHO. The SOHO project is a joint ESA/NASA Observatory. Credit for photograph of launch NASA/ESA
SOHO was designed to study the internal structure of the Sun, its extensive outer atmosphere and the origin of the solar wind,
•The Solar and Heliospheric Observatory (SOHO) was one of ESA and NASA's most ambitious projects for the 1990's. It helped us to understand the Sun and also the interactions between the Solar Wind and the Earth's magnetosphere better than had been previously possible. It gave solar physicists their first long term, uninterrupted view of the mysterious star that we call the Sun.
•Whilst the Sun is called a yellow star it is actually giving out radiation over a whole range of wavelengths. The Sun was viewed through a number of filters enabling its activity to be observed at a number of different wavelengths. Cameras using these special filters took whole series of photographs of the Sun.
The Source of Energy of the Sun - Thermonuclear Burning
The definition of the word burning
In everyday language the term burning refers to CHEMICAL REACTIONS. For example when fat is heated in an atmosphere of air it reacts with the oxygen to give carbon dioxide and water and gives out energy in the form of heat and light (flame)
In thermonuclear burning hydrogen nuclei (proton) collide and react together to give (eventually) helium and energy as gamma radiation.
•Thermonuclear Burning in the Sun
Hydrogen burning – The Sun was formed from a gas cloud which collapsed under its own gravitational attraction. When a temperature of about fifteen million degrees were reached that the main burning reactions kicked into action. There are three main pathways by which hydrogen is converted into helium and energy. They are known as the PP1, PP11 and PP111 pathways.
The same amount of energy is given out whichever reaction pathway is followed.
The simplest (the PPI chain) takes place in three stages.
Proton + Proton → Deuteron + Positron + neutrino + energy as gamma ray photon 0.42 million electron volts
Proton + Deuteron → Helium 3 + energy as gamma ray photon 5.49 million electron volts
Helium 3 + Helium 3 → Helium 4 + Proton + Proton + energy as gamma ray photon 12.86 million electron volts
The overall reaction is 4 Protons → Helium 4 + energy as gamma ray photon 18.77 million electron volts
4 hydrogen nuclei are converted into one helium nucleus ( the helium 4 will be explained later under isotopes) + energy
There is onother process known as the CNO Catalytic Process
As stars go our Sun is well above average mass. Although most stars perhaps 85% are less massive than the Sun there are stars even more massive than our Sun. In these more massive stars most of the burning of hydrogen to helium takes place via the CNO catalytic cycle. In the case of the Sun, detailed considerations suggest that it is producing about 98-99% of its energy from the PP chains and only about 1% from the CNO cycle. However, if the Sun were but 10-20% more massive, its energy production would be dominated by the CNO cycle.
The Life History of the Sun
Like all stars our Sun has undergone and will undergo changes from its birth to final ‘death’ as a white dwarf.
The new star steadied down and entered the MAIN SEQUENCE PHASE. Our Sun will stay in that phase for about ten thousand million years. It started its life a little over four and a half thousand million years ago so its almost half way through its main sequence phase. During this time it will burn hydrogen at an ever increasing rate to prevent the Sun condensing and is slowly growing brighter. It is about 30% brighter than it was at the beginning of the main sequence phase.
The last stages Red Giant Stage - Helium Burning Formation of 'Planetary Nebula' and Carbon/Oxygen White Dwarf
Whilst vigorous burning will continue in the helium core the rest of the star will expand enormously until its outer regions engulf first Mercury and then Venus. It may just miss engulfing the Earth. This is called the red giant stage. It will only last about one hundred million years until most of the helium has burnt to carbon and oxygen. During this time the heat around the moons of Jupiter may be so great that liquid oceans of water may form on Europa, Ganymede and Callisto. Life may even begin for a brief period of a few million years out as far as Titan (Saturn's Moon).
Photograph Credit Hubble Space Telescope NASA/ESA
With the failure of Advanced Camera aboard
Hubble, scientists are beginning to get more use of the previously seldom used
Wide Field Planetary Camera 2 (WFPC2). NGC 2440 was chosen to show off the
capabilities of WFPC2 and get scientists used to the A-Z operation of the
This image on the right illustrates the colourful "last hurrah" of a star like our Sun. The star is ending its life by casting off its outer layers of gas, which formed a cocoon around the star's remaining core. Ultraviolet light from the dying star makes the material glow. The burned-out star, called a white dwarf, is the white dot in the center. Our Sun will eventually burn out and shroud itself with stellar debris, but not for another 5 billion years.
Our Milky Way Galaxy is littered with these stellar relics, called planetary nebulae. The objects have nothing to do with planets. Eighteenth- and nineteenth-century astronomers named them planetary nebulae because through small telescopes they resembled the disks of the distant planets Uranus and Neptune. The planetary nebula in this image is called NGC 2440. The white dwarf at the center of NGC 2440 is one of the hottest known, with a surface temperature of nearly 400,000 degrees Fahrenheit (200,000 degrees Celsius). The nebula's chaotic structure suggests that the star shed its mass episodically. During each outburst, the star expelled material in a different direction. This can be seen in the two bow tie-shaped lobes. The nebula also is rich in clouds of dust, some of which form long, dark streaks pointing away from the star. NGC 2440 lies about 4,000 light-years from Earth in the direction of the constellation Puppis.
The image was taken Feb. 6, 2007 with Hubble's Wide Field Planetary Camera 2. The colours correspond to material expelled by the star. Blue corresponds to helium; blue-green to oxygen; and red to nitrogen and hydrogen
The White Dwarf Stage
Stars with a mass of less than eight times the mass of the Sun (8 solar masses) end their lives by collapsing into white dwarfs. This will be the eventual fate of our own Sun. The Sun like other similar stars goes through a series of changes during it life time.
1) Birth of the Sun from the gravitational collapse of the interstellar gas and dust
2) Thermonuclear ignition of hydrogen nuclei (protons) commences at around 15,000,000 degrees Celsius
3) The newborn star undergoes a violent T-tauri phase in which it sheds considerable amounts of material
4) The Sun quietens down and enters the longest part of it's life called the Main Sequence Phase. This is by far the longest part of it's life and for our star the Sun this phase will last for about 10,000 million years. As it burns hydrogen to helium it slowly gets hotter. It has lasted about 4,500 million years already and has grown about 30% hotter.
5) Towards the end of its life it will swell up and engulf Mercury as it enters its first red giant stage.
6) It will by now have used most of it's hydrogen fuel and will contract again until the temperature in its inner regions will rise to about 100,000,000 million degrees Celsius. At this point helium will start to burn to carbon.
7) For our Sun and similar stars of around one solar mass the temperature will not rise high enough to burn any further and contraction will begin again. (There may be a little burning to oxygen but no further). The core contracts and releases energy which causes the outer layers of the star to expand. The envelope expands again in a second red giant phase and may engulf Venus and even the Earth. When the core of the star contracts, it causes a release of energy that makes the envelope of the star expand. Now the star has become an even bigger giant than before! Our Sun's radius has become larger than Earth's. This process continues until the star finally blows its outer layers off. The core of the star, however, remains intact, and becomes a white dwarf. The white dwarf is surrounded by an expanding shell of gas forming an object known quite wrongly as a planetary nebula.
Since the lower mass stars make the white dwarfs, this type of remnant is the most common endpoint for stellar evolution. If the remaining mass of the core is less than 1.4 solar masses, the pressure from the degenerate electrons (called electron degeneracy pressure) is enough to prevent further collapse.
Credit for diagram to Nick Strobel http://www.astronomynotes.com/evolutn/s11.htm
Subrahmanyan Chandrasekhar predicted while a very young man in the 1930s that there was a limiting mass for white dwarf stars: no white dwarf could be stable against gravitational collapse if it exceeded this mass, which is about 1.4-1.5 solar masses, depending on the detailed composition of the white dwarf. The idea was not very well received by many established astronomers, who considered it absurd that a white dwarf could have a limiting mass. Much later, Chandrasekhar's brilliant idea was completely accepted within the astrophysics community and he eventually won a Nobel Prize for his deep theoretical contributions to astrophysics. Today, all textbooks on stars describe the Chandrasekhar Limiting Mass for white dwarfs as central to the structure of white dwarfs. Beyond this limit the star collapses still further in a supernova explosion to form a neutron star. If the mass of the remnant exceeds about 3 solar masses it contracts further and forms a black hole. It must be clearly understood that this limits apply to the mass of the core remnant - the original star from which the core was derived was obviously far greater - at least 8-9 solar masses.
Because the core has about the mass of the Sun compressed to something the size of the Earth, the density is tremendous: around a million times denser than water. Despite the huge densities and the "stiff'' electrons, the neutrons and protons have room to move around freely---they are not degenerate.
White dwarfs shine simply from the release of the heat left over from when the star was still producing energy from nuclear reactions. There are no more nuclear reactions occurring so the white dwarf cools off from an initial temperature of about 100,000 degrees. The white dwarf loses heat quickly at first cooling off to 20,000 degrees in only about 100 million years, but then the cooling rate slows down: it takes about another 800 million years to cool down to 10,000 degrees and another 4 to 5 billion years to cool down to the Sun's temperature of 5,800 degrees.
Subrahmanyan Chandrasekhar. 1910 -1995 Nobel Prize in Physics 1983
"Chandra was one of the great astrophysicists of our time," said Hans Bethe, a fellow Nobel laureate and a professor of physics emeritus at Cornell. "He showed that white dwarf stars cannot grow beyond a certain mass-the same mass that triggers the explosion of supernovae, the most brilliant display in the sky. Chandra was also the greatest master of the English language that I know."
Known affectionately as "Chandra" to his friends and colleagues, he was admired not just for his enormous scientific achievements but for his deep and broad knowledge of literature and the arts.
The Chandra X-ray Observatory is a space telescope launched by NASA on 23 June 1999. It was named in honour of Subrahmanyan Chandrasekhar. The name 'Chandra' means moon or luminous in Sanskrit.