History of Science-15.
Jean Richer-(1630-1696).
Giovanni Cassini-(1625-1712).
James Gregory-(1638-1675).
Friedrich Wilhelm Bessel-(1784-1846).
Scot Thomas Henderson-(1798-1874).
Ejnar Hertzsprung-(1873-1967).
Henry Norris Russell-(1877-1957).
Arthur Eddington-(1882-1944).
Edward Pickering-(1846-1919).
Henrietta Swan Leavitt-(1868-1921).
George Ellery Hale-(1868-1938).
Vesto Slipher-(1875-1969).
Edwin Hubble-(1889-1953).
Milton Humason-(1891-1972).
Harlow Shapley-(1865-1972).
Marcel Grossmann-(1878-1936).
Bernad Riemann-(1826-1866).
Friedrich Gauss-(1777-1855).
William Clifford-(1845-1879).
Georges Lemaitre-(1894-1966).
George Gamow-(1904-1968).
Ralph Alpher-(1921-2007).
Robert Herman-(1922-1997).
Arno Penzias-(1933-).
Robert Wilson-(1936-).
Cecilia Payne-(1900-1979).
Our understanding of the Universe rests upon two foundations.
Measuring the distance to the stars.
Measuring the composition of stars.
In the 18th century Edmond Halley, realised that some of the fixed stars had moved,
since they were observed by his predecessors in ancient Greece.
At that time astronomers began to make measurements of distances,
across the solar system, using the process of triangulation.
This technique had been used to measure the distance to the moon,
which was just 384,000 km away.
For more distant objects you need longer baselines, to make accurate measurements.
French astronomer Jean Richer, went to French Guiana, to observe the position of Mars, against the background of fixed stars.
His colleague in Paris, Giovanni Cassini made similar observations.
This made it possible to calculate the distance to Mars.
By combining this with Kepler’s law of planetary motion, it was possible to calculate,
the distance from the Earth to the Sun.
Cassini calculated this to be 140 million kilometers, which is close to the modern value,
of 149.6 million kilometers.
This is called as astronomical unit, or AU.
At this time astronomers had a good idea of the scale of the solar system.
The Earth moves from one side of the Sun to another, every six months.
This is a distance of 300 million kilometers, or 2 AU.
Yet the positions of the stars do not change when viewed,
from either end of this enormous baseline.
We would expect the nearer stars to seem to move,
against the background of more distant stars.
This is known as parallax.
One parallax second of arc, is called a parsec.
Parsec is the distance to a star which would show a displacement of 1 second of arc,
from opposite ends of a baseline, 1 AU long.
So a star, 1 parsec away, would show a displacement of 2 seconds of arc,
from opposite ends of 300 million kilometre baseline.
Such a star would be 3.26 light years away.
No star is close enough to show this much parallax displacement,
as the Earth moves around the sun.
Christiaan Huygens measured the distance to Sirius, the brightest star in the night sky.
He estimated that Sirius must be 27,664 times further away from the Sun.
In 1668, James Gregory used improved techniques to estimate the distance of Sirius,
as 83,190 AU.
Isaac Newton came up with a revised estimate of 1 million AU.
This was published in 1728.
The actual distance is 550,000 AU or 2.67 parsecs.
Measuring distances to stars using parallax technique,
required the positions of the stars to be measured to a high degree of accuracy.
Flamsteed’s catalogue gave positions to an accuracy of only 10 seconds of arc.
It is only in the 1830’s that the technology was available,
to measure the distance to the stars, with some accuracy.
The german scientist Friedrich Wilhelm Bessel,
was the first person to use stellar parallax technique,
to measure the distance to a star, in 1838.
He estimated the distance to the star 61 cygni, to be .3136 sec of arc,
or 10.3 light years.
Modern measurements give a distance of 11.2 light years.
Scot Thomas Henderson measured the distance to Alpha Centauri as 4.3 light years.
Alpha Centauri is the closest star to the Sun.
Once we know the distance to a star, we can determine its true brightness,
called the absolute magnitude.
We now know that Sirius is much brighter than the Sun.
This was not known by Newton and his contemporaries.
In the end of the 19th century , it was possible to measure the distance to stars,
with improved techniques, using photographic plates.
Earlier the measurements were done by using the eyes.
By 1950 the distances to 10,000 stars had been determined.
Towards the end of the 20th century Hipparcos measured the distance to 120,000 stars,
to an accuracy of .002 arc seconds.
Modern astronomy, or astrophysics started at the beginning of the 20th century.
This was possible by the use of photographic techniques,
to preserve the images of the stars.
Photography also provided a way of recording images of the spectra of stars.
Spectroscopy was developed in the 1860’s.
This enabled astronomers obtain information about the composition of the stars.
One other vital piece of information that was needed was the masses of the stars.
This was obtained by studies of binary systems.
Binary systems are where two stars orbit around one another.
The invaluable doppler effect, seen from the stars in the binary system,
tells astronomers how fast the stars are moving around the another.
This along with Kepler’s laws enables astronomers, to work out the masses of stars.
Two astronomers came up with the simple most important insight,
into the nature of stars.
This is a diagram which relates the colour of the stars to their brightness.
Danish scientist Ejnar Hertzsprung was born in 1873.
He graduated from the Copenhagen polytechnic in 1898.
He later studied photochemistry.
He worked at the observatory of the University of Copenhagen.
He discovered the relationship between the brightness of a star and its colour.
He published this in 1905, and 1907 in a photographic journal.
It was unnoticed by professional astronomers around the world.
He carried out astronomical research well into his 80’s.
He died in 1967.
Henry Norris Russell was born in New York in 1877.
He studied at Princeton.
In 1911 he became a professor of astronomy, in Princeton.
He independently made the same discovery as Hertzsprung,
about the relationship of the colour of stars and their brightness.
In 1930, he published it in a journal read by astronomers.
This graph is now known as Hertzsprung-Russell, or HR diagram.
He made other important contributions to the study of binary stars,
and the composition of the atmosphere of the Sun, using spectrography.
He retired in 1947 and died in 1957.
The temperature of a star is closely related to the colour.
Astronomers define the colour of a star very precisely,
in terms of the amount of energy, it is radiating at different wavelengths.
This tells us the temperature of the surface emitting the light.
Using the known properties of black-body radiation, the surface temperature of a star,
can be determined, from measurement of just three wavelengths.
Astronomers call the brightness of the star, as magnitude.
The absolute magnitude of the star tells us how much energy it is radiating overall,
regardless of the temperature.
It is possible for some red stars to be both cool and bright, because they are very big.
Small stars can be bright, only if they are blue or white hot.
Small orange stars like the Sun, are intrinsically less bright,
than hot stars of the same size, or large stars with the same temperature.
When the temperatures and magnitude of stars are plotted on the HR diagram,
most stars lie on a band running diagonally across the diagram.
The hot massive stars about the same size as the Sun, is at one end of the band.
The cool dim stars, with less mass than the Sun, lie at the other end.
The Sun itself is an average star, roughly in the middle of the so-called main sequence.
The large, cool but bright stars, or red giants, lie above the main sequence.
The dim, small but hot stars, or white dwarfs, lie below the main sequence.
The main sequence gave astrophysicists their first insight,
into the internal working of the stars.
British astronomers Arthur Eddington is regarded as the first astrophysicist.
He was the person who discovered the relationship between the mass of a star,
and its position on the main sequence.
Eddington was born in 1882.
He studied at the Owens college in Manchester, and at the University of Cambridge.
He worked at the Royal Greenwich Observatory till 1931.
He became professor of astronomy, and experimental philosophy,
at the University of Cambridge.
In 1914, he also became the director of the University Observatories.
He was a brilliant theorist.
He had a gift of communicating scientific ideas, in a clear language to a wide audience.
He was the first to popularise Einstein’s theory of Relativity.
Eddington is best remembered for two key contributions.
Einstein’s general theory of relativity was first presented,
to the Berlin Academy of Sciences, in 1915.
Eddington received a copy of this during the world war,
through Netherlands which was a neutral country.
Eddington broke the news to the Royal Astronomical Society.
Einstein’s theory among other things, predicted that light from distant stars,
should be bent by a certain amount, as it passed close to the Sun.
This could be observed during a eclipse.
There was a suitable eclipse in 1919, which was visible,
in Brazil and the west coast of Africa.
The Royal Astronomical Society made a contingency plan,
to send expeditions to observe and photographs the eclipse.
The British government had introduced conscription, with all able bodied men,
eligible for the draft.
Eddington was exempted since he was a key scientist.
In the 1920’s Eddington gathered data on the stellar masses,
and linked this with data from the brighter stars.
He showed that brighter stars are the most massive.
For example, a main sequence star, 25 times the mass of the Sun,
is 4000 times as bright as the Sun.
More massive stars burn more fuel, to generate pressure, to counteract gravity.
This generates heat, which eventually escapes from the surface of the star,
as more light.
The temperature at the heart of a star can be calculated from its brightness,
mass and size.
Eddington’s analysis provided a profound insight.
All main sequence stars, have roughly the same central temperature,
even though they have a range of masses, from 1/10th of the Sun’s mass,
to tens of times the mass of the Sun.
It was as if the stars have an inbuilt thermostat.
In the 19th century, there was a debate between the geologists, evolutionists,
and physicists, about the age of the Sun and Earth.
Physicists like Lord Kelvin, pointed out, that there was no known process,
to keep the Sun shining for the long times scales,
required for the evolution of life, on Earth.
By the end of the 19th century, scientists knew a new energy,
in the form of radioactive isotopes.
In the 1920’s, armed with Einstein’s special theory of relativity, and his equation,
E=mc squared, Eddington was able to spell out the source of energy for the Sun,
as sub-atomic energy.
He pointed out that there is sufficient source of energy in the Sun,
to last for 15 billion years.
He went on to say that the process of hydrogen transmuting to helium,
releases energy for the Sun.
When the hydrogen in the Sun fuses to form more complex elements,
it releases enough energy for the Sun.
It would take decades, to work out how this energy will be liberated.
The calculations would need quantum mechanics, which was fully developed,
only in the end of 1920’s.
The colour-magnitude relationship in the HR diagram,
gives a guide to the distance to stars.
If we measure the colour of the star, we know where it belongs in the main sequence.
This tells us the absolute magnitude.
If we measure the apparent magnitude, we can workout how far away it is.
In actual practice dust in space, interferes with the measurement,
of colour and brightness.
Edward Pickering became the director of Harvard University observatory in 1876.
Henrietta Swan Leavitt joined the Harvard team, in 1895 as an unpaid volunteer.
She later became the head of the department of photographic photometry.
Pickering gave her the task of identifying variable stars in the southern skies.
Such variations can happen for two reasons.
First the star is actually a binary system, and one star,
can partially eclipse the other star as it moves in front of it.
Second, stars may be intrinsically variable, changing their brightness,
as a result of change in their internal structure.
Some stars swell up and shrink back, pulsating in a repeating cycle.
One such class of pulsating stars, is known as Cepheids.
Some have short periods of a day or so,
and some have periods of more than hundred days.
Leavitt was studying two clouds of stars,
known as the Large and Small Magellanic clouds.
They are now known to be small satellite galaxies associated with the Milky way galaxy.
Leavitt noticed that the Cepheids in the small Magellanic cloud, or SMC,
showed a pattern where the brighter Cepheids, went through the cycle more slowly.
In 1912, Leavitt established a mathematical model for the period-luminosity relationship.
She realised that the SMC is so far away, that the stars in it are all effectively,
at the same distance from Earth.
The light from them is dimmed by the same amount en-route to our telescope.
The distances between the stars in the SMC might be dozens to hundreds of light years.
But this is only a small percentage of the distance from Earth.
So they only effect the apparent brightness of the stars by a small percentage.
Leavitt found a clear mathematical relationship, between the apparent brightness,
of a Cepheid in the SMC, and its period.
This meant that the absolute magnitudes of Cepheids are related in the same way,
since the distance is essentially the same.
It was now needed to find the distance to 1 or 2 Cepheids,
so that their absolute magnitudes could be determined.
Then the absolute magnitudes of other Cepheids could be determined,
from the Leavitt’s period-luminosity law.
Hertzsprung was the first to measure the distances to nearby Cepheids, in 1913.
It provided the calibration that the Cepheid distance scale needed.
Hertzsprung’s calibration implied a distance to the SMC of 30000 light years,
or 10000 parsecs.
The modern figure which takes into account, reddening and extinction effects,
is 170 thousand light years, or 52000 parsecs.
Hertzsprung’s estimate provided the first indication of how big space real is.
If there is a cluster of stars, and just one Cepheid in the cluster,
the distance to all the stars are known.
In probing beyond the Milky Way, Cepheids influenced our appreciation,
of our place in the Universe.
The probing of the Universe became possible,
thanks to the development of a new generation of telescopes.
The astronomer George Ellery Hale, was instrumental in building,
some of the new generation telescopes.
Notable among them was the 100 inch diameter Hooker telescope,
built in 1918, and still in use today.
It was the largest telescope for 30 years,
and transformed our understanding of the Universe.
Two prominent astronomers, Edwin Hubble and Milton Humason,
were responsible for this.
The first person to produce a map of the Milky Way galaxy,
which resembles the modern one was Harlow Shapley.
He used the 60 inch telescope in Mount Wilson, from 1908 to 1918,
which was the biggest telescope at that time.
Some of the stars that Shapley thought were Cepheids,
were members of a different family, known as RR Lyrae stars.
They behave in a similar fashion to Cepheids,
but had a different period-Luminosity relationship.
It was increasingly clear at this time, that the Milky Way,
is a flattened disc shaped system, with the vast number of stars.
The Sun is just one star among them.
It was widely thought that the Sun lay at the centre of the Milky Way.
There is also a concentration of stars known as globular clusters,
which lie above and below the plane of the Milky Way.
By mapping the distance to the globular clusters, Shapley established that the Sun,
is not at the centre of the Milky Way.
By 1920, his measurements indicated that,
the Milky Way was 100 thousand parsec across,
and the centre of the Milky Way lay 10,000 parsec,
or 30,000 light years away from the Sun.
The current estimate of the diameter of the Milky Way is 28,000 parsecs.
The centre of the Milky Way is about 8,000 to 9,000 parsecs away from the Sun.
The disc of the Milky Way is only couple of 100 parsecs thick, which is very small,
compared to its diameter.
Shapley established that the Sun is located in the suburbs of the Milky Way,
which has an estimated several hundred billion stars.
The beginning of the 1920’s, it was widely thought that the Milky Way,
dominated the Universe.
Other fuzzy patches of light, in the night sky, like the Magellanic clouds,
were thought to be smaller satellites of the Milky Way,
or glowing clouds of gas within the Milky Way.
Only a few astronomers like Heber Curtis, pointed out that these spiral nebulae,
were actually galaxies, in their own right.
They were so far away, that individual stars in them, could not be resolved,
with the best telescopes available.
He presented his view that the Milky Way is just one galaxy,
among many in the Universe.
In 1923, Hubble using the 100 inch telescope, was able to resolve individual stars,
in a large spiral nebula, known as the Andromeda galaxy.
He was able to identify several Cepheids, in it, and calculate the distance.
He estimated it as 300,000 parsecs, almost a million light years.
The modern estimate is that the Andromeda galaxy is at a distance of 700,000 parsecs.
Hubble found Cepheids in other similar nebulae.
He established that Curtis, was essentially correct about the Universe.
Other techniques were developed to measure distances to galaxies.
One of them was observations of exploding stars, or supernovae.
Supernovae have roughly the same absolute maximum brightness.
It eventually became clear that there are hundreds of billions of galaxies,
in the visible Universe.
The Universe extends for billions of light years in all directions.
The solar system is an insignificant speck, within a insignificant speck of the Milky Way.
The distance and sizes of all galaxies, beyond the Milky Way,
were at first underestimated.
It appeared that the Milky Way was the biggest galaxy in the Universe.
It was only in the late 1990’s, using data from the Hubble space telescope,
it was established that our Milky Way galaxy, is just average in size.
In the 1930’s, Hubble extended his measurements of distances to galaxies,
as far as possible using the 100 inch telescope.
He could calibrate other features of galaxies, and use them as secondary indicators,
to give distances to more distant galaxies, in which Cepheids could not be resolved,
even with the 100 inch telescope.
During this time, he made the discovery for which he is remembered.
He discovered the relationship between the distance to a galaxy,
and the redshift in the spectrum of light from it.
Redshifts in the light from galaxies, had actually been discovered by Vesto Slipher.
His 24 inch telescope could not resolve individual stars in these galaxies.
So he could not measure the distances of the galaxies.
By 1925, Slipher had measured 39 redshifts, but only two blueshifts.
This was interpreted to be as a result of the Doppler effect.
That is most galaxies were moving away from us, and just two were moving towards us.
Hubble and Humason measured distances to galaxies observed by Slipher,
to test their own apparatus and confirm Slipher’s results.
They extended their investigation to other galaxies.
They did not find any new blueshifts.
They discovered that the distance of a galaxy, is proportional to its redshift.
This was reported in 1929, and now known as Hubble’s law.
To Hubble and Humason the redshift was a distance indicator.
But the significance of the discovery went far deeper than that.
The explanation for the discovery made by Hubble and Humason,
came from Einstein’s general theory of relativity, published in 1916.
The special theory of relativity deals with objects moving in straight lines,
at constant speed.
General theory of relativity deals with accelerations.
It was Einstein’s great insight to appreciate,
that there is no distinction between acceleration and gravity.
Einstein realised that a person falling from the roof, would be weightless,
and would not feel the pull of gravity.
The acceleration of the downward motion cancels out, the feeling of weight,
because the two are exactly equal.
We know that when a lift accelerates upwards, we feel heavier.
When a lift decelerates we feel lighter.
Einstein’s genius was to find a set of equations,
to describe both acceleration and gravity in one package.
He showed that, special theory of relativity, and even Newtonian mechanics,
were special cases of the general theory.
Einstein did many things between 1905 and 1915,
when the special and general theory of relativity was formulated.
He left the patent office in 1909, to become a full time academic,
in the university of Zurich.
He devoted a lot of effort to quantum physics till 1911.
He settled in Berlin in 1914.
The key to the mathematics which underpinned, the general theory of relativity,
was given to him by a friend, Marcel Grossmann.
Grossmann lent his notes to copy, when Einstein couldn’t be bothered to attend classes.
By 1912, Einstein had accepted Hermann Minkowski’s representation,
of the special theory of relativity, in terms of the geometry of flat,
four dimensional space time.
He needed a more general form of geometry, for more general form of physics.
Grossmann pointed Einstein to the work of 19th century mathematician,
Bernad Riemann.
Riemann had developed a mathematical model, to described curved surfaces,
called non Euclidean geometry, in as many dimensions as required.
Non Euclidean geometry had a long pedigree.
Friedrich Gauss worked on the properties of geometries,
where parallel lines can cross each other.
He did not publish this, and it became known only after his death.
The Earth is an example, where lines of latitude are parallel at the equator,
and cross at the poles.
Other mathematicians rediscovered this later.
These models dealt with special cases of non-Euclidean geometry,
such as the geometry of the surface of a sphere.
Riemann’s outstanding contribution was to present a general mathematical model,
for the whole of geometry.
British mathematician William Clifford, translated Riemann’s work,
and used it as the best way to describe the Universe at large, in terms of curved space.
He presented a paper, on variation in the curvature of space.
He made the analogy, that small portions of space, are in fact analogous,
to little hills on the surface, which is on the average flat.
After Einstein the analogy is made the other way around.
Concentrations of matter, such as the Sun are seen as making, little dimples,
in the spacetime of an otherwise flat Universe.
Clifford died of TB in 1879, the year Einstein was born.
He could not further develop his ideas.
The general theory of relativity, describes the relationship,
between spacetime and matter, with gravity as the interaction that links the two.
The presence of matter bends spacetime.
The way material objects, are even light, follow the bends in spacetime,
is what shows up to us as gravity.
The aphorism summarises this principle.
Matter tells spacetime how to bend.
Spacetime tells matter how to move.
Einstein applied his equations to the Universe, and published the results in 1917.
He found that the equations had one unexpected bizarre feature.
In their original form, they did not allow for the possibility of a static Universe.
The equations insisted that space itself, must be stretching as time passed,
or shrinking, but could not stand still.
At that time, the milky way was thought to be essentially the entire Universe,
and it showed no signs of either expanding or contracting.
Einstein added another term, to his equations, to hold still the Universe they described.
This is represented by the Greek letter lambda.
This is often referred to as the cosmological constant.
The equations setup by Einstein allowed one to choose different values of lambda.
Some of which would make the model Universe expand faster.
At least one would hold it still.
Some of them would make it shrink.
Einstein thought he had found a unique mathematical description,
of matter and spacetime, which matched the known Universe of 1917.
After the equations of the general theory was published,
other mathematicians used them, to describe different model Universes.
The Russian scientist Aleksandr found a whole family of solutions to the equations,
and published them in 1922.
Some described expanding Universes, and some contracting Universes.
Einstein was somewhat irritated, since he hoped his equations,
would provide a unique description of the Universe.
Belgian astronomer Georges Lemaitre, who was also a priest,
published similar solutions in 1927.
There were contacts between Lemaitre and Hubble.
Lemaitre came to know about the discovery of Cepheids in the Andromeda galaxy.
Lemaitre also corresponded with Einstein.
By the 1930’s, when Hubble and Humason published redshifts and distances,
for nearly a hundred galaxies, it became clear that the Universe is expanding.
It was also clear that there is a mathematical model,
with a choice of cosmological models, to describe the expansion.
Cosmological redshifts is not caused by galaxies moving through space.
It is not therefore a Doppler effect.
It is caused by the space between the galaxies, stretching as time passes.
It matches with Einstein’s equations, but Einstein refused to believe it in 1917.
If space stretches while light is en route to us, from another galaxy,
then the light itself will be stretched to longer wave lengths.
For visible light this mains moving it towards the red end of the spectrum.
The existence of the observed redshift-distance relationship, of Hubble’s law,
implies that the Universe was smaller in the past.
This in turn implies a beginning to the Universe.
This concept was repugnant to many astronomers, including Eddington, in the 1930’s.
The catholic priest, and astronomer Lemaitre whole heartedly embraced this idea.
He called it the primeval atom, or the cosmic egg.
According to this, all the matter in the Universe was initially in one lump,
which then exploded and fragmented.
The idea gained attention in the 1930’s, but most astronomers including Eddington,
thought that there could not be a beginning to the Universe, now know as the Big Bang.
The idea of the Big Bang gains some acceptance,
only after the work of the Russian scientist George Gamow,
and his colleagues, in the 1940s.
The distance scale worked out by Hubble, we now know,
was in error by about a factor of 10.
This meant that he thought the Universe,
was expanding 10 times faster then we now think.
Using the cosmological equations derived from the general theory of relativity,
a model of the Universe was developed by Einstein and de Sitter, in the 1930’s.
It is called the Einstein-de Sitter model.
Using this it is straightforward to calculate how long it has been since the Big Bang,
from the redshift-distance.
Because Hubble’s data implied that the Universe was expanding 10 times too fast,
the calculation gave the age of the Universe as 1.2 billion years.
This is less than 1/10th of the current estimate.
It was also only 1/3rd of the well determined age of the Earth.
Something was clearly wrong.
Till the age issue was resolved, it was difficult for many scientists,
to take the idea of the Primeval Atom seriously.
Because of the age problem, the scientists Fred Hoyle, Herman Bondi,
and Thomas Gold, came up with an alternative to the Big Bang.
It was known as the steady state model.
This model the envisaged the Universe as eternal, always expanding,
but looking much the same as it does today, because it was new matter,
in the form of hydrogen, is continuously being created,
in the gaps left behind as galaxies move apart.
It was a viable alternative model, to the Big Bang, right through the 1950’s and 1960’s.
New techniques developed, including radio astronomy,
showed that far away galaxies are different from nearby galaxies.
This proved that the Universe is changing as time passes, and galaxies age .
The age question was gradually resolved, as better techniques,
like the 200 inch telescope, on Mount Palomar, became available in 1947.
The confusion between Cepheids and other kinds of variables stars, was resolved.
It was only in the 1990’s, the expansion rate of the Universe,
was estimated with a uncertainty of 10%, with the aid of the Hubble space telescope.
By the end of the 20th century, the age of the Universe was determined,
to be between 13 billion and 16 billion years.
George Gamow, who was a student of Friedmann, in the 1920’s,
made a significant contribution to the development of quantum physics.
He showed how quantum uncertainty could enable alpha particles,
to escape from radio active atomic nuclei, during alpha decay.
The alpha particles are held in place by the strong nuclear force.
They have nearly enough energy to escape, but not quite enough,
according to classical theory.
Quantum theory, however says that alpha particles can borrow enough energy,
to do the job, from quantum uncertainty.
The particle escapes, and then repays the borrowed energy,
before the world has time to notice, that it had borrowed.
It was one of the many oversights of the Nobel Committee,
that Gamow did not receive, the Nobel prize, for his profound contributions,
to understanding nuclear physics.
Under Gamow’s supervision, his student Ralph Alpher, investigated for his doctorate,
the way in which complicated elements could be built from simple elements,
under the conditions that existed in the Big Bang.
This was the time, the entire Universe was packed into a volume, of our solar system.
Gamow guessed that the raw material for the manufacture of the chemical elements,
must have come from a hot fireball of neutrons.
This was the time when the first nuclear bombs was exploded,
and the first nuclear reactors were being constructed.
Information was available, how different kinds of nuclei, irradiated with neutrons,
absorbed neutrons one by one, to become the nuclei of heavier elements.
They got rid of the excess energy by gamma radiation.
Sometimes unstable nuclei created in this way, would adjust their internal composition,
by emitting beta radiation, or electrons.
The raw material for the Universe was assumed to be neutrons.
Neutrons themselves decay to produce electrons and protons,
which together make the first element hydrogen.
Adding a neutron to hydrogen gives deuterium or heavy hydrogen.
Adding another proton makes helium-3.
Adding another neutron makes helium-4.
Nearly all the deuterium and helium-3, is converted to helium-4.
Gamow found that the nuclei formed most easily, was the common elements.
Nuclei that did not form easily, corresponded to rare elements.
They found that this process would produce an enormous amount of helium,
compared to other elements.
This matched with the observations of the composition of the Sun.
Alpher’s work provided the material for his Phd dissertation.
It also formed the basis for publishing a scientific paper in the journal, Physical Review.
Gamow was a inveterate joker.
He decided to add the name of his old friend Hans Bethe, as co-author,
simply because he liked the sound of the names Alpher, Bethe, Gamow,
which corresponded to alpha, Beta, Gamma.
The paper appeared in the journal on 1st April, 1948.
This publication marks the beginning of Big Bang cosmology as a quantitive science.
After the publication of the alpha-beta-gamma paper,
Alpher and his colleague Robert Herman came up with a profound insight,
into the nature of the Big Bang.
They realised that the hot radiation that filled the Universe, at the time of the Big Bang,
must still fill the Universe today.
But it would have cooled by a quantifiable amount,
as it expanded along with the universal expansion of space.
We can think of this as an extreme redshift.
It stretches the wave length of the original gamma rays and x-rays,
far into the radio part of the electro magnetic spectrum.
In 1948, Alpher and Herman published a paper,reporting their calculations of this effect.
They found that the temperature of the background radiation today,
should be 5 kelvin, or about minus 268 degree centigrade.
The credit for the prediction of the existence of background radiation,
should go to Alpher and Herman.
In the early 1960’s, two radio astronomers, working with the horn antenna,
at the Bell laboratories research station, found they were plagued by faint radio noise,
coming from all directions in space.
This corresponded to a black body radiation, with a temperature of 3 degrees kelvin.
The two astronomers Arno Penzias and Robert Wilson,
had no idea of what they had discovered.
In Princeton University Jim Peebles, was actually building a radio telescope,
specifically intended to search for the echo of the Big Bang.
Peebles had independently carried out similar calculations.
When news came of what the Bell researches had found,
Peebles was able to explain what was going on.
The discovery was published in 1965.
It marked the moment when most astronomers,
started to take the Big Bang model seriously.
Penzias and Wilson shared the Nobel Prize for the discovery.
Alpher and Herman should also get the credit for this discovery,
though they did not get the Nobel prize.
Cosmic microwave background radiation has been observed by many instruments,
including the COBE satellite.
It has been confirmed to be perfect black-body radiation,
with the temperature of 2.725 degree kelvin.
This was the most powerful single piece of evidence, that there really was a Big Bang.
The Universe experienced the extremely hot dense phase about 13 billion years ago.
Cosmologist in the 21st century are tackling the puzzle, of how this super hot fireball of
energy, came into existence.
The ideas are still speculative.
It was in 1992, when the COBE satellite results were announced,
that the Big Bang model, came to become the Big Bang theory.
Alpher and Herman, worked on what actually emerged from the Big Bang.
They discovered there was a problem with nucleosynthesis,
by the repeated addition of neutrons, to a nuclei.
It turned out that there is no stable nuclei with masses of five units or eight units,
on the atomic scale.
Starting with a sea of protons and neutrons, it is easy to make hydrogen and helium.
It is now thought that these protons and neutrons,
were manufactured out of pure energy, in line with the equation e=mc squared.
The calculations of Gamow’s team, showed that a mixture of 75% hydrogen,
and 25% helium, could be made in this way, in the Big Bang.
If we add a neutron to helium, we get a very unstable isotope.
A little lithium 7 can be made by rare interactions,
in which a helium-3 nucleus merges with a helium-4 nucleus.
The next step is to produce a nucleus of beryllium-8.
Beryllium-8 immediately breaks into 2 helium-4 nuclei.
If we could make only hydrogen and helium, in the Big Bang,
than all other elements must have been manufactured elsewhere.
This could have been inside the stars.
This understanding came about in 1930’s, when it became clear,
that the Sun and the stars, are not made as the same mixture of elements as the Earth.
The idea that the Sun is made from the same elements as Earth, has a long history.
In the 5th century BC, Greek philosopher Anaxagoras,
encountered a meteorite which fell to the Earth.
It was largely made of Iron.
He concluded that the meteorite came from the Sun,
and the Sun was a ball of red hot iron.
In the beginning of the 20th century, after understanding of nuclear energy,
it was realised, that radio active decay of radium could keep the Sun shining.
This encouraged the idea, that most of the Sun’s mass,
might consist of heavy elements.
Some astronomers started to investigate how nuclear fusion might provide the energy,
for the Sun and stars to be hot.
They investigated processes in which protons, fuse with nuclei of heavy elements.
The process by which protons penetrate heavy nuclei,
is the opposite of the process of alpha decay.
In alpha decay, an alpha particle, which is a helium nucleus,
escapes from a heavy nucleus.
It is governed by the same rules of quantum tunnelling, discovered by Gamow.
Gamow’s calculations of the tunnel effect was published in 1928.
Two scientist Robert Atkinson, and Fritz Houtermans, published a paper describing,
how protons could fuse with heavy nuclei.
They thought that charged particles can penetrate the atomic nucleus.
Eddington used the laws of physics to calculate the temperature,
at the heart of the Sun from its mass,
radius and the rate at which it was releasing energy.
Without the tunnel effect this temperature was about 15 million kelvin.
This was too low to allow two nuclei to come together with sufficient force,
to overcome their mutual electrical repulsion.
Progress in understanding the internal workings of the stars,
could be made only after the quantum properties of entities like protons,
were understood.
In the 1920’s scientists were assuming that the sun was rich in heavy elements.
After spectroscopy became more sophisticated,
this assumption started to become doubted.
In 1928, British astronomer Cecilia Payne used spectroscopy,
to discover that composition of stellar atmosphere is dominated by hydrogen.
The astronomers Albrecht Unsold and William McCrea,
independently established that there are a million times more hydrogen atoms,
present in the atmosphere of stars, than there are atoms of everything else put together.
By the end of the 1920’s, scientists started to begin understanding,
what keeps the stars shining.
It took longer to understand the likely nuclear interactions, and appreciate fully,
that hydrogen dominates the composition of the visible Universe.
It is only after scientists discovered the processes which can turn hydrogen into helium,
they realised that heavy elements are rare in stars.
They realised that hydrogen and helium makeup 99% of stars.
The nuclear fusion process was identified independently by different scientists.
German scientists Hans Bethe, and Carl Von Weizsacker, identified two processes,
which could operate at the temperatures inside stars.
They made allowance for quantum processes such as tunnelling,
to convert hydrogen into helium, with the appropriate release of energy.
One of them known as the proton-proton chain, turns out to be the dominant interaction,
in stars like the Sun.
It involves two protons coming together, with a positron being ejected,
to make a nucleus of deuterium, or heavy hydrogen.
When another proton fuses with this nucleus, it falls helium-3.
When two helium-3 nuclei come together and eject two protons,
the result is a nucleus of helium-4.
The second process operates more effectively at higher temperatures found in stars,
at least one and half times as massive as the Sun.
In many stars both process are at work.
The second process, the carbon cycle operates in a loop.
It requires the presence of a few nuclei of carbon,
involving protons tunnelling into these nuclei.
Because the process operates in a loop, these heavy nuclei emerge,
at the end of the cycle unchanged, effectively acting as catalysts.
Starting with a nucleus of carbon-12, the addition of a proton makes unstable nitrogen-13,
which ejects a positron to become carbon-13.
Adding a proton makes nitrogen-14.
Adding a third proton to nitrogen-14, makes unstable oxygen-15.
Oxygen-15 ejects a positron to become nitrogen-15.
The addition of the 4th proton, ejects an alpha particle and reverts to being carbon-12,
which was the starting ingredient.
Alpha particle is a nucleus of helium-4.
The net effect is 4 protons are converted into a single nucleus of helium.
A lot of energy is ejected along the way.
These processes were identified in the late 1930’s.
The work on nuclear weapons during world war II, helped to improve the understanding,
of nuclear processes.
In the 1950’s scientists started looking at the problem of how the heavy elements,
is manufactured in stars.
The key insight came from the British astronomer Fred Hoyle in 1953.
The proton fusion puzzle was solved by quantum tunnelling.
Hoyle suggested that the nucleus of carbon-12 must possess a property known as resonance.
This greatly increases the probability of three alpha particles fusing, to form carbon-12.
Such resonances are states of higher than usual energy.
If the base energy is likened to the fundamental note played on a guitar string,
resonances can be likened to higher nodes played on the same string.
Only certain notes or harmonics is possible.
In order for this to work, carbon-12 had to have a resonance,
with the certain very precise energy, corresponding to a pure note.
Willy Fowler carried out experiments, to test for the existing of such a resonance,
in carbon-12.
It turned up exactly as Hoyle had predicted.
The existence of this resonance allows 3 alpha particles to merge together.
This creates a energetic nucleus of carbon-12, which radiates away excess energy,
and settles into the basic energy level, known as the ground state.
This was the key discovery which explained how elements heavier than helium can be,
manufactured inside stars.
Hoyle, Fowler, Geoffrey Burbidge, and Margaret Burbidge, published in 1957,
a definitive account, of how heavier elements are built up in stars.
Following this scientists were able to model the detailed inner working of stars.
By comparing these models, with observations of stars, they were able to determine,
the life cycles of stars, and work out among other things, the ages of stars,
in our galaxy.
The understanding of nuclear fusion processes in stars,
explained how all the elements upto iron,
can be manufactured from the hydrogen and helium, produced in the big bang.
The proportions of different elements predicted to be produced,
match the proportions seen in the Universe at large.
This however could not explain the existence of elements heavier than iron.
The iron nuclei represents the most stable form of everyday matter, with the least energy.
To make heavier elements such as gold, lead, or uranium,
energy has to be put into force the nuclei, to fuse together.
This happens when stars more massive than the sun, reaches the end of their lives,
and run out of nuclear fuel, which can generate heat, to hold them up.
When their fuel runs out, such stars collapse dramatically in upon themselves.
When they do so, enormous amounts of gravitational energy is released,
and converted to heat.
This makes a single star shine, for a few weeks, as brightly as a galaxy.
It becomes a supernova.
This provides the energy which fuses nuclei together, to make the heaviest elements.
In the end a huge explosion occurs, in which most of the material of the star,
is scattered in interstellar space.
This forms a raw material for new stars and plants.
In 1987, a supernova was seen to explode, in our neighbouring galaxy,
the Large Magellanic cloud.
This was the closest supernova observed since the invention of the astronomical telescope.
The processes of this supernova, match closely with the theoretical models.
It was the most important and exciting discovery, concerned with the origin of elements.
Scientists were able to calculate the great accuracy, how much material of different kinds,
is manufactured inside stars.
These are scattered into space by supernova, and stellar outbursts.
Scientists confirmed these calculations,
by measuring the amount of different kinds of material,
in clouds of gas and dust in space, using spectroscopy.
This is the raw material from which new stars and planetary systems form.
They found that apart from helium, the four most common elements in the Universe,
are hydrogen, carbon, oxygen and nitrogen.
They are collectively known as C H O N.
The process of enquiry started with Galileo, and ended with the observation,
of the supernova in 1987.
Another line of investigation, which seemed to have nothing to do with stars,
was started by Vesalius.
Vesalius put the study of the human body on a scientific footing.
This culminated with the investigation of DNA in the 1950’s.
This showed that there is no special force, and all life is based on chemical processes.
The four most common elements involved in the chemistry of life,
are hydrogen, carbon, oxygen and nitrogen.
We are made out of exactly the same material, which is most abundant in the Universe.
The implication is that the Earth is not a special place.
Life forms based on C H O N, are likely to be found across the Universe.
It is the ultimate removal of humankind from any special place in the cosmos.
It completed the process started by Copernicus.
The Earth is a ordinary planet, orbiting and ordinary star, in the suburbs of an average galaxy.
Our galaxy contains hundreds of billions of stars, and there are hundreds of billions of galaxies,
in the visible Universe.
It removes the Pre-Renaissance idea that the Earth was the centre of the Universe,
and humankind as the unique pinnacle of creation.
Science itself is undergoing a qualitative change.
A small child can learn the rules of chess.
That does not make the child a grandmaster.
Even the greatest grandmaster whoever lived,
would not claim to know everything there is to know about the game of chess.
Four and a half centuries after the publication of De Revolutionibus by Copernicus,
we are in the same situation, of the small child who has learned the rules of the game.
We are just making our first attempts to play the game,
with developments such as genetic engineering and artificial intelligence.
We do not know what the centuries to come, will reveal to us.