A Glance at Our Cosmos
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Although Cosmos and Universe are commonly used as synonyms, physically is Cosmology primarily a branch of Astronomy and deals with the study of the heavenly bodies and the origin and growth dynamics of the universe. This implies that Cosmology covers studies from the atom and beyond to the galaxy and beyond because the formation of the heavenly bodies will not be fathomed without the understanding of how matter was originally formed from a highly ionized state where neutrons,protons and electrons prevailed immediately after the big bang, which has gained a sweeping scientific consensus nowadays, as the beginning of the formation of our universe. Cosmology has to do, thus, with Astrophysics as well as Particle Physics.
The Big Bang (BB)

The BB is simply a theory about how the universe began and developed. Modeling, observations and experimentation boosted and consolidated its credibility within astronomy circles.. Here, [print], is a glance at the historical background of the BB.
The theory states that the universe originated from a state of singularity, which was a fabulously hot and dense point. The unleash of the stored energy and the ensuing process of cooling, over thousands of years, led to a process of energy conversion to subatomic particles, (such as protons, neutrons and electrons), which were the ABC, i.e. the initial blocks, of matter formation in the following stages of the universe development. Those subatomic particles gave way to the formation of atomic nuclei and  atoms, which were the vital basis for matter formation. Elemental Hydrogen H, with 1 as its atomic number,  was the first matter, in its plasma state, that appeared and, in its elemental form, constituted about 75% of the universe.
Plasma refers, simply, to a hot, ionized gaseous state and has its own idiosyncratic characteristics that distinguish it  from solids, gases and liquids. Elemental hydrogen, in its plasma state, was the first overwhelmingly developed matter just after the BB. Those hydrogen clouds were merged, under the influence of gravity, to form stars, which are massive luminous and spherical celestial bodies. Spectroscopic measures and observations led to the conclusion that traces of  He (Helium) and Li (Lithium) did coexist with H shortly after the BB. Other forms of matter, heavier metals, were formed later as a result of the nuclear reactions, known as nucleosynthesis, taking place within stars:

The concept of nucleosynthesis in stars was first in 1946 realized by Fred Hoyle, an English astronomer and mathematician. In 1950, Hoyle was heading a group of scientists that set forth the basics of the concept that chemical elements in our universe have been manufactured within stars as a result of a nuclear fusion. This concept formed what became known as the field of nucleosynthesis in stars.

It is well known that, besides H, He is the next most abundant matter in the universe: 24%. This is consequent of the fusion of H to He taking place in stars that act as nuclear furnaces fuelled by the fusion-released energy. There are two, generally referred to, fusion processes in stars that yield He from H:
The first  is the
PP Chain, or the Proton-Proton Chain, and is mainly present in sun-size stars and smaller, while the second is the C-N-O Catalytic Cycle, or the Carbon-Nitrogen-Oxygen Cycle,which exists in bigger than sun-size stars and demands much higher temperature levels in order for the fusion process to be initiated. [ For the pp-process, the temperature threshold is 4,000,000 K, while it is 15,000,000 K for the cno-process. By taking our Sun as an example, the temperature threshold is 15.7,000,000 K, and rises, with the ensuing quickly heating up process, to 17,000,000 K when it becomes the main energy source fuelling the Sun's nuclear furnace. (http://dx.doi.org/10.1088%2F0004-637X%2F701%2F1%2F837, Schuler, S. C.; King, J. R.; The, L.-S. (2009). "Stellar Nucleosynthesis in the Hyades Open Cluster". The Astrophysical Journal 701 (1): 837–849.)] Further illustration for the performances of these two fusion processes is illustrated below:


The first stars are anticipated to have existed about 500 million years after the BB, which is generally

viewed as the metal-free, or at least metal-poor, phase in the development of our universe from the BB onwards. This means that the first generation of stars was poor-metal with anticipations of having member stars of up to 250-sun-size. With the presence of heavy metals in the universe, sizes of stars dropped down so that, nowadays, stars of higher than 150-sun-size are, generally, assumed not to exist, and it would.be a surprise-phenomenon if such a "young" star is detected.                                                                                                                                                                                                                                                                                                                                                                                                                                                   

Currently, active research efforts go on relentlessly in order to find "very metal-poor" stars. If discovered, such stars would have dated back to Population III, or at least to "early" stars of Population II .Note that Population III refers to the first generation of stars, Population II refers to the second generation of stars and Population I refers to the recent, young, generation of stars.
[Here, [print], is a fresh fruit of such research efforts: A 2nd-generation star discovered?]

This raises the question: How old is our universe, really, and how reliable is such a classification of stars ? In fact there is no talk, here, about precision and accuracy. Man has invented technologies for assessing this and that, and these technologies are developing with time. This means that the figures available about our universe remain fluid and contentious. Do not forget that we talk about fabulously wide universe and distant stars. To give a rough illustration about what surrounds us, think about the following:

*- The age of the universe is assumed to be between 12 and 15 billion years. Generally, it is taken to be
    around 13.7 billion years.
*- The distance to our Sun is taken to be 150,000,000 km, or 1 AU (astronomical unit)  Its light reaches us
     in about 81/3 minutes.
*- The nearest star to us is 41/3 light years away.
*- The nearest galaxy to us is about 150,000 light years away.
     (Light Year is the distance that light travels through a vacuum in one year, (9.46 x 1012 km).
*- Our galaxy (the Milky Way) is about 100,000 light years in diameter.
*- The distance to the galaxy M87 in the Virgo cluster is 50 million light years.
*- The average separation between stars in a galaxy like our own is in the order of parsecs. A parsec, pc, is
    equal to 3.26 light years.
*- The diameter of a galaxy is typically in the order of 100 kpc.
*- The separation between galaxies in a cluster of galaxies is typically several Mpc.
*- The separation between clusters of galaxies is typically in the order of 10 Mpc.
*- The most distant galaxies observed are thousands of Mpc away from us.
*- The distance to most distant object detected in the universe is about 18 billion light years (18 x 109 light
   years). [
http://csep10.phys.utk.edu/astr162/lect/distances/units.html] Note the discrepancy between this
   figure, 18 billion years, and the generally adopted age for the universe. Another very recent, 2009,
  estimate is 13.63 billion years, [
http://www.sciencedaily.com/releases/2009/10/091028142231.htm]. Yet
  another estimate, 2003, is: 12.79 billion years, [

Theoretically, this means that if man were able to launch a rocket to stars at the speed of light, which is rounded to 300,000 km/sec, it will reach
# the Sun after 81/3 minutes,
# the nearest star to us after 41/3 years,
#the nearest galaxy to us after 150,000 years,
# galaxy M87 after 50 million years, and
# the most distant object detected in our universe after 12.79, 13.2,13.65 or 18 billion (18x109) years!

The BB, despite its acquired credibility as a model for the origin of our universe, has, however, its own shortcomings. A very serious one, to me, has to do with the so called
dark matter (dm). It is taken for granted that celestial bodies, as they spin and orbit, actually "swim" in a medium whose ins and outs remain a black box. This medium is transparent and inert to any form of interaction with light and electromagnetic forces. Nevertheless, this medium is, generally, thought to be held coherent by forces of gravity. Hereabout, two situations arise:
> Was this
dm available before the BB? If "yes", then the BB was an incident in an "existing" medium, and it did not signal the beginning of our universe. Rather, it signaled the beginning of the creation of the perceived-by-man matter in a by-man-unfathomed medium, but failed to offer any clue whatsoever for an explanation for its prehistory. 
> Was this
dm not available before the BB? If "yes", then the BB failed, as a model, to offer an explanation for the "creation" of that matter.

                                          B- Spectroscopy and Astronomy

Before moving to metals and their generation in stars, it will be worthwhile to go over a brief survey of the role of spectroscopy in defining the chemical composition of stars and other celestial bodies.
Spectroscopy, in its broad sense, studies the interaction between radiated (electromagnetic) energy and matter. In astronomy, this interaction is embodied in a
spectrum of electromagnetic radiation, or light, including visible light, which is radiated by heavenly bodies such as stars and meteors. Generally, the word spectrum refers to a plot of the desired response against either the frequency or the wavelength of the received energy. From such spectra, valuable info may be obtained about the chemical composition, densities, temperatures and velocities of meteors, stars and galaxies.
The received light will be dismantled/decomposed into its sundry wavelengths by the spectrometer. This results in displaying its corresponding spectrum. This decomposition of light into its wavelengths is reminiscent of the prism effect on the visible light passing through it. The prism effect displays white light as a visible spectrum, which may be illustrated as follows:

The white light emerges decomposed into its wavelengths
Red, Orange, Yellow, Green, Blue, (Indigo) and Violet. However, Indigo is no more in use and is not defined as an independent colour in the visible spectrum.

We already know that atoms may be ionized by absorption as well as by emission of photons/energy. Every atom has its own orbital layout around the nucleus. These energy levels in a magnesium (mg) atom, for illustration, are different from those levels in a copper (cu) atom. Besides, this atomic layout means that an atom of an element absorbs and/or emits at specific energy wavelengths and frequencies at which no other atom of another element absorbs and/or emits photons/energy. This fact  leaves every element in nature with its own "fingerprint" regarding the corresponding absorption and emission lines that appear in its energy/light spectrum. Here lies the basic concept that astronomers use to identify the chemical composition of heavenly bodies. Light from celestial bodies acts like a messenger loaded with precious information about the "sender". The prime task of the astronomers is to decipher those received messages. It is needless to stress that the chart of the spectra of elements has already been identified and documented on Earth, and it is used as the basic reference in understanding and identifying the received light spectra from skies. To ensure a better understanding of spectra, we need to go briefly on the two concepts
absorption and emission of energy by atoms. Generally, and graphically, the emission zone is where peaks exist and the absorption zone is where troughs, or sinks, exist, as illustrated below:

By going to an atom, the following is noted:

Assume that an atom of an element is hit by a jet of light/photons. The atom has a set of energy levels characteristic of that element. It absorbs, and emits, energy/photons only according to specific wavelengths and frequencies. This means that a photon whose energy is exactly (E2-E1) will be absorbed by the atom, which pushes an electron to its next higher energy level. The remaining jet of light/photons will proceed without the absorbed photon, which corresponds not only to a specific wavelength and frequency, but also to a specific color. By taking the original jet of light as representative of the continuous spectrum (cs), then the element has "absorbed" a strip of a specific color from the cs. What will be left after absorption will be a dark strip/line on the cs. This is what we call "an absorption line".  The resulting spectrum will be referred to as the "absorption line spectrum".
However, the ionized atom will, shortly after absorption, emit the absorbed energy, not necessarily in the same direction of the jet, and its electron will go back to its normal state. This process of ejecting the absorbed energy will appear as a glowing strip/line on a black background, stressing the fact that the jet, embodying the cs, has already gone. The glowing strip/line will bear the exact color originally stripped off the cs during the absorption phase. The resulting spectrum will be referred to as the emission line spectrum".
Discussion, above, was limited to one energy level. However, a light jet with higher intensity will be able to push an electron to a higher energy level. The mechanism remains, nevertheless, the same.
The corresponding spectra will look as follows:

Generally, the continuous spectrum (cs) refers to light emission at all wavelengths, and this covers the whole electromagnetic spectrum, where wavelengths range from radio waves, and longer, to gamma rays, and shorter. The visible light spectrum makes only a slight portion of the whole, ranging from 400 to700 nanometers (nm), or, equivalently, from 4.3x1014 to 7.5x1014 Hz, as seen next:


As we already know, light is electromagnetic waves. Consequently, waves with longer wavelengths are waves with lower energy/temperature levels, and waves with shorter wavelengths are waves with higher energy/temperature levels.
The above stated information explains further the significance of plotting the spectral data from stellar bodies graphically as intensity vs wavelength. These plots display a lot about temperature, chemical composition, density, motion, etc., of the emitting bodies.

Note that the colors and wavelengths of the emission lines for an element correspond to the same colors and wavelengths on the cs from which they were stripped off during absorption. Note also the correspondence between the absorption lines and the "troughs"and between the emission lines and the "peaks".
Our Sun is a star. The information presented in the section about the Sun, [print], displays that absorption lines may be associated with specific elements, temperatures, color ranges, etc. Here, [print], is a list of the emission spectra for some known elements, which play a role in identifying the chemical composition of stellar bodies. There the chance is also available to virw higher-pitch-clarity versions of those lines.

                                                  C- Classification of Stars

Stars are, generally, classified
according to age as well as according to surface temperature, also known as
spectral sequence, of stars.


                                                  1- According to Age:  Star Population

By taking our Galaxy, The Milky Way, as basis, the American astronomer Walter Baade was able, in 1944, to discover a norm for defining the star population, Population I and II, as follows:

Population I Member Stars: Luminous, hot and young

i- Lie in the galaxy disk and execute orderly circular orbits around the center.
ii- Age is between 0-10 billion years.
iii- Are loosely bound together, not concentrated, and are, thus, referred to as
open clusters.
iv- Are close to mid-plane of the galaxy.
v- Have greater abundance in elements heavier than He.
vi- Old members have about 0.1xsma, "sma"= solar metal abundance, and their orbits are slightly elliptical. They may rise, on their orbits, to about 3000 ly from the disk plane.
vii- Middle-aged members, like our Sun, have metal abundance, ma, of 0.5 to 1.0xsma, [x means "times"]
viii- May rise up to 1100 ly from the disk plane..
ix- Young members have ma of about 1 to 2xsma. They stay within 650 ly from the disk plane.
x- Members of age < 100 million years exist in the spiral arms of the galaxy and within 100 ly from the spiral disk. Their ma is about 1 to 25xsma.

Population II Member Stars: Less luminous, cooler and ouder

i- They lie in the halo and bulge of the galaxy.
ii- Age ranges from 10 to 13 billion years.
iii- Their masses are up to 0.8 solar mass.
iv- Their orbits are much less orderly and highly elliptical.
v- The so called
globular clusters, which are clusters of old members, belong to this population. They are used as a guide to locate the center of the galaxy.
vi- The halo members are metal-poor: 0.001 to 0.03xsma, and this increases towards the center. Bulge members have the highest ma.

                                                                Population III Member Stars  

The first generation of stars began dawning about 500 million years after the BB. They were massive ranging from 140 to 260 times Sun-mass.  The lives of those massive stars extended to about three million years, after which they imploded resulting in the ejection of enormous amounts of heavier-than-H-and-He elements into the space. They, or some of them, are thought to have ended in the creation of black holes, quasars and proto-stars and proto-galaxies, which are supposed to have coalesced to form our contemporary population of galaxies. The implosions, referred to as supernovae, are thought to have resulted in the emergence of highly metal-enriched galaxies within about 1 billion years from the BB. That metal-rich environment is assumed to have changed the cooling properties of the widely spread , metal-rich clouds of interstellar gas, thus paving the way to the birth of Population II and, then, Population I of reduced-size celestial bodies.

                                            2- According to Spectral Sequence, or Surface Temperature

Here, stars are classified according to their surface temperatures, which are derived from their collected spectral data. This classification, in the decreasing surface temperature order, takes the form:
O-stars, B-, A-, F-, G-, K-, and M-stars. Sirius, for example, is a hot, type A, star at about 10, 000 K. [
The classification of stars, based on their spectral sequence, is referred to as the Harvard Spectral sequence (HSS). It may be presented as follows:

O Ionized Helium and metals; weak Hydrogen
B Neutral Helium, ionized metals, stronger Hydrogen
A Balmer Hydrogen lines dominant, singly-ionized metals
F Hydrogen weaker, neutral and singly-ionized metals
G Singly-ionized Calcium most prominent, Hydrogen weaker, neutral metals
K Neutral metals, molecular lines begin to appear
M Titanium Oxide molecular lines dominant, neutral metals
R, N CH, CN, and neutral metal lines
S Zirconium Oxide, neutral metal lines

These main categories are further subdivided into 10 families, from 0 to 9, as follows, for illustration: O1, O2, ... O9, etc. The HSS is, in fact, a temperature sequence, which may be graphically presented as follows:

For illustration, note that ionized He, corresponding to (very) high surface temperatures, belongs to class O-stars, while ionized molecules, corresponding to low surface temperatures, belong to class K- and class M-stars.
Since the color fingerprint is also a measure of surface temperature, this sequence may also be interpreted in terms of spectral color data, with the color index values going, in decreasing order, from higher , on the left, to lower, on the right. This interdependence between the color and temperature spectra may be presented, graphically, as follows:

Note that stars may be taken as black bodies emitting their maximum energy.radiation at the peak value. The Sun, as can be seen, emits this output within the visible light range. Note also that the emission is "
bluer"at higher temperatures and "redder" at lower temperatures, and that at 3500 K and lower, and at 6ooo K and higher, the emission falls outside the visible light band on the long wave and on the short wave band sides respectively.

The colour-temperature spectrum may be further illustrated as follows:


D- Generation of metals in Stars

We have seen that the BB resulted in the generation of huge clouds of H, and He, (with traces of Li and Be), in the plasma state. Shortly after the BB, those clouds cooled to a point where gravity forces became dominating and were able to hold those clouds together in the form of huge gaseous spheres/balls. During this phase of growth, contraction of the cloud generated heat whose resulting internal pressure counterbalanced the gravity pressure and saved the newly emerging star from collapsing under its own gravity. 
With time, the contraction-generated heat will rise the internal temperature to a level that will trigger a fusion reaction. This (hydrogen) fusion will be the energy source for the star during its lifetime, which is referred to as the star’s
main sequence. From now on, the born stars will act as nuclear-fuelled “lamps” illuminating a universe which, otherwise, would have been pitch-dark.
It is generally assumed that the first stars emerged to existence about 500 million years after the BB, and that the fusion processes going on in the core generated an internal pressure that counterbalanced the gravity forces.

However, and as the star burns its fuel, there will be loss in the internal pressure, which tips the balance in favour of the growing influence of gravity forces. This decline in the internal pressure will be accompanied by an upsurge in core temperature and density. At a point, this will trigger the fusion of He to C and O. This ionized medium in the core will lead to the formation of yet heavier elements …. At the same time, the rising pressure and energy in the core will result in spewing H to the outskirts of the star, thus augmenting its size. Ultimately, the star will explode dispersing its metal content into the galactic space, which is full of giant galactic clouds, and that will signal the beginning of the generation of a new, but now metal-rich, generation of stars, (Population II). Very massive stars will go beyond the He fusion and will fuse C, then O, then Ne, then Si, then Fe to fuel its furnace before collapsing. This will happen because (Fe) fusion, then, demands more energy that will be available in order to maintain the nuclear reaction in the core. At this point, gravity forces will overcome the internal pressure, thus leading to the inevitable collapse of the star. The enormous energy resulting from this collapse will lead to the creation of yet heavier elements such as Au, Pu and U. Schematically, this process of metal generation in stars may be presented as follows:

The Periodic Table shows that the known elements are 118. From these, 94 exist naturally in our planet, Earth, either free or in the form of compounds with other elements. The rest are lab-generated. These naturally existing elements vary widely in terms of their abundance. By taking H as the most abundant element, then the relative (to H) abundance of the other elements, in our sol, may be graphically presented as follows, in the increasing order of their
atomic numbers:

The following list gives theses elements in ascending order of their
atomic numbers.

However, the most abundant elements in our Milky Way are:

Z          Element          Mass Fraction in ppm
1           H                    739,000       71m
2           He                   240,000      23m
8           O                    10,400          m
6           C                    4,600           0.4423m
10         Ne                   1,340          0.1288m
26         Fe                   1,090          0.1048m
7           N                    960             0.0923m
14         Si                    650             0.0625m
12         Mg                 580              0.0588m
16         S                    440              0.0423m

                                                         E-Evolution of Stars

We have seen that the Population III stars were rather massive. Their lives were short, about 4 million years. The collapse of those stars is referred to as
supernova resulting in shedding huge amounts of elements and radiation energy. The elements found lodging in surrounding, huge clouds of molecular hydrogen, which began to fragment and under the effect of the radiant energy began to collapse triggered by their gravitational fields. These would form the protostars. Under the influence of the ongoing collapse, a point was reached at which the fusion of hydrogen nuclei started. That declared the birth of stars, which were nothing less than nuclear furnaces, or simply nuclear-fuelled lamps. The period to reach a fully-fledged star might have extended tot about 100 million years!
The lifespan of a star is known as its
main sequence. By taking our sun as reference, the sun-size stars live about 12 billion years. The less massive stars live (much) longer: up to hundreds of billions, or even trillions, of years! These are known as red dwarf stars. The more massive stars live shorter.
At the terminal end of its main sequence, the star would have consumed its hydrogen fuel. Internal pressure would be decreasing while the gravitational forces would be increasing. The rising core temperature will eventually trigger the fusion of He to C. Here, the outer shell of the star, which is mainly hydrogen, grows/expands outwards , cools and shines red. The star, at this point, would become
red giant. This stage of evolution is common to all stars.
Massive stars, like
blue supergiants, would be able to use an alternative fuel beyond C, etc.
After losing its fuel, the star would, eventually, collapse: to a
white dwarf, neutron star or black hole for less massive stars, and to supernovae for massive, about 20 sun-size and higher, stars. Our Sun is expected to degenerate to a white dwarf, which would, eventually, degenerate to a black dwarf.

From Dictionary:

Giant molecular clouds of hydrogen (GMC): “Massive clouds of gas in interstellar space composed primarily of hydrogen molecules (two hydrogen atoms bound together), though also containing other molecules observable by radio telescopes. These clouds can contain enough mass to make several million stars like our Sun and are often the sites of star formation.”
Red giant: “A star that has low surface temperature and a diameter that is large relative to the Sun.”
White dwarf: “A star that has exhausted most or all of its nuclear fuel and has collapsed to a very small size. Typically, a white dwarf has a radius equal to about 0.01 times that of the Sun, but it has a mass roughly equal to the Sun's. This gives a white dwarf a density about 1 million times that of water!”
Neutron star: “The imploded core of a massive star produced by a supernova explosion. (typical mass of 1.4 times the mass of the Sun, radius of about 5 miles, density of a neutron.) According to astronomer and author Frank Shu, "A sugar cube of neutron-star stuff on Earth would weigh as much as all of humanity!" Neutron stars can be observed as pulsars.”
Black hole: “An object whose gravity is so strong that not even light can escape from it.”
Supernova explosion: “(a) The death explosion of a massive star, resulting in a sharp increase in brightness followed by a gradual fading. At peak light output, these type of supernova explosions (called Type II supernovae) can outshine a galaxy. The outer layers of the exploding star are blasted out in a radioactive cloud. This expanding cloud, visible long after the initial explosion fades from view, forms a supernova remnant (SNR).”
“(b)  The explosion of a white dwarf which has accumulated enough material from a companion star to achieve a mass equal to” about 1.4 solar masses.
Black dwarf: “A non-radiating ball of gas resulting from a white dwarf that has radiated all its energy.”

                                                                                                                         Expanding Universe and Redshift