Comets - the formation(nutshell)

Comets are believed to be coagulations of material left over during the formation of the solar system through gravitational contraction and spinning up of the solar nebula. The spinning up is a consequence of conservation of angular momentum. In this process , a time came when the spinning material was concentrated into an ever-thinking disc. The shape was ordained by the fact that the epuitorial regions of the spinning cloud could overcome the gravitational contraction while the polar regions could  not. Finally when the central core of  the cloud became dense and hot inside, thermonuclear reactions started, thereby converting hydrogen into helium and producing a lot of energy.

Outflow of this energy produced the outward pressure to stabilise the sun against the contracting force of gravity.

Density fluctuations in the disc led to coalescence of mass into planets going around the sun. But a lot of mass in the outer fringes of the thin disc escaped congregation into large aggregates and was retained as dispersed material in the far reaches of the solar system. The additional pressure of the solar wind probably aided dispersal . This cloud is known as the Oort cloud .

The mass in the thinning edge of the pre -solar disc was too sparse to form large bodies through coagulation and was left alone to form it's own small colonies. Comets could be considered as colonies  of this kind and are the transformed components of Oort cloud .  They also go around the sun in long period elliptical orbits that are often perturbed when they travel into the inner solar system. In the early history  of the solar system, they occurred more frequently  and there is a feeling that much of the water on earth and other planets was brought in by comet collisions.

The Mars & Life

Mars , some basic information

The scientists have not found any evidence for remnants of plants, animals or other significant life forms on mars. But search for life beyond the earth is a natural urge. It is true that our Earth is a wonderful planet but it is only a tiny little rock around an ordinary star. There are billions upon billions such stars in the universe. Even if life is an extremely rare and wondrous natural happening, surely it could have also found a foothold at some other places. One finds that organic molecules of the type used by life forms are found in abundance in many molecular clouds in our own galactic vicinity. If life is not a unique miracle but a natural outcome of far-from-equilibrium thermodynamics in the evolution of matter and energy, one cannot stop Looking for its presence in all sorts  of near - congenial locations .

If we look at other planets of our solar system , we do not find many good candidates. The moon is too small and cannot sustain an atmosphere. Mercury is horribly hot and cold during its days and nights respectively, and cratered much like our moon because it lacks the protection of an atmosphere. Venus seems to have done something wrong during its evolution . It is subject to a runaway greenhouse effect, raising it's temperature to hundreds of degrees Celsius . Jupiter is enormous and has thick atmosphere , with its lower parts at thousands of degrees , altogether in congenial from the point of an earth-centric scientist, even if we were to grant a latitude of few hundred degrees Celsius for survival of life. The moons of Jupiter (like Europa) are beginning to look attractive  because they might have pools of liquid water. Mars on the other hand , seems like an attractive  candidate . There is evidence that it did have flowing water at some time. It probably still has some frozen water below the surface. It's atmosphere is thin , but might not  be completely hopeless in terms of supporting life. We certainly cannot expect elephants , monkeys or men running around there. Signs of even rudimentary life forms would be useful and extremely instructive . Because of all these reasons , I am very partial towards scientists who are devoting an enormous amount of effort to detect evidence of life processes on Mars . Our solar system itself is a tiny part of the universe. We could go on looking for evidence of life having emerged somewhere or the other . Some searches have been made but we have not looked hard enough. Universe is vast, even bigger than we can imagine , search for life by us and giving conclusion that there is no other intelligent life form or simply other life form in the universe is like that taking a glass of water from the Pacific ocean and concluding that there are no fishes in the ocean. The search for life is just beginning , humanity has a very long way ahead to discover more.

DARK ENERGY-THE DARK SECRET

We are passing through a unique period in the history of science. Based on observations and experiments in our laboratories and on internal consistency of the of the theories we had built, we were convinced that our universe is structured and controlled only by four types of forces . These were gravitation, electromagnetism, the weak force, and the strong force. These forces seemed to account for everything we saw or measured , here on earth or amongst stars and galaxies . There was no measurement or observation that was considered unexplainable. The basic principles incorporating these theories were not violated. The world of physics, chemistry, biology, geology, and astronomy threw up nothing that required the existence of an other type of force or energy. With e had also worked out theories that helped us to unify at least theree of these four interactions . The only residual agenda in this connection was the unification of gravity with all the other forces . Some ingenious experiments looked for a possible fifth force . These could only put upper limits of the order of a millionth the strength of gravity. Then came some surprising observations in astronomy and from cosmological theories. Some radical changes in the Constitution and the history of universe has been indicated , even though we are far from understanding the science behind them. The proposal for existence of dark energy was one of the new hypothesis .
This was entirely new . In addition, we did have mounting evidence that a dominant fraction of some matter in the universe is not visible to us. It has gravity like all other matter but it is dark, in the sense that it does not emit any detectable radiation. The nature of dark matter is still under discussion even though it's existence is observationaly strongly indicated.
I find a present situation rather uncomfortable. Till recently, we did our physics through observation and theory, connected with things happenings on earth. We had tremendous success in understanding the external universe using the very same physics. Now the distant universe is beginning to suggest objects and laws that are difficult, if not impossible to test here on earth! I , for one, am beginning to have some doubts about the validity of the evolutionary or bigbang model of the creation of the universe, in spit of the fact in many respect it seems so simple and compelling . In this regard, let me describe a piece of evidence brought forth to suggest that the bulk of the energy in the universe resides in a repulsive field called Dark energy.One of the methods used for determining the rate of expansion of the universe is to explore the relation between the red shift of astronomical objects and their distance . It is clear that we could get an approximate idea of the distance of an object if it were assumed to be a standard "candle" and it's brightness was measured. The rate at which it is receding from us would be known from the red shift of its spectral lines. A class of supernovae has been used to provide standard candles in this regard. It has been recently discovered that some of these supernovae at large red shifts appear to be too dim to be at the distance indicated by their red shifts if we use the standard value of the Hubble constant. This has been interpreted as indicating that universe has started to expand much faster than initially believed. A large repulsive force embedded in the fabric of the universe is believed to be responsible . The energy associated with this force is the so called Dark Energy!!.I cannot help being a little sceptical .Do we have enough confidence that the few supernovae we have used are truly standard light sources? Are we sure that the dimness of the sources is not due to absorption of their light by intervening dust clouds? Perhaps these doubts would be removed In time. However one is still left with a feeling that parameters are being chosen somewhat arbitrarily, to make the universe completely consistent with the dominant model of big bang  cosmology . We might end up getting a good parametric fit without a true physical foundation . That would be , at best , some tentative progress, not a proper understanding.

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Earth history

How old is Earth

The clock used for measuring the geological age of the earth

Is a nuclear clock. 

The most common is called U238 pb206 clock. U238 has a half life of 4.51 billion years. 

After a long chain of decays the end element is pb206 and U238 in an old rock. This would tell us how long the uranium has been decaying in that rock that gives us the age of the rock. 
This has been done very carefully. Some other radioactive dating methods have also been used. 
The current consensus is that the age of the earth and the solar system is 4.6+0.1 or 4.6-0.1 billion years.

Weight of earth

Since the earth is suspended in space,"weighing" it is not the same thing as putting an object on a scale.
when we speak of the weight of the Earth, we mean the amount of matter that makes it up.
this is called its mass.

The earth's mass is about 6600 trillion tons. to give you an idea of how that number looks here it is ----       
 66 00000000000000000000 . 

how did scientist find out that this was the mass of the earth,?

To do this, they used a principle based on the fact that any two objects attract each other. 
this is what the force of gravity depends on. put in simple terms, the law of gravity states that two objects are attracted by force That depends on their mass and their distance apart .
the bigger the objects the greater the force that pulls them together . the farther apart they are, the smaller the Force.

To measure the weight of the Earth, the following is done- a small weight is suspended from a string .the exact position of that weight is measured .
now a tonne of lead is brought near the Hanging weight. there is an attraction between the weight and the lead and this causes the weight to be pulled just a tiny bit out of line . actually it is less than 0.00002 mm so you can see how carefully the measuring must be done.

After this is measured scientist can use mathematics to figure out the weight of the earth.
 they have measured the power of the earth's attraction on the weight and they have measured the power of the 1 ton leads attraction on the suspended weight. 

the relative difference can be calculated and tells them the mass of the earth.

What is this mass made of?

There is the crust of solid rock ; then a layer called the Mantle which is also solid Rock and goes down about 1800 miles ; and then the innermost part, which is the core and is about 2100 miles in radius . the material of the core is liquid because of the great heat at the centre of the earth.

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OUR SUPER STAR-THE SUN

The sun and its speed

The sun is the basic source of life on earth, earth and all other planets revolves around it, but does sun also revolves around something?? 

The answer is yess!!

It revolves around our galaxy, the milky way, and its speed of revolution is 220   km per second!, it sounds insane , but its true, but even at this speed itt takes about a million years to go around once, because the galaxy is so large. Humanity has not been around even for one revolution! It is useful to know that the matter and stars in the galaxy take on a spiral shape.

By analyzing the differential rotation of the stars as a function of distance from the centre of the galaxy, astronomers have concluded that besides the visible stars and clouds, there must be a lot more invisible matter that exerts a gravitational force on the visible matter in the galaxy. This invisible matter is referred to as the so-called dark matter. We still do not know the nature or identity of this matter.

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