|By Stany D'Souza, USA [ Published Date: February 22, 2006 ]|
When we think about a birth of any living being, we think in terms of instantaneous reward (in terms of human life span), when it comes to its span of time to take birth. Although, it is not so instantaneous for us, let me say that in terms of Cosmos and its origin and ongoing redefining of Cosmic evolution, it does seems like instantaneous. We have heard thousands of times in TV serials like ‘Star Trek’ the phrase “The space the final frontier”. This frontier is so vast and filled with such wonderful and amazing physical bodies, it is sometimes falls under the category of, “Believe it or not”, although it is based on pure scientific findings and current theoretical understanding.
We all know our nearest star, the Sun is just 93 million miles (one astronomical unit – 1AU) away from Earth and it is an average star compared to its, let me say in simplistic layman terms – height (size), weight (mass), brightness (magnitude) and age. Our Sun is an average (main sequence) star. Stars can be broadly classified (for our purposes) into five categories: Dwarfs, Average (main sequence) stars, Giants, Super giants and Hypergiants. Our Sun is an average, main sequence star which can hold about 13 lakhs (1.3 million) Earths. As we say, whatever we see is not whatever we get. In the same way, whatever the brightness of the star up above in the night sky (apparent magnitude) is not exactly its actual brightness (absolute magnitude) in its real comparative analysis with that of our Sun.
Orion Nebula, a birth place of several stars
Although white dwarfs are 100 times lesser in size and in brightness than from that of Sun, some of the stars that we see during night sky are thousands to millions of times larger than our Sun and are called as Hyper giants. One of the hyper giant star is Pistol Star in the constellation of Saggittarius, the Hunter. The largest hypergiant is presumed to be LBV 1806-20, which is about 30-49 thousand light years away at the other side of the center of our Milkyway galaxy. If this star is placed at the place of our Sun, it may engulp the entire orbit of Pluto! There are very bright stars like Deneb (blue super giant) in Cygnus (Swan) constellation, also, popularly known as Northern Cross and tip of this cross is the star Deneb, which is 60,000 candle power more brighter than our Sun.
To give you a comparative analysis of how big the hypergiant star LBV(luminous blue variable) 1806-20: Imagine the recent spacecraft New Horizons sent by United States in January of this year, traveling at a great speed of about 66,000 Kms/hour, will reach Pluto in ten years which will cover the distance of about 6 billion Kms (six hundred crores). So, if we travel at the speed of about 100 Kms/hour (60 miles/hour), it will take us about 12 - 13,000 years to travel across the diameter of this star (at light speed, it will take only 5hrs 35mins)! This by itself will give you some idea about how big some of the stars are in our night sky. Our own galaxy has about a staggering number of 100 billion stars. And our galaxy which we call as Milkyway galaxy is just an average elliptical galaxy with 100,000 light years across in diameter. One light year is the distance covered by the light in one year. Light travels at a great speed of about 3 lakhs kilometers/sec (186,000 miles/second). It can take 7 rounds around the world in one second. Our Moon is just about 1.25 light second away and our Sun is about 8 light minutes away and our nearest star to our solar system, Proxima Centauri is about 4.2 light years away! These facts will provide you some basic information which will guide you in understanding the actual phenomenon which was and is taking place as we speak/read about birth of a star.
(L)LBV 1806-20 compared to our Sun, speck at the bottom left hand side
(R) Eagle Nebula a stellar nursery
Now, coming to the issue of star birth, let us have some understanding of what is a star is. Star is a condensed ball of matter, where at the core of it there is fusion reaction which gives rise to solar energy and all other phenomenon allied to that process. Sun’s core is fuelled by the fusion of two hydrogen atoms to helium. Sun spends about 40 tons of matter into energy/second. A kilogram of matter if it is completely converted into electrical energy (mass into energy), it can supply electricity for the United States for 3 months! You can imagine how vast amount of energy is generated by our simple twinkling stars in the night sky.
The birth of star begins with very, very diffused molecular clouds of gas present in the galaxy into which they are formed. The density of a gas in such cloud is as low as 1000 molecules per cubic centimeter, and the gas is in the form of molecules due to its very cold condition of about 10 degrees Kelvin. The cloud is very large, and may extend to even more than hundred light years across and has enough mass to form thousands of stars. These clouds are generally called by the name ‘nebulae’. We see in our night sky 3 stars (Alnilam, Alnitak and Mintaka – star names – all these are giant stars about thousand or more light years away from us or our solar system) in equal brightness in a single file. They are popularly called as “Theg Rai” meaning, ‘Three Kings’ in Konkani language. Just go out at about 8 or 9 PM in the first or second week of February and you will see these stars right above the sky close to Zenith. You can see on the left hand side of it there are three small faint stars (in an angle to the main stars) in almost in a file and if you look closely (20-20 vision or better, in a clear night devoid of Moon) you can see a tiny faint cloud (fourth magnitude) in the middle of them. This is an Orion nebula and all these stars aforementioned are members of Orion constellation. This great nebula is about 1, 500 light years away from us and is vast cloud of about 30 light years across. Just imagine the light at its great speed of 3 lakhs kilometers/second shall travel for about 30 years to cover this cloud from one end to the other!
These nebulae are generally called as stellar nurseries. These nebulae are consisting of interstellar gas of dust and gas (mostly hydrogen). These stellar nurseries are abundant in the arms of spiral galaxies. These giant clouds at it core undergo gravitational collapse and compress to form a rotating gas globule. The globule is cooled by emitting radio waves and infrared radiation. It is then compressed by gravitational forces and the globule collapses and rotates at high speed. The process of collapse takes from between 10,000 to 10 lakh (one million) years.
As the collapse proceeds, the temperature and pressure within the globule increases, as the atoms are in closer proximity. Also, the globule rotates faster and faster. This spinning action causes an increase in centrifugal forces that causes the globule to have a central core and a surrounding flattened disc of dust (called protoplanetary disk). The central core becomes the star; the protoplanetary disk may eventually evolve into orbiting planets, its satellites, asteroids, etc.
As the spinning of the central core progresses and the contracting cloud heats up due to friction, it transforms into a glowing protostar and this stage lasts for roughly 50 million years (5 crores). If there is enough material in the protostar, the gravitational collapse and heating continue. When the temperature at its core reaches for about 27 million degree Fahrenheit, nuclear fusion begins and a star is born! This is the nuclear reaction in which hydrogen atoms are converted into helium atoms giving out enormous amount of different forms of radiations, which includes gamma rays, x-rays, ultra-violet rays, visible light, etc. Young star emit jets of intense radiation that heat the surrounding matter to the point at which it glows brightly. These narrowly-focused jets can be trillions of miles long and can travel at 5 lakhs (500, 000) miles per hour.
Pleides star cluster – a part of Taurus, the Bull, constellation, newly born stars. Commonly known as “shimti Neketr” in Konkani, a cluster of stars of which 7 can be seen for the naked eye.
The protostar is now a stable main sequence star which will remain in this state for sbout 10 billion years. After that, the hydrogen fuel is depleted and the star begins to die! Let us now wait for the very interesting and immensely intriguing phenomenon of death of a star!!