Friday, September 19, 2014

Death of a Star

Death of a Star
May 2010 -Dale Alan Bryant

  A few stars that are overdue and just may go supernova at any moment: "Alnilam" or ε (eta) Orionis, the middle star in Orion's belt; "Betelgeuse" or α (alpha) Orionis, the right shoulder of Orion and "Eta Carina" (ε Carinae), 7th brightest star in constellation Carina. When these stars go, they will be seen during daylight. This is because they are all relatively nearby, within our Milky Way galaxy, at distances of about 700 to 800 light-years. A typical extra-galactic supernova is so bright that, even though it is millions of light-years away, it outshines its entire 'host' galaxy of several billion stars.
  Most known supernovae have occurred in other galaxies but have been bright enough to be seen from Earth.
  It is supernovae that provide the universe with the heavier elements, like iron, nickel and most other metals. These metals are not present inside the star, however; they are forged during the star's collapse and subsequent explosion. The term 'supernova' is used to describe a normal star's catastrophic end-of-life event. 'Supernovae' spend most of their lifetimes, which can be several millions of years, as normal, stable stars, but they are considerably more massive than the sun. Like all stars, they start out as balls of hydrogen.
 At birth, a star is a cold sphere of hydrogen. Because of its large mass, its gravitational center begins to collapse under its own weight, causing the star to compress, gravitation overcoming the nuclear forces that work against collapse. At this point of nuclear and gravitational imbalance, the star's temperature rises with the increase in pressure and it reaches a state where, because of these conditions, thermonuclear ignition occurs and the star begins burning brightly, converting hydrogen into helium. The star remains in this stable hydrogen-helium conversion state for many millions of years.
 At last, by conversion, the hydrogen is depleted and only helium remains. At this time the star undergoes a second collapse where the temperature begins to rise once more under the enormous pressure of the helium, to the point where the remaining helium again reaches thermonuclear ignition and - the star's only remaining fuel is ignited. It continues to burn the helium for millions more years, all the while contracting, until, at last, the helium is finally depleted. At this point, the star collapses even further and its atoms become so densely packed that it becomes a solid; the star's atoms cannot support the weight of the continually condensing mass and finally break down to a state where individual neutrons are actually pressed against one another - the star is literally a ball of neutrons, called a "neutron star". In this state, the neutrons are so tightly packed together that a cubic inch of material from the star, containing trillions more neutrons than normal, would weigh several billion tons!
  With each collapse event, a star shrinks in size; as it shrinks, the entire mass of its atoms is inclined to move towards the center. Gravity increases with the ever-compressing atoms to the point where they can no longer support their own weight - a catastrophic event takes place - the star blows itself up. This is the point at which the star becomes a 'supernova'. Within the supernova, temperatures and pressures become so high that brand new elements are forged, from light elements to heavy elements. The heaviest elements are transformed into all of the known metals: zinc, copper, iron, etc. The lighter elements are the silicates and various other particles of 'dust'. These elements are distributed throughout the cosmos and frequently end up in a nebula of gases and dust particles, where they contribute their rich assortment of the heavier elements.
  Back in February of 1989, I had the good fortune of co-discovering an extra-galactic supernova (later designated SN1989b) that occurred in the spiral galaxy M66 in the constellation Leo, using a reflector telescope. The galaxy itself appears through a telescope as a dim, uniform oval of light. On this particular February night, M66 showed just a hint of a pinpoint or knot of light near one of its outer edges, a tell-tale sign of a very distant supernova explosion. Its not any wonder that this supernova was so dim - M66 is 60 million light-years away!

  Eventually, supernova remnants from our own galaxy become the beautiful nebulae that can be seen through telescopes. Many of these nebulae have areas of uneven mass distribution and appear to have small knots throughout. These knots, or, nodules may contain enough gaseous mass to one day condense into proto-stars and enough metals and silicates to condense into proto-planets.

  This is how stars are born and die. As for our star the sun, it will never go supernova; it doesn't contain enough mass. To become a supernova, a star needs to be roughly 3.2 solar masses. Our sun, however, being only one solar mass, will become a red-giant star after it has used up its nuclear fuel (hydrogen and helium). This will take place in about 4.5 billion years... BTW: our sun's proper name? - "Sol" (pronounced "soul"). Hence, our planetary system, we call, the "Sol"-ar system. Technically, there are no other 'Solar' systems because there are no other stars by that name. There are, however, lots of other 'planetary' systems. Their names are derived using the host star's name as a prefix, e.g., the "Vegan" system, the "Alpha Centauri" system, the "Rigellian" system and so on. These systems contain the now well-known 'exoplanets' (planets outside of our 'Solar' system). The Kepler orbiting telescope has, to date, discovered several thousand exoplanets and their host stars. Hundreds of these exoplanets are Earth-like and may harbor alien life.

No comments:

Post a Comment