ASTRONOMY: SUPERNOVA

Image of a supernova
Courtesy psxextreme.com

On November 11, 1572 CE (i.e. 1572 AD) Tycho Brahe, the leading astronomer of that time observed a new star appearing in the constellation of Cassiopeia. At that time the world view was that the sky was immutable above moon and atmosphere, that is the sky never changes above moon and atmosphere and remains same from the time God created it. Tycho Brahe carefully measured the parallax of this new object but there was none (a parallax is an angular measurement to find out the distance of an object). This led him to doubt the so called immutable world. This new star became bright for some time and then slowly faded away. He wrote about it in a small book and dubbed it as “nova stella” meaning “new star” in Latin.

In 1604 another new star appeared in the sky and this time another great astronomer Johannes Kepler made very systematic observations of it.

Tycho Brahe
Courtesy theworld.org

Johannes Kepler
Courtesy en.wikipedia.org

It was in 1930s, when the real big telescopes became available to mankind the real nature of these “new stars” became understood bit by bit. The first detailed work was done by astronomers Walter Baade and Fritz Zwicky at Mount Wilson observatory near Los Angeles. Zwicky coined the name ‘supernova’ for these objects.

As we know now they are actually cosmic fireballs caused by the giant explosion of unbelievably mammoth proportions of a star in its death throe. These cosmic cataclysms shine 100 billion times brighter than the sun, equal or more than a whole galaxy for a very brief period.

Not all stars explode at its death. A star like our sun will let off most of its gaseous atmosphere at its and become a white dwarf , only a few thousand miles in diameter and radiate its left over heat for billions of years.

However when a star (particularly a white dwarf) of mass 1.44 times the mass of sun will die a different thing will happen. When this limit exceeds (known as Chandrasekhar Limit ,named after the great Indian physicist Subrahmanyan Chandrasekhar , who discovered it in 1930s) the star will die a different kind of death .It will either explode in to a supernova leaving behind a remnant and a neutron star at its centre or implode into a black hole.

Fritz Zwicky
Courtesy ned.ipac.caltech.edu

Subrahmanyan Chandrasekhar
Courtesy starchildgs.nasa.gov

A supernova explosion expels much of the star’s material at a tremendous velocity of 30000 kilometers /second creating shockwaves in the surrounding interstellar medium.

This shock wave sweeps up an expanding shell of gas and dust called a supernova remnant .

Supernovas are of mainly two categories, Type-1a and Type -2. Type -1a supernova occurs when a white dwarf accretes mass from a nearby binary star so that after sometime it crosses Chandrasekhar Limit. At this point the internal matters are heated to an extent that carbon fusion occurs and in a runaway nuclear fusion of the substantial mass of the star, it releases enormous energy within a very very short time and the star explodes.

The Type-1a supernova is conspicuous by absence of the hydrogen lines in its spectrum. Also all Type-1a supernovas has almost similar mass and almost same peak luminosity so that they can be used as standard candles to measure distances in our universe.

White dwarf sucking material from companion star and eventually becoming supernova
Courtesy news.harvard.edu

Type-2 supernovas develop differently. Very massive stars reach a point of the nuclear fusion of its core that it produces iron and fusion becomes unsustainable because fusion of iron produces a net energy loss.As there is no energy from inside to counteract the gravitational pressure the star collapses creating a massive shock wave which blows the star apart. The collapse causes violent expulsion of its outer layer into a supernova leaving a neutron star behind. In certain cases the gravitational force may be too great so that the whole star collapses in to a tiny little space becoming a black hole. The gravitational pull of the black hole is such that even the light cannot escape it and remains invisible to the outer world (hence the name “black hole”).Type-2 supernovas are conspicuous by the presence of hydrogen lines in their spectrum.

In the aftermath of a supernova two very interesting thing happens. The heated gas and dust released from supernova become fuel for births of new stars. Also the nuclear fusion creates all the heavier elements like sodium, calcium, oxygen, iron and so on.

These heavier elements propagate through the interstellar space and ultimately make a planet like earth and creatures like human beings. We, who are made of many different elements like sodium, calcium etc and breathe oxygen, owe our lives to supernovas.

Although many observations for a long time are attributed to supernova explosions (the first concrete observations were made by the Chinese in 185 CE) human beings observed a supernova “real time” in 1987 after modern telescopes were built.

Oscar Duhalde, a telescope operator doing regular observation in Chile at Las Campamanas observatory on February 23, 1987 went outside to have a coffee break. As he looked at sky he suddenly saw a “new star” in Large Magellanic Cloud, a nearby dwarf galaxy visible from southern hemisphere. Subsequent observation detected it to be a supernova. This time all the astronomical fraternity focused on this event and mankind was able to observe it not only real time but in a lots of different way; from radio wavelength to high energy wavelengths like x-rays.

From all these observations which are going till date scientists have amassed lots of information about supernovas. In fact in University of Chicago scientist have managed to simulate a supernova explosion (using supercomputers equivalent to power of 128,000 desktops and using 60,000 hours of computing time). One of the significant results of a supernova is the birth of a neutron star. In the neutron stars the protons and electrons are combined to produce neutron stars and are squished together due tremendous gravity. One tea spoonful of neutron star stuff would weigh a billion tons on earth. Also births of neutron stars generate a significant amount of neutrinos, a ghostly particle without electrical charge invisible and able to pass through about everything (steady streams of neutrinos are passing through us and earth at every moment). Scientists have envisaged neutrinos long ago but never able to catch them. However on February 23^rd 1987 the labs made specially to catch neutrinos deep underground earth caught a number of neutrinos confirming both the existence of this particle and also the fact that the new star was a supernova giving birth to a neutron star.

Supernovas not only eject material space but emit electromagnetic waves of high energy like x - rays and gamma rays. These rays, if generated near earth in sufficient proportions have the capacity to alter the atmosphere of the earth as also the basic structures life like DNA. A sufficiently close encounter with a supernova will destroy earth; even more distant supernova will cause our DNA to alter to change irreversibly, so that our basic properties of life will change, for better or worse. In fact it is believed the Ordovician–Silurian extinction of ocean life which happened around 450 million years ago is due to a close enough supernova explosion.

On an average a supernova occurs in our Milkyway galaxy every 50 years. There are quite a few candidates in the Milkyway which may explode any day into supernova. Conspicuous and closest of them are Eta Carinae, 7500 light years away and Betelgeuse in the constellation of Orion, (Indian names: Ardra Nakshatra at the shoulder of Kalpurush) 500 light years away from us. These supergiant stars are due to explode at any moment, but that ‘any moment’ may be now or a million years away in future. But when it happens will be a spectacular thing on earth, shining even on day time, out shining perhaps even moon.

Betelgeuse
Courtesy ancientvisitors.sk

The existence of Type-1a supernova has given us an insight to the distances, early history and nature of universe which was not there before. As I said, Type-1a supernova has certain same peak luminosity, that is, their true brightness is same. Measuring how dim it looks to us we can gage its distance from us and the galaxy it resides in. That is they are standard candles by which we can measure distances far into the deepest corners of our universe. By interpolating these distances and their velocity (by the Doppler Effect in spectrum analysis) scientists arrived at a conclusion that the existing universe began roughly about 13.7 billion years ago with a Big bang. Also they have found out that the more distant the galaxies are the more velocity with which they expand. Most startling is the discovery that the more distant objects are flying away from us and each other with increasing acceleration. This means at a distant future everything in this universe will be alone and at a certain very distant point of time (trillions of years) there will be nothing to be seen in the sky except darkness and everything in this universe will die in cold darkness. Also it has given rise to the concept of dark energy, the invisible energy which is causing these cosmic objects to accelerate beyond grasp anymore.