The Star is Dead, Long Live the Star

The Star is Dead, Long Live the Star

Debi Prasad Choudhary

Los Angeles, 4/1/2014

All that is born must die. That is the ancient oriental wisdom. It is always observed for the living creatures on earth, but is true for the stars, galaxies and perhaps for the universe it self? These are big questions. We made considerable progress in past one hundred years to answer some of these questions. In fact, two Nobel Prizes have been awarded for understanding the dead stars!!

So what happens in the end of a star’s life? That was a question pondered by a young Indian student Subramanian Chandrasekhar in 1930 while travelling from Madras to London by boat. He used the newly discovered physical laws that govern the property of matter at very very small and very very large scales in extreme situations.

The energy produced at the center by converting hydrogen to helium flow out and exert pressure on the matter, which is countered by the pressure due to gravitational force during the main part of stars life that keeps it stable. Towards the end of its life, more hydrogen is burnt to produce more energy. As the energy flows out, the outer part of the star expands making the star a bigger sphere. These are called red giants, as they look redder due to relatively cooler surface temperature. Betelgeuse is an example of such a star.  Later, the material from the outer envelops blows away with varying speed. The outer most part is blown away at a slower speed compared to the inner part, which eventually overtakes. This process produces a shell like structure, called planetary nebula, as they can be seen through small telescopes as planets. 

As the outer layer becomes loosely bound to the star and ultimately blown out, the core of the star starts collapsing after the exhaustion of hydrogen to burn and become a dense. Typically a star like the size of sun reduces to an object of the size of earth and become white dwarf stars. They are hot and small, so look dimmer compared to their temperature. They are so dense that the weight of a teaspoon of material from such objects would be equal to the weight of a cruise ship!!

Chandrasekhar worked out the property of matter in such extreme situations. Following the discovery of the “particle” property of light waves by Einstein in 1905, in 1924 a French PhD student de Broglie postulated the wave nature of electrons in his 17-page thesis, which was awarded Nobel Prize. According to this, all particle of matter is associated with a wave, the wavelength of which decreases with its mass and velocity. Using the physics of “small world” or “quantum mechanics”, it becomes apparent that particles cannot be brought closer than their wavelength. When the matter is squeezed enough in a white dwarf stars, they come closer and the velocity increases approaching the velocity of light. The velocity of light has special significance, as nothing can cross this speed limit. As objects approach this speed limit, they become heavier and shorter and the time slows down!! Chandrasekhar showed that in such extreme conditions, if the mass of a star were 1.72 ´ (mass of the sun), the pressure due to the electron gas would play the role of radiation pressure for a normal star and stop the further collapse of the star. This is called Chandrasekhar limit, for which he received Noble Prize in 1983, a year after I joined for PhD and took few classes from him!!

I still remember his simple style of teaching complex mathematics using just chalk and blackboard!! Question may arise, why did it take more than 50 years to award Noble Prize to Dr. S. Chandrasekhar, professor of physics in University of Chicago? My understanding is that, observational verification of his theory was difficult and took a long time. One such star accompanies the brightest star in the sky called Sirius.  The stars like the sun and 3 to 4 times heavier, would die to create a Planetary Nebula, which would eventually disappear, but their core would survive for eternity as white dwarf!!

For heavier stars with masses in the range of 4 to 8 times the mass of the sun, the death is more violent. The electron gas cannot stop the collapse of the core. At the end of the life, the core would collapse suddenly such that it would be denser than the nucleus of an atom. As, that would not be sustainable; the core would immediately relax to a slight bigger size, so that a star like sun shrinks to the size of San Fernando Valley. This would send shock waves to blow away the outer envelop of the star at very high speed. The outer ejecta, known as supernova remnant, would shine an entire galaxy, brighter than 10 billion suns. Chinese astronomers recorded one such event in our galaxy in 1048 AD, which is known as Crab nebula. I witnessed such an event in 1987, in a nearby galaxy Large Magellanic cloud. Recently in Mid January 2014, one such event was observed in a nearby spiral galaxy M82. These ejecta fads away quickly depending on the death of a normal or a binary star with strange companions.

These ejecta prepare elements heaver that iron. We are made out of these elements, so we are star stuff!!

If the core of the dead star is below 5.6 times the mass of the sun and squeezed to a size of 10 to 15 km in size, the matter would be so dense that all of them would be in the form of neutrons. At this stage the neutrons would resist further collapse, similar to the electrons in case of white dwarfs. Neutrons are neutral particles inside the nucleus of an atom and play very important role in nuclear reaction that produce energy in power plants. After receiving a telegram about their discovery by Chadwick in 1932 at University of Cambridge, Lev Landau of USSR immediately stated that there must be neutron stars. Two years later Caltech astronomers Walter Baade and Fritz Zwicky published a paper proposing the core of some of the supernova ruminants to be neutron stars. These dense objects rotate fast with periods of about few seconds and emit light, mostly in radio wavelengths, through cones along the rotation axis. Some such stars are aligned towards us in such a way that we see them as lighthouse beacons and known as pulsars. While in graduate school, I had a rare privilege to work briefly with Dr. Anthony Hewish, who received Noble Prize in 1974 for the discovery of pulsar by his PhD student Jocelyn Bell in 1967 under dramatic situations. It was a pleasure to hear many stories surrounding this discovery.

When the core mass exceeds 3 to 5 times the mass of the sun (the limit is not precisely known), even the neutrons cannot stop the collapse. The star eventually becomes a Black Hole. If the light enters a sphere of about 10 km of a Black Hole of 5 solar masses, it cannot escape. So, anything that happens within this sphere can never be known. This sphere is known as event horizon.

Every thing that is born is certain to die. When and how is not precisely know, but we surely know the rough circumstances of all these events. So, the stars die but their cores long live.