A BILLION years from now, Earth will be a barren, lifeless planet: Because both the luminosity and the temperature of the Sun is gradually increasing. So what happens three billion years beyond that, four billion years into the future, is academic—but not irrelevant.
Quite to the contrary, it is both relevant and instructive. Otherwise, neither Bethe nor Chandrasekhar would have been awarded the Nobel Prize. Keep in mind, that our Sun and the Sirius system are not isolated entities. They share in the chemistry and physics of the universe.
So too, do all the other objects that appear in the eastern sky during the dry season—stars like Rigel and Betelgeuse, the Hyades and Pleiades clusters and the Orion Nebula. These are places we will visit, before the serial ends.
But to do so, without some tutoring from Chandrasekhar, would be pointless. That’s because the varied colours, luminosities, physical properties, life-spans and ultimate fates of these entities are, to a very large extent, consequences of the Chandrasekhar limit.
Internally, main sequence stars are said to be in a state astrophysicists describe as “hydrostatic equilibrium”. What this means, is best depicted through a widely employed personification: Gravity and heat don’t like each other a lot and, accordingly, spend the productive life of the star in combat.
Heat wants the star to expand, while gravity tugs and jerks every ion or atom towards the center. The advantage shifts back-and-forth, from heat to gravity, creating a dynamic equilibrium—at least for a time. But over the long haul, gravity will usually prevail.
The showdown comes, astronomers say, when the star exhausts its hydrogen fuel and moves off the main sequence of the HR diagram. During the long hydrogen-burning epoch, helium was building up in its core. But whereas hydrogen started to fuse at 10 million K, it takes a temperature of 100 million K to burn helium.
At this juncture, fusion shuts down in the core, temporarily. With no heat being generated, gravity gets the upper hand; and the star starts to contract—generating heat in the process. Then, in a violent reaction, which astrophysicists call “triple alpha,” helium starts to fuse and the star expands again.
Two things need to be noted here. The first is that the chemical change in the star’s core induces structural changes that give it a different appearance. Whether it was originally yellow, like our Sun, or blue like Sirius-B, its outer layers expands and cool; and it becomes a red giant.
As the Denver Museum of Nature and Science (U.S.A.) point out, in its Technical Paper of 2005, “The process of core contraction—as the nuclear fires shut down in the center but fusion beginning again in the intermediate layers—result in the star expanding again into a red supergiant.
“At this stage,” it continues, “the stellar envelope swells up enormously to become as large as the orbit or Earth or Mars”.
This is an important clue: Both to the enormous visual diversity of the objects visible in the eastern sky, which we are about to examine individually, and to the importance of Chandrasekhar’s delineation of dying stars, on the basis of their mass and chemistry.
Secondly, helium burning lasts only about 10 million years—a fleeting astronomical moment. This is the end of the line for stars with less than roughly 1.4 times the mass of our Sun. They cannot generate the 600 K temperatures in their cores, which is required to fuse carbon.
Consequently, low-mass become white dwarfs: Glowing spheres of superheated carbon. These astronomical cinders will continue to cool, over billions of years, with their colour changing to yellow, orange, red and eventually black.
The conventional wisdom, in Chandrasekhar’s day, was that this is the fate of all stars—which, of course is not the case. The white dwarf is just one of three possible ends to a star’s life: Depending on the mass it has left, at the end of its energy-producing regime.
It’s one thing for me to write this now, when the theory being taught in secondary schools around the world. But for a 19 or 20-year-old Indian physicist to assert in 1931, that the fates of stars rested on the chemistry and mass of their cores, was heretic.