JOIN USThe knowledge from books and other sources that twinkling stars are as massive or may be more than the sun fails to impress us when we gaze at the night sky.
The fact that some of them are doublets add to the amazement. The earliest suspicion of the double nature (binary nature, to be precise) of stars takes us back to several centuries when John Goodricke, a deaf and dumb boy, watched the periodic variation of the brightness of Algol (which means demon’s eye). Subsequently, the list started growing – almost 50% of the star are binaries.
The binary nature of the stars reveals itself in various ways. For example, the companion of Sirius, the brightest star in the sky was suspected of an unseen companion by its wobbling motion, barely recognisable with naked eye. Called Sirius B, it is a typical white dwarf with a mass about the same as that of our Sun. Its colour, blue-white, indicates that its temperature is rather high: 29,200 K, compared to 5,500 K for the Sun and 10,000 K for Sirius A. But it is not bright. Its diameter is 9,800 km, slightly smaller than that of Earth. Thus, effectively it is a star of the size of a planet.
The Chandrasekhar Limit
Although a precise theory for the formation of a binary star cannot be formulated, the subsequent evolution has been worked out. The mechanism of mass transfer between the stars plays a very crucial role. The star with larger mass evolves faster as per the norms. Thus, after a few billion years, a binary which had both ‘normal’ hydrogen burning stars, will have an ‘old’ and ‘a young’ star going round each other.
A normal star like the Sun produces energy by fusion of protons into helium (proton — proton (p-p) reactions) in the star’s core which generates energy. The pressure supports its weight. The Sun is made of mostly hydrogen, enough to last like this for about 15 billion years. However, when it begins to run out of hydrogen in its core, where the fusion is in progress, the core of the star gradually begins to collapse.
As the core collapses, it gets hotter and it triggers new fusion reactions involving helium, the ‘ash’ of the previous p–p reactions. It gets so hot that the energy from the core causes the outer parts of the star to expand and get less dense. As a result, the star looks cooler (redder). The star is now becoming a red giant. The process continues so that it is possible for carbon to fuse into oxygen. Eventually, the core is all carbon and oxygen, no additional heat and gas pressure is generated and the core begins collapsing again. The electron degeneracy pressure begins to increase significantly as the collapse proceeds.
Electron degeneracy pressure brings the collapse of the core to a halt before it gets hot enough to fuse carbon and oxygen into magnesium and silicon. The unstable outer parts of the star fall apart altogether and they are ejected and ionised by light from the core. This produces a planetary nebula. The planetary nebula’s material are ejected with very high velocities and in a few thousand years, the former core of the star — which is now about the size of Earth — is all that is left behind. This is the hot, white (like Sirius B) or even blue in colour white dwarf. Its weight is supported against further collapse by electron degeneracy pressure and it cannot do anything but cool off. Some of the oldest ‘white dwarfs’ in our galaxy, around the age 12 billion years, have had enough time to cool down to temperatures in the few thousands of degrees and therefore look red.
Astrophysicist Subrahmanyan Chandrashekhar’s monumental theory on these white dwarfs predicted sizes reasonably close to that determined from the observations of Sirius B. He also arrived at the peculiar relation between mass and size: higher-mass degenerate stars are smaller than lower mass ones, the opposite of what happens with normal objects. In simpler words, the scenario can be explained as - more mass implies more pressure is required to balance gravity, and more degeneracy pressure requires more tightly-confined electrons (smaller star). This sets a limit on the mass of the core, beyond which the ‘white dwarf’ reduces to a point object. This limit of about 1.4 times the mass of the Sun (the term ‘mass of the Sun’ is akin to kg), is called the Chandrashekhar limit.
Opportunities for mass transfer
The scenario is applicable to stars in binary systems too. However, after one of the two becomes a white dwarf, there are ample opportunities for a mass transfer. We have examples of cases where there is no influence of the companion — the two stars evolve independently. The other extreme is when the transfer process happens quite often and they will finally merge into a common envelope phase. The accretion of mass can result into sporadic or periodic outbursts giving a false impression of a new star called ‘nova’ in the sky. This should not be confused with supernova which is the penultimate stage in the evolution of a massive star where the core burns through a succession of nuclear fusion fuels at different time scales.
Finally, an inert iron core builds up and grows until its mass reaches about 1.2-1.4 Msun. Core temperature reaches 10 billion K and density around1010 g/cc. The energy is quickly consumed by two processes: nuclei photo-disintegrate into He, p & n, and protons and electrons combine to form neutrons and neutrinos. The neutrinos escape and carry away energy. Both processes consume energy from the core, hastening the final collapse which is catastrophic.
The core with radius around 6000 km (~Rearth) and density around 108 g/cc collapses within a second to radius around 50 km and density around 1014 g/cc. Thus, the speed of collapse is about one fourth that of light. The star shines with the brightness of a billion suns in minutes. It can outshine the entire galaxy; the ejecta blasted off at few 10000 km/s. The end product may be a neutron star or a black hole. An event which occurred in 1054 AD resulted in the Crab nebula.
Crossing the threshold
These scenarios are altered by the presence of the companion, which would not have met its end yet. The mass transfer to it can gradually accrete the mass considerably. On the white dwarf, the accretion can lead to the Chandrasekhar limit. Thus, the peaceful dead star is now disturbed. This explosive event of crossing the Chandrashekhar limit is also termed supernova. A characteristic feature of this is the absence of hydrogen in the spectra.
The two phenomena are distinguished as Type II (single star) and Type I (white dwarf). The signature of the physical process is identifiable in the spectrum. Type I shows no hydrogen lines. The historical supernovae recorded so far in our galaxy have provided examples of both types. The one named after Kepler (1604 AD) and the other which occurred 1000 years ago (1006 AD) are examples of Type I category.
What if both stars have comparable masses and evolve together? We have a variety of binary systems. Both can be white dwarfs or both can be neutron stars or both black holes. The other possibility of a combination of the two of the three also exists. How would these evolve? That takes us to the interesting scenario of the merger of the black holes. The Laser Interferometer Gravitational-Wave Observatory (LIGO) detected gravitational waves last December and more recently in June. These waves came from the merging of two black holes. The second result hinted at smaller black holes. The duration of the waves was about a second. The waves came from the merging of black holes with masses of about 14 and 8 times that of the Sun, 1.4 billion light years away. The first detection involved masses of about 29 and 36 solar masses.
This new branch of astronomy has generated lot of excitement among the observers and theoreticians. The challenge lies in addressing the systems with very small mass ratios. So far, the attempts are successful to about 0.01; but the challenging value is 0.00001. This will address the question on the super massive black holes sitting at the centres of galaxies.
About a year ago, a very distant quasar PSO J334.2028+01.4075, was found to be periodically varying in brightness every 542 days. The analysis soon revealed that there were two massive black holes orbiting each other within a separation of Sun – Mars distance. The discovery was very exciting because the right candidate for the merger and subsequent emission of gravitational waves was spotted.
(The author is director, Jawaharlal Nehru Planetarium, Bengaluru)