Lectures

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	Lecture 30 - Millisecond Pulsars If one tries to describe the development of our understanding of ``Millisecond Pulsars'', it would look very much like the work report of the Astrophysics Group of Raman Research Institute in the 1980s-'90s. The group was headed by Prof. G. Srinivasan at that time, and it was he (alongwith Prof. V. Radhakrishnan) took the main initiative in this activity. In fact, great strides were made from this group in the overall area of the evolution of Neutron Stars during this period. A neutron star spinning 642 times a second was discovered in 1982! Although it was a `solitary pulsar', a bold conjecture was made that it must be a ``recycled pulsar" - like the Hulse-Taylor pulsar - spun up in a low-mass binary, which somehow `got rid' of its binary companion. This scenario… Read More
	Lecture 29 - The Gravitational Radiation On 25 November 1915, Einstein published the gravitational field equations of the General Theory of Relativity (GTR). Almost exactly a century later, on 14 September 2015 came the first direct detection of gravitational waves, made by an extremely sensitive instrument called the LIGO (Laser Interferometer Gravitational-Wave Observatory), providing the most direct proof of GTR. The observed gravitational waves were generated by the merger of two massive Black Holes some 1.3 billion years ago! This lecture is about the recent detections of gravitational waves. Just as Maxwell's Theory of Electromagnetism predicted the existence of transverse electromagnetic waves, Einstein's General Theory of Relativity predicted the existence of ``Gravitational Radiation". Although this should have… Read More
	Lecture 28 - The case of the missing neutrinos On many occasion Astrophysics has played a pivotal role in changing the direction of fundamental physics. The problem of the `missing' solar neutrinos has been one such. The solution has taken more than four decades to arrive at and has the startling implication that the netrinos have mass (even if tiny). The most significant consequence of this is the need for a major revision of the current `Standard Model' of Particle Physics! This lecture is about a most remarkable `detective story'. The relentless hunt for the missing neutrinos from the Sun. In August 1920, Sir Arthur Eddington made the remarkable suggestion that the energy radiated by the sun and the stars is produced when Hydrogen is transmuted to Helium - the… Read More
	Lecture 27 - Supermassive Black Holes "Black Holes are macroscopic objects with masses varying from a few solar masses to millions of solar masses. Macroscopic objects are governed by a variety of forces derived from a variety of approximations to a variety of physical theories. In contrast, the only elements in the construction of Black Holes are our basic concepts of space and time. They are, thus, almost by definition, the most perfect macroscopic objects there are in the universe. - S. Chandrasekhar" The compact sources at the centre of radio galaxies have incredible luminosities. Invoking the "Eddington Luminosity Limit", astronomers concluded that the central engine must have a mass of the order of a billion solar masses. The time variability of the compact central source led one to… Read More
	Lecture 26 - Radio Galaxies Radio emission detected for the first time from an external galaxy Cygnus A and the suggestion that it could be a 'double radio source' compelled the radio astronomers to find ways of attaining very high angular resolution of celestial radio sources. This led to the discovery of the "Aperture Synthesis" telescope by Sir Martin Ryle, for which he was awarded the Nobel Prize in 1974 (first Nobel in Astronomy, along with Anthony Hewish)., The Giant Meterwave Radio Telescope (near Pune) is largest among such telescopes in the low frequency regime. Nowadays GMRT is routinesly used for observing Radio Galaxies, sometimes revealing interesting morphologies where the energetic jets emitted by these galaxies twist, wrap, overlap and bend. Radio emission from an external galaxy was first detected around 1950 from the powerful galaxy Cygnus A. There was a suggestion that it was a `double radio source'. This discovery catalyzed the quest for achieving higher angular resolution. This led to the discovery of "Aperture Synthesis"… Read More
	Lecture 25 - Recycled Pulsars The Astrophysics group of Raman Research Institute (Bangalore) played an important role in Neutron Star research in the 1980 - '90s under the leadership of Prof. G. Srinivasan. In particular his pioneering ideas about `pulsar recycling' have been (and remain) some of the truly major milestones in the area of Neutron Star Astrophysics. Although the Hulse-Taylor binary discussed in the previous lecture has two neutron stars, only one of them was detectable through its pulsed radio emission. This pulsar was not only the first pulsar to be discovered in a binary, it was also the most peculiar. Its very rapid spin rate suggested… Read More
	Lecture 24 - Binary Neutron Stars In 1974, a binary neutron star system was discovered - two neutron stars going around a common centre of mass. Russel Hulse, a young PhD student working under the supervison of Joseph Taylor, had been at the center of this discovery which gave us the first evidence (albeit indirect) of Gravitational Waves! This lecture describe neutron star binaries of different types and their signficance for Einstein's General Theory of Relativity. In 1974, a binary neutron star system was discovered - two neutron stars going around a common centre of mass. This raised the following question: ""How did such a system form?"" The first part of this lecture is devoted to the evolution of massive stars in 'binary systems', and the eventual… Read More
	Lecture 23 - Black Holes II Chandrasekhar once remarked - The black holes of nature are the most perfect macroscopic objects there are in the universe: the only elements in their construction are our concepts of space and time.'', Very appropriately, one of the most remarkable developments in physics in the later part of the twentieth century, the formulation of the theory of black holes, has depended entirely upon considerations of theoretical physics and abstract mathematics, without hardly any observational corroboration. This lecture describes the path to the most important milestones of this theory in simple terms. As we saw in Lecture 21, in 1939 Oppenheimer and his student, Snyder, made the spectacular discovery that if a star contracts to the critical radius, and becomes BLACK, it will continue to collapse till it disappears - all that will be left is a Black… Read More
	Lecture 22 - Black Holes I There are two kinds of black holes. The first is formed in supernovae, as the evolutionary end products of the most massive stars. The stellar black holes can be as heavy as a few tens of solar masses. Then there are the super-massive black holes, such as the one sitting at the center of the Milky Way and most other galaxies, which can be more than a million times more massive than the Sun. , On 10 April 2019, eighty years after Oppenheimer & Snyder predicted the existence of black holes, scientists of the ‘Event Horizon Telescope (EHT) released the first-ever image of a black hole - the super-massive object at the heart of the M87 galaxy. This impressive image is a rather remarkable indication of our technological prowess as well as the capability of our scientists to form and work in gigantic collaborations. The idea that light may not be able to escape from sufficiently dense bodies dates back to 1783 when the English Pastor John Michell first advanced this notion. This found a prominent mention in the great treatise by the French mathematician Laplace (1798). This conclusion was within the… Read More
	Lecture 21 - Maximum mass of neutron stars Academic pursuit has always been influenced by the course of human society and never more so than the effect of the two world wars on the twentieth century physics. Before the world came to know Robert J. Oppenheimer as the Gita-quoting head of the 'Manhattan Project', his attention was focussed on something entirely other-worldly. Oppenheimer, along with his students Volkoff and Snyder, was busy calculating the maximum mass of neutron stars., Today, advances in nuclear physics has but only validated and refined their pioneering work. Their calculations also led them to the conclusive prediction of Black Holes, but then the war began. By 1934, Chandrasekhar had clearly established that white dwarfs cannot exist with mass greater than a limiting mass, now referred to as the Chandrasekhar Limit. Soon after the ‘neutron’ was discovered, physicists made stars entirely of neutrons, and it appeared for a while that very… Read More