Lectures

Lecture 10 - Quantum Statistics and the Fermi-Dirac distribution

Fermi-Dirac statistics describes the energy distribution of a non-interacting gas of identical particles with half-integer spin angular momentum. Despite the many successes of this in explaining various terrestrial systems, the most spectacular application of Fermi-Dirac has been, without doubt, in the area of stellar physics, in particular in explaining the physics of stellar remnants (white dwarfs, neutron stars).

The dependence of the pressure of the gas on the density and temperature plays a crucial role in the evolution of the stars as they age. The evolution is very different for stars in which the gas obeys classical Boyle’s law, compared to stars in which the gas obeys the rules of quantum physics.… Read More

Lecture 9 - Spiral Structure of Galaxies

Traffic jam in the galaxy! The beautiful spiral patterns seen in a large number of galaxies are, in fact, regions of higher density that move around the galaxy slower than the individual stars, much like a 'moving' traffic jam created by a slow-moving truck chugging along a fast lane.

Nearly 60% of the galaxies in the present universe have well defined spiral arms. Recently formed clusters of massive stars, as well as giant molecular clouds, are closely associated with these spiral arms. In the 1960s, an important idea was advanced concerning the nature of these spiral… Read More

Lecture 8 - Rotation of the Milky Way Galaxy

Dark matter, whose presence can only be inferred from its gravitational attraction, is pushing the boundaries of our known physics. This lecture explains how the first hint of the existence of this 'Dark Matter' came from examining the nature of the Rotation of our Galaxy.

All spiral galaxies rotate, and so does our Milky Way Galaxy. Around 1920, it was discovered that our galaxy rotates ‘differentially’, like the planets in the solar system. But in order to determine the precise law of rotation – how the angular velocity depends on the distance from the galactic… Read More

Lecture 7 - The Realm of the Galaxies

The `Great Debate' or the `Shapley–Curtis Debate' between the astronomers Harlow Shapley and Heber D. Curtis set the tone of our investigations into the nature of galaxies in the early twentieth century. This lecture delves into the questions addressed at that debate and many others in the light of the progress made since then.

A hotly debated question at the dawn of the twentieth century concerned the size of our universe, and whether there are other galaxies besides our Milky Way Galaxy. Another important question concerned the different stellar populations in our Galaxy, and the spatial distribution of stars. This… Read More

Lecture 6 - Interstellar Medium - Molecular Gas

The ratio of intensities suggested rotational temperature of 2.3K, which, of course, has a limited meaning." A remark made by Herzberg on the basis of the observations on the molecular spetra of cyanogen (CN), 24 years before the discovery of Cosmic Microwave Background radiation which, we now know, has that characteristic temperature.

Apart from giant clouds of atomic hydrogen, there are also giant clouds of molecular hydrogen in interstellar space. These molecular clouds are the sites of the formation of new stars and hence have a special role in the life cycle of the birth and death of stars. This lecture is devoted… Read More

Lecture 5 - Interstellar Medium - Atomic Gas

More than a century has elapsed since the discovery of ISM in 1904 to its appearance in popular culture ('Arora' - an SF novel) today. But the science behind our understanding of this tenuous medium remains as fascinating.

Ultraviolet radiation from newly formed hot stars ionizes the relic gas from which they formed. When the ions 'recombine' with electrons, spectacular gaseous nebulae are produced. The vast space between the stars is not empty. There is widespread diffuse gas, as well as gas clouds - just like in… Read More

Lecture 4 - Energy Generation in Stars

Stellar Nucleosynthesis was formally born around the middle of the twentieth century. Yet, at a time when the discovery of neutrons was ten years in the future, Sir Arthur Eddington accurately predicted that the Sun’s source of energy comes from the fusion of Hydrogen into Helium.

How do stars generate the energy they radiate?

Stars radiate an enormous amount of energy for a very long time - from tens of millions of years to billions of years, depending on their mass. In 1920, Sir Arthur Eddington advanced the idea that the energy radiated by the stars is… Read More

Lecture 3 - Why are the stars as they are ?

Why are the stars as they are? Sir Arthur Eddington showed how it is the fundamental laws of physics that define the basic characteristics of stars.

In the first lecture, we described stars as globes of gas in which the inward pull of gravity is balanced by the pressure of the gas. In this lecture, we shall describe Eddington's essential modification of this idea with the introduction of the pressure of the radiation trapped in the star. As… Read More

Lecture 2 - Principles of Radiative Transfer - Absorption and Emission of Radiation

Electromagnetic radiation, from low-energy radio waves to ultra high energy gamma rays.., acts as our window to the Universe. This second lecture discusses the propagation of electromagnetic waves through a material medium it must pass through before reaching us, starting from distant astronomical objects.

This lecture introduces the FUNDAMENTAL PRINCIPLES OF RADIATIVE TRANSFER: Most of the information concerning the physical nature of celestial bodies (such as their chemical composition, density, temperature, etc.) comes from detecting and analyzing the electromagnetic radiation from them at… Read More

Lecture 1: What are the Stars ?

This first lecture takes us through the early evolution of Astrophysics when amazing discoveries began to take place through scientist's efforts to explain celestial observations using known laws of physics.

This lecture begins with the reason for the "limb darkening" of the Sun, and the discovery of Helium in the Sun. It then explains the late 19th century ideas on "Stars as globes of gas", supported against gravity by the pressure of the gas. The equation of "hydrostatic equilibrium" is derived.… Read More