Particle physics, condensed matter physics and astrophysics are arguably the three major research frontiers of physics at the present time. It is generally thought that a physics student's training is not complete without an elementary knowledge of particle physics and condensed matter physics. Most physics departments around the world offer one-semester comprehensive courses on particle physics and condensed matter physics (sometimes known by its more traditional name ‘solid state physics’). All graduate students of physics and very often advanced undergraduate students also are required to take these courses. Very surprisingly, one-semester comprehensive courses on astrophysics at a similar level are not so frequently offered by many physics departments. If a physics department has general relativists on its faculty, often a one-semester course General Relativity and Cosmology would be offered, though this would normally not be a compulsory course for all students. It has thus happened that many students get trained for a professional career in physics without a proper knowledge of astrophysics, one of the most active research areas of modern physics.

Of late, many physics departments are waking up to the fact that this is a very undesirable situation. More and more physics departments around the world are now introducing one-semester comprehensive courses on astrophysics at the advanced undergraduate or beginning graduate level, similar to such courses covering particle physics and solid state physics.

Setting the time table

The present uniform expansion of the Universe suggests that there was an epoch in the past when the Universe was in a singular state with infinite density. Since most of the known laws of physics become inapplicable to such a singular state, we cannot extrapolate to earlier times before this epoch of singularity. We therefore concern ourselves only with what happened after this epoch of singularity, which is called the Big Bang. In the solutions discussed in §10.6 and §10.7, the time t was measured from the Big Bang.

The spacetime dynamics discussed in Chapter 10 sets the stage of the Universe. Now we shall look at the dramatis personae who were involved in the grand drama which unfolded and is still unfolding against this background stage of spacetime. How the temperature of the early Universe varied with time is given by (10.67) and (10.69). At times earlier than 1 s after the Big Bang, typical photons had energies somewhat larger than 1MeV. Since such photons are known to produce electron-positron pairs, the Universe at these early times must have been full of electrons and positrons which would have been approximately as abundant as photons. When photons had energies larger than 2GeV at still earlier times, they would have given rise to proton-antiproton pairs and neutron-antineutron pairs along with pairs of many other particles listed in elementary particle physics textbooks and their antiparticles.

The possibility of nuclear reactions in stars

We have seen in the previous chapter that many aspects of stellar structure can be understood without a detailed knowledge of stellar energy generation mechanisms. This is indeed fortunate because not much was known about energy generation mechanisms when Eddington was carrying out his pioneering investigations of stellar structure in the 1920s. Eddington (1920) correctly surmised that the Kelvin-Helmholtz hypothesis of energy generation by contraction (see§3.2.2) could not possibly be true and stellar energy must be produced by sub atomic processes. Nuclear physics, however, was still in its infancy and details of how the stellar energy is produced could not be worked out at that time. With the rapid advances in nuclear physics within the next few years, it became possible to work out the details of energy-producing nuclear reactions inside stars. To build sufficiently detailed and realistic models of stars and stellar evolution, a good understanding of energy generation mechanisms is essential. We turn to this subject now.

Let us consider a nucleus of atomic mass A and atomic number Z. It is made of Z protons and A Z neutrons. The mass mnuc of the nucleus is always found to be less than the combined mass of these protons and neutrons.

Interested in amateur astronomy? What are the first steps?

Astronomy is not just for professionals – the sky belongs to everyone! Amateur astronomy is a fascinating hobby, running from the simple pleasure of gazing at the night sky, to learning to appreciate the phenomena and mysteries of the Universe, all the way to making quasi professional observations. Perusing books and magazines on astronomy, even studded with spectacular photographs, can never match the emotional impact of engaging directly with the heavens.

Pastime or true passion, here is an activity that is within almost everyone's means. Even the most sophisticated amateur astronomers spend significantly less money on their hobby than do many sportsmen, boating enthusiasts, and golfers. It is important to start out on the right foot, though. If you just rush in and buy a cheap “toy” telescope, you will quickly be disappointed and lose interest. On the other hand, if you acquire the biggest, most expensive instrument on the market, you are likely to find yourself overwhelmed.

The best way to start is to observe the sky with the naked eye from a dark site using a star chart or one of the new handheld devices using GPS technology (such as SkyScout). Once you have learned to find your way in the sky and are familiar with the major constellations and planets, you can move up a step, to binoculars. Standard models, 8 χ 40 or 7 χ 50, are good choices. They are relatively inexpensive, convenient to use, and offer a generous field of view, making it easy to locate objects.

Introduction

We have pointed out in §6.1 that astronomers in the early twentieth century thought that our Milky Way Galaxy is the entire Universe! Even a small telescope shows many nebulous objects in the sky. The great German philosopher Kant already conjectured in the eighteenth century that some of these nebulae could be island universes outside our Galaxy (Kant, 1755). However, astronomers at that time knew no way of either establishing or refuting this conjecture. In 1920 the National Academy of Sciences of USA arranged a debate on this subject – Shapley arguing that these nebulae are within our Galaxy and Curtis arguing that they are extragalactic objects (Shapley, 1921; Curtis, 1921). We discussed in §6.1.2 how the distances of Cepheid variable stars can be determined. Using the newly commissioned Mount Wilson telescope, which was much more powerful than any previous telescope, Hubble (1922) resolved some Cepheid variables in the Andromeda Galaxy M31 and estimated its distance, clearly showing that it must be lying far outside our Milky Way Galaxy. Our current best estimate of the distance of M31 is about 740 kpc. It soon became clear that many of the spiral nebulae are galaxies outside our Galaxy, heralding the subject of extragalactic astronomy and establishing that galaxies are the building blocks of the Universe.

Normal galaxies

Light coming from a typical simple galaxy seems like a composite of light emitted by a large number of stars.

Introduction

We have seen in the previous two chapters that the gravitational attraction inside a normal star is balanced by the thermal pressure caused by the thermonuclear reactions taking place in the stellar interior. Eventually, however, the nuclear fuel of the star is exhausted and there is no further source of thermal pressure to balance gravity. We have pointed out in §4.5 that such a star keeps on contracting – unless some kind of pressure other than thermal pressure is eventually able to balance gravity again. The aim of this chapter is to discuss the possible end configurations of stars which have nonuclear fuel left in them.

We have to make use of one very important property of Fermi particles. In a unit cell of volume h3 in the six-dimensional position-momentum phase space, there cannot be more than two Fermi particles (one with spin up and the other with spin down). The electrons inside the stellar matter make up a Fermi gas, and when the density inside the contracting star becomes sufficiently high, this electron gas becomes ‘degenerate’. This means that the theoretical limit of twoparticles per unit cell of phase space is almost reached. We shall show in §5.2 that such a degenerate Fermi gas exerts what is known as the degeneracy pressure. White dwarf stars discussed in §3.6 are believed to represent stellar configurations in which the inward pull of gravity is balanced by the degeneracy pressure of the electron gas.

How do refracting and reflecting telescopes differ?

Generally speaking, a telescope† is an instrument that enhances the observation of celestial objects by increasing their apparent size and luminosity. This applies to the entire electromagnetic spectrum, from radio waves to gamma rays, including of course the “optical domain,” which covers visible light and radiation in the infrared and ultraviolet. Optical telescopes can be very diverse, but basically they work just like a photographic camera: they focus the light of a celestial object to form a real image‡ on film or on an electronic detector. In the visual version, an “eyepiece,” which is basically a magnifying glass, is used to observe the image directly.

The terms “refracting” and “reflecting” simply refer to the composition of a telescope's optics. If lenses are used, the instrument is a refracting telescope; if mirrors are used, it is a reflecting telescope.

Which system is better? Well, in general, the larger the telescope – that is, the larger its main mirror or lens – the more light it can collect and the fainter the object it can observe (Q. 205). Reflecting telescopes can be built very large indeed, whereas refracting telescopes have serious size limitations.