Stars contain huge amounts of hydrogen, and the gravitational forces near their centre are extremely high. This creates a pressure which pushes the hydrogen nuclei closer and closer, eventually fusing pairs of them together, creating helium. Since one helium nucleus has slightly less mass than the two hydrogen nuclei that formed it, the surplus mass has to be released. This occurs, but the release is not in the form of a particle of matter; the mass difference is radiated away as energy, and many such fusings quickly raise the sun's core temperature (which stabilizes at about twenty million degrees Celsius). Physicists replicate this process in hydrogen bombs, and they are attempting to do the same thing in the laboratory. (Here the biggest problem is how to contain and control the high temperatures involved ”about 100 million degrees, or over five times the temperature of the sun's core, in some research reactors.)

The average star today burns for about ten billion years. As a star's hydrogen continues to form helium, its core gradually shrinks. This causes it to become hotter, and its nuclear fusions become more complex. Through fusions, helium is converted into carbon and oxygen, then into other elements (those in approximately the first quarter of the periodic table). Further core shrinkage eventually creates enough radiation to induce a big expansion of the outer layers, and stars that are about the size of our sun become red giants, enveloping any orbiting planets in incandescent gases. Eventually, fusion can no longer be sustained in the core, and red giants shrink, lose their outer layers of gases, and become compact white dwarfs. This is likely to be the fate of our sun and its planets, some six billion years in the future.

A number of stars are large enough to avoid this kind of relatively slow death. Their extra mass creates higher core temperatures and heavier elements are formed. However, forming nuclei of elements beyond iron requires an input of energy (because extra energy is needed to hold together the many mutually repulsive protons such nuclei contain). This reduces the energy released during fusion to a point where it can no longer counterbalance gravitational attraction, and the star collapses. This sudden implosion releases tremendous amounts of energy and everything immediately heats up again. This, in turn, rapidly fuses many of the existing elements into the larger and more complex elements that exist beyond iron (over eighty of them; copper, silver, gold and mercury, for example). The core's raging furnace builds in intensity and soon explodes, scattering the star’s chemical elements into space. Some massive stars undergo several cycles of explosions and collapses. (Astronomers occasionally witness these events; each explosion is termed a nova.) Other giant stars explode completely in one detonation; what is then seen is called a supernova. Both appear abruptly as bright patches of light in the sky, often intense enough to be visible in daylight, occurring in the spot formerly occupied by a star.

Observers find and study one or two new novae each year, and witness gigantic supernovae blossoming within our galaxy, the Milky Way, an average of twice a century. In the ten billion years of our galaxy's existence, about two hundred million supernovae have spewed out the chemical elements we know from spectroscopic evidence to be present throughout its volume. The dark patches (mentioned earlier in this chapter), observed within interstellar and intergalactic space, are due to the presence of vast clouds of these minute dust particles. This dust (altogether amounting to hundreds of times more matter than is contained within the total of every galaxys collection of stars and planets) is mostly composed of the elements formed and ejected during novae and supernovae explosions. These particles absorb and obscure light from the stars and galaxies that lie beyond, and it is the absence of this light that produces regions that appear to be dark.

The visible universe is estimated to contain some 100,000,000,000 (100 billion) galaxies, and an average galaxy (such as ours) accommodates a collection of some 2-300,000,000,000 (200-300 billion) stars. Thus, there are twenty to thirty thousand billion billion (i.e., 20-30,000,000,000,000,000,000,000, that is, 20-30 sextillion, or 2-3x1012x10²²) stars all told in our universe.

As mentioned, stars of all descriptions have been found. Some, only about a half-million years old, have been photographed via the infrared radiation they emit. These newly formed stars already show a spin (imparted by the kinetic energy of condensing gases as they are drawn into the star by gravitational attraction), which the star retains for its lifetime. Observations suggest that about half of all newly formed stars are also accompanied by a rotating disk of gases, particles, dust and debris. Matter that is not pulled into the star is gradually pushed away by the star's radiation and most eventually disperse into space. However, gravity also pulls some of the disk's matter and dust together to form aggregates; the largest of these we call planets.

The brightness and remoteness of stars generally prevent us from directly observing any planets that might orbit them. However, the presence of planets can be inferred by various techniques, and some nearby stars are now known to have orbiting bodies. One way to determine if a star has one or more planetary companions is to measure its wobble (caused because the orbiting bodies together rotate about their common centre of mass, i.e., the star no longer rotates about its own centre and therefore looks as if it is œwobbling. A similar wobble can occur when a car tire is unbalanced.

Another way to find planets is to look for lensing effects, when light from distant stars becomes bent due to gravitational pull as it passes close to large masses, such as those of giant (e.g., Jupiter-size or greater) planets. Astronomers also look for emission intensity variations and dark spots transiting the face of stars (caused by planets crossing in front of a star and so preventing some of its light from reaching the observing telescopes). The presence of rings of matter surrounding some stars gives observers yet another way to infer that planets have been (or are being) formed about a star, and circular gaps within such rings of dust almost certainly mean that planets are present. (Clumps of particles within a ring gravitationally sweep up additional dust as they travel around a star; this causes the orbiting bodies to gradually enlarge and leaves the gaps that are seen. These dust aggregates may ultimately become large enough to form asteroids and planets. Most stars, including our own, lose much of their dust halo due to this process [as well as due to pressure from the star’s radiation] in the first 400 million years following their birth.)

Planets may also be sought and even studied by examining the doppler-shifted starlight scattered by their atmospheres. Over one hundred exoplanets (as planets outside our solar system are called) have been found to date, a number that is being added to every few weeks. Undoubtedly, with time and as astronomers refine their planet-finding.

LIFE CYCLE OF LOW AND HIGH MASS STARS

The life cycle of a low mass star (left oval) and a high mass star (right oval): Image from NASA

"The illustration above compares the different evolutionary paths low-mass stars (like our Sun) and high-mass stars take after the red giant phase. For low-mass stars (left hand side), after the helium has fused into carbon, the core collapses again. As the core collapses, the outer layers of the star are expelled. A planetary nebula is formed by the outer layers. The core remains as a white dwarf and eventually cools to become a black dwarf."

"On the right of the illustration is the life cycle of a massive star (10 times or more the size of our Sun). Like low-mass stars, high-mass stars are born in nebulae and evolve and live in the Main Sequence. However, their life cycles start to differ after the red giant phase. A massive star will undergo a supernova explosion. If the remnant of the explosion is 1.4 to about 3 times as massive as our Sun, it will become a neutron star. The core of a massive star that has more than roughly 3 times the mass of our Sun after the explosion will do something quite different. The force of gravity overcomes the nuclear forces which keep protons and neutrons from combining. The core is thus swallowed by its own gravity. It has now become a black hole which readily attracts any matter and energy that comes near it. "... source -Life Cycles of Stars -- NASA.