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 galaxy’s 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 2-3x1022) 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 “baby” 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 “wobbling.” 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 techniques, many more will be discovered.
- Published in 1905, Einstein’s E = mc2 equation explains the origin of the large amounts of energy our sun and the stars release, although the particular sequence of events occurring in a star’s core was not deduced until theoretical work was conducted leading to the atomic bomb in the 1940s.
- The power of an atomic bomb comes from atomic fission (splitting apart), whereas the vastly greater power of a hydrogen bomb comes from atomic fusion (joining together).
It is relatively simple to make atomic bombs using radioactive isotopes of uranium, because they are constantly splitting apart (with each split releasing energy, other emissions, and neutrons, which then bang into and split other atoms—producing the so-called chain reaction). All that’s required is to drive together pieces of uranium (the more radioactive, “enriched” isotope U-235, is used). (The difficulty stems from finding a way to force the lumps together sufficiently quickly that many atoms split before the resultant energy release pushes everything too far apart [which stops the chain reaction]. This problem was solved by detonating a containing shell of conventional explosives.)
It is slightly more difficult to force enough hydrogen atoms together to make a hydrogen bomb. Physicists succeeded by exploding an atomic bomb and using the extremely high pressure this developed to compress surrounding tritium (an isotope of hydrogen that contains two, rather than one, neutrons in its nucleus). Squeezed tightly enough together, tritium atoms fuse to form slightly lighter atoms of helium; the small amount of extraneous matter is expelled in the form of large amounts of energy.
- The announcement that “cold fusion” was possible created much excitement in 1989. Many thought, for a while, that everyone could one day have a little fusion reactor in their home, turning deuterium into cheap energy. (Energy generated this way would be cheap because the deuterium used as fuel can be produced by processing water, which requires much less energy than that released when deuterium is fused to form helium.)
Unfortunately, the experimental results were difficult to reproduce, and no practical means of generating power from low energy nuclear reactions has been demonstrated. (This cost-benefit does not apply to hydrogen fuel cells. The amount of electrical energy required to produce the fuel hydrogen exceeds that released when it is later combined with oxygen in fuel cells. Automobile companies are gearing up to use fuel cells in transportation because fuel cell emissions are pollution-free [and because of government legislation], not because hydrogen provides low-cost energy.)
- Hydrogen was more plentiful earlier in the universe’s life, and stars were generally bigger than they are today. Larger stars burn faster and have a shorter life.
- It was earlier proposed that hypernova, about a hundred times larger than the average supernova, could occur and be the source of extremely intense gamma-ray bursts (GRBs) of energy that have been detected, but GRBs are now thought to signal the birth of black holes.
- The remnants of a supernova recorded by Chinese astronomers as occurring July 4, 1054 CE, can still be seen in the Crab Nebula. When first observed, it remained bright enough to be visible during the day for more than three weeks. (Since the Crab Nebula is 6,300 light-years away, the supernova actually exploded 6,300 years before it was first observed on Earth.)
On February 24th, 1987, astronomers observed a star, known to have been about twenty times more massive than our sun, exploding as a supernova. The emissions from its remains are being carefully monitored to learn more about the processes involved.
- The visible universe is calculated to be approximately 13.7 billion years old.
- There are so many stars in the universe that modern telescopes would be able to detect supernovae occurring every minute, if they were aimed in the right direction.
- Most stars pair up to form binary (or larger) systems and orbit each other around a common centre of gravity. Single stars, like our sun, are the exception, rather than the rule. Binary systems would produce complex effects on orbiting planets, and this might affect the number of planets supporting life forms.
- Part of Orion’s sword, the Great Orion Nebula (about 1500 light-years distant from us) contains a star-forming region. About 700 young stars lie within the centre of this nebula, and some 150 of these are surrounded by rings of gas and dust particles that herald the future formation of planetary systems (see The Earth of this chapter).
- Asteroids are pieces of matter that have been left over from this process. Most of our sun’s asteroids move in an elliptical orbit (the asteroid belt) between the orbits of Mars and Jupiter. Significantly more of the early dust from which our planetary system originated still orbits the sun as chunks of dust and ice outside Pluto (the Oort Belt). These lumps can be displaced from their orbits by passing stars, and some have taken up elliptical orbits around the sun, occasionally becoming visible as comets.
- However, some planets have been observed directly by telescopes (see http://www.eso.org/outreach/press-rel/pr-1998/pr-18-98.html).
- Two planets (with a strong likelihood of there being a third occupying a life-favourable position) have been found to orbit the star 55 Cancri, by this method.
- Light-bending was predicted by Einstein’s General Theory of Relativity (published in 1916). When a solar eclipse occurred in 1919, a team of astronomers used the opportunity to check the theory’s accuracy. As the moon blocked the sun’s radiance, they were able to photograph light coming from stars located behind the sun. Since the sun itself lay on a straight line drawn between these stars and the Earth, this could only happen if the light from these stars bent, as predicted, as it travelled close to the sun.
- Some of the light from the star HD 209458 is periodically blocked by a planet that orbits it.
- The thousands of gaps in Saturn’s rings are likely to have been created by satellites of various sizes. Saturn has about twenty confirmed moons, an additional dozen or so possible ones, plus millions of smaller chunks formed from frozen gas and water.
- The planet that orbits Boötis has been investigated in this manner.