Galaxies, stars and planets

Element Relative abundance Element Relative abundance 

The majority of the Universe is made from hydrogen and helium produced during the Big Bang, although some He has been made subsequently. The relative cosmic abundance of some of the elements relevant to the formation of life is given in Table 1.2, with all elements heavier than H, He and Li made as a result of fusion processes within stars, as we shall see later. The cosmic abundance is assumed to be the same as the composition of the Sun. 

The Sun formed some 4.5 Gyr ago (Gyr is a Gigayear or 109 years) from its own gas cloud called the solar nebula, which consisted of mainly hydrogen but also all of the heavier elements that are observed in the spectrum of the Sun. Similarly, the elemental abundance on the Earth and all of the planets was defined by the composition of the solar nebula and so was ultimately responsible for the molecular inventory necessary for life. The solar system formed from a slowly rotating nebula that contracted around the proto-sun, forming the system of planets called the solar system. Astronomers have recently discovered solar systems around 

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For remote sensing, spectroscopy at THz frequencies holds the key to our ability to remotely sense enviromnents as diverse as primaeval galaxies, star and planet-fonuing molecular cloud cores, comets and planetary atmospheres. 

COBE has produced some of the most dramatic scientific achievements in infrared astronomy. COBE measured the spectral shape of the cosmic background and found it to be a perfect blackbody spectrum at a temperature of 2.37 K. COBE also made the first observations of structure in the distribution of cosmic background radiation that are the probable first step in changes that produced the galaxies, stars, and planets we know today from the primordial smooth distribution of matter produced by the Big Bang. 

Matter from exploding supernovas was blown throughout the galaxy, forming a new generation of stars and planets. Our own sun and solar system formed only about 4.5 billion years ago from matter released by former supernovas. Except for hydrogen and helium, all the atoms in our bodies, our planet, and our solar system were created more than 5 billion years ago in exploding stars. 

In the present state of the universe, only a very small part of the energy is in the form of protons, neutrons and electrons that make up ordinary matter in all the galaxies. The rest consists of thermal radiation at a temperature of about 2.8 K and particles called neutrinos that interact very weakly with other particles. The small amount of matter which is in the form of stars and galaxies, however, is not in thermodynamic equilibrium. The affinities for the reactions that are currently occurring in the stars are not zero. The nuclear reactions in the stars produce all the known elements from hydrogen [2-4]. Hence the observed properties such as the abundance of elements in stars and planets cannot be described using the theory of chemical equilibrium. A knowledge of the rates of reaction and the history of the star or planet are necessary to understand the abundance of elements. 

The composition of the Earth was determined both by the chemical composition of the solar nebula, from which the sun and planets formed, and by the nature of the physical processes that concentrated materials to form planets. The bulk elemental and isotopic composition of the nebula is believed, or usually assumed to be identical to that of the sun. The few exceptions to this include elements and isotopes such as lithium and deuterium that are destroyed in the bulk of the sun s interior by nuclear reactions. The composition of the sun as determined by optical spectroscopy is similar to the majority of stars in our galaxy, and accordingly the relative abundances of the elements in the sun are referred to as "cosmic abundances." Although the cosmic abundance pattern is commonly seen in other stars there are dramatic exceptions, such as stars composed of iron or solid nuclear matter, as in the case with neutron stars. The... 

Studies in chemistry or any realm of science commonly consist of a series of directed examinations of parts of nature s realm called systems. A system is an identifiable fragment of the world that is recognizable and that has attributes that one can identify in terms of form and/or function. We can give examples at any level of size and complexity and in essentially any context. Indeed, a dog is a system at a pet show whereas the human heart is a system to the cardiologist a tumor cell is a system to the cancer specialist a star or planet or galaxy is a system to an astronomer a molecule or a collection of molecules is a system to a chemist and an atom or group of atoms is a system to a physicist. A system is, then, whatever we focus our attention upon for study and examination. 

Concerning gas losses, we must subtract gas transformed into stars and the matter imprisoned in stellar corpses. The latter occur in three forms white dwarfs, neutron stars and black holes. We must also include matter going into planets and aborted stars (brown dwarfs), forever frozen and permanently withdrawn from the (nuclear) chemical evolution of the Galaxy. 

There are also brown dwarfs to consider. Brown dwarfs are astronomical objects somewhat between a planet and a star and have a mass less than 0.08 times the mass of our sun and a surface temperature below 2,500 K. (As comparison, the cool red dwarfs are about 3,000-3,400 K). A large number of brown dwarfs would not change how bright the Galaxy appears in optical observations but would change its total mass quite substantially. 

So take your pick. We can say that the world is the way it is because the laws of nature are the way they are. Or we can say that the world is the way it is because hydrogen is the way it is. Whichever you select, one or the other, is a matter of preference. Either way, the litde hydrogen atom commands the stage on which the long and enchanting drama of our universe, the story of galaxies, stars, planets, and life, unfolds. 

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