In order to understand spectroscopy, it is essential to have some knowledge of the workings of matter and light at the atomic level-- it is important to know some quantum chemistry. A fundamental discovery about the atomic realm indicated that all matter and energy (i.e., light) comes in little packets, called quanta. (This is called quantization, for the vocabulary mongre.) This discovery was in sharp conflict with everything known by physcists at the time. Before this discovery, it was believed that matter and energy could be infinitely divisible; mathematically speaking, matter and energy were considered to be continious.

We now know that all matter is made up of atoms, which in turn are made up of protons, neutrons, and electrons. These are indivisible elements of which all matter is composed. We know that in the center of the atom lies the nucleus (made up of positively-charged protons and nuetrons), and 'orbitting' the nucleus are negatively-charged electrons. The world 'orbitting' is intentionally placed in quotes because electrons do not exactly 'orbit' the nucleus at all; rather they are attracted to the nucleus' positive charge, and are probabilistically distributed around it in 'clouds' called orbitals (not orbits).

Each orbital can contain several electrons, and the orbital which an electron occupies corresponds to a certain amount of potential energy within the given atom. An electron can move between the different orbitals, but that would require a change in the potential energy of the nucleus-electron system. By the Law of Conservation of Energy, energy can not be created or destroyed, so the missing potential energy has to go somewhere -- it is given off as radiation (i.e., light). The converse can also occur: photons of light, stricking an electron, may cause the photon to be absorbed, and pushing the electron into a higher energy orbital.

What does this have anything to do with observing galaxies and galaxy clusters? Good question; the answer is Everything!

Each element has only discrete energy orbitals that an electron can occupy; this means that there exist certain discrete energy differences between orbitals; and when electrons move from a higher to a lower energy orbital, the energy is released as light. Since the amount of energy carried by a photon of light is proportional to its frequency (or inversly proportional to its wavelength; i.e., a low, spread-out wave carries less energy than a high, packed wave), only certain frequencies of light are emitted from a particular element -- precisely those corresponding to the energy transitions of the electrons.

So what? Well, we have established what the mathematician would call a one-to-one correspondence between photon frequencies and elements. This means that if a series of photons of certain frequencies are observed, the electron energy transitions can be calculated; these can then be paired uniquely with the various elements (hydrogen, oxygen, carbon, barium, etc.). This is precisely how the composition of our Sun was first measured at the beginning of this century -- and it was found to be almost entirely hydrogen and helium (in 3:1 ratio).

In my research this summer, I looked at clusters of galaxies (from now on, I will refer to them simply as clusters). These are objects with names such as Abell-1795, Abell-0401, and Abell-2657 (commonly abbreviated as A1795, A0401, and A2657). Most clusters are simply given numbers, prefixed with the word 'Abell' in honour of the astronomer George Abell, who catalogued the first clusters in 19..

For this reason, in order to do x-ray astronomy, we must bypass the atmosphere alltogether, and measure the photons in space! This explains why x-ray astronomy is a relatively new science -- it was not until the 1960s, with the advent of earth-orbiting sattelites, has it been possible to measure cosmic x-rays.

There are two most recent and most popular x-ray satellites currently in orbit -- ROSAT and ASCA. ROSAT (which stands for ROentenberg [the German physicist we met earlier] SATellite ), made by Germany and launched by the United States in 1990, was shutdown at 9 successful years in orbit in Febraury of this year. ASCA (which stands for Advanced Satellite for Cosmology and Astrophysics), was made by Japan and also launched by the United States in 1993. It is near the end of its mission, but continues to make x-ray measurements of the Universe. Recently (on July 23, 1999), a third United States-made x-ray sattellite named Chandra (in honour of the Indian-American astrophysicist Chandra....) was launched. For the first few months, calibration measurements will be made, after which Chandra will begin to take x-ray data.

The Position-Sensitive-Photon-Counter (PSPC) is a cylindrical gas chamber that measures the positions and energies of incoming photons.

Since the detector is not perfect, there are uncertainties in the location and energy of each photon that is measured. In addition, there are various sources near Earth that may affect the operation of the detector. Examples include intermittent solar flares, the Earth's magnetic field, and constant solar wind. These factors are external to the objects that we are observing, which are outside of the our Solar System, and even outside of our own galaxy.

The first step before any scientific knowledge can be deduced, these external affects must be accounted for. The process of accounting for and subtracting these affects is known as data reduction.

Data reduction is an extremely difficult task, since it requires an broad and deep understanding of not only galaxy clusters, and not only of local external affects as we saw earlier, but also the structure and function of the intrumentation and the detector. There are also very subtle interactions between these three domains that make this part of the task even more difficult.

As we learned earlier, each element has a unique quantum spectrum. But since galaxy clusters are enourmously complex objects, they has many different combinations of elements -- hydrogen and helium being the most abundant. Using quantum physics, we can create models of the spectra based on various properties of the cluster. Since we can measure the spectra, we can attempt to "fit" the model to the data by adjusting the various parameters, which refer to properties of the clusters we can not directly measure. Such properties include temperature, redshift (a measure of distance from Earth), relative abundances of various elements, and nuetral hydrogen absorption column (a measure of the amount of matter between Earth and the cluster which absorbs x-rays).

Well, firstly, thank you for spending the time reading this far. As for "so what?", I do not have an answer. In my opinion, knowledge is its own reward. If you want money for looking at A1795 and determining its elemental abundance, forget it! Don't go into astrophysics; chose a career another career such as business, medicine, or politics. If you want fame, also forget it! Pursue acting or athletics.

No, really, you shouldn't believe strangers you meet on the web. Unless they are honest strangers like myself, in which case you should believe them.

Come on! Stop being paranoid! Modern man has lost faith in the goodness of humanity. So trust me when I say that what I write here is accurate to the best of my knowledge. (Although my poor spelling should indicate the short range of my knowledge :-).)

I hope to publish my results in the Astrophysical Journal, and hopefully that will give me some more credibility. In the mean time, you can refer to John's paper published in ApJ -- whose results I am trying to confirm: