Lars Lindberg Christensen




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Spectral line radiation


The revolution of quantum mechanics at the start of the 20th century changed our understanding of the Universe forever, and gave us amazing tools to probe the structure of matter even from great distances. Spectral lines are specific wavelengths of light that are emitted and absorbed by every kind of atom and molecule and act as a unique fingerprint. Identifying known spectral lines in distant stars and galaxies allows us to measure chemical composition and determine physical properties like temperature, density and motions from afar.
A fundamental principle of quantum mechanics is that when we start looking at the smallest scales in the Universe, we find that energy comes only in discrete packets, or quanta. Within an atom, the electric forces that bind the negatively charged electrons that whirl around the positively charged nucleus only permit certain orbits at specific levels of energy. These levels vary, depending on the element (and how many protons are in the nucleus) and how many electrons are bound to it.
Nothing comes for free, however, and the tally of energy must always balance. An electron in a lower energy level can be bumped up to a higher level if it gobbles up a passing photon that has just the right amount of energy. Conversely, if an electron in a higher energy level drops to a lower one, it must emit a photon of an exactly matching amount of energy.
Since the energy of a photon is directly related to its wavelength, each energy transition in the atom (or molecule) corresponds to a precise wavelength of light. This light is known as a spectral “line” because of how exact the wavelength has to be; when plotted on a graph such a transition appears as a narrow line-like mark.
Spectral lines can be seen as an emission line if the electron is dropping from a high level to a low level, or as an absorption line if the electron absorbs a passing photon of the right wavelength from a background source.
Fluorescence is a common term used to describe a process where a high energy photon is absorbed by a body — which need not be hot — and gets transformed into one or more lower energy (redder) photons. This is familiar in fluorescent — or cold — lights where ultraviolet emission from an electrically-excited gas, like mercury, excites a material on the inside of the glass envelope to produce visible light. This can be a very passive process, like the use of fluorescent paints to capture blue light and radiate it as a vivid green, yellow or red. Even white writing paper contains a fluorescent dye that responds to blue/ultraviolet light and makes it glow “whiter-than-white”.
Astronomers use their knowledge of the various chemical fingerprints of known atoms and molecules to identify the composition of distant stars and nebulae.


Figure 12: Examples of fluorescence

This image shows different minerals emitting visible light when exposed to (“invisible”) ultraviolet radiation. The same thing happens across the Universe as clouds of dust and gas re-emit longer wavelength light when exposed to higher energy radiation from nearby stars.

Non-thermal radiation


There are a number of other processes in the Universe that create light in more exotic ways, exotic at least compared with our day to day experience. For instance, charged electrons and protons that are passing through magnetic fields will move along oscillating spiral paths that produce electromagnetic waves (synchrotron radiation). Fast-moving charged particles that deflect one another from their electric field interactions can also generate light (bremsstrahlung radiation). Such processes are particularly evident in the radio part of the spectrum and will be discussed at greater length in Chapter 7.


2. The View from the Ground


Figure 13: The Australian Radio Telescope Array

This beautiful image shows parts of the Australia Telescope Compact Array (ATCA) near the town of Narrabri in rural New South Wales. It was taken just before sunrise with Mercury, Venus, and the Moon all appearing close together in the sky behind the array. Mercury is the highest of the three bright celestial beacons. The ATCA consists of six radio telescopes, each one larger than a house. Together they form one of the highest resolution measurement devices in the world. This is truly an astounding sight as impressive planetary conjunctions occur every few years.
Credit: Graeme L. White & Glen Cozens (James Cook University)

Astronomy is an observational science. Apart from the use of space probes in the Solar System, it is not possible to carry out experiments in situ, and information must be gleaned from light signals collected by telescopes and analysed with instruments such as cameras and spectrometers, which spread out the light into its constituent wavelengths and allow a closer study.
The telescope was invented in the early 17th century by Dutch spectacle makers and used for astronomical research for the first time by the Italian Galileo Galilei in 1609. Galileo pioneered the scientific method by thoroughly documenting all the new astronomical bodies and phenomena he saw with the telescope: craters on the Moon, Jupiter’s moons, spots on the Sun.
Since Galileo, thousands of observatories have been built around the world, and, since the 1960s, also in space. There are many advantages to be gained by observing from space (see Chapter 3), but it is expensive to launch telescopes and, with the notable exception of the Hubble Space Telescope, it is not possible to repair and upgrade them once they are there. Consequently, good sites on the ground are very attractive places to build large and powerful telescopes. These can then be continuously upgraded as new technology becomes available. Ground-based telescopes, working at visible, infrared and radio wavelengths, are forefront devices that usually work in a such a way as to complement the expensive, and usually smaller, space telescopes.

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