Above: a computer simulation of stellar flares. Download larger version.
This article reviews some of the energetic processes in the Cosmos. Energy can be defined as the ability to cause change.
Dynamic processes (i.e. those involving change) are energetic. Without energy there would be change and hence no time.
Your sensory systems are systems designed to detect certain types of energy. When you sit on a chair, it is the energetic
repulsion between the negatively charged electrons in the atoms of the chair and the similarly charged electrons in your
buttocks that stop you falling through the chair and allow you to feel its presence! These energetic repulsions give matter
perceivable solidity. Most of your senses, however, work at a distance. For example, your ears detect mass vibrations of air
molecules, called sound waves and your eyes detect certain wavelengths of electromagnetic waves.
When we observe the planets, stars and galaxies, we are detecting energy that these bodies radiated away into space. Of all
the forms that energy can take, radiation is the only one that can travel through a vacuum (and outer space is a near
vacuum). Cosmic phenomena can be viewed so far away because they are very energetic and emit tremendous amounts of
radiation. This need not be the ionising radiation that most people think of as 'radiation' (such as alpha, beta and gamma
rays as might be radiated by uranium metal) but could be visible electromagnetic (EM) radiation (light) such as the
predominantly yellow light radiated by the Sun, or frequencies of electromagnetic radiation that the unaided human eye
cannot detect, such as X-rays, ultraviolet, infrared, gamma, micro and radio waves. Particles of matter may also be radiated,
such as neutrinos, electrons and protons, or their antimatter equivalents and other more exotic particles.
Many objects in the universe are hot enough to radiate their own energy, in fact all objects will radiate some energy of their
own. Take a cold metal iron, one can see it because light reflects off it and enters the eyes, this is reflection and not
emission, but put on a pair of infrared (IR) goggles and you will see that it emits its own radiation as IR light (some define
'light' to be strictly that part of the electromagnetic spectrum that the unaided human eye can see, whilst others consider
other parts of the electromagnetic spectrum as 'light', especially those parts that certain other animals can see, such as IR
and ultraviolet (UV)).
Colder objects emit longer wavelengths of electromagnetic energy. Our cold metal iron emits a wavelength too long for us to
see. Heat the iron in a fire, however, and will begin to emit shorter wavelengths of light that we can see. It might at first begin
to emit red light. Heat the iron to higher temperatures, perhaps in a furnace, and it will begin to emit orange, then yellow light.
Heat it still further and it will emit mostly white light (a mixture of colours) and finally blue light and violet light, the shortest
wavelengths the human eye can detect. Even hotter objects will emit predominantly in the UV and hotter still in the X-ray
region (our metal iron might well melt before that)! In fact, hot iron glows red at 1000K, yellow at 1200K and white at 1400K
and melts at 1811K.
What causes these thermal emissions?
A changing spatial pattern of electric charges can absorb or emit EM radiation such as light. Light consists of waves of
electric energy and waves of magnetic energy, with the two at right angles to one-another. It is, therefore, not surprising that
when light interacts with matter, it interacts with the electric charges in matter. Since light is an oscillating wave, it interacts
with oscillating electric charges. Such oscillating electron charges may be present in solids, liquids or gases. Such a gas can
absorb and emit certain wavelengths of light (depending on how rapidly its electric charges oscillate). Accelerating electric
charges generate magnetic fields (and similarly a changing magnetic field generates an electric field) and so with moving
electric and magnetic fields present, it is no surprise that electromagnetic energy can be emitted. The wavelength of the EM
radiation emitted depends on the thermal kinetic energy of the atoms or molecules. Kinetic energy is the energy of
movement, and when particles are hot they move about faster and so have more kinetic energy. In thermal emission, some of
this kinetic energy is converted into EM energy in the form of EM radiation. It also depends on the incident light hitting the
object and which wavelengths of that light get absorbed, scattered or transmitted through the material. One way to
circumvent such complexities is to consider thermal black bodies. A black body is an object in thermal equilibrium with its
surroundings (meaning it emits as much energy as it absorbs) which absorbs all the radiation incident upon it and emits this
energy perfectly at the same rate. Such a body need not be black! Rather its colour (by which I mean the main wavelength
emitted) depends on its temperature and is given by a mathematical equation, called Planck's Radiation Law which gives
the amount of radiation emitted at each wavelength. This predicts, for example, that a blackbody at 2000K will emit
predominantly in the infrared, one at 6000K will emit mostly yellow light. (Note the difference with our iron which emitted yellow
light at a much lower temperature of 1200 K, this is because iron metal is not a black body!).
Black bodies are idealisations that do not occur exactly in Nature. However, stars are good approximations to black bodies
and so by observing the colours of stars we can estimate the star's surface temperature reasonably accurately. (This is not
the only way to estimate its temperature, for example, the thermal broadening of its spectral lines can also give us its
In hot objects, electrons within the atoms or molecules can get excited to high energy states by absorbing energy (perhaps
from a collision with another molecule, or directly by absorbing EM energy). When an electron in an atom absorbs energy in
this way, then it moves further from the nucleus into a higher energy level. This might happen in outer space when radiation
from a nearby star strikes a cloud of gas (nebula) irradiating and heating it. Sooner or later these excited electrons de-excite
and give up some or all of their excess energy and drop down into a lower energy level or all the way to the lowest energy
level they can occupy (the ground state). In doing so they emit a photon of energy. The larger the drop back down in energy,
the higher the energy of the photon and the higher the frequency (smaller the wavelength) of the emitted light. Since the
distance of the available energy levels from the atomic nucleus (and hence their energy) differs according to the type of
atom, different elements emit different spectra of light. Each element produces a characteristic emission spectrum made-up
of characteristic wavelengths of light. This allows us to determine the types and abundances of the elements present in the
Ions also have different spectra. For example, cations, those that have a net positive charge have not only missing one or
more electrons, but the excess positive charge, which resides in the protons of the nucleus, pulls more tightly on the
remaining electrons, pulling the energy levels closer to the nucleus.
The most abundant element in the Universe is hydrogen, the simplest element with only one electron and one proton in its
atom. When we look at interstellar space we see gas clouds of hydrogen with two distinct emission spectra - these are 1) HI
regions and 2) HII regions.
Hydrogen-1 (HI) is ordinary atomic neutral hydrogen. HI regions are neutral hydrogen gas clouds that are typically around 5
pc across, 50 solar masses and at temperatures of 70K ( a cool -193 oC). Between the clouds are more tenuous regions of
neutral hydrogen at about 800K (537 oC). These cold clouds emit radio waves with a wavelength of 21 cm (corresponding to
a frequency of 1.4204 GHz) though Doppler shifts may change this characteristic wavelength. The Doppler shift means that if
the cloud is moving towards us, then its wavelength is shortened by a factor depending on how fast it closes on us; similarly if
it is receding, then its wavelength is lengthened. This allows us to determine the speed and direction of motion of the cloud.
This particular emission is due to a characteristic jump-down of the electron in hydrogen as it de-excites and is a transition
that is forbidden (a forbidden transition giving rise to a forbidden spectral line). It is forbidden in the sense that it is not
normally observed in terrestrial conditions, because the transition is so slow, that the atom is much more likely to lose energy
differently by colliding with another atom. The fact that such a transition dominates the spectra of these nebulae tells us that
these nebulae are tenuous and thin with a very low density of hydrogen atoms that rarely bump into one-another as they
move about due to their thermal kinetic energy.
Hydrogen-2 is ionised hydrogen, H+, or a plasma of protons and electrons and in these clouds most of the hydrogen is
ionised. These regions are often associated with star-birth and stellar nurseries. In these conditions the hydrogen becomes
photoionised (ionised by photons striking the atoms) due to UV, cosmic-rays, X-rays or shock waves striking the clouds. All
these energies can be produced in a nebula when a star forms - newly born stars are extremely energetic!
Young HII Clouds are typically more-or-less spherical with a sharp boundary and are found around hot O or B stars,
massive and relatively short-lived stars. UV light emitted by these hot stars ionises the hydrogen. The ionisation front
expands outwards from the star at about 10 km/s until the Stromgren radius is reached. The Stromgren radius is the point
at which the radiation has weakened sufficiently during its transit through the gas that the rate of ionisation equals the rate of
recombination of electrons and protons to form hydrogen (electrons and protons have unlike electric charges and so attract
one-another to form H atoms if allowed to do so). The Stromgren radius is usually about 200 km and is approximately
constant, allowing these clouds to be used to estimate the distances to other galaxies.
Bremsstrahlung Radiation (thermal free-free transitions)
When an electron is free of the atomic nucleus it still has a definite energy level, but rather than having a discrete number to
chose from it, like when it is bound in an atom, it has a very large number to chose from (potentially infinite but not likely in
reality) called a continuum of energy levels. Electrons can still change their energy, effectively jumping up or down the
continuum. When an electron jumps down from one level in the continuum to another it emits a photon in a so-called
free-free transition. This occurs, for example, when a moving electron passes close to a proton and loses some of its energy
due to the attractive pull of the proton. The EM radiation emitted has a characteristic spectrum and is called Bremsstrahlung
radiation (lit. 'breaking' radiation). If the electron loses enough energy then it will be trapped by the proton and recombine
with it to form an atom. This atom will likely be in an excited state and the electron may further de-excite, jumping down to
lower energy levels within the atom (until it reaches the ground state or becomes excited again). This the spectrum produced
is characterised by a continuum of frequencies from the many possible free-free transitions plus recombination emission
lines and produces a spectrum in the radio, IR and optical parts of the spectrum. This is a form of thermal emission, due to
the thermal motions of electrons and nuclei in a hot plasma. The other classic type of thermal emission is blackbody radiation.
These nebulae, then, are mostly ionised hydrogen plasma characterised by free-free and recombination/de-excitation
emissions. They include the HII regions, for example the Orion Nebula, where new giant stars have been formed and have a
temperature of about 10 000 K and a spectrum characterised by forbidden line emissions mostly of oxygen (O) and nitrogen
(N). They also include planetary nebulae in which the gas has been UV ionised and supernova remnants. These clouds are
often red and green in colour. The red colour is due to red H-alpha emission and a red NII (N+) forbidden transition. The
green is due predominantly to a forbidden transition in OIII (O++, oxygen atoms that have lost 2 electrons, due in this case
to UV photoionisation).
The Orion Nebula
Is an emission nebula (M42) is one of the brightest emission nebulae as seen from Earth and is 400 pc distant in the centre
of Orion's sword (and is just visible to the naked eye) and is a HII region associated with the Trapezium cluster - 4 hot young
stars that photoionise the nebula which emits both radio and optical radiation and X-rays. This is a region of active star birth.
Synchrotron Radiation (Magnetobremsstrahlung)
Electrons moving in a magnetic field emit EM radiation (see particle paths) as they accelerate (change direction) in the
magnetic field. Very high energy electrons moving at relativistic speeds (speeds that are an appreciable fraction of the
speed of light). The radiation emitted is nonthermal (it has a different spectrum to thermal black body and thermal
bremsstrahlung radiation) and polarised. Examples of such emission include radio emission from extragalactic radio sources,
supernova remnants including pulsars. Other relativistic effects may be apparent, such as relativistic beaming in which the
emitted radiation is beamed forwards in the direction of electron motion. The radiation may also be reabsorbed before it can
escape, especially at low frequencies, in a process called synchrotron self-absorption (which causes a characteristic
low-frequency dip or turnover in the spectrum).
Seyfert Galaxies and Quasars
Active galaxies, like Seyferts and Quasars, have bright active galactic nuclei at their cores which are thought to be
supermassive black holes. Quasars are about 100-times more powerful than Seyferts and may tend to represent an earlier
stage in galaxy formation, although the nucleus may undergo periodic episodes of increased activity when fresh matter falls
into it. About 10% of giant spiral galaxies are Seyferts (giant spiral galaxies are a class of galaxy that includes the Milky Way
Galaxy). These regions emit massive amounts of non-thermal radiation, such as radio-frequency synchrotron radiation as
hot plasma is accelerated by the intense magnetic fields surrounding the central object. The nucleus is surrounded by hot
ionised gas (such as the BLR and NLR clouds - see AGN) that emit optical (visible) light with broad emission lines indicative
of fast-moving clouds. These probably represent matter spiralling into the central black hole at faster and faster velocities
(BLR) as well as clouds of gas ionised both by radiation emitted from the central source and by accelerated plasma
impacting the gas clouds, creating ionising shock waves.
This is the scattering of photons by charged particles. The photon interacts with a charged particle (such as an electron)
and gives-up some of its energy to the charged particle and so the scattered photons have less energy and hence a lower
frequency than the incident photons and the photons are said to be Compton scattered. (Since: energy of a photon (E) =
Planck's constant (h) x frequency (f), E = hf, and the speed of light in a vacuum (c) = frequency (f) x wavelength (L), c = fL
which gives f = c/L and L = c/f).
In inverse Compton scattering, the charged particle gives some of its energy to the photon and the scattered photons
gain energy and so have a higher frequency (and lower wavelength) and are Compton upscattered. In the synchrotron
self-Compton (SSC) mechanism photons emitted as synchrotron radiation are inverse Compton scattered by the very
electrons that emit them. The emission spectrum predicted to be generated by such SSC processes gives a good match to
the spectra of active galactic nuclei.
Left: Synchrotron emission. A high
speed electron (red) traces a helical path
around a magnetic flux line (cyan) as it
accelerates in the magnetic field.
Acceleration means a change in speed
and/or direction, in this case it is the
direction that is changing as the electron
winds around the field line. This
acceleration causes the electron to emit
Seen end-on the electron traces a circular
path making v = eB/2(Pi)m revolutions per
second (where e is the magnitude of the
electron charge, B is the magnetic flux
density, m is the electron mass and Pi is
approximately 3.14. This frequency is
called the gyrofrequency and matches
the frequency of the EM radiation emitted,
which is known as cyclotron radiation. If
the electron is moving with relativistic
speeds then Einstein's theory of relativity
must be applied and other effects occur,
such as relativistic beaming in which the
photons are beamed forwards in the
direction of electron motion. This latter EM
radiation is called synchrotron radiation.