A molecule is a discrete unit of atoms joined together by chemical bonds. If the atoms are of the same type then
an amount of that element will be formed. For example, hydrogen consists of molecules, each of which consists
of two hydrogen atoms joined together by strong chemical bonds. Thus, the symbol for a single hydrogen atom
is H, but that for a hydrogen molecule is H2. In hydrogen gas, almost all the atoms are paired up in this way,
since the H2 molecules are chemically more stable than H atoms. Most substances, however, contain atoms of
different elemental types joined together to form
compounds. For example, steel is an alloy containing atoms of
iron and atoms of carbon; most rocks contain atoms of silicon and oxygen and one or more metals joined
together. Water consists of molecules of two hydrogen atoms (H) joined to one oxygen atom (O), giving the
molecules the formula H2O.


Manoeuverability rating; how manoeuverable a spaceship is. The MR tells us how far a spaceship can turn in a
ten minute period, in units of 60 degrees. For example a starship with an MR of 4 can turn by up to 240
degrees in ten minutes.


A subatomic particle of neutral (i.e. no) electric charge. Neutrons are a constituent of atomic nuclei. A neutron
is made up of two down quarks and one up quark (ddu).

Nucleus (plural nuclei)

This has several meanings in science, but in the current context it often refers to the centre of the atom where
most of the atom's mass resides in the neutrons and protons. The nuclear radius is less than 0.005 %
(1/20,000th) of the radius of the atom, or about 0.000000000001 mm. Since this tiny volume contains most of
the atom's mass, the density of the nucleus is immense. One cubic centimetre of nuclear material  would have a
mass of around 200 000 000 tonnes. The world's strongest man could only deadlift about 0.000002 cubic
millimetres of nuclear matter, which is about the size of a single microscopic human cell. We also talk about
stellar nuclei - the nuclei of stars refer to the central core region of the star.


A photon is a particle or quantum of light. A photon is not a particle in the ordinary sense - an electron, proton
or neutron has a measurable mass, but photons only have mass because they are moving, if one were to stop
moving then it would have no mass (we say that the photon has
zero rest mass).


A subatomic particle of positive electric charge and a constituent of atomic nuclei. The number of protons in the
nucleus determines the element that the atom belongs to. A proton is made up of two up quarks and one down
quark (uud).

Quantum (plural quanta)

The smallest divisible unit of something - the smallest building block of a thing. According to quantum physics,
'things' like energy exist as a multiple of very small units (rather like matter is made up of atoms) each unit
being called a quantum. Loosely speaking the word quantum is sometimes used in place of particle, for
example a photon is a quantum of light, though a particle may carry several quanta of energy. Atoms may be
the smallest building block of an element, such as the iron atoms in iron, but atoms are composed of even
smaller units.

Spectral Class

Stars are classified into groups or spectral classes, according to the characteristics of their stellar spectra. This
classification is based primarily on absorption lines and absorption bands produced by the different atoms and
molecules in the star's atmosphere. This gives us information about a star's atmospheric temperature, as does
the colour of the star. The coolest stars glow red, the hottest are blue. Going from hottest to coolest stars are
classified as types O, B, A, F, G, K and M. Type O stars are the hottest stars and are blue. Type B stars are
less hot, but are also blue. Type A stars are blue-white stars. Type F are white stars, type G are yellow, type K
are orange and type M are the coolest red stars. The Sun (Sol) is a type G star.

Stellar Spectrum

A display of the light coming from a star, which shows the intensity of the electromagnetic radiation at each


When an object disappears and reappears somewhere else, more or less instantly, we say it has teleported
from one place to the other. This may sound strange, but on the scale of the very tiny, such as atoms and
electrons, teleportation happens frequently. An atom can be teleported from one place to another, as can a
photon. Indeed, these particles can appear in several places at once! The subatomic world is, to coin a phrase,
'simply different' to the large-scale world that we know so well. However, since we are made up of atoms, then it
is possible in principle to build a device that teleports people.

TW (terrawatt, as a measure of laser power)

Terrawatt; one terrawatt = 1000 000 000 000 watts. A watt is a unit of power. Power is the speed at which a
machine can do work. For example, a one killowatt electric heater (one kilowatt, kW, = 1000 W) is quite a
powerful heater, but a 2 kW heater is twice as powerful and will heat a room quicker. Laser power is often
measured in TW, for very powerful military lasers. For example, the Nova laser at the
Livermore labs on Earth,
isa 100 TW laser. This is an enormously powerful laser! The trouble is that it is rather large and it gets so hot
that it can only fire six times a day, and it can only fire for very brief periods of time. Military lasers can drill
through 2 cm of steel in 2-6 seconds. There are two ways for a laser to do more damage: 1, to increase the
wattage; 2, to increase the time that the target is exposed to the laser beam. The latter has problems with laser
over-heating and vapour from the target gets in the way of the beam. Using a pulsed laser, which fires in
bursts, gives time for the vapour to dissipate. It also requires precision targeting to keep the laser focused on
the same spot of a moving target for several seconds. Nevertheless, as lasers get more powerful and so can be
miniaturised, army jeeps on Earth should be able to carry lasers of real destructive power within a few years.
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