Lexicon of some useful terms



Accretion disk
Flat circular structure, made of gas and dust, which appears around a gravitational centre of attraction.
This structure swirls around the center of attraction. The disk spirals all the more faster when the attraction is strong.
A particle which exhibits the same characteristics as an ordinary particle, apart from an opposite electrical charge. These antiparticles form antimatter. An encounter between matter and antimatter ends up in a complete annihilation of the two of them producing nothing but photons.
The smallest part of a pure material.
An atom is made of a nucleus, which makes up the largest part of its mass, composed of protons (electrically charged) and neutrons (neutral), around which orbit negatively charged electrons.
In a neutral atom, the number of electrons is the same as the number of protons. If these two numbers are not equal, the atom is electrically charged, and it's called an ion.
The number of protons establishes the nature of atom. For instance, 1 proton = hydrogen, 2 protons = helium... up to 92 = uranium, which is the heaviest natural nucleus.
Barnard objects
Gas and dust clouds, which are not lit up by stars, and which hide the stars which lie behind them.
Baryons are heavy particles, like the proton or the neutron, which belong to the family of the hadrons.
Black body
A body which is in thermal equilibrium with its own radiation.
electromagnetic spectrum of a black body
In this case, the curve : radiated intensity vs. wavelength of the radiation exhibits a typical shape, and the frequency of the maximum is only dependent on the equilibrium temperature.

(note : temperatures are expressed in degrees Kelvin. 0°C=273 K)


For a black body, the Wien's law gives the relationship between the peak wavelenght λ and the temperature
λ * T = 2897.8
And the Stefan's law gives the radiated power (by unit of surface) :
RT = 5,67*10-8 * T4


Bok globules


globule de Bok globule de Bok (infrarouge)

The Bok globule Barnard68, seen in optical wavelenghts (on the left), and in the infrared (on the right).
The cloud is transparent in the infrared, revealing the stars behind it.

Source : ESO

Chandra Telescope
Observing satellite for the X-ray part of the spectrum.
Launched in 1999, it is the most recent and sensitive telescope for X-rays.
Conservation of angular momentum
Angular momentum measures an object's tendency to continue to spin.
If the size of a system is shrinking, its spinning speed must increase, because angular momentum is conserved.
With this principle, a skater spins faster when she folds her arms back.
Cosmological redshift
Shifting toward longer wavelengths of the spectrum of an object, because of its escape velocity, due to the Doppler effect. Since the works of Hubble and the Big Bang theory, we know that this escape velocity is the result of the expansion of the universe, and is proportional to the distance of the object.
This redshift is usualy denoted z.. It corresponds with the relative shift of the wavelenght : -
By taking relativistic effects into account, we can obtain the complete formula of the redshift :-
where v is the escape velocity, and c the speed of light.
relation redshift-distance
Distance of an object depending of its redshift, in accordance with the value of the Hubble's constant.

The most recent measurements seems to agree with a value of about 65 km/s/Mpc.

We know that v is proportional to the distance with the Hubble formula, so we can obtain the distance as a function of the redshift : -

 The higher a body's redshift is, the further it is. Light travels at a finite speed, so when you look far away in space, you also look far away into the past.
If we look at an object located 10 billion light years away, we see it as it was 10 billion years in the past.

There exists another redshift, called gravitational redshift, which appears when the light source is inside a gravitational field.

Dark matter (or black matter)
Dark matter has escaped all detections until now. Its existence is however deduced from its gravitational influence on phenomena like the rotation of the spiral galaxies, or the evolution of the galactic superclusters.
The composition of this invisible matter is the assumption of many hypothesis : WIMP (Weakly Interacting Massive Particles), MACHOs (MAssive Compact Halo Objects), primordal black holes, heavy neutrinos and other ideas.
Degeneracy pressure
FForce exerted by atomic particles when you try to confine them. Following Pauli's exclusion principle, two particles are not allowed to be in the same place with the same energy.
If you try to force them, they will develop the equivalent of a pressure in order to resist.
It is this force which prevents a white dwarf or a neutron star collapsing under gravity.
Degenerated matter
degeneracy of matter with the pressure
Very high density matter, the structure of which is modified by the intense gravity.
Particles, which must "squeeze", create the degeneracy pressure.


Density wave
Region in space where the density is higher than outside it.
Areas in front of the arms in spiral galaxies are typical of these waves. The moving of the arm creates a shock wave, able to compress the interstellar medium.
Doppler effect
Shifting of the electromagnetic spectrum due to the motion of the light source.
If the source is moving away from the observer, the light is shifted to longer wavelengths (redshift). If it moves toward, you get a blueshift.
Electromagnetic sprectrum
Totality of electromagnetic radiation, from radio waves, to X-rays and gamma rays.
Visible light is only a very small part of this spectrum.


Electromagnetic sprectrum

Energy is directly correlated with temperature. The hotter a body is, the more energetic the radiated "light" becomes.
The entropy of a physical system is a measurement of its disorder. It is the number of microscopic configurations that let its macroscopic state unchanged.
Escape velocity
Minimal velocity needed to escape a massive body's gravity.
On the Earth's surface, this velocity is about 11 km/second.
European Southern Observatory : european observatory build in Chile. It consists of four telescopes, each one having an 8.2m mirror.
These telescopes are called the VLT : Very Large Telescope.
The four telescopes are fully operational today, and will be combined in order to obtain pictures by means of interferometry.
The first trials of this new technique were conducted in October 2001. The results are available on the ESO pages.
Elementary particle, messenger of the strong nuclear interaction. Gluons have no mass, but they carry a "color" charge (which is only a name, without any relationship with usual colors).
HII areas
Cloud mainly made up of hydrogen, where most of the atoms are fully ionized by the ultra-violet radiations of stars of class O and B.
These clouds form emission nebula, recognizable with their red color.
A particle which is under the influence of the strong nuclear interaction. It is made of quarks. Nucleons made of three quarks are hadrons; the mesons are built with two quarks.
Heisenberg's uncertainty principle
Another fundamental principle of quantum mechanics, which postulates that you cannot know with an infinite precision two independant parameters of an object at the same time.
For example, you cannot precisely measure simultaneously the speed and the position of an object, or the energy and the duration of an electromagnetic impulse.
In particular, if an object is at rest, its position must have uncertainty.
The Hubble Space Telescope, orbiting around Earth. Since it is above the atmosphere, it can obtain a much better resolution than a terrestrial telescope, since images are not perturbed by atmospheric turbulence.
Hydrostatic equilibrium
Hydrostatic equilibrium is the balance between the thermal pressure from the heat source pushing outwards and gravity trying to make the star collapse to the very center.
Thermal pressure arises from the nuclear fusion, which is the engine of the star.
Interferometry is a method of measurement which uses the interferences between two or more waves coming from the same source.
To get, not a picture but interference fringes, we must use two or more receivers which became equivalent to a single receiver whom diameter is egal to their distance.
In order to be efficient, the received waves must be coherent. Practically, if this is quite feasible for radio waves, it is very difficult to use with visible light.
Kepler's laws
Laws for the orbits of the planets, expressed by Kepler, and explained by Newton. There are three laws :
A particle which does not feel the strong nuclear interaction, in contrast with hadrons. Electrons, muons, taus and their associated neutrinos are leptons.
Length units in astronomy
Here are the main length units used in astronomy :
Magnetic monopole
Theoretical particle with only one magnetic pole. A magnet always has two poles.
Grand Unified Theory predicts the existence of monopoles. They should have a heavy mass.
Visual magnitude
The apparent brightness of a star observed from the Earth.
Absolute magnitude
If a star was 32.6 light years away from us, then its apparent (or visual) magnitude would be equal to its absolute magnitude.
This figure is useful in order to compare stars together, regardless of their distance.
Molecular cloud
Wide area, filled with molecular hydrogen. Its size can be as great as tens, even hundreds of light years. Their masses can reach millions of solar masses.
They are the birthplaces of stars.
Elementary particle, belonging to the lepton class, with a very little mass (supposed to be nul for a long time).
Neutrinos are generated in the nuclear reactions inside the stars, and in a great amount during the gravitational collapse of a supernova.
Because of their very weak interaction with the matter, they are difficult to observe.
Nuclear fusion
Merging process of the nucleii of two atoms, in order to obtain a heavier third one.
This reaction emits a large amount of energy. The simplest one is the fusion of two hydrogen nucleii in order to obtain a deuterium nucleus (one proton+one neutron). Fusion of a deuterium nucleus and another hydrogen one will give a helium nucleus.
This is the process in use within an H bomb, in a much smaller way than inside a star.
Particle inside an atomic nucleus : there is the proton, electrically charged, and the neutron, without charge.
Pauli's exclusion principle
Essential principle of quantum mechanics, which postulates that two particles (of fermion kind) can't be in the same quantum state.
That means that two electrons, or two neutrons, can't be at the same place with the same energy. If an external force, such as gravity, forces them to be at the same place, they can not have the same energy, i.e. the same velocity.
If a lot of fermions are compelled to be in the same area, they will have to move at different speeds, faster and faster : but relativity limits these speeds to the speed of light, preventing the volume to decrease. This appears as a force which resists any collapse : the degeneracy pressure.
Elementary particle, messenger of the electromagnetic interaction. Photons have no mass and don't carry any charge.
They carry the energy of electromagnetic waves, including the light.
State of the matter where the different elements of atoms - nuclei and electrons - are dissociated.
Population I and II stars
Classification of stars, made by a German astronomer Walter Baade, around 1944.
Type I are young stars, rich in metal, which are found in the arms of spiral galaxies.
Type II are old stars, with a low metal content. These stars are mainly found in elliptical galaxies, globular clusters, and outside the spiral arms.
The very initial stars, which were created at the beginning of the universe are called Type III stars.
Quantum mechanics
The field of physics which deals with atomic particles, and their interactions.
Elementary particle, building neutrons and protons among others. A quark has an electrical charge multiple of 1/3.
The quarks, which exist in 6 varieties, carry a so called "color" charge and are therefore subject to the strong nuclear interaction.
Quarks can usually not exist as isolated particles, they always clump together by 3 (phenomenon of quark confinement).
Quark deconfinement
Quarks are carrying a particular charge called 'color'. Theory predict that such particles can't stand alone, they are necessarily grouped to obtain a neutral color. This phenomenon is called confinement.
So deconfinement is the possibility for such a particle to stand alone. Of course, this can only happen in some particular conditions, very far from usual conditions.
Radiation pressure
Pressure created by photons on a particle of matter.
The higher the energy of the photon, the higher the pressure.
Roche lobe
lobes de roche
The volume around a star in a binary system in which, if you were to release a particle, it would fall back onto the surface of that star.
The point at which the Roche lobes of the two stars touch is called the inner Lagrangian or L1 point.


Scalar field
Quantum mechanics links every particle with a field. This field covers the whole of space. At every point of space there are one or more numbers which represent this field. For instance, you can associate a number with each point in a room which represents the temperature at that point.
These fields are called "scalar" when you can characterize them with only one number at each point.
Schwarzschild radius
Radius inside which you must compress a mass in order to obtain a black hole. This radius increases with the mass.

This is the size of the horizon of a black hole, beyond which nothing can escape, not even light.

If G is the gravitational constant, c the speed of light, hence the Schwarzschild radius of a black hole whose mass is M
is expressed as -


Stellar evolution
The final evolution of a star is mainly dependent upon its initial mass.
This is a short summary of this evolution.


compared stellar evolutions

Stellar wind
The ejection of gas off the surface of a star. The ejected particles move along the magnetic lines of the star.
The stellar wind from the Sun is responsible for the Northern Lights (among other things).
Synchrotron radiation
Electromagnetic radiation given off when very high energy charged particles encounter magnetic fields.


synchrotron radiation

Triple alpha process
Nuclear fusion reaction which occurs in the helium core of a star.
Two helium nuclei merge together to give an unstable beryllium nucleus. If a third helium nucleus merges with this one, before it splits, there appears a stable carbon nucleus.
The name 'alpha' is given because the helium nuclei are the 'alpha ray' particles, observed in the radium decay.
Types of supernovae
Supernovae are divided into two basic physical types, named Ia et II.
The supernovae of type Ia are the results of the explosion of a white dwarf inside a binary system, and the types II occur at the end of the life of a supergiant star.
Type Ia do not show hydrogen lines in their spectrum, because the white dwarf stars have no atmosphere, but instead silicon.
There are also two other types of supernova :