Re: Space matter, energy, anti-matter

A full list of the contents of space is the subject of astronomy and is still far from complete. Let's begin with matter. We only know 10% of the matter that composes the Universe. The other 90% has not been observed by currently used methods. It is assumed to be there because it produces gravitational attraction on other bodies. This 90% of matter is called "dark matter" because it apparently does not emit light. Some fraction of it could be composed of regular matter like asteroids, planets, brown dwarfs (stars to small to produce nuclear reactions at their centers) or elementary particles like neutrinos. Neutrinos are a likely candidate because there are a lot of them in the Universe and we don't know exactly what their mass is. However, a substantial amount of dark matter must be composed of something we don't yet know, such as particles that have been predicted by theories, but have not been detected in the laboratory yet. The fact is that we don't really know what this 90% of matter is made up of.

The matter in the space that is among the stars in a galaxy is called interstellar matter. The matter in the space among galaxies is called intergalactic matter. The density and composition of those matters varies from place to place. Near the Sun the density of interstellar matter is less than 3×10^-24 grams per cubic cm or g/cm³. That's about 1.7 hydrogen atoms per cubic centimeter. To convert to cubic kilometers multiply those numbers by 10¹⁵. That gives a density of 3×10^-9g/km³ or 0.000000003 g/km³. To this interstellar matter you must add about 4×10^-9g/km³ in the form of stars and planets. To explain the motions of stars in our Galaxy, which are produced by the gravitational attraction of matter, we need 10×10^-9g/km³. Therefore we have not detected about 3×10^-9g/km³, which is assumed to be "dark matter" in our Galaxy. However measurements of the matter in the whole Galaxy suggest that the missing matter in our Galaxy may be much more than this.

The chemical composition of interstellar matter is also variable. Interstellar matter is composed of gas and dust. Most interstellar gas is composed of about 90% hydrogen atoms, and 10% of helium atoms. The rest of the elements have less than 0.03% of atoms, and because of this, it is best to list their abundances in parts per million, that is how many atoms of a given element there are for every million atoms. This is a short list of average chemical abundances in parts per million in several places of the Universe: the interstellar gas :

The Earth crust is mostly formed by silicates, which are molecules with one silicon and four oxygen atoms. The human body chemical composition can be explained by the fact that it is formed mostly by water, which is 66.6% hydrogen and 33.3% oxygen (remember two hydrogens and one oxygen atom form a water molecule), and organic compounds, which are mostly carbon and hydrogen atoms.

There is an interesting difference between the chemical composition of the Sun (and other stars) and the interstellar gas. Stars are formed by the condensation of interstellar matter. Stars also convert hydrogen and helium in heavier elements in their interiors; therefore their composition should be mostly similar to that of the interstellar gas. However you can see that most elements have lower abundances in the gas than in stars, but especially refractory elements (those that can exist in solid form in space) like carbon, magnesium, silicon, iron and others are severely depleted in the interstellar medium. The current assumption is that those elements have formed solid grains, also known as interstellar dust, that range in size from 50 angstroms to about 0.25 microns. It has been very difficult to make a consistent theory of dust in space that can explain all its observed properties as well as the observed depletions of chemical elements in the interstellar medium. Current theories suggest that most iron, magnesium and silicon in space are locked in dust grains, while 2/3 of carbon and 20% of oxygen are also in grains. It is possible that grains in space can be covered by ice mantles of molecules that can account for the missing oxygen, nitrogen and carbon in interstellar space. The cores of the dust grains are probably formed of silicates, metal oxides (MgO, SiO, FeO), graphite, crystalline silicon, or pure iron, but there is no definitive answer to this question. Some astronomers have suggested that soot (aggregates of carbon atoms) may be part of the interstellar dust. Of course all the elements that compose the interstellar gas and dust are identical here and in space. It's the molecules that they form what may be different.

The density of dust grains in space is very small. The most abundant grains are the small ones of some 50 angstroms in size. There may be about 10⁹ hydrogen atoms for every dust grain in space.

Vladimir Escalante Ramírez
Institute of Astronomy, UNAM