What is 'ordinary matter'?
Look around you. Look into the sky -- at night. Everything you see,
the sun, the moon, distant galaxies, your body, your shoes, are all
made of what we call ordinary atomic matter. It has another name which
will be explained below: baryonic matter.
The Atomic Model
All atomic matter is made from the same basic building blocks: A central
nucleus surrounded by a cloud of electrons.
In a little more detail, every atom consists of:
- a nucleus, which:
- has some number 'Z' protons -- positively charged, these determine the atom's chemical identity
- has some number 'N' neutrons -- neutral but participate in the nuclear binding force. These determine the isotope.
- The sum of N+Z is called A, the atomic mass number.
- an electron cloud -- negatively charged. A neutral atom has the same
number 'Z' of electrons as protons. If there are less or more electrons
attached, then we call the atom an 'ion'.
This model explains everything out there (except for dark matter, but hold your horses -- we'll get to that later). Here's how:
Galaxies, ignoring dark matter for the moment, are made of stars and dust.
How are stars and dust
made from the atomic building blocks? Well, stars are just big, hot
collections of different combinations of different atoms. In particular, they
start out as mostly hydrogen and helium, the simplest possible atoms.
Hydrogen is defined as a nucleus with Z=1 proton.
Most hydrogen has N=0, meaning no neutrons. So, hydrogen, the simplest element,
is just a proton. In fact, we often write equations involving hydrogen
in the sun using the symbol "p" meaning "proton" for hydrogen. If it has
a neutron bound to the proton in the nucleus,
we call it deuterium and its symbol is 2H. It can sometimes
have even N=2 neutrons bound to the proton. That is called tritium and its
symbol is 3H. A helium nucleus, He, has two Z=2 protons
and either N=1 or N=2 neutrons, with the symbols 3He and
Inside the stars, the nuclei of hydrogen and helium and whatever else might be
around will fuse together to form bigger, more complex nuclei. In other
words, if you take two nuclei with a nucleus Z=2 protons (helium), you can
fuse them together to make a new nucleus with Z=4 protons (which happens
to be called beryllium). And so on...
of fusing together nuclei with only a few protons and neutrons into nuclei
with many protons and neutrons also happens to be the energy source which
stars: thermonuclear fusion. By this stellar alchemy
all of the other atoms of the universe were formed!
Everything from the carbon (Z=6) in your body to the iron (Z=26) in the
Earth to the sulfur (Z=16) in the atmosphere of Venus was formed somehow
by a star. Even uranium with a whopping Z=92 protons and (usually) N=146
neutrons was made by a star (probably in a supernova explosion as opposed
to regular stellar fusion, but that is stellar alchemy nonetheless).
The point is that in the `old days', most of the universe was made of
these atomic building blocks. The universe was just made of either
empty space or some heap of atoms (which make the heap of clothes on your
floor). Most of the atoms which constitute the heaps of stuff in the
universe were built up from the lightest atoms through thermonuclear fusion
inside of stars or during supernova explosions. And that explained all the
galaxies and dust and stars and planets: the universe.
Another important point here is that the negative electrons balance the positive
charge of the protons in the nucleus, but they are very much lighter than
protons and neutrons. The proton has about the same mass as a neutron,
but the electron's mass is about 1/1837 of the proton mass.
Therefore, when considering the cosmological mass scales, we are concerned
only with the contributions of nuclei of atoms, since their electrons are
so light. In other words, if you wanted to know how much all of the
atomic matter in the universe weighed, you could leave out the electrons
and it would weigh
about the same. That's why we call A=Z+N the atomic mass number, since the
mass of each atom is by far just the weight of all of the protons and
Nucleons, baryonic matter
Since protons and neutrons are generally found in the nuclei of
atoms, we call them nucleons. A nucleon means either a proton
or neutron. Another name for a proton or neutron is baryon.
There are other baryons besides the proton and neutron -- it is a more
general class of particle. This is explained later.
The important point is that cosmologists refer to ordinary atomic
matter as "baryonic matter" since most of the mass of every atom or ion is
in the nucleus and the nucleons (protons and neutrons) are technically
classified as "baryons". So, when you hear or read about "baryonic
matter" in the universe, it is really just ordinary atomic matter being discussed.