is regarded as relatively pristine relics of the primor-
dial solid matter of the early Solar System, and from
their chemistry we can estimate its overall composition
(except, of course, for the most volatile elements and
compounds, which may have been expelled by high sur-
face temperatures generated during descent through the
by astronomical observation. From such measure-
ments, astronomers are able today to estimate, within
quite narrow limits, the overall density of visible
baryonic matter in our galaxy and in the universe as
a whole, at least those parts whose emissions are
The mass of baryonic matter in our galaxy calculated
in this way, however, contrasts starkly with estimates
of overall galactic mass that we get from gravity.
How can galactic mass be estimated from gravity?
Planets in our Solar System orbit the Sun at velocities
inversely proportional to the square root of their dis-
tance from the Sun: those close to the Sun orbit more
quickly (and have shorter years) than those further
out. Planets close to the Sun experience stronger grav-
ity, so must have higher orbital velocities (greater 'cen-
trifugal force') to resist this stronger attraction and
remain in orbit. This regularity in orbital velocity is a
consequence of nearly all of the Solar System's mass
being concentrated at its centre, in the Sun itself.
If, on the other hand, we measure the analogous
velocities of our Sun and other stars as they orbit
around the centre of the Milky Way (our galaxy), we
Meteorites other than chondrites are products of the
segregation of metal from silicate (forming 'irons' and
achondrites 6 respectively), and are described as differ-
entiated meteorites (Box 11.1). It is generally assumed
that this segregation is a consequence of incorporation
into small planetary bodies, perhaps a few hundred
kilometres across (Box 3.5), in which high internal tem-
peratures facilitated gravitational segregation of the
two phases, as indeed happened with the separation of
the metallic core of our own planet.
Differentiated meteorites - the result of such bodies
being broken up by later collisions - are poor repre-
sentatives of primordial matter, but shed light on the
internal development of the rocky planets. A few dif-
ferentiated meteorites have chemical signatures indic-
ating a Martian (e.g. the Nahkla achondrite illustrated
in Plate 6) or a lunar origin.
(a) stellar velocities, unlike orbiting planets, vary little
between stars closer to, and further from, the cen-
tre of the galaxy; and
(b) stars move much faster than the mass of visible
matter in the galaxy would lead us to expect.
Current estimates lead us to believe that there is almost
100 times more dark matter (in the universe) than visible
matter . (Ferreira, 2006)
From these discrepancies, astrophysicists conclude
that the galaxy must contain much more mass than we can
directly see . To account for (a) above, they envisage a
dispersed halo of invisible mass, extending to the outer
reaches of the Milky Way, whose gravity exerts the
dominant control on the motions of the visible stars in
the galaxy. Astrophysicists refer to this invisible matter
as dark matter , and believe that it far outweighs the
visible matter in the Universe.
What might this invisible 'dark matter' consist of?
There are reasons to believe that perhaps 5-10% of
it consists of baryonic matter (i.e. familiar chemical
elements) that just happens to remain dark - 'ordinary
stuff that doesn't shine' as Ferreira (2006) puts it. It
happens that celestial bodies with masses less than
about 8% of the solar mass (~80 Jupiter masses) are
unable to ignite the nuclear fusion reactions that power
So far in this chapter, we have assumed that all matter
in the universe consists of atoms of the chemical
elements that are familiar to us on Earth, that is to say,
consisting ultimately of protons, neutrons and elec-
trons. Protons and neutrons in the nucleus, making
up nearly all of the atom's mass, are known collec-
tively as baryons , 7 so matter of this familiar atomic
kind is commonly referred to as baryonic matter . It
reveals its existence across the universe by emitting
or absorbing recognizable atomic spectra (Chapter 6),
and thus its abundance can be estimated from Earth
Achondrites contain negligible metallic iron.
From the Greek barus meaning heavy.