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A star ignites when its formative gas ‘heavy hydrogen’ — has a neutron
cloud collapses, due to its own grav- and a proton in its nucleus, unlike
itational pressure or some outside normal hydrogen, which only has a
force (such as the shockwave of a single proton. The fusion of deute-
supernova blast), to the point where rium is one of the very simplest of
the mass of the core reaches the fusion reactions and requires only a
temperature and pressure needed fraction of the temperature needed
for hydrogen fusion. But what hap- for hydrogen fusion. Translated into
pens if such a cloud core just isn’t mass and gravitational pressure,
dense or massive enough to form a this temperature requirement (give
fully-fledged star? What if the result- or take a couple of tens of thousands
ing ball of gas, called a ‘protostar’, of kelvin, depending on the sur-
lacks the ‘final punch’, so to speak, rounding dust, debris and magnetic
to begin the process of fusing hydro- fields) yields the ‘thirteen Jupiter’
gen? threshold. These protostars are hot
In 1975, the and dense enough to fuse deuterium
For a protostar to begin hydrogen with a proton into helium-3. It’s also
fusion it needs a core temperature astronomer Jill the reason why explorers find some
of over one hundred million kelvin. Tarter came up very big gas giants to be rich in he-
That’s a very high bar which requires lium. The fusion process may have
immensely strong gravitation. The with the name stopped long ago, but the helium is
generally accepted lower mass limit ‘brown dwarf’. retained.
is eight percent of the mass of Sol.
Looking for a suitable classification
Below this, hydrogen fusion just for these substellar objects, 20th
isn’t possible as the protostar’s core century astronomers considered a
temperature and pressure are too number of terms. ‘Dwarf’ seemed ap-
low. Astrophysicists have adopted a propriate, as the cosmos had already
different scale for such protostars: seen red dwarfs and white dwarfs.
one based on the mass of Jupiter. They weren’t white, though — rather,
Protostars with between thirteen they were very, very dim. The terms
and eighty Jupiter masses become ‘red’ and ‘black’, on the other hand,
brown dwarfs. were already occupied; so in 1975,
the astronomer Jill Tarter came up
Thirteen Jupiter masses is an im- with the name ‘brown dwarf’. In the
portant threshold for astronomers, absence of better alternatives, this
because above that point, another name has stuck, and it is certainly
interesting thermonuclear process in-keeping with astronomers’ knack
begins: the fusing of deuterium. for dramatic terms.
Deuterium — sometimes known as
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