Page 26 - Ultimate Visual Dictionary (DK)

THE UNIVERSE

Small stars
STRUCTURE OF A
MAIN SEQUENCE STAR
SMALL STARS HAVE A MASS of up to about one and a half Core containing hydrogen
times that of the Sun. They begin to form when a region of fusing to form helium
higher density in a nebula condenses into a huge globule of Radiative
gas and dust that contracts under its own gravity. Within zone
a globule, regions of condensing matter heat up and
Convective
begin to glow, forming protostars. If a protostar
zone
contains enough matter, the central temperature
reaches about 27 million °F (8 million °C). At this
temperature, nuclear reactions in which hydrogen
fuses to form helium can start. This process releases
REGION OF energy, which prevents the star from contracting
STAR FORMATION
more and also causes it to shine; it is now a main
IN ORION
sequence star. A star of about one solar mass remains
on the main sequence for about 10 billion years, until much of the hydrogen in Surface temperature
the star’s core has been converted into helium. The helium core then contracts, 10,000°F (5,500°C)
and nuclear reactions continue in a shell around the core. The core becomes hot Core: 27 million °F
enough for helium to fuse to form carbon, while the outer (15 million °C)
layers of the star expand and cool. The expanding
star is known as a red giant. When the
STRUCTURE OF A NEBULA
helium in the core runs out, the outer
layers of the star may be blown
away as an expanding gas shell
called a planetary nebula. The
remaining core (about 80 Young main
sequence star
percent of the original
star) is now in its final
Dense region of dust and
stages. It becomes
gas (mainly hydrogen)
a white dwarf star condensing under gravity
that gradually cools to form globules
and dims. When it
finally stops shining
Hot, ionized hydrogen
altogether, the dead gas emitting red light
star will become due to being stimulated
by radiation from hot
a black dwarf.
young stars
Dark globule of dust and
gas (mainly hydrogen)
contracting to form protostars
LIFE OF A SMALL STAR OF ABOUT ONE SOLAR MASS
About 1.4 million km
Cool cloud of
gas (mainly Natal cocoon
hydrogen) Star
(shell of dust
and dust Glowing blown away by producing
ball of gas energy by
(mainly radiation from
Dense globule hydrogen) protostar) nuclear fusion
condensing to in core
form protostars
NEBULA PROTOSTAR MAIN SEQUENCE STAR
Duration: 50 million years Duration: 10 billion years
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