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microwave radiation, which makes
it hard to see. Eventually, however,
the protostar gathers enough mass
for fusion to begin, but initially only
the deuterium, a heavy isotope of
hydrogen, begins to burn. Unlike
an “adult” star, a protostar releases
its heat entirely by a process of
convection. Heat from its core rises
up to the surface in the same way
that hot water in a pan rolls around
as it boils. The convection and
rotation of the star create a strong
magnetic field, which pushes out
from each pole, clearing a narrow
hole in the envelope of gas and
dust. The growing protostar’s heat
and a stellar wind of plasma are
directed away from the star via
these polar jets. These features,
explained by Shu’s model, have
been confirmed by observations.
Becoming a star, almost
A star with the mass of the sun
spends about 10 million years as
a protostar. As its mass increases,
the angle of its polar jets widens,
pushing away more of the gas
cloud. Eventually, the protostar’s
stellar wind blasts out from the
entire star’s surface, and it clears
its gas cloud away completely. At
this point, the young stellar object
is revealed for the first time. Giant
stars (above 8 solar masses) have
already started burning hydrogen
by this point and have become
THE TRIUMPH OF TECHNOLOGY
full-fledged stars destined to
live short, bright lives. However,
smaller stars—those less than 8
solar masses—have not begun a
full fusion process and so are known
as pre-main-sequence (PMS) stars.
A PMS star still has a disk of
material spinning around it. Some
of that will be dispersed by the
stellar wind into the wider GMC.
What remains, around the smaller
stars especially, is likely to form
into gas giant planets, and perhaps
later, rocky ones as well.Final ignition
The final phase of star formation is
a contraction of the fast-spinning
PMS star. Red, orange, and yellow
dwarfs (M, K, G, and F type stars)form from PMS stars that are
less than 2 solar masses. They are
considerably wider and less dense
than their adult forms, and appear
much brighter as they give out
light from larger surface areas,
frequently punctuated by high-
energy outbursts of X-rays.
This energy is the product of
gravitational contraction, not
nuclear fusion. It takes about
100 million years for the PMS star
to compress itself enough to begin
burning hydrogen, and by that
time, it will have lost half to three-
quarters of its initial mass. Larger
PMS stars (those between 2 and 8
solar masses) take a different route
to achieving fusion and form rare
blue dwarfs (A and B type stars).
PMS stars are the earliest
stage of star formation that have
been seen clearly. Infrared space
telescopes such as Spitzer and
Hubble have given faint glimpses
of protostars but mostly they are
too heavily shrouded by the dark
dust clouds. NASA’s new infrared
James Webb Space Telescope is
designed to be sensitive enough
to see through that dust, so perhaps
soon the moment when a star is
born may at last be observed. ■A protoplanetary disk surrounds
the young star HL Tauri in the
constellation Taurus. The dark patches
are thought to represent the possible
positions of newly forming planets.An infant star sits at the center of
two nearly symmetrical jets of dense
gas. Known as CARMA-7, the star is
about 1,400 light-years from Earth.