Star Formation

Stars are born within the clouds of dust and scattered throughout most galaxies. A

familiar example of such as a dust cloud is the Orion Nebula. Turbulence deep within these

clouds gives rise to knots with sufficient mass that the gas and dust can begin to collapse

under its own gravitational attraction. As the cloud collapses, the material at the center

begins to heat up. Known as a protostar, it is this hot core at the heart of the collapsing

cloud that will one day become a star. Three-dimensional computer models of star formation

predict that the spinning clouds of collapsing gas and dust may break up into two or three

blobs; this would explain why the majority the stars in the Milky Way are paired or in groups

of multiple stars.

As the cloud collapses, a dense, hot core forms and begins gathering dust and gas. Not all of

this material ends up as part of a star — the remaining dust can become planets, asteroids,

or comets or may remain as dust.

In some cases, the cloud may not collapse at a steady pace. In January 2004, an amateur

astronomer, James McNeil, discovered a small nebula that appeared unexpectedly near the

nebula Messier 78, in the constellation of Orion. When observers around the world pointed

their instruments at McNeil's Nebula, they found something interesting — its brightness

appears to vary. Observations with NASA's Chandra X-ray Observatory provided a likely

explanation: the interaction between the young star's magnetic field and the surrounding

gas causes episodic increases in brightness.

Main Sequence Stars

A star the size of our Sun requires about 50 million years to mature from the beginning of

the collapse to adulthood. Our Sun will stay in this mature phase (on the main sequence as

shown in the Hertzsprung-Russell Diagram) for approximately 10 billion years.

Stars are fueled by the nuclear fusion of hydrogen to form helium deep in their interiors. The

outflow of energy from the central regions of the star provides the pressure necessary to

keep the star from collapsing under its own weight, and the energy by which it shines.

As shown in the Hertzsprung-Russell Diagram, Main Sequence stars span a wide range of

luminosities and colors, and can be classified according to those characteristics. The

smallest stars, known as red dwarfs, may contain as little as 10% the mass of the Sun and

emit only 0.01% as much energy, glowing feebly at temperatures between 3000-4000K.

Despite their diminutive nature, red dwarfs are by far the most numerous stars in the

Universe and have lifespans of tens of billions of years.

On the other hand, the most massive stars, known as hypergiants, may be 100 or more
times more massive than the Sun, and have surface temperatures of more than 30,000 K.

Hypergiants emit hundreds of thousands of times more energy than the Sun, but have

lifetimes of only a few million years. Although extreme stars such as these are believed to

have been common in the early Universe, today they are extremely rare - the entire Milky

Way galaxy contains only a handful of hypergiants.

Stars and Their Fates

In general, the larger a star, the shorter its life, although all but the most massive stars live

for billions of years. When a star has fused all the hydrogen in its core, nuclear reactions

cease. Deprived of the energy production needed to support it, the core begins to collapse

into itself and becomes much hotter. Hydrogen is still available outside the core, so

hydrogen fusion continues in a shell surrounding the core. The increasingly hot core also

pushes the outer layers of the star outward, causing them to expand and cool, transforming

the star into a red giant.

If the star is sufficiently massive, the collapsing core may become hot enough to support

more exotic nuclear reactions that consume helium and produce a variety of heavier

elements up to iron. However, such reactions offer only a temporary reprieve. Gradually, the

star's internal nuclear fires become increasingly unstable - sometimes burning furiously,

other times dying down. These variations cause the star to pulsate and throw off its outer

layers, enshrouding itself in a cocoon of gas and dust. What happens next depends on the

size of the core.

Average Stars Become White Dwarfs

For average stars like the Sun, the process of ejecting its outer
layers continues until the stellar core is exposed. This dead,
but still ferociously hot stellar cinder is called a White Dwarf.
White dwarfs, which are roughly the size of our Earth despite
containing the mass of a star, once puzzled astronomers - why
didn't they collapse further? What force supported the mass of
the core? Quantum mechanics provided the explanation.
Pressure from fast moving electrons keeps these stars from
collapsing. The more massive the core, the denser the white
dwarf that is formed. Thus, the smaller a white dwarf is in
diameter, the larger it is in mass! These paradoxical stars are
very common - our own Sun will be a white dwarf billions of
years from now. White dwarfs are intrinsically very faint
because they are so small and, lacking a source of energy
production, they fade into oblivion as they gradually cool
down.

This fate awaits only those stars with a mass up to about 1.4
times the mass of our Sun. Above that mass, electron pressure
cannot support the core against further collapse. Such stars
suffer a different fate as described below.
White Dwarfs May Become Novae
If a white dwarf forms in a binary or multiple star system, it
may experience a more eventful demise as a nova. Nova is
Latin for "new" - novae were once thought to be new stars.
Today, we understand that they are in fact, very old stars -
white dwarfs. If a white dwarf is close enough to a companion
star, its gravity may drag matter - mostly hydrogen - from the
outer layers of that star onto itself, building up its surface
layer. When enough hydrogen has accumulated on the
surface, a burst of nuclear fusion occurs, causing the white
dwarf to brighten substantially and expel the remaining
material. Within a few days, the glow subsides and the cycle
starts again. Sometimes, particularly massive white dwarfs
(those near the 1.4 solar mass limit mentioned above) may
accrete so much mass in the manner that they collapse and
explode completely, becoming what is known as a supernova.

Supernovae Leave Behind Neutron Stars or Black Holes

Main sequence stars over eight solar masses are destined to
die in a titanic explosion called a supernova. A supernova is
not merely a bigger nova. In a nova, only the star's surface
explodes. In a supernova, the star's core collapses and then
explodes. In massive stars, a complex series of nuclear
reactions leads to the production of iron in the core. Having
achieved iron, the star has wrung all the energy it can out of
nuclear fusion - fusion reactions that form elements heavier
than iron actually consume energy rather than produce it. The
star no longer has any way to support its own mass, and the
iron core collapses. In just a matter of seconds the core shrinks
from roughly 5000 miles across to just a dozen, and the
temperature spikes 100 billion degrees or more. The outer
layers of the star initially begin to collapse along with the core,
but rebound with the enormous release of energy and are
thrown violently outward. Supernovae release an almost
unimaginable amount of energy. For a period of days to
weeks, a supernova may outshine an entire galaxy. Likewise,
all the naturally occurring elements and a rich array of
subatomic particles are produced in these explosions. On
average, a supernova explosion occurs about once every
hundred years in the typical galaxy. About 25 to 50
supernovae are discovered each year in other galaxies, but
most are too far away to be seen without a telescope.

Neutron Stars
If the collapsing stellar core at the center of a supernova
contains between about 1.4 and 3 solar masses, the collapse
continues until electrons and protons combine to form
neutrons, producing a neutron star. Neutron stars are
incredibly dense - similar to the density of an atomic nucleus.
Because it contains so much mass packed into such a small
volume, the gravitation at the surface of a neutron star is
immense. Like the White Dwarf stars above, if a neutron star
forms in a multiple star system it can accrete gas by stripping
it off any nearby companions. The Rossi X-Ray Timing Explorer
has captured telltale X-Ray emissions of gas swirling just a few
miles from the surface of a neutron star.

Neutron stars also have powerful magnetic fields which can

accelerate atomic particles around its magnetic poles
producing powerful beams of radiation. Those beams sweep
around like massive searchlight beams as the star rotates. If
such a beam is oriented so that it periodically points toward
the Earth, we observe it as regular pulses of radiation that
occur whenever the magnetic pole sweeps past the line of
sight. In this case, the neutron star is known as a pulsar.
Black Holes
If the collapsed stellar core is larger than three solar masses, it
collapses completely to form a black hole: an infinitely dense
object whose gravity is so strong that nothing can escape its
immediate proximity, not even light. Since photons are what
our instruments are designed to see, black holes can only be
detected indirectly. Indirect observations are possible because
the gravitational field of a black hole is so powerful that any
nearby material - often the outer layers of a companion star -
is caught up and dragged in. As matter spirals into a black
hole, it forms a disk that is heated to enormous temperatures,
emitting copious quantities of X-rays and Gamma-rays that
indicate the presence of the underlying hidden companion.

From the Remains, New Stars Arise

The dust and debris left behind by novae and supernovae
eventually blend with the surrounding interstellar gas and
dust, enriching it with the heavy elements and chemical
compounds produced during stellar death. Eventually, those
materials are recycled, providing the building blocks for a new
generation of stars and accompanying planetary systems.