The Lives

of Stars:
From Birth to Death and Beyond

BAADES WINDOW, NGC 6528, AND NGC 6522 IN SAGITTARIUS. COPYRIGHT I993 DAVID MALIN AND THE ANGLO-AUSTRALIAN TELESCOPE BOARD

Many of the skys most dramatic showpieces

are but chapters in the lives of stars.

By Icko Iben Jr.

and Alexander V.Tutukov

1997 Sky Publishing Corp. All rights reserved.

(PART I)

In todays picture of star formation, our galaxys spiral

arms play a critical role. They are the result of soundlike density waves that travel through the galaxys disk,
compressing matter along the way. The disk is in a continuous state of flux. Rarefied matter (gas and dust) is
compressed into stars; later, some of it is returned by
these same stars to the interstellar medium, albeit in
different forms.

Young Stars within their Gaseous

Cocoon: NGC 2024 (Infrared Image)

Gas between spiral arms is, as a rule,

rather hot and highly ionized, and its relatively high pressure prevents gravity
from inducing collapse. In a galaxy arm, however, the
gas density is large enough for collisions of ions and
atoms with dust grains to cool gas clouds, initiating
the formation of hydrogen molecules. Molecular gas
then concentrates itself into giant clouds (60 to 300
light-years across) with 100,000 to 1,000,000 times the
mass of our Sun.
Magnetic fields prevent molecular clouds from collapsing immediately, as do tidal forces from our galaxys
bulge. Nevertheless, the density of matter in such clouds
increases slowly with time and, at some point, gravity

Photo

COPYRIGHT I993 DAVID MALIN AND THE ANGLO-AUSTRALIAN TELESCOPE BOARD

The Birthplaces of Stars

IAN McLEAN / UCLA; INSET: W. A. TREMBLY JR.

look at the star-filled night sky produces one of the deepest impressions
of eternity and infinity accessible to
humankind. People have probably always attempted to understand the nature of stars and the universe, thereby
looking for their place in the world. The modern scientific picture of the universe has deep historical roots
reaching back to the ancient Greeks. Not only did they
contribute many of its fundamental elements, they also
populated the sky with mythological heroes whose
memory is preserved to this day in the names of the
constellations.
The role of stars in astronomy is evident from the name of the science itself.
In Greek, astro means star and nomos
means law. And though modern astronomy includes the study of the interstellar medium, galaxies, and the universe as a whole, stars remain prime
objects of study. This is understandable,
Visible-light
given the fact that our Sun, though a
rather ordinary star, is an entity without
which we would not exist!
Yet many of the global physical attributes of stars
were unknown until the last century drew to a close.
By then, some stellar distances had been measured,
and an understanding of electricity and magnetism
enabled estimates of stellar luminosities. A full appreciation of stellar structure was not possible until the
beginning of this century, when developments in
atomic physics permitted realistic mathematical models of stars to be made.
Even then, however, one ingredient was missing from
the nascent science of stellar evolution: the source of a
stars energy. This lack was only remedied in 1938,
when Hans Bethe, Carl von Weizscker, and Charles
Critchfield identified several of the nuclear-fusion reactions that keep the stars shining. The transformation of
hydrogen and helium into heavier chemical elements
was recognized as the driving force of stellar evolution,
and models incorporating newly measured nuclearreaction rates allowed estimates of stellar lifetimes to
be made for the first time.
Since this fundamental turning point, astronomers
have devoted countless hours to the problems of star
birth, star death, and all that takes place in between. The
resulting paradigm of stellar evolution accounts for
many of astronomys most provocative subjects white
dwarfs; black holes; supernovae and connects them to
the countless stars we see on a clear, moonless night.

M83 in Hydra: A Spiral Galaxy with Starbirth in its Arms

1997 Sky Publishing Corp. All rights reserved.

Sky & Telescope December 1997

37

wins out and the clouds fragment

A Star Is Born
into smaller, denser bodies, each bear- In the story line above, we glossed over the means by
ing 1,000 to 10,000 Suns in mass. In
which stars actually form from collapsing, fragmentary
else, a stars initial
turn, the collapse of these smaller
gas clouds. So lets examine this crucial birth event in
clouds leads to the formation of com- more detail.
pact groups of young stars.
As the material destined to form a solitary star colallotment of mass
The star-formation efficiency in
lapses, it develops a dense, central core of gas and dust.
these groups the fraction of the gas This core temporarily stops collapsing when it becomes
sets the course of its and dust actually converted into stars opaque to its own infrared radiation. This opaque core
is about 25 percent, and this has
weighs in with a mass 110 our Suns, and it spans about
fateful consequences for most new
1,000 times our Suns diameter. The gas-dust core consubsequent evolution. open clusters. The massive stars in
tracts slowly for about 3,000 years, accreting matter
such a cluster heat the remaining gas
from overlying layers all the while.
to a temperature of about 10,000
As the core collapses, its interiKelvin, driving the gas out of the
or heats until the dust evaporates.
cluster. As a result, the young clusHaving temporarily become transStellar Evolution
ters total mass is reduced to the
parent, the core collapses once
extent that the stars can escape its
more until hydrogen becomes
gravitational grip and run away
ionized, making the core opaque
Dusty gas cloud
collapses
from their place of birth. Thus
yet again.
paradoxically, though most stars
The collapse phase that follows
are formed in open clusters, almost
lasts only 10 to 30 years. This
95 percent of these clusters die at
means that at any given time only
the moment of their birth.
a few dozen stars in the entire
Protostar becomes
The few open stellar clusters we
galaxy are in this phase! The raopaque; envelope
see, such as the Pleiades and the
dius of the young, completely concollapse continues
Hyades, are very rare long-term survective stellar core is about 3 to 5
vivors, but even they are currently
times our Suns. With a surface
in the process of evaporating. Pretemperature of 3,000K, the boilsumably the lives of these clusters
ing core radiates at visual and inConvective stellar
body develops within
have been prolonged because the
frared wavelengths. But this radiacollapsing envelope
process of star formation in them
tion is deeply hidden by the dust
was more efficient than usual.
in the still-collapsing envelope,

More than anything

Mass between 0.8 and 11 Suns

Main-sequence star
(type B, A, F, or G)

Red giant with

helium core

Red supergiant with

carbon-oxygen core

Mass between 11 and 50 Suns

Main-sequence star
(type O or B)

Red or blue
supergiant with
helium core

Mass greater than 50 Suns

Main-sequence
O-type star
with strong wind

Wolf-Rayet star

Red supergiant
with iron core
Type Ib supernova

Planetary nebula
with central star

Type II supernova

Black hole
White dwarf

Neutron star
Not to scale

38

December 1997 Sky & Telescope

1997 Sky Publishing Corp. All rights reserved.

COPYRIGHT I993 DAVID MALIN AND THE ANGLO-AUSTRALIAN TELESCOPE BOARD

Young Stellar Adults:The Pleiades

From Dwarf to Giant

104

Life Cycle of a Sun-like Star

Planetary
nebula

103

N
AI

102

Asymptotic
branch
giant
and
ends

Core
helium
flash
begins

Red
giant

E
QU
SE

10

Mainsequence
star

101

E
NC

After spending billions of years as a stable nuclear-fusion

reactor, a low-mass star like our Sun follows a complicated
path in the Hertzsprung-Russell diagram a crucial diagnostic tool that uses the temperatures and brightnesses of
stars to track stellar evolution. Courtesy the authors.

105

The main source of energy that keeps stars shining is

the transformation of hydrogen into helium. How long
this process can last depends on a stars mass, which
determines the temperature and pressure in its core.
Ironically, smaller stars live longer than their high-mass
siblings even though they have less fuel to burn
(that is, to fuse into heavier elements). Thats because
their cores are cooler, and nuclear reaction rates are
very sensitive to temperature.
As it turns out, at most 5 percent of all the stars yet
born have been privileged to evolve beyond the main
sequence. On the other end of the scale, the main-sequence lifetime of the most massive stars (those with
more than 11 solar masses) is 10,000,000 years or less.
As a result, thousands of generations of those heavyweights have come and gone since the Big Bang. Note
that when we say these stars have come and gone, usually that means only that they have been transformed
into a dead remnant that no longer feeds on nuclear
energy. But before acceding to astronomical anonymity,
most stars go through dramatic changes that produce
some of the skys most stunning showpieces.
A low- or intermediate-mass star spends between 80
and 90 percent of its life in the main-sequence phase.
This comes to an end when a large fraction of the stars
hydrogen has been converted into helium. Temporarily
too cool to tap the next source of nuclear energy, the
stars core now composed of helium contracts. In
the meantime, hydrogen burns at an even greater rate
than before in a shell surrounding the core. This causes
the stellar envelope to expand until it spans some 100 to
1,000 present-day solar diameters, transforming the star
into a red giant.
Eventually, core temperatures rise until helium nuclei

begin to fuse into those of carbon and oxygen. In stars

that start out with less than about 2.3 solar masses, core
helium burning begins abruptly, engendering a brief
thermonuclear runaway, or core helium flash. When this
happens, hydrogen burning actually slows in the surrounding shell, because it expands and cools; this makes
the stars luminosity drop. By contrast, helium burning
begins more gradually in stars with masses between 2.3
and 11 Suns.
As helium burns in the core of a low- or intermediatemass star, hydrogen continues to burn in a surrounding
shell, and this provides most of the stars luminosity. The
core helium-burning phase lasts from 10 to 25 percent of
the main-sequence lifetime that preceded it.
In the end, when helium in turn is exhausted within
an appreciable portion of its center, a star that started
out with anywhere from 0.8 to 11 solar masses forms a

Luminosity (Suns)

and the star continues to be seen only in

the infrared portion of the spectrum.
The accretion of envelope matter
onto the core proceeds for 100,000 to
1,000,000 years, until the envelope matter is either exhausted or expelled by the
stars radiation or by its solar-wind
particles. Intermediate-mass stars (those
with 1 to 11 times our Suns mass) contract over a period of 100,000 to
10,000,000 years until they reach the socalled main sequence. In that prolonged
state of stellar adulthood, gravity balances gas pressure and the energy lost
from the stellar surface matches that liberated by nuclear reactions in the core.
Stars more massive than 11 Suns are essentially born on the main sequence,
skipping adolescence entirely.

White
dwarf
102

103
100,000

10,000

1,0

Surface temperature (degrees Kelvin)

1997 Sky Publishing Corp. All rights reserved.

Sky & Telescope December 1997

39

ing the dust grains that so efficiently

cool molecular gas clouds, AGB stars
pave the way for the formation of future generations of stars and planets!

COPYRIGHT I993 DAVID MALIN AND THE ANGLO-AUSTRALIAN TELESCOPE BOARD

The Ghosts of Sun-like Stars

Stellar Death Shroud:The Helix Nebula in Aquarius

As their lives come to

After several hundred thousand years on

the asymptotic giant branch, a low-mass
star expels its hydrogen-rich envelope. The
remnant star contracts rapidly, raising its
surface temperature as high as 100,000 K.
At such high temperatures, the dying stars
surface emits copious ultraviolet light and
low-energy X-rays. This radiation agitates
atoms and molecules in the ejected gas,
causing them to fluoresce. The result: a
planetary nebula that remains visible for
typically 10,000 years.
The name planetary nebula came
about because some of these stellar
shrouds resemble the planet Uranus
when viewed at the eyepiece of a telescope. There are roughly 10,000 planetary nebulae in our galaxy.
Ultimately, the stellar cinder at a planetary nebulas heart ceases to burn hydrogen and
evolves into a cooling white dwarf. In a white dwarf,
the gravitational forces compressing matter are balanced by the quantum-mechanical pressure of degenerate electrons. As it turns out, a white dwarf counterintuitively gets smaller as you increase its mass! Typically
white dwarfs are comparable in size to the Earth. Although 10 billion white dwarfs are believed to inhabit
our galaxy, with todays telescopes we can see only
those within about 600 light-years of the Sun. Siriuss
faint companion is such a star, as is Procyons.

nearly degenerate core of carbon and

oxygen nuclei. Above, hydrogen and
helium burning continue to power
an end, stars seed
the swollen star. The star spans several
hundred solar radii (exceeding the radius of Earths orbit) and glows with a
the galaxy with the
relatively cool surface temperature of
3,000 K. Such a star is called an aselements upon which ymptotic giant branch (AGB) star,
thanks to the shape of its evolutionary track on the Hertzsprung-Russell
life depends.
diagram (see page 39).
Roughly 97 percent of the single Going Out with a Bang
stars (or those in wide binaries) that evolve off the The most massive stars follow a very different life cycle
main sequence within a 15-billion-year time frame be- from that of their lightweight counterparts. After a relcome AGB stars. The nuclear reactions that occur in atively brief main-sequence phase, a single star that
AGB stars are responsible for at least half of the carbon started out with 11 to 50 solar masses also forms a carin the universe, and for
approximately 200 neutron-rich isotopes of elements like tin, cadmium,
A Red Supergiants Core
and lead.
Observations show
that AGB stars pulsate
with typical periods of
200 to 600 days, and that
they lose matter from
their surfaces in the form
Ir o n
x y S ilic o n , S u lf u r m
of powerful winds. These
ge
u
Ca n, N eo n, S o di n
o
rbo
winds carry out freshly
n, O xyg e n, N e
H e li u
n
e
g
o
m
r
t
,
i
N
made carbon and neuH y d ro
gen, Helium
tron-rich elements as
well as dust grains that
have been formed in the
stars relatively cool outer
Diagram not to scale
atmosphere. By produc40

December 1997 Sky & Telescope

1997 Sky Publishing Corp. All rights reserved.

J. C. HAYES / A. BURROWS

pelled at high velocities. Far away, astronomers witness a Type II supernova

like the explosion that occurred in the
Large Magellanic Cloud in 1987.
A typical Type II supernova shines for
several weeks with 10 to 100 billion
times our Suns luminosity comparable to that of an entire spiral galaxy.
Most of what we know about supernovae comes from outbursts in other
galaxies. Observations suggest that a
Type II supernova should occur roughly
every 50 years in our Milky Way.
Type II supernovae return to the interstellar medium (ISM) the chemical
elements produced by their precursor
stars. In doing so, they enrich the ISM
with the heavy elements that are so important to our everyday existence. It has
been discovered recently that the heavyelement abundance in intergalactic gases
is comparable to that within galaxies
themselves. Since there are no stellar
sources of heavy elements in the intergalactic medium, about half of the
heavy elements produced by supernovae
must leave their parent galaxies. This
probably takes place by way of fountains that form
when dense clusters of supernovae punch holes in the
disks of their parent galaxies.

bon-oxygen core. But this core is not degenerate, and

hence it can contract until another set of reactions
kicks in, producing neon nuclei as well as more oxygen.
In turn, the resulting oxygen-neon core contracts and
heats until neon-burning reactions are ignited. The
cycle of contraction, heating, and ignition continues
until a core of iron-peak elements is formed. (Iron-peak
elements are those that have roughly 26 protons and 30
neutrons in one of their isotopic nuclear configurations.) Creating yet heavier elements requires expending
energy rather than producing it. As a result, a massive
stars iron core ceases to be a source of nuclear energy.
As lighter elements continue to burn in shells above
it, the iron core grows in mass until its mass exceeds
the Chandrasekhar limit the maximum possible
mass of a white dwarf, some 1.4 Suns. The core then
begins to collapse. The stellar cores iron-peak nuclei
decompose into those of helium, which then fragment
into neutrons at the price of the stars gravitational potential energy. The core collapses, producing a neutron
star a degenerate ball of neutrons somewhat akin to
a white dwarf, but far, far denser. (A neutron star typically packs 1.5 to 2 solar masses into a ball the size of a
small city.)
Core collapse lasts only 110 of a second, and almost all of
the gravitational potential energy released (some 1053
ergs) is converted into neutrinos, which leak out from the
core over a period of about 10 seconds following collapse.
Most of these neutrinos continue unimpeded through the
rest of the star at the speed of light or nearly so. A small
fraction of these ultralight particles scatter against the
atomic nuclei that the dying star took so long to produce.
This imparts so much energy and momentum to the
matter above the core that the stars outer layers are ex-

COPYRIGHT I993 DAVID MALIN AND THE ANGLO-AUSTRALIAN TELESCOPE BOARD

A Supernova Begins (computer simulation)

A Supernovas Aftermath:The Vela Remnant

1997 Sky Publishing Corp. All rights reserved.

Sky & Telescope December 1997

41

Evidence that Type II

supernovae make neutron
stars was discovered about
formed when the
30 years ago when pulsating sources of radio waves
were found. Once it bebrief lives of the
came clear that they were
not beacons from extratermost massive stars restrial civilizations, the
radio sources were named
pulsars. Most astronomers
come to an end.
agree that the clocklike
pulses are emitted from
spinning neutron stars. The Crab Nebula
and its pulsar offer a well-known example
of a neutron star within the ejecta of a
documented supernova explosion. More
than 500 pulsars have been identified,
and theory suggests that there may be up
to one billion neutron stars in our galaxy.

R. WAINSCOAT AND J. KORMENDY / COPYRIGHT 1997 ASTRONOMICAL SOCIETY OF THE PACIFIC

Black holes are

The Crab Nebula in Taurus: Remains of a Shattered Star

At the Top of the Scale

The stars starting out with the very highest masses
50 to 100 Suns shine with the brightness of 100,000
to 1,000,000 Suns during their brief lives as nuclear-fusion reactors. These exceptional stars lose mass at such
high rates that only a helium core remains by the time
they leave the main sequence. These relatively rare objects are known as Wolf-Rayet stars.
The interiors of Wolf-Rayet stars follow qualitatively
the same evolutionary path as do the interiors of their
lower-mass (11 to 50 solar-mass) cousins. However,
their iron cores are so massive that their collapse cannot be halted by neutron degeneracy. Instead, collapse
continues until the stars gravity prevents light waves
from escaping. This is one of the ways nature creates
JASON WARE

black holes. The explosive collapse of a Wolf-Rayet star

appears as a Type Ib supernova one that lacks any
spectral evidence for hydrogen.
The black hole created by a Type Ib supernova will
be visible only for a limited time, as leftover stellar
debris dribbles down its ultrasteep gravitational
well. Thereafter, even the most massive of single
stars becomes one more dead remnant whose existence can be discerned only when it momentarily
bends the light of background stars as it wanders
through the galaxy.

A Cycle of Creation
The stars we have considered here solitary ones and
those in wide binaries evolve into white dwarfs,
neutron stars, or black holes that eventually fade away
without a trace. However, even in their death throes,
stars play a continuing role in the life cycle of our
galaxy. They seed interstellar space with the chemical
elements out of which we and our home planet are
made, and they produce the dust grains that play such
a vital role in forming further generations of stars and
planets. Our lives and those of the stars are inextricably
intertwined.
An upcoming second installment will examine how
stars lives are altered when they orbit one another at
close range.
Icko Iben Jr. is Distinguished Professor of Astronomy and
Physics at the University of Illinois. Alexander V. Tutukov
heads the Department of Stellar Evolution at the Institute for
Astronomy in Moscow, Russia. They have collaborated on numerous projects in the last 14 years.

Further Reading
A Wolf-Rayet Star Sheds its Atmosphere:
NGC 6888 in Cygnus
42

December 1997 Sky & Telescope

Kaler, James B. Stars and Their Spectra: An Introduction to the Spectral Sequence. Cambridge, U.K.: Cambridge University Press, 1989. Several chapters of this
book first appeared as articles in Sky & Telescope.

1997 Sky Publishing Corp. All rights reserved.