Star (astronomy), massive shining sphere
of hot gas. Of all the stars in the universe, our Sun is the nearest to Earth
and the most extensively studied. The stars visible to the naked eye all belong
to the Milky Way Galaxy, the massive ensemble of stars that contains our solar
system (the Sun and its nine planets).
About 5,000 stars can be seen with the
naked eye, although not all of these stars are visible at any given time or
from any given place. With a small telescope, hundreds of thousands of stars
can be seen. The largest telescopes disclose millions of galaxies, which may each
contain over 200 billion stars. Modern astronomers believe there are more than
1 x 1022 stars in the universe (this number is very large, a 1
followed by 22 zeros). The largest stars, if placed at the Sun's position,
would easily engulf Earth, Mars, Jupiter, and Saturn. The smallest white dwarf
stars are about the size of Earth, and neutron stars are less than about 20 km
(about 10 mi) in diameter.
All stars are composed of hot glowing
gas. The outer layers of some stars are so empty that they can be described as
red-hot vacuums. Other stars are so dense that a teaspoonful of the material
composing the outer layers would weigh several tons. Stars are made chiefly of
hydrogen and a smaller amount of helium. Even the most abundant of the other
elements present in stars—oxygen, carbon, neon, and nitrogen—are generally
present in very small quantities.
The Sun, our nearest star, is
about 150 million km (about 93 million mi) from Earth. It appears different
from the stars visible in the night sky because it is about 250,000 times
closer to Earth than the next closest star. The next nearest star is Proxima
Centauri, which is more than 30 trillion km (20 trillion mi) from Earth. While
light from the Sun takes only about eight minutes to reach Earth, the farthest
stars are so distant that their light takes billions of years to reach Earth.
The color of stars—ranging from the
deepest red through all intermediate shades of orange and yellow to an intense
white-blue—depends directly on their temperature. The coolest stars are red and
the hottest stars are blue. Most stars make light by several different kinds of
thermonuclear fusion, a process in which the nuclei of atoms combine to form a
heavier element and release energy (see Nuclear Energy). One of the most
common thermonuclear fusion processes occurs in stars when four hydrogen atoms
combine into a helium atom, releasing energy that is transformed into light and
heat.
In the 1990s astronomers discovered
planets orbiting stars outside our solar system. Planets outside our solar
system are difficult to detect, because they are much fainter than stars are.
However, astronomers located these planets by measuring the wobble of a star’s
motion created by the slight gravitational pull that is exerted on the star by
a planet. Although scientists can only speculate how many Earthlike planets
with continents and oceans exist in the universe, they believe that many stars
have planetary systems (See also Gravity).
II
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CHARACTERISTICS OF
STARS
|
Astronomers learn about the physical and
chemical properties of a star by studying the energy it emits. They can
directly observe the atmosphere, or outermost gaseous layer, of the star.
Astronomers can infer many of the properties of the star’s interior by studying
the star’s atmosphere, its size, and the energy the star releases. The
properties more closely related to the interior structure of the star are its
mass and chemical composition. Astronomers also observe the motions of stars to
learn more about star and galaxy formation.
A
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Stellar Atmosphere
|
The only visible part of a star is its
gaseous outer region, or atmosphere. The atmosphere of the Sun is about 320 km
(about 200 mi) thick, while the Sun’s diameter is 1,392,000 km (865,000 mi). Even
though the atmosphere is relatively small compared to the size of the entire
star, astronomers can learn a great deal about a star by studying its
atmosphere.
Light emitted by a star has
several properties of interest to astronomers. Magnitude is how astronomers
measure a star’s brightness. Luminosity is the total amount of light a star
emits. Astronomers analyze a star’s light to classify the star’s spectral type,
which provides clues to the temperature and chemical composition of the star.
A1
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Magnitude
|
Astronomers first ranked stars in the night
sky by their apparent magnitude, or relative brightness. They grouped
stars visible to the naked eye into six classes, or magnitudes, that each
correspond to a factor of about 2.5 in brightness. The brightest stars are
classified as 1st magnitude, 2nd-magnitude stars are about 2.5 times less
bright, while stars just visible to the naked eye on a clear night are 6th
magnitude. The faintest stars observable with the Hubble Space Telescope are
fainter than the 28th magnitude.
Astronomers distinguish between apparent magnitude
and absolute, or intrinsic, magnitude. Apparent magnitude is the
brightness of a star as viewed from Earth, and the absolute magnitude of a star
is its actual brightness as viewed from a set distance away from the star. The
difference between these two types of magnitude helps astronomers distinguish
between stars that appear bright only because they are relatively close to
Earth and stars that appear bright because they are intrinsically bright or
highly luminous. For example, Sirius, which is only 8.6 light-years distant,
has the greatest apparent magnitude of any nighttime star. (A light year is the
distance light travels in a year—9.5 trillion km, or 5.9 trillion mi.) Other
stars may appear fainter because they are farther away, even if they actually
shine much more brightly than Sirius does. Such stars have a greater absolute
magnitude. The absolute magnitude is closely related to the physical conditions
of the star.
A2
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Luminosity
|
The luminosity of a star is its
intrinsic brightness, or the total energy radiated per second. For most stars,
this energy is generated by thermonuclear reactions occurring deep within a
star’s interior. Luminosity often depends on where a star is in its
evolutionary sequence, so it is important to astrophysicists who study the
evolution of stars (see Astrophysics). Stars emit energy in the form of
electromagnetic radiation, which includes ultraviolet radiation, visible light,
infrared, and radio waves. Because Earth’s atmosphere blocks the ultraviolet
radiation emitted by stars, calculating the exact luminosity of stars is
difficult for astrophysicists. In order for astrophysicists to determine a
star's luminosity, they must estimate the amount of unobserved ultraviolet
radiation or measure it directly from space craft orbiting above Earth’s
atmosphere. Although luminosity calculations are made partly by observation and
partly from theory, values have been established for many stars. The luminosity
of stars varies greatly. While some stars are only one five-hundredth as bright
as the Sun, others are 500,000 times brighter.
A3
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Spectral Type
|
Astronomers determine the spectral type of a star
by passing the star’s light through an instrument called a spectroscope. The
spectroscope usually breaks the light down into a continuous band of colors
that is crossed by numerous dark lines called Fraunhofer lines. A set of dark
lines corresponds to an element in the star that is absorbing the missing
colors of light. For example, the set of dark lines made by hydrogen includes a
dark red line, the set of dark lines made by sodium includes a pair of dark
yellow lines, and the set of dark lines made by iron includes lines of nearly
every color. Each element in the gaseous outer layer of a star produces its own
particular pattern of dark spectrum lines, depending on the temperature and
pressure of the gas. Astronomers have observed spectrum lines, or spectra, for
hundreds of thousands of stars. The appearance of each spectrum depends
primarily on the star’s temperature. Differences in chemical compositions of
stars produce more subtle effects, and require careful analysis for
astrophysicists to find them (see Spectroscopy).
After looking at the spectra for
many different stars, astronomers found that they could arrange almost all the
spectra into a continuous sequence based on the relative intensity of the dark
absorption lines in the spectra. They classified the majority of stellar
spectra into a sequence of seven standard categories, or types. Because the
strength of the spectral lines identifies the physical state of atoms and
molecules composing the star, astronomers were able to correlate the spectra
with the colors and temperatures of different stars. Astronomers arranged these
stars in a continuous sequence according to their surface temperature. From
hottest to coolest, these types are O, B, A, F, G, K, and M. Each color type is
further divided into ten subclasses based on gradations in their spectral
pattern. These subclasses form the sequences O0, O1, O2 ... O9, B0, B1, B2 ...
and so on.
While stars within the standard O, B,
A, F, G, K, and M sequence vary slightly in composition, they have different
spectra, mainly because of their different temperatures. The spectrum of a star
is therefore a good indication of its temperature. Astronomers also use a
star’s color to help determine its temperature. Just as a piece of hot iron or
glass will glow dull red, orange, or yellow depending on its temperature, so
will a star glow a certain color depending on its temperature. Type G stars,
similar to the Sun, are yellow stars that have surface temperatures around
6000° C (11,000° F). Hotter, type A stars, are white and have temperatures
around 10,000° C (18,000° F). Still hotter B and O-type stars are blue. Red
type M stars, at the other end of the sequence, can have surface temperatures
as low as 3000° C (5400° F).
The apparent magnitudes of stars depend on
the particular colors astronomers measure in the stars. Astronomers use colored
filters to select the color they wish to measure. The difference between
magnitudes in different colors for the same star is called the color index
of the star. It is a numerical indicator of the star's color, and it is
correlated with a star's temperature and spectral type.
The correlation between color and spectral
type does not hold strictly true for many distant stars, whose light is
reddened by interstellar dust. For example, a very distant B star may appear
yellow or orange instead of blue-white. For this reason, the spectral type is a
more fundamental quantity than is the color index because it does not change
with distance. On the other hand, for stars grouped together in space, such as
in a cluster, the reddening is the same for the entire group and the color
index can give a reliable indication of the relative colors of the stars within
the group.
In the early 20th century, Danish
astrophysicist Ejnar Hertzsprung and American astrophysicist Henry Norris
Russell independently developed a graph now known as the Hertzsprung-Russell
(H-R) diagram, which plots absolute brightness against spectral type. In this
diagram, the brightest stars lie near the top of the diagram and the hottest
stars lie to the left. On the H-R diagram, most of the stars, including the
Sun, fall along a diagonal line that goes from the upper left to the lower
right of the diagram. This line called the main sequence. The great
majority of stars neighboring the Sun fall on the lower part of the H-R
diagram’s main sequence, and relatively few lie on the portion of the main
sequence above the Sun. This means that most of the Sun’s neighboring stars are
both cooler and fainter (in absolute magnitude) than the Sun. A smaller
population of brighter but cooler stars known as supergiants occupies the
uppermost region of the diagram. Some stars, which are difficult to discover
because they are so intrinsically faint, lie near the bottom of the H-R
diagram. These faint stars are called white dwarfs.
A4
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Effective
Temperature
|
Every star varies in temperature
from that of the core, the temperature of which is measured in the millions of degrees,
to that of the atmosphere that is relatively cool. For example, the Sun’s core
reaches 15 million degrees C (27 million degrees F), while its outer layer is
about 5800° C (about 10,000° F). Astronomers determine the temperature of a
star’s surface (its outer layer) by comparing its spectrum with that of a black
body (a theoretical body that perfectly absorbs all the radiation striking
it). Scientists know how to correlate a black body spectrum with its
temperature. From the known temperature of the black body spectrum that agrees
most closely with the star’s spectrum, astronomers can determine the star’s
surface temperature.
A5
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Size
|
In 1920 scientists measured the angular
diameters of a few giant and supergiant stars with an instrument called a
Michelson stellar interferometer. The angular diameter of a star is its
diameter as observed from Earth, expressed in degrees and seconds of the arc it
sweeps out in the sky. Astronomers combined this data with the known distances
from Earth to the stars to calculate linear diameters of these stars.
Astronomers calculated that Arcturus, the fourth brightest star in the sky
located in the northern constellation Boötes, has a diameter of 23 solar
diameters, or 23 times bigger than that of the Sun (the Sun’s diameter is 1.39
x 106 km/8.65 x 105 mi). Betelgeuse, which marks the
right shoulder of the hunter in the constellation Orion, has a diameter of
about 1,000 solar diameters.
Another procedure for measuring stellar
sizes depends on eclipsing binary stars (binary stars are two stars that orbit
about a common center of mass). The orbits of these double stars are aligned so
that one or the other of the stars periodically passes behind the other when
they are viewed from Earth. Astronomers can measure the decrease in light
emitted during the eclipse to determine the relative radii of the two stars. If
measurements of the Doppler shift are also available, astronomers can determine
the absolute sizes of the stars. Doppler shifts are changes in the wavelength
(distance between waves) of a star's light caused by the star's movement.
If a star is moving away from Earth, each light wave emitted by the star leaves
from slightly farther away than did the previous wave, lengthening the distance
between waves. If a star is moving toward Earth, each light wave is emitted
from slightly closer to Earth than the previous wave was, shortening the
distance between waves. By measuring these changes in wavelength of the lines
in the star’s spectrum, astronomers can determine the star’s movement. From the
star’s movement, astronomers can convert the relative radii of the two
eclipsing binary stars into absolute sizes.
The amount of energy a star
radiates per unit of surface area depends on how hot the star burns (its
temperature). Therefore, if two stars are burning at the same temperature, the
larger star will have more surface area and hence greater luminosity than the
smaller star has. For example, the Sun and Capella, both G-type stars, have
equal effective temperatures of 5800° C (10,000° F). However, because of its
greater luminosity, Capella lies much higher on the H-R diagram. The total
surface area of Capella must therefore be greater than that of the Sun, and in
fact, Capella’s diameter is 16 times larger than the Sun’s diameter. In
contrast, the A-type and F-type white dwarf stars, which lie well below the
main sequence, must have comparatively little surface area and very small
radii. In fact, some white dwarf stars are as small as Earth itself.
B
|
Stellar Interior
|
Although the three interrelated
properties of luminosity, temperature, and size are essential for describing a star,
its mass and chemical composition are far more fundamental to its behavior. For
example, the mass and chemical composition of a star can determine its core
temperature and therefore the outward pressure exerted by the burning gases. If
these outward forces exceed the inward force of gravity (which depends on the
star’s mass), the star will expand until a balance is reached. In this way the
mass and chemical composition of a star determine both the size and luminosity
of the star.
B1
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Mass
|
The strength of the gravitational
force acting inside a star (the attraction of its matter for itself) depends on
the mass and distribution of the matter contained in the star. Astronomers can
calculate the masses of binary stars by measuring how closely the stars orbit
each other and how long it takes them to complete an orbit. These measurements
help astronomers determine stellar masses, because the orbits of the binary
stars depend on the gravitational attraction between them, an attraction that
depends on the masses of the stars and their distances from each other.
Three types of binary stars yield
information about stellar masses. The first type of binary star system, known
as visual binaries, describes two stars that can be individually discerned through
a telescope. If visual binaries are close enough to Earth to allow astronomers
to determine the size and inclination (tilt) of the orbit of the two stars
around each other, they can calculate the mass of the two stars. Astronomers
identify the second type of binary star system, known as spectroscopic
binaries, by Doppler shifts in the spectrum lines of two stars created as the
two stars orbit each other.
Astronomers can only determine the
lower limit of the masses of the stars in a spectroscopic binary system by
measuring how much their light is shifted as they move around each other. They
cannot make a better estimate without knowing the orientation of the stars as
they orbit each other, because they cannot measure the size of the orbit
without knowing how much the orbit is tilted with respect to Earth. Because the
orbits of binary stars are not limited to one plane (with respect to Earth),
they can circle each other so that neither star ever obstructs the other from
Earth’s view. When a binary star in a system passes in front of the other star
in the system, this is called an eclipsing binary. Because one star passes in
front of the other, astronomers know that at this point in the orbit, the two
stars line up with Earth. This information reveals the orbit’s orientation.
Knowing the stars’ orientation allows astronomers to make a more definite
calculation of their mass. Astronomers have found that virtually all measured
stars have masses that range between one-fiftieth and 50 times the Sun’s mass,
which is 1.99 x 1030 kg (4.39 x 1030 lb).
Astronomers also use a relationship
known as the mass-luminosity law to help determine a star’s mass from its
brightness. This law states that main-sequence stars with greater mass are
brighter (more luminous) than stars with less mass. The more massive a star is,
the more tightly the core material is pulled together by gravitational
attraction. The greater the central pressure is, the hotter a star’s core
becomes. Since the rate of thermonuclear reactions occurring in the star’s core
increases at higher temperatures, more massive stars produce more energy and
burn more brightly (are more luminous) than less massive stars do. Scientists
have confirmed this correlation through observation and found that it applies
to stars fueled by the nuclear fusion of hydrogen atoms (stars located along
the main sequence). Stars not located along the main sequence deviate from the
mass-luminosity law. For example, because white dwarfs have exhausted their
supply of nuclear fuel, they are dim for their mass.
B2
|
Chemical
Composition
|
Although all main-sequence stars consist
primarily of hydrogen and lesser amounts of helium, they differ somewhat in
their chemical composition. For example, recent evidence suggests that younger
stars contain higher proportions of metals. Certain unusual stars, such as
older white dwarf stars, may contain large amounts of helium and very little
hydrogen. Red giant stars—expanding stars in the late stages of the
evolutionary sequence of a normal star—have exhausted their supply of hydrogen
fuel and are burning helium and heavier elements. Much of the carbon and
particulate matter ejected from red giant stars provides crucial chemical
building material for solar systems throughout the universe.
Astronomers have used variations in chemical
composition from star to star to identify different generations of stars in the
universe. While some stars formed from new material, others formed from
material ejected into space during the death sequence of old stars and
therefore belong to the next generation. Massive stars that formed early in the
history of the Milky Way finished their principal stages of evolution several
billion years ago. Near the end of the existence of these stars, heavier
elements, fused from hydrogen by nuclear reactions, may have been spewed back
into the interstellar gas and dust. Consequently, later-generation stars
forming from this enriched material contain a relative abundance of metals
(heavy elements). Thus, the structure and evolution of stars that are members
of different generations vary, as revealed by differences in their chemical
composition.
These second- and later-generation
materials are also important for the formation of planets (for more
information, see the Formation section of this article). The planet
Earth formed from gas and dust ejected by ancient, dying stars. These elements,
including carbon, oxygen, nitrogen, and iron, form all the known substances in
our world.
C
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Motion
|
Although stars appear fixed in the
apparently flat patterns of the constellations, they are actually moving at
high speeds measurable over time by small changes in position. The movement of
stars over time is known as proper motion. This movement is separate from the
apparent motion of stars across the sky throughout the night. That apparent
motion is actually caused by Earth’s rotation. Astronomers can determine how
quickly a star is moving toward or away from Earth (its radial motion) by
examining its spectrum. This technique for determining motion in the line of
sight uses the Doppler effect, a change in the spectrum of a star created by
the star’s motion.
Astronomers have found that stars
neighboring our solar system are moving in random directions at an average
speed of about 24 km/s (15 mi/s) with respect to each other. The Sun's motion
with respect to neighboring stars is 26 km/s (16 mi/s) in the direction of the
constellation Hercules, near the bright star Vega.
III
|
HOW STARS PRODUCE
ENERGY
|
For many years astronomers were puzzled
about how the Sun provided energy. While Earth’s fossil record indicates that
the Sun has been shining for hundreds of millions of years, efficient chemical
reactions known to early scientists—such as burning coal—could only provide
energy from a similar mass for a few thousand years. Not until the 1920s did
astronomers discover that nuclear reactions (energy released by the fusing of
atomic nuclei) were a star’s principal source of energy.
Nuclear reactions can occur inside stars,
because the interior temperatures of stars are in the millions of degrees. For
example, the temperature of the core of the Sun reaches 16 million degrees C
(29 million degrees F). At such high temperatures the electrons are completely
stripped away from the nuclei of atoms, and the matter is neither solid,
liquid, nor gaseous but exists in a fourth state called plasma (a gaslike state
in which the atoms lose their electrons and become ions). At the high
temperature, pressure, and density of star interiors, atomic nuclei crash into
one another at tremendous speeds, creating temperature-controlled thermonuclear
reactions.
A
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Hydrogen Burning
|
Hydrogen, the simplest of the elements and
chief constituent of most stars, furnishes the fuel for stars like the Sun.
Because the core of a typical star is so violent and hot, hydrogen nuclei are
separated from their electrons. In the star’s core, the great pressure of
overlying material forces the protons to collide so violently that the nuclei
fuse together. The nuclear reactions fuse the nuclei of four hydrogen atoms
into a single helium nucleus, liberating energy in the process and producing a
star’s light and heat. In this fashion, more than 4 million tons of the Sun’s
mass are destroyed and turned into energy every second. For a more detailed
description of the hydrogen-burning process that occurs in stars like the Sun, see
Sun: Nuclear Fusion in the Core.
B
|
Carbon Cycle
|
A more complex sequence of reactions,
involving the nuclei of carbon atoms, produces the same net effect in some
stars as that of the fusion of hydrogen nuclei. The carbon cycle starts with
carbon-12 and hydrogen and ends with carbon-12 and helium. Carbon-12 acts as a
catalyst (a substance that speeds a chemical reaction) in the production of
helium from hydrogen. Because the carbon cycle is only triggered by
temperatures that exceed 20 million degrees C (40 million degrees F), these
reactions only occur in more massive stars. Carbon speeds the fusing of
hydrogen nuclei into helium in the carbon cycle, so these stars burn faster and
more brightly than do other hydrogen-burning stars.
C
|
Helium Burning
|
If a star’s core temperature rises
to about 100 million degrees C (about 200 million degrees F), helium, which is
inert (unreactive) in cooler stellar interiors, will participate in certain
nuclear reactions. The collision of two helium nuclei can release energy and
form a beryllium nucleus. The beryllium nucleus is unstable, but sometimes
another helium nucleus collides with the beryllium nucleus before it can
disintegrate, forming a carbon nucleus. Bluish-white supergiants and yellow
giants use helium as fuel. Stars with insufficient mass never achieve high
enough internal temperatures to fuse helium atoms.
At still higher temperatures, carbon
and helium can combine to form oxygen, and this fusion process can continue,
forming elements of successively higher atomic number (number of
protons) up to iron. Compared to the long, stable hydrogen-burning stage in
most stars, these processes predominate for relatively short periods of time
and only in fairly massive stars. Astronomers believe that most of the elements
found on Earth could have been formed in this way deep within stars that
existed long before the Sun formed.
IV
|
MULTIPLE STAR
SYSTEMS
|
When viewed through a telescope, many
stars appear in systems of two or three. At first, astronomers thought that observed
double stars might be accidental pairings of a nearby and a distant star in the
same line of sight. However, continued observation reveals that few stars occur
singly and that these systems of multiple stars orbit about each other. The
number of stars composing these groups ranges from a few stars to clusters of
tens of millions of stars. Much larger aggregations of stars, gas, and dust are
called galaxies. Galaxies may contain hundreds of billions of stars, all
gravitationally interacting and orbiting about a common center.
A
|
Open Star Clusters
|
An open, or galactic, cluster is a
group of relatively young, loosely bound stars. Astronomers have cataloged more
than 1,000 galactic clusters within the Milky Way. Some clusters are quite
sparse, containing only 20 or 30 stars, while others, such as the h and Chi
Persei clusters each contain several hundred stars. The Pleiades is an
open cluster located about 380 light- -years from our solar system (a
light-year is the distance light travels in a year, 9.46 trillion km/5.88
trillion mi) and it contains about 500 stars. Astronomers believe that open
clusters contain relatively few stars because there is insufficient mutual
gravitational attraction to hold the stars together over long periods of time.
Thus, by astronomical time standards, existing galactic clusters are
comparatively young and have not yet been torn apart (see Open Cluster).
B
|
Globular Star
Clusters
|
Globular clusters are spherical, compact groups
of stars containing thousands or perhaps even millions of stars. The 120
globular clusters recorded in the Milky Way galaxy generally contain hundreds
of thousands of stars. In contrast to open clusters, globular clusters are
highly stable and represent very old aggregations of stars. Whereas open
clusters are distributed throughout the plane of the Milky Way, globular
clusters are concentrated in a spherical distribution around the nucleus of the
galaxy, especially in the direction of the constellations Scorpius and
Sagittarius.
C
|
Galaxies
|
Galaxies are massive assemblies of
hundreds of millions or even hundreds of billions of stars orbiting a common
center. The arrangement of several hundred billion stars into the disk-shaped Milky
Way system is only one possible configuration of a galaxy. Some galaxies are
highly regular spheres or ellipsoids (bodies shaped like a rounded football),
while others have no regular structure.
V
|
LIFE CYCLES AND
AGES OF STARS
|
Stars form from dust and gas
clouds and may spawn planets in the process. Once formed, the chemical
composition and mass of a star plays an important role in the life stages of
the star and how long it will live.
A
|
Formation
|
Astronomers believe that stars form when
enormous compression waves traveling through gas clouds create dense knots of
gas in the cloud. Gravitational forces of these denser areas attract nearby gas
particles. As a knot grows, its gravitational force increases, and it attracts
more gas particles. Eventually, the knot coalesces into a growing sphere of
compressed gas that reaches internal temperatures of a few million degrees C.
At this point the gases in the knot’s interior become so hot that their atomic
nuclei begin fusing, creating large amounts of nuclear energy and forming a new
star. Such brilliant newly formed stars outline the spiral structure of the
Milky Way Galaxy. Pressure from the radiation of new stars in turn causes more,
higher-density zones to form in the gas cloud, which initiates the birth of
more stars. Small, starlike objects called brown dwarfs are similarly formed.
Brown dwarfs are larger than planets are, but they do not have enough mass to
initiate thermonuclear reactions and become true stars.
Astronomers have developed a theory that
planets form from gas and dust encircling young stars. As the density of an
emerging star increases, the surrounding gas and dust slowly condense into a
spinning disk. Some areas of this disk are thicker than others are, and the
gravitational pull of these dense areas attracts nearby dust and gas as the
disk orbits the star. Over a period of several millions of years, these dense
areas consolidate and grow in size, forming the planets of a solar system. By
the end of the 20th century, astronomers had located more than a dozen other
star-planet systems outside our own. Astronomers believe many stars have
planets orbiting around them.
B
|
Evolution of Stars
|
In its earliest stage, a typical star such
as the Sun is large and emits infrared light. Within a million years, the
gravitational attraction of the star’s material for itself causes the star to
shrink to the present size of the Sun. The added pressure caused by this
collapse in size raises the star’s internal temperature high enough to trigger
nuclear reactions in the core.
B1
|
Main-Sequence Stars
|
About a million years after a new star
like the Sun forms, it stabilizes in the hydrogen-burning, or main-sequence, phase,
in which it remains for about 10 billion years. Main-sequence stars fall along
the diagonal line that goes from the upper left to the lower right on the H-R
diagram. During its main-sequence phase, a star gradually exhausts its hydrogen
supply.
B2
|
Red Giants and
Supergiants
|
The next stage of a star’s
evolution involves dramatic stages of expansion and contraction as the star
approaches the end of its life cycle. After the star has consumed the hydrogen
in its innermost core, the core begins shrinking, converting hydrogen into
helium in ever-larger shells around the inner core. The star’s core shrinks
because the outward pressure of heat generated by the nuclear reactions no
longer balances the inward gravitational attraction of the star’s mass for
itself.
Although the core of a star
gradually shrinks as it exhausts its hydrogen supply, the star itself begins
expanding. It resorts to burning the hydrogen in a shell around its helium
core, which inflates the outer layers of its atmosphere. Eventually, the star
expands into a red giant, possibly attaining a diameter from 10 to 1,000 times
the diameter of the Sun. For example, in its red giant stage, the Sun will
expand to the size of the orbit of Earth or beyond and become 2,000 times
brighter than it is now. The shrinking core increases the star's internal
pressure. The increase in pressure makes the star's temperature increase again
until it is hot enough to trigger nuclear reactions between previously inert
helium nuclei present in the star. This new series of nuclear reactions
releases more energy and the star's core stops contracting. At this point, the
star's outer atmosphere begins to contract.
Although rare, the most massive stars
can evolve into stars called supergiants. In such a star, radiation released by
the fusion of helium into carbon causes the red giant to expand into a
supergiant—a star at least 500 times the Sun’s size.
B3
|
White Dwarf
|
When a low- to medium-mass star exhausts
the nuclear fuel in its core, it collapses under the gravitational pressure of
its own weight into an extremely compact, dense star known as a white dwarf. As
a more massive star (6 to 8 solar masses) collapses to a white dwarf, it blows
off more than half of its outer layer into space as a planetary nebula—gas and
dust that may provide building material for planets in newly forming solar
systems. Although dimmer than the original star, a white dwarf will continue
radiating light for several billion years from thermal energy (heat energy)
trapped in its interior.
Most white dwarfs have only slightly
larger radii than that of Earth, but the density of a typical white dwarf is
about 600,000,000 kg/m3 (4,000,000 lbs/ft3), and the mass
of a typical white dwarf is about 70 percent of the Sun’s mass. As a white
dwarf slowly loses energy and cools, it changes color from blue-white, to
white, to yellow, to orange, and finally to dull red. After several billion
years, the white dwarf exhausts its energy supply, and becomes what is known as
a black dwarf.
B4
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Neutron Stars and
Black Holes
|
When the core of a supergiant
has exhausted its helium, the core will again contract, and if the core is sufficiently
massive, additional nuclear reactions will be triggered during this
contraction. These nuclear reactions convert carbon and other elements into
increasingly heavier elements, until the core may consist largely of iron. Some
supergiant stars then form an astronomical body known as a neutron star.
Neutron stars form when a supergiant continues to collapse and the material in
the stellar core becomes more and more dense. The atomic nuclei are forced so
close together that they fuse to form neutrons. When this occurs, the core
stops contracting and remains as a neutron star, a rapidly spinning, extremely
dense star consisting mainly of closely packed neutrons. A neutron star may
contain a mass that is equal to 1.4 to 3 times the Sun’s mass and that is
compressed into a volume about 20 km (about 10 mi) in diameter.
Still more massive supergiants, with a
mass more than 5 times that of the Sun, may continue collapsing until their
nuclei are crushed into even denser matter. This matter forms a body so dense
that it forms a black hole—an extremely dense, invisible celestial body with a
gravitational field powerful enough to prevent the escape of light.
During the collapse of a supergiant,
the outer layers of the star are ejected into space by a massive explosion
known as a supernova (for more information, see the Supernova section of
this article). This ejected gas and dust contain hydrogen and heavier elements,
such as carbon, oxygen, nitrogen, and iron, that formed in the supergiant’s
core. Supergiants are a major source of heavy elements throughout the universe.
Astronomers believe Earth and all its living organisms are composed of elements
formed in the interiors of stars, especially supergiants that exploded as
supernovas.
C
|
Age
|
Astronomers have identified stars that are
as young as 25,000 years old and others that are more than 10 billion years
old. The Sun is 4.6 billion years old. Astronomers believe that once
medium-sized stars are fully formed, they may last up to 10 billion years.
While the Sun’s core will probably run out of hydrogen in about 7 billion
years, the very hottest stars spend their energy much faster and die—or become
dark and cool—much more rapidly.
The locations of different stars can
help astronomers determine the age of these stars. Most O (blue) and B
(blue-white) main-sequence stars are not randomly distributed throughout the
sky. Instead, these stars tend to be grouped into associations lying along the
spiral arms of the Milky Way. Some of these groups, such as one in the
constellation Perseus, appear to be expanding. By extrapolating its expanding
motion backward, astronomers can determine that the age of the expansion (and
therefore the stars) is less than two million years. On an astronomical
timescale, O and B stars are extremely young.
VI
|
IMPORTANT TYPES OF
PECULIAR STARS
|
Although most stars are normal members
of the main sequence, astronomers have also identified stars with variations in
brightness (known as variable stars) and stars with unusual spectra.
A
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Pulsating Variables
|
Pulsating variables are stars that
rhythmically brighten and fade due to changes in the stars’ outer layers. Pulsating
variables change in luminosity, temperature, and other characteristics as they
expand and contract. Astronomers have identified two types of pulsating
variables: periodic stars, such as Cepheid variables and RR Lyrae stars, and
semiregular stars. Light emitted by periodic stars increases and decreases in
regular cycles, while light emitted by semiregular stars fluctuates in
irregular cycles.
A1
|
Cepheid Variables
|
Cepheid variables are a type of yellow
supergiant star that vary in size and brightness in regular intervals, or
periods, that last from 1 to 50 days. Typical Cepheid variable stars contain
from 5 to 20 times more mass than the Sun does and shine about 10,000 times
more brightly than does the Sun. As a Cepheid variable star pulsates, it
expands beyond its equilibrium size, the size at which the inward force of
gravity is offset by the outward pressure of the burning gases. The star’s rate
of expansion slows as the force of gravity exceeds the outward pressure of the
burning gases. At a certain point, the star begins contracting and continues
until the outward pressure inside the star forces it to begin expanding again.
This process causes the star to expand and contract, changing up to 30 percent
in size over regular intervals. Delta Cephei, a member of the northern
constellation Cepheus, is a Cepheid variable about 950 light years from Earth
with a period (the time it takes for a cycle of expansion and contraction) of
5.37 days. Delta Cephei expands and contracts rhythmically, heating and cooling
as a result of internal instability. The longer the period of a Cepheid
variable is, the greater its intrinsic or absolute brightness is. Astronomers
use this correlation to calculate the distance between Earth and remote Cepheid
variable stars, which provide valuable distance indicators for measuring the
extent of the Milky Way and the distance to other galaxies.
A2
|
RR Lyrae Stars
|
Named after the prototype star in the constellation
Lyra, these pulsating variable stars have periods ranging from 88 minutes to
just less than a day. RR Lyrae stars are old, giant stars mostly found in
globular clusters. These stars all have about the same luminosity, regardless
of the length of their period. As a result, these stars are important distance
indicators for establishing the size of our Milky Way galaxy.
A3
|
Mira or Long Period
Variables
|
Mira variables, named after Mira Ceti, the
type star, are cool but luminous stars, with absolute brightness typically 3000
times that of the Sun. Mira variables change in brightness by at least 10
times, and sometimes by as much as 1000 times. These pulsational changes go in
cycles of 100 to 500 days, so they are often called long period variables.
A4
|
Semiregular and
Irregular Variables
|
Nearly all cool giants and supergiants
show some variability, and are classified as semiregular or irregular variable stars.
The brightness variations of these stars are neither quite punctual nor
regular. Some irregular stars have variations that appear to be random or
chaotic rather than appearing as pulsations. Betelgeuse, a red supergiant star
marking the right shoulder of the constellation Orion the Hunter, is an
irregular variable that has random fluctuations of brightness superimposed on a
six-year cycle of brightness variation that is so small it is seldom noticed by
visual observation.
B
|
Novas
|
A nova is an explosion in a binary
star system composed of a main-sequence star and a white dwarf. The explosion
dramatically increases the brightness of the system (to about 300,000 times
brighter than the Sun) within an interval of about a day. Novas tend to remain
this bright for a few days or weeks and then slowly fade. Novas are different
from supernovas, explosions that typically destroy or radically alter a star,
because the stars in a nova continue to exist much as they were before the
outburst. Astronomers believe that a nova explosion is caused when the
gravitational pull of one star drags the atmosphere of the other star onto its
own surface over a period of between 10,000 to 100,000 years. When the
collected layer of atmosphere becomes thick enough, a thermonuclear explosion
occurs. After a nova outburst, the stars return to their original brightness
relatively unchanged. –Between 25 and 75 novas erupt each year in the Milky Way
Galaxy. One of the brightest nova explosions of the 20th century occurred in
1992 in the northern constellation Cygnus. Named Nova Cygni, this nova was
bright enough to be observed from Earth with the unaided eye.
C
|
Supernovas
|
Astronomers have observed two types of supernovas:
thermonuclear explosions and core collapse supernovas. They believe
thermonuclear explosions occur when a white dwarf exceeds its limiting mass, or
the highest mass at which it can remain stable, and collapses, triggering a
thermonuclear runaway (fusion of higher and higher elements). The energy
released in these thermonuclear reactions causes an explosion that ejects large
amounts of radioactive nickel into space. This radioactive nickel then decays
into iron and cobalt.
Core collapse supernovas occur in stars
containing a core completely transformed by nuclear fusion into iron, which is
incapable of generating more energy. At this point, the core may collapse and
become a neutron star or a black hole (if the star is massive enough). The core
collapse happens very quickly, in a few tenths of a second. Astronomers believe
that as the core collapses, the collapse causes shock waves, which travel from
the star’s core to its outer layers, causing the supernova explosion. Only five
supernovas have been observed in the Milky Way during the past 1,000 years. The
first supernova visible from Earth to the unaided eye in almost three centuries
was SN 1987A, a star that erupted in 1987 in the Large Magellanic Cloud, a
galaxy near the Milky Way. This supernova, which was an erupting blue
supergiant, reached a visual magnitude of 2.9. This is surprisingly bright for
a star that is in another galaxy. (Vega, in the Milky Way, is the brightest
star in the northern hemisphere and has a visual magnitude of 0.3—about 9.7
times brighter than SN 1987A).
D
|
Flare Stars
|
Flare stars are red dwarf stars (M
and K main-sequence stars that are both cooler and smaller than the Sun) that
may suddenly and unpredictably release bursts of light, particles, and radio
waves that can increase the brightness of these stars by a factor of 100 or
more. These bursts typically subside after 10 to 60 minutes. Flare stars are
also known as UV Ceti stars, named after the star UV Ceti in the constellation
Cetus.
E
|
Pulsars
|
Astronomers believe that certain neutron
stars form pulsars, objects that are sources of powerful, pulsating radio waves
in space. As one of these neutron stars condenses, it gathers rotational speed
until it forms a small, dense ball that spins several hundred times per second.
The collapse of the neutron star concentrates its magnetic field until it is
about 1 trillion times stronger than Earth’s magnetic field is. The rapidly
spinning motion of the neutron star and its powerful magnetic field create an intense
beam of radio waves that sweeps out across space as the star rotates. This
sweeping motion creates pulses of radio waves that vary in period (the time
between pulses) from 1.6 milliseconds to 4 seconds. Most of the radio pulsars
in the Milky Way are concentrated in the outer spiral arms of the galaxy’s
disk.
F
|
X-Ray Stars
|
As their name implies, the most
conspicuous energy emitted by X-ray stars is from the invisible X-ray region of
the spectrum. In contrast, nearly all the energy from a typical star such as
the Sun is emitted from the visible part of the spectrum. Approximately 100
X-ray stars associated with neutron stars have been detected in the Milky Way
Galaxy. By the late 1990s, astronomers had discovered a few dozen X-ray stars
that each shared a common orbit with an ordinary star. As the two stars orbit
about a common center of mass, light emitted by the visible star is distorted
by the X-ray star and is observable to astronomers as a cyclical change in
light. Some astronomers believe that X-ray stars produce X rays by attracting
material from their visible star companion. This gaseous material then heats up
as it falls toward the X-ray star until the gas emits X rays.
G
|
Magnetar
|
In the late 1990s, astronomers
found evidence for a new class of neutron stars known as magnetars, so-called
because they have a magnetic field 100 times stronger than the typical neutron
star (see Neutron Stars and Black Holes section above). The magnetic fields of
magnetars, which are about one thousand trillion times more powerful than the
Earth’s magnetic field—are the strongest magnetic fields known in the Universe.
This magnetic field causes a magnetar to behave differently than a typical neutron
star. The magnetic field of a magnetar slows the star’s rotation. In addition,
the magnetic field periodically wrinkles and cracks the star’s solid crust,
releasing gamma rays and X rays from the star’s interior.
During the night of August 27,
1998, scientists detected a burst of X-ray and gamma-ray radiation in the
Earth’s upper atmosphere. Astronomers traced the source of this radiation to a
magnetar located in the Large Magellanic Cloud. This wave of radiation lasted
about five minutes. The energy burst was about equal to the daytime radiation
released by the Sun, and was the most powerful wave of radiation ever detected
on Earth’s surface from outside the Solar System.