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＞＞＞When nature is left alone, a balance is reached among the an..
When nature is left alone, a balance is reached among the animals and plants living in one area. But when man starts his work in nature, the balance is likely to be destroyed. He grows a crop and
then there are no dead leaves to fall on the ground, holding water while it sinks into the surface, or decaying (腐烂) and adding humus (腐殖质) to the soil. Unless a farmer acts with knowledge and skill, he is therefore most likely to make the land poorer. To take the place of the useful matter in the crops that he removes, he uses some kind of fertilizer. Chemical fertilizers are of great help, but the waste products of animals and decaying remains of plants should also be put on the land. In some places, it is a habit to burn waste material lying about, but such burning destroys the useful matter in the dead plants. Although the ashes that are left are valuable when put on the land, a better practice is to bury the waste so that it decays and increases the humus in the soil.In the past, when the world population was much lower than it is now, a man had little difficulty in ordinary times in growing the food that was needed. When a field had been used some years and had become tired, the farmer could move to another place. The tired land then slowly recovered. Gradually grasses and other plants would appear on it and its productive power would slowly return to normal through their decay. But nature, left alone, would take a long time to bring back the land the length of time required would depend on local conditions, but it might well be ten years.It is a bad practice to grow the same crop in a field year after year. If the crop is changed, the land will suffer less because it is treated and used in a different way. Different plants have different effects on the soil. Therefore, a change of crop will do less harm than the growing of the same crop year after year and a regular change to grass will do good to the soil. Much will therefore be gained if different crops are grown one after another, a method known as the rotation (轮作) of crops.72. According to the passage, the land will become poorer________. A. if all the dead leaves are cleared away&&&&&&&&&B. if the humus is increased after the harvestC. if dead leaves decay in the soil by themselves&&&D. if waste plant material lying about is buried73. We can learn from the passage that the tired land has gradually recovered_______.A. when grasses and other plants appear againB. when the treatment is given by nature aloneC. after new grasses and other plants have decayed againD. after nature has been left alone for several months 74. A modern farmer can hardly move to another place as he did before because_______.A. the productive power of a new field isn't higher than that of an old oneB. there are few free fields left for him to do farmingC. it takes a farmer more than ten years to start farming in a new fieldD. there will be too many grasses in a new field to grow crops 75. It is most likely that the author will go on to ______ in the paragraph following the passage above.A. introduce other methods of planting crops &&&&&&B. deal with how to prevent land getting tiredC. start another topic of how to make use of land D. explain what the rotation of crops is&&&&&&
题型：阅读理解难度：偏易来源：不详
小题1:A小题2:C小题3:B小题4:D略
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据魔方格专家权威分析，试题“When nature is left alone, a balance is reached among the an..”主要考查你对&&日常生活类阅读&&等考点的理解。关于这些考点的“档案”如下：
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日常生活类阅读
日常生活类阅读的概念：
日常生活这一话题主要涉及人们衣食住行等方面的活动。这一话题的选材主要针对人们日常的工作，生活以及学习情况。做这一类题时，最主要的是要把握好人物的活动内容，时间和地点。 日常生活类阅读题答题技巧：
【题型说明】该类文章内容涉及到人们的言谈举止、生活习惯、饮食起居、服饰仪表、恋爱婚姻、消遣娱乐、节日起源、家庭生活等。文章篇幅短小，追根溯源，探索各项风俗的历史渊源，内容有趣。命题也以送分题为主，如事实细节题、语义转换题、词义猜测题和简单推理判断题等。虽然这类文章读起来感觉轻松，试题做起来比较顺手，但绝不能掉以轻心。因为稍不留神，就会丢分。 　　【备考提醒】为了保证较高准确率，建议同学们做好以下几点： 　　1、保持正常的考试心态。笔者在教学中发现，越是容易的试题，同学们越是容易失分。为什么呢？因为在这种情况下，同学们极易产生麻痹思想，认为题目好做，就不引起高度重视，于是思维不发散、不周密。而命题人就是利用同学们的这一弱点，设计陷阱题。所以，无论试题难易与否，我们都要保持正常的考试心态。试题容易，不欣喜；试题难，不悲观。 　　2、根据前面讲到的方法，认认真真、细细心心做好事实细节题。 　　3、做好语义转换题。这类题是根据英语中一词多义和某些词语在文中能表达一定的修辞意义的原则而设计的。要求同学们解释某生词的含义，确定多义词或短语在文中的意思，确认文中的某个代词所指代的对象，或者对英语中特有的表达、格言、谚语进行解释。这种题要求同学们一定要根据上下文猜测词义或理解句子，切不可望文生义。 　　4、做好简单推理判断题。简单推理判断题要以表面文字为前提，以具体事实为依据进行推理，做出判断。这种推理方式比较直接，只要弄清事实，即可结合常识推断出合理的结论。
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365528220532399163213212390742366493StarsIntro
THE NATURES OF THE STARS
The stars surround you.
At night they are everywhere,
in the daytime, one, our Sun, dominates, its
brilliant light washing the others away until twilight yields to
They are the givers of light and life.
ourselves, we must know the stars."
This site is linked to Spectra and The Hertzsprung-Russell (HR)
For a broader continuous story see From the Sun to the Stars.
Return to .
The purpose of this site is to provide a deep, non-technical
review of stars and their natures for the beginner.
presents facts about stars as we know them without delving into
the details of discovery.
Parallel sites that explore the spectra of the stars and the HR Diagram examine how we have learned so
much of what is presented here.
The three sites are linked,
allowing you to go back and forth among them to see how stars are
born, live their lives, and die, in the process creating other
stars, perhaps other earths, and all that is around us.
A star is a body that at some time in its life generates its
light and heat by nuclear reactions, specifically by the
fusion of hydrogen into helium under
conditions of enormous temperature and density.
When hydrogen
atoms merge to create the next heavier element, helium, mass is
lost, the mass (M) converted to energy (E) through Einstein's
famous equation E = mc squared, where "c" is the speed of light.
is powered by hydrogen fusion, as
are many of the other stars you see at night.
The fusion does
not take place throughout the star, but only in its deep
interior, in its core, where it is hot enough.
The temperature
at the center of the
is 15.6 million degrees Kelvin (K =
centigrade degrees above absolute zero, -273 C), and the density
is 14 times that of lead.
About 40% of the mass of the Sun,
occupying about 30% of the radius, is capable of fusing hydrogen.
Even under these extreme conditions, the Sun (as well as all
other stars) is still a gas throughout.
That said, things get
more complicated, as nature creates "substars" called "brown dwarfs" that do not have enough mass and therefore internal heat to run full
Even though they do not abide by the formal definition
of a "star," they are still referred to as "stars" even if their
masses are not much greater than those of planets.
In the second century BC, the Greek astronomer Hipparchus divided
the stars into six brightness groups called magnitudes
(now apparent visual magnitudes (m or V), first magnitude
the brightest, sixth the faintest.
The system is still used
today, though with a mathematical definition (a star of one
magnitude is 2.512... times brighter than the next fainter) that
takes the very brightest stars and planets through magnitude zero
and into negative numbers.
Through the telescope we see much
fainter, to near 30th magnitude (4 billion times fainter than the
human eye can see alone).
Though stars bear some resemblance to
the , they appear as points in the sky
because they are so far away, the nearest, , four light years away.
light year is the distance a ray of light will travel in a
year at 300,000 kilometers (186,000 miles) per second, so one
light year is about 10 trillion kilometers (63,000
Astronomical Units, where the AU is the average distance
between the Earth and the Sun).
The stars are so far that
distances were not measured until 1846, by means of
parallax (viewing the star from opposite sides of the
Earth's orbit).
The most distant stars the unaided eye can see
are over 1000 light years away, which is about the practical
limit of parallax measures.
The apparent visual magnitude of a
star depends on true visual luminosity (in watts) and distance.
To compare true visual luminosities, astronomers calculate the
absolute visual magnitude (M), the apparent magnitude the
star would have were it at a distance of 32.6 light years (10
parsecs, where the parsec is the professional unit of
distance, equal to 3.26 light years).
The absolute visual
magnitude of the Sun is +4.83.
Absolute visual magnitudes range
from around -10 (a million times more luminous than the Sun) to
below +20 (a million times fainter).
We need to set the stars in context.
All those you see at night
are part of our local collection of stars, all part of our
Trillions of other galaxies flock the Universe, ours one of the
larger ones.
The principal part of our Galaxy (our own with a
capitol "G") is in the shape of a flat disk about 100,000 light
years across that contains some 200 billion stars.
Our Sun is
toward the nominal edge, about 25,000 light years from the
center, the whole structure at the Sun's distance rotating with a
period of 200 million years.
The "edge" is not sharp, but just
gradually fades away to much greater distances.
A large portion
of the disk's stars are set within pinwheel-like spiral arms that
over millions of years come and go, the stars moving in and out
of them as they orbit the Galaxy's center.
The arms seem to come
off of a central bar.
Since we are in the disk, we see the
combined light of its billions of stars around our head as the
famed "," the
(which contains a supermassive
of three million solar
masses) located behind the thick star clouds of .
of the disk is about 10 billion years.
Surrounding the disk is a
thinly populated rather spherical halo that seems to date to
about 14 billion years.
All the 's stars orbit the Galactic center in .
If they did not, the Galaxy would
collapse into itself.
Orbits of disk stars (including the Sun)
are more or less circular, while those of halo stars (including
) are more elliptical.
At a velocity of 240 kilometers per second, the Sun (25,000 light
years from the center) takes about 200 million years to go all
the way around.
Because the orbits are all different (even if
only slightly so), all stars move relative to each other.
long enough period of time (far longer than the human written
record), the
dissolve and new ones will appear.
Stellar motions have two
The proper motion is the annual angular
displacement of the star across the line of sight relative to the
It's usually measured in fractions of a second of arc per
year, where a second of arc is 1/3600 of a degree.
At 10 seconds
of arc per year,
the angular speed record.
From the star's distance, we can
calculate the true speed across the line of sight (the
transverse velocity) in kilometers per second.
The Doppler shift in the star's
spectrum then gives the radial velocity, that along the
line of sight.
Combination with the transverse velocity yields
the star's true velocity (again, relative to the Sun).
speeds for disk stars relative to the Sun are in the low tens of
kilometers, while those of halo stars can be in the upper tens or
even hundreds of km/s.
High velocity stars tend to have lower
than does the Sun
and are generally older.
From the combination of local motions
of stars, we find that the Sun is moving at about 15 km/s toward
a point (the ) that lies between
Motions of other stars
relative to us allows their Galactic orbits to be calculated,
which yields a picture of the Galaxy's dynamics.
To create the conditions for such "thermonuclear fusion," stars
must be massive.
The Sun has the mass of 333,000 Earths.
can range up to about 100 times the mass of the Sun (at which
point nature stops making them) down to around 7.5% that of the
Sun, at which point the internal temperature is not high enough
to run the full range of
(which requires at least 7 million degrees Kelvin).
"Substars"
below the 7.5% limit, called ","
do exist in significant numbers however, and down to around 1/80
the solar mass (13 Jupiter-masses) can fuse their natural deuterium (heavy hydrogen, with
an extra neutron).
The lower limit to brown dwarf (substellar)
masses is not known.
Masses are
and can be calculated
from luminosity and temperature using the theories of stellar
structure.
Stars are made of the same
as found in the Earth, though not in the same
proportions, the
found from the stars' spectra.
Most stars are made almost
entirely of hydrogen (about 90% by number of atoms) and helium
(about 10%), elements that are relatively rare on our planet.
About a tenth of a percent is left over, that tenth containing
all the other elements found in nature.
Of these, oxygen usually
dominates, followed by carbon, neon, and nitrogen.
metals, iron usually dominates.
Nevertheless, there is only one
atom of oxygen in the Sun for every 1200 hydrogen atoms and only
one of iron for every 32 oxygen atoms.
However, within this
tenth of a percent, the proportions of the numbers of atoms in
the Sun is rather similar to what we find here, in the Earth's
Other stars can deviate considerably, depending on their
states of aging or upon where they are in the Galaxy.
Halo stars, including globular
clusters, typically have heavy element contents only a hundredth
that found in disk stars, the result of their being older.
Stars are supported, kept from shrinking under their own gravity,
by energy generated by internal fusion of light atoms into heavier ones.
hydrogen into helium can take place only under the extreme
conditions of temperature and density found in a star's deep
The "proton-proton chain" operates in ordinary stars (those that have not yet begun
the ) with masses more or
less like that of the Sun and under (while higher mass stars do
it by the .
It begins when two
protons (bare hydrogen atoms) ram together strongly enough to
overcome the mutual repulsion caused by their positive electric
charges and get close enough to stick together under the "strong
force" (which operates only over a very short range).
One of the
protons ejects its positive charge in the form of a "positron," a
positive electron that hits a normal electron to generate energy
in the form of .
The conversion creates a deuterium (heavy hydrogen)
nucleus as well as a tiny particle called a "neutrino."
Detection of neutrinos on Earth allow us to "see" directly into
the solar center.
The fusion of the deuterium with another
proton produces a light form of helium (with two protons and one
neutron), while the fusion of two light helium atoms into a
normal helium atom with two protons and two neutrons (with the
ejection of two protons) completes the process, each reaction
generating heat and light as a result of a slight loss of mass.
Higher mass stars (with masses greater than about 1.5 times that
of the Sun) fuse hydrogen into helium via the "carbon cycle,"
which works only under high-temperature conditions, but is then
more efficient than the .
It begins when a normal carbon atom (C-12, with 6 protons and 6
neutrons) picks up a proton to make radioactive nitrogen-13, one of whose
protons ejects a positron (positive electron) to make stable
carbon-13 (with the additional ejection of a neutrino).
13 plus a proton makes normal nitrogen-14, while an additional
proton collision makes oxygen-15, which (like N-13) decays into
nitrogen-15.
The N-15 collects another proton, and then falls
apart into the original carbon-12 and a helium nucleus.
event produces some energy either itself or through the
collisions of positrons and electrons.
The space between the stars is not empty.
Here we find the
chaotic, lumpy interstellar medium (ISM), 99
percent of which (by mass) is made of gas, the rest of
interstellar dust.
The gas has about the same chemical
as do the stars: 90
percent hydrogen, 10 percent helium, and a small remainder made
of all of the heavier chemical elements.
In our part of the Galaxy there is on the average only about one
atom of gas per cubic centimeter and one grain of dust per cubic
meter, but volumes are so large that the ISM constitutes more
than 10 percent of the Galaxy's mass.
The dust grains are tiny,
typically only about a thousandth of a millimeter in diameter.
They are usually made either of silicates or of carbon into which
are mixed ices and metals absorbed from the gas.
The silicates
were mostly produced in the winds of
in which oxygen dominates carbon, while the
carbon grains came from Mira-type
or their higher mass versions.
The vast majority of
the ISM resides in the inner part of the Galaxy's disk, and most
of that in the spiral arms, where
of cold, opaque, molecular hydrogen birth stars.
The dense molecular
clouds contain more than 150 species of molecules that
include carbon monoxide, water, formaldehyde, ammonia, alcohols,
acetic acid, and odd carbon structures not seen on Earth.
molecules can form because the thick dust keeps out the energetic
starlight that would break them up.
Complexes of the dark clouds
are easily visible to the naked eye and make much of the
the Incas made
"dark constellations" of them.
They are set into a partially
ionized tenuous intercloud medium, which also harbors less dense
neutral hydrogen clouds that radiate a powerful radio spectrum emission line at a wavelength of
21 centimeters, from which we can determine much of the Galaxy's
structure, including the location of its spiral arms.
this messy mixture are hot (100,000 degrees or more) expanding
bubbles of very thin gases produced by the blast waves of
exploding stars ().
density of the ISM diminishes rapidly perpendicular to the
Galaxy's disk, and is nearly absent in the halo except for hot
gas blown out by supernovae, which then cools and falls back.
Hot stars ionize lumps of the ISM, mostly the stars' surrounding
birth clouds, to produce .
The light from cooler stars can scatter
off the dust grains to make .
Interstellar dust
scatters and absorbs starlight, making distant stars of the disk
look too faint.
It also makes the stars look redder than they
should be for their classes or temperatures, which allows us to
determine the amount of dimming and to correct observed
luminosities to their true values.
The space between the stars is filled with dusty gas.
Thick dust
clouds can even be seen with the naked eye within the Milky
Way blocking the light of distant stars and providing much of the
Milky Way's structure.
Interstellar matter is compressed by the
Galaxy's winding spiral arms.
The clouds can be further
compressed through collisions or by blast waves from exploding
high-mass stars ().
of matter therefore form within the interstellar clouds.
their gravity is great enough, they can condense into one or more
Contraction causes more rapid spin, which creates a disk
around the birthing star, from which it can draw matter.
condensation within the disk can create planets (or even stellar companions).
The contraction of forming
stars raises the internal temperature, finally to the point of
ignition of .
Gravity would
like to make the star as small as possible, but the fusion
reactions stabilize it and keep it from contracting any further.
The whole life story of a star from here on out is told by the
battle between gravity and nuclear fusion, first one, then the
other getting the upper hand.
New high mass stars commonly light
up their surroundings to produce
like the .
For decades, astronomers predicted the existence of substars we
now call "brown dwarfs," stars too small and light (of
insufficient mass, less than 0.073 solar masses) to run the full
(from ordinary hydrogen
to helium).
After all, the
process should "know" nothing of the conditions under which
nuclear reactions should turn on.
Brown dwarfs (which are still
called "stars") turned out to be so cool that only new infrared
technologies could find them.
We now know they are very common,
so common that new , L and T
(cooler than M) had to be made for them.
Between 0.073 solar
masses (78 Jupiter-masses) and 13 Jupiter-masses, brown dwarfs do
fuse their natural
(heavy hydrogen, with an extra neutron) to helium.
Jupiters, fusion stops altogether.
As noted above, the lower end
of brown dwarf masses is not known.
They quite likely overlap
the masses of planets.
current definition made from the "bottom up," accumulated from
dust in disks surrounding new stars, while stars (including brown
dwarfs) are made from the "top down," by direct condensation from
interstellar gases.
But here even the definitions become
confused and might overlap as well.
As a new star condenses from a gaseous lump in interstellar
space, it spins faster, the outer parts of the contracting cloud
spinning out into a dusty disk.
The dust particles, in orbit about the new star, accumulate,
building themselves into planets.
Here at home, the planets that
formed close to the Sun (Mercury through Mars) were in an
environment too hot to incorporate much water or light atoms like
hydrogen, so they are made of heavy stuff like iron, silicon, and
In the outer System, the planets contain huge amounts of
hydrogen and helium and could grow large, their satellites made
largely of water ice.
grow planets too,
could be quite different from our own and that are now being discovered.
There are many kinds and
Those that are actively fusing hydrogen
into helium in the middle, that is, in their cores (either
through the
or the carbon cycle), are called "main sequence" stars.
(For historical reasons, main sequence stars are also commonly
referred to as "dwarfs").
The main sequence is the first stage
following birth.
In general, main sequence stars have chemical
compositions similar to that of the Sun.
The higher the mass of
the main sequence star, the greater its diameter and the higher
its surface temperature.
Dimensions range from about 10% the
size of the Sun (which is 1.5 million kilometers -- 109 Earths --
across) to just over ten times solar, and surface temperatures
from under 2000 degrees Kelvin to about 49,000 K (the Sun's
surface is at 5780 K).
Around the beginning of the 20th century,
astronomers divided the stars (of all kinds including giants, ,
and others) into seven basic lettered groups that they later
learned were related to their surface temperatures, which for the
main sequence are: O (above 31,500 K), B (10,000 - 31,500 K), A
(7500 - 10,000 K), F( K), G(5300 - 6000 K), K(3800 -
5300 K), and M (2100 - 3800 K).
A century later, two more
classes were added to account for faint red stars turned up by
new technologies: class L (1200 - 2100 K) and T (below 1200 K),
the whole set now OBAFGKMLT.
Class L is a mixture of dwarfs and
substars, while class T consists
entirely of brown dwarfs.
The Sun is a G star.
The system is
decimalized, making the Sun class G2.
Examples of main sequence stars are Acrux, , Sirius, Porrima, ,
The classes are
actually derived from the stars' spectra.
The stellar astronomer's
greatest tool is the , a plot of
absolute visual magnitude against spectral class, in which we can
see nearly all of the stages of stellar life and death.
the main sequence is a band that runs from the highest-mass
hydrogen-fusing stars at the upper left to the lowest masses at
the lower right.
Since the color of a heated body depends on temperature, the
take on different,
though subtle, colors, from slightly reddish or orange for class
M to orange-yellow for K, through yellow-white to bluish for
classes B and O.
Star colors can be noted rather easily even
with the unaided eye, especially when those close together (a in
pairings) contrast against
each other.
Stars of classes L and T, none of which are visible
to the naked eye, range from red through deep red to "infrared"
(these optically invisible under any circumstances).
stars like , whose blue spectra
have been removed by , are also deep red.
Most of these are advanced giants.
Color can be expressed
numerically by the difference in magnitudes measured at different wavelengths.
The observed color
of a star compared to the color expected from the spectral class
allows the calculation of the dimming of the starlight by interstellar dust.
Main sequence (dwarf) stars have only a certain amount of
internal fuel available within their hot cores.
hydrogen fuel has all turned to helium, the stars begin to die
and to produce a number of other different kinds: lower mass
stars become , while those of higher
mass (above roughly 8 or 9 solar masses) into supergiants.
Giants then die as white dwarfs, while supergiants explode
The whole process is
commonly known as stellar evolution.
Because higher mass
stars use their hydrogen fuel much more quickly than lower mass
stars, those of higher mass live shorter lives.
The Sun has a 10
billion year main sequence lifetime (of which half is gone).
most massive stars live only a couple million years, the least
massive for trillions, so long that no star with a mass less than
about 0.8 solar masses has ever died in the history of the
From theory, we calculate that such a 0.8 solar mass
star should live for about 12-13 billion years.
The Galaxy
should be about as old as its oldest stars, and is thus 12-13
billion years old, in accord with the 13.7 billion year age of
the Universe found from its .
Begin with stars more or less like the Sun, those with masses
from about 0.8 times that of the Sun to about 5 times the solar
When the fuel in a solar-type star's core runs out, the
helium core contracts under the effect of gravity and heats up.
Hydrogen fusion then expands into a shell around the old
burnt-out core, and so much energy is produced that the star
brightens and expands by many times over, the expansion cooling
the surface, turning the star into a class M red giant.
When the core temperature hits around 100 million degrees Kelvin,
the helium is hot enough to fuse into carbon (through the near-
simultaneous collision of three helium atoms) and even a bit
further, into oxygen.
This new power source stops the core's
contraction and the star stabilizes for a time, dimming and
heating somewhat at the surface.
We commonly see these helium-
fusing stars as yellow-orange type K giants.
Good examples of giant stars are Aldebaran and Arcturus.
Such stars can have diameters
tens of times that of the Sun.
The giant and subsequent stages
up to the actual death of the star (the end of nuclear fusion)
takes roughly 10 or 20 percent of the main sequence lifetime.
From about 5 solar masses to 9 or so, helium fusing stars have
higher temperatures and may appear as class F and G giants and
Giants can also be defined strictly by their spectra.
On the HR diagram, the giants run roughly from the
middle toward the upper right (higher luminosity), where they are
fusing helium, are about to do so, or have already done so.
Class A and B giants are only somewhat cooler than dwarfs of the
same absolute visual brightness and are not yet fusing helium.
Among G and K giants, because of their lower gas densities,
temperatures are up to a few hundred degrees cooler than they are
for main sequence dwarfs of the same class.
Between the dwarf and giant stages, stars appear as
subgiants.
Like , dwarfs, and supergiants, they can be defined by their
spectra and position on the .
In the context of stellar evolution, they are stars that have
just given up core hydrogen fusion or are about to do so and,
with helium cores, are making the transition to becoming true
When more massive stars (2 to 8 times that of the Sun) pass
through mid-temperatures either on their way to fusing helium or
during various stages of helium fusion, they can become unstable
and pulsate in size, temperature, and luminosity.
The first of
these discovered, , gave
the name "Cepheid" variable to the group.
Cepheids, usually
classed as F and G supergiants (though not as massive as true supergiants), vary by a couple to a few
magnitudes over periods of one to 100 days.
A strict relation
between absolute magnitude and pulsation period allows us to
determine their distances (period gives absolute brightness, and
comparison to apparent brightness yields distance.)
Cepheids are
the major keys to learning distances to .
The brightest Cepheid in the sky is
Polaris , though the variations are too
small to be seen by eye.
Cepheids occupy the upper range of the
's "instability strip."
When the helium in the core has turned to carbon and oxygen, the
core shrinks again, and the helium begins to fuse to carbon and
oxygen in a shell around the old core, this shell surrounded by
another one fusing hydrogen into helium, the two turning on and
off in sequence.
The star now brightens again, expands even
more, and becomes cooler and even redder than before.
star brightens it becomes unstable and begins to pulsate, the
pulsations making it vary, or change in brightness.
become so huge, near or greater than the orbit of the Earth, that
the pulsations can take a year or more.
The first of these
in , changes from second or third magnitude to
tenth, becoming quite invisible to the naked eye.
Such stars are
now called "long-period variables" (LPVs) or "Mira
variables."
Thousands, all cool class M giants, are known.
On the , such advanced giants
are at the cool end of the "giant branch," the Miras occupying the coolest and
brightest portion.
In astronomical jargon, such stars are called
asymptotic giant branch stars (or AGB stars)
because of the appearance of their distribution on the HR diagram.
The gases of red giants can circulate upward to the tops of the
stars, carrying the by-products of nuclear fusion with them.
Oxygen is normally more abundant than carbon.
If conditions are
right, the surfaces of some stars can change their chemical
compositions, some becoming very rich in the carbon that was made
below by helium fusion, resulting in the reversal of the normal
Mira variables and other old red giants thus divide into
oxygen-rich stars like
itself and carbon stars such as 19 Piscium and
Raised up along with the carbon are elements such as
zirconium and many others that have been made in a huge variety
of nuclear reactions that go on at the same time as helium
Other stars' surfaces are enriched in helium and
Such huge giant stars have low gravities and lose mass through
powerful winds that blow from their surfaces.
Some of the gas
condenses into molecules and dust.
There may be so much that the
star can be buried in it and become invisible to the eye, the
glow of the heated dust seen only by its infrared (heat)
radiation.
Oxygen-rich giant stars make silicate dust, while
carbon stars make carbon-dust similar to graphite and soot.
of the dust that inhabits interstellar space began this way,
though since inception it has been highly modified in the freezer
of interstellar space.
These stars therefore play a powerful
role in later star formation.
The winds are so strong during the
giant stage of a star's life that it can lose half or more of its
mass back into space, whittling itself down to little more than
the parts that underwent nuclear fusion.
As a giant star loses almost all of its remaining outer hydrogen
envelope, it comes close to revealing its intensely hot core.
fast wind from the core first compresses the inner edge of the
old expanding wind.
High-energy radiation from the hot core then
lights up this inner compressed portion, which is now many times
the size of the whole Solar System.
These illuminated clouds,
which can be quite beautiful, were discovered by
William Herschel around 1790, who termed them"planetary nebulae" for their disk-like
appearances (they have nothing else to do with planets).
best known is the
Their complex appearances depend to
a degree on how matter is lost from the giant stars that make
Expanding at rates of tens of kilometers per second, they
last no more than a few tens of thousands of years.
From their
analyze their chemical compositions, and find that many are
enriched in the by-products of prior nuclear fusion in the parent
advanced .
dissipates into the
gases of , it leaves behind
the spent, old core that now includes the dead nuclear fusing
These stars, made of carbon and oxygen and compressed
under their own gravity, have shrunk to about the size of Earth.
The first ones found (, Procyon-B, and ) were fairly hot and white, so the class acquired
the name "white dwarf" to discriminate it from the main sequence
of stars (which were originally called "dwarfs" to distinguish
them from the giants).
Though small, white dwarfs still contain
near the mass of the Sun, giving them astonishing average
densities of a metric ton per cubic centimeter.
The tremendous
outward pressure provided by tightly packed "degenerate
electrons" (which behave like waves that keep them from getting
closer) prevents gravity from shrinking white dwarfs any further.
White dwarfs are therefore also called degenerate stars.
These small stars, the remains of stars that began their lives
between 0.8 and 9 or so solar masses, no longer have any source
of energy generation and are destined only to cool.
The cooling
time is so long, however, that all white dwarfs ever created are
still visible, though the oldest are becoming cool, dim, and
(There is no such thing as an invisible, cold "black
The age of the Galaxy calculated (with the aid of
theory) from the oldest white dwarfs roughly agrees with that
derived from the coolest (lowest mass) evolved main sequence
On the , they fall in a
line rather parallel to, but far fainter than, the main sequence
Masses of white dwarfs are tied to the original stellar
birth masses and range from about half a solar mass (for a birth
mass roughly solar) to a limit of 1.4 times that of the Sun for a
birth mass of 8 or 9 solar.
Beyond 1.4 solar masses, the
degenerate electrons can no longer provide support, and the core
must collapse, the star exploding as a supernova.
Overflow of the limit by mass
accreted from a close
also produce collapse and a
supernova.
As they start to die, higher mass stars (those with masses over
about 9 or 10 times that of the Sun) initially develop the same
way as giants, but then their course of evolution becomes very
different.
High mass stars are already large and luminous.
their dead helium cores contract, heating and firing to fuse the
helium to carbon and oxygen, the stars expand to approach the
sizes of the orbits of the outer planets, becoming distended red
"supergiants."
first magnitude
in Orion and Antares in .
Supergiants are so massive, in spite of
great mass loss through huge winds, that nuclear fusion can
proceed farther than it can in ordinary giants.
When the helium
runs out, the carbon and oxygen mixture compresses and heats,
causing it to fuse to a mixture of neon, magnesium and oxygen.
Hydrogen and helium fusion had already moved outward into nested
shells around the core.
When carbon fusion dies out in the core,
leaving a mix of neon, magnesium, and oxygen, it too moves
outward into a shell.
The neon-magnesium-oxygen mixture now in
the core then heats and fuses into a mix of silicon and sulfur,
each fusion stage taking a shorter period of time.
During the
course of their evolution, red supergiants can also contract some
and heat to make blue supergiants.
The great mass-loss suffered
by supergiants can strip some of them of their outer envelopes to
the point that we see huge surface enrichments of helium,
nitrogen, and carbon that have been made by nuclear fusion.
for them scattered across the top of the .
Finally, the silicon and sulfur fuse to iron, an element that is
incapable of energy-generating fusion reactions.
Gravity now
wins the war that has been going on for the star's lifetime, and
since the iron refuses to support itself, the core
catastrophically collapses.
The iron breaks down into its
component particles, protons, neutrons, and electrons (the
constituents of atoms), and the whole mass gets compressed into a
tight ball of neutrons only a few tens of kilometers across.
collapse produces a shocking blast wave that rips through the
surrounding nuclear fusing shells and the remaining outer
envelope, and rips the rest of the star apart.
On Earth we see
the star explode in a grand "
supernova," an event so powerful it is easily visible even in
another galaxy a huge distance away.
The part of the star that
is exploded outward is so hot that nuclear reactions produce all
the chemical elements, including a tenth of a solar mass of iron,
which then blend with the gasses of interstellar space, out of
which new stars are formed.
also be caused by the collapse of a
There are ways of making
than through core collapse.
Nevertheless, supernovae are still
rare, taking place in our
or three times a century.
Most are hidden from us by the vast
clouds of dust that birth the stars.
On Earth we observe about
five supernovae per millennium, and have not seen one since
Kepler's Star of 1604
(probably created in the collapse of a white dwarf, as
described later).
The great supernovae of 1006
(the "Chinese Guest Star"), and 1572
(Tycho's Star)
were visible in daylight.
Our knowledge of supernovae comes
almost entirely from observing them in other galaxies, the best of these exploding in
1987 () in the Large
Magellanic Cloud, a companion to our Galaxy some 165,000
light years away.
But keep your eye on
or Antares, which are quite good candidates
for core collapse.
An even better candidate is the southern
hemisphere's , which
underwent a huge eruption in the 19th century and produced a
surrounding nebula, a vast
cloud of dusty gas.
The star should go off within the next
million years or so.
At their current distances, the explosions
of such stars would rival the brightness of a crescent Moon.
blast is so powerful that it if occurred within 30 or so light
years, it would probably damage the Earth.
Fortunately, no
candidate is nearly that close (though such nearby events have
almost certainly happened in the past).
As the debris of a supernova clears, we see a gaseous expanding
shell around the old star, the "supernova remnant," which
consists of the debris of the explosion that is rich in the by-
products of myriad nuclear reactions mixed in with local
interstellar matter that is compressed by the mighty blast.
Supernova remnants are readily identifiable by their X-rays and
radio radiation.
We believe all the iron in the Universe has
come from such (and related) explosions.
Indeed, between
ordinary , , and supernovae, all the elements other than hydrogen
and helium (and some lithium) were created in or by stars.
most famous supernova remnant is the Crab
Nebula in , the remains of
the great supernova of 1054, which was well observed by Chinese
astronomers.
Tens of thousands of years after a supernova event,
we may still see the blast waves sweeping through the gases of
interstellar space, compressing and heating them and perhaps
making new stars.
At the center of the expanding cloud is a lone neutron star
spinning many times per second, with a mass greater than the Sun,
a diameter the size of a small town, and an amazing density of
100 million tons per cubic centimeter.
As white dwarfs are supported by "degenerate
electrons," neutron stars are supported by degenerate neutrons.
The magnetic fields of such collapsed stars are magnified along
with the density to strengths millions of millions of times that
The magnetism is so strong that radiation is beamed
out the magnetic axis.
The axis is tilted relative to the
rotation axis (like that of the Earth), and wobbles around as the
little star spins, the beamed energy spraying into space.
distance, the star looks like a lighthouse: if the Earth is in
the way, we get a blast of radiation, and from here see the
neutron star as a "pulsar.
Young pulsars emit from low-energy radio waves through high-
energy X-rays and gamma rays.
As the pulsar ages, it slows, and
finally emits only radio waves, which is the case for most of the
600 or so pulsars known.
When the rotation period is about 4
seconds there is insufficient energy for the pulsar to be seen at
all, and it disappears from view.
Not fusing anything, the
neutron star is held up forever against gravity by pressure
exerted its own extreme density.
The collapsing star of a supernova will turn into a neutron star
only if its mass is less than about two or three times that of
If the mass is greater, then even the star's huge
density cannot hold gravity back, and instead of a neutron star
the supernova creates a "star" that nothing can support against
gravity, and the body contracts forever.
At a small enough
radius, the gravitational force becomes so great that light can
not escape, and the star disappears forever into a collapsing "black
What we refer to as the black hole is actually a kind
of "surface" at which the velocity required for escape equals
light-speed.
What goes on inside is unknown.
The center of our
, 26,000 light years away, contains a
that carries some three million solar
Most of stars you see at night have companions, with a great many
obviously double ("binary") even through a modest telescope.
components of some double stars are nearly equal in mass and
brightness.
More commonly, one dominates the other, sometimes to
the point where a little companion is not really visible at all,
and detectable only with the most sophisticated techniques.
the lowest end, we have stars with low-mass brown dwarfs for
companions.
The stars of some doubles are so far apart that they
take thousan others are so close that they revolve around
each other in only days or even hours.
Gravitational theory
allows us to measure the masses of the stars from the orbits'
indeed such measurements are the only way in which we
can find stellar masses.
Examples of visually-seen double stars
are , Acrux, , Albireo, and Mizar.
Double stars are vitally important in the measure of stellar
masses, which are derived from Kepler's Laws as generalized by Isaac Newton.
lesser star goes around a stationary more massive one, as seen
for example for , Castor, or Algieba.
The first law states that the
orbit must be a conic section (circle, ellipse, parabola, or
hyperbola), here specifically an ellipse with the more massive
member at one focus, the second law that the orbiting star speeds
up in a known way as it gets closer to its mate, slows down as it
gets farther away.
The crucial third law states that the square
of the orbital period in years equals the cube of the average
distance between the stars divided by the sum of the masses (in
solar masses), which can then be found.
In reality, the two
stars orbit a common center that lies on a line between them
positioned from each in inverse proportion to the mass ratio.
The sum of masses along with the location of the center of mass
(and thus the mass ratio) then gives the individual masses, which
can be used to test theory.
The mass of the Sun is found using
the orbit of the Earth (whose mass is inconsequential).
Stars can also bond into more complicated multiples.
two kinds, stable "hierarchical" systems and unstable "trapezium"
In the first, a distant star orbits an inner double
(which it senses gravitationally as one) to make a triple (as in
system), or two
doubles may orbit each other as a quadruple, of which Epsilon Lyrae (THE famed "Double-
Double") is the prime example.
In more complex systems, a star
or even another double can orbit an inner triple or double-double
to make a quintuple or sextuple system (like Mizar-Alcor or Castor).
The structures of the orbits
will depend on relative masses.
In the second kind of multiple,
named after the
Orionis) in 's , the member stars are all rather mixed
together, which allows close encounters to eject stars until some
kind of stability is achieved.
Trapezium systems must all
therefore be young.
Formation is still contended.
The oldest idea involves simple
When a new star condenses from the interstellar gases,
it spins faster.
If the contracting blob is spinning rapidly
enough, it can separate or otherwise develop into a pair or stars
rather than a single star.
Each of these contracting components
can further separate into a double, producing a "double-double"
star, the most famous of which is fourth magnitude Epsilon Lyrae.
This idea is now widely
discounted.
More likely scenarios involve capture within a dense
stellar environment, fragmentation of the collapsing birthcloud,
and condensation of a companion from a the circumstellar disk
that surrounds a new-born star.
Formation of multiples is even less understood, especially of
stars with distant fragilely-bound members of the sort we find in
If the two stars of a pair are fairly close together, and if the
plane of the orbit is close to the line of sight, each star can
get in the way of the other every orbital turn, and we see a pair
of eclipses, one of which is usually of much greater visibility
than the other.
Eclipsing systems are very important in stellar
astronomy, and are used to help determine masses, to find the
stars' diameters, temperatures, and even to assess shapes in the
cases that the stars' mutual gravities distort each other.
Eclipsing doubles are quite common, the most famous second
In a double star system in which the two have significantly
different masses (by far the most common), the higher mass star
will use its internal hydrogen fuel the fastest and become a
giant first.
We then see a red giant, or maybe a helium-fusing,
orange class K giant coupled with a main sequence star, also very
Eventually, the giant produces its planetary nebula and
dies as a white dwarf.
Good examples of such systems are Sirius and Procyon, each of which are orbited by the
tiny dead stars.
For each of these systems, and for many others,
the white dwarf is by far the LESS massive of the pair, proving
that stars really do lose a great deal of their mass back into
interstellar space.
If the two stars of a double are close together, they can
When the more massive becomes a giant, its surface
significantly approaches that of the other star.
The lower-mass
main sequence star can then raise tides in the giant, distorting
If the two are close enough, matter can flow from the giant
to the main sequence star.
Good examples that display such
behavior are
and Sheliak.
In more extreme cases, the lost
matter can encompass both stars, creating a "common envelope."
Friction will then bring the stars even closer together, making
the process go yet faster.
The stirring of the lost mass can
create unusually distorted planetary nebulae.
At the end, the
white dwarf created from the giant finds itself very close to the
remaining main sequence star.
In high mass double stars, the
higher-mass component can explode and produce a nearby neutron
star or even a black hole companion.
Some giant stars have the masses and internal constructions that
allow them to bring by-products of deep nuclear fusion to the
stars' surfaces, in the most extreme examples creating carbon stars.
Mass lost from
one of these enriched giants to a close companion can contaminate
the companion with the giant's newly-formed chemical elements.
When the giant becomes a white dwarf we are left with a seemingly
single star (main sequence or evolved giant) with an odd chemical composition.
with determined observation can we tell that a dim white dwarf is
Among the most prominent examples are "barium stars"
( an example), giants that have
very strong absorptions -- and great overabundances -- of the
heavy element barium among several others.
All seem to be
companions of what were once mightier stars that had become
carbon stars and that are now reduced to white dwarfs.
If the white dwarf and main sequence remnant of a close double
are close enough, the white dwarf can raise tides in the main
sequence star, and mass will flow the other way, from the main
sequence star to the white dwarf.
Theory and observation both
show that the flowing matter first enters a disk around the white
dwarf from which it falls onto the white dwarf's surface.
Instabilities in the disk can make such a star "flicker" over
periods of days and weeks, even producing sudden outbursts of
The star that became the white dwarf had lost almost all
of its hydrogen envelope during its own evolution.
When enough
fresh hydrogen from the main sequence star has fallen onto the
white dwarf, it can, in the nuclear sense, ignite, fusing
suddenly and explosively to helium.
The surface of the white
dwarf blasts into space, the star becoming temporarily vastly
On Earth we see a "new" star or "
nova" (meaning "new in Latin) erupt into the nighttime sky,
not a new star at all but an old one undergoing eruption.
Novae are common, 25 or so going off in
the Galaxy every year, once a generation one close enough to
reach first magnitude.
Nova Cygni in 1975 rivalled
, giving the celestial Swan two
In a massive double star system, the more massive of the pair may
develop an iron core and explode as a supernova, becoming either
or a black hole.
Either of these stellar
remains in turn may raise tides in the more-normal companion,
causing matter to flow into a disk around the collapsed body,
from which it falls into an immense gravitational field.
in the disk is so hot it can radiate X-rays.
From the motion of
the normal star, we can calculate information on the mass of the
collapsed one.
If the mass of the dark orbiting companion is
below a two-to-three solar mass limit, as in X Persei, it is a neutron star.
But if the
mass is great enough, we can infer the existence of an orbiting
, the best actual proof we
Fresh hydrogen falling from the disk onto a neutron star
can produce great , become
compressed and fuse to helium, and then explode violently as the
helium fuses to carbon. The result is an X-ray burst similar in
nature to a nova.
The term "supernova" is derived from "nova" in that the supernova
is vastly brighter, no matter that the mechanism of the core
collapse of a supergiant is completely different from the
mechanism of nova production.
White dwarfs, however, can also
produce supernovae.
No white dwarf can exceed a mass of 1.4
times that of the Sun, a limit discovered in the 1930s by Subramanyan Chandrasekhar when he applied relativity
theory to the gases in white dwarfs.
If the limit is exceeded,
even the white dwarf's enormous pressure cannot hold gravity back
and the white dwarf must collapse into a neutron star or a black
hole or perhaps even annihilate itself.
There are two
alternative theories for such an event.
A massive white dwarf
may accept enough mass from a close main sequence companion and
be pushed over the edge before a nova eruption can take place.
The white dwarf then collapses, creating a supernova that is
grander even than one produced by the collapse of a supergiant's
iron core.
The main sequence star of a double that contains a
white dwarf can also evolve through the giant stage to become a
white dwarf, creating a DOUBLE white dwarf system.
If the two
have been drawn close enough together by interaction during a
common envelope phase, they can spiral together by the radiation
of gravitational waves predicted by relativity theory.
dwarfs then merge, again producing a spectacular supernova.
either case, the collapse and resulting explosion makes nuclear
reactions that again create all the chemical elements and even
more iron than in the type of
produced by the collapse of the iron core of a massive star.
Kepler's supernova of 1604, the last seen in this Galaxy, was
probably of this kind.
Stars have a strong tendency to be born in groups, in whole
If they are bound together tightly enough by their own
gravity, they can survive for millions, even billions, of years,
even for the lifetime of the .
are two kinds, open clusters and globular clusters.
Open clusters, the sparser but by far the
more numerous of the two, are found in the disk of the Galaxy,
and therefore lie largely in the plane of the Milky Way.
Many of the closer ones, such as
and , are easily visible to the naked eye.
are angularly so large that they make constellations of their own, or at least
significant parts of them.
Thousands of open clusters dot the
Galaxy's disk.
Though their sizes vary greatly, they typically
contain a few hundred loosely arranged stars packed within a
diameter 10 or so light years across.
And though bound together
by their own gravity, most open clusters gradually break up as a
result of random encounters among stars that speed members to the
escape velocity, and because of stretching by tides raised by the
Open clusters thus tend to be young, under a billion
years of age (indeed, some are just born), though in the far
reaches of the outer Galaxy they can survive for far more than a
billion years.
Somewhat related to ,
stellar associations (commonly called OB
associations) are large, loosely organized, stellar groupings
that lie within the
defined by their young, blue .
Gravitationally unbound, OB associations are
expanding systems, their stars moving away from individual common
centers that may have core open clusters.
Though they contain
stars with a full range of masses, associations are best
recognized through their most massive, luminous, and hottest
members, which cannot get far away from their birthplaces before
they expire, rendering ageing associations essentially invisible.
OB associations are mostly named for their constellations of residence, for example
OB2, Upper , and so on.
Several constellations, in
particular
and Scorpius, owe their prominence and sparkle to
being made largely of OB associations.
Having massive stars,
associations are prime sources of supernovae.
While there are thousands of
in the , there are but
150 or so known .
Distinct from open clusters, their home is a huge spheroidal
halo that surrounds the Galaxy's disk.
And while open
clusters are sparse, loose, and comparatively young, globulars
are compact, closely spherical, and can contain over a million
stars packed into a volume only a hundred or so light years
Their compactness gives them (at least those that have
survived) long lifetimes.
With ages of 11 to 12 billion years,
formed when the metal content of the Galaxy was much less than it
is today (the increase the result of stellar evolution), they are
among the oldest things known and among the first things to be
created after the Big Bang, the event that formed our Universe.
Clusters of both kinds are profoundly important in establishing
the distance scale in astronomy and in testing and guiding the
theories of stellar evolution -- the ageing process.
are born with an intact array of stars that occupy the entire main sequence (dwarf sequence) of the HR diagram, in which the higher the mass, the
greater the luminosity.
Since high mass stars die first, a
cluster evolves by losing its dwarf sequence from the top down.
Application of evolutionary theory can then tell us the age of
any given cluster by the most luminous and hottest dwarfs still
The 's hot
class O stars tell that it is young.
The most luminous stars of
the somewhat older
class B, while the still older
has lost even these.
Open clusters range in age from just born
to nearly 10 billion years, which gives the age of the Galaxy's
By contrast, globular clusters have lost the entire upper
main sequence (dwarf) population down to stars somewhat below a
solar mass, which gives them ages 11 to 12 billion years or so,
nearly the age of the Galaxy itself.
In most dense globular
clusters, and even in some open clusters, some stars with masses
higher than the main sequence cutoff linger, refusing to evolve.
Since these stars are also bluer in color than the majority of
evolving dwarfs, they are called blue stragglers.
Blue stragglers are believed to be
caused by stellar mergers within the dense cluster environments,
either by direct collision or by the mergers of close double stars, which increases their masses
beyond the cutoff and thus seems to hold back their evolution.
Stars can range in size, depending on mass and age, from only a
few kilometers across to the diameter of the orbit of perhaps
They can range in temperature from near "cold" at only
2000 K for an extreme red giant through far over 100,000 K for
the star inside a planetary nebula to over a million K for a
neutron star.
All the stars you see in the sky will eventually
expire, some soon, some not for aeons.
Lower mass stars create
planetary nebulae and white dwarfs, while higher mass stars make
supernovae that result in neutron stars or black holes.
stars add spice to the product, making novae and a different kind
of supernova.
All these endings send newly made chemical
elements into the interstellar stew, out of which new stars are
As a result, the heavy element content of the Galaxy
increases with time.
Ancient main sequence stars, the subdwarfs, and their
giant star progeny have a low abundance of metals, whereas
younger stars like the Sun have higher metal contents, allowing
us to track the oldest and youngest stars and to determine the age of the Galaxy.
therefore contain the by-products of the old, our Earth a
distillate of earlier generations.
Our Sun will someday make its
own contribution, however modest it may be, to generations of
stars and planets yet unborn.
Stars are a fundamental component of other galaxies, other
collections of stars that work and act at least something like
Were there enough time and
astronomers, nearly a trillion galaxies could
be counted extending to billions of light years away.
and a number of different classification schemes.
Among the larger galaxies there are two broad groups,
spiral (disk) galaxies like ours (class "S") and
elliptical galaxies (class "E") that have no disks.
galaxies are more like the halo of our own galaxy, but are much
more thickly populated.
Each kind has its own subgroups.
subgroups are based on whether or not there is a central
bar from which the arms emanate ("SB") and on the degrees at
which the arms open outward.
Elliptical subgroups depend on the
flattening of the elliptical shapes.
Spirals are mostly larger
systems like ours, whereas ellipticals can range from small dwarfs
giant ellipticals much bigger and more massive than ours.
Set against these are small
irregulars with no well defined shapes.
The dusty gases
fill the inner disks of
most spiral galaxies and constitute good portions of the masses
of irregular galaxies, whereas they are largely absent from
ellipticals and small spheroidals, which then also lack star
formation.
There being no massive blue stars, ellipticals take
on reddish colors.
Bound by their mutual gravity (and that supplied by hot
intracluster gas), galaxies tend to cluster together. Our Galaxy
belongs to the small Local
It is dominated by us and the
Messier 31), which is similar to ours, if not somewhat more
Though at a distance of 2.5 million light years, M 31
is still easily visible to the naked eye.
Messier 33, a bit farther away and roughly a tenth the size
of our Galaxy, plays a lesser role in holding the Local Group
Under excellent sky conditions, it too is visible
without aid.
More than three dozen dwarfs and irregulars scatter
among them.
Included are our Galaxy's larger satellites, the
Large and Small
Magellanic Clouds, which are respectively just under
160,000 and 200,000 light years away.
appear to the unaided eye as broken-off pieces
Among M 31's prominent
satellites are bright, round
Messier 32 and elliptical, fainter, NGC 205.
nearest large cluster is the Virgo Cluster.
Around 55 million light years away, it contains up to 2000
galaxies, including the two prominent ellipticals Messier 84 and M
86 plus the giant elliptical
(M 87, a prime example of an
active galaxy, contains a
supermassive black hole of around six billion solar masses
that ejects a jet at near the speed of light.
By comparison the
black hole at the center of our own Galaxy measures only four
million solar masses.)
Larger galaxies were probably built
through mergers with smaller systems.
Such collisions go
on yet today.
Giant ellipticals at the cores of dense clusters
of galaxies seem to be the product of repeated collisions among
lesser members that send galactic debris to the clusters'
Taken over the nearby Universe, ten times as much
matter as is found in stars is bound up in hot intracluster gas
(known through its X-ray radiation).
Clusters of galaxies
assemble into even larger unbound superclusters and "walls" of
The overriding feature of the Universe is its expansion, the
study of the Universe at large called cosmology.
exception of a few galaxies in the , all galaxies are moving away from us (as told by the
redward shifts of the absorption or emission lines in their spectra) with speeds directly
proportional to their distances (as discovered by Edwin Hubble).
At low velocities, the shifts of spectral lines to the red end of
the spectrum (the "redshifts") mimic the Doppler effect, but they are
really caused by the expansion and stretching of the fabric of
space in which the galaxies are embedded.
The rate of expansion
(the Hubble Constant) is 72 kilometers per second per
megaparsec (with a few km/s uncertainty), where a
megaparsec is 32.6 million light years.
The effect is
explained by the concept of the Big Bang, in which our Universe began in a
hot dense state from which it rapidly expanded, the expansion
going on yet today.
The reality of the Big Bang is confirmed by
(CMB), which pervades all
space and comes from the original "fireball" now cooled to 2.7
degrees above absolute zero (theory agreeing with observation).
The age of the Universe from the time of the initial expansion as
found from the Hubble Constant as interpreted through the CMB is
13.7 billion years, which fits well with the ages of the oldest
known stars.
The expansion is seen only across the largest
scales, between clusters of galaxies.
The Galaxy, the Local
Group, and other clusters are held together by their own gravity,
which over short distance scales overwhelms the expansion.
Nobody knows what came before the Big Bang or how the hot dense
state originated.
Tiny fluctuations of just a hundred thousandth
of a degree in the temperature map of the CMB over the whole sky
are probably the predecessors of the walls and clusters of
galaxies that we see today.
Galaxies within clusters orbit their centers of gravity at
velocities that are much too high for the combined masses as
found from the members' visual brightnesses plus the mass of the
hot intercluster gas.
Moreover, spiral galaxies, including ours,
rotate too fast for their masses, that is, stars orbit too fast
for the amounts of matter seen to be internal to their orbits
(which control their rotation speeds).
Some invisible substance
dark matter must produce the additional gravity required.
Nobody knows what it is, though exotic atomic particles lead the
list of suspects.
Moreover, the expansion rate of the Universe
seems to be to be accelerating, which implies the existence of
some sort of dark energy, which has an equivalent mass
hrough Einstein's famous equation of e=mc**2, where c**2 is the
square of the speed of light (that times itself).
average, only 0.4 percent of the total mass-energy of the
Universe is tied up in stars, 3.6 percent in hot intercluster
gas, 22 percent in dark matter, and 74 percent in dark energy
(the exact numbers being dependent on the source).
In spite of
their low mass contribution, stars are still the main tracers of
mass and energy in the Universe both in space and, since all
chemical elements but hydrogen and helium come from stellar
processes, in time too.
Calculations of the expanding hydrogen gas (ionized to protons
and electrons) in the first few minutes after the Big Bang show
that nuclear processes created only helium and a tiny fraction of
The first stars to condense from this cooling gas
should then have been made of nothing but hydrogen and helium.
Since the relatively high metal stars of the Galaxy's disk (like the Sun) are called
Population I and the stars of the halo (subdwarfs and
globular clusters) are Population II, these are often
called Population III.
They are responsible for making
the first batch of heavy elements that littered the gas from
which the current crop of extreme subdwarfs were born.
back in time to find very low metal stars with just a hundred
thousandth the amount (relative to hydrogen) of solar metals, but
we cannot in our Galaxy find the zero-metal Population III.
Presumably (supported by theory and the chemical compositions of
the oldest stars we can find), the first stars must have all been
so massive that they blew th