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The Earth and the Universe 1
The Earth and the Universe
Ever since the first humans looked in wonder at the stars in the
night sky, we have longed
to know more about the Universe. The first use of astronomy was
to measure time. The
early Egyptians used the appearance of the brightest star,
Sirius, to mark the start of the
season when the Nile River was in flood. The Greek astronomer
Ptolemy made accurate
measurements of the movement of the planets in about 180AD.
Before the invention of
the telescope, people could only use their eyesight for looking
at heavenly objects and
measuring positions of the stars.
But for around 400 years, most of our knowledge about the night
sky came from
telescopes, mainly the refracting telescope that uses only
lenses and the reflecting
telescope that uses mirrors and lenses. Galileo Galilei
(1564-1642) made his first
observations using a telescope in the early 17th century.
Sir William Herschell (1732-1822) started observing the night
sky in 1771, using a small
reflecting telescope. He discovered the planet Uranus, the first
planet to be discovered
with the help of a telescope, since we are not able to see this
planet with the naked eye.
From the 1930s, radio telescopes were able to detect other rays
and waves from space as
we shall see later on.
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The Earth and the Universe 2
The Universe Today, we have the technology to explore deeper
into the Universe than ever imagined.
Our Earth is only just one very tiny part of the vast universe -
a small rocky planet travelling around a medium sized star, the
Sun, in one of billions of galaxies. A Galaxy is a huge group of
millions of stars in space. But galaxies are often millions of
times further
apart than the stars within the galaxy. Normal galaxies come in
three different shapes;
spiral, elliptical and irregular. The universe is made up of
more than 10 000 million such
galaxies. Nobody knows really where the universe begins or ends.
We have so far
explored only a very small part of the universe.
The universe is everything that exists, mostly made up of empty
space. It stretches further
almost than the human mind can imagine - we already know that
the universe reaches at
over 13 billion light years in every direction so we see the
stars as they were 13 billion
years ago! So the universe is even older than this. It is filled
with matter in many different
shapes, sizes and forms. It contains dust and gases; planets
such as the Earth and billions
of stars such as our Sun, our Milky Way Galaxy and countless of
other galaxies.
We can see about 5000 individual stars in the night sky. On a
really clear night, it is also
possible to see one or two galaxies. The most visible part of
our own Milky Way Galaxy
is the part of the sky that looks like a misty cloud. It is
really a band of millions of stars.
The Milky Way Galaxy looks like a thin disc with a dense,
bulging centre. It is about
2000 light years thick and about 100 000 light years across.
Gravity keeps the stars to-
gether in a galaxy and like most things in the universe, the
galaxies all rotate. It contains
an estimated 100 000 million stars and they orbit its centre at
a speed of 274 km/s.
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The Earth and the Universe 3
There are other galaxies, the most famous being the Andromeda
Galaxy, which is close to
the constellation Pegasus. It is the most distant object visible
to the naked eye and looks
like a small fuzzy patch. The Andromeda galaxy is a huge spiral
galaxy, 2.2 billion light
years away and is thought to contain at least 300 billion
stars.
The light year
The universe is so huge that we cannot measure it in kilometres,
or even millions of
kilometres. Imagine writing down in kilometres the distance of
the Andromeda galaxy
from earth! So astronomers measure the size of the universe or
the distance of stars by
using the speed at which light from these objects travel. In
empty space, light travels at a speed of 3 x 108m/s. Using this
speed for a year will give you a distance of 9.5 x 1012km
So the distance that light travels in one year is 9.46 million,
million kilometres. This is
called a light year and it is a measurement of distance not
time! In fact light from the Sun,
which is about 150 million kilometres from Earth, takes 8
minutes to reach us. The
nearest star, Proxima Centauri, is 4.225 light years from
Earth.
The Solar System The sun is a very ordinary star among thousands
of millions of other stars in the Milky
Way Galaxy. Its outer layer or coronas temperatures reach as
high as 2 million oC. It has
a planetary system, called the Solar System, which is made up of
nine planets and other left over material that did not form into
planets surrounding it. The planets start off with
Mercury, which is at an average distance of 58 Mkm from the Sun.
then comes Venus, Earth, Mars (these planets are known as the four
rocky inner planets), Jupiter, Saturn, Uranus, Neptune and Pluto.
Between the orbits of Mars and Jupiter lies an area of one
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The Earth and the Universe 4
Kind of left over material, the asteroids. These are lumps of
rock, some of which are many kilometres long.
The planets vary enormously in size. They also vary in their
distance from the Sun.
Planets dont give their own light (they are not stars), they
reflect the Suns light. Because
they are much nearer than stars, they appear to move slowly
against the background of
the stars. The planets are held in their orbits by the
gravitational pull of the sun. The orbit
of each planet is not quite a circle. It is a slightly squashed
circle called an ellipse. All planets in our solar system orbit in
the same plane, except Pluto. They are not all travel-
ling at the same speed, the planets nearer to the Su travel more
quickly so they have a
shorter year.
1 Mercury Mercury is closest to the Sun and small for a planet
(about the size of our Moon). It has
no atmosphere and is covered in craters. The side of planet
facing the Sun is very hot,
about 430 oC.
2 Venus Venus is almost as big as the Earth, but very
unpleasant. It is covered in clouds of
sulphuric acid, with an atmosphere of carbon dioxide at a very
high pressure. Because of
the CO2 and the Greenhouse Effect it is even hotter than
Mercury.
3 Earth From space, Earth is a blue planet with swirls of cloud.
It is the only planet with water
and oxygen and living things. It is at the right distance from
the Sun, with the right
chemicals, to support life. Other stars may have planets with
the same conditions.
4 Mars Mars - the red planet - is a cold desert of red rocks,
with huge mountains and canyons.
There is no life on Mars. It has a thin atmosphere of carbon
dioxide, and two small
moons. Between Mars and Jupiter there are thousands or rocks,
called asteroids.
5 Jupiter Jupiter is the cold giant of the planets. It has no
solid surface, being mainly liquid
hydrogen and helium, surrounded by gases and clouds. The Great
Red Spot is a giant
storm three times the size of Earth. Some of the 16 moons have
volcanoes.
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The Earth and the Universe 5
6 Saturn Saturn is another gas giant, very like Jupiter. The
beautiful rings are not solid. They are
made of billions of tiny chunks of ice, held in orbit by the
pull of Saturns gravity. As
well as the rings, Saturn has more than 20 moons.
7 Uranus Uranus is another giant planet. It looks pale green,
with very faint rings and 15 moons. It
was discovered by William Herschel in 1781. It is unusual
because its axis is tilted right
over, so that it is lying on its side as it goes round the
Sun.
8 Neptune Neptune is the twin of Uranus. They are both about 4
times the size of Earth. Neptune is
bluish, due to the thick atmosphere of cold methane. It has 8
moons, one with volcanoes.
The Great Dark Spot is a storm about the size of Earth.
9 Pluto Pluto is the smallest of all, discovered in 1930. We
dont know much about it. Plutos
orbit is not circular as the others. Most of the time its orbit
is outside Neptunes but
between 1979 and 1999 its orbit was inside Neptunes. Some
astronomers think there is a
planet 10 beyond Pluto.
The
planets
Avr. Dist. From Sun
Mkm Earth=1
Diameter
(Earth = 1)
Density
(kg/m3)
Avr. Temp oC
Mass
Earth=1
Gravity
(N/kg)
orbit time
(years)
No. of
moons
1-Mercury 58 0.4 0.4 5500 +430 to 0.1 4 0.2 0
2-Venus 108 0.7 0.95 5200 +470 0.8 9 0.6 0
3-Earth 150 1 1 5500 +15 1 10 1 1
4-Mars 228 1.5 0.5 4000 -30 0.1 4 1.9 2
Asteroids
5-Jupiter 778 5 11 1300 -150 320 26 12 16
6-Saturn 1427 9.5 9 700 -180 95 11 30 20 + rings
7-Uranus 2870 19 4 1300 -210 15 11 84 15 + rings
8-Neptune 4497 30 4 1700 -220 17 12 165 8
9-Pluto 5900 39 0.2 500 -230 0.002 4 248 1
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The Earth and the Universe 6
Moons Moons are heavenly bodies which orbit planets. It is the
force of gravity which holds the
moons in orbit. The Earth has only one moon but other planets
have quite a few. We can
only see the moon because it reflects sunlight. The phases of
the moon happen, depending
on how much of the illuminated side of the moon we can see.
Beyond Pluto is another kind of left over material, the comets
such as Halleys Comet. These are bodies of rock, ice and dust. They
are very highly elliptical orbits, which bring
them close to the Sun and then far out in the solar system
beyond Pluto. When they fall
near the Sun they speed up as the pull of gravity increases. The
dust and gas are blown
away from the Sun and shine in the sunlight, to form a long
tail. When some comets pass
close to the Sun they heat up and give off material, forming a
tail. Comets also give off
small rocky particles which form many of the meteors or shooting
stars that we see.
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The Earth and the Universe 7
The Earth Look up at the sky on a clear night. The sky seems to
arch overhead like a vast
star-studded dome. Ancient people believed that the stars were
actually stuck on the
inside of a sphere, surrounding the Earth. Each day, the Sun and
stars seem to move
across the sky, rising in the East and setting in the West.
Ancient civilisations attribute
this motion to the rotation of the celestial sphere around the
Earth. The apparent rotation of the celestial sphere means that the
sky changes in appearance
continually during the night. The night sky also looks
different, from season to season,
according to the latitude of your position. To many people
today, the continually varying appearance of the sky is as
confusing as it was to ancient man.
Of course, the Sun and stars do not really move of their own
accord around the heavens
each day; it is actually the daily rotation of the Earth on its
axis that makes them appear
to move so.
The Earth takes 24 hours to rotate once on its axis (one
complete revolution), then for each hour of time the stars will
appear to move 15o across the sky. The Earths daily
rotation, obtained from the observations of stars, is the basis
of our time keeping.
But the Earth does not only spin on its axis. It is also moving
on its orbit around the Sun
and it is pulled into orbit by a centripetal force. This is the
gravitational force between the mass of the Sun and the mass of the
Earth. The time taken for the Earth to orbit the Sun once is known
as a year, it lasts approximately 365 days and 6 hours. Because the
Earth does not orbit the Sun in an exact number of days, we have to
add an extra day to
the calendar every four years to keep the calendar with the
seasons. Each year con-
taining an extra day is called a leap year.
An additional confusion, is that the Earths axis of rotation is
not perpendicular to the
plane of its orbit, but instead tilts at an angle of 23.5o from
the vertical. This tilt of the Earths axis causes the seasons.
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The Earth and the Universe 8
In June, the earths North pole is tilted 23.5o towards the
Sun
and so there is summer in the northern hemisphere. Six
months later, in December, the South pole is presented to
the
sun, causing summer in the southern hemisphere. In summer,
the days last longer than the nights, while the opposite is
true
in winter. But on two days each year, night and day are
equal
in length. These are the occasions on which the sun crosses
the celestial equator, and are known as the equinoxes. The
vernal equinox occurs on 21st March and the autumnal occurs on the
23rd September.
There is one more effect we need to take into account to
complete our understanding of
the appearance of the sky, and that is the effect of changes in
latitude. An observer at one
of the Earths poles would see only half the sky. Above would lie
the celestial pole, the
celestial equator would be just on the horizon. Each night, the
stars would spin around the
pole, with none appearing to rise or set. An observer standing
at the Earths equator, by
contrast, would see the celestial poles lying on the north and
south horizon respectively,
and the celestial equator would be above. Each night, stars
would appear to rise in the
East, move overhead and set in the west as the Earth turned.
During one year, an observer
would see the entire sky that can be observed form that
latitude.
Observations stationed at latitudes on Earth midway between the
poles and the equator
see an intermediate amount of the sky. Some stars appear to
circle the pole each night
without setting (circumpolar stars) while others rise and set.
Inhabitants of the Earths
northern hemisphere can easily check their latitude, because a
fairly bright star, Polaris,
lies close to the north celestial pole. Unfortunately there is
no equivalent star near the
south celestial pole.
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The Earth and the Universe 9
Gravitational Forces About 1666, at the early age of 24, Isaac
Newton investigated the motion of a planet
moving in its orbit round the Sun. Newton concluded that a
universal law could be stated
for the attraction between any two particles of matter, not just
planets! In fact, all objects attract each other and gravity is the
force of attraction which acts between all masses. He found that
the force of attraction between two given particles of a certain
mass is
inversely proportional to the square of their distance apart.
From this law it follows that
the force of attraction F, between two particles of masses m and
M respectively, at a distance d apart, is given by the following
equation.
F = G m M M m m d2 Force : 1 1/4
Distance: 1 2
G is a universal constant known as the gravitational constant.
This expression for F is Newtons law of gravity. This law is
applied to the motion of planets round the Sun, to satellites round
the Earth and to the moon.
From this law we can conclude that:
1. The greater the mass, the greater the force of
attraction.
2. The greater the distance, the smaller the force of
attraction. In fact if the distance
doubles, the gravitational pull is 1/4. This is the inverse
square law. The graph illustrates how the force of gravity varies
as you get closer to a planet.
We have already seen that the Suns
force of gravity holds the planets in
orbit, but planets are not equally
attracted by the Sun. Mercury is
very near to the Sun. To counteract
this strong force of gravity, the
planet must move faster and cover
its orbit quicker. So mercury moves
at a faster speed than Pluto since it is
nearer to the Sun.
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The Earth and the Universe 10
Satellites Imagine firing a cannon from the top of a high
mountain. The canon ball would fall back to the
Earth just like when you shoot a ball. But if the
cannon ball is fired faster, it falls farther away. If
the cannon ball is fired at 8 km/s (25 times the
speed of sound), then it would still fall towards
the Earth, but because of the curving of the
Earth, the ball would stay the same height above
the ground. It is then a satellite in orbit and the
force of gravity keeps pulling it into orbit.
So an object becomes a satellite when it is propelled fast
enough - at a speed of 28 800
km/s - to be able to fly in a continuous fall or arc around the
Earth without being pulled
back by gravity. Satellites fly above the Earths atmosphere at a
height of at least 160 km.
The higher the orbit, the slower the speed of the satellite and
the longer it takes for one complete orbit. A close-look
observation satellite will fly in a low orbit around the Earth so
that its cameras can take high-resolution images. However, a
communications
satellite is situated in a high orbit so that it can provide
services to as large an area of the
world as possible.
The speed of a satellite can be calculated very easily. If
the radius of the orbit is R, then the circumference of the
circle is 2 R. If the time for one orbit (the periodic time) is T,
then:
Speed = Distance travelled = 2 R time taken T Example: A
satellite at a height of 700 km above the Earths surface, orbits
with a
period of 100 minutes. What is its speed if the radius of the
Earth is 6400 km?
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The Earth and the Universe 11
A Geostationary Satellites used for Communications A satellite
moves very fast, but it can seem to be standing still! If the
satellite is put at
just the right height and speed, it will take 24 hours to go
round the Earth. This is the
same time as the Earth takes to spin once, so the satellite
appears to stay over one place.
Most communication satellites are in geostationary orbit above
the Equator (equatorial orbit) at a height of 36 900 km so that
they appear stationary when viewed from Earth.
Communications satellite technology has developed at a rapid
rate. Just 30 years ago the
rather basic Telstar satellite was transmitting TV signals to a
ground station for onward
transmission by land-line to peoples homes. Today, satellites
can beam TV pictures di-
rectly into millions of homes - and from not just one channel
but hundreds of different
ones at the same time. Some important scientific advances have
made this possible. Also
the satellites, with their perfectly fashioned antennas, have
extremely high transmitting
power - they are supplied with large amounts of electricity by
advanced solar panels.
Lastly, the high power transmissions mean that
satellite-receiving dishes can be smaller.
Satellite provide numerous communications services other than
TV. Some are able to han-
dle 30 000 simultaneous phone, data and fax calls.
Modern communications technology
allows us to keep in touch with each other
from almost anywhere in the world. The
network of satellites in space provides
instant communications whether by
telephone, mobile phone, fax, pager,
computer or e-mail. If you telephone to
America, a microwave radio signal is transmitted from a dish
aerial up to the satellite.
The satellite then transmits it down to another aerial dish in
America. Microwaves are used because they travel in a narrow beam,
in a straight line and pass through the Earths
atmosphere. The satellites are positioned in such a way that a
mobile phone user is within
the range of at least one satellite at any time. Today a typical
communications satellite
weighs 3500 kg providing high power transmission in many
wavebands. Using 6000W of
power from solar cells, it transmits to thousands of receiving
dishes as small as 1m in
diameter or smaller.
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The Earth and the Universe 12
B Low Polar Orbit Satellites Satellites can be launched into
polar orbits which are usually lower. As the Earth turns on
its axis, the satellite passes over a different part of the
Earth on each orbit. The time taken
for each full orbit is just a few hours and so it allows the
whole surface of the Earth to be
monitored each day. This finds many applications:
1 Weather Satellites A fleet of polar weather satellites from
USA, Russia, Europe, India and Japan provide a
daily World Weather Watch service to the worlds population. The
satellites help
meteorologists to make instant weather forecasts including early
warnings of weather
extremes such as hurricanes. These early warnings enable
thousands of people to
evacuate areas about to be hit. The familiar weather satellite
pictures we see on our TV
screens are only one part of the weather story. Satellites also
take images in different
wavelengths, highlighting other aspects of the weather, such as
temperature and
atmospheric content.
2 Earth Observation Satellites Daily maps from Earth observation
satellites show the surface temperature anywhere on
the Earth, both on land and at sea. Other images reveal the
water content in the atmos-
phere and its pattern of circulation, and monitor the damage to
the earths ozone layer
caused by the emission of greenhouse gases. So international
meteorological satellites in
polar and geostationary orbit above the earth are continually
monitoring the worlds
weather as well as changes to our environment. Meteosats
successor, called Metop, is a
typical example of todays environmental satellite.
3 Navigational Satellites Navigation satellites can provide
accurate information to within a few metres of a per-
sons location anywhere in the world - on land, at sea or in air.
They can also work out the
speed of a moving person or object at within 0.1m/s. This
technology is vital for all types
of military operations, from guiding a missile to its target or
telling on undercoat agent exactly where he or she is. Russia and
the USA operate fleets of navigational satellites.
The US Air Force operates a navigational service called the
Global Positioning System, GPS. It consists of 24 Navstar
satellites, which are at all times equally spaced apart in six
different paths around the Earth.
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The Earth and the Universe 13
Many satellites have been launched for purely scientific
purposes - to learn as much as possible about the Earth and its
place in space. The discovery of radiation belts around
the Earth by Americas first satellite, Explorer 1, is a classic
example of this. Other
satellites have been launched to study the radiation belts, the
Earths magnetic field and
its upper atmosphere.
Many research and development satellites have been launched to
test out new equipment
to be used on future operational spacecraft, while other
satellites have conducted experi-
ments using the lack of gravity in space. Space processing could
lead to a revolution in
the worlds pharmaceuticals and electronics industries.
4 Astronomical Satellites New astronomical satellites have now
been developed that observe the Universe not as
we would see it with our eyes but using different wavelengths,
such as x-rays and UV
light. Some of these telescopes have been launched into space to
obtain a better view of
the Universe, without the interference of the Earths atmosphere.
An example of such a
satellite is the Hubble Space Telescope (HST), which was
launched into space in April 1990. This telescope is probably the
greatest advance in astronomy. It can see 50 times
deeper into space than the most powerful telescope on Earth.
Using a telescope on Earth
to see into space is rather like being under water in a swimming
pool and trying to see
outside the pool. It is very difficult to see through the thick
atmosphere. In space the HST
is not influenced by any atmosphere. With the help of the HST,
astronomers are now able
to see sites they used to dream of - black holes, quasars and
even possible planets moving
around other stars. Many of the visible light images are
combined with other observations
made in other wavelengths by the HST, producing more complete
photos.
Life of stars Stars are incandescent balls of gas, giving out
their own heat and light, similar to our own Sun. While the stars
stay fixed in position, the planets orbit a central star, mov-ing
slowly across the sky. The planets appear bright to us because they
reflect light from
the Sun - they do not emit their own light. But how are stars
born? What keeps them
glowing? And how do they grow old and die? Astronomers believe
they now have
answers to most of these questions.
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The Earth and the Universe 14
Stars are, thought to be, formed from giant clouds of dust and
gas in space, known as nebulae. One famous site of star formation
is the great nebula in Orion, visible to the naked eye as a hazy
patch, and particularly impressive viewed through a telescope.
Stars
are forming today inside the Orion Nebula. Radiation from the
largest and hottest of these
stars makes the whole nebula glow.
Stars start to form when part of a gas cloud like the nebula
breaks up into individual blobs
as a result of random swirling motions in the cloud. These blobs
collapse under the
inwards pull of their own gravity. As they shrink, becoming
smaller and denser, pressures
and temperatures build up inside them until nuclear reactions
switch on at their cores.
When this happens, a gas blob becomes a true star.
Our star had a similar birth, then the planets began to form.
The heat of the Sun pushed
some of the lighter gas and ice into a doughnut-shaped cloud,
leaving the heavier dust in
the gap. Because of gravity the dust began to stick together,
eventually making the four
rocky inner planets. In the same way, the outer planets formed
from ice, then collected the
gas to become gas giants like Jupiter.
Stars, it seems, do not come into being on their own, but in
vast groups. The Pleiades
cluster in the constellation of Taurus is an example of a star
group that has formed from a
giant gas cloud. Our Sun was probably a member of a similar
cluster when it was born
4700 million of years ago, but the stars of that cluster have
long since drifted apart. Some
nebulae contain no illuminating stars, and therefore are dark.
These can only be seen in
silhouette against a brighter nebula or the Milky Way star
field.
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The Earth and the Universe 15
Stars do not burn in the usual sense of the word, to give us
light. They are powered by nuclear reactions at their centres,
which turn hydrogen to helium, releasing huge amounts of energy in
the process. This is similar to the reaction, which occurs in a
hydrogen bomb, but in stars the reaction occurs in a more
controlled way.
Stars are made largely of hydrogen, which is the simplest and
most abundant element in
the Universe. For instance 70% of the sun (by weight) is
hydrogen, and most of the
remainder is Helium, the second simplest and most abundant
element. All other elements
make up less than 2% of the Suns mass. These figures are typical
of other stars.
In the nuclear furnace at the centre of the sun, atoms of
hydrogen are crushed together to
make atoms of Helium, a process called fusion. The temperature
at the core of the Sun where this reaction takes place is estimated
to be 15 million oC. Each second 600 million
tons of hydrogen in the sun are turned into helium, with 4
million tons going to produce
energy. Even at this rate, the sun has enough hydrogen stocks to
last for about another
5000 million years. The sun is believed to be in stable middle
age, roughly half way
through its life cycle.
Eventually a star starts to run out of hydrogen at its core,
having turned it
all into helium. Burning hydrogen then moves out into the
surrounding
zone. When this happens, the star gets hotter inside and the
result of this
extra energy release is that the star swells up in size. As it
swells, its
surface temperature drops so that it becomes red in colour. The
star has
become a red giant. So the stars colour and brightness depend on
its temperature. Blue stars are the hotter, while red stars are the
coolest. It
follows that most red stars are very old and have expanded to a
huge size
compared to their original size.
A red giant can be as much as 100 times the size of the present
Sun.
When our Sun reaches this stage, in billions of years, it will
engulf the
Earth, thereby ending life on our planet. For the next stage in
the stars
development, the helium core also becomes sufficiently heated to
ignite
and partake in nuclear reactions, this time forming carbon. The
extra
energy released expands the star still further - so much so that
its outer
layers drift off into space, forming a stellar smoke ring or
planetary nebula. Planetary nebulae have nothing to do with
planets, they get their name from the fact that through a telescope
they resemble the disc of a planet.
At the centre of the expanding shell of gas lies the exposed
core of the former red giant
star. This small, hot core is known as a white dwarf. A white
dwarf star may contain as much mass as the Sun, compressed into a
ball the size of the Earth. White dwarfs are
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The Earth and the Universe 16
Made of such dense materials that a handful of material would
weigh several tons. Since
white dwarfs are so small, they are very faint and difficult to
spot through telescopes.
They cool off slowly like a dying fire to become a black dwarf,
eventually fading into invisibility. This is predicted to be the
final fate of the Sun.
All stars of less than about four times the mass of the Sun go
through this life cycle,
although at different rates, depending on their masses. A star
like Sirius, for example,
with about twice the mass of the Sun, can live for no longer
than 1000 million years,
which is 1/10 the suns predicted life time. At the other hand of
the scale, red dwarf stars -
far smaller and cooler than the sun - are predicted to live at
least 10 times as long as the
sun, because they use up their nuclear fuel more slowly. Red
dwarfs, being long-lived, are
probably the most abundant stars in the Galaxy, although they
are so faint they are
difficult to spot.
Stars with more than about 4 solar masses die a spectacular
death. A
large star burns hotter (blue giant) and runs out of fuel
sooner. They
expand and cool to red super-giants, larger and brighter even
than red
giants, and then a series of run-away nuclear reactions sets in
at their
core. The result is that the star erupts in a gigantic nuclear
explosion
known as a supernova. In a supernova explosion, the star may
flare up in brightness by thousands of times, so that for a few
days or
weeks it is giving out as much light as an entire galaxy!
As the star erupts, complex nuclear reactions occur which give
rise to
all the known chemical elements. These atoms are scattered
into
space by the explosion to mix with gas clouds and then to gather
up
to make new stars. One famous supernova was seen by Middle
Eastern and Oriental astronomers in 1054AD. Some stars blow
up
themselves completely to bits in a supernova.
But in many cases the heavy core of the dead star remains as
an
object even smaller than a white dwarf which then becomes a
neutron star (or pulsar). If this still has a big mass it
continues to collapse under its own gravity. The pull of gravity
becomes so strong that nothing can escape it, even light. It
be-
comes a black hole.
Cloud of gas gathered gas becomes more compact nuclear fusion
process where
by gravity as a result it get hotter hydrogen is changed
into
helium - a star is born!
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The Earth and the Universe 17
small stars expand to outer layers expand to
red giants become planetary nebulae.
Core collapses to become a
white dwarf
Medium stars expand to supernova explosion
Hydrogen runs out yellow/orange super giants leaving a neutron
star
large stars expand to supernova explosion
become white super giants leaving black hole
Beginnings and endings Edwin Hubble, the astronomer who
established the existence of galaxies outside our own,
announced in 1929 that the galaxies seemed to be moving apart
from each other at speeds
that increased with distance, as though the entire Universe was
expanding like a balloon
being inflated.
This amazing and fundamental discovery, was made by analysing
the spectrum of light
from distant galaxies. Their light turned out to have increased
in wavelength, a common
effect caused by rapid motion of a light source away from the
observer, and named the
Doppler effect after the German physicist Christian Doppler who
described this principle in 1842. The effect is known as a red
shift, because red light lies at the long - wavelength end of the
spectrum and so an increase in wavelength means that light is
shifted towards
the red end. In other words, the frequencies are all slightly
lower than they should be. Its the same effect as a car horn,
sounding lower-pitched when a car is travelling away
from you. The sound drops in frequency.
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Hubble found that the amount of red shift in a galaxys light
depends on its distance from
us, with the farthest galaxies having the greatest red shifts
and hence they must be mov-
ing away from us the quickest. The relationship between red
shift and distance is called
Hubbles Law and it means that by measuring the red shift in
light from a galaxy we can
estimate its distance. Hubbles discovery might seem to imply
that our local group of
galaxies is the centre of the Universe from which all else is
receding, but this is not the
case. An observer on any galaxy would see exactly the same
effect as we do. The entire
universe is expanding and hence all the galaxies are moving
apart from each other. There
is no known central point in the universe.
Scientists have three main theories about the creation of the
Universe:
1 the steady state theory The steady state theory says that the
Universe has always been existing, expanding and
that new material is constantly being created and there is the
same amount of matter in
the same place. This theory is no longer supported since
astronomers can see that the uni-
verse looked different in the past. A further blow to the steady
state theory is that radio as-
tronomers have heard what they believe is the echo of the Big
Bang - an explosion
which marked the origin of the Universe. In 1965, the American
physicists Arno Penzias
and Robert Wilson detected a faint radio noise coming from all
over the sky. In 1978, this
earned them a Noble Prize. The reason for this noise is that
space in not entirely cold, but
has a temperature of 2 or 3 degrees above absolute zero. The
slight warmth pervading the
Universe is interpreted as being the energy left over from the
Big Bang.
2 The pulsating Universe theory According to the pulsating
Universe theory, all matter is flying apart from a heavily com-
pacted mass and will eventually slow down, begin to contract and
become so condensed
that it will explode again.
3 the Big Bang theory The Big Bang theory suggests that the
universe began in an explosion about 15 billion
years ago and will continue to expand forever. According to this
theory, the early universe
was very hot and dense, with all its matter and space packed
into a very small area, then it
exploded. As it cooled down, the Universe expanded and
astronomers believe it is still
expanding. These estimates are not very accurate because it is
hard to tell how much the
expansion has slowed down since the Big Bang. The rate at which
the expansion is
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The Earth and the Universe 19
Slowing down is an important factor in deciding the future of
the Universe. Without the
force of gravity, the universe would expand forever. However the
attraction between all
masses in the universe tends to slow the expansion down.
The eventual fate of the universe depends on how fast the
galaxies are moving apart and
how much total mass there is in it. We can measure the speed
with which the galaxies are
separating but we just dont know how much mass there is in the
universe as most of the
mass appears to be invisible (in black holes and interstellar
dust). There are two ways the
universe could go:
1. If theres enough mass compared to how fast the galaxies are
currently moving, the
universe will eventually stop expanding and begin contracting.
This would end in a
Big Crunch, which would be followed by another Big Bang and the
endless cycles of explosions and contractions.
2. If theres too little mass in the Universe to slow the
expansion down, then it could
expand forever with the universe becoming more and more spread
out.
Who knows what and when will be the end?!