The universe is a huge wide-open space that holds everything from the
smallest particle to the biggest galaxy. No one knows just how big the Universe
is. Astronomers try to measure it all the time. They use a special instrument
called a spectroscope to tell whether an object is moving away from Earth or
toward Earth. Based on the information fromthis instrument, scientists have
learned that the universe is still growing outward in every direction.
Scientists believe that about 13.7 billion years ago, a powerful explosion
called the Big Bang happened. This powerful explosion set the universe into
motion and this motion continues today. Scientists are not yet sure if the
motion will stop, change direction, or keep going forever.
Evolution Of Universe
The threemain theories put forward to explain the origin and evolution of the
- The Big Bang Theory
- The Steady State Theory
- The Pulsating Theory
The Big Bang Theory: Le Maitre and Gammow
proposed this theory. According to this theory, at the beginning of the
universe, the whole matter of the universe was once concentrated in an
extremely dense and hot (~10 12K) fireball. Then about 20 billion years
ago a vast explosion (big bang) occurred. The matter was broken into
pieces, which were thrown out with high speed in all directions forming
stars and galaxies; which are still moving way from one another.
According to Hubble’s law, the velocity of recession of a galaxy becomes
equal to the velocity of light at a distance equal of 20 billing light
years. Itmeans, the light rays fromstars and galaxies, which are
situated at a distance of 20 billion light years or more, can never
reach us. Thus this distance becomes the boundary of observable
universe. On account of continuous recession, more and more galaxies
will go beyond this boundary and they will be lost.As a result of this,
the number of galaxies per unit volume will go on decreasing and
ultimately a time may come when we may have empty universe
Steady State Theory: Bondi, Gold and Fred
Hoyle developed this theory. According to this theory, the number of
galaxies in the observable universe is constant and new galaxies are
continuously being created out of empty space, which fill up the gaps
caused by those galaxies, which have crossed the boundary of the
observable universe. As a result of it, the overall size of mass of the
observable universe remains constant. Thus a steady state of the
universe is not disturbed at all.
Pulsating Theory: According to this theory,
the universe is supposed to be expanding and contracting alternately
i.e. pulsating. At present, the universe is expanding. According to
pulsating theory, it is possible that at a certain time, the expansion
of the universe may be stopped by the gravitational pull and they may
contract again. After it has been contracted to a certain size,
explosion again occurs and the universe will start expanding. The
alternate expansion and contraction of the universe give rise to
A galaxy contains stars, gas, and dust which are held
together as a group by gravity. There may be millions, or even billions, of
stars in one galaxy. There are billions of galaxies in the universe.Galaxies are
labeled according totheir shape. Some galaxies are called “spiral”, because they
look like giant pinwheels in the sky. The galaxy we live in, the Milky Way, is a
spiral galaxy. Some galaxies are called “elliptical”, because they look like
flat balls. A galaxy may be called “irregular” if it doesn’t really have a
shape. A new type of galaxy was discovered recently, called a “starburst”
galaxy. In this type of galaxy, new stars just seem to ‘burst out’ very
(I) THE MILKY WAY
The Milky Way is over 100,000 light-years wide. It is called
a spiral galaxy because it has long arms which spin around like a giant
pinwheel. Our Sun is a star in one of the arms. When you look up at the night
sky, most of the stars you see are in one of the Milky Way arms.
Before we had telescopes, people could not see many of
the stars very clearly. They blurred together in a white streak across the
sky. A myth by the ancient Greeks said this white streak was a “river of
milk”. The ancient Romans called it the Via Galactica, or “road made of
milk”. This is how our galaxy became known as the Milky Way.
A light-year is the distance light travels in one year.
It is 9.5 trillion (9,500,000,000,000) kilometers. The size of a galaxy may
be as little as a thousand light-years across or as much as a million
Quasars are farther away from Earth than any other known
object in the universe. Because they are so far away from us, it takes
billions of years for the light they give off to reach Earth. The light
stays the same, it just has to travel a long time to get to us. When we look
at a quasar, it is like we are looking back in time. The light we see today
is what the quasar looked like billions of years ago. Some scientists think
that when they study quasars they are studying the beginning of the
Quasars give off huge amounts of energy. They can be a
trillion times brighter than the Sun! Astronomers think that quasars are
located in galaxies which have black holes at their centers. The black holes
may provide quasars with their energy. Quasars are so bright that they drown
out the light from all other stars in the same galaxy. The word quasar is
short for quasi-stellar radio source. Quasars give off radio waves, X-rays,
gamma-rays, ultraviolet rays, and visible light. Most of them are larger
than our solar system.
Quasars give off more energy than 100 normal galaxies
Matter is anything that takes up space and has mass. We
are used to matter which we will call visible matter. Visible matter can be
seen because it gives off light or reflects light given off by another
object. Dark matter cannot be seen. It does not give off light or reflect
Scientists believe that over ninety-percent of the matter in the universe is
dark matter. They also believe that by studying dark matter they will gain
new information about the universe. Some of the information they hope to
discover is the size, shape and future of the universe. Scientists also hope
to learn about how galaxies formed by studying dark matter.
Scientists cannot see dark matter, so they have a special
way of studying it. Scientists study dark matter by looking at how it
affects visible matter. Scientists use computers and satellites to study
dark matter. The Hubble Space Telescope has taken pictures that have helped
scientists discover where dark matter can be found.
Dark matter was once called “missing matter”. It was
called this because scientists looking at the sky could not find it.
A star is a huge, shining ball in space that produces a
tremendous amount of light and other forms of energy. The sun is a star, and
it supplies Earth with light and heat energy. The stars look like twinkling
points of light — except for the sun. The sun looks like a ball because it
is much closer to Earth than any other stars. Stars are formed initially
from gas and dust. They are composed mainly of the hydrogen gas. Gas are
very hot and give off huge amounts of energy in the form of heat and
light.Our Sun is a medium sized star. Stars have a life-span of about 10
billion years, after which they will cease to exist. Stars are very
far away from Earth. The closest Star is about 23.5 trillion miles away.
WHY ARE STARS HOT AND BRIGHT?
Nuclear Fusion and Nucleosynthesis Stars are giant nuclear
reactors. In the center of stars, atoms are taken apart by tremendous atomic
collisions that alter the atomic structure and release an enormous amount of
energy. This makes stars hot and bright. In most stars, the primary reaction
converts hydrogen atoms into helium atoms, releasing an enormous amount of
energy. This reaction is called nuclear fusion because it fused the nuclei
(center) of atoms together, forming a new nucleus. The process of forming a new
nucleus (and element) is nucleosynthesis.
WHAT IS THE CLOSEST STAR?
The closest star to us is the sun! Other than that, the
closest star is Proxima Centauri, aka Alpha Centauri C (the dimmest star in the
Alpha Centauri system). Proxima Centauri is 4.3 light-years from the Sun.
WHY DO STARS TWINKLE?
The scientific name for the twinkling of stars is stellar
scintillation (or astronomical scintillation). Stars twinkle
when we see them from the Earth’s surface because we are
viewing them through thick layers of turbulent (moving) air in the Earth’s
Stars (except for the Sun) appear as tiny dots in the sky; as their light
travels through the many layers of the Earth’s atmosphere, the light of the star
is bent (refracted) many times and in random directions (light is bent when it
hits a change in density - like a pocket of cold air or hot air). This random
refraction results in the star winking out (it looks as though the star moves a
bit, and our eye interprets this as twinkling).
Stars closer to the horizon appear to twinkle more than stars
that are overhead - this is because the light of stars near the horizon has to
travel through more air than stars overhead and subject to more refraction.
Also, planets do not usually twinkle - they are big enough that this effect is
not noticeable (except when the air is extremely turbulent).
Stars would not appear to twinkle if we viewed them from
outer space (or from a planet/moon that didn’t have an atmosphere)
The Solar System
The word “solar” refers to the sun; the sun is one of the 150
billion stars of the Milky Way. It moves through space taking with it a larger
family of objects. The whole group is called the solar system. Our solar system
is elliptical in shape. The sun is the center of the solar system. Solar system
is always in motion. Its largest and most important members are the nine known
planets and their moons, along with smaller objects called comets, asteroids,
and meteoroids that orbit the sun. The sun is the biggest object in our solar
system. It contains 99.8% of the solar system’s mass. Many scientists believe
that our Solar System is over 4.6 billion years old.
Scientists believe that the solar system was formed when a cloud of gas
and dust in space was disturbed, may be by the explosion of a nearby star
called SUPERNOVA. This explosion made waves in space that squeezed
the cloud of gas and dust. Squeezing made the cloud start to collapse, as
gravity pulled the gas and dust together, forming a solar nebula. The sun’s
nuclear fires, ignited at the dense center of this nebula. The planets were
born in the swirling currents of the great cloud.
The planets Mercury, Venus, Earth, and Mars, evolved as globes of rock
that are present near the Sun. They were too small and their gravitational
fields too weak to capture. However, far from the sun, the massive planets
Jupiter and Saturn, with powerful gravitational fields, did attract and hold
thick gaseous atmospheres of Hydrogen and Helium.
- The Sun is our closest star. It is a member of the Milky Way galaxy. The
diameter of the Sun is 1,392,000 kilometers. It is believed to be over 4
billion years old. The Sun is a medium sized star known as a yellow dwarf.
The Sun spins slowly on its axis as it revolves around the galaxy. The Sun
is a large ball of gas consisting mostly of hydrogen and helium. The Sun is
about 109 times larger than Earth.
- The center, or core, of the Sun is very hot. The temperature in its core
is estimated to be over 15,000,000 degrees Celsius. A process called
“nuclear fusion” takes place there. Nuclear fusion produces a lot of energy.
Some of this energy travels out into space as heat and light. Some of it
reaches the Earth! We can see storms on the Sun’s surface called as
“sunspots” because they look like dark spots on the Sun’s surface. The Sun
also produces big explosions of energy called solar flares. These flares
shoot fast moving particles off the Sun’s surface. These particles can hit
the Earth’s atmosphere and cause a glow called an Aurora.
The Sun has several layers: the core, the radiation zone, the convection
zone, and the photosphere (which is the surface of the Sun). In addition,
there are two layers of gas above the photosphere called the chromosphere
and the corona. The following are the events that occur on the Sun
frequently: sunspots, solar flares, solar wind, and solar prominences.
Without the Sun, the Earth would be a lifeless ball of rock and ice. The
Sun warms our planet, creates our weather, and gives energy to plants
providing food and energy to support life on Earth. The Sun is a large ball
of gas consisting mostly of hydrogen and helium. The Sun is about 109 times
larger than the Earth. Scientists estimate that the temperature at the
center of the Sun is about 15 million degrees Celsius. This is similar to
exploding a hydrogen, or nuclear, bomb. Large explosions on the Sun’s
surface cause solar flares that shoot up high into space. The surface
temperature is about 4000 degrees Celsius. Energy released from the Sun
radiates in all directions, reaching the Earth and other planets. The
further the planet is from the Sun, the less energy it receives.
Other Objects in the Solar System
Asteroids: Asteroids are rocky and metallic objects that orbit the Sun
but are too small to be considered as planets. They are known as minor
planets. Most of the asteroids in our solar system can be found orbiting the
Sun between the orbits of Mars and Jupiter. This area is sometimes called
the “asteroid belt”. A few asteroids approach the Sun more closely.
Asteroid belt: The asteroid belt is a doughnut shaped
concentration of asteroids orbiting the Sun between the orbits of Mars and
Jupiter, closer to the orbit of Mars. Comets: A comet is made of dirty ice,
dust, and gas. Scientists believe that comets are made up of material left
over when the Sun and the planets were formed. When a comet gets close to
the Sun, part of the ice starts to melt. Scientists think about 100,000
million comets orbit the Sun. There are some comets orbiting the Sun like
planets. Their orbits take them very close to and very far away from the
Sun.Comet can be seen only when it comes close to the Sun. The Sun’s heat
melts the comet’s ice to form glowing gases. The gases stream out into a
long tail that can extend to millions of kilometers.
Meteorites: Besides asteroids some smaller pieces of
rocks and dust also orbit the Sun. These pieces of rock or dust enter the
Earth’s atmosphere. As they pass they encounter great friction, which causes
them to heat up and burn out. These burning pieces of rock or dust are
called as meteors. Although they are not stars, people call them as shooting
stars, because they flash light across the sky. Most of the meteors burn up
before they reach the Earth. Some are so large that a part of it reaches the
ground as a meteorite. Any leftover part that does strike the Earth is
called a meteorite. A meteorite can make a hole or crater in the ground when
it hits it. The larger the meteorite, the bigger the hole.
Sun Reference Data
1.4 million km (870,000 miles)
4.5 billion years
330,000 x Earth
Distance from Earth:
149.6 million km
Distance to Nearest Star:
4.3 light years
3 million km/hr.
390 billion billion
8 - 11 years
(22.5 milliono F)
Rotation Period at Equator:
25 Earth days
Rotation Period at Poles:
35 Earth days
By the current count of astronomers, our solar system
includes 8 planets and 5 dwarf planets. The planets were formed during the
process of solar system formation, when clumps began to form in the disk of gas
and dusk rotating about our young Sun. Eventually, only the planets and other
small bodies in the solar system remained. The four rocky planets at the center
of the solar system Mercury, Venus, Earth, Mars, are known as the inner planets.
Jupiter, Saturn, Uranus, and Neptune are all composed primarily of gas and are
known as the outer planets.
Mercury, the planet nearest the Sun, is also the smallest
planet in the Solar System. Only slightly larger than the Earth’s moon,
Mercury’s surface is covered with craters. This tiny planet does not have any
rings or moons.
Fair but inhospitable Venus, a “Sister” planet to the Earth
that is very different from our home. Venus does not have any moons or rings.
Earth, our home planet, is a beautiful blue and white ball
when seen from space. The third planet from the Sun, it is the largest of the
inner planets. Earth is the only planet known to support life and to have liquid
water at the surface. Earth has a substantial atmosphere and magnetic field,
both of which are critical for sustaining life on Earth. Earth is the innermost
planet in the solar system with a natural satellite – our Moon.
Mars, Earth’s outer neighbor, is the fourth planet from the
Sun. Mars’ bright appearance and reddish color stand out in the night sky.
Impressive surface features such as enormous volcanoes and valleys are
frequently obscured by huge dust storms.
Jupiter is the largest planet in the solar system. When
approached from afar, its fantastic striped atmosphere gradually reveals
intriguing clouds that move around the planet. Rich in historical and cultural
connections, Jupiter is the site of recent comet impacts and continuing
Saturn, the sixth planet from the Sun, has the most
spectacular set of rings in the solar system. We now know that Saturn has 62
moons in addition to its complex ring system
Uranus, the seventh planet from the Sun, has its spin axis
almost in the plane of its orbit about the Sun. This produces unusual seasons
and also causes unique magnetic and electric field structures. Uranus has a
faint ring system and 27 known moons.
Neptune, the eighth and furthest planet from the Sun, is a
very cold place. Its bluish color is caused by small amounts of methane gas in
its atmosphere. The planet has 13 moons and a very narrow, faint ring system.
In 2006 the International Astronomical Union (IAU) approved a
new classification scheme for planets and smaller objects in our Solar System.
Their scheme includes three classes of objects: “small solar system bodies”
(including most asteroids and comets), the much larger planets (including Earth,
Jupiter, and so on), and the new category of in-between sized “dwar+f planets”.
There are currently five official dwarf planets. Pluto, formerly the smallest of
the nine “traditional” planets, was demoted to dwarf planet status. Ceres, the
largest asteroid in the main asteroid belt between Mars and Jupiter, was also
declared a dwarf planet. The three other (for now!) dwarf planets are Eris,
Makemake, and Haumea. Pluto, Makemake, and Haumea orbit the Sun on the frozen
fringes of our Solar System in the Kuiper Belt. Eris, also a Trans-Neptunian
Object, is even further from the Sun.
What’s the difference between regular planets and dwarf
It’s partly an issue of size, with dwarf planets being
smaller. But just how big does a planet need to be to become a full-fledged
planet instead of a dwarf? You might think the minimum size requirement is
arbitrary, but the size cutoff is actually based on other properties of the
object and its history in the Solar System. Both planets and dwarf planets orbit
the Sun, not other planets (in which case we call them moons). Both must be
large enough that their own gravity pulls them into the shapes of spheres; this
rules out numerous smaller bodies like most asteroids, many of which have
irregular shapes. Planets clear smaller objects out of their orbits by sucking
the small bodies into themselves or flinging them out of orbit. Dwarf planets,
with their weaker gravities, are unable to clear out their orbits.
Though there are just five dwarf planets now, their number is expected to grow.
Scientists estimate there may be 70 dwarf planets amongst outer solar system
objects that have been discovered already. Since we don’t know the actual sizes
or shapes of many of the objects we’ve found (because they are so far away), we
can’t yet determine whether they are actually dwarf planets or not. More
observations and better telescopes will help us determine which other objects
are dwarf planets.
The Earth is the third planet from the Sun in our Solar
System. It is the planet, we evolved on and the only planet in our Solar System
that is known to support life. It is 149.6 million kilometers away from the Sun.
It has one moon. The Earth is the fifth-largest planet in our Solar System
(after Jupiter, Saturn, Uranus, and Neptune)
The Earth’s atmosphere is ideal for supporting life. Most
of the Earth’s surface is covered with water. From the sky, the Earth
appears blue. Earth is the only inner planet in our solar system that has
liquid water on its surface. Seventy percent of the Earth’s surface is
covered with water. Mountains, volcanoes, deserts, plains, and valleys cover
the remaining 30 percent. Earth has an atmosphere made up of many different
gases. The atmosphere gives us air to breathe. We live on the planet Earth.
Each day on the Earth takes 23.93 hours (that is, it
takes the Earth 23.93 hours to rotate around its axis once). Each year on
the Earth takes 365.26 Earth days (that is, it takes the Earth 365.26 days
to orbit the Sun once).
i Rotation and Revolution of the Earth
Rotation of the Earth: The Earth orbits around the
Sun. It takes one year to go around the Sun one complete time. The Earth
also rotates, or spins, on its axis. It takes one day to spin around
itself one complete time. The Earth’s axis is not straight up and down,
but tilted at an angle of 23.5 degrees. The rotation is what causes the
change from day to night. This tilt is responsible for having seasons.
If the Earth were not tilted, we would have the same season all year
Revolution of the Earth: The movement of the Earth
around the Sun in a fixed orbit is called as revolving. One full orbit
around the sun is one revolution. The Earth takes 365 days or 1 year to
complete one revolution. The Earth revolves around the Sun because of
gravity. The Earth really rotates 365 ¼ times during each revolution.
The calendar always has 365 days, after every 4 years, the earth has
made one extra rotation. How ever, one extra day is added to the month
of February once in every four years forming a leap year with 366 days.
ii Day and Night
The Earth exhibits two different kinds of motion. One
is rotation around its own axis and another movement is it revolves
around the sun in a fixed orbit. The spinning of the Earth around its
own axis causes day and night. It takes 24 hours to complete one
rotation. However, sunlight shines only on the half of the Earth facing
the Sun. That half has day the other half is dark and has a night.
The moon is a satellite that revolves around the Earth in an
oval shaped orbit. Earth’s moon is smaller than the Earth. It is smaller than
the sun. It looks bigger because it is much closer to the Earth. It is 240,000
miles (384,000 kilometers) away.
Moon appears in the nighttime. The Moon turns so slowly that
a moon’s day is two Earth-weeks long. Then it is night for the next two weeks.
The moon is very hot during daytime and gets very cold at night. It appears
hotter than boiling water because there is no air to protect the moon from the
hot sunlight. It gets very, very cold during night because there is no air to
hold heat on the moon. The moon has no atmosphere, air or water, to support
life. Moon can be seen clearly with your eyes, binoculars, or a telescope.
Many manned and unmanned spacecrafts from the United States and Russia, Japan,
China, India, have landed on the moon to study its surface. The surface of the
moon has many craters caused by being hit by large meteoroids and asteroids. The
moon is much like Earth in some ways. Its rocks are similar to Earth rocks. Lava
rock from underground volcanic eruptions look like the lava rocks found on
Rotation of the Moon
The Moon takes 29 1/3 days to make one orbit around the
Earth. It also takes 29 1/3 days to complete one rotation on its axis.
The phases, or changing appearance, of the Moon depends
on its position in relation to the position of the Sun.
Atmosphere of the Moon The moon is much like the Earth in
some ways. Its rocks are similar to Earth rocks. The moon has no atmosphere,
air or water, to support life. There are tall mountains and flat, dusty
plains on the moon. The big holes on the moon are called Craters, which are
made when space rocks hit the moon.
(a) Light from the Moon
We always see the one side of the Moon from the Earth. You
have to go into space to see the other side. Moon does not make its own light.
We can see it because it reflects light from the sun.
(b) Eclipse of the Moon
Every object, the Sun shines on casts its shadow. The Earth
and moon also cast their shadows on each other. Most of the times these shadows
fall on empty space. But sometimes we can see them from the Earth. As the moon
revolves around the Earth, at times the Sun, the Earth, and the Moon are in a
The Earth blocks the sun’s light falling on the Moon. The Moon appears to be
dark for a brief period, after which you would see a full moon. This is called
as “Lunar Eclipse”.It occurs when the shadow of the Earth falls on the Moon. It
occurs only on a full moon day, but not on all full moon days.
When the moon casts its shadow on the Earth it is called as “Solar Eclipse”.
It occurs only on a new moon day, but not on all new moon days.
You might have observed that the seasons change as a year progresses. You
might have felt the heat of the summer and the chill in the winter. What causes
these seasonal changes?
The Earth takes about one year to revolve around the Sun, and also it rotates on
its axis. The axis on which the Earth rotates is slightly tilted, and this
causes the cycle of seasons. While the Earth is revolving, if the Northern
Hemisphere gets the direct sunlight, it has summer and on the other side, the
Southern Hemisphere, will have winter. When the Southern Hemisphere faces direct
sunlight it will have summer and it will be winter in the Northern Hemisphere.