1. Stars can be measured using parallax, which is the apparent shift in a star's position relative to distant objects as seen from different positions. Nearby stars have a larger parallax and can be used to determine distances. 2. Astronomers classify stars based on properties like color, size, brightness, chemical composition, and mass. Stars are placed on the Hertzsprung-Russell diagram according to these characteristics. 3. Stars are formed from dense clouds of gas and dust called nebulae. Nuclear fusion powers stars for most of their lives on the main sequence. When the fuel runs out, stars evolve in different ways depending on their mass.

1. THE ME: THE UNIVERSE/ ASTRONOMY
STARS
If you look into the sky at night, you’ll see that stars look like points of light. You will probably also notice
that some stars are brighter than others. If you look closely, you’ll see that some stars have different
colors. However, you can’t tell how large or how far away a star is simply by looking at it. You can’t poke
the sun, which is considered to be a fairly average star.
DISTANCES TO THE STARS
Although the sky seems full of stars, most of the universe is nearly empty space. The seeming
contradiction exist because most stars are separated by vast distances.
The Light-Year ; you wouldn’t measure the distances between two distance cities in centimeters.
Similarly, because stars are so far apart, it’s not practical to measure their distances in units that might
be used on Earth, such as kilometers. Instead, astronomers use much larger units, including the light-
year is the distance that light travels in a vacuum in a year, which is about 9.5 trillion kilometers.
Proxima Centauri, the closest star to the sun, is about 4.3 light-years away.
Parallax; Stars are so far away that astronomers cannot measure their distance directly. Astronomers
have developed various methods of determining their distances to stars. Different methods are used for
stars at different distances.
To understand how astronomers can measure distances to nearby stars, hold your thumb up at arm’s
length in front of you. Close your left eye and look at your thumb with just your right eye open. Then
cover your right eye and look with just your left eye open. Even though you didn’t move your thumb, it
appeared to move relative to the background because you look at it from slightly different angles. The
apparent change in position of an object with respect to a distance background is called parallax.
As earth moves in its orbit, astronomers are able to observe stars from two different positions.
Imagine looking at the stars in winter and then six months later in summer. During this time, Earth has
moved from one side of its orbit to the other-a distance of a distance of about 300 million kilometers.
Because people on Earth is looking from a different angle, the nearby star appears to move against the
more-distant background stars.
Before the invention of the telescope, astronomers couldn’t measure a star’s position very
accurately. They couldn’t detect the apparent movement of a single nearby star as Earth moved around
the sun. With the invention of the telescope, astronomers could measure the position of stars with
much greater accuracy .Astronomers measure the parallax of nearby stars to determine their distance

2. from Earth. The closer a star to Earth, the greater is its parallax. Astronomers have measured the
parallax of nearby stars and determine their distances from earth. However, if a star is too far away, its
parallax is too small to be measured. With present technology, the parallax method gives reasonable
accurate distance measurements for stars within a few hundred light-years. Astronomers have
developed other ways to estimate distances to more-distance stars.
PROPERTIES OF STARS
There are many types of different stars. Astronomers classify stars by their color, size, and brightness.
Other important properties of stars include their chemical composition and mass.
Colour and Temperature: Have you ever looked closely at a candle flame? The hottest part of the flame
near the wick is blue or white, while the cooler flame tip is orange. A propane torch flame is blue. Dying
campfire embers are red. You can estimate the temperature of a flame from its color. In the same way, a
star’s color indicates the temperature of its surface. The hottest stars, with surface temperature above
30, 000 k, appear blue. The surfaces (photosphere) of relatively cool are red stars are still toasty 3000 k
or so. Star with surface temperature between 5000 and 6000 k appear yellow, like the sun. The
differences between hot blue stars and cool red stars can be seen with the unaided eye. More precise
measurement of stars’ temperatures can be made by studying stars’ spectra.
Brightness ;When you walk along a street at night, look up at a row of street lights. The closer lights the
closer lights look bright and the more distant lights look dim. However, the more distant lights are not
really dimmer. They appear dim to you because at greater distance, their light is spread out over a
greater area, so a smaller portion enters your eyes. The same is true for the light emitted by stars.
You might think that closer stars will always appear brighter than more distant stars. Astronomers
have discovered, however, that the brightness of stars can vary by a factor of more than a billion. So,
stars that look bright may actually be farther away than stars that appear dim .
Although the sun appears to be the brightest star in our sky, it is really a star of only average brightness.
The sun appears very bright to us because it is much closer than other stars. The brightness of a star as it
appears from Earth is called its apparent brightness. The apparent brightness of a star decreases as its
distance from you increases.
If you move away from a street light or a star, it shines just a brightly as before-but to you it appears
fainter. Absolute brightness is how bright a star really is. A star’s absolute is a characteristic of the star
and does not depend on how far it is from Earth. You can calculate a star’s absolute brightness if you
know its distance from Earth and its apparent brightness.
Size And Mass; Once astronomers know a star’s temperature and absolute brightness, they can
estimate its diameter and then calculate its volume. However, there is no direct way of finding the mass
of an isolated star. Instead, astronomers are able to calculate the masses of many stars by observing the
gravitational interaction of stars that occur in pairs. From such observations, astronomers have
determined that, for the most stars, there is a relationship between mass and absolute brightness.
Astronomers have found that many stars are similar to the sun in size and mass.Composition; A
spectrograph is an instrument that spreads light from a hot glowing object, such as a light bulb or a star,
into a spectrum Astronomers can use spectrographs to identify the various elements in a star’s
atmosphere.

3. Each star has its own spectrum. The elements within a star’s atmosphere absorb light from the star’s
photosphere. Each element absorbs light of different wavelengths from the star’s continuous spectrum.
The result is bright spectrum, such as absorption lines that show where light has been absorbed. Just as
fingerprints can be used to identify a person, a star’s absorption lines can be used to identify different
elements in the star.
Absorption lines of most elements have been identified in the spectra of stars. Observations of
such lines in many stars have shown that the composition of most stars is fairly similar. Most stars have
chemical makeup that is similar to the sun, with hydrogen and helium together making up 96 to 99.9
percent of the stars’ mass.
Questions
A)
1. How can the distance to a star be measured ?
2. How do astronomers categorize stars?
3. What elements are found in stars?
B) Terminology; write the meanings of the following words
1. star
2. light-year
3. parallax
4. apparent brightness
5.absolute brightness
6. absorption lines
C)
mass
Properties
stars
1.
include
2.

4. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
How Stars Formed
The space around stars contains gas and dust. In some regions this matter is spread thinly; in others it is
packed densely. A nebula is a large cloud of gas and dust spread out over a large volume of space. Some
nebulas are glowing clouds lit from within by bright stars. Other nebulas are cold, dark clouds that block
the light from more distant stars beyond the nebulas.
Stars form in the densest regions of nebulae. Stars are created by gravity. Gravity pulls a nebula’s
dust and gas into a denser cloud. As the nebula contracts, it heats up. A contracting cloud of gas and
dust with enough mass to formed a star is called protostar. As a protostar contracts its internal pressure
and temperature continue to rise. A star is formed when a contracting cloud of gas and dust became so
dense and hot that nuclear fusion begins. Pressure from fusion supports the star against the
tremendous inward pull of gravity. This new energy source stabilizes the young star, and it joins the
main sequence.
Adult Stars
Stars spend about 90% of their lives on the main sequence. In all main-sequence stars, nuclear fusion
converts hydrogen into helium at a stable rate. There is an equilibrium between the outward thermal
pressure from fusion and gravity’s inward pull. A star’s mass determines the star’s place on the main
sequence and how long it will stay there.
The amount of gas and dust available when a star forms determines the mass of each young star.
The most massive stars have large cores and therefore produce the most energy. In a large, young star
with 30 times the sun’s mass, gravity exerts a huge inward force, increasing the star’s internal
temperature and pressure. High-mass stars become the bluest and brightest main-sequence stars.
Typically these blue stars are about 300,000 times brighter than the sun but, like gas-guzzling hot rods,
large stars pay a price. Because blue stars burn so brightly, they used up their fuel relatively quickly last
only a few million years
Stars similar to the sun occupy the middle of the main sequence. Yellow star like the sun has a
surface temperature of about 6000 k and will remain stable on the main sequence for about 10 billion
years.
Small nebulas produce small, cool stars that are long-lived. A star can have a mass as low as a tenth
of the sun’s mass. The gravitational force in such low-mass stars is just strong enough to create a small
core where nuclear fusion takes place. This lower energy production results in red stars, which are the
coolest and least bright of all visible stars. A red main-sequence star, with a surface temperature of
about 3500 k, may stay on the main sequence for more than 100 billion years.
The Death Of Star
Stars don’t last forever. When a star’s core begins two run out of hydrogen gravity gains the upper hand
over pressure and the core starts to shrink. Soon, the core temperature rises enough to cause the
hydrogen in a shell outside the core to begin fusion. The energy flowing outward increases, causing the
outer regions of the star to expanding atmosphere moves farther from the hot core and cools to red.

5. The star becomes a red giant. Eventually, the collapsing core will grow hot enough for helium fusion to
occur, producing carbon, oxygen, and heavier elements. In helium fusion, the star stabilizes and its outer
layers shrink and warm up. In this period, the star remains in the upper right part of the H-R Diagram.
The dwindling supply of fuel in a star’s core ultimately leads to the star’s death as a white dwarf,
neutron star, or black hole. The final stages for a star’s life depend on its mass.
Low- And Medium-Mass Star Low mass and medium mass stars which can be as eight times as massive
as the sun, eventually turn into white dwarfs. Such stars remain in the giant stage until their hydrogen
and helium supplies dwindle and there are no other elements to fuse. Then the energy coming from the
star’s interior decreases. With less outward pressure to support the star against gravity’s inward pull,
the star collapses. The dying star is surrounded by a glowing cloud of gas. Such a cloud is called a
planetary nebula because the first ones founded looked like planets when viewed through a small
telescope. As the dying star bows off much of its mass, only its hot core remains. This dense core is a
white dwarf. A white dwarf is about the same size as Earth but has about the same size as Earth but has
about the same mass as the sun. White dwarfs don’t undergo fusion, but glow faintly from leftover from
thermal energy. When a white dwarf becomes to cool to glow visible light, it is called a black dwarf. But
it takes about 20 billion years for a white dwarf to cool down, so the universe hasn’t been here long
enough for any black dwarfs to form yet.
High-Mass Star The life cycle of high-mass star (those with a mass of more than eight times that of the
sun) is very different from the life cycle of lower-mass star. As high-mass star evolve from hydrogen
fusion to the fusion of other elements, they grow into brilliant supergiants. This creates new elements,
the heaviest being iron. A high-mass star dies quickly because it consumes fuel very rapidly.
As fusion slows in high mass star, pressure decreases. Gravity eventually overcomes the lower
pressure, leading to a dramatic collapse of the star’s outer layers. This collapse produces a supernova,
an explosion so violent that the dying star becomes more brilliant than an entire galaxy. Supernovas
produce enough energy to create elements heavier than iron. These elements, and lighter ones such as
carbon and oxygen, are ejected into space by the explosion. The heavier elements in our solar system,
including the atoms in our body, came from a supernova that occurred in our galaxy billions of years
ago.
As a supernova spews material into space, its core continues to collapse. If the remaining core has a
mass less that about three times the sun’s mass, it will become a neutron star. A neutron star is the
dense remnant of a high-mass star that has exploded as a supernova. In a neutron star, electrons and
protons are crushed together by a star’s enormous gravity to form neutrons. Neutron stars are much
smaller and denser than white dwarfs. A spoonful of a neutron star would weigh nearly a billion tons on
Earth! A neutron star with the mass of the sun would be only about 25 kilometers across, the size of a
large city.
Like a spinning ice skater pulling in his arms his arms, a neutron star spins more and more rapidly as
it contracts. Some neutron stars spin hundreds of turns per second! Neutron stars emit steady beams of
radiation in narrow cones. If the neutron star is spinning, these emissions appear to pulse on and off at
regular intervals, like the spinning beacon on a light house. Each time one of these beams of radiation
sweeps across Earth, astronomers can detect a pulse of radio waves. A spinning neutron start that
appears to gives strong pulses of radio wave is called pulsar.

6. As impressive as pulsars are, very massive star can have even more dramatic ends. If a star’s core
after a supernova explosion is more than about three time the sun’s mass, its gravitational pull is very
strong. Gravity causes the core to collapse beyond the neutron-star stage. As the collapse continues, the
pull of gravity increases and the speed required to escape the star’s core reaches the speed of light.
Beyond this point, nothing can escape and a black hole is formed. A black hole is an object whose
surface gravity is so great that even electromagnetic waves, traveling at the speed of light, cannot
escape from it
BLACK HOLE
A black hole is a region of space containing so much matter that it collapses to an infinitely dense point.
The force of gravity in a black hole is so great that anything falling into it, including electromagnetic
waves, becomes trapped
BLACK HOLE
Astronomers now think that many large galaxies, including Milky Way, have black holes in their centers.
The Milky Way’s center is hidden from view by interstellar dust in the spiral arm lying between the Sun
and the galaxy’s center. Radio and infrared telescopes can penetrate the dust, however. Images show a
cluster of several million stars and great turbulence at the Milky Way’s core in a region about three light-
years across. The high mass of These Region indicates the presence of a black hole. However, the area
does not exhibit many typical black hole behaviors, such as x-ray emissions, detected in the centers of
other galaxies
Interpreting Diagrams An object that falls into a black hole would be crushed by its intense gravitational
force.

7. ANSWERS
1. Astronomers measure the parallax of nearby stars to determine their distance from Earth
2. Astronomers use several properties to classify stars.
3.Most stars have composition similar to the sun
1. A star is a large glowing ball of gas in space, which generates energy through nuclear fusion in its core.
2. A light year is the distance that travels in a vacuum in a year, which is about 9.5 trillion kilometers.
3. The apparent change in position of an object with respect to a distant background is called parallax
4. The brightness of a star as it appears from Earth is called its apparent brightness.
5. Absolute brightness is how bright a star really is. It is the characteristic of the star and does not
depend on how far it is from the earth.
6. Absorption lines contain a set of dark lines that show where light has been absorbed.
7. H-R diagrams are used to estimate the sizes of stars and their distances, and to infer how stars change
over time.
8. main sequence

8. GALAXIES
A galaxy is a huge group of individual stars, stars systems, star cluster, dust, and gas bound together by
gravity. Our own galaxy is called the Milky Way. In the 1800s and early 1900s, many astronomers
thought that the universe did not extend beyond the Milky Way. However observations of odd spiral-
shaped objects sparked a spirited scientific debate. The debate focused on whether there so-called
“spiral nebulae” were within or outside the Milky Way. The question was settled only in 1920s, after a
large new telescope was built on Mount Wilson in California. Astronomers could see individual stars,
proving that spiral nebulae were actually distant galaxies consisting in billions of individual stars.
Astronomers now know that there are billions of galaxies in the universe. The largest galaxies
consist of more than a trillion stars. Galaxies vary widely in size and shape. Astronomers classify
galaxies into four main types; spiral, barred-spiral, elliptical, and irregular.
The Milky Way Galaxies On a clear, dark night far from city lights, you can see a faint white band
stretching across the night sky. This is the Milky Way. The Milky Way galaxy has an estimated 200 to 400
billion stars and a diameter of more than100, 000 light years. Every individual star that you can see with
the unaided eye is in our galaxy. The solar system lies in the Milky Way’s disk within a spiral arm, about
two thirds of the way from the center. From Earth we are looking at the rest of the Milky Way edgewise,
so it appears as a band of our night sky, rather than a spiral.
The Milky Way’s flattened disk shape is caused by its rotation. The sun takes about 220 million years
to complete one orbit around the galaxy’s center. At the center of the galaxy is a bulge of stars
surrounded by an immense halo of globular clusters. Recent evidence suggest that there is a massive
black hole at our galaxy’s center. Extending outward and winding through the galaxies disk are spiral
arms of gas, dust, and young stars. Stars are performing in these spiral arms.
Quasars In the 1950’s astronomers were mystified by the discovery of distant objects they called
quasars. By studying their spectra, astronomers have determine that quasars (KWAY zahrz) are the
enormously bright centers of distant, young galaxies. Quasars produce more light than hundreds
galaxies the size of the Milky Way. What makes a quasar so bright? The most likely explanation involves
matter spiraling in to a supper massive black hole with the mass of a billion suns. The gravitational
potential energy of this matter is transformed in to electromagnetic radiation as it falls into the black
hole.
THE BIG BANG THEORY
If the universe is expanding , where were the galaxies in the distant past? Hubble observed that in every
direction you look, galaxies at a given distance are moving away at the same rate. This is what you
would expect if the universe were expanding uniformly.
Astronomers theorize that the universe came in to being at a single moment, in an event called the
big bang. According to this theory, all the matter and energy of the universe were at one time
concentrated in a incredibly hot region smaller than the period at the end of this sentence. The big bang
theory states that the universe began in an instant, billions of years ago, in an enormous explosion.
After The Big Bang The universe expanded quickly and cooled down after the big bang. After a few
hundred thousand years of expansion, the universe was still much smaller and hotter than its now, but
cool enough for atoms to form. Gravity pulled atoms together into gas clouds that eventually evolved

9. into stars in young galaxies. The sun and solar system is formed about 4.6 billion years ago when the
universe was about two thirds of its present size.
Evidence for the theory In 1965, two American physicist, Arno Penzias and Robert Wilson, noticed a
signal on their radio telescope that they couldn’t explain. They eventually realized that they were
detecting a faint distant glow in every direction. Today this glow is called the cosmic microwave
background radiation. This glow is energy produced during the big bang, still travelling throughout the
universe. The existence of cosmic microwave background radiation and the red shift in the spectra of
distant galaxies strongly support the big bang theory.
The big bang theory describes how the expansion and cooling of the universe over time could have
led to the present universe of stars and galaxies. It offers the best current scientific explanation of the
expansion of the observable universe. Variations of the theory continue to be proposed and are being
tested with new observations.
Age Of The Universe Since astronomers know how fast the universe is the is expanding now, they can
infer how long it has been expanding. If you travelled backward in time, all of the matter in the universe
would be at its starting point 13 to 14 billion years ago. Recent measurements of the microwave
background radiation have led to a more precise age. Astronomers now estimate that the universe is
13.7 billion years old.
Asteroids
They are minor planets or remains of a planet found in the solar systemThe asteroid belt is the region
of the Solar System located roughly between the orbits of the planets Mars and Jupiter. It is occupied by
numerous irregularly shaped bodies called asteroids or minor planets.

10. Spiral Galaxy
Barred spiral galaxy

11. Elliptical galaxy, Irregular galaxy

12. Milky Way
The Milky Way, or simply the Galaxy, is the galaxy in which the Solar System is located. It is a
barred spiral galaxy that is part of the Local Group of galaxies. It is one of billions of galaxies
in the observable universe.
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SPM PRACTICES
Objective questions;
In 1929, Edwin Hubble introduced the Big Bang theory. Stars and planets are too far away
that if we are to measure the distance in kilometers, the number would be enormous.
Instead scientists measure such distances in light years. Light travels at 300,000 km per
second so a light year is the distance light travels in one year- 9,460,000,000,000 km.
1. Distance in space are measured in
A) seconds B) kilometres C) light years D) million years
Most heavenly bodies, such as comets and asteroids, are too distant to be reached by
human beings. Hence , most missions have incorporated the use of space probes. Some
space probes are only able to orbit around the planet in order to make observations and
take photographs, such as the space probe Magellan. Others, such as the Galileo, can

13. examine the atmosphere of a planet before landing on the surface. Some space probes can
land and send back information on the chances of life on a planet.
2. Why do space missions incorporate the use of many space probes?
A) To study the chances of life on a planet
B) To examine the atmosphere of the planet
C) To make observations by orbiting around planets
D) To obtain a wide range of information about a distant planet
A. What are the followings?
 Comets
……………………………………………………………………………………………………………………………………
…..
 Meteoroids…………………………………………………………………………………………………………………
………………..
B. Writing; Should the moon be developed?
Giving viewpoints,
Develop the moon Do not develop the moon
*many advantages *waste of time and money
*to gain scientific knowledge-early history of *government funds better spend on more
solar system immediate effect projects eg svhools,
feeding the hungry
*tourist destination
*development of the moon would be too
*cost would be high but many people will be costly and risky
willing to pay for the unique chance to
experience the moon’s low gravity, airless *techniques of mining on the moon is
environment uncertain.
*rich in minerals-iron, aluminum and *such resources would be too expensive to
calcium. produce
*they could be mined for Earth
*stepping stone for future mission to Mars
*technology can improve people’s lives and
create new job.

14. Answers:
B.Comets: chunks of rock and ice.
Meteoroids consist of rock or metal and they can be any size from tiny particles
to huge boulders. When in space they are meteoroids, as they enter the Earth’s
atmosphere, they are meteors and when strike the Earth’s surface they are
meteorites.
Sample answer;
C.Develop the moon;
Developing the moon will have many advantages. One is to gain scientific
knowledge, especially about the early history of solar system. The moon will be
the ultimate tourist destination. Although the cost would be high, many people
would be willing to pay for the unique chance to experience the moon’s low
gravity and airless environment.
The moon is rich in a variety of minerals, including iron, aluminum and calcium.
Once permanent lunar base is established, these resources could be mined for
use on Earth or for the eventual colonization of space.
The moon is a logical stepping stone for future missions to Mars. A lunar vase
would provide a valuable testing ground for technology that would eventually be
useful on Mars. Such technology will also have applications on earth, where it will
improve people’s lives and create new jobs.
Do not develop the moon;
The development of the moon would be a waste of time and money. Scarce
government funds would be better spent on projects with a more immediate
benefit, such as feeding the hungry, improving schools, and building better
housing. In the short run, government funding for space should be focused on
space probes and telescopes. A mission sending crew to Mars would be of much
greater interest and scientific value.
The proposed development of the moon would be too expensive and risky to
receive private funding. Few tourists are likely to be able to pay the high costs of
visiting a resort on the moon .The techniques for mining minerals and energy
resources on the moon are uncertain, and any such resources would probably be
very costly to produce.