Astronomy

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Astronomy

• Searching beyond Earth's boundary

The Andromeda Galaxy, one of the two galaxies
heading towards our Milky Way Galaxy
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What are stars?
Stars are fiery balls of gases (mainly hydrogen
and helium) racing through outer space at
thousands of kilometers per second. Near the
Earth, stars appear to stay at the same place in
outer space because they are so far away that it
will take thousands of years for any changes in
their positions to become noticeable.
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What are stars?
Our Sun is also a star. It looks so different
from the other stars because it is comparatively
very near to Earth. The next nearest star
visible to the eyes is about 4 light-years (LY)
away. No wonder they all look like point light
sources.
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What are stars?
There are billions of stars in the Universe. We
cannot see all the stars in this picture with our
naked eyes because they are too dim. The dark
patch in this picture is an interstellar dust and
gas cloud where stars are born
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What are stars?
Although stars remain practically stationery in
outer space, from Earth, we see them rising in
the East in the evening and setting in the West
in the morning, just like what the Sun does in
the day. This apparent motion of the stars is
due to the rotation of Earth.
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What are stars?
There is one star directly above the North Pole
which remains at the same position no matter how
the Earth rotates. This star is called Polaris
and is a very important navigation guide for
people in the past. We will discuss more of this
under the topic on constellations.
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What are stars?
The twinkling of stars are caused by air movement
in our atmosphere. We don't see this effect
with our Sun or planets in the Solar System
because these are very much brighter than those
far away stars.
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What are stars?
To locate stars (and other objects) astronomers
have developed star charts based on the celestial
sphere model. In this model, the Earth remains
stationery in the centre while the whole Universe
revolves around it.
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What are stars?
This is a typical astronomer's star chart. All
objects are located by their Right Ascension (in
hours) and declination (in degrees). There are
simpler versions for the amateurs.
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What are constellations?
From Earth, stars appear not only stationery,
they also appear to be equally far away. Our
imaginations give rise to images of star patterns
known as constellations.
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What are constellations?
Constellations are used to divide the vast sky
into smaller manageable regions. They are useful
as memonics. The ancients relied on the
appearance of different constellations to keep
track of the time of the year.
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What are constellations?
What is this constellation? Answer Cynus the
swan
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What are constellations?
If you draw a line joining the two stars Dubhe
and Merak in the Big Dipper and extend it to the
handle of the Small Dipper, you can locate the
North Star or Polaris. This star remains at the
same position no matter how the Earth rotates.
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What are constellations?
Once Polaris is located, we can measure its angle
of elevation. This is equal to the latitude from
which Polaris is measured.
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What are constellations?
As the Earth orbits around the Sun, the latter
appears to move along a path known as the
ecliptic. Constellations located along this
apparent path of the Sun are known as the
constellations of the Zodiac. These are the
signs used in astrology.
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What are constellations?
Because of the "flatness" of the Solar System,
most of the planets appear to move along a path
close to the ecliptic. Their positions in the
sky can therefore be located with the help of the
Zodiac constellations.
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Sources of stars energy
How do stars generate so much energy for so
long? Astronomers have discovered that all stars
have roughly the same composition as our Sun
71 hydrogen 27 helium and 2 other heavier
elements. Every second, our Sun converts 600
million tons of hydrogen into helium to produce
the energy needed to maintain a pressure high
enough to prevent its collapse from the crushing
effect of gravity.
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Sources of stars energy
The conversion of hydrogen to helium takes place
at very temperature in a process known as nuclear
fusion. In our Sun, this is about 15 million
degree centigrade and occurs at the core. We
therefore know that the Sun, or any other stars
will not last forever!
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How did we find out so much?
Astronomers use spectroscopy to determine the
amount of light produced by a star (luminosity)
and its temperature. When measurements from a
cluster of stars are plotted on a graph (known as
Hertzprung-Russell (HR) diagram), they fall into
distinct groups.
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How did we find out so much?
The Hertzprung-Russell (HR) diagram shows that
90 of the stars in a cluster belong to the group
called "main sequence". These are stars which
still have a long way to go. The blue ones are
the ones that are burning out at the fastest rate.
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How did we find out so much?
The group known as "red giants" are stars
reaching the end of their lives and the "white
dwarfs" are leftover of small stars which have
expired. In that sense, "white dwarfs" are not
real stars. They shine because of their very high
temperatures.
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How did we find out so much?
Stars are born in clusters at about the same time
instead of at random. From their HR diagrams,
astronomers can estimate the age of each cluster.
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How did we find out so much?
The data which astronomers used to plot the HR
diagrams are obtained from spectroscopic analyses
of the star light. Most of us are familiar with
the continuous spectrum when light from a heated
solid passes through a prism.
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How did we find out so much?
However, light from a heated gas produces a
unique emission spectrum (colourful lines against
a dark background). Since stars are gases, their
spectrum tells us what these gases are.
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The evolution of stars
Stars are formed from interstellar gas and
dust. As the gas and dust collapse, it generates
tremendous amount of heat and temperature. When
hydrogen begins to fuse to helium, a star is born.
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The evolution of stars
Stars do not last forever. When a low mass star
uses up its hydrogen, it undergoes a series of
changes and finally turns into a white dwarf.
This continues to shine for millions of years
until it becomes too cold to give out light.
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The evolution of stars
When a high mass star uses up its hydrogen, it
too undergoes a series of changes. The end
however, is a massive explosion known as a
supernova. This leaves behind either a neutron
star or a black hole.
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The evolution of stars
Black holes are found by detecting the tremendous
amount of x-rays being emitted when it sucks in a
nearby star. This discovery was made possible
when x-rays telescopes were into outer space.
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The evolution of stars
These pictures show the explosion of a massive
star. The first supernova was observed in China
about 1000 years ago. At that time, the
astronomers dubbed it as a "guest star" because
it appeared only for a short while.
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The evolution of stars
This photograph shows a close up view of the crab
nebula (the term nebula is used to describe fuzzy
objects). It is made up of debris from a
supernova.
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The End Thank You