Introduction
The night sky is much more than just dots of light against a
dark screen. I would say it's more like a completely different world out there.
Let me take you on a journey to that dark yet illuminated world.....
Stars are huge fiercely burning balls of gas with powerful
nuclear reactions* at their cores. They
are the most widely recognized astronomical objects, and represent the
most fundamental* building blocks of galaxies.
Star Life and Death
Stars aren't forever, they do die, and the story of their life
is actually pretty interesting.
Most stars are born in enormous star nurseries called nebulae. A
nebula is basically made up of hydrogen, helium and dust. Gravitational pull
in denser parts of the nebula causes
matter to clump together increasing in
mass and temperature, generating its own gravity.
Protostars
As a large clump of gas shrinks and grows hotter, it is called a
protostar - a star in the making. Most protostars become hot and dense* enough
to trigger off nuclear reactions and start to shine.
Main Sequence
The stage where a star starts to shine is called the main
sequence and lasts for billions of years. The star generates energy through
nuclear fusion (turning hydrogen into helium in the core). The sun has been in
its main sequence for more than four billion years.
When a star of the sun's size has used up most of the hydrogen
fuel in its core, it swells up to become a red giant, and when bigger stars (8
or more times bigger than the sun) start to swell, they become supergiants.
Betelgeuse (Alpha Orionis) is a red supergiant so huge, that if it replaces the
sun in our solar system, its outer atmosphere would extend past the asteroid
belt
Fade Away
Some protostars don't have enough mass to trigger off nuclear
reactions and become stars. Instead, they generate smaller amounts of energy
through continuing contraction*. These failed stars are called brown dwarfs.
They radiate remaining heat into space, slowly fading until there is no energy
left. At this point they are known as black dwarfs.
Planetary Nebula
After completing the main sequence, stars around the same size
of the sun start to collapse, increasing in density and temperature. These
stars then swell to an enormous size and throw off their outer layers as giant
gas clouds. They clouds cool to form a planetary nebula surrounding the star's
remains, which are known as white dwarfs. More than 3,000 planetary nebulae
have been observed. Some examples of nebulae are: Orion Nebula, Dumbbell
Nebula, Horse Head Nebula, etc.
White Dwarf
Although white dwarfs have run out of hydrogen or helium fuel to
burn in their cores, they still continue to shine for millions of years. They
can range from hot white to cool red. Scientists estimate that a white dwarf is
so dense that one teaspoonful of its matter would weigh around 5,000 tonnes.
Just like brown dwarfs, white dwarfs fade over time to become black dwarfs.
Supernova!
As some massive stars die, their cores contract sharply and rise
in temperature by millions of degrees. The cores absorb more and more energy
and then erupt in a gigantic explosion. In the first ten seconds of typical
supernova explosion, a hundred times more energy is released than the sun will
ever generate in its entire lifetime.
Neutron Star
Some massive star deaths result in their core collapsing in
itself to form a neutron star: the smallest and densest stars in the Universe.
Neutron stars may be as small as under 20km (12 miles) in diameter yet contain
the same mass as the sun. Rapidly spinning neutron stars send out radio waves
that we can pick up on earth. These
stars are known as pulsars.
Black Holes
Some stars collapse even further into a dense point called a
singularity. The space immediately around a singularity is called a black hole.
Black holes are so dense and the pull of gravity so strong that nothing, not
even light can escape from it. Astronomers cannot observe a black hole
directly, only its effects on nearby objects, such as the pull of gas into the
hole, which can release powerful X-Rays*.
Now that you know about the life cycle of a star, you must be
wondering about the types of stars in the Universe. Luckily, that's what the
next chapter is all about.............
Star Types
Stars are just like people, they come in all sorts of colors and
sizes, and most of them have names too.
There are 7 basic kinds of stars:
Type O
-
hottest, brightest, biggest kinds of stars
-
Color: violet- blue
-
These stars end their lives quickly in supernova explosions,
ending up as black holes or neutron stars
-
Examples: Naos and Mintaka
Type B
-
Slightly cooler and less massive
-
Examples: Spica A, Rigel
Type A
-
Due to their high mass, these stars are several times brighter
than the sun
-
Examples: Sirius, Vega, Zaniah
Type F
-
Color: yellowish white
-
Slightly smaller in size compared to type A stars
-
Examples: Pole Star (Polaris) , Canopus
Type G
-
Almost the same mass as the sun
-
Examples: the sun, Alpha Centauri A, Capella
Type K
-
These stars are:
-
Generally somewhat lighter than the sun, tend to be long lived
and stable
-
Color: Orange
-
More common than other stars
-
Examples: Arcturus, Albedran
-
These stars are the most common type of stars
-
Examples: Proxima Centauri (Nearest star to the earth after the
sun), Mira
So just what are these constellations you keep hearing about? A
constellation is a group of stars that form patterns when seen from earth. There
are 88 modern constellations recognized by the International Astronomical
Union. A few common ones are: Orion, Cassiopeia, Canis Major, Andromeda, etc.
-
The Zodiac is a group of 12 constellations that lie along the
ecliptic (the plane in which most of our Solar System lies)
-
The constellations that are part of the Zodiac are: Capricorn,
Aquarius, Pisces, Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpius
and Sagittarius
-
Largest constellation: Hydra
-
Some constellations are
visible only from the Northern Hemisphere
and some are visible only from the Southern Hemisphere.
-
Constellations that are visible in both hemispheres may appear
upside down in the Southern Hemisphere.
So now whenever you look up at the night sky, you'll know that
there is much much more to it than meets the eye................
Glossary
-
Fundamental: basic, foundational
-
Dense: having the component parts closely packed together
-
Contraction: the process of becoming smaller
-
X- Rays: a kind of ray part of the electromagnetic spectrum.
X-rays are used for taking images inside the body for diagnosis and also to discover
paintings hidden under other artwork.
Bibliography
Books:
WOW!
Science
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Me How
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Me Why
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