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Constellations
Stars
 Our sun is one of about 200 billion stars in our galaxy
(the Milky Way)
 The sun is considered a very average star should have
enough fuel to last another 4-5 billion years
 There are other stars in our galaxy which are just like
our sun but also ones which are smaller (the size of
earth) and much larger (the size of the orbit of Jupiter)
 Each of these stars have their own life cycle – some
lasting only a few million years and others lasting
billions of years
Do stars actually move?
 Just like planets, stars also move.
 We cannot see this movement, though, because the
distance is so far that in order to see a change in
position of the stars takes years and years
 If we could travel ahead in time 20 millions years the
constellations as we know them would look much
different from what we see today.
Constellations
 Stars form patterns in the sky
 Using these patterns Greeks told stories of mythology
about their Gods
 There are 88 constellations that make up the sky
 The best way to describe a constellation is a connectthe-dot drawing of picture in the sky
The North Star
 The earth is tilted on an axis of 23.5 degrees
 The top of the axis points North at the North star (also
known as Polaris)
 Polaris is located at the end of the handle of the Little
Dipper
 Other constellations fall around
Polaris
Apparent movement of the
constellations
 Just like how the sun appears to rise in the east and set
in the west, stars appear to move in our night sky
 This is due to the fact that earth is tilted on an axis and
rotates on it
 As we rotate, everything in the sky seems to move!
 It is from our position on earth that constellations seem
to rotate and change position (see next slide)
 The stars seem to rotate around the North star, which
itself does not move
Different seasons – different
stars
 Stars stay where they are all year, so we can only see
certain stars during certain times of the year
 For example, I was born in the month of September which
means my zodiac sign is Virgo. Every September you are
able to see this constellation. At the opposite time of year,
you cannot see it.
 The reason why we can’t see the same stars all year
round is because we are revolving around the sun. The
sun blocks the view of the stars on the other side of it. It
is so large that we can’t see any of the stars that stayed
on the other side.
Circumpolar stars
 We can see a few stars all year round – of course we
can always see the North star. That is why it is so
helpful for navigation.
 We are also able to see the constellations close to the
North star all year round
 The big dipper and the little dipper
 The stars we can see all year round are called
circumpolar stars. At all times of the year we are able
to see them.
12:00 am
6:00am
6:00pm
12:00pm
From our
perspective on
earth this is
how one full
rotation would
make the big
dipper look in
one night.
Since it takes
24 hours for
one rotation,
each quarter
turn would
take 6 hours.
January
April
October
July
This
movement of
constellations
could also be
how they
“move”
throughout the
year as we
revolve
around the
sun. Each
quarter turn
would then be
three months
apart from
each other
 Why Do We See Different Constellations During the Year?
 If observed through the year, the constellations shift gradually to the west.
This is caused by Earth’s orbit around our Sun. In the summer, viewers are
looking in a different direction in space at night than they are during the winter.
 What is the Zodiac? Earth orbits our Sun once each year. Viewed from
Earth, our Sun appears to trace a circular path. This path defines a plane
called the plane of the ecliptic (or just the ecliptic). The zodiac is the group
(or “belt”) of constellations that fall along the plane of the ecliptic. It is through
these constellations that our Sun appears to “pass” during the year. While
there are 12 astrological constellations of the zodiac, there are 13
astronomical zodiac constellations: Capricornus, Aquarius, Pisces, Aries,
Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpius, Sagittarius, and
Ophiuchus. The annual cycle of the zodiac was used by ancient cultures to
determine the time of year.
Vocabulary
 Constellation: a group of stars that have been given a name
 Zodiac sign: constellations that appear during certain months
 Circumpolar: stars that are visible in the night sky all year round
(Ursa Major, Ursa Minor, Cassiopeia)
 Polaris: a star that points to the north. Our axis always points to
Polaris. It is right above the north pole. You can see it all year
round.
Star Brightness (Extension)
 Star brightness is described in terms of magnitude
 The brightest stars may be as much as 1 million times
brighter than our sun
 White dwarves are about 1000 times less bright