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Constellation: Group of stars that form a distinctive pattern in the sky› International Astronomical Union (IAU)
designated 88 constellations
Asterism: Smaller groups of stars that form patterns within a constellation, from the Greek word aster, meaning star
The larger the ‘dot’, the brighter the star
Apparent magnitude: star’s brightness as seen from Earth› If all were placed at same distance from
Earth
Apparent magnitude scale first developed by Greek astronomer (around 130 BCE)
He assigned a magnitude of 1 to the brightest star he could see› Faintest star was 6
Today, stellar magnitude extends well beyond 6 and into the minus range› Sun now recognized as -26
Stars in constellations appear to be close together – same line of sight
› May be light years apart!
Light year – represents the distance light travels in one year› At 300 000km/s, light travels about 9.5 x
1012 km in one year
Best known group of stars (Asterism)› Not a constellation! Part of a large
constellation, Ursa Major
Recognized by many cultures› Ancient Chinese – chariot› Early Egyptians – bull leg› Aboriginal - bear
The Big Dipper’s two end stars are known as pointer stars› Help to navigate the night sky› Point toward Polaris
Polaris (“North Star”) points toward the North› Does not appear to move› Stars appear to rotate counterclockwise
around Polaris
Latitude: angular distance north or south of the earth's equator, measured in degrees
Latitude determines which constellations are observable› Move northward,
constellations along southern hemisphere slip below horizon
More Constellations – The Twins
The Twins as you see them in the sky
Allegorical representation
More Constellations – The Twins
Geometrical
Note: the stars are still in the exact same position, but now the shape has meaning!
Graphically
Constellation Charts
Big Dipper
Little Dipper
Dragon (Draco)› Large, but not
very bright
› Seen best from late May - early Nov
Constellation Charts
Great Bear› Ursa Major –
very large
Lion› 3 bright stars
Hunting dogs
Little Lion › Looks more
like a mouse
Constellation Charts Cassiopeia
› In the milky way
› “W” shape – easy to remember
› Draw a line from where handle joins bowl, through Polestar and beyond
Cepheus Giraffe
› Hard to see
Learning Check:
Please answer the following questions on a
separate sheet of paper:
1. What does the term “celestial object” mean?
2. What unit of time does one revolution of Earth around the Sun correspond to?
3. Explain how calendars were helpful to ancient civilizations
4. Imagine you are standing at the North Pole and see a star directly overhead. Where do you think the star would be if you were standing at the equator?
5. Compare and contrast the terms constellation and asterism in a Venn diagram
6. What is a star’s apparent magnitude?
7. Define the term light year