CONSTELLATIONS

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WHAT ARE THE CONSTELLATIONS• In popular usage, the term constellation is used to denote a

recognizable grouping of stars.

• Astronomers have redefined the constellations as 88 regions of the night sky, while referring to the groupings as asterisms .

The constellation Orion

popular usage astronomers usage

ORIGINS

• Star patterns named by ancient peoples after gods, goddesses, animals, monsters, and mythic heroes.

• The constellations used by Western Culture today originated in Mesopotamia around 3,000 B.C.

CELESTIAL MAP

3D CONSTELLATIONS

• The stars of a constellation only appear to be close to one another

• Usually, this is only a projection effect.

• The stars of a constellation may be located at very different distances from us.

Land marks - When people first started going out in boats they tended to stay close to shore so they could use landmarks to guide them.

STARS AND NAVIGATION

As people ventured further (the Hawaiians 3000BC could travel over 1000km between islands) they needed to find other fixed marks, so they turned to the stars.

CELESTIAL NAVIGATION

HOW DO WE KNOW WHERE WE ARE?

You are here.

Huh?

X

THE CELESTIAL SPHERE• The Celestial Sphere is an imaginary hollow globe with

the Earth at its center, to which all the stars seen in the night sky appear to be fixed .

NORTH AND SOUTH CELESTIAL POLES

• The North Celestial Pole is the point on the celestial sphere directly above the Earth's North Pole.

Similarly, the South Celestial Pole is directly above the Earth's South Pole.

• The star Polaris, in the constellation Ursa Minor, is located very close to the North Celestial Pole.

Polaris is therefore also called the North Star.

Zenith = Point on the

celestial sphere directly

overhead

Nadir = Point on the

c.s. directly underneath

(not visible!)

Celestial equator =

projection of Earth’s

equator onto the c. s.

North celestial pole =

projection of Earth’s

north pole onto the c. s.

ZENITH AND NADIR

ANGULAR MEASURE IN THE SKY

• 360o = A COMPLETE CIRCLE

• 1o = 60’ (minutes)

• 1’ = 60” (seconds)

LATITUDE AFFECTS VISIBLE SKY

• What we see in the sky depends on our latitude.

• At the north pole, the north celestial pole is at the zenith.

• At the equator, the north celestial pole is at the horizon.

• The altitude of the north celestial pole = latitude.

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RIGHT ASCENSION AND DECLINATION

Declination (latitude)

is measured from

the Celestial Equator towards the North Celestial Pole.

Right Ascension (longitude)

is measured from the

Vernal Equinox (see below).

THE NORTH CELESTIAL POLE

• The stars appear to circle around the North Celestial Pole (near Polaris) once in 24 hours.

• Stars within the angle of the observer's latitude away from the North Celestial Pole never rise or set.

• They are always above the horizon (circumpolar stars).

16AUSTRALIAN VIEW OF THE SOUTH CELESTIAL POLE

• In the Celestial Sphere picture, the stars all rotate from east to west about the line through the celestial poles.

• Thus, in Australia, near the Earth’s south pole, the stars all appear to rotate about the South Celestial Pole.

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THE APPARENT MOTION OF THE NIGHT SKY

Equator USA North Pole

West West

The motion of the stars in the night sky may be visualized as a rotation

of the celestial sphere from east to west about a north-south axis.

CELESTIAL EQUATOR AND ECLIPTIC

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Since the Earth is considered to be at rest at the center of the Universe, the ecliptic is defined as the annual path of the Sun around the celestial sphere.

19THE ECLIPTIC - HELIOCENTRIC VIEW• In the heliocentric view, the ecliptic is defined as the path

of the Earth’s orbit around the Sun.

• The Earth rotates from east to west, about an axis tilted by 23½o from the normal to the ecliptic plane.

Normal to

the ecliptic.

North-south

rotational

axis.

23½o

READING THE LIGHT FROM THE STARS

• the direction and speed a star is moving

• its mass

• its brightness or luminosity

• its chemical composition

• its size

• its age

• its temperature

• its distance from us

• its stage of life

• how it makes its energy

• even whether it has companions or not (planets or other stars orbiting it)!

If we look at the sky, we see that different stars are different brightness, and even different colors. With the right instruments, astronomers can tell a lot about a star, just from the light.

BRIGHTNESS OR MAGNITUDE• Even thousands of years ago,

ancient peoples noticed that the stars weren’t all the same brightness.

• Hipparchus, a Greek philosopher, invented a system of magnitudes. He called the brightest stars in the sky (like Sirius), first class or first magnitude stars.

• The next brightest group of stars were 2nd class or magnitude 2 stars, and so forth, down to magnitude 6 stars, which were just barely visible to the naked eye.

• Hipparchus also estimated that the brightest (mag. 1) stars were 100 times brighter than the faintest (mag. 6) stars.

MAGNITUDE SCALE

• Because Hipparchus called a difference of 5 magnitudes (from 1 to 6) equal to a 100-fold change in brightness, each change of one magnitude = 1001/5 = 2.512 change in brightness.

• In other words, a Mag. 1 star is 2.512 times brighter than a Mag. 2 star, but 2.5122 (or 6.31 times) brighter than a

Mag. 3 star.

APPARENT AND ABSOLUTE MAGNITUDE

• A star’s apparent magnitude (m) is how bright it appears at the earth’s surface.

• A star’s absolute magnitude (M) is how bright it appears from a standard reference distance of 10 pc or 32.6 LY.

MAGNITUDE

CIRCUMPOLAR CONSTELLATIONS

Ursa Major Ursa Minor Cassiopeia

Cepheus Draco Camelopardalis

A Constellation that NEVER rises or sets as seen at a certain latitude

The constellations that lie along the

ecliptic are collectively called the

Zodiac.

From the perspective of the Earth, The

sun would be in different constellations

of the Zodiac at different times of the

year (if you could see the stars in

daylight).

13 Constellations of the Zodiac

BIG DIPPER POINTERS

• The two “pointer stars” furthest from the handle of the Big Dipper point to Polaris (the North Star).

• The next two stars point in to Regulus, the brightest star in the constellation Leo.

• The pattern may appear upside-down because it rotates about Polaris.

29THE WINTER TRIANGLE

• The Winter Triangleconnects three bright stars: Betelgeuse (in Orion), Procyon (in Canis Minor) and Sirius (in Canis Major).

• This triangle is almost equilateral, but slightly stretched in the direction of Sirius.

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THE SUMMER TRIANGLE

• The Summer Triangleconnects three bright stars: Vega (in Lyra), Deneb (in Cygnus) and Altair (in Aquila).

• This triangle is stretched in the direction of Altair.

THE WINTER HEXAGON

THE GREEK ALPHABET• Stars within a constellation are usually lettered from

Alpha (α) to Omega (ω) roughly in order of brightness, but there are numerous exceptions. For instance, the stars of the Big Dipper are lettered clockwise around the bowl and then out the handle.

URSA MAJOR AND URSA MINOR

RESOURCEShttps://www.youtube.com/watch?v=oMuHpb7EUv0

https://www.youtube.com/watch?v=Hm2MKez7atI

https://www.youtube.com/watch?v=GqV7KMjw9jY

http://www.space.com/27303-lunar-solar-eclipses-october-meteor-shower.html

https://www.youtube.com/watch?v=9P8Veb_AlJ0

http://earth.google.com./sky

http://www.astro.wisc.edu/~dolan/constellations/

http://en.wikipedia.org/wiki/Constellation

http://amazing-space.stsci.edu

Live webcast

http://www.ccssc.org/webcast.html