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Astronomy 161
An Introduction to Solar System Astronomy
Prof. Scott Gaudi

Lecture 3: The Night Sky

The Starry Night

About 6000 stars are visible to the naked eye on moonless nights over the course of the year
Nearly 200 Billion Stars make up our Milky Way Galaxy

Figures in the Sky

Most constellations are composed of bright stars that stand out from the others
Many look like what they are named for
Peoples greatly separated in distance and/or time often made the same connections:
Orion depicted as a male human
Scorpius called a scorpion by desert peoples
Various celestial rivers and snakes

The Classical Constellations

Ancient Babylonian, Egyptian, and Greek civalizations identified constellations.
The Greek Astronomer Ptolemy made a catalog of 48 ''classical'' constellations in the 2nd century AD
This catalog included all of the constellations visible from the middle latitudes of the northern hemisphere
16th and 18th century travelers to the southern hemisphere filled in the rest of the sky

Modern Constellations

IAU officially adopted a list of 88 constellations in 1922 that are used today.
Boundaries were set in 1930 -- these constellations cover the entire sky

Purposes of Constellations

Story-telling Mnemonics
Navigational Aides

What Constellations are Not

Not physical groupings of stars.
Not permanent -- appearence will change due to relative motion of the stars

Modern Constellations

Official IAU list of 88 constellations as of 1922
Definitive boundaries drawn in 1930
Every piece of the sky is in a constellation
What are the 88 constellations?
men, women, animals, fictional beasts, inanimate objects, a river, and a head of hair.


Bright constellation that appears in the winter sky
Nearly always seen as a human figure


A generally recognized pattern that is not an official constellation
Big dipper
Little dipper
"W" in Cassiopeia

Star Names

The brightest stars all have proper names
Arabic and Greek names reveal the chain of western astronomical tradition:
Mesopotamia - Classical Greece - Roman Empire - Islamic Cultures - Renaissance Europe - Today
Other cultures have also named some of the brightest stars

Common Names

Most star names are Arabic, but there is a mix of Greek and Latin for a few famous stars:
Arabic: Rigel, Aldebaran, Deneb, Betelgeuse
Greek: Sirius, Arcturus
Latin: Polaris (Northern Pole Star)

Bayer (Greek Letter) Names

In 1601, German astronomy Johannes Bayer developed a system of naming stars using lower-case Greek letters in approximate order of brightness (though he didn't always get it right). A "Bayer Name" for a star consists of two parts
  1. Greek letter to indicate brightness, in order of brightest to faintest.
  2. Genitive (possessive) form of the constellation name

Examples: Orion

And so forth.

See A Note about Graphics to learn why some of the graphics shown in the lectures are not reproduced with these notes.

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