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
- 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
- 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
- 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
- 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
- Greek letter to indicate brightness, in order
of brightest to faintest.
- Genitive (possessive) form of the constellation name
And so forth.
- Betelgeuse = Alpha Orionis (brightest star)
- Rigel = Beta Orionis (2nd brightest star)
- Bellatrix = Gamma Orionis (3rd brightest star) etc.
See A Note about Graphics to learn
why some of the graphics shown in the lectures are not reproduced with
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