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29.2 Measuring the Stars
Constellations
• 88 constellations
• Many civilizations
looked at the
brightest stars and
named them after
animals,
mythological
characters, or
everyday objects
Classifying Constellations
• Constellations are classified as summer,
fall, winter and spring constellations
• People in years past used the
constellations to know when to prepare for
planting, harvest and ritual celebrations
Greek Letters- Bayer
Letters (Johann Bayer)
• The Alphabet of Greek
letters is generally
assigned to stars in a
constellation in order of
brightness.
• The alpha star is the
brightest (alpha, beta, gamma,
delta, epsilon)
Sagittarius
Leo
Leo- Regulus as Alpha Star
Star Clusters
• When a group of stars are gravitationally
bound to each other it is called a cluster
Binary Stars
• When two stars are gravitationally bound
to each other, they orbit a common center
of mass
• Often appear bound to each other, even
with a telescope
Properties of a Star
• Mass, diameter and luminosity (energy output of
the star per second) are the basic properties of a
star
• Magnitude: how bright the star appears; +1 was
the brightest star, +2 was the next brightest star,
+3 was the third brightest star
• Absolute magnitude: takes into account
distance and measures how bright a star would
appear if they all were at the same distance; the
brightest star has a -40 value and the dimmest
star has a value around +40
Temperature of a Star
• Classified using letters: O, B, A, F, G, K,
and M and further subdivided into
numbers. Example: A4 or A5
• O stars are the brightest whereas M stars
are the coolest
• The sun is a G2 star with a surface
temperature of 5800 K
Composition of Stars
• All stars are
approximately 73%
Hydrogen,
approximately 25%
Helium and the
remaining 2% varies
between stars;
however, these
numbers change as
the star ages