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Chapter 10
The Nature of Stars
What do you think?
• What color are stars?
• Are most stars isolated from other stars, like
the Sun?
A star’s color reveals its surface temperature
Photometry
• Technique used to accurately measure the color
of stars using UBV filters
– U - ultraviolet wavelengths
– B - blue wavelengths
– V - visible wavelength
• astronomers compare combinations of U minus
B (U-B) and B minus V (B-V)
• these combinations are called color indices
Stars are classified by their spectra as
O, B, A, G, K, and M spectral types
Stars are classified by their spectra as
O, B, A, G, K, and M spectral types
•
•
•
•
O BAF G KM
hottest to coolest
bluish to reddish
An important sequence to remember:
– Our Best Astronomers Feel Good Knowing
More
– Oh Boy, An F Grade Kills Me
– Oh Be a Fine Guy (or Girl), Kiss Me
The HertzsprungRussell diagram
identifies a definite
relationship between
temperature and
absolute magnitude
HR DIAGRAM
Absolute magnitude vs
temperature
or
luminosity vs spectral type
Determining the Sizes of Stars from an HR Diagram
• Main sequence stars
are found in a band
from the upper left to
the lower right
• Giant and supergiant
stars are found in the
upper right corner
• Tiny white dwarf stars
are found in the lower
left corner of the HR
diagram
Luminosity
classes
• Class I includes
all the supergiants
• Class V includes
the main
sequence stars
• e.g., the Sun is a
G2 V
Monitoring how binary stars move provide
information about stellar masses
Monitoring how binary stars move
provide information about stellar masses
There is a
relationship
between mass
and luminosity
for mainsequence stars
Bigger is brighter!
There is a
relationship
between mass
and luminosity
for mainsequence stars
Bigger is brighter!
The orbital motion of binary stars
affects the wavelengths of their
spectral lines
Radial-Velocity Curve
showing the positions of
the stars and their
spectra at four instants in
an orbital period.
Some binary stars eclipse each other
Mass transfer in close binary systems
can produce unusual double stars
• Close binary systems are where only a few
stellar diameters, or less, separate the stars
• Mass can be dramatically transferred
between the stars
– detached binary (no mass transfer)
– semidetached binary(material can flow across
along a path called the Roche lobe)
– contact binary (the two stars share a common
envelope of material)
What did you think?
• What color are stars?
Stars are found in a wide range of colors, from
red through violet as well as white.
• Are most stars isolated from other stars, like
the Sun?
In the vicinity of the Sun, two-thirds of the stars
are found in pairs or larger groups.
Self-Check
1: Describe the observations necessary for the determination of basic
physical properties of stars such as mass, temperature, luminosity,
chemical composition, sizes, and motions.
2: Explain the classification of stars according to their spectra and their
luminosity.
3: Draw and describe the H-R diagram and its value for displaying physical
properties of stars and in expanding our knowledge about stellar sizes
and distances.
4: List the classes of binary stars and state the criteria for determining
membership in each.
5: Draw, label, and describe a graph of the mass-luminosity relation and
specify limitations on its applicability.
6: Explain the characteristics of the radial-velocity curves for binary stars.
7: Describe the differences between the light curves for total and partial
eclipsing binary stars.