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CHAPTER 17 MEASURING THE STARS
17.1 The Distances to the Stars
Stellar Parallax
 Parallax (recall from ch. 1) is used to measure distances to terrestrial and solar
system objects.
 Parallax is an object’s apparent shift relative to some more distant background as
the observer’s point of view changes.
 Measuring: observe object from either end of some baseline and measure the angle
through which the line of sight to the object shifts
o Parallax decreases as distance increases
o Parallaxes of even the closest stars are very small, so more convenient to
measure in arc seconds rather than in degrees
o Parsec: (“parallax in arc seconds)
 Distance (in parsecs) = 1/parallax (in arc seconds)
 A star with a measured parallax of 1” lies at a distance of 1 pc from
the Sun.
Our Nearest Neighbors
 Closest star to Earth (excluding the Sun) = Proxima Centauri (part of a triple-star
system known as the Alpha Centauri complex)
 Next nearest star to the Sun = Barnard’s Star
17.2 Stellar Motion
 In addition to the apparent motion caused by parallax, stars have real motion too!
 Stellar Motion has 2 components
1. Transverse Component: measures a star’s motion perpendicular to our line of
sight (its motion across the sky)
2. Radial Component: measures a star’s movement along our line of sight (toward us
or away from us)
 Proper Motion: annual movement of a star across the sky, as seen from Earth
17.3 Stellar Sizes
Direct and Indirect Measurements
 Giants: A star with a radius between 10 and 100 times that of the Sun
~The color of any 3000 K object is red, a star that is the size of a giant and burns red is
known as a red giant
o Red giant: A star whose surface temperature is relatively low, so that it
glows red
 Supergiants: A star with a radius between 100 and 1000 times that of the Sun
 Dwarf: Any star with radius comparable to, or smaller than that of the Sun
(including the Sun itself)
~The color of any 24, 000 K object glows white
o White Dwarf: A dwarf star with sufficiently high surface temperature that
it glows white
17. 4 Luminosity and Apparent Brightness
Luminosity is an intrinsic property of a star – it does not depend in any way on the
location or motion of the observer – it is sometimes referred to as the star’s absolute
brightness.
 Absolute Brightness: How bright a star would appear if it were placed at a standard
distance of 10 parsecs from Earth
When we look at a star we see NOT its luminosity but rather its apparent brightness.
 Apparent Brightness: Brightness a star APPEARS to have, as measured by an observer
on Earth (depends on our distance from the star)
The Magnitude Scale (Page 387 Figure 17.9)
Instead of using SI units to measure brightness (watts/sq. meter) astronomers find it
more convenient to work in terms of the magnitude scale
 Magnitude Scale: System of ranking stars by APPARENT brightness
o Old system followed these rules:
 Magnitudes range from 1 (brightest) through 6 (faintest)
 Notice that a LARGE magnitude means a FAINT star
 Scale has been modified:
1. Defined a change of 5 in magnitude of an object (going from magnitude 1 to
magnitude 6 or from magnitude 7 to magnitude 2) to correspond to exactly a
factor of 100 in apparent brightness.
2. B/c we are really talking about apparent brightness (rather than absolute)
the numbers in the ranking system are called apparent magnitudes.
 Apparent Magnitude: apparent brightness of a star, expressed using
the magnitude scale
3. Scale is no longer limited to whole numbers
4. Magnitudes outside the range 1-6 are allowed
Absolute Magnitude: The apparent magnitude a star would have if it were placed at a
standard distance of 10 parsecs from Earth.
17. 5 Temperature and Color
Color of a star is due to the temperature of a star’s surface. Temperature is determined
by measuring its APPARENT BRIGHTNESS and matching the observations to the
appropriate blackbody curve
17. 6 The Classification of Stars
Spectral Classes: Classification scheme, based on the strength of stellar spectral lines,
which is an indication of the temperature of a star.
o Stellar spectral lines deal with the different elements burned in a star
o The different classes are represented by a letter (p. 391 table 17.2)
o O, B, A, F, G, K, M (in order of DECREASING temperature)
o Mnemonic Device: Oh, Be A Fine Guy/Girl, Kiss Me
17.7
o
o
o
The Hertzsprung-Russell Diagram
Astronomers use luminosity and surface temperature to classify stars
Figure 17.13 plots luminosity versus temperature for well-known stars
We abbreviate Hertzsprung-Russell Diagram to H-R Diagram
o H-R Diagrams: A plot of luminosity against temperature (or spectral class)
for a group of stars (see insert after p. 458 in book)
o Vertical Scale = luminosity (sun at middle of scale with a luminosity of 1.
o Surface temperature plotted on the horizontal axis (increasing to the left –
unconventional). Color is almost used to express temp.
o Often referred to as color-magnitude diagrams.
THE MAIN SEQUENCE
 As more and more stars are plotted on an H-R diagram a relationship (trend) is
seen.
 Stars are NOT uniformly scattered
 Most are confined to a fairly well defined band stretching diagonally from
top left (high-temp, high-luminosity) to bottom right (low-temp, lowluminosity)
 Cool stars tend to be faint and hot stars tend to be bright
 This “band” of stars spanning the H-R diagram = Main Sequence
 A clear trend is seen as you travel the main sequence from top to bottom.
 At one end stars are large, hot, and bright = Blue Giants
 The largest stars = Blue Supergiants
 At the other end, stars are small, cool, and faint = Red Dwarfs
 Our Sun lies right in the middle of the main sequence
WHITE DWARFS AND RED GIANTS
 White Dwarf Region: The bottom left-hand corner of the H-R Diagram where
white dwarf stars are found
 Red Giant Region: The upper right hand corner of the H-R diagram, where redgiant stars are found
17.9 STELLAR MASS
BINARY STARS
 Most stars are members of multiple-star systems – groups of two or more stars in
orbit around one another
 Binary-star systems: A system which consists of 2 stars in orbit around
their common center of mass, held together by their mutual gravitational
attraction.
 Most stars are found in binary-star systems.
 Eclipsing Binaries: Rare binary-star system that is aligned in such a way
that from Earth we observe one star passing in front of the other, eclipsing
the other star
17.10 STAR CLUSTERS
Star Cluster: grouping of anywhere from a dozen to a million stars, which formed at the
same time from the same cloud of interstellar gas. Lie at roughly the same distance from
Earth.
Open Cluster: Loosely bound collection of tens to hundreds of stars, a few parsecs
across, generally found in the plane of the Milky Way
Globular Cluster: Tightly bound, roughly spherical collection of hundreds of thousands,
and sometimes millions, of stars spanning about 50 parsecs. Globular clusters are
distributed around the Milky Way and other galaxies.