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BROCK UNIVERSITY
ASTR 1P01
Introduction to Astronomy I
Summary Notes for Chapter 13, Measuring the Properties of Stars
Dr. S. D’Agostino
Contemplating the stars, their enormous distances from us, their enormous sizes (some are
so large that they would swallow up the entire orbit of the Earth!), their complex workings,
and their interesting life stories, never fails to elicit a cosmic feeling. The universe is vast,
and we are but a small part of it. There is so much going on in the beautiful universe, and
it’s such a pleasure to know some small part of it.
There is an old Joni Mitchell song about a music festival in 1969 (Woodstock ), covered
famously by CSN, and one line of the song proclaims that we are stardust. It’s true, too! The
Big Bang created hydrogen and some helium, along with a slew of other subatomic particles
and electromagnetic radiation, but all the heavier elements were created inside stars. When
the early massive stars “died” (their lifetimes were very short due to their enormous mass)
the resulting explosions (“supernovae”) spread heavier elements throughout the universe,
where some of them coalesced into our lovely planet, and some of them became us. It gives
one pause for reflection, doesn’t it, when one realizes that the very stuff that makes up our
bodies is the dust resulting from stellar explosions!
The details about this fascinating process will be explored further in ASTR 1P02, but
for now we will begin to learn some of the basics of stellar structure, and also how we came
to know the distances, sizes, and masses of stars. This in itself is quite a detective story, and
well worth learning about.
Textbook Reading: Chapter 13, Measuring the Properties of Stars, Pages 328–359.
Learning Objectives:
• Explain how astronomers measure the distances to stars using parallax and describe
the typical sizes of parallax shifts.
• Calculate a distance using the parallax formula.
• Explain why a star’s colour indicates its surface temperature, and use Wien’s law to
determine this temperature.
• Describe the difference between the luminosity and brightness of a star, and calculate
one from the other.
• Understand the temperature dependence in the Stefan-Boltzmann law, and describe
how surface temperature and radius of a star determine its luminosity.
• Show how the radius of a star can be determined from its surface temperature and
luminosity, as well as by interferometer or eclipse measurements.
• Describe the magnitude system for star brightness.
• List the characteristics of stellar spectra, and explain how the classification scheme
indicates surface temperature.
• Describe luminosity classes and how they are determined.
• Explain how the masses of stars are estimated, and apply Newton’s version of Kepler’s
third law to determine such masses.
• Describe the layout of the Hertzsprung-Russell (H-R) diagram, and infer a star’s size
and evolutionary state based on its location in the diagram.
• Describe how Eddington’s mass-luminosity relationship was derived and which class of
stars it can be applied to.
• Estimate a star’s distance based on the method of standard candles.
Questions to Guide Your Reading and Review:
• How can distances be determined by triangulation?
• What is parallax, and how is it used to determine the distances to nearby stars?
• What is Wien’s law, and how do astronomers typically use it?
• What is a star’s luminosity?
• What is a star’s brightness?
• How are a star’s brightness and luminosity related to its distance from us? What is
the usefulness of this relationship?
• How are a star’s luminosity, temperature, and radius related? What is the usefulness
of this relationship?
• What is the magnitude system for stellar brightness? What is the difference between
apparent magnitude and absolute magnitude?
• What is a star’s spectrum? How does a star’s spectrum tell us about its composition?
• What is the difference between a star’s emission spectrum and absorption spectrum?
What do each tell us about a star’s composition?
• How are stellar spectral types classified? What is the main physical property that
determines a star’s spectral type?
• How can a star’s motion be inferred from the amount of Doppler shifting in its spectrum? What is redshift? What is blueshift?
• What is a binary star system? What are visual binaries? What are spectroscopic
binaries? What are eclipsing binaries?
• How can stellar masses be determined using measurements of binaries?
• What is the Hertzsprung-Russell (H-R) diagram? What is its use? Where are various
types of stars located in the H-R diagram?
• What is the main sequence of the H-R diagram? What kinds of stars are on the main
sequence of the H-R diagram?
• What is Eddington’s mass-luminosity relation? To which kinds of stars does it apply?
What is its use?
• What are luminosity classes? What physical property distinguishes the various luminosity classes?
• What is the method of standard candles? What is its purpose?