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Chapter 17 Homework

Course Materials are always under revision! Weblecture content may change anytime prior to two weeks before scheduled chat session for content.

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Scholars Online Astronomy - Chapter 17: The Nature of Stars


Reading Preparation

Reading: Astronomy, Chapter 17: The Nature of the Stars

Study Guide

Key Formulae to Know

Calculate Parallax d = 1/p d = distance in parsecs
p = parallax angle in arcseconds
Absolute luminosity, distance, and apparent brightness L = 4πd2b L = Luminosity
d = distance from star
b = brightness
Relative luminosity, distance, and apparent brightness L/L* = (d/d*)2 * b/b* L = luminosity in watts
d = distance in any units
b = brightness
* refers to the Sun
Luminosity is often calculated relative to LSUN for convenience.
Brightness and Magnitude b1/b2 = 2.5(m2-m1) b = apparent brightness
m = apparent magnitude
Brightness to Magnitude m1 - m2 = 2.5 log(b1/b2) m = apparent magnitude
b = apparent brightness
Distance Modulus m - M = 5 log(d) - 5 m = apparent magnitude
M = absolute magnitude
d = distance in parsecs
UBV Photometry bV/bB

Relative brightness indicates dominant wavelength location.
bV / bB < 1, bB/bU > 1
Star is cool

bV / bB < 1, bB/bU > 1
Star is hot
Stefan-Boltzmann Law L = 4πR2σT4 L = Luminosity
R = stellar radius in meters
T = surface temperature in K in Watts
σ = 5.67 * 10-8 W/m2K4
Kepler's Third Law M1 + M2 = a3/p2 M1 and M2: stellar masses in solar mass units
a = semimajor axis of one star's orbit around the other in AU
p = orbital period in years
Mass-Luminosity Relationship L ∝ M3.5
Use: L/L* =M3.5/M*
L = Luminosity
M = Stellar Mass
Note that the logical connector is a proportion, not an equal sign. To use this relationship, we need a ratio with known values, such as those of the Sun(*).

Web Lecture

Read the following weblecture before chat: Observing Stars: What we can learn from starlight

Study Activity

Planetarium Program: Use your planetarium program to explore the 10 brightest stars in Earth's night sky. If your program does not list these, check the list of 26 Brightest Stars seen from Earth.

  • Which of these stars are visible from your location?
  • Given the temperature of each star, what color do you expect each star to be?
  • If possible, use the planetarium program to investigate the Hertzprung-Russell diagram.
  • If possible, use your planetarium program to view the sun and record its apparent magnitude from 1 AU (Earth distance), 2 AU, 3 AU, 4 AU, 8 AU, 16 AU.

Website of the Week: Read about the Gaia Mission, a follow-on mission to Hipparcos from the European Space Agency. Hipparcos ( the high precision parallax collecting satellite) operated between 1989 and 1993 into a geostationary transfer orbit. Because of a failure in a boost motor, it never reached its target orbit, but scientists were still able to gather data from it for over 3 1/2 years. The Gaia mission was scheduled to be launched in the summer of 2012 and placed into orbit at the Sun-Earth L2 Lagrangian point, but launch was delayed until 19 December 2013. Its mission is to create a precise three-dimensional map of stars throughout the Milky Way galaxy and determine their motions, luminosity, effect of temperature, gravity, and chemical composition.

Chapter Quiz

Lab Work

Read through the lab for this week; bring questions to chat on any aspect of the lab, whether you intend not perform it or not. If you decide to perform the lab, be sure to submit your report by the posted due date.