Hand in this pre-lab at the beginning of the Stars, Spectra, and the H-R Diagram Lab.

Much of what we know about the Universe is based on what we know about stars. Our knowledge of light and spectra enables us to study the properties of stars without being able to visit them directly. In this lab, we'll look at how we use the information contained in starlight to determine their sizes, temperatures, distances and luminosities.

The Relationship between Luminosity, Distance, and Brightness-

The luminosity of a star is the amount of energy a star gives off each second. The brightness is the amount of that energy that reaches Earth. Before jumping into the luminosity and brightness of stars, let's start with an analogy that's more down to earth.

You are standing near a road at night. You see a distant car driving towards you. As the car approaches you, do the car's headlights appear brighter or dimmer than they were when the car was far away?

Does the total amount of light emitted by the car's headlights (i.e., the luminosity of the headlights) change as the car approaches you? Why or why not?

An intrinsic property is a fundamental property that does not depend on the observer. With the previous two answers in mind, is luminosity an intrinsic property of a light source or does it depend upon the observer? How about brightness?

In this analogy, the car's headlights represent a star. Astronomers often measure the brightness of a star in order to determine more useful information about the star: its distance and luminosity. How bright a star appears to observers on Earth depends on both its luminosity and distance.

Imagine that you look at the night sky and see two stars of equal brightness. Can you be sure that they have the same luminosity? Why or why not?

The relationship between a star's luminosity and brightness is described by the inverse square law:

B ∝ L/d²    Or, in words   Brightness ∝ Luminosity/(distance)²

Let's use this expression to solve a problem. Star X and Star Y are identical stars. If Star X is located 10 light-years from Earth and Star Y is located 30 light-years from Earth, what is the ratio of their brightness's, B _{Star X}/B _{Star Y}? Show your work.