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Name:__________________________________________ Date:_________________
PHYS 110L Lab # 11 Distance to M100 as Determined by Cepheid Variable Stars
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Instructions: Please read and follow the steps described below and answer all questions. Feel
free to use Excel for your calculations.
Introduction:
Part #1 Using Cepheids as Distance Estimators
Measuring the distance to an astronomical object is very difficult and is one of the greatest
challenges facing astronomers. Over the years different distance estimators have been found.
One of these is a class of stars known as Cepheid variables that change in brightness.
Cepheids are rare and very luminous stars that have a very regularly varying luminosity. They
are named after the star Delta-Cephei in the constellation Cepheus, which was the first known
example of this particular type of variable star.
In 1912 the astronomer Henrietta Leavitt observed 20 Cepheid variable stars in the Small
Magellanic Cloud (SMC), a dwarf galaxy orbiting the Milky Way. The small variation in
distance to individual Cepheid variables in the SMC are negligible compared with the much
larger distance to the galaxy. Henrietta Leavitt uncovered a relation between the intrinsic
brightness (luminosity) and the pulsation period (time between peak brightness) of Cepheids and
showed that more luminous Cepheids have a longer period
(known as the Period-Luminosity relation). By observing
the period of a Cepheid, one can deduce its luminosity and
so, by observing its apparent brightness, calculate its
distance. This is done by recalling that brightness decreases
with the square of the distance (e.g., if the distance to a star
were to double, it would look only ¼ as bright). In this way
Cepheids can be used as a “standard candle” to measure
distance. Cepheid stars can be distinguished from other
variable stars by their characteristic light curves (a plot of
brightness versus time; see Figure 1).
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Modified from The Distance to M100 as Determined by Cepheid Variable Stars, from The ESA/ESO Astronomy
Exercise Series Exercise 2.
Learning Objectives:
In this lab assignment you will use observations of Cepheid variable stars to measure the
distance to the galaxy M100. You will then compare your result with the published value.
Figure 1: The light curve of a
Cepheid has a shape with brightness
rising sharply, followed by a gentle
decline.
period
