Exploring Stellar Light and Temperature
Learning Objectives:
1. To examine the nature of radiant energy from the sun as displayed in the visible light
spectrum.
2. To demonstrate how spectral analysis of starlight can be used to determine the relative
surface temperature of a star.
Background Information
Radiant Energy and the Sun:
To life on Earth, the sun represents a source of light and heat upon which all
living things depend for their survival. Except for its proximity to Earth, the sun is a
rather ordinary star in a universe of trillions of stars. Each star represents a body of gases
that gives off a tremendous amount of radiant energy. While nearly all known natural
elements are found in analysis of the sun’s rays, hydrogen (92.1%) and helium (7.8%)
constitute 99.9% of the sun’s atoms. With a core ten times denser than iron, the sun’s
mass is some 333,000 times greater than the Earth’s. The sun’s massive outer layers
press inward on the core with a force more than 200 billion times the atmospheric
pressure on Earth.
The enormous pressure in this dense core environment squeezes the hydrogen
atoms, generating temperatures high enough to ignite and sustain the fusion of hydrogen
into helium. As energy is released from the fusion of the sun’s core, it radiates upward in
the form of invisible gamma and X-rays. These rays are then absorbed by dense
hydrogen gases in the solar interior and reemitted to rise still further. Because of cooler
temperatures in the sun’s outer layers, the absorbed and reemitted energy changes in
wavelength from X-rays to ultraviolet radiation, and ultimately to visible light as it
reaches the sun’s surface. Astonishingly, the transition from initial fusion reaction in the
core to release of light at the solar surface takes about 10 million years!
This new radiant energy released from the sun’s surface travels through space in
the form of electromagnetic waves at a speed of 300,000 kilometers per second, reaching
the Earth in about 8 minutes. Classified by their wavelengths, the longest waves include
radio and microwaves. Infrared, visible light, and ultraviolet waves possess shorter
wavelengths; while the shortest wavelengths are found in X-rays and gamma rays. Of all
the kinds of radiant
energy released by the
sun, we are able to sense
only two forms. The
first and most obvious
form,
visible
light,
makes up only a small
part of the spectrum of
radiant energy.
The
second, infrared rays,
can be sensed on the skin
as heat. Visible light and
infrared rays make up
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more than 90% of the radiant energy released by the sun. Nearly all of the remaining
10% is made up of potentially harmful ultraviolet rays, gamma rays, and X-rays.
Ultraviolet rays in sunlight cause sunburn and tanning of skin, and excessive
exposure can pose health risks. Fortunately, the gases in the Earth’s upper atmosphere
absorb gamma and X-rays and most of the ultraviolet rays (UV). Recent concerns over
the depletion of a layer of the Earth’s atmosphere known as the ozone, are based on the
fact that this layer is responsible for blocking out most of the sun’s harmful ultraviolet
rays.
Color, Magnitude and Temperature of Stars
We know that stars like our sun produce radiant energy in the form of visible
light. By analyzing this starlight, astronomers can learn much about a star’s composition,
distance from the Earth, brightness, and temperature. By directing starlight through a
spectroscope (a device for separating visible light into its component wavelengths), a
display of colors and lines called a spectrum is produced. There are three types of
spectra: emission, or bright-line; absorption, or dark-line; and continuous. Dark-line
spectra are particularly useful in revealing certain characteristics of stellar composition
and temperature.
Absorption lines were discovered first by an English chemist called William
Wollaston in 1801 but he failed to recognize the significance of them, and it was their
rediscovery by Joseph von Fraunhoffer in 1814, and they still carry his name today, being
known as Fraunhoffer lines.
Emission lines were discovered as chemists (initially John Herschel and William Fox
Talbot) starting to examine the patterns of colored lines given off as chemicals were
burned, and gradually the emission spectra for the elements emerged.
Just as with the hydrogen spectrum above, each element in turn emits its own
signature spectra. This enables scientists to determine the constituent elements making
up a particular star, simply by using spectral analysis.
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The connection between the emission lines and absorption lines was provided by
Robert Bunsen and Gustav Kirchoff and Kirchoff worked very hard to ensure that other
people would be able to understand and use spectral analysis. In so doing he clarified 3
important truths that tie together the absorption and emission spectra:
1. A hot solid object or hot dense gas produces a continuous spectrum - a rainbow
2. A hot tenuous gas produces a series of brightly colored lines (depending on its
chemical composition) - an emission spectrum
3. A hot solid body surrounded by a cooler tenuous gas gives an absorption
spectrum
Stars appear at night in a wide variety of colors. These color differences are
indicative of the star’s surface temperature. Just as an iron poker placed in a fire first
becomes red, then changes to yellow, and finally blue-white with progressive heating, so
too the color of a star changes with surface temperature. The hottest stars shine with a
blue to blue-white light, while stars emitting a red light are among the coolest. Our sun is
a yellow star.
Stellar Color and Temperature
Color
Blue
Blue-White
White
White-Yellow
Yellow
Orange
Red
Surface Temperature (˚C)
Above 30,000
15,000-30,000
8,000-11,000
7,500
5,000
4,000
3,000
Example
Rigel
Algol
Vega
Canopus
Our sun
Arcturus
Betelgeuse
Some stars also appear to be brighter than others. The brightness of a star as seen
from the Earth is called its apparent magnitude. The apparent magnitude of a star
depends on the star’s temperature, size, and distance from the Earth. The true brightness
of a star is called absolute magnitude and reflects how bright the star would appear from
a fixed distance of 32.6 light-years (a light-year is 9.5 x 1012 km). Two stars can have the
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same absolute magnitude but different apparent magnitudes because one star may be
farther from the Earth. Conversely, two stars with the same apparent magnitude can also
have different absolute magnitudes for similar reasons. By comparing a star’s absolute
magnitude to its apparent magnitude, astronomers can calculate its distance from the
Earth.
Scientists have
also discovered that a
relationship
exists
between a star’s surface
temperature and its
absolute magnitude. In
most cases, this means
that the higher the
surface temperature of a
star, the higher the
absolute magnitude. To
study this relationship,
stars are often plotted on
a chart according to their
surface temperature and
absolute
magnitude.
The resulting chart is called a Hertzsprung-Russel or H-R diagram after its creators.
The chart reveals a pattern of three main groups of stars, with most stars falling into a
narrow band running through the middle of the chart. This band extends from hot, bright,
blue stars at the upper left, to cool, dim, red stars at the lower right. Termed the main
sequence stars, the stars in this band include the sun and over 90% of the visible stars in
the night sky. In the upper right corner of the chart are a group of large, bright red stars
called red giants. Some of these stars are so large, they are known as supergiants. While
their red color indicates a low surface temperature, their large surface area generates
surprising brightness. A third group of stars near the bottom center and left of the chart
are very hot and produce white light. However, due to their small size, these stars (called
white dwarfs) are not very bright. Some white dwarfs are no larger than the Earth.
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Overview
In this special investigation, you will first observe how visible light can be
separated into various colors using a spectroscope. You will describe the colors in the
visible light spectrum and relate them to wavelength. The light source you will be using
in this lab is tiny, and for best results you should aim the spectroscope directly at the
filament of the bulb. While you will not be looking at the sun, it is important to point out
that this is very hazardous and any attempt could do permanent damage to your eyes.
You will also be investigating how color is related to stellar temperature by using
a simple variable resistance board connected to a light bulb and battery. Working in lab
groups, one student should move the battery’s position on the board while the other
observes the light bulb through the spectroscope. Then, switch so that each person gets
to view. You will then use the results of your investigation along with a bar graph, to
analyze the relative surface temperatures of three stars.
Materials Required
Spectroscope
Miniature light bulb with socket
Variable resistance board
1.5 V D-Cell battery
Black paper
Two lengths of bell wire
Electrical tape
Procedure (Part 1)
In this investigation you will explore how radiant energy in the form of visible light may
be broken down into various wavelengths (seen as colors) and analyzed with the aid of a
spectroscope.
1. Locate the miniature light bulb and socket. Check to see that the two lengths of wire
have been secured to the two light socket terminals. Now connect the free ends of the
wire to the positive and negative ends of the D-cell battery with tape. The light bulb
should now light.
2. Place your light bulb assembly on a sheet of black paper. With the light bulb now on,
hold the spectroscope provided with your kit up to your eye with the slit end pointed
toward the bulb. With the spectroscope positioned about 12” from the bulb, line up the
slit opening so that it runs horizontally and is centered on the bulb’s filament. You
should be able to see colors spread out in a narrow line to the right or left in the
spectroscope tube. In your journal, list the colors you see in the order they appear to
radiate outward from the slit opening. Sketch and color the spectrum you see as well.
Questions for part 1
1. In your spectroscopic analysis, the shortest wavelength of light will appear nearest the
light bulb. View the bulb through your spectroscope once again. Which color appears
closest to the light bulb and has the shortest wavelength?
2. Which color appears farthest from the light bulb and has the longest wavelength?
3. The sun releases several invisible forms of radiant energy. Which form is found just
outside the visible spectrum on the short wavelength end? On the long wavelength end?
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Procedure (Part II):
In the second part of your investigation, you will use a variable resistance board to
explore the relationship between star color and the surface temperature of stars.
1. Locate the variable resistance board (12” board wrapped with wire). Connect the bare
wire lead from one end of the board to one bell wire lead from the miniature light bulb
assembly by twisting it slightly.
2. take the end of the second bell wire and tape it to the base (negative end) of the D-cell
battery. Place the board, light bulb assembly, and battery on the sheet of black paper.
3. Just like stars, our miniature light bulb emits visible light across a continuous
spectrum. Using the variable resistance board, you will control the amount of current
reaching the bulb and its temperature by touching the center post of the battery (positive
terminal) to different places on the board.
4. Note the place on the board where the bulb shines brightest. Now slide the battery
along the board until you reach a spot where the bulb just barely glows. Repeat this
process, and at the same time have your partner view the bulb through the spectroscope
as it is dimming. Record your observations.
Questions for part II:
1. At what point on the board did the bulb glow brightest? Dimmest?
2. When viewed through the spectroscope, were all the color bands of the spectrum
visible when the light was brightest? Dimmest?
3. Looking at the filament wire in the bulb, what color did it appear when the light was at
its brightest? Dimmest?
4. At which point do you think the bulb was hottest? The coolest?
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Procedure (Part (III)
We’ve learned that a star’s color is an indication of its surface temperature. Three stars
in the winter constellation Orion (Betelgeuse, Rigel, and Capella) all appear differently in
the night sky. Ranging from bright red, to bluish, and yellow-white respectively, these
stars emit wavelengths of light which reflect differences in their surface temperatures.
Study the bar graphs for these three stars below:
Questions for part III
1. Which of these stars gives the brightest violet-blue (v-b) color band?
2. Which of these stars has a yellow-orange (y-o) that is brighter than its other colors?
3. From your earlier investigations, which of these three stars do you think is the hottest,
and why?
4. Why does Betelgeuse appear red?
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