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Twinkle twinkle little star, the children's nursery rhyme begins. How I
wonder what you are! Up above the sky so bright, like a diamond in the
night. Have you ever wondered what stars are, and what makes them
twinkle? I know I did! I also wondered why some stars looked white and
some stars looked red or blue. The main reason why stars are
differently colored is that some are hotter than others. Deep in their
interior all stars are hot. Their temperatures are measured in tens and
hundreds of millions of degrees. Their temperature lessens toward their
outer layers, and the coolest stars put most of their radiation in the
red part of the spectrum. These coolest stars appear red. Hotter
stars, stars like our Sun, appear yellow. Still hotter stars appear
white and the hottest stars of all appear blue.
The most familiar form of electromagnetic radiation is visible light.
'White' light, such as sunlight, is a mixture of light at different
wavelengths. On passing through a prism, white light is split into the
colors of the rainbow. This is a result of refraction, the change in
direction of light as it passes from one medium, such as air, into
another, such as glass.
We can see the spectrum of a star easily. Light from a star is
collected in a telescope, passed through a slit, diffracted through a
prism or across a diffraction grating, and focused in an arrangement
called a spectrograph. Perhaps you have used a diffraction grating,
mounted on a slide, to show students the spectrum of various light
sources in the classroom. There are several terrific classroom
activities about the size of stars, color of stars and types of stars in
the Astronomical Society of the Pacific's activity and resource
notebook, "The Universe at your Fingertips". Information about ordering
the notebook is at the end of the article.
I want to take you out under the stars, however. At your class star
party, your students can see with very own their eyes the colors of
stars! They don't need a telescope or anything!
The constellation of Orion and its neighbors contain what is called the
Winter Circle of Stars. Look on any star chart and find Orion, Canis
Major, Canis Minor, Gemini, Auriga and Taurus, circling westward as
winter ends. These six constellations contain a visible "circle" of very
bright and colorful stars. If you can imagine the circle as a clock,
we'll begin with Capella, yellow like our sun in the one o'clock
position. Capella is the bright yellow star in the constellation
Auriga, found above the shoulders of Orion. Red Aldebaran, the eye of
the bull Taurus is at the three o'clock position. Red stars are the
oldest and coolest. At five o'clock, stands Rigel, the brilliant blue
knee of Orion, the hunter. Rigel is young and very hot! Diamond white
Sirius, the brightest star in the sky, is below Orion in the
constellation Canis Major, the great dog. It fills the seven o'clock
position. At nine o'clock is Procyon, another yellow star like our sun,
in the constellation Canis Minor. The Gemini Twins, Castor and Pollux
complete the circle at eleven o'clock. Castor is white and Pollux (the
brighter of the twins) is red. Within the circle are red Betelgeuse,
the shoulder of Orion, and blue/purple stars Alnitak and Mintaka, the
pretty belt stars of the constellation Orion.
Through a telescope these stars give away many secrets. I have a nifty
little star spectroscope that I can attach to an eyepiece. When I bring
my star spectroscope to a school star party, and a student looks at one
of these brilliant stars, she sees the color spread out in a spectrum.
The bar she sees is broken into the colors of the rainbow, purple at one
end and moving through the spectrum of blue, green yellow, orange and
red. She can take the temperature of the star, and tell you its age
from the information on the spectrogram she sees! Bars of shadow called
absorption lines and bars of concentrated illumination called emission
lines are the main indicators of age and temperature.
A simple exercise I often demonstrate is to show one red old cold star
like Betelgeuse and one white hot young star like Sirius. Thick black
molecular absorption bands of titanium oxide - the evidence of a star
that is cool and that has lost its ability to ionize the elements show
vividly on old red stars. It is clear in the eyepiece.
A hot young white star like Sirius does not show this thick absorption
band. Instead, delicate shadows, which are hair like in thickness show
instead. This is evidence of some of the four Balmer hydrogen emission
lines. They show in and near the blue area of the spectrum. I could go
on and on. Old cool stars have big thick molecular bands and hot young
stars do not. That's a pretty interesting observation to share with
your students!
Universe at your Fingertips:
Contact ASP at
390 Aston Avenue
San Francisco, CA 94112
(415) 337-1100
or email the coordinator at astro@aspsky.org
Rainbow Optics Star Spectroscope:
Contact Jim Badura at
1593 E Street
Hayward, CA 94541
(510) 581-8266
Jane Houston is an amateur astronomer and telescope mirror maker. She
volunteers with several Northern California schools as part of The
Astronomical Society of the Pacific's (ASP's) Project Astro program.
She is
also the president of the 25 astronomy club strong Astronomical
Association
of Northern California. http://aanc-astronomy.org. Her email is
jane@whiteoaks.com.
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Jane Houston Jones
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