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Week 7 - Stars and Constellations
What is the most important celestial body in the solar system? Did you answer Earth? There is
no question that our planet is special, but it would completely lifeless without the sun. The
most important object in our solar system is not a planet, but a star. Without the sun's energy,
life on Earth could not exist. We owe our existence to a star. In this lesson, you will learn about
stars and constellations.
Definition
Almost everyone knows that the sun is a star, but what exactly is a star? A star is a ball of gas
held together by its own gravity. This gravitational force continually tries to collapse the star by
pulling gas molecules toward its center. Gravity is counteracted by the pressure of the hot gas
inside the center of the star. Gravity pulls inward, and the pressure pushes outward. The two
forces balance each other and the star maintains a spherical shape. This balance is called
hydrostatic support.
Many planets, like Jupiter and Saturn, are made of gas, so what makes a star different? Unlike
planets, stars generate their own energy through nuclear reactions deep within its core. The
incredible mass of a star creates the perfect condition for the fusion of hydrogen into helium.
The energy from this reaction is released by the star as light and heat. When you look into the
night sky and see a star, you are seeing self-generated light. When you look at the moon or a
planet, the light you see is reflected light from the sun. A star is like a flashlight, while the other
objects are simply mirrors. Stars are made mostly of hydrogen and helium. Since they generate
energy by fusing hydrogen into helium, the percentage of each element changes throughout
the stars' life cycles.
Magnitude
Which star do you think is the brightest in the Milky Way galaxy? The sun? Would it surprise
you to learn that the brightest star, Canopus, is almost 14,000 times brighter than the sun? Yet,
this star is only the second brightest object in the night sky. Why does the sun seem so bright in
comparison? Brightness is determined by both distance and luminosity. A star which radiates a
lot of light, but is far away from the Earth, can appear less bright than a less luminous star
closer to Earth. Imagine sitting outside at night with a candle. The light from the candle will
seem much brighter than the light from a streetlight a mile down the road. However, if the two
light sources were placed next to one another, the candle's luminosity would be much less than
the streetlight's.
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The brightness of a star as called its magnitude. Magnitude is measured on a numerical scale,
with a lower number indicating a higher magnitude. A star with a magnitude of 5.5 is less bright
than a star with a magnitude of -2.7. There are two types of magnitude - apparent and actual.
The brightness of a star it appears from Earth is called apparent magnitude. Apparent
magnitude measures a star's brightness based on luminosity and distance. Close stars usually
have higher apparent magnitudes than more distant ones. The sun, for example, has the largest
apparent magnitude (-26.73) of any other star in the universe. The actual brightness of a star is
called its absolute magnitude. Unlike apparent magnitude, absolute magnitude does not
consider distance. It measures a star's brightness based on its luminosity alone.
Apparent and absolute magnitude. The magnitude of a star depends on two factors. The first
factor is the intensity of the radiation emitted from the star. The second factor is the star's
distance from earth. A star that is far away may seem dimmer than a star that is close. For
example, the light from a lamp on your desk appears very bright. The same light appears
dimmer when seen from across the street. Although the lamp seems dimmer, its actual
brightness has not changed.
Astronomers use the term apparent magnitude to describe how bright a star appears from
earth. Apparent magnitudes are written with a lower case "m." The term absolute magnitude is
used to describe how bright a star would appear one A.U. from earth. Absolute magnitudes are
written with an upper case "M." With absolute magnitude, the brightness of a star is measured
independent of distance. The sun is extremely close to the earth and has an apparent
magnitude of -26.7. However, the absolute magnitude of the sun is 4.8. In contrast, the supergiant Betelgeuse is very far away. It has an apparent magnitude of 0.41. However, this star is
actually much brighter than the sun, and has an absolute magnitude of -5.6.
Classification
Have you ever been outside for too long without sunscreen? Did you get sunburned? The heat
from the sun is extremely hot, but the sun itself is only a medium temperature star. Stars are
classified by temperature and color into seven main categories - O, B, A, F, G, K, and M. O stars
are the hottest and M stars are the coolest. O and B stars are extremely bright, and very rare. M
stars are very common, but fairly dim. Each letter is followed by a number from 0 to 9. This
number represents a sub-classification of temperature, from hottest (0) to coldest (9). For
example, the sun is classified as a G2 star. The color of a star is connected to its temperature.
Although it seems backwards, the hottest stars are blue in color, while the coolest are red. In
between, from hottest to coolest, are white, yellow, and orange stars. Since the sun has only a
medium temperature, it is a yellow star.
Average Luminosity
Star Type
Color
Example
(Sun = 1)
O
Blue
1,400,000
10 Lacertra
B
Blue
20,000
Rigel Spica
A
Blue
80
Sirius
2
F
G
K
M
Blue -white
White - yellow
Orange - red
Red
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1.2
0.4
0.04
Canopus
Sun
Aldebaran
Betelgeuse
Constellations
Orion. Ursa Minor. Scorpius. Do you recognize these names? They are the names of major
constellations in the night sky. A constellation is a group of stars which form a pattern when
seen from Earth. The stars are divided into 88 constellations, ranging from the large Centaurus
constellation (which includes 101 stars) to the small Crux, or even Southern Cross, a
constellation made of only four stars. Constellations have been passed down by ancient
civilizations, most prominently the Greeks and Romans. Many constellations have myths and
legends associated with them. The Pegasus constellation, for example, represents the winged
horse that flew out of Medusa's head after she was killed by Perseus in Greek mythology.
Constellations only make sense from Earth. Stars which appear close together in a constellation
are not often close to each other in space. The relative position of the stars depends on a
person's point of view. In other words, a person looking up at the stars in New York City will not
see the same constellations as a person stargazing in Santiago, Chile. This is because the Earth is
a sphere. The Southern Hemisphere looks into space at a different angle than the Northern
Hemisphere. The Earth's rotation also causes constellations to "move." Constellations which
appear to rise and set are called seasonal constellations and can only be seen during certain
times of the year. Constellations which never seem to move are called circumpolar.
Let's Review
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Stars are massive balls of gas held together by gravity. They generate their own light and
heat through nuclear fusion reactions.
Apparent magnitude describes a star's brightness as seen from Earth. Absolute
magnitude describes a star's actual brightness.
Stars are classified according to temperature (O, B, A, F, G, K, M) and color (blue, white,
yellow, orange, red).
The sun is a medium sized, medium temperature, yellow star (G2).
Constellations are groups of stars that form patterns when viewed from Earth. Different
constellations are seen in the Northern Hemisphere than in the Southern Hemisphere.
HISTORICAL OBSERVATIONS
Before electric lights obscured the sky, people could easily see the night sky. In ancient times,
man would gaze at the magnificent night sky. In his mind he imagined the outlines of people
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and animals. The early Greeks gave names to these star-shaped figures from their religious and
cultural stories. These stories are known today as myths.
For centuries men have looked up at the night sky and wondered whether those numberless
specks could affect their lives. People who studied the stars were called astrologers. Early
astrologers may have started as true scientists. However, astrology quickly became a religious
practice. Astrology became part of the Babylonian religion. To them the zodiac was a sacred
pathway for the sun and the planets.
Stars became familiar signposts in the sky to sailors and travelers. Some of the first inventions
were used by sailors to use the stars as a means to find their way in the vast oceans.
Ancient peoples told time by the sky. The sun and stars marked the time of day and night. In
other words, celestial clocks! The passage of time may be more accurately recorded by the stars
than could even be expected from the precision of our modern atomic clocks.
The moon measured the month. Stars marked the year and its seasons. Observers noticed that
stars set about four minutes earlier each night and concluded that the star day is four minutes
shorter than the sun day.
As people studied the sky, they observed at least four different motions:
1. Most stars rise in the east and set in the west;
2. Both the sun and moon rise in the east and set in the west;
3. The sun appears over the horizon farther north in summer and farther south in winter;
4. The moon does not always rise at the same time.
CONSTANT MOTION
Do you think you are sitting still in your chair right now? If you could look at yourself from outer
space, you would see that you are turning a giant somersault once every 24 hours as the earth
turns on its axis at about 1,600 kilometers per hour. You are also making a circle around the sun
at the speed of 107,200 kilometers per hour. Also, our solar system revolves around the center
of our galaxy, the Milky Way, at the rate of 69,200 kilometers an hour. The whole galaxy is
traveling toward the star Vega at about twenty kilometers per second. So you see, you are
definitely not sitting still in your chair.
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The Milky Way is the galaxy which contains our solar system. On a summer evening you can see
a dusty trail of stars stretching across the sky. This is
our Milky Way galaxy consisting of hundreds of billions
of stars. Instead of seeing each star individually, the
combined light appears as a faded band if the sky is
very dark. With a telescope you can see many more
stars. People in ancient times thought that all stars
were part of the Milky Way. Today, we know of many
other galaxies similar to the Milky Way.
To study the Milky Way as a whole is difficult for
scientists on the earth because we are located within
it. We cannot look at the Milky Way galaxy from the
outside to observe its size and shape. Outer space is so
staggering that a unit of measurement was invented to
express the great distances. A light year represents the
distance that light travels in one year. One light year is
almost 6 trillion miles. This distance is based on the
apparent speed of light. Measurements by sensitive
instruments indicate that the current speed of light is
about 186,000 miles a second. The distances to stars therefore appear to be very great. The
nearest star, other than the sun, is Proxima Centauri, which is one of three stars in the Alpha
Centauri system. It is 4.3 light years away. Imagine that if the earth was only the size of a small
marble, the nearest star would be about 24,000 miles away!
Because light travels so fast, 4.3 light years is a vast distance. Many stars appear to be even
further from the earth. For example, 300 or 1,000 light years away or greater.
It is important to understand that a "light year" is a measure of distance and not time. It is the
distance that light would travel in one year.
STAR CHARTS
The constellations, or imaginary figures, that the ancients observed are star groups that
seem to travel together in space. Star charts have been developed to help follow the everchanging picture formed by the heavenly bodies. Each season has its own pattern of stars. Once
you learn to recognize the principal constellations of each season, you can use them as a guide
along with a star chart to find other stars.
The Winter Sky. Of all the months of the year, February is best for stargazing. It has an exciting
parade of stars.
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Orion, the hunter, is one of the best-known constellations in the sky. Three stars make up
Orion's belt. Betelgeuse, a giant red star, marks his right shoulder. Betelgeuse is one of the
largest stars man has found. Its diameter is 400 million miles. Rigel, a brilliant blue star, is
positioned at Orion's left knee. He appears to be holding his right arm over his head. If you
imagine a line drawn along Orion's belt toward the southeast, you will find the star, Sirius, the
dog star. Sirius is the second brightest star in the sky. Sirius is the nose of one of Orion's hunting
dogs, Canis Major.
The imaginary line made by Orion's belt toward the northwest passes just under the horn of
Taurus, the bull. The horns of Taurus form a V-shape in the sky and contain the star, Aldebaran.
Pleiades, the seven sisters, are located on the shoulder of Taurus. Usually only six stars can
readily be seen; but when viewed with a telescope, many more stars can be seen.
The horns of Taurus point toward two stars located above Orion's second dog, Canis Minor.
These stars are the Gemini twins, Pollux and Castor.
The Spring Sky. The spring sky contains the best-known of all the constellations: the Big Dipper,
also known as Ursa Major, the big bear. It can be seen all night because it does not rise or set.
The spring sky is not as spectacular as the winter sky. Locate the Big Dipper. How many stars
are in the Big Dipper? The two end stars in the bowl of the Dipper are called the pointer stars
because they point to Polaris, the North Star.
Polaris is not one of the brightest stars, but it is one of the most useful to man. The North Star
can be used to find direction. It is located almost directly over the North Pole. All other stars
appear to rotate around it during the night. Polaris is the end star in the handle of the Little
Dipper. The Little Dipper is also called Ursa Minor, the little bear.
Notice the gentle arc in the handle of the Big Dipper. Follow that arc toward the southeast to a
bright star called Arcturus. Arcturus is in the constellation Boötes.
To the east on the horizon is the constellation Libra, the scales. This constellation is the symbol
for justice.
The most prominent constellation in the spring sky is Leo, the lion. Leo is almost overhead at
the zenith. Leo's head resembles a backward question mark. The star Regulus is the period. The
rear end of the lion is shaped like a small triangle.
The Summer Sky. Darkness comes much later in the summer, so your observation of the stars
will be at a later hour than during other seasons. The summer sky has three bright stars that
form what is known as the summer triangle.
The constellation Lyra, the harp, will be found near the zenith. This group of stars is easy to
locate because it contains the bright star Vega.
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East of Lyra is the constellation Cygnus. Cygnus means swan; but because of its shape, this
constellation is often called the Northern Cross. The brightest star in Cygnus, Deneb, is located
in the tail of the swan.
Below Cygnus to the southeast is Aquila, the eagle. Aquila's brightest star is called Altair. Altair,
Vega, and Deneb make up the summer triangle.
West of Lyra is found the constellation that immortalized Hercules, a most popular hero of
ancient mythology. With his foot on the head of Draco, the dragon, he is about to club him. In
his other hand Hercules holds the golden apples from the garden of Hesperides which Draco
guarded.
To the south of Lyra is Sagittarius, the archer. Sagittarius was a mythological centaur who lived
on a mountain in Thessaly.
On the southwestern horizon is Scorpius, the scorpion. Orion appears in the winter sky and
Scorpius in the summer sky.
The Autumn Sky. The parade of stars across the autumn sky is the least spectacular of the four
seasons. The summer triangle has moved off to the west, and Orion is just rising on the eastern
horizon.
The Perennial Sky. The polar stars are the stars found around Polaris, the North Star. In the
northern hemisphere, these stars belong to every season because they never set. They are
called circumpolar stars because they seem to revolve around the North Star, and because they
do not go below the horizon. The constellations are these: the Little Dipper; the Big Dipper;
Draco, the dragon; Cassiopeia, the queen; Cepheus, the king; and Camelopardalis, the giraffe.
Cepheus resembles a house; Cassiopeia, his queen, looks like the letter W.
Early evening darkness is the best time for stargazing. Hold the star chart over your head. Make
sure that north on your chart is lined up with true north. Use a flashlight covered with red
cellophane to read your chart and compass. White light deadens the ability of your eyes to see
the stars.
Do not be in a hurry to find all the constellations at once. Locate one or two at a time and
watch them for several nights. Then look for another group of stars. The constellations on your
star map are extremely small when compared to the constellations in the sky. The spaces
between the real stars are much larger than you imagined, and they will cover huge areas of
the sky.
GEOCENTRIC THEORY
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The geocentric theory of the universe originated with Aristotle and was later modified by
Ptolemy. According to these men, the earth was the center of the universe.
Study these drawings. On the right is a drawing of the universe as Aristotle described it. The
other represents Ptolemy's view of the universe. Examine how they are alike and how they
differ.
Aristotle. For almost 2,000 years after his death, Greek philosopher Aristotle reigned as the
supreme authority in scientific matters. Among these ideas was the geocentric theory, which
held that the earth was the center of the universe, and that all planets revolved around it.
According to the theory, the planets are fastened to a series of crystalline spheres concentric
with the earth (see illustration above). The nearest sphere is the atmosphere. The next four
spheres are the four "elements:" earth, water, air, and fire. Beyond the sphere of fire is a region
containing ether. The planets occupy other spheres stretching away from Earth in this order:
the moon, Mercury, Venus, the sun, Mars, Jupiter, Saturn, and the fixed stars. At the outermost
limits of the universe is the sphere of the Prime Mover, the First Cause. The Prime Mover is the
source and origin of all motion and life. The Prime Mover is the force that keeps each of the
spheres in motion.
Aristotle's written work, On the Heavens, presents his cosmology. He argues entirely from
theoretical principles.
The geocentric theory, however, contradicts observations. Several planets periodically show a
slower, or even retrograde, motion. The apparent movement of each planet was ingeniously
explained as the result of the combined motions of a number of spheres. Altogether, over 50
separate circular motions were required to explain the puzzling phenomenon.
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The simplest explanation is generally the most accurate one. Everything that moves is moved
from outside itself, but where do celestial bodies derive their motion?
A problem in Aristotle's concept of the universe was obvious from the first. Several stars did not
move in the same direction and with the same regularity as did all the others. These few
mavericks appeared to wander through the constellations--sometimes moving forward,
sometimes standing still, sometimes even moving backward against the backdrop of the regular
stars. Aristotle's spheres could not explain these wanderers and their sometimes retrograde
travel.
Ptolemy. In the third century AD, Ptolemy modified Aristotle's
model and developed a series of mathematical explanations
for the unusual movements of the wanderers. Like Aristotle,
Ptolemy taught that the earth was fixed at the center of the
heavens. The sun, moon, and known planets revolved around
the earth in circles. Beyond them the stars were attached to a
crystal-clear sphere. Beyond all was the Prime Mover.
Ptolemy's modification contained the explanation that each
planet moves in a small circle, the center of which is carried on
the circumference of a larger circle. The center of this larger
circle is the earth. By this system of epicycles, Ptolemy explained the apparently erratic
planetary movements.
WANDERERS
If you watch the night sky for some time, you will find five bright objects that do not move the
same way that the fixed stars move. They do not belong to constellations. They move among
the constellations at different speeds. They even change directions. The Greeks called these
bodies wanderers. We call them planets.
Like the sun, the planets appear to travel eastward through the fixed stars. Periodically, they
appear to slow down. They seem to stop briefly and then move backward. They slow down
again, stop, and then move eastward again. This different pattern of movement makes the term
wanderer appropriate.
Mars, Jupiter, and Saturn always appear brightest when they are halfway along their backward
paths.
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Most of the sky-watchers long ago thought that the earth was the center of the universe. Their
conclusion was based on appearances. The sun, moon, stars, and planets were thought to be
fastened to spheres that spun around the earth. This explanation did not satisfy Aristarchus, a
teacher at Alexandria, Egypt, in 290 BC. If the planets were fastened to spheres, he reasoned,
they would always be the same distance from the earth. Then why did they change in
brightness, he asked? Aristarchus suggested that the sun was at the center of the universe. He
said that the earth was a planet, and that the planets moved around the sun. Very few scholars
agreed with him, so in time his idea was forgotten.
METEORS
Most people have seen streaks of light in the sky and called them "shooting stars," or "falling
stars." A better name for the light is meteor.
Space is full of metal and rock that move at high
speed. When they are in space, the small pieces
of metal and rock are called meteoroids. When
they enter our earth's atmosphere, they heat up
and give off light. If the piece manages to get
completely through the atmosphere without
being consumed by heat, the portion of a
meteor that hits the ground is called a
meteorite.
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The average speed of a meteoroid in space is about 85,000 miles per hour. When a meteoroid
enters the atmosphere, friction with the atmosphere causes it to heat up. Most meteoroids
vaporize and disappear well above the earth's surface. Meteoroids range in size from grains of
sand to great boulders. Approximately two tons of meteoric dust is estimated to fall on the
earth every 24 hours.
Most of the dust is composed of iron and nickel. Since both iron and nickel-iron are magnetic,
small meteoritic particles can be recovered by dragging a small magnet over the ground. About
10% of the material clinging to the magnet will be dust from outer space. Examine some of it
under a low-powered microscope if one is available.
Meteor "showers" are named according to the constellations from which they appear to fall.
Meteors radiating from the constellation Leo are called Leonids; those from Orion are called
Orionids. The following table lists the main annual meteor showers and the dates of maximum
shower activity.
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Table of the Main Annual Meteor Showers
Number of
Name of
Days It Will
Shower
Last
Quadrantids
4
Lyrids
4
Eta Aquarids
8
Pons?
Winnecke
Delta Aqaurius
3
Perseids
25
Oraconids
1
Orionids
14
Leonids
7
Andromedes
?
Geminids
14
Date of
Maximum
Activity
January 3
April 22
May 5
Hours of
Maximum
Activity
12-4 a.m.
12-4 a.m.
12-4 a.m.
Approximate
Number
of Meteors Per
Hour
28
7
7
June 29
12-4 a.m.
?
July 29
August 12
October 10
October 20
November 17
November 21
December 13
12-4 a.m.
12-4 a.m.
12-4 a.m.
12-4 a.m.
12-4 a.m.
12-4 a.m.
12-4 a.m.
27
69
?
21
21
?
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HUBBLE CONSTANT
The distances between celestial bodies require unique methods of measurement. These
methods are used to measure tremendous distances, sometimes even in the billions of lightyears. One such method is triangulation. This technique uses parallax angles to measure
distances, but it is difficult to calculate and limited in range.
Another technique is called the red shift method. It was developed by the scientist Edwin
Hubble in the 1920s. Hubble discovered that observed galaxies were heading away from the
earth in all directions. He used spectral analysis of the light emitted from stars in the galaxies to
support his finding. Hubble showed that the more distant galaxies were proceeding away from
us at faster rates than closer galaxies. The red shift was greater for distant galaxies. Hubble
established a relationship between distance and velocity called the Hubble Constant. The
Hubble Constant is expressed in kilometers per second per megaparsec, or km/sec/Mps. Radial
outward velocity (line-of-sight motion from us) is represented by v, and the distance of the
galaxy from the earth is represented by d.
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Great care is needed in calculating this constant since the distances being considered are so
vast that any slight variation would lead to errors of great proportions.
CEPHEID VARIABLES
During the development of different measuring methods, astronomers discovered a unique
type of star. Cepheid Variables, also called Cepheids, are stars which brighten and dim
periodically. The time they take to brighten, dim, and brighten again is called a period. Cepheid
periods are regular and can be used to determine distance. The longer the period, the greater
the star's absolute magnitude, or true brightness. Once absolute magnitude is determined, it is
compared with apparent magnitude, or what we see from earth. Distance is calculated from the
difference.
Cepheids are considered to be stellar "yardsticks." By comparing distances of objects against
the assumed distances of Cepheids, distances of far-away objects can be calculated. There have
been several Cepheids found in the Large Magellanic Cloud and the Small Magellanic Cloud
galaxies. These two small galaxies are companion galaxies whose distances have been
measured by triangulation. Triangulation is thought to be reliable to about 300 light-years. If
this is the case, distance measurements to the Magellanic Clouds (150,000 and 173,000 lightyears) would be subject to an error factor of roughly 500. With an error factor this large,
reliable results would not be attainable.
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