The Stars and Constellations
Standing outside on a dark and moonless night, one can see about 3,000 stars peppered across the sky. There are an equal number of stars hiding below the horizon, and one need only wait as new stars rise in the east the same time others set in the west. The stars by themselves are very hard to tell apart from one another, but the mind begins to perceive patterns in the random placement of the stars. Three bright stars near one another might seem to form a triangle, four a cross. With so many stars, they can be linked together by imaginary lines creating complex shapes like figures of men or animals. These patterns in the stars became the constellations, and each ancient culture had their own set of unique constellations that were usually closely linked to their own mythology. With such a mythological backing, each constellation had a story associated with it, making it easy to remember, so the constellations became a useful mnemonic for locating particular stars among the thousands. For example, if I were to ask you to find a bright red star at the celestial coordinates of 5 hours 53 minutes right ascension, positive 7 degrees 24 minutes declination, it might take you quite a while. However, if I tell you that same star is in the constellation Orion, one of the most familiar, you will probably find it in a few seconds.
There are 88 official constellations used by astronomers today. 48 of the constellations are based on Greek myths and were published in 150 AD by the Greek astronomer Ptolemy. Since Greece is located north of the equator, he missed all constellations near the south celestial pole. Astronomer Johann Bayer included 12 new constellations of the southern regions of the sky when he published his first star atlas in 1603. He tried to keep with the tradition of the ancient names and mainly connected these constellations to the sea and its creatures. Later, astronomers began to create new constellations in the regions between the existing constellations that are relatively poor in stars. German astronomer Jakob Bartsch added three new constellations located between existing constellations in 1624. Later in the 17th century Tycho Brahe declared Coma Berenices, Berenice's Hair, to be an official constellation, as it was previously considered to be either a part of Leo or Virgo. In 1687, German astronomer Johannes Hevelius added seven more constellations in the northern skies. A visit to the Cape of Good Hope by the French astronomer Nicolas Louis de Lacaille resulted in 14 new southern constellations. Lacaille broke with tradition and mostly used scientific instruments as names of these constellations. This brought the count of constellations to 85. In the mid 1800s it became the practice to break Ptolemy's largest constellation Argo Navis (Argo the Ship) into the four smaller constellations representing the ship's keel (Carina), stern (Puppis), and sails (Vela), as well as Lacaille's invention Pyxis (The Compass). The current list of 88 constellations was made official in 1928 by the International Astronomical Union.
Ancient astronomers referred to the constellations as fixed stars maintaining permanent positions in the sky. (They also observed the wandering stars which were the planets.) They indeed seem fixed in place, keeping the same positions night after night for centuries. Hipparchus discovered long ago that precession of the equinox moves the stars gradually into a different position on the celestial sphere. More recently, we have discovered that the stars are all moving relative to the Sun, and the constellations will gradually change their shape after many thousands of years.
Ask people to point out a constellation in the northern sky, and many will choose the Big Dipper. The Big Dipper, however familiar it may be, is not actually a constellation. It is a small part of the constellation Ursa Major. It is what is called an asterism, which can be made up of part of one constellation, or stars from many constellations. The Little Dipper is also an asterism, and is part of Ursa Minor. The Summer Triangle is another asterism, made from the stars of three constellations, Deneb in Cygnus, Altair in Aquila, and Vega in Lyra. The Pleiades, the Seven Sisters, is an asterism that is part of the constellation Taurus.
The constellations can be explored on the following sites:
Munich Astronomy Archive
The zodiac is the belt of stars that follow the circle of the ecliptic on the celestial sphere, the path of the Sun. The word zodiac means "figures of animals" in Greek. There are 12 constellations on the zodiac, one for each month of the year, as the Sun appears to spend about one month in the position of each constellation. In astrology, which has very little to do with astronomy, your astrological sign is the constellation of the zodiac which was behind the Sun on your birthday in the classical age of Greece. The constellations of the zodiac are no longer synchronized with the astrological signs due to the precession of the equinox discussed earlier. In fact, the zodiac will slip backwards by one sign every 2,200 years. The 12 constellations of the zodiac are Capricornus, Aquarius, Pisces, Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpius, and Sagittarius.
The Names of the Stars
The above picture is from a modern star map showing the region near the constellation Orion. As you can see the constellations are still used to identify the stars. They are also handy for finding other celestial objects like nebulae which show up as the irregular shapes on the map. Johann Bayer came up with the useful convention of naming the brightest stars in the constellations by assigning letters from the Greek alphabet to them in the order of their brightness. So, for example the brightest star in the constellation Gemini is alpha Gemini, also known as Castor, and the next brightest star is beta Gemini, which is also called Pollux. This convention is still used on star maps today, but it is interesting to note that the brightness of some stars changes over the years. So alpha Orion, the star Betelgeuse, is actually dimmer than beta Orion, the star called Rigel. This suggests that Betelgeuse must have been brighter than Rigel when it received its designation as the alpha star and has since become dimmer in the past few centuries.
The brightest stars in the sky have older proper names still in use today. Most of these stars have Arabic names like Aldebaran and Betelgeuse. This is because the greatest Arabic astronomer Al-Sufi published his own version of Ptolemy's star catalog in Arabic in the 10th century, giving many of the stars their Arabic names. The original meaning of many of these names had been forgotten by Al-Sufi's time. Some of the names that Al-Sufi published were direct translations of Ptolemy's descriptions. For example the star Fomalhaut in the constellation Pisces comes from Arabic for "mouth of the southern fish," which is how Ptolemy described it in his book. After the tenth century, the works of Greek astronomers like Ptolemy were introduced into Europe by Islamic Arabs, and the books were translated from Arabic into Latin, which at the time was the European language of science. So, today we have a system of Greek constellations with Latin names containing stars with Arabic names.
Planets: The Wandering Stars
In contrast to the thousands of fixed stars on the celestial sphere, the ancients closely watched five bright wandering stars that moved about relative to the rest of the stars, trying to discover the secret of their mysterious meanderings. The word planet comes from the Greek word planetes meaning "wanderers." Most ancient civilizations thought of them as gods. The names of the planets are the names of Roman gods, and the five original planets, which can be seen with the naked eye, are Mercury, Venus, Mars, Jupiter, and Saturn. The planets move along the ecliptic, as they share the ecliptic plane with Earth in their orbit around the Sun, the orbits forming concentric circles (ellipses). The planets all orbit the Sun in the same direction (counterclockwise), so they appear to move eastward along the celestial sphere.
Despite their odd behavior, the most striking attributes of the planets are their brightness and their different colors. With the exception of Saturn, the original planets are brighter than all the stars in the sky (except the Sun), even Sirius the brightest star. Like the Moon, the planets are visible because they reflect sunlight; none of them emanates their own light. The different compositions of the planets causes them to reflect different portions of the visible light spectrum, giving them different hues. Also like the Moon, the planet Venus shows phases, waxing from crescent to full and then waning back again, since it is closer to the Sun than the Earth, but a telescope must be used to see the phases of Venus.
Galileo was the first to point a telescope toward the planets, and after he discovered four small moons orbiting around Jupiter, he saw the planets were other worlds, like our own Earth. When Galileo pointed his telescope at Saturn, he was surprised to discover a planet that clearly was not round. Later, we see that the rings of Saturn gave this impression. The telescope allowed William Herschel to discover another planet, Uranus, in 1781. The final two planets, Neptune and Pluto, were first discovered by mathematics, to explain subtle perturbations in the orbits of first Uranus and then Neptune which were clearly caused by a gravitational influence. It turns out the mathematics predicting Pluto were in error, but they still started the search for another planet and Pluto was discovered by Clyde Tombaugh at the Lowell Observatory. Neptune and Pluto were discovered in 1846 and 1930, respectively. This brought the count of planets to nine, including the Earth.
Double Stars and Variable Stars
The stars in the heavens appear as a single point of light. However, most stars that appear to be single are actually composed of two or more stars revolving around each other. The brilliant star Sirius, for example, is actually a system of two stars called a double or binary star. The bright Sirius has a very faint companion called Sirius B revolving around it. Any star system with two or more stars is called a double star, and most cannot be discerned without binoculars or a telescope.
A favorite double star is in the Big Dipper asterism (Ursa Major), and both stars in the pair can be seen with the naked eye. The stars in this pair are called Mizar and Alcor, in the crook of the Dipper's handle seen above. Mizar and Alcor were called the Horse and Rider by Native Americans. It takes a very clear night and a sharp eye to "split" this pair (meaning to see both stars). Mizar and Alcor are not a true double star. They are merely an optical pair, appearing very near to each other from our perspective on Earth but are actually very far apart. However, Mizar itself is a double star, though it takes a telescope to split Mizar into its components Mizar A and Mizar B, pictured below. It has been discovered that both Mizar A and Mizar B are themselves double stars, though this cannot be seen with an amateur telescope. Also shown below is the double star Albireo in the head of Cygnus the swan. In a telescope, the different colors of the two stars can be seen, and this is one reason why amateur astronomers enjoy double stars.
Variable stars are stars that periodically change their brightness. One type a variable star is also a double star and is called an eclipsing binary. An eclipsing binary appears to dim when one of the stars in the system periodically blocks the other as they orbit. Some variable stars vary their brightness by themselves and these are the true variable stars. These stars periodically change in size and therefore brightness.
The Milky Way
The Milky Way is the dense smoky band of billowing stardust and pinprick stars arcing across the sky. It can be seen passing through Perseus and Cassiopeia in Johann Bode's star atlas pictured above. We now know that when we look at the Milky Way, we are peering far into our own galaxy. Before the invention of the telescope, though, the Milky Way was a complete mystery. Attempts to explain it are found in many ancient mythologies. For example, the ancient Mesopotamians thought it was the smoke from the sacrificial offerings rising to the gods. To the Chinese, it was a heavenly river where the mothers of the Sun and Moon bathe their children before they enter the sky. Many thought it was the path that souls take on their way to the afterlife. The name Milky Way most likely comes from Greek mythology. It is described in the story of Hercules. The infant Hercules suckled milk from Hera, goddess wife of Zeus, to gain her wisdom. When Hera realized that Hercules was the bastard son of Zeus and another woman, she pushed the baby away from her breast, spurting milk into the heavens which became the Milky Way.
Ancient astronomers were perplexed by the Milky Way, as its celestial circle had nothing to do with the positions of the equinoxes or other important celestial points. "Anaxagoras saw it as a collection of stars whose light was partially obscured by the shadow of the Earth. Pythagoras said it was thousands of suns that were extremely far from the Earth and not very bright. Aristotle thought it was a great mass of luminous vapor higher than the ether but lower than the planets. Metrodorus claimed it marked the former path of the sun and Theophrastus thought it was the seam in the sky where the two hemispheres of the celestial vault didn't quite fit together. The light was that of the celestial beyond peaking through the ill fitting seam." (L. Stefaniak, Stories of the Constellations) It was not until 1609 when Galileo pointed his telescope at the Milky Way that it was seen to be composed of thousands and thousands of individual stars.
Meteors, often called "shooting stars," are quick streaks of light that do in fact resemble a star streaking across the sky in less than a second. Meteors are in fact small pieces of dust and ice in space (called meteoroids) that come into contact with the Earth's atmosphere and burn up causing bright streaks of light. If any are large enough to survive the fiery trip through the atmosphere and strike the ground, they are called meteorites. On any given night of the year, you are likely to see a random meteor, called a sporadic meteor, about every 10 minutes. On some nights of the year, however, the Earth crosses the path of a greater concentration of meteoroids in space, and a meteor shower will occur with a meteor appearing ever few minutes. These stream of particles in space are believed to be debris mostly from defunct comets. Meteor showers appear to originate from the same part of the sky as a particular constellation, so they are named after that constellation. Here is a table of the larger regular meteor showers:
|Shower||Date of Maximum Activity
|Time (EST)||Associated Object||Average Hourly Rate|
|Lyrids||Apr 21||11 p.m.||Comet 1861 I Thatcher||15|
|Eta Aquarids||May 4||2 a.m.||Comet Halley||20|
|Delta Aquarids||Jul 28||5 a.m.||20|
|Perseids||Aug 11||8 p.m.||Comet 1862 III Swift-Tuttle||50|
|Orionids||Oct 21||midnight||Comet Halley||25|
|South Taurids||Nov 3||-||Comet Encke||15|
|Leonids||Nov 17||7 a.m.||Comet Temple-Tuttle||15|
|Geminids||Dec 13||7 p.m.||Asteroid 3200 Phaethon||50|
|Ursids||Dec 22||1 a.m.||Comet Tuttle||15|
|Notes: Since the year is 365.25 days long, the date and time starting in March is for leap years. Add 6 hours for each year following in the four-year sequence. Most showers are named after the constellation in which their radiant is located, or after the bright star their radiant is near. The Quadrantids were named after Quadrans Muralis (in the northern part of Boötes) that is no longer an accepted constellation.|
Comets often appear to be fuzzy smudges in the sky. On rare occasions they come near Earth on their long trip around the Sun, and they grow brighter and form a spectacular tail. Comets are bodies made mostly of dirty ice with a rocky core left over from the formation of the solar system. These bodies are concentrated out beyond the orbit of Neptune in a region called the Kuiper Belt. (The ninth planet Pluto and its companion moon Charon are also Kuiper Belt objects.) Occasionally one of these Kuiper Belt objects gets nudged by distant and tenuous gravitational effects, and it begins to move toward the Sun. It has become a comet and is in an elliptical orbit taking it around the Sun in a period less than 200 years, making its appearance predictable. This is called a short-period comet. Less predictable are long-period comets which arrive from a distant (and mostly hypothetical) region called the Oort Cloud way out at about 100,000 astronomical units (AU) from the Sun, near the edge of the Sun's gravitational influence. It is estimated that a long-period comet may take as long as 30 million years to orbit the Sun.
As the comet gets closer to the Sun it begins to heat up, and ice on the surface melts and then turns to gas. Vents on the side nearest the Sun release fountains of dust and gas stretching for tens of thousands of kilometers. The gas forms an atmosphere called the coma, and the gases in the coma begin to flouresce and become visible from Earth. The coma can become hundreds of thousands of kilometers in diameter.
Sunlight and the solar wind, which is composed of electrically charged particles, push the coma materials away from the Sun forming the long bright tail of the comet. The tail is often seen as two components like Comet Hale-Bopp shown above. The blue tail is ionized gas and the white tail is dust. The tail of a comet always points away from the Sun. Most comets travel a safe distance from the Sun, but occasionally a comet gets too close causing it to break up and finally vaporize.
When Comet Shoemaker-Levy 9 passed Jupiter 1992, the planet's strong gravity tore it to pieces, shown below in a picture taken by the Hubble Space Telescope.
In 1994, Comet Shoemaker-Levy 9 collided with Jupiter, and the resulting explosions threw plumes of dusty cometary debris into Jupiter's atmosphere. The dark spots in the image below show the impact sites of different pieces of Comet Shoemaker-Levy 9.
Aurorae and the Solar Wind
Light and heat are not the only things that emanate from the furnace of nuclear fusion that is our Sun. The Sun also creates the solar wind, a stream of electrically charged particles, that flows out across the solar system. The Earth has a magnetic field that extends out into space, called the magnetosphere. The magnetosphere deflects some of the solar wind, but it also captures large amounts of solar wind, and the energy generated as these particles enter our atmosphere causes spectacular light displays in the sky called the Aurora Borealis and Aurora Australis at the north and south poles. The poles of the Earth happen to be very near the magnetic poles, and the magnetic field lines draw the charged particles toward the poles and into the atmosphere. Occasionally the Sun generates solar flares that shoot extra material out into the solar wind. This increased solar activity can cause the aurorae phenomena to extend further toward the Earth's equator, providing rare and beautiful light displays to many people.
The solar wind is traveling at incredible speeds. Fortunately, the Earth's magnetic field deflects most of the solar wind so that it does not hit our atmosphere head on. The planet Mars has little or no magnetic field, and it is believed that most of its former oceans and atmosphere have been lost to space. This loss was at least partially caused by the direct impact of the solar wind on the upper atmosphere of Mars. The planet Venus also has no magnetic field. It is thought to have lost most of its water to space in a similar manner.
This is a photograph of an aurora as seen from space. It was taken by astronaut Don Pettit from the International Space Station.
Jupiter, like Earth, has a strong magnetic field. It too experiences aurorae in its atmosphere, though some of the charged particles come from its moon Io, as Jupiter's magnetic field strips off and electrically charges some of Io's surface material as it rotates.
Zodiacal Light: The False Dawn
Often well before dawn, the sky seems to glow faintly, appearing triangular and near the horizon. Assuming this is not skyglow from the lights of a nearby city, this faint glow is something called zodiacal light. It is sunlight reflecting off dust particles orbiting the solar system in the ecliptic plane, much of which was left by passing comets. It was called a false dawn by ancient civilizations as it tends to appear before dawn. Zodiacal light is best seen in September and October just before sunrise from a very dark location. It can also be seen just after sunset, as it never strays farther than 90 degrees from the Sun.
Images are shown here for noncommercial educational purposes.
The beautiful illustration of Perseus and Andromeda is from Johann Bode's 1801 Uranographia star atlas, which is now in the public domain.
The photograph of the Big Dipper is a NASA image taken by astronaut Don Pettit from the International Space Station.
The telescope sketches of Mizar and Alcor and Albireo in Cygnus are from Wes Stone's website Sky Tour.
The image of the Milky Way was photographed by amateur astronomer Ginger Mayfield of Colorado and can be found on this site. I read the Colorado Springs Astronomical Society's mailing list and am a big fan of Ginger's work.
The picture of Comet Hale-Bopp is by A. Dimai and is from NASA's Astronomy Picture of the Day site.
Comet Shoemaker-Levy 9 images are taken from two NASA Astronomy Picture of the Day articles:
A String of Pearls
Comet Impacts on Jupiter
The aurora over Alaska photograph is from NASA's Astronomy Picture of the Day site.
The image of an aurora from space is also from a NASA Astronomy Picture of the Day article and was taken by astronaut Don Pettit from the International Space Station.