The Sun and our Solar System
The Sun


Our sun is one of billions and billions just like it in the universe. It is a middle-aged, yellow, main sequence star. Our sun is composed primarily of Hydrogen and Helium gases. At the heart of our sun is a nuclear inferno. Our sun has been described as a nuclear explosion being held in check by gravity. Our sun is also very large, though its density if fairly low. If our sun were hollow, it is estimated that one million Planet Earth's would fit inside.


Our sun is the center of our solar system and is the source of all energy for almost every organism on the face of the Earth. The sun provides a vast spectrum of energy. Everyone knows that the sun produces lots of heat and light. But our sun produces energy that is much more important. The vast amount of energy that our sun produces is called RADIANT ENERGY. Radiant energy is defined as invisible and invisible light, it is also energy that travels in waves. directly or indirectly, the sun provides nearly all of the energy required by the organisms on Earth.


The Sun's Structure



Our sun has a diameter of approximately 1.35 million kilometers. It is composed primarily of Hydrogen and Helium. In its core, thermonuclear fusion of hydrogen is taking place. While it is a gas, four distinct parts of the sun have been identified.



The Core: The core is the innermost part of our sun. It is the source of the sun's energy. The temperature of the core is approximately 15,000,000 degrees Celsius. You may recall that the process by which Main Sequence Stars like our sun perform to produce energy is called Thermonuclear Fusion of Hydrogen.  In this process, Hydrogen atoms are fusted into Helium atoms by the sun's tremendous gravity.  In this process, a small amount of Hydrogen is actually converted into lots and lots of energy. (Remember E=mc2.  Every second, about four million tons of matter is converted to energy in the sun's core. Even at this tremendous rate of consumption, the sun has enough fuel to continue this process for about another 5 billion years.


The Photosphere: The photosphere is the visible surface of the sun. When you look at the sun under normal conditions, this is the only part that you see. It is the layer of the sun that gives off light. It has a temperature of approximately 5,500 degrees Celsius. The photosphere also has a textured or grainy appearance. This is primarily due to the presence of Granules (areas on the photosphere where hotter gases rise to the surface and cooler gases sink). The photosphere also has Sunspots, which are darker, cooler areas on the photosphere (temp: 4,000 degrees Celsius). sunspots are probably caused by storms in the sun's lower atmosphere. Sunspots vary in diameter from about 16 km to 160,000 km. Sunspots produce ions, which interfere with radio communications on Earth. sunspots occur in approximately 1-0-12 year cycles. No-one seems to know exactly why this is the case, but it is.


The Chromosphere: The chromosphere is the inner atmosphere of the sun. It extends several thousand kilometers above the photosphere. The chromosphere exhibits a red coloration due to the presence of burning hydrogen gas. The chromosphere is much dimmer than the photosphere, and is usually overshadowed by it. As a result, the chromosphere is only visible during a total solar eclipse, when views of the photosphere are blocked by Earth's moon.


The Corona: The corona is the sun's upper, or outer, atmosphere. It can extend for millions of kilometers into space. The corona is actually a lot hotter than even the surface of the sun and temperatures can reach into the millions of degrees Celsius. The corona is another layer of the sun that is only visible during a total solar eclipse. The corona also produces the Solar Wind, which is a stream of fast-moving ions which fly from the corona in all directions.


Features of the Sun:

The sun rotates on its axis like most celestial objects. Since the sun is composed of gases, the sun does not all rotate at the same rate. At its equator, the sun rotates once in 25 Earth Days. We first learned of the sun's rotation by observing the movement of sunspots across its surface.

Other features of our sun include Prominences, which are large streams of gases which shoot up from the sun and then curve back down to the surface forming great arches. Others go millions of kilometers into space and appear as thin, dark streaks when viewed against the sun's disc. These prominences are called Filaments. Solar Flares are explosive increases in the brightness of the sun, generally seen near sunspots. Flares last over an hour and release tremendous amounts of energy. They are one source of the particles which cause the Auroras in Earth's atmosphere. Auroras are bands of light which result when particles from the sun get trapped in Earth's atmosphere, causing a neon light type of effect. Solar flares also impact Earth's communication systems.



The Planets in our Solar System:



In ages past, ancient astronomers noted that some stars seemed to move across the night sky. They termed these special stars "wanderers", and their word for wanderer was "Planet". We know today of course that the planets are not stars at all. They are something entirely different. Our solar system consists of our sun and at least nine planets that revolve around it. The planets are broken into two groups. Those that are closest to the sun are called the Inner Planets. Those furthest away from the sun are called the Outer Planets. 

All of the inner planets are Terrestrial, or Earth-like. It was long suspected that many of these planets would harbor life of some sort. Space probes or exploratory spacecraft have been sent in recent years to study these planets, our closest neighbors. All of the inner planets are small and rocky, much like our own.

With the exception of Pluto (which is terrestrial), all of the outer planets are Gas Giants. They are much, much different than the inner planets. They are primarily composed of Hydrogen, Helium, Methane and Ammonia gases.


The Inner Planets


Mercury: Mercury is a small, desolate planet. It is about 4878 km in diameter and roughly 58,000,000 kilometers from the sun. Mercury is too close to the sun to be easily studies with telescopes from the Earth. Most of what we know about Mercury is the result of the Mariner-10 spacecraft, which was launched from Earth on March 2, 1978. Mariner actually went to Mercury and set up in orbit around the small planet and radioed information back to Earth. The surface of Mercury is similar to that of Earth's moon. It is cry and rough and pockmarked with many craters. Mercury has practically no atmosphere. Either the planet is too small and therefore has too little gravity to hold an atmosphere, or the planet is simply too close to the sun and the atmosphere burned away many billions of years ago. Because of its effective lack of an atmosphere, Mercury has no weather, no rain, no wind. This means that the surface of Mercury doesn't change very much as time passes. Recently, small amounts of what scientists believe to be ice has been found in the shaded portions of at least some of Mercury's craters.  This frozen water is believed to have come from passing comets.  Atmospheres also tend to protect a planet from collisions with rocks and the like from space. Since this planet doesn't have much of an atmosphere, it doesn't get all that much protection. We can see that Mercury's surface is literally covered with the evidence of such collisions. When you look at the surface of Mercury, you literally see craters, within craters, within craters. Mercury also experiences a great change of temperature from daytime to night. In the day, the temperature can approach 950 degrees Fahrenheit. At night, the temperature plummets to -346 degrees Fahrenheit.


Planets as seen from Earth

Venus- Venus has been named Earth's sister planet, since in many ways it is very much like Earth. It is nearly the same size, mass and density as is our own planet. Venus also comes closer to Earth than does any other planet in the solar system. Venus is approximately 12, 104 km in diameter and about 108 million km from the sun. The surface of Venus is obscured from our view by Venus' thick atmosphere. Droplets of sulfuric acid are contained in clouds of carbon dioxide, giving new meaning to the term "acid rain". This thick atmosphere also traps most of the heat energy that strikes the planet. The process of this heat energy being trapped by gases in the atmosphere is called the Greenhouse Effect. As a result of this greenhouse effect, Venus experiences a constant temperature of 900 degrees Fahrenheit. This is enough heat to melt lead! Much of our knowledge of Venus comes from various space probes. The Mariner series probes began with a flyby of Venus in the 1960's and the Pioneer-Venus probe is currently in orbit and still sending information back to Earth. Even more exciting are the reports from the Magellan probe, which was sent to map the surface of Venus using Radar. Because of its oppressive atmosphere, we people of Earth have had a difficult time actually landing a space probe on the surface.  The harsh conditions within the atmosphere tends to take its toll on our relatively fragile probes from Earth.  The only probe to ever reach the surface was the Russian space probe Valerian, it landed and for a brief time radioed images to Earth of a strange and alien landscape.  Our vision was distorted because of the thickness of the atmosphere, and the very landscape seemed to ripple and waver, almost like distant stretches of a highway on a hot day here on Earth.  An interesting fact about Venus is that it experiences Retrograde Rotation. Venus rotates in the opposite direction from the other planets. Venus rotates backwards, or from East to West. We think that at some point in the distant past, Venus must have experienced one of those huge collisions that we think were relatively common back then.  This collision with some mysterious object apparently tipped the planet completely upside down.  This means that the north pole of venus is where you would expect the south pole to be.  This little oddity is only one of the unusual things we could expect ot witness on venus.  Another interesting fact is that one rotation of Venus on its' axis takes 243 days, one revolution around the sun takes 225 days. This means that on Venus, a day is actually longer than a year! Another interesting discovery about Venus is that it is one of only three places in the solar system where there are Active Volcanoes!  This is evidence that the planet is still an active one, and the future may well hold other changes for lovely Venus.


Infrared Venus image. Note the cloud layers.


Earth- The third planet from the sun. A special, unique and wonderful world that we are studying for the entire school year! Earth is approximately 150,000,000 km from the sun and 12,756 km in diameter. Earth has one moon, called Luna. As far as we know, Earth is the only place in the solar system where life ever definitely developed. THAT is especially wonderful! 


Mars- This planet was named for the Roman god of war. What else would be a better name for a planet that is the color of dried blood?! Mars is about 228 million km from the sun and about 6878 km in diameter. Ever since Giovanni Schiaparelli observed what he thought were canals on the Martian landscape, humans have been enamored with the idea of "little green men on Mars". Schiaparelli saw straight lines running across the Martian landscape and thought he was looking at canals, built by a dying race of Martians to bring their limited supply of water from the poles to more equatorial areas. What he actually observed were the remains of once raging rivers, now only dry river beds. At one time, Mars was awash with running water. Today, that water is all gone, perhaps all contained in great polar ice caps, or contained just below the surface of the soil in a permafrost layer. The planet was originally explored by two famous probes, Viking I and II. One of the Viking probes remained in orbit, sending back telemetry and valuable data. The other Viking probe actually landed on the surface and took samples or rocks and the like with a robotic arm. It performed tests on the samples it collected. All of this data was sent back to the scientists on Earth. In fact, the Viking lander is STILL sending back data even today. Mars is tilted on its axis, so the planet experiences seasons, much like we do here on Earth. Mars rotates at a rate about equal to Earth, therefore the length of its days and nights is close to what we have here on Earth. However, Mars takes 23 months to circle the sun, so its seasons; spring, summer, winter and fall last approximately twice as long as they on Earth. The atmosphere of Mars is composed primarily of Carbon Dioxide and monatomic Oxygen (O). The Viking spacecraft currently serve as weather stations, sending back daily information. Current data indicates that summer temperatures are around -21 degrees Fahrenheit, and winter temperatures drop to -191 degrees Fahrenheit. This planet has been more recently explored by the Mars Pathfinder mission. You can find out more about this exciting, and Internet accessible, mission, by following the link. In orbit around Mars are two, small and lumpy satellites, named Phobos and Deimos. You may have heard something in the news recently about a meteorite, discovered on Earth in Antarctica, which originated on the planet Mars. The exciting thing about this little 12 million year old piece of rock is that it may contain evidence of past life preserved in fossil form. Fossilized remains of a type of bacteria have been confirmed in this sample, and there's at least a chance that this simple life form originated on Mars! If it's true, it will be the first confirmed example of life developing elsewhere than on our wonderful world. Wouldn't THAT be exciting? Think of it, if in our tiny, solar system, we know that life developed in not one, but two places, what would that mean? Would it mean that if it could happen here it probably happened "out there" too? Yes indeed. Tiny Mars might once have been blessed with life at one time in the ;past, just like Earth! Unfortunately, at some point in time, that fragile thread of life was broken. Perhaps when Mars lost most of its atmosphere, forever changing its climate. Still, it makes you think though, huh?  Yep, and it makes a lot of other people think as well.  Currently, NASA has a number of new missions whose focus is the tiny red planet of Mars.  Be sure to check out the Astronomy Links section for the latest information from the Mars Rover project, as well as other recent robotic adventures to this fascinating planet.

Olympus Mons, a large volcano on the Martian surface


This image illustrates the many dried up river and stream beds visible on the Martian surface.

An actual view of the Martian Landscape. Taken by our Viking probe on the surface.

A view of Mars from space.

A happy crater on the Martian surface.

The Outer Planets:


With the exception of Pluto, these planets are all much larger than the inner planets. Except for Pluto, these planets are all GAS GIANTS, composed mostly of Hydrogen and Helium gases with smaller amounts of Methane and Ammonia. 


JUPITER-The largest planet in our Solar System. Jupiter contains twice the mass of the rest of the planets combined. Jupiter is 778,300,000 km from the sun and is about 142,800 km in diameter. Jupiter rotates very quickly for a planet its size, making a complete rotation every 10 hours! This rapid rotation has caused the middle of the planet to become bulged outwards and the polar regions have become flattened. Jupiter has no solid core, though gases at its center may be compressed into the liquid state. Jupiter's most renowned feature is the GREAT RED SPOT, a large, cyclonic storm at Jupiter's equator. The great spot stands out against Jupiter's brightly colored bands and can be seen easily from Earth with a telescope. The spot is a massive storm caused by gases rising from Jupiter's depths. You could fit about four planet Earth's side by side, within this massive spot. Jupiter was visited by the Voyager space probes in the 1980's. Voyager discovered a thin, faint ring system around the planet as well as at least 23 moons. The four largest moons were discovered in the 1650's by the famous astronomer Galileo. These moons were first described by him and today are named in this honor today. The GALILEAN MOONS are: Io- a small rocky and volcanically active moon. Ganymede- a larger moon composed of rocks and ice. Europa- a large moon that is noteworthy because it is covered with a thick, smooth layer of ice. Large cracks have appeared in the ice, leading to speculation for years that beneath the ice could be found a vast ocean of some liquid. We've recently confirmed this fact. There is a large, liquid ocean beneath the ice of Europa! Who knows what secrets lie beneath this surface layer of ice? Recently the GALILEO space probe visited Jupiter and its moons, and has made some interesting discoveries. You can get to the Galileo site by following the link. The other GALILEAN moon of Jupiter is Callisto- a moon which is interesting because of its pockmarked surface. There are so many impact craters on Callisto's surface,that the moon looks sort of like a golf ball. Callisto stands as a mute reminder of the violence with which our solar system was born.


This is a photo of the famous Shoemaker-Levy comet about to impact Jupiter

Saturn- This is the second largest planet in our solar system and perhaps the most famous one of them all! Saturn displays the most extensive and beautiful system of rings that we have ever seen! Saturn is about 1,427,000,000 km from the sun and is 120,000 km in diameter. Saturn probably has a small solid core about the size of the Earth. Like Jupiter, Saturn rotates very rapidly causing the poles to become flattened. Saturn's rings are its most impressive feature. The rings are about 40,000 km wide, but only about a kilometer or two thick. When you look at the rings edge-on, they are barely visible. (Like looking a sheet of paper, edge-on) The rings are composed of various particles of rocks and ice. The size of the particles range from about 30 meters in diameter, to pebbles the size of gravel. Saturn has at least 30 moons in addition to its ring system. One of these moons has been particularly interesting to scientists. Titan, one of Saturn's largest moons was studied fairly closely by the Voyager probe. Thick, orange clouds mask its surface. Scientists theorize that Titan may hold some clues to how life developed first on Earth. Life may be developing on the surface of Titan even now for all we know. We have never been able to see its surface because of that thick cloud layer. Future space probes have been planned to visit this fascinating moon.

An early view of Saturn's lovely rings. Note the separations.

An infrared image of Saturn



Just beautiful, isn't it?



Uranus- Uranus is a beautiful planet with a greenish coloration due to the presence of Methane gas. This planet also exhibits strange rings which run " up and down", rather than side to side, as do the rings of other planets. Uranus is 2,869,000,000 kilometers from the sun and about 51,800 kilometers in diameter. It has ten dark rings which run from North to South and at least 15 moons. The entire Uranian system bespeaks of the great days of violence that preceded the development of our solar system. It's moons have been repeatedly destroyed and reformed. A particularly interesting moon is Miranda. Miranda is one of the coldest places in the solar system. Here we see huge cliffs of ice, mountains laying on their side or even upside down! Miranda was a witness to the violence of the early days of our solar system. With further study, what stories could Miranda tell of the origins of our solar system? Uranus itself was once probably hit by a large comet nucleus and as a result, is now flipped on it's side. (This is why the rings are arranged the way they are--)


Neptune is a beautiful blue gas giant only recently seen "up close and personal" by the Voyager probe. this planet wasn't even discovered until 1845, and since then, has no completed one orbit around the sun..Neptune is 4,496,000,000 kilometers from the sun and 49,5000 km in diameter. It was recently discovered that Neptune has a large storm of the sort seen on Jupiter. Thus, Neptune's Great Dark Spot, was named and discovered. It is a storm very similar to the Great Red Spot on Jupiter. This large tornado like storm would literally swallow our entire planet, it's that big! It was previously thought that on such a cold planet as Neptune, large storms such as this couldn't exist, due to the extremely cold temperatures. Neptune has at least two moons, one of which, TRITON, is one of the largest in the solar system, temperatures there are lower than anywhere else in our solar system.


Cloud bands on Neptune


Tiny Pluto is usually the most distant planet in our solar system. It is also the smallest planet at just 3,000 km in diameter. Pluto is about 5,900,000,000 km from our sun. It is a small, rocky or terrestrial world. Our sun appears as no more than a bright star in it's sky. Because of its terrestrial nature, Pluto is thought to be an escaped moon of Neptune, which has taken up it's own orbit around the sun. Pluto was not discovered until around 1930. Pluto has just one moon, which is nearly as big as it is. The moon is called CHARON, which was discovered in 1978. Charon and Pluto are pretty much the same size, leading some scientists to call the Pluto/Charon system a double planet, rather than a planet and a moon. Of all the planets in our solar system, only Pluto will not be visited by space probes by the year 2000. In fact there are no current plans to ever send a probe to Pluto.

Planet X, Sedna and other distant members of our Solar System

It's long been theorized that there are more planets in our solar system out there, in the dark reaches of space, still awaiting our discovery.  "Planet X" has long been the term given to that "tenth" planet, still eluding our sight here on Earth.  Many scientists even dispute the fact the Pluto was classified as a planet in the first place.  These scientists propose that Pluto more on par with an asteroid, orbiting the sun, but too small to be a planet, and actually a member of a vast cloud of objects known to inhabit the outer reaches of our solar system.  This cluster of objects is called Kuiper  Belt, and mostly consists of fragments of ice and rock.  Perhaps Pluto, and it's moon Charon, are merely big Kuiper Belt Objects after all.  There is a fair bit of disagreement as to exactly what the qualities of a planet should be in the first place.  To summarize, it all has to do mostly with orbits.  Objects that orbit the sun, and whose gravity has made into a spherical object, if they are of sufficient size, and they don't share their orbits with too many other objects, are considered a planet.  Moons are generally smaller, and usually spherical objects, but they orbit a planet.  Then, together with their planet, they also revolve around the sun.  Simple really, right?  Thrown into the mix are several areas of "rubble" in our solar system.  The first area is an asteroid belt between the orbits of Mars and Jupiter.  Most of the chunks of rock and ice in this asteroid belt are quite a bit smaller than a planet, but they do orbit the sun.  Some are even close to the size of Pluto, but they share their orbit with lots of other largish chunks of rock, so that disqualifies them from "planetary status", and makes them an asteroid instead.  The Kuiper Belt begins in the the vicinity of Neptune, and has an inner region, as well as a more distant outer region.  This belt also contains largish chunks of rocks and ice, some of which nearly as large as Pluto, but these chunks aren't called planets because they share their orbit with numerous similar objects, both large and small.  One such object, recently discovered, has been named Sedna.  Sedna is about 90AU's from the sun, or about 4.6 billion miles further away from the sun than Pluto.  It's about 3/4 the size of Pluto, and may have other objects (moons) orbiting it.  Many consider Sedna to be our TENTH PLANET, others believe Sedna to be only a large Kuiper Belt Object, and in fact, believe that's what Pluto should have been classified in the first place.  And then, way beyond tiny Sedna, lies the realm of comets, called the Oort Cloud.  Long theorized, but never proven to actually exist.  What secrets lie awaiting discovery in this distant realm where our sun wouldn't even be the brightest star in the sky?  Time will certainly tell... 


And the most beautiful planet of all...