.. The Night Sky Long ago, people looked into the night sky and wondered what they were looking at? How far away are those twinkles in the sky? Could they all be stars, or maybe, could they be something else? What makes certain lights brighter than others, and how does distance affect their intensity? These questions and other interesting facts will be reviewed in the following pages. One of the most common curiosities regarding the night sky is distance, which can be very hard to determine.
Because space is so vast, scientists must use mathematical methods to determine how far away, how large, and how bright something actually measures. However, because of the constantly changing position of Earth and the solar system in relation to the galaxy, and the incredible distances that separate objects in space, scientists have developed a different standard unit of measure. The most common unit of measure is a light year. A light year is the distance that light can travel in one year (Giancoli 1000).
Using very sophisticated tools, scientists have measured the speed at which light travels and have found the distance in one second to equal 3 x 10^8 meters. From this discovery, they have the ability to determine that one light minute equals 18 x 10^9 meters, which calculates one light year to equal 9.46 x 10^15 meters (roughly 10^13 kilometers). To promote a better picture of how far ten trillion kilometers stretches, imagine the distance between the earth and the moon to measure 384,000 kilometers, or 1.28 light seconds; the distance between the earth and the sun is 150,000,000 kilometers, or 8.3 light seconds; and the distance from Earth to the farthest planet, Pluto, measures 6 billion kilometers or 6 x 10^-4 light years. To envision the almost unimaginable distances of space, the closest star to the sun, Proxima Centauri, is 4.3 light years away, which is over 10,000 times farther away than the most distant planet in Earths solar system (Ibid). Of all the distant objects that are seen in the night sky, the closest objects to Earth are planets.
There are a total of nine planets in the earths solar system, including Earth, along with a few other stellar objects, such as comets, that pass by every so many years (Ridpath 59). The planets vary greatly in size and composition. Some that shine very brightly can be seen all year round, while others are very hard to locate and distinguish. The order of the planets in the solar system goes as follows: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and finally Pluto (Ibid). Mercury, the smallest planet in the solar system, has a diameter of only 2,900 miles. This first planet is solid with very little atmosphere and has only six percent the volume of Earth.
Mercurys mass, however, is not known since there are no satellites, or in other words, moons orbiting the planet, that would reveal its gravitational influence upon them . If there were moons orbiting Mercury, then they could help us in determining the planets mass by using Einsteins Universal Law of Gravitation. However, even without the moons, scientists have been able to estimate its mass to be about four percent of that of the earth (Pickering 44). Mercury can be seen in spring and fall, at sunrise or sunset, which is when it is at its greatest distance from the sun.
It can only be seen for a short period of time, though, and is very near the horizon even when it can be seen (Pickering 46). The next planet is sometimes known as the Morning Star because it is the brightest object in the night sky. Because of Venus rapid orbit of the sun, every 225 days, the time of year that it can be seen varies from year to year. Venus is so easily visible because it is incredibly bright compared to normal stars. At its brightest angle, Venus can be up to 4.4 times brighter than Sirius, the brightest star in the night sky.
Venus has a diameter of 7,700 miles, and is very comparable to the size of Earth. Its mass is 82% of that of the earths and its gravitational pull is 89% the strength of the earths (Pickering 47). Even though the proportions to Earth are so close, Venus has one very distinct difference, and that is its atmosphere. Venus atmosphere is so dense that it actually hides the surface of the planet completely and permanently. Probes that have been sent to Venus have found the atmosphere to be composed of 95% carbon dioxide and the remaining atmosphere composed up of other trace toxins. This atmosphere is also 90-120 times more dense than that of the earths, and this would eliminate almost all of the air from ever reaching the surface of Venus.
Because of the dense atmosphere, the light heat that penetrates the planet is trapped, and Venus surface temperature has been found to be 900 degrees Fahrenheit. From the extreme temperature, the lack of light and water, and the extreme density and toxicicity of Venus atmosphere, scientists have concluded, almost beyond a shadow of a doubt, that there is no chance of life on the planet or that man will ever be able to land there (Pickering 48-50). The fourth planet in the solar system is Mars, otherwise known as the red planet. Mars has a diameter of 4,213 miles and is only fifteen percent of the volume and about ten percent of Earths mass. The time of year that Mars can be seen varies greatly as well, because it orbits the sun once every 687 days (Pickering 50-51).
However, when the planet can be seen, it is the second brightest object in the night sky, just a little more than three times that of Sirius. Mars has a very thin atmosphere that is made up mostly of nitrogen and carbon dioxide in the upper atmosphere. The density of the atmosphere is comparable to that of the earth. There are polar ice caps on Mars but these have been found to be composed of mainly carbon dioxide with just traces of H2O. There is evidence that at one time there were oceans and large amounts of water on Mars, but due to its weak gravitational hold on its atmosphere, most of the water vapor has escaped.
Many probes have been sent to Mars to find an answer whether or not there is, or ever was, life on the red planet. The results show that the planet is very seismically active and that nothing there suggests that Mars ever has had any intelligent life (Pickering 51-53). Then comes the largest planet in the solar system, Jupiter. It has a diameter of 98,329 miles.
Its volume is 1,318 times that of Earth, but its mass is only 318 times that of Earth (Pickering 57). Jupiter is made mostly of atmosphere. Even though scientists have not been able to send a probe to Jupiter as of yet, there are two predominant theories as to what the planet consists of. What can be agreed upon is that the outer atmosphere consists of hydrogen, methane, and ammonia. Then the one theory suggests that layer is only 8,000 miles deep and then there is a 10,000 mile thick layer of ice over the central core, whose composition of metals remains a mystery. The other theory is that beyond its outer layer of gas is a sphere of hydrogen that is so compressed towards the core that it has certain metallic properties (Pickering 58).
Jupiter has twelve satellites, four of which are very bright and can be easily distinguished, however, the four will almost never be seen together on the same night because of the incredible velocity that they orbit the planet with (Pickering 58, 59). Jupiter is the third brightest object in the night sky, almost three times brighter than Sirius, and is only behind Venus and Mars (Pickering 58). The next planet in the solar system is Saturn. Saturn is the second largest planet behind Jupiter with a diameter of 75,021 miles.
Oddly enough, it only has a mass of 95.3 times that of the earth and has such a low density that it could float in water (Pickering 60). Saturns atmosphere is very similar to that of Jupiter except that the outer temperature of the planet has frozen all of the ammonia (Pickering 61). Saturn is famous for its rings which have been determined to be the remains of satellites that have been pulled too close to the planet and have been torn apart by the two bodies of opposing forces being the gravitation of the planet and the initial velocity of the satellite. As of this day, Saturn still has ten satellites (Pickering 61-63). Even farther out than these gas giants lies Uranus, which is similar to that of its larger neighboring planets. Uranus has a diameter of 29,300 miles and has a mass of more than 14.68 times that of the earth (Pickering 63).
Uranuss atmosphere is almost identical to those of Jupiter and Saturn only being colder, 305 degrees Fahrenheit below zero (Pickering 65). Uranus is visible from Earth, but is no brighter than any other star due to the great distance that the light has to travel (Pickering 66). Uranus has five satellites and a faint green ring around it, but other than those two features, Uranus is just a small replica of its two larger neighbors (Pickering 65-66). Then the next planet is Neptune. Neptune has a diameter of 27,700 miles and a mass of 17.3 times that of the earth (Pickering 67).
Neptune, like its three neighbors, is a gas giant. Its atmosphere consists of the same gaseous material as Jupiter, Saturn, and Uranus (Pickering 68). Neptune has two satellites and a faint bluish disk that can barely be seen with a high power telescope (Pickering 68). Neptunes elliptical orbit crosses over Plutos, and they change positions as to which planet is farther away from the sun. However, they will never collide because they are not on the same plane of rotation. Pluto has a 17 degree angle difference in planes of rotation, and the two planets will never be closer than 240 million miles of each other (Pickering 71).
Pluto is currently the farthest planet from the sun (Pickering 69). There is little known about Pluto, but from the information that scientists have, they are fairly sure of their assumptions. The estimated diameter of Pluto is 3,700 miles, and this would make Pluto the second smallest planet in the solar system. It is not believed to have any atmosphere, and even if it did, it would be frozen solid to the surface due to the extreme cold of deep space (Pickering 69-70).
Pluto has no satellites and is even thought to have been, at one time, a satellite of some other planet that had sufficient force to break the orbit of that planet only to be caught in the gravitational pull of the sun. Pluto was actually found through the use of mathematics. There were irregularities in the orbit of Neptune and Uranus that led scientists to believe that there had to be another planet outside the orbit of these two planets to affect there even progress around the sun. This is what led scientists to search for years upon years for the tiniest planet that couldnt even be seen with the naked eye (Pickering 70-71). Scientists have gathered this information through the use of high powered telescopes, satellites, and space probes that have been lunched throughout the years, and have determined the distances of nearby object, such as the moon and the satellites by using sonic waves and measuring the pause in time for the waves to bounce back.
Using what they know about the earths location relative to the sun, the moon, and the satellites that they send out, they have determined the distances of planets far from the sun using parallax. Parallax is the apparent motion of object against the background of more distant objects due to the earths rotation around the sun (Giancoli 1003-1004). Parallax angles can be used to determine the distances of things up to 100 light years away; beyond this, they become too minute to receive an accurate reading (Giancoli 1004). To determine the distances of objects farther than 100 light years, scientists used the red shift or blue shift of an object.
Red shift is the lengthening of the wavelength of spectral lines caused by the moving away of two objects from each other, and blue shift is the shortening of the wavelengths of spectral lines caused by two objects moving closer to each other (Clark 40). Red shift is therefore used to calculate the distances of extremely far-away objects such as distant stars and other galaxies. Scientists used the Doppler Effect to determine why there was a difference in wavelengths and why the pitch of the spectrum would increase or decrease as objects moved farther apart or closer together, thus the red shift technique was developed (Clark 25). Those are just the closest thing to Earth, farther out lies the majority of what people see, stars. A star is a gaseous self-luminous body in space (Pickering 133).
Stars vary in brightness, mass, density, volume, and surface temperature. Scientists use parallax to determine the distance of all of the stars in the Milky Way. The Milky Way is a rotating spiral shaped disc and is about 100,000 light years in diameter and 2,000 light years thick with around 100 billion stars (Giancoli 1000). All stars are made up of one basic element, hydrogen. In the beginning, after the Big Bang, the moment thought to be the beginning of time when all of the matter in the whole universe was together in one large body (Fraser, Lillestol, Sellevag 100), all of the parts of atoms combined and thus one neutron, one proton, and one electron combined to make a hydrogen atom (Clark 118). There is a certain gravitational pull that all objects have that cause things to pull together, and over many billions of years they begin to form clouds of particles called nebulas, and then contract even more until a star is born.
During this first stage of a stars life, it is called a protostar. These gradually heat up due to the gravitational pull and the amount of collisions occurring yet as of this point there is no nuclear fusion taking place. As the temperature rises, the gas begins to move faster and thus creates more pressure which gradually balances the inward pull of gravity and halts the collapse of the protostar (Clark 118). The protostar now has a significant amount of gravitational pull and collects more and more gas thus creating more pressure and thus more heat. Finally once enough pressure and heat build up they gravitational pull on the atoms will force the protons close enough together for nuclear fusion to take place (Ibid).
From here the protostar now joins the Main Sequence of stars and depending on how much mass it has accumulated it will follow one of many different variations. One odd thing about stars is that the more massive the star, the shorter its life will be (Clark 122). The largest stars may have the most hydrogen in them but they burn with such efficiency and at such a high temperature that they burn themselves out in just a few tens of millions of years. The smallest of the stars are the most numerous but are hardly ever noticed because they lack the mass to create any significant amount of light. Oddly enough these tiny stars, called red dwarfs, will continue to exist for many tens of billions of years.
The star that Earth orbits is medium sized and will exist for around nine billion years in the main sequence (Ibid). Newer techniques now exist for gathering more data about stars, one of which is the inverse square law, that states that the intensity of light drops off as the inverse square of the distance (Giancoli 1005). Scientists can also determine the mass of the star by the light spectrum that the earth receives because there is a direct correlation between mass of the star, surface temperature, and the pitch of the wavelength of light (Giancoli 1006). The differences in surface temperature favor certain areas of the light spectrum. From all of this, scientists have determined how hot, how massive, and how far away each star measures, and since they know this, they can determine at what stage in the stars life cycle that it is currently in.
During the main sequence the star remains stable while the internal nuclear reactions continue to combine hydrogen atom to hydrogen atom creating helium atoms, which are denser than hydrogen atoms, and thus are pulled into the core of the star. The nuclear reactions can only take place in the very center of the star however, because this is the only place that has a high enough gravitational pull to create the amount of pressure for nuclear fusion. These fusion reactions emit incredible amounts of energy which is the only thing that prevents the star from contracting further. After about ten percent of the hydrogen fuses to create a fairly large helium core, the lack of nuclear reactions in the core cause it to begin to contract causing greater pressures and higher temperatures to occur. Because of these higher temperatures the layer of hydrogen around the helium core burns with even greater fury than the core previously did, creating even larger amounts of energy overcoming the gravitational pull of its own body, causing the outer shell of the star to expand rapidly (Clark 124-125). The star is now leaving the Main Sequence and entering the expansion phase moving towards the Red Giant period of its life.
During this expansion phase the star can swell from ten to one hundred times its original size (Clark 124). As this happens the outer temperature will cool and the surface will be a very bright red. Eventually the hydrogen layer around the core combined with the continuing contraction of the helium core will be so great that fusion can occur between the helium atoms. This will create unstable beryllium isotopes, that will react with another helium atom almost instantly, and finally create stable carbon atoms. This process will continue until all of the helium has been combined, and the core is made of solid carbon (Ibid).
This is where the mass of the star determines what will happen to it. If the star has less than 1.4 solar masses, no more fusion can take place, and the star will collapse on itself due to its own gravitational pull. This collapse will cause the star to lose its outer gaseous layers that will disperse into a planetary nebula around the star and eventually disappear into space, leaving behind a white dwarf (Ronan 122-123). When the sun turns into a white dwarf it will have about half of its original mass and will only be the size of Earth.
The density of a white dwarf star is so great that the electrons no longer exist in orbits around the nuclei (Clark 125). They are now so compressed that they try to occupy the lowest energy level possible resulting in a very hot surface temperature that yields a very low amount of light. The white dwarf cools gradually over the course of the next few billion years, until it becomes a body known as a black dwarf. The largest stars, though, suffer a much more dramatic fate and can leave even more compact objects than white dwarfs (Clark 125). In the case where the stars mass is greater than 1.4 solar masses, there will be enough heat energy in the red giant core that nucleosyntheseis can occur creating very dense nuclei that eventually turn into iron and then nuclear fusion comes to a halt (Giancoli 1009). From these series of reactions there is a large build up of iron in the core of the star and layer upon layer of other elements in order of density.
The amount of pressure is incredible at this point and the electron pressure begins to build up and the core becomes unstable. Once the gravitational pressure becomes too great the electron shell around the nuclei begin to collapse causing the core to collapses even farther, and the star crashes down upon itself. When this happens it loses control of the tremendous amount of energy that had been building up during the fusion process and the red giant explodes, virtually blowing itself to bits. This is known as a super nova (Clark 126-127). While this occurs, there is enough energy produced to create elements much heavier than iron and from this explosion all planets and complex elements are made. From this, scientists have determined that all of the dense elements that are in the human body were, at some point in time, involved in the explosion of a star (Clark 127).
Some of the most powerful bodies in space are created from these explosions. If the original mass of the star was between 1.4 and 3.2 solar masses, the core of the star will collapse due to the tremendous gravitational pressures that were on the elements, and the atoms contract beyond the level of the electron so that the electrons combined with the protons thus creating neutrons. This is where the star gets the name, neutron star (Clark 128). As a result the entire star is now made of neutrons and lacks the electron shell to separate the nuclei of one element to the next thus creating an object similar to one giant nuclei made entirely of neutrons.
Neutron stars of this nature are considered to have more than one and a half the mass of the sun but only the diameter of 10 to 20 kilometers (Clark 128). In the rare case that the core would be greater than 3.2 solar masses, the mass of the core would be so great that the neutrons would not be able to resist their own gravitational pull, and the star would collapse beyond the atomic level to become a black hole. The gravitational pull of a black hole is greater than that of the speed of light, and thus no light can ever escape from one of these bodies, making them very difficult to study or even find (Clark 130). Not much is known about these mysterious bodies, but many scientists have developed theories as to what could be done with such a body of unimaginable power. With the use of red shift scientists have been able to determine the last few twinkles in the sky as other galaxies. So far, more than 100 billion galaxies have been discovered, the closest being 2 million light years away and the farthest being about 10 billion light years away (Giancoli 1002).
Very little else can be determined from other galaxies because of the vast distances of space between the Milky Way and all of the other galaxies. It is known that many are spiraling on a disc the same as the Milky Way, and that the universe is still expanding (Clark 136). Scientists have determined this from the findings of the differences in the red shift, and it supports the Big Bang theory. From all of this man can determine how far away a star is from Earth using parallax, and how far away another galaxy is using red shift. He can also use the light spectrum that is received and use the inverse square law to determine how massive, how hot, and how far away a star measures from Earth.
From that information scientists have found the life cycles that stars undergo and have learned a great deal about how the sun, the solar system, the Milky Way, and the universe operate as a whole. This information is very helpful in understanding the origins of the everything and in predicting what may come long after man is gone.