Twinkle, Twinkle Little Star, How I Wonder What You Are?
If you were to go out into space and observe the Sun and one of the nine planets side by side, what differences do you think you would see? Both stars and planets are spherical objects floating in space, however, the Sun would appear substantially larger than even Jupiter, the largest planet in our Solar System. But the differences don’t stop there. The Sun also appears to be like a big ball of fire that emits light, while the planets in our Solar System do not produce their own light, but merely reflect it from the Sun. When looking up at the night sky, you can usually tell whether an object is a planet or a star because stars twinkle and planets do not. The main differences and most obvious difference between stars and planets is the fact that stars have the ability to emit light and heat, something life can not survive without on Earth. When an object has a mass 0.084 times the mass of our own sun (85 times the mass of Jupiter), the core is able to start the process of nuclear fusion at its core. If the mass is smaller than this, the lowest temperature can not support nuclear fusion and the object will never be able to produce light or heat.
Stars also remained fixed in the night sky, only rising and setting due to the Earth’s rotation. Planets, on the other hand, are constantly moving around, due to the fact that the other planets and our own are physically moving in orbit. And although stars also revolve around the center of the galaxy, it takes an extremely long time for them to move that it is not noticeable from Earth.
It Started With A Bang
Stars and planets also differ on how they were created. A star forms when a big cloud of gas, called a nebula, contracts under the influence of its own gravitational force. While it contracts, the object begins to emit fall energy.’ The contractions causes the core to become much denser and hotter, and when the core reaches a temperature of about 3 million Kelvin, it begins to produce light as a result of nuclear fusion at the core. After this happens, the gas cloud stops contracting because the gravitational force is in equilibrium with the pressure that was built up by the hot gas; resulting in a new star.
Planets are formed by the build up of the dust that surrounds a star. When a star forms, a disk of gas continue to surround it. When this gas cools, it condenses to form solid grains. The grains eventually form large bodies, called planetesimals, which collide with each other to form protoplanets. It is from these protoplanets where our existing planets have evolved.
There are two different types of planets. The inner planets (Mercury, Venus, Earth, and Mars) can all be classified as terrestrial planets because they are solid and smaller. The outer planets (Jupiter, Saturn, Uranus, and Neptune) are called Jovian or Gas Giants due to their large masses and liquid and gas interiors. Although planets usually form from the gas that surrounds a newly form star, the Jovian planets may have formed in a different way. Although they are considered planets, they may have formed out of a gas cloud that contracted under the influence of the gravitational force, similar to stars. Their very thick atmospheres that contain hydrogen and helium, surround a rocky core. Stars also consist of hydrogen and helium, but Jovian planets can not be classified as stars because they do not produce their own light and heat; nor are they large enough to do so.
Brown Dwarfs – Star or Planet?
However, not everything is black and white, as there are bound to be some grey areas (or should I say brown?). Brown dwarfs are the exception to the rule and don’t seem to be characterized as neither a star or a planet, but rather the middle ground between them. Brown dwarfs are formed in the same way as stars, but they are too small to sustain the nuclear fusion process that takes place inside a star. However, brown dwarfs do reach a high enough temperature for the fusion of protons into deuterium to take place. Because of this, it emits light during the first period of its lifetime. But because very little fusion takes place, the core of the star can not build up enough pressure in order to prevent the star from further contractions under the influence of its own gravitational forces. As a result, the gas in the center of the star becomes so dense that it begins to degenerate. When this happens, brown dwarfs are eventually built up by the pressure of degenerated matter and as a result it can no longer contract.
Brown dwarfs resemble the Jovian planets relating to their structure and the fact that their chemical compositions strongly depend on the temperature of the gases in their atmospheres. However, the fact that brown dwarfs are able to sustain at least some type of fusion in its core is something that distinguishes it from planets. Overall, brown dwarfs have some similarities between both planets and stars, and can be seen as a transition between these two objects.
Resources:
http://www.astro.rug.nl/~onderwys/sterIIproject97/huisman/index.html
http://curious.astro.cornell.edu/question.php?number=548
