In a dark, clear, moonless night.We look up into the sky.We see thousands of stars arranged in patterns or constellations.The light from these stars has traveled great distances to reach Earth. But what are stars? How far away are they? Are they all the same? Are there other planets around them? When stars initially form from large balls of gas, they contract under the influence of gravity, and heat up because that gravitational energy goes into the motion of the gas.Stars get their heat from two sources - gravity, and nuclear fusion.Once they get hot enough, the hydrogen nuclei in the plasma in the center of the star start to occasionally merge to form helium, releasing a lot of energy from that nuclear fusion process. All this heating, from both sources, creates a pressure that causes the star to quit contracting at some point.
If the star is big enough to start with, the central temperature will be high enough to really keep that fusion going, and the heat gradually will seep out to the outside parts of the star, so the outer surface regions will also rise to quite high temperatures. We all know how important it is that our Sun gives us light! But, why does this happen? To understand, we need to look at the structure of stars.
What causes stars to shine?
Because stars are so massive, the density and pressure in the cores are extremely high! It is the temperature of the outer surface that determines the way the stars shine - our sun has an outer temperature of around 5000 degrees, but other stars can be as hot as 50,000 degrees, producing much bluer light, while red dwarf stars are considerably cooler and produce mostly red and infrared light. The process that causes stars to shine is the same one that we have harnessed for use as a weapon: nuclear fusion.Stars start as huge regions of gas, mostly hydrogen. This gas will start to contract, and it heats up. In fact, the Sun's core is so hot and has such high pressure that it undergoes nuclear fusion. Fusion is the combining to two lightweight elements into a heaver element. In our Sun, the core converts hydrogen into helium. This conversion of hydrogen into helium is the first reaction that happens in every star, it is called the main sequence. This reaction actually ends up with a surplus of energy according to the equation E=mc2. This is because the mass of the combing hydrogen is greater that the end product of helium. Therefore, the mass is converted to energy, and that energy is the electromagnetic radiation (light) that we get.
