EXPLORE THE UNKNOWN

There are hundreds of billions of stars in the Milky Way alone, so what are stars and how did they get there? Stars are "luminous celestial bodies, made up of plasma (particularly hydrogen and helium) and having a spherical shape" (sourced from: Star. (n.d.). Retrieved January 2, 2016, from https://en.wiktionary.org/wiki/star). Stars are formed in clouds of dust and gas called nebulae which are spread throughout many galaxies. Turbulence (which is created by Alfven waves, moving disturbances in plasma or magnetic fields. Coronal mass ejections, highly charged particle emissions from stars, are the main cause of space turbulence.) within nebulae cause "knots", a result caused from a collision of gases. These knots have enough mass to attract gas and dust from their nebula, which begin to collapse inwards into a dense core because of the gravitational attraction. The core and all the material surrounding it begin to rotate, faster and faster (while pulling in more gas and dust) so that the core heats up, forming what is known as a protostar. Eventually, the protostar will become so hot and dense nuclear fusions will occur (combining two lightweight elements into a heavier element), and once temperatures exceed 10 million Kelvin, the protostar will become a new star. The process of nuclear fusion generates energy, providing radiation pressure to balance the inward pull of the protostar's gravity, stopping the newly formed star from collapsing in onto itself. 

 

And how do these stars shine? Basically, stars shine because they are hot, and in order to replace the energy expended they need an internal source of energy. Because, stars are so massive (our Sun is 10 times larger than Jupiter, the largest planet in our solar system), their cores are extremely dense, and the pressure is also extremely high. This allows nuclear fusion to take place, combining the nuclei of hydrogen into the heavier element - helium. Our Sun fuses 620 million metric tons (one metric ton is equal to 1000 kg) of hydrogen into helium per second. The weight of the hydrogen fused by our Sun is equal to the weight of over three million adult blue whales! Stars exist because a balance between the star's gravity, trying to make the star collapse inwards, and the heat produced by the star, pushing back against the forces of gravity, has been reached. This is called hydrostatic equilibrium. Since nuclear fusion generates so much energy scientists have tried to recreate nuclear fusion on Earth to generate electricity. While they have succeeded, the process consumed more electricity than it could generate therefore, it was not efficient. Once the star has begun fusion, it is now in the main sequence phase.

 

Celestial bodies that are 0.08 of the Sun's mass cannot reach the nuclear fusion stage because there is not enough pressure in its core. These bodies become brown dwarves. 

 

Main sequence stars are the most common kind of stars in the Universe. These stars fuse hydrogen into helium and have a mass ranging from 200 solar masses (200 of our Suns) to a tenth of a solar mass. Any smaller, and it would be considered a brown dwarf, or a failed star. Now that the star is in the main sequence phase, the star will have a core, where the nuclear fusion occurs, a radiative zone, where photons carry the energy away from the core, and a convective zone, where hotter substances rise carried by convection currrents, and colder substances fall. The "life" of a main sequence star depends on its mass. The greater its mass, the faster it will fuse hydrogen into helium (since there would be more pressure in the core and the temperature would be higher) and the smaller the star the slower the process of nuclear fusion will be. This is why red dwarf stars, which are about 0.08 solar masses or more, can "live" for over 80 billion years. The Hertzprung-Russel diagram, which classifies the spectral classes of stars, organizes stars by their luminosity and surface temperature. Interestingly, the diagram shows that most stars lie on a continuous, diagonal curve from the top left corner (hot and bright stars) to the bottom right (cool and dim stars) corner, as you can see from the image on the right. On the diagonal curve of the main sequence stars are between 30 to 0.1 solar masses.

 

Stars eventually begin to "die" once they run out of hydrogen to fuse into helium. The "death" of a star depends on how much mass it has. Stars with about the same mass as the Sun, will collapse from the inwards pull of their own gravity once they run out of hydrogen fuel. This is because the star's gravity is no longer balanced from the creation of helium through nuclear fusion. As the star shrinks, the pressure increases in the core, allowing nuclear fusion to continue to occur in the outer layers - heating them up. The outer layers will begin to expand from the heat, causing the star to become a red giant. The core of the star will now be hot enough to allow it to fuse helium into carbon. When the helium runs out, the star will expand and cool, ejecting gase, forming a planetary nebula. The core will then turn into a dense, white dwarf, which will eventually cool into a black dwarf and fade away. This process wil taked billions of years.

 

Stars more massive than the Sun (10 solar masses +), fuse hydrogen into helium, like our Sun, but at much faster rates due to the increased pressure and temperature in the core (since the star is so massive). When the core runs out of hydrogen, these stars also begin to fuse helium into carbon. However, since the mass of these stars is so enormous, once the helium runs out, they can begin to fuse carbon into heavier elements like oxygen and silicon. Once the star has fused iron, then it is no longer able to continue fusing iron into heavier elements, since it takes more energy to fuse iron than the process produces. This causes the star to collapse from its own gravitational force, and sends the outer layers out in a supernova. These supernovae, are bright enough to outshine an entire galaxy of stars. If the original star had a mass of 10-30 solar masses, then it will form a neutron star. If the mass of the original star was greater than 30 solar masses, then it will form a black hole. In both cases, gases drift off as a nebula to form new stars.