Facts About Stars

• The Sun in the center of our solar system is a star.
• There are around 200 billion stars in the Milky Way alone.
• VY Canis Majoris is the largest known star in our galaxy, if this star was in the center of our solar system its outer atmosphere would reach the orbit of Saturn.
• One of the smallest known stars in our galaxy is VB 10, it is only around 20% larger than Jupiter.
• Very large stars have a lifespan of only a few million years while very small stars can exist for trillions of years.
• The lifespan of our own star, the Sun, is around 10 billion years.
• Supernovas are explosions generated by large stars when they come to the end of their lifespan.
• There is a maximum of 2,500 stars visible to the naked eye at any one time in the night sky.
• The nearest star to our solar system is Proxima Centauri which is 4.2 light years away.
• The Sun is part of a single star system but there are also binary and multiple stars where two or more stars orbit around each other.

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Stars are born inside clouds of gas and dust known as nebulas which exist throughout the galaxy. Some nebulas form from the gravitational collapse of gas in the interstellar medium while others are the result of the death throws of a massive star. Hydrogen clumps together inside these clouds of gas growing ever larger and hotter until eventually the early stage of a star called a protostar is formed. As gravity collapses the protostar temperatures and pressure in its core become high enough to trigger nuclear fusion. The star is now fusing hydrogen atoms creating an enormous amount of energy, this stage of a star's existence is known as its main sequence and depending on its size it could remain in this state for billions or possibly even trillions of years. Together the stars light up the universe in a variety of colors and most importantly our own star also provides the energy which allows life to flourish on our planet.

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When a star is in its main sequence it is fusing hydrogen atoms in its core which creates energy. Stars in their main sequence are referred to as 'dwarfs', the sun being an example of a dwarf star, all stars will spend the majority of their lifespan in this state before they exhaust their supply of hydrogen. After this stage most stars become giants, for example when our sun can no longer fuse hydrogen in its core it will begin to fuse hydrogen in its outer shell causing it to expand greatly and become a red giant.

The color of a main sequence star depends on its surface temperature, which is usually related to its mass. Low mass stars generally have a lower surface temperature and appear red, high mass stars generally have higher surface temperatures and appear blue. It’s worth noting that all stars emit light from every color of the spectrum but will appear as one color to us.

Stars are classified with a letter depending on their surface temperatures, either A, B, F, G, K, M, or O. They are not in alphabetical order, stars with similar surface temperatures to our sun are classified as G, whereas a much hotter star may be classified as B or O. These are by far the most common type of star in our galaxy. Red Dwarfs have less than 50% the mass of our own Sun, as a result they are much cooler and emit far less energy. As Red Dwarfs burn their fuel at a very slow rate their lifespan is much longer than those of other stars, it is possible some could exist for over a trillion years. Some red dwarfs with a high enough mass may become giants but will never achieve core temperatures high enough to begin fusing helium. Red dwarfs are classified with the letter M. Orange dwarfs are also very common in our galaxy, they generally have a mass of between 0.5 and 0.8 of the sun and have lower surface temperatures. Orange dwarfs stay in the main sequence period of their lifespan up to three times longer than yellow dwarfs such as our sun. As a result orange dwarf systems are considered very stable environments for the development of planets and the evolutionary process of life. Orange dwarfs are classified with the letter K. The Sun in the center of our solar system is a Yellow Dwarf, these are stars that have approximately between 0.8 to 1.2 the mass of our sun. Most Yellow Dwarfs are actually more whitish in color, our sun only appears yellow due to the light interacting with Earth’s atmosphere. The lifespan of a Yellow Dwarf in its main sequence is around 10 billion years. Yellow-White dwarfs have a mass of approximately between 1.2 and 1.4 times that of the sun and have slightly higher surface temperatures. The stellar classification for Yellow Dwarfs is G whereas Yellow-White Dwarfs are classified as F type stars. Stars which appear white or whitish-blue generally have a mass approximately between 1.4 and twice that of the sun. Their surface temperatures can be almost twice as high as the sun and they are usually classified as A type stars. Blue stars generally have a mass of more than twice that of the sun, in some cases they can have a mass of more than a hundred times that of the sun with surface temperatures up to 10 times hotter and thousands of times brighter. Blue stars burn through their fuel at a far quicker rate than smaller stars meaning they have a lifespan of only a few million years, they are generally classified as B or O type stars.

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Stars create energy by fusing hydrogen atoms into helium in their core, at some point though the hydrogen available for fusion will eventually run out signalling the beginning of the end for any star. A Yellow Dwarf such as our sun has enough hydrogen to last 10 to 12 billion years, after this point fusion activity in its core will cease. Instead the star will begin to fuse hydrogen in a shell surrounding the core causing it to expand to hundreds of times its original size and cooling its surface, becoming what is known as a red giant.

During this process the star’s helium rich core contracts and heats up, at a certain point the core reaches a high enough temperature to begin fusing helium into carbon and oxygen. After the star has exhausted its supply of helium it will shed its outer layers leaving only the core remaining. At this stage the star is now a White Dwarf (pictured above), the burning embers of a dead star around the size of the Earth. It will exist in this state for billions of years until it eventually cools down.

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Larger stars have a very different fate to Yellow Dwarfs, the death of these stars results in some of the largest explosions in the Universe called Supernovas (picture left). When a very large star exhausts its supply of hydrogen and helium it has enough power to continue to fuse elements in its core, so helium will be fused into carbon and oxygen, carbon and oxygen into magnesium and neon and so on through the common elements. As a result of this process the star will create layers of elements with an outer shell of hydrogen, below that helium, then carbon and so on. The star will continue this process until it creates iron in its core, as the star is unable to fuse iron it becomes unstable and collapses. The iron core will collapse from the size of Earth to only around 10 miles in diameter, the energy then rebounds causing a massive explosion that rips the star apart. In the few seconds after the supernova enough heat is generated to create even heavier elements than iron such as uranium and plutonium.

The elements thrown off by these explosions form into nebulas which are clouds of gas and other materials that go on to form more stars, planets, moons and even life itself. We ourselves are made from these elements, the iron in our blood or the calcium in our bones were at one point part of a star.

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After a Supernova the core of the star is left intact, what happens to the core all depends on how massive it is. A core remnant with a mass of between 1.4 and 5 times larger than the Sun will form into a neutron star. As the core collapses it combines electrons with protons to form neutrons allowing gravity to force the core to become even smaller. Some neutron stars can be as small as 10 miles (16 km) in diameter and are incredibly dense, one teaspoonful of a neutron star would weigh hundreds of millions of tonnes. Neutron Stars spin incredibly quickly, they can make hundreds of rotations per second, combine this with their intense magnetic field and neutron stars sometimes produce a beam of light. The light is caused by charged particles streaming along the axis of the magnetic field, theses types of Neutron Stars are called Pulsars. Another type of Neutron Star is a Magnetar. These stars possess a magnetic field which is one thousand times stronger than a normal Neutron Star’s and one thousand trillion times stronger than Earth’s. The field is so strong it heats up the stars surface to 10 million C (18 million F), the temperature on the surface of the sun is only 5,500 C (10,000F). If the remaining core of an exploded star has a mass more than five times that of the sun they do not form into Neutron Stars, the force of collapse is just too great and nothing can stop gravity crushing the core into an object of infinite density and zero volume, these are known as Black Holes. The gravitational force of a Black Hole is so strong that not even light can escape from it. The point of no return for any object that strays too close to a Black Hole is called the Event Horizon, past this point any object will be pulled inside and ripped apart.

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