(Astronomy) Stellar Evolution

Welcome to your first lesson in astronomy. In this guide and those that are to come, I will try to focus more on the basics and do my best in simplifying everything so that it will be easier for you people to understand. Comments are welcome. Hopefully I will help more people understand the wonders of the universe.

Today's topic will be on Stellar Evolution

It's true! Stars do die! just that it takes a freakin long time to do so. At more than 5 billion years old, our own Sun (Our Sun is a Star) is middle aged. There are stars that have longer/shorter lifespans than ours, depending on how massive they are...more on that later. Better to ask questions on the tagboard than as a comment, can have livelier discussions that way.

This is a simple chart that shows the life cycle of stars. Note the two different
types of Giants. Also note the different fates awaiting the Super Red Giant.

All stars start out of gas clouds. Most of these are called Molecular Clouds. These
are HUGE clouds of gas (of mainly H2, or Hydrogen) that span billions of light years across. In fact, Nebulae is usually just a tiny portion of the molecular cloud that is visible because of stars that are residing/being formed in it. The energy given of by these stars causes the nearby gases to glow, lighting the surrounding area up. Eg. The Eagle Nebula size, as seen here is probably only 0.0001% o the entire molecular cloud of which it is part of. Inside the eagle neb are the famous 'pillars of life', where many stars are being born.

Anything that has mass has gravity...including you and me, just that our mass is too
small to make any signifanct amount of attraction. Well, the Earth is big enough, that is why we are stuck to the ground no?

Also Imagine running your hands through water that has been dropped with some coloured dye, you'd see spirals right? Other stars would affect the surrounding gases in this manner as well. That, plus the gases own gravitional attraction, causes the surrounding material to spiral towards the centre. As this bundle of gas starts gaining mass and momentum, it starts to glow, primarily from the increasing friction caused by the movement of the spiralling material. At this stage, the growing bundle star is known as a proto-star. The real magic starts when it has reached critical mass and goes through puberty at an instant : the start of nuclear fusion.

Nuclear Fusion would start with a bang. In an instant, energy released pushes outward
from the within the newborn star, pushing gases and other materials outward. Heavier bodies of gas and rock that manage to withstand the blast would later be known as planet.

Nuclear Fusion is the heartbeat of the star.

A very popular manifestation of a nuclear reaction is the Nuclear Bomb. It makes use of a reaction called Nuclear FISSION, which creates energy by splitting the molecules. Nuclear FUSION, as its name suggests, is the direct opposite. In this process, molecules(primarily Hydrogen from the core of the star) are joined, or 'fused' together. Much more energy is produced using this method. It is also very stable when compared to Nuclear Fission, which is why researches are spending so much money to try and replicate the process.

So far, attemps are replicating the Fusion process has been frustrating, to say the least, as scientists has only been able to maintain the process for less than a fraction of a second, and has only been successfully reproduced in a Tokamak, which uses powerful magnetic forces to control the extremely high temperature plasmas that would otherwise melt any known material while the process is underway. More about it here.

Remember that proto-stars start nuclear reaction when it reaches critical mass? This critical point is when a star has enough gravity to compress 2 molecules into each other, fusing them and creating a new element which releases huge amounts of energy and starts a chain reaction. Once this starts, energy and light is released. This signals the real birth of a star.

Now, when two molecules are joined, would it create a heavier molecule? Remember the Periodic table, which lists all the different chemical compounds according to their molecular weight? There you go, now you know how all the materials like metals and rocks are formed, all inside stars. The slide above shows the respective elements that are created...hydrogen combined with hydrogen to form...Helium..then helium and helium joined to form...Carbon...etc etc. Notice that it stops at iron?

So I am not lying when I say that we were all born in the heart of a star hmm? Now go, run along and fool some blondes with that info.

Even stars don't last forever. All stars have a limited supply of fuel which they can burn. There will come a point in time when a star would have run out of hydrogen in its core as most of it has become Helium. Once this happens, the star would switch to its backup store of hydrogen, which is located in the outer layer of the star.

Let's change the subject for a little while. Remember that a star is basically this huge ball of gas in space? Now, if it is not solid, how does it stay as it is...a ball of gas in space? The answer is Gravity. Gravity is what holds this ball together and prevents the gases inside from being blown away. Back to topic. Now that the reaction is taking place OUTSIDE the core, more hydrogen is being fused at one time (due to the larger surface area involved) , which means more energy is released. This causes the entire star to swell significantly in size. There is always a tug of war between the energy released pushing outwards and gravity pushing inwards towards the core, maintaining the form of the star.
Depending on the mass of the star, it will swell to become a Giant, or a Super Giant.

(Giant: 10 or less Solar Masses. Super Giant: More than 10 Solar Masses. 1 Solar Mass
is the mass of the Sun)

The process will keep repeating itself until most of the material has been fused into Iron, yo-yo-ing in size as it changes from core to outer layer, material to heavier material. Eg. Core hydrogen - (expand) outer hydrogen - (shrink) core helium - (expand) outer helium - etc etc...

Why is it red instead of yellow or blue? Because the temperature on the surface has actually dropped. Why is it still brighter than it was? Because it now has an exponentially bigger surface area, which means that overall, more light is still being released at once.

As you can see in the picture above, the swelling is no joke. So much more hydrogen is fused that the increase in energy release is exponential, which causes the swelling to be enermous. In real life, Earth would have been sucked in the Sun because of the gravitational pull. It has been retained in the pic to give you a sense of scale. Talk about global warming...ouch

First, we will talk about the death of Giants.

Giants, which are formed from stars equal to or smaller to 10 times the mass of our sun, die a relatively peaceful death.

As the fusion process dies down, gravity will begin winning the tug of war and continue pushing inwards. There will come a point in time where it will burst, like a balloon being squished. Gravity will lose its grip on the material, usually around the poles of the star, allowing the unprocessed elements from the outer core to peel away, forming a planetery nebula. After it has peeled away, an small hot iron core, known as a dwarf, is left behind. When this happens, a giant/star is said to have gone Nova.

As its earliest and brightest (hottest) stage, a dwarf will usually be known as a white dwarf. All other dwarfs you hear about and just at different stages of cooling. Like white dwarf, to grey, then brown and finally black dwarfs..which is basiacally a cool dwarf with no more energy to glow.

More mass = more gravity.

Suger Giants are formed from stars that are at least 10 times heavier than our sun and because it has so much mass, it has an extremely strong gravitational field as well. A glorious death fit for the gods await them; an explosive Supernova.

As the super giant's fusion process dies down to a certain level, gravity will force itself inwards with a vengeance. The entire star shrinks to a fraction of its suger giant size. The real drama though, is in the core. The iron core implodes on itself as its electrons and protons combine into neutrons. The outer layer elements that was falling into the core impacts it at high speed and bursts outward in a spectacular fashion. These explosions are so big that it can outshine entire galaxies at the time of occurance. Heavier rare elements like gold and platinum are created in the explosion

What's left is a small, dense neutron star that is probably no bigger than an average city on earth, though one teaspoon of this material can weigh a few hundred tonnes!

Here comes, everybody's favourite part: If the star is massive enough, a neutron star would further collapse into a black hole! Nobody really knows how a black hole really looks like, because in these objects, the gravity is so strong that even light is unable to escape from it, and no light means that you can see nuts.

Hopefully, you enjoyed this brief lecture on the birth and death of stars. Note that this is meant for beginners, so if you are a seasoned astronomer....well maybe next time yea? ^^ Remember, questions can be asked on the tagboard to the top right of this page.

Enjoy, and see you next time!