Tracing Your Cosmic Origins

30 07 2009

The Crab Nebula, the remains of a supernova which exploded in 1054 AD.  This picture was taken by the Hubble Space Telescope.

The Crab Nebula, the remains of a supernova which exploded in 1054 AD. This picture was taken by the Hubble Space Telescope.

You are a star.

I am too.  Every single one of us is.  Every single living thing on Earth- not to mention the Earth itself- is made from materials forged in the furnace of a red super-giant star that lived more than four and a half billion years ago.


When the Universe was formed in the Big Bang, the only elements produced were hydrogen and helium.  Yet all around us we see a world made from heavier elements.  Iron and silicon are in the rocks, calcium is in our bones, oxygen is in the air and the water, and carbon is the basis for every single life-form on the face of this pale blue dot we call a planet.  None of these were produced in the Big Bang; they were produced much later, in the centers of red super-giant stars.

All stars, regardless of size or color, spend the majority of their lives powered by hydrogen fusion.  This is when atoms of hydrogen are combined to form atoms of helium, releasing energy in the process.  It takes enormous temperatures and pressures for this process to happen (a thermonuclear bomb, which uses the same principle, requires a regular nuclear bomb to set it off), so fusion only occurs in the hottest, densest part of a star- the very center.  The rest is just dead weight.

An average star, like our sun.

An average star, like our sun.

Eventually, the core of the star runs out of fuel.  There is not enough hydrogen left in the center to fuse into helium, so fusion temporarily shuts down.  This causes two things to happen: the dead weight part of the star expands to hundreds of times its previous size, absolutely obliterating any planets which are unlucky enough to get too close (this is how the Earth will be destroyed, in about five billion years), while the core collapses to a fraction of its former size, causing a drastic increase in both heat and pressure.  Eventually the heat and pressure in the core get high enough to ignite the next level of fusion, helium into carbon.  The heat released by this fusion increases the temperature of the bottom layer of the dead weight hydrogen gas until it gets hot enough to begin fusing into helium.  The star then has a core of helium-to-carbon fusion surrounded by a shell of hydrogen-to-helium fusion, surrounded by a lot of dead weight hydrogen.  This is called a red giant.


A Red Giant Star. Our sun will end its life as one of these.

For stars that started out small, this is where life ends.  They don’t have the heft to get their cores up to the next level of fusion.  But for one of the big boys – a star that started out at least eight or nine times as massive as the sun- this is just the beginning.  A large star goes beyond being a red giant to become a red super-giant star.  It gets its core so hot and pressurized that it can fuse carbon with hydrogen or helium to make nitrogen or oxygen.  Then it fuses carbon with oxygen to make silicon.  Once it has done that, the super-giant is free to make all sorts of different fusions, and it ends up producing all the elements on the periodic table which are lighter than iron.  After that, fusion no longer releases any energy.  Once it reaches iron, the super-giant is finished.  It put up a gallant fight; it hung on to the very end.  But its life is finally over.

A Red Super Giant Star. This diagram is simplified. The interior of a super giant star is chaotic and difficult to predict.

A Red Super Giant Star. This diagram is simplified. The interior of a super giant star is chaotic and difficult to predict.

The death-throes of a red super-giant are awesome to behold.  After years of violent instability, the core collapses into a neutron star or a black hole.  This releases a great deal of gravitational potential energy,* igniting one last round of explosive fusion and sending the rest of the star careening out into space.  The super-giant absolutely obliterates itself in a massive, planet-shattering detonation called a supernova.  This explosion is so powerful that for a few weeks, a star going supernova can outshine an entire galaxy. That means that a single star produces more light than the roughly 200,000,000,000 stars in an average galaxy. In all the chaos of this cosmic cataclysm, unprofitable forms of fusion can occur, creating elements heavier than iron.  The gold in your wedding-ring and the copper wires in your computer were both formed in the death throes of a great cosmic giant.

Supernova N63A in the Large Magellanic Cloud.  This picture was also taken with the Hubble Space Telescope.

Supernova N63A in the Large Magellanic Cloud. This picture was also taken with the Hubble Space Telescope.

From the ashes, new life is born.  The debris spewed into interstellar space by the supernova is chock full of heavy elements.  There is carbon and oxygen, silicon and iron, and even rarities like copper, gold and uranium.  These elements mix into the interstellar gas, enriching it.  And as the aftershocks of the supernova begin to cool, this gas begins to clump.  Some of these clumps collapse into discs, and from these discs new solar systems are born.  From the silicon and iron blasted forth by the supernova, planets form.  And, if one of those planets is just the right distance from its sun, then from the carbon and nitrogen and oxygen fused in the core of the red super-giant, life may form.  Four and a half billion years ago, the new planet was Earth, and our bacterial ancestors were the new life.  Like roses rising from a grave, each and every one of us owes our existence to the death of a super-giant, the greatest of stars.  As Carl Sagan would say, we are all starstuff.


*Gravitational potential energy is the energy you get when matter falls from a high elevation to a low elevation.  It is this energy which does the damage when you drop an anvil on someone’s head.




3 responses

30 07 2009
Sam T

Hey Mike,

This is really cool and interesting! I found the diagrams particularly helpful for understanding the concepts. I have a few questions:

1) What is the time frame we’re talking about in the progression from star to red super giant/super nova? How long does each stage take? Does it just progress through the stages as it burns through its hydrogen?

2) How do the more complex elements (iron, silicon, etc etc) “congeal” to form planets? It seems like there’d just be a lot dust spread out over a lot of space.


31 07 2009

1) The time frame depends on the mass of the star, but generally speaking the later stages are quicker. A star like our sun will spend ten billion years as a normal star, and a few hundred million to a billion years as a red giant. A very massive star has a much shorter lifespan, usually spending from a few tens to a few hundreds of million years in the first stage, with subsequent stages proportionally shorter as well. Each successive stage in a super-giant’s lifetime has less fuel than the one before it (a star has more hydrogen than it has helium, more helium than it has carbon, etc), so each successive stage takes less time than the one before it. The final, iron producing stage of a super-giant’s life will be over in a few thousand years (I think), and the supernova explosion itself lasts mere weeks, although its remnants can continue to glow for a long time (like the Crab Nebula).

2) Basically, gravity is the key to planets “congealing”. When a cloud of gas and dust first begins to form a new solar system, it begins by collapsing gravitationally. As this happens, any initial spin it may have had by chance gets magnified enormously (the same way a figure skater spins faster by pulling in her arms) and this squeezes the cloud out into a disc in orbit around its own center of mass (where the next star forms). Over time the heavy elements, which were in the form of ice and dust grains in the disc, begin to collide with each other. Sometimes they stick together, and once any globs of them achieve a certain mass they begin attracting other globs gravitationally. Over time there are a lot of collisions and gravitational accretion, until finally only a few planets remain.

31 07 2009

Ah, the “pale blue dot.” I’m a fan of Mr. Sagan’s as well. Loved the post!

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