By Bill Pellerin

Houston Astronomical Society

GuideStar Editor

Last month, I wrote an article about the way we categorize nebulae (clouds in the sky). There are emission, reflection, and nebulae, which tells you how they are visible to us. The other way to categorize nebulae is as pre-stellar (star forming) and post-stellar (a remnant of a burned out star). I recently viewed the Wonders of the Universe video series by Brian Cox (highly recommended) and one of his subjects is the lifecycle of the universe and how objects in space fit into that story. That is, if we look at how the universe began, how it got to where it is today, and the future of the universe we get another perspective on nebulae.

So, let’s go all the way back to the beginning of the universe, the Big Bang. Various light elements came into being as the result of the Big Bang, but the heavier elements did not. The Big Bang, while extremely hot was also of short duration. In other words, there was not enough time for heavier elements to form as a result of the Big Bang. This formation of elements as a result of the Big Bang is called ‘Big Bang nucleosynthesis’.

What the universe began with, and what were the building blocks of the future universe, were (75%) hydrogen, (25%) helium, and small amounts of lithium and beryllium. That’s it. No carbon, oxygen, or anything else existed yet existed. The universe had to wait for these elements to be formed.

By Bill Pellerin

Houston Astronomical Society

GuideStar Editor

Last month, I wrote an article about the way we categorize nebulae (clouds in the sky). There are emission, reflection, and nebulae, which tells you how they are visible to us. The other way to categorize nebulae is as pre-stellar (star forming) and post-stellar (a remnant of a burned out star). I recently viewed the Wonders of the Universe video series by Brian Cox (highly recommended) and one of his subjects is the lifecycle of the universe and how objects in space fit into that story. That is, if we look at how the universe began, how it got to where it is today, and the future of the universe we get another perspective on nebulae.

So, let’s go all the way back to the beginning of the universe, the Big Bang. Various light elements came into being as the result of the Big Bang, but the heavier elements did not. The Big Bang, while extremely hot was also of short duration. In other words, there was not enough time for heavier elements to form as a result of the Big Bang. This formation of elements as a result of the Big Bang is called ‘Big Bang nucleosynthesis’.

What the universe began with, and what were the building blocks of the future universe, were (75%) hydrogen, (25%) helium, and small amounts of lithium and beryllium. That’s it. No carbon, oxygen, or anything else existed yet existed. The universe had to wait for these elements to be formed.

So, how do we get to the present day? The chemical composition of the universe hasn’t changed much, but it has changed enough to allow life forms, like us, to exist. The elements in the universe today are (74%) hydrogen, (24%) helium, (1%) oxygen, (.5%) carbon, (.1%) neon, (.1%) iron, (.1%) nitrogen, (.06%) silicon, (.06%) magnesium, and smaller fractions of other elements.

Where did these heavier elements come from? Stars. They came from stars, and stars were formed in the star-forming nebulae that I called ‘pre-stellar nebulae’ in last month’s article. Today’s pre-stellar material is mostly the same as the pre-stellar material that existed soon after the Big Bang, but the important difference exists in the 2% of material that is not hydrogen or helium. This is the material that allows the formation of solar systems, planets, asteroids, comets, and most significantly you, me, and the other life forms we see on earth. There’s a bit of a division of labor when creating heavy elements between low mass stars and high mass stars.

Low mass stars (less than 8 solar masses) create helium from hydrogen fusion, but more significantly they create carbon and oxygen from helium fusion. Low mass stars are not hot enough to fuse carbon and oxygen, so their job is done. They have seeded the universe with some of the important heavier elements and end their lives as planetary nebulae with white dwarfs. The white dwarf stars cool to become black dwarf stars and disappear from view forever. So, when you look at a planetary nebula, you’re looking at the elements of the star that have been cast off into the void to become raw material for new stars, new planets, and new life forms. We earthlings are late to the party because the elements that are needed to make us did not exist in the very early universe. The universe is 13.7 billion years old, and the solar system is about 5 billion years old. This means that the universe had 8 billion years to leisurely create the chemical elements that were required to make us.

The other part of the story involves high mass stars. A high mass star can fuse elements heavier than helium, and over time these stars create all the elements up to and including iron. Iron is the end of the road because it cannot be fused to create stellar energy. Where do elements heavier than iron come from?

The end-of-life event for high mass stars (greater than 8 solar masses) is called a supernova. The supernova event creates the elements beyond iron in the periodic table.

When our Sun, a low-mass star, dies, it will provide the products of its fusion process to the universe and blow-off the raw materials for the next generation of stars and the next generation of life. Whether these materials will be used is a story yet to be told.