The Chemistry of the Solar System CHAPTER 21 The Chemistry of the Solar System 21.1 The Sun and the Stars

Is the sun a giant ball of fire? What is fire? Is the sun a giant ball of fire?

Is the sun a giant ball of fire? What is fire? On Earth, fire is a chemical reaction (combustion in the presence of oxygen) The Sun is something completely different Is the sun a giant ball of fire?

More than meets the eye What “dark matter” is, is still a mystery We can estimate the amount of matter that explains the apparent strength of gravity, but we only “see” a small fraction of the matter. What “dark matter” is, is still a mystery

Elemental composition of the visible universe 75% hydrogen 23% helium 1% oxygen Atomic # Element ppm 1 Hydrogen 750,000 2 Helium 230,000 8 Oxygen 10,000 6 Carbon 5,000 10 Neon 1,300 26 Iron 1,100 7 Nitrogen 1,000 14 Silicon 700 12 Magnesium 600 16 Sulfur 500 0.5% carbon

The chemical makeup of the Sun changes over time, as hydrogen is converted to helium by nuclear fusion reactions in its core

The Sun’s interior Most elements on Earth are combined into compounds. In the Sun, no compounds exist at all! Thermal energy is so high that chemical bonds cannot form. Sun’s core Sun’s surface

The Sun’s interior Sun’s core Sun’s surface Above the convection zone is the photosphere, which is what we see and call the Sun’s “surface”

Inside the Sun, the temperature is so high that atoms become ionized and exist in a state called plasma plasma: a hot, energetic phase of matter in which the atoms are broken apart into positive ions and negative electrons that move independently of each other.

99% of the visible universe is plasma The remaining 1% that is not plasma includes all the planets, ourselves, and the matter between the stars that might one day become planets

Energy production The production of He from H occurs in a sequence called the proton-proton chain. Each step involves 2 particles: Step 1 a neutrino, like an electron with no charge the positron immediately annihilates with an electron to form gamma rays

Energy production The production of He from H occurs in a sequence called the proton-proton chain. Each step involves 2 particles: Step 1 Step 2

Energy production The production of He from H occurs in a sequence called the proton-proton chain. Each step involves 2 particles: Step 1 Step 2 Step 3 3 possible reactions

Energy production Step 3: Three possible reactions The helium-4 nucleus can get involved in additional reactions

Energy production Step 3: Three possible reactions

Energy production Step 3: Three possible reactions

The Big Bang theory Our galaxy, the Milky Way, contains ~200 billion stars. Outside the Milky Way are billions more galaxies. There is evidence that the universe is expanding, so galaxies are moving farther away from each other. Thermal energy remains constant, so the universe is also cooling down. There is a large body of evidence supporting the Big Bang theory: 13.7 billion years ago all matter and energy was concentrated in one point. The “Big Bang” refers to the extraordinary explosion from that infinitesimal point.

The Big Bang theory Beginning of the universe Initially Temperature too high; protons and neutrons could not form nuclei At 1 min Temperature had cooled enough; nuclei began to form At 3 min Temperature continued to drop; any free neutrons now bound to protons; already formed nuclei fused together After 3 min The elemental composition of the universe was “frozen” in place Calculations predict that hot nuclear plasma “freezing out” would give 75% hydrogen and 25% helium, with tiny amounts of lithium and beryllium – almost exactly what is observed in the visible universe.

The Big Bang theory

Synthesis of carbon The Sun has been converting hydrogen to helium for the past 4.5 billion years, and continue to do so for the next 4.5 billion years. As the Sun runs out of hydrogen, the process will slow down, and the Sun’s core will get hotter and denser, and a new series of reactions will take place: Heavier nuclei will be created

With heavier nuclei being formed by fusion, the core becomes hotter and more compressed. When it gets to the point of fusing iron (the most stable nucleus), the Sun will become unstable. Larger stars can explode as supernovas.

Interstellar medium - By mass: 99% gas, 1% dust The interstellar medium is the space in between stars and planets. - By mass: 99% gas, 1% dust - The elemental composition changes over time. When a star reaches the iron-burning phase, it explodes and the interstellar medium is enriched with heavier elements. Today: 89% H, 9% He, 2% heavier elements

Interstellar medium The interstellar medium is the space in between stars and planets. Slow, random motion sweeps interstellar matter into vast, cold molecular clouds. Molecular clouds are believed to be the birthplace of stars and planets. A cluster of hot, young stars formed from a molecular cloud in the giant nebula NGC 3603

In the Sun - No compounds exist at all - The state of matter is called plasma (atoms are split apart) - Fusion reactions convert hydrogen to helium - Heavier elements will be formed until the Sun becomes unstable. Exploding stars can form supernovas. Sun’s core Sun’s surface

In the Sun - No compounds exist at all - The state of matter is called plasma (atoms are split apart) - Fusion reactions convert hydrogen to helium - Heavier elements will be formed until the Sun becomes unstable. Exploding stars form supernovas. The Big Bang Theory - 17.3 billion years ago all matter and energy in the universe was in one indefinite point - Minutes after the “Big Bang” explosion, the elemental composition was “frozen” in place - Since then, the universe has been expanding - The Milky Way is the name of our galaxy