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Small Objects in the Solar System. The Sun. Asteroids and Comets Our Own Star. Lecture 35. Chapter 17.4 17.7. Remnants of the Solar Nebula. Small bodies remain virtually unchanged since their formation 4.5 billion years ago
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Small Objects in the Solar System. The Sun. Asteroids and Comets Our Own Star Lecture 35 Chapter 17.4 17.7
Remnants of the Solar Nebula Small bodies remain virtually unchanged since their formation 4.5 billion years ago They carry history of the Solar system in their compositions, locations, and numbers. Asteroid means starlike, a rocky leftover Comet means hair (Greek), an icy leftover Meteor – a thing in the air (falling star) Meteorite – any piece of rock than fell to the ground from the sky
Asteroids Undetectable to the naked eye (a.k.a. minor planets) The first asteroid Ceres was discovered by Piazzi in 1800 in an attempt to find the missing planet between Mars and Jupiter Ceres has a diameter of ~500 km (~ half of Pluto) There are more than 100,000 asteroids larger than 1 km in diameter The asteroid belt lies between 2.2 and 3.3 A.U.
Origin and evolution of the Belt The most likely reason for the belt existence is orbital resonances A resonance occurs if an object’s orbital period is a simple ratio of another’s object period (1/2, 1/4,etc.) Asteroids with resonance periods will be pushed out of their orbits by large planets (e.g., Jupiter)
Meteorites Meteorites are pieces of rock falling from the sky. Seen as fireballs (sometimes with sound) Primitive meteorites composed of a random mix of flakes from the solar nebula, contain pure metals, which are bound in minerals on Earth. Processed meteorites – parts of a larger object. Some resemble the Earth’s core, others are similar to the Earth’s crust and mantle.
Comets Comets are basically dirty snowballs where ice mixes with rocky dust. Their mean size is a few kilometers across. Comets change appearance when they approach the Sun. The comet body is called nucleus. Sublimating ices create coma. A tail pointing away from the Sun appears. There are two tails: plasma tail and dust tail.
Pluto Pluto was discovered in 1930 by Claude Tombaugh. Its radius is 1,195 km and mass is 0.0025 Earth mass. Its orbital period is 248 years. It was closer to the Sun than Neptune in 1979 – 1999. At aphelion it is 50 AU from the Sun. Its moon Charon was discovered in 1978.
Summary of Small Objects Asteroids and comets are the best evidence of how the solar system formed The small bodies are significantly affected by planets gravity. Impacts with comets and meteorites are spectacular events, but may even alter life. Pluto has more similarities with the Kuiper belt comets.
Why Does the Sun Shine? Ancient view: a hot, glowing rock of the size of Massachusetts Early XIX century: cooling or chemical reactions Enough energy for a few thousand years Late XIX century: gravitational contraction Enough energy for ~25 million years XX century: nuclear fusion Can shine for ~10 billion years
Current State of the Sun The Sun’s size is stable, maintained by a balance between the force of gravity and gas pressure. This balance is called hydrostatic or gravitational equilibrium. Gravitational equilibrium implies that the pressure increases with depth. This makes the Sun extremely hot and depth in its core.
Big Picture of the Sun The Sun contains 70% hydrogen, 28% helium, and 2% heavier elements. The total power output (luminosity) is 3.8 1026 W. 1Watt = 1 joule/second The radius is 700,000 km. The mass is 2 1030 kilograms (300,000 times more massive than the Earth). The surface temperature is 5,800 K
Nuclear Fusion Nuclear fusion is the process of combining nuclei to make a nucleus with a greater number of protons and neutrons. Particles in a nucleus are held together by the strong force. This is the only force that can overcome the electromagnetic repulsion between two positively charged nuclei.
Hydrogen Fusion in the Sun The proton-proton chain
Solar Neutrino Neutrino is a subatomic particle. It is a by-product of the solar proton-proton cycle. It barely interacts with anything. Counts of neutrino coming from the Sun are crucial to test our knowledge about solar physics. Neutrino observatories use huge amounts of different substances to detect nuclear reactions with neutrino. So far theory predicts more neutrino than is seen.
How does the Light Comes Out? Photons are created in the nuclear fusion cycle. They collide with other charged particles and change their direction (random walk). They also decrease their energy while walking. It takes ~10 million year to get outside. The random bouncing occurs in the radiation zone (from the core to ~70% of the Sun’s radius). At T<2 million K, the convection zone carries photons further towards the surface.
Sunspots and Other Solar Activity Sunspots have T~4,000 K, cooler than the 5,800 K surrounding plasma. Sunspots are kept together by strong magnetic fields. Usually sunspot appear in pairs connected by a loop of magnetic field lines. The loops rising into the chromosphere or corona may appear as solar prominences. Solar flares are events releasing a lot of energy where magnetic field lines break.
The Sunspot Cycle Observations of the Sun since the beginning of the telescopic era revealed that the number of sunspots gradually rises and declines. An average period is 11 years (from 7 to 15 years). The magnetic fields in sunspots reverse their direction when a cycle is over. No sunspots were observed in 16451715, when a Little Ice Age took place in Europe and America.
Summary of the Sun The Sun shines with energy generated by fusion of hydrogen into helium in its core. Gravitational equilibrium determines the Sun’s interior structure and maintains a steady nuclear burning rate. The Sun is the only star near enough to study it in great detail.
