Key Concepts: Lecture 22: The Sun Basic properties of the Sun The outer layers of the Sun: Chromosphere, Corona Sun spots and solar activity: impact on the Earth Nuclear Fusion: the source of the Sun s energy The Surface of the Sun Visible surface is called photosphere Temperature of visible surface is about 6000 K Granular structure - gas motion (convection) carries energy to surface Dark cool regions are called Sun Spots Structure of the Sun (and other stars) The Sun The Sun is huge compared to the Earth 110 times bigger in diameter 300,000 times as massive Slightly denser than water, 1400 kg/m3 - mostly H & He Contains 99.9% of the mass in the solar system Emits 3.9x1026 watts of power Large flux at Earth. e.g. an area equivalent to Florida absorbs ~5x world energy usage G 2 Main Sequence Star The Chromosphere of the Sun Outside photosphere Cooler than photosphere hydrogen atoms exist here Causes absorption lines in the spectrum of the Sun (recall Kirchoff s 3rd law)
Sun Spots Solar Activity Sun spots have an 11 year cycle Number of spots changes Position of spots changes Two sun spot cycles make up a solar cycle of 22 years (the magnetic field reverses twice to return to its original orientation). Solar Activity Sunspots are regions where the solar magnetic field breaks through surface Cycle due to twisting of magnetic field Equator rotates faster than pole Fields twist up like rubber bands When fields cross they connect Field flips every 11 years Sun Spots and the Earth The amount of energy the Sun releases is not perfectly constant The number of Sun spots may be related to the amount of energy the Sun emits More spots = more energy 1% more UV at solar maximum
The Maunder Minimum From 1650-1700 almost no Sun spots occurred Known as Little Ice Age in Europe Thames river in London froze 11 times Ice seen off the coast of England Another similar minimum may have occurred in the 1400s The Corona of the Sun Corona extends far out Visible only during eclipses Very tenuous Much hotter 500,000 to 2,000,000 K Emits mostly in X-rays How does it get so hot? Puzzle - Heat only flows from hot to cold Clue - Corona is hottest near sun spots Corona is fed by sporadic flares and prominences from the Sun s surface, which can lead to coronal mass ejections The Corona is the source of the Solar Wind. Strong ejection events in the wind can affect communications on Earth. Where does the Sun get its Energy? Skating in the Town Moat of Brussels - Robert van den Hoecke (1649 during the Maunder Minimum) Lifetime for the Sun to shine is related to source of energy: lifetime = (energy available) / (luminosity) Gravitational Contraction? Contraction due to gravity releases energy Total energy due to contraction 10 42 Joules (from Newton s Laws) lifetime = (10 42 Joules)/(4x10 26 Joules/sec) = (1/4)x10 16 sec ~ 10 8 years (short compared to age of solar system) So gravitational contraction cannot be supplying Sun s energy. Radioactive decay, similar to heating of Earth? Sun is mostly H and He, which do not decay. Need heavy nuclei Even if solid uranium it would only produce 1/2 of energy seen. So not enough radioactivity to supply Sun s power.
Nuclear Fusion Several light nuclei (H) collide and combine to form a heavier nucleus (He) Energy is released Need high temperatures and densities Must overcome electrostatic repulsion of positively charged atomic nuclei Core of Sun is 15,000,000 K Density is 150,000 kg/m 3 Where Does the Energy Come From? Mass is converted directly into energy Einstein s famous relationship Mass and energy are the same thing! The speed of light, c, is very big so you get a lot of energy from a little bit of matter 4 H 1 He Mass of H = 1.67252x10-27 kg Mass of He = 6.64258x10-27 kg 4xH - He = 0.04750x10-27 kg E = mc 2 = 4.75x10-29 kg (3x10 8 m/sec) 2 = 4.3x10-12 Joules [1 watt = 1 Joule/sec] E=mc 2 1st step towards He How much mass is destroyed each second to keep the Sun shining with its luminosity of 4x10 26 Watts? Nuclear Fusion: The Proton-Proton Chain Energy From Fusion Overall, fusion in stars involves destruction of about 1% of the total mass, but this is still enough to make fusion one of the most efficient means of energy generation (much more energy can be released compared to chemical reactions or radioactive decay) If 10% of the Sun s Hydrogen is converted into Helium, the Sun will shine at its current rate for 10 Billion Years = 10 x 10 9 yr Note this process also produces neutrinos.
Question Remember that the luminosity of a main sequence star was found be to related to its mass (L M4). Now we know that stars get their energy by converting their mass directly into energy so the total amount of energy a star has is proportional to its mass (E M). Journey to the Stars, part 1 Will a massive star live a longer or shorter time than a low mass star? Answer given and described in class lecture. The Lifetime of Stars on the Main Sequence tlifetime depends on amount of fuel (M), divided by rate of using fuel (L M4) Therefore tlifetime M / (M4) M-3 So how long will a star that is twice the mass of the Sun be able to shine on the main sequence? And a star 10x the mass of the Sun? Or a star 0.1x the mass of the Sun? Solar Neutrinos Neutrinos Very low-mass particles produced as a side product of nuclear fusion. They hardly interact with matter so they can travel completely out of the Sun undisturbed. Millions pass through you every second! Detection of Neutrinos Difficult since they interact so weakly with matter. Takes very large detectors. Several have been built to detect different types of neutrinos from inside the Sun. >Confirms fusion is occurring in center of Sun
The Structure of the Sun SAME BASIC IDEAS ALSO APPLY TO OTHER STARS Stars arrange themselves to balance the forces within them Gravity tries to pull all matter together Pressure of the gas prevents collapse Pressure of escaping radiation also helps to prevent collapse If not enough pressure, star will contract until pressure increases to balance gravity If too much luminosity and pressure, star will expand until pressure drops Interior Structure of the Sun Pressure increases as you move deeper into the Sun Must hold up all the overlaying material Density and temperature increase Only in core is it hot and dense enough for nuclear fusion. Fusion needs very hot conditions because the protons need to move fast enough to overcome their electrostatic repulsion. All energy is generated in core Gas Pressure The pressure of a gas is due to motion of gas atoms Higher temperature - faster speeds - higher pressure Higher density - more atoms - higher pressure Compressing a gas increases pressure Heating a gas increases pressure Energy Escaping From the Sun Energy always flows from hot regions to cold regions. So heat flows from the core to the outer layers and then into space Two major ways to transport energy out Photons (Radiation Transport) Deep interior relatively transparent to light so photons can carry the energy out Currents of hot gas (Convection) Outer parts are opaque to light so gas motions must move the energy out
Random Walking Even in the relatively transparent interior of the Sun a photon only travels about 1 cm before it is absorbed A photon is absorbed and re-emitted billions of times before it wanders out of the Sun Re-emitted in random directions each time It takes about 1 million years for a photon to work its way out The Main Sequence Revisited Like the Sun, all stars arrange themselves to balance the force of gravity and their interior pressure As mass increases gravitational forces increase Pressure increases inside the star Stars generate more energy and are more luminous Stars are hotter and larger to let the energy out This equilibrium sequence of mass is the Main Sequence Question In the Star Trek movie Generations, The evil Dr. Zorron launches a missile into a star. The missile stops fusion inside the star so it blows up during a life and death battle between Zorron, Kirk and Picard. How long would they really need to be fighting before they would notice fusion had stopped in the star?