Our Star Chapter 14 Notes

Our Star A Closer Look at the Sun - In the 19th century, astronomers suggested that the Sun generates energy by slowly contracting in size - Gravitational contraction: the process in which gravity causes an object to contract, thereby converting gravitational potential energy into thermal energy - This is known as Kelvin-Helmholtz contraction - A gradual shrinking Sun would always have some gas moving inward, converting gravitational potential energy into thermal energy. - This thermal energy would keep the inside of the Sun hot. - Gravitational equilibrium: a state of balance in which the force of gravity pulling inward is precisely counteracted by pressure pushing outward - Also referred to a hydrostatic equilibrium - Gravitational equilibrium works much the same way in the Sun, except the outward push against gravity comes from internal gas pressure. - The Sun’s internal pressure precisely balances gravity at every point within it, thereby keeping the Sun stable in size. - The weight of overlying layers is greater as we look deeper into the Sun, the pressure must increase with depth. - Deep in the Sun’s core, the pressure makes the gas hot and dense enough to sustain nuclear fusion. - The energy released by fusion, in turn, heats the gas and maintains the pressure that keeps the Sun in balance against the inward pull of gravity. - Energy balance (in a star): the balance between the rate at which fusion releases energy in the star’s core and the rate at which the star’s surface radiates this energy into space - Energy balance is important because without it, the balance between pressure and gravity would not remain steady. - If fusion in the core did not replace the energy radiated from the surface, thereby keeping the total thermal energy content constant, then gravitational contraction would cause the Sun to shrink and force its core temperature to rise. - Energy liberated by fusion has maintained gravitational equilibrium and energy balance within the Sun, keeping it shining steadily. - Sunspots: blotches on the surface of the Sun that appear darker than surrounding regions - The Sun releases an enormous amount of radiative energy into space - Power: the rate of energy usage, usually measured in watts (1 𝑤𝑎𝑡𝑡 = 1 𝑗𝑜𝑢𝑙𝑒/𝑠) - Luminosity: the total power output of an object, usually measured in watts or in units of 26 solar luminosities (𝐿𝑆𝑢𝑛 = 3. 8 × 10 𝑤𝑎𝑡𝑡𝑠) - - Solar wind: a stream of charged particles ejected from the Sun - The solar wind helps shape the magnetospheres of planets and blows back the material that forms the plasma tails of comets Corona (solar): the tenuous uppermost layer of the Sun’s atmosphere; most of the Sun’s x-rays are emitted from this region, in which the temperature is about 1 million K - - Chromosphere: the layer of the Sun’s atmosphere below the corona; most of the Sun’s UV light is emitted from this region, in which the temperature is about 10,000 K Photosphere: the visible surface of the Sun, where the temperature averages just under 6,000 K - The photosphere is where you’ll find sunspots, regions of intense magnetic fields that would cause your compass needle to swing about wildly Convection zone (of a star): a region in which energy is transported outward by convection Radiation zone (of a star): a region of the interior in which energy is transported primarily by radiative diffusion - The temperature rises to almost 10 million K 14.2 - Nuclear Fusion in the Sun - Nuclear fission: the process in which a larger nucleus splits into two (or more) smaller particles - Nuclear fusion: the process in which two (or more) smaller nuclei slam together and make one larger nucleus - Fusion occurs within the Sun because 15 million K plasma in the solar core is like a “soup” of hot gas full of positively charged atomic nuclei (and negatively charged electrons) whizzing about at extremely high speeds. - In most cases, electromagnetic forces deflect the nuclei, preventing collisions, because positive charges repel one another. - If nuclei collide with sufficient energy, they can stick together (fuse) to form a heavier nucleus. - ✩ Nuclear fission splits a nucleus into smaller nuclei, while nuclear fusion combines smaller nuclei into a larger nucleus. - Strong force: one of the four fundamental forces; it is the force that holds atomic nuclei together - The strong force is more like glue or velcro → it overpowers the electromagnetic force over very small distances but it is insignificant when the distances between particles exceed the typical sizes of atomic nuclei - The key to nuclear fusion is pushing the positively charged nuclei close enough together for the strong force to overcome electromagnetic repulsion. - Proton-proton chain: the chain of reactions by which low-mass stars (including the Sun) fuse hydrogen into helium - Neutrino: a type of fundamental particle that has extremely low mass and responds only to the weak force; neutrinos are leptons and come in three types (electron neutrinos, muon neutrinos, and tau neutrinos) - The Solar Thermostat Nuclear fusion is the source of all energy the Sun releases into space - If fusion rate varied, so would the Sun’s energy output, and large variations in the Sun’s luminosity would almost surely be lethal to life on Earth. - Instead, the Sun fuses hydrogen at a steady rate, thanks to a natural feedback process that acts as a thermostat for the Sun’s interior - - - Fusion reaction converts four hydrogen nuclei into one helium nucleus - The total number of independent particles in the solar core therefore gradually decreases with time. - This gradual reduction in the number of particles causes the solar core to shrink - The slow shrinkage then gradually increases the core temperature and fusion rate, keeping the core pressure high enough to counteract the stronger compression of gravity. The Solar thermostat balances the Sun’s fusion rate so that the amount of nuclear energy generated in the core equals the amount of energy radiated from the surface as sunlight Radiative diffusion: the process by which protons gradually migrate from a hot region (ex, the solar core) to a cooler region (ex, the solar surface) Energy released by fusion moves outward through the Sun’s radiation zone primarily by way of these randomly bouncing photons. 14.3 - The Sun-Earth Connection - Solar activity (or solar weather): the short-lived phenomena on the Sun, including the emergence and disappearance of individual sunspots, prominences, and flares - Sunspots appear dark in photographs only because they are less bright than the surrounding photosphere - They are less bright because they are cooler: the temperature of the plasma in sunspots is about 4,000 K, which is significantly cooler than the 5,800 K plasma that surrounds them - Magnetic field lines: lines that represent how the needles on a series of compasses would point if they were laid out in a magnetic field - Solar magnetic field lines act somewhat like elastic bands, twisted into contortions and knots by turbulent motions in the solar atmosphere. - Sunspots occur where tightly wound magnetic fields poke nearly straight out from the solar interior - Solar prominences: vaulted loops of hot gas that rise above the Sun’s surface and follow magnetic field lines - Some prominences rise to heights of more than 100,000 kilometers above the Sun’s surface - Individual prominences can last for days or even weeks - Solar flares: huge and sudden releases of energy on the solar surface, probably caused when energy stored in magnetic fields is suddenly released - Coronal holes: regions of the corona that barely show up in x-ray images because they are nearly devoid of hot coronal gas - Coronal mass ejections: bursts of charged particles from the Sun’s corona that travel outward into space → particles that travel outward from the Sun in huge bubbles - The bubbles have strong magnetic fields and can reach Earth in a couple of days if they happen to be aimed in our direction - Once a coronal mass ejection reaches Earth, it can create a geomagnetic storm in Earth’s magnetosphere - Sunspot cycle: the period of about 11 years over which the number of sunspots on the Sun rises and falls - At the time of solar maximum, when sunspots are most numerous, dozens of sunspots can be seen at once - At the time of solar minimum, when sunspots are fewer, very few sunspots can be seen - The frequency of prominences, flares, and coronal mass ejections also follow the sunspot cycle, with these events being most common at solar maximum and least common at solar minimum