| What is there more in space between stars, gas or dust? Solution: 99% of the interstellar medium is gas; only 1% is dust. What do the following notations represent: Li I, C II, O III? Solution: Li I is neutral (non-ionized) lithium. C II is ionized carbon (with one electron missing). O III is ionized oxygen (with two electrons missing). Note that this notation is only used in astronomy; in other sciences, you may have seen these atoms written as Li, C+, O++. Is H III a valid atom? Solution: No. H III would mean hydrogen with two electrons missing. But hydrogen only has 1 proton, so neutral hydrogen (H I) has 1 electron and ionized hydrogen (H II) has 0 electrons. H III would have -1 electrons, which is impossible. Why do H II regions glow? Solution: They are composed of ionized atoms, which are missing an electron. When they recapture electrons, and the electrons drop down to lower energy levels, they emit photons. Why are H II regions rare? Solution: Because they only exist when interstellar matter is close enough to a very hot star, and this doesn't happen often. How is the 21 cm line produced? Solution: When an electron undergoes a spin flip transition. In this process, the electron does not change energy levels, it just flips the direction of its spin. The electron's direction of spin was originally parallel to the spin of the proton, but after the spin flips it is now in a direction opposite to the spin of the proton. This configuration of spins has a slightly lower energy, so a photon with a tiny amount of energy is released in the process in order to conserve the total amount of energy. This photon has a wavelength of 21 cm. What is the difference between emission, absorption, and reflection nebulae? Solution: Emission nebulae emit light, so they glow and we can see them from Earth. Absorption nebulae do not emit light, they absorb it, so they block light from objects behind them. We can see them because they look like dark patches in the sky (similar to how a dark object placed in front of a flashlight can be detected because it blocks some of the light). Reflection nebulae don't emit their own light, but they reflect light from nearby stars, so we can see them due to this reflection. What is interstellar extinction? Solution: The process where light emitted by stars is absorbed or scattered by dust clouds located between us and the stars, making the stars look dimmer. Are cosmic rays the same as light rays? Solution: No, light rays are made of photons while cosmic "rays" are actually a form of matter. The word "rays" is only used for historic reasons, since people originally thought they were just energetic light rays. In the Milky Way, what kind of interstellar matter occupies the smallest volume? Solution: H II regions. What is a baryon? Solution: A particle made of an odd number of quarks. For example, a proton is made of 2 up quarks and 1 down quark, for a total of 3 quarks. 3 is an odd number, so a proton is a baryon. What are the Local Bubble and Local Fluff? Solution: The Local Bubble is a "cavity" in the interstellar medium, which means it has a lower density and higher temperature than the interstellar matter outside it. The bubble is a few hundred light-years in size, and contains our solar system as well as some other stars and star clusters. The Local Fluff (or Local Interstellar Cloud) is a cloud located inside the Local Bubble which is defined by its higher density, and contains our solar system (at least for a few more thousand years) and only a few other stars. What are the steps of star formation? Solution: A clump forms inside a molecular cloud, and a dense core forms inside the clump. At first, there is equilibrium between gravity (which wants to compress the core) and pressure (which wants to expand the core). However, as the core acquires more mass, it also has more gravity, so eventually gravity overcomes pressure and the core collapses under its own gravity and forms a protostar and a disk. The protostar continues to acquire more mass from its surroundings until the gas and dust are dispersed by stellar wind. Finally, hydrogen fusion begins and the star transitions to the main sequence. What is a T Tauri star? Solution: A protostar with mass less than or similar to that of the Sun, which is still in the process of contracting to form a main-sequence star, and has almost reached its final mass. What are jets, and how are they related to HH objects? Solution: Jets are beams of particles shooting out of both poles of a protostar. If these jets collide with a region of gas, they excite the atoms in the gas and cause them to glow. These glowing regions are known as Herbig-Haro (HH) objects. How does a protostar's mass affect the location, duration, and shape of its path to the main sequence in the H-R diagram? Solution: By looking at the evolutionary tracks of protostars on the H-R diagram, we see that if a star is more massive, it will be located higher in the H-R diagram (since it will have higher luminosity), have a less steep initial drop in luminosity, and have a longer path (since it will go through a wider range of temperatures), but the time spent in this path will be shorter (since it will burn fuel faster). How hot does a star's core need to be in order to fuse hydrogen? What happens if a protostar fails to reach this temperature? Solution: Around 12 million kelvins. If the protostar does not reach this temperature, it will become a brown dwarf. Explain what it means for the star to be in equilibrium, and how this affects the star throughout its life, in particular with relation to the main sequence stage and the transition to the red giant stage. Solution: There are two competing forces fighting for dominance in every star throughout its lifetime. Gravity is pushing inwards and trying to make the star collapse and shrink, while pressure is pushing outwards and trying to make the star grow and expand. When the star is in equilibrium, such as during the main sequence stage, its size doesn't change. However, at certain times during a star's lifetime, it can go out of equilibrium. For example, when the star reaches the end of the main sequence stage, it can no longer fuse hydrogen in its core. This reduces the pressure, so gravity wins and the star begins to contract. However, later the star will begin fusing hydrogen in a shell around the core, so now the pressure wins and the star begins to expand and become a red giant or supergiant. What happens when a star can no longer fuse hydrogen in its core? Solution: See previous answer. The star leaves the main sequence. The core gets compressed, since it can no longer hold against its own gravity. This causes it to heat up, which then causes hydrogen fusion in higher layer of the star, outside the core. What are red supergiants, and where do they come from? Solution: Red supergiants reside at the top right of the H-R diagram, above and to the right of the main sequence. This means they are more luminous, and colder, than main sequence stars. Red supergiants are a later stage in the evolution of main-sequence stars that are significantly more massive than the Sun. How does helium fusion work? Solution: Through the triple-alpha process. Two helium-4 nuclei fuse into one beryllium-8 nucleus, which then fuses with another helium-4 nucleus to produce a carbon-12 nucleus. In this process we fused a total of 6 protons and 6 neutrons from 3 different helium-4 nuclei into one carbon-12 nucleus. A photon was emitted in each stage of the process (fusion to beryllium and further fusion to carbon) to account for the lost mass in each stage. The carbon-12 can later fuse with another helium-4 to form oxygen-16. What is a helium flash? Does it happen in every star? Solution: The helium flash is a sort of feedback loop that causes the entire helium core of a star to begin helium fusing almost simultaneously. It happens in low-mass stars, but not in massive stars. In which stage of its life will a star with mass similar to the Sun have the hottest temperature? What about highest luminosity? Solution: The hottest temperature will be at the main sequence stage. However, the highest luminosity will be at the second red giant stage (after helium fusion). What is a planetary nebula? Under what circumstances is it created? What makes it glow? Solution: A planetary nebula is material ejected from a very hot dying star, which is then ionized by the strong stellar winds and UV radiation from that star, causing it to glow (similar to how H II regions glow). It is created when a low-mass star dies, but not when a massive star dies. What kind of stars can fuse elements heavier than helium? Solution: Only a massive star. A low-mass star cannot get hot enough to ignite fusion in its caron-oxygen core. What is stellar nucleosynthesis? Solution: The process in which atomic nuclei are created in stars by nuclear fusion. Nuclei from helium up to iron are created by fusion in the cores and shells of stars. The more massive the star is, the heavier the nuclei it will create. |