Astronomy 201 Review 1 Answers What is temperature? What happens to the temperature of a box of gas if you compress it? What happens to the temperature of the gas if you open the box and let the gas expand? Describe some astronomical examples where we see these things happening. Temperature is a measure of vibrational energy. If you compress a gas, the temperature will increase. Conversely, allowing a gas to expand will cause the gas to cool. Stars form from the gravitational contraction of cool gas clouds. While the gas contracts it heats up and when it heats up to the point where it can begin fusing H to He, a star has formed. The universe as a whole is expanding and therefore cooling. If you trace the universe back, it was much hotter and denser early on and the expansion has caused it to cool to what we see today. Explain why the universe consists mostly of hydrogen and helium, the two simplest elements. Where do the more complex elements come from? In the very early Universe the temperature was so great that all matter was fully ionized and dissociated. Roughly three minutes after the Big Bang itself, the temperature of the Universe rapidly cooled (due to expansion of the universe) from 10 32 K to approximately 10 9 K. At this temperature, nucleosynthesis, or the production of light elements, could take place. In a short time interval, protons and neutrons collided to produce deuterium (one proton bound to one neutron). (Most of the deuterium then collided with other protons and neutrons to produce helium and a small amount of tritium (one proton and two neutrons). Lithium 7 could also form from the coalescence of one tritium and two deuterium nuclei). However, the universe was still expanding while this was taking place and cooled to the point where no further fusion was possible, thereby locking the preliminary mass fractions of the different elements and leaving mostly H and He. The heavier elements are produced through nuclear fusion in stars. The more massive the star, the higher the gravity which means the more it can contract and therefore the higher the temperature, which allows fusion of more complex elements to occur. Once a star has a core of iron, no further fusion can take place because iron is the most stable element. Every element heavier than iron was formed in supernovae explosions. What conditions must be satisfied in order for us to see a solar or lunar eclipse? What phase must the moon be in for a solar eclipse? What phase for a lunar eclipse? Why is a lunar eclipse characterized by a reddish tint? Why don t we see eclipses every month? What is the umbra? What is the penumbra? In order for us to see an eclipse, the line of nodes must be aligned. This means that the sun, moon, and earth are aligned and lay in the same plane. A solar eclipse occurs when the moon is at the new moon phase and is between the sun and the earth. A lunar eclipse occurs when the Earth is between the sun and the moon, which means the moon is full. The reddish tint associated with lunar eclipses is due to the bending of light from the sun by the Earth. Since red light has the longest wavelength (compared to light in the optical region) it is not scattered as much as the other colors and so the red light can pass through the atmosphere while the shorter
wavelengths are scattered within the Earth s atmosphere. Since the moon s orbit is inclined at approximately five degrees, we do not see eclipses every month. The umbra is a central cone of darkness which tapers away from the Earth or moon, whilst the penumbra is an outer cone of partial shadow which diverges instead of tapering. What is the zenith? What is a meridian? What is the celestial equator? What is the ecliptic and why is it tilted relative to the celestial equator? What are the north and south celestial poles? What are circumpolar stars? The zenith is the point directly overhead. The meridian is the line that passes through the north celestial pole, through your zenith, and through the south celestial pole. The celestial equator is the projection of Earth s equator onto the sky and lies halfway between the north and south celestial poles. The ecliptic is the intersection of the plane of Earth s orbit and the celestial equator. It is tilted because of the tilt of the Earth. The sun appears to travel along the ecliptic about one degree per day. The north and south celestial poles correspond to the rotation axes of the earth. Circumpolar stars are stars that do not rise or set. If you at either of the celestial poles, you will see a lot of circumpolar stars. What direction, relative to the horizon, does a planet move in one night? What direction do planets move relative to the stars in direct motion? What direction, relative to the stars, does a planet in retrograde motion move? Relative to the horizon, a planet moves East to West over the course of one night. This motion is attributed to the diurnal motion due to Earth s rotation. Relative to the stars, a planet moves West to East in direct motion and East to West in retrograde motion. Why do we have seasons? Why is it warmer in the summer than in the winter? Seasons are due to the Earth s tilt which causes the northern hemisphere to be receiving more direct sunlight while the southern hemisphere is tilted away and receiving less direct sunlight, and vice versa. In the summer, we have more daylight and the sun can reach a higher point in the daytime sky, which causes the sun s rays to strike more perpendicularly (solar heating). What is an equinox? What is the vernal equinox? What is the autumnal equinox? An equinox occurs when the sun crosses the celestial equator. The two points of equinox are thus where the ecliptic intersects the celestial equator. The vernal equinox occurs when the sun is moving from South to North across the celestial equator which means summer is coming. The autumnal equinox occurs when the sun is crossing from North to South across the celestial equator, signaling the coming of fall and winter. During an equinox the length of the day equals the length of the night. What is precession? What causes precession? What is the tropical year and how is it related to precession?
The Earth s axis rotates and this is known as precession. The axis makes one rotation approximately every 26,000 years. Precession is caused by the gravitational pull on the Earth by the sun and moon. The tropical year is the time it takes the sun to go from one vernal equinox to the next, or from one autumnal equinox to the next. The tropical year changes with precession because the equinox changes with precession due to the projection of the Earth s equator (celestial equator) changing with precession. Why does the moon have phases? What are the moon s phases? Why does Venus have phases? The moon s phases are due to the observed angle of the moon from the Earth. One side of the moon is always fully illuminated, but the angle we see the moon changes over the course of a month. The moon s phases are: new, waxing crescent, first quarter, waxing gibbous, full, waning gibbous, last quarter, waning crescent, new. Venus has phases because the viewing angle changes relative to us. It is full when it is opposite the sun and new when it is between the sun and us. What are the inferior planets? What are the superior planets? What general path do the planets follow in the sky? The inferior planets are those that lay within Earth s orbit. The superior planets lay outside Earth s orbit. The planets are basically following the ecliptic as they move across the celestial sphere. Explain all three of Kepler s laws. 1. The planets orbit the sun in ellipses with the sun at one focus. 2. A line drawn from a planet to the sun sweeps out equal areas in equal time intervals (conservation of angular momentum). 3. The square of the sidereal period equals the cube of the semi major axis. Explain the Doppler Effect. The Doppler Effect is the perceived shift in frequency of a wave to the relative motion of the observer and the source. If a source is moving away, you will see fewer peaks per unit time (redshift), but if a source is moving toward you, you will see more peaks per unit time (blueshift). What is inertia? What is gravity? What does it mean for angular momentum to be conserved for an object in orbit? Inertia is the tendency of a mass to resist a change in motion. Gravity is the mutual attraction between masses. Conservation of angular momentum for an orbiting object is what Kepler s second law states. What are tidal bulges? Why do we have tides? What are spring tides? What are neap tides?
Tidal bulges are caused by a stronger gravitational pull on the side of Earth closer to the moon or sun. The water on the side of Earth feeling a greater pull will be pulled up causing tides. Spring tides occur when the sun and moon are on opposite sides of the Earth creating high tides on both sides, whereas neap tides occur when the sun and moon are ninety degrees apart causing weaker tides. What is light? What are photons? What is wavelength? What is frequency? How are wavelength and frequency related? How is frequency related to energy? Light has properties of both a wave and a particle. As a wave, light is an electromagnetic wave (perpendicular electric and magnetic field components), but as a particle, light is comprised of photons, or discreet packets of light energy. Wavelength is the distance between two crests in a wave, while the frequency is the number of crests that pass a given point in a given time. Wavelength and frequency are related by the equation governing the speed of light: the speed of light equals the frequency times wavelength. The energy of a photon is directly proportional to the frequency; the higher the frequency, the more energetic the photon. Something with a high frequency has a small wavelength, which means that something with a comparatively small wavelength will be more energetic than something with a longer wavelength. This is why blue light is more energetic than red light. If you look at a hot blackbody, what kind of spectrum will you see? If a cloud of cold gas is in between you and the blackbody, what kind of spectrum will you see? What kind of spectrum will you see if you look at a hot gas? The spectrum of a hot blackbody will have all the colors in the rainbow; it is a continuous spectrum. If a cold gas is put in between you and the blackbody, then you will see an absorption spectrum because some of the atoms in the cloud have electrons in just the right atomic orbitals to absorb photons of specific frequency (or wavelength) from the blackbody.. An absorption spectrum looks like a continuous spectrum with black lines in specific places. A hot gas will give you an emission spectrum which will be mostly black with a few bright lines in it. The lines in the emission spectrum correspond to electrons that are falling from an excited state within an atom to a less excited state, thereby losing energy and emitting it as a photon of energy exactly equal to the difference in energy between the two energy levels. Explain why anything with temperature above absolute zero will emit thermal radiation. Matter is made of atoms. Atoms are made of charged particles. Anything with temperature will be vibrating because temperature is a measure of vibrational energy. This will cause the charged particles within the atoms to be vibrating, or oscillating, as well. An oscillating charged particle creates an electromagnetic wave which is radiation. Therefore, everything with temperature will be emitting radiation. What is Wien s law? What is the Stefan Boltzmann law? What is the Planck distribution?
Wien's Law tells us that objects of different temperature emit spectra that peak at different wavelengths. Hotter objects emit most of their radiation at shorter wavelengths; hence they will appear to be bluer. Cooler objects emit most of their radiation at longer wavelengths and therefore will appear redder. The Stefan Boltzmann law relates the total amount of energy radiated by a blackbody to the fourth power of it s absolute temperature. The Plank distribution governs the intensity of radiation emitted from a blackbody as a function of wavelength for a fixed temperature. Explain how line emission and line absorption are related to atomic orbitals. An emission spectrum is comprised of specific bright bands in an otherwise black spectrum. These bands correspond to photons of very specific energy. When an electron from an excited energy level (atomic orbital) falls to a less energetic orbital it loses energy and that energy is lost in the form of a photon. The energy of the photon is exactly equal to the difference in energy between the two orbitals. An absorption line corresponds to a black band in an otherwise continuous spectrum. When we see an absorption line at a given wavelength it means that an electron absorbed the photon of that wavelength. When an electron absorbs a photon it gains energy because the photon had energy and the energy must be conserved. When an electron absorbs a photon, it means the photon had exactly the right energy to bump the electron to a higher energy level. The energy of the photon in this case went into bumping the electron to a higher orbital and therefore we will not see it in the spectrum. Why is the sky blue? Why is it redder during sunrise and sunset? The sky is blue because blue light is scattered in our atmosphere and we see the scattered light. The reason blue light is preferentially scattered is because our atmosphere is more than seventy percent nitrogen, and the size of the nitrogen molecules is comparable to the wavelength of the blue light. During sunrise and sunset, the sun is at the horizon. At the horizon the light is passing through more of the atmosphere so the blue light is still being scattered by the nitrogen and the redder light is passing through, but at the horizon it is also lower to the ground. Near the ground, there is more dust and heavier particles that will scatter the red light. What do telescopes do? What are the two types of optical telescopes? What is chromatic aberration? How does seeing depend on the atmosphere? Telescopes collect light; they are buckets that collect photons. The two primary types of optical telescopes are refractors and reflectors. In the simplest sense, refractors use a lens to focus the light to the object whereas a reflector uses a mirror to focus the light at the objective. Chromatic aberrations are differences in the focal length of different wavelengths. The idea is that different wavelengths will be refracted differently when traveling through something like a lens, and so they will focus at different places. The more atmosphere you look through, the worse the seeing. So the best place to look at a star is outside the atmosphere; next best is at high altitude. Also, when the star is directly overhead, the light passes through the least amount of atmosphere so the seeing is best at the zenith. If the atmosphere has a lot of turbulence, the object you are looking at will appear wavy and this is bad seeing.