Physics 101 - Astronomy

Study guide and notes for Ch. 5 through 10

Here are some factual statements that you might want to know for the exam.  This gives you an idea of similar important ideas that you might review in the book. If you are in a hurry, review the chapter summaries. I have left out some similar statements that are in the day notes, so you should look at them too.

Ch. 5 Radiation and Spectra

The period (denoted by T for Time) of repetitive periodic motion is the inverse of frequency (we use f to denote frequency): T = 1/f

Frequency, wavelength, and velocity of waves are related by an equation:  l = v/f   This equation can be derived from the idea that the crest of the wave travels to the position of the next crest of the wave (it travels one wavelength l ) during a time interval of one period T. Since it goes at velocity v, the distance l = vT. (This comes from the fact that distance = velocity times time.) Then since T = 1/f we can make a substitution l = v/f = vT.

Suggested reading in Ch. 5 is sections 5.1 - 5.6, pp. 145-186. You may skip the detailed math derivations in the boxes, but skim through that to see what we are trying to calculate.

Ch. 6 Astronomical Instruments

To make telescopes we need to get images of astronomical objects. Images are formed by bringing light to a focus, either with a lens or with a concave mirror. To make a telescope we can use lenses or mirrors, or a combination of both. A telescope without a mirror is called a refractor; if it has a primary mirror, it is called a reflector.

A Refracting Lens can form an image by using the principle of the refraction of light. Refraction of light occurs when light passes from one type of material into another, in which the speed of light is different. The light is then bent from its original path.
The main types of reflector are
Prime focus
Newtonian
Cassegrain
See the PowerPoint for diagrams of these.

Telescopes should be large to give lots of sensitivity to dim light, so the only way to do this is with reflectors, since glass lenses cannot be made too large or they lose their shape and don't work well. Hence in general, reflectors can have larger apertures (the openings at the end of the telescope where light enters) than Refractors.

Resolution is limited by Diffraction, the spreading out of light when it goes through a small opening (a small aperture). Resolving power is the ability of a telescope to distinguish one point object from a nearby point object. A large aperture results in better resolving power.

One cause of poor images is Atmospheric Turbulence, which is reduced at high altitudes. So many large telescopes are placed on mountains. This also puts them above most of the clouds, humidity, and turbulence of the lower atmosphere.

Adaptive Optics (also called active optics) use a laser beam as a “guide star” and continually change the shape of the mirror to compensate for atmospheric turbulence and distortion.

Various types of devices are used to collect electromagnetic radiation in different parts of the spectrum: radio, IR, visible light, UV, X-rays, and Gamma rays. Radio telescopes can be used in the daytime because radio waves travel right through the atmosphere. IR, UV, X-ray, and Gamma ray telescopes can only be used in space since the atmosphere will absorb most of these types of radiation.

Suggested reading for Ch. 6 is sections 6.1 - 6.6, pp. 189-224. However, there are long lists of various telescopes, etc., which you can just skim over.

Ch. 7 Other Worlds: An Introduction to the Solar System

I suggest that you read quickly through Ch. 7 when you can. This is just a summary of the solar system, and we will study these topics in detail later in the course.

Ch. 8 Earth as a Planet

The Earth's atmosphere is layered. Starting at the ground, the layers are called Troposphere, Stratosphere, Mesosphere, and Ionosphere. Most of our weather is in the troposphere, near the ground up to about 7 miles above the ground. Above that the stratosphere has very thin air that begins to have ozone in it at about 30 miles altitude. The ozone layer is important, because it absorbs UV light that could cause sunburns and skin cancer. To protect the ozone layer, certain gases known as CFCs (chlorofluorocarbons) have been banned, because they caused the destruction of ozone. All weather and most of the mass of the atmosphere is in the troposphere. Air is mostly nitrogen, oxygen, and argon. Greenhouse gases are mainly carbon dioxide and water, with some small contribution from methane.

Earth is unique in many ways, but one is the existence of a hydrosphere (the oceans), which no other planet has.

The surface of the Earth is very weathered, and has been altered by tectonics, or continental drift, so very few craters remain on Earth. Evidence of tectonic activity is revealed by mountain ranges, like the Cascades or Andes, mid-Atlantic ridge, African rift valley, fault lines like the San Andreas fault in California, and frequent earthquakes near the edges of the plates. The mantle of the Earth is about 80% of the volume of the Earth. Convection in the mantle causes the movement of the tectonic plates that float on the crust of the Earth.

The structure of the interior of the Earth is deduced from study of the waves produced by earthquakes. Seismic waves pass through the entire Earth whenever there is an earthquake. These waves are really like sound waves traveling through the interior of the Earth. Some of them are not heard on the other side because part of the core of the Earth is liquid and transverse sound waves cannot travel through liquid. This causes a shadow zone, which gives us information about the core. Scientific study of these seismic waves allows us to deduce the structure of the core of the Earth. The Earth's core is believed to be mostly iron and nickel, but could have many of the heavier elements. There is certainly some Uranium and other radioactive elements which produce heat that keeps the core hot and molten. The liquid iron outer core of Earth causes the magnetic field, which causes the aurora, and traps radiation in space around the Earth as in the Van Allen belts.

Suggested reading in Ch. 8 is sections 8.1 - 8.3, pp. 265-282. Also read about the greenhouse effect in section 8.4. We will skip section 8.5 for now and come back to look at it when we study meteors and asteroids.

Ch. 9 Cratered Worlds: the Moon and Mercury

The Moon was probably created in an impact which tore off part of the earth's mantle and crust, since the Moon has very little iron core and no magnetic field. Lunar surface features are maria and highlands on the near side, and mostly highlands but with a big impact basin on the far side. There is no atmosphere to cause weathering, so most of the craters are still visible.

Mercury is hard to see because it is close to the sun. It rotates slowly, and has the greatest variation of temperature of the planets. Mercury's rotation is locked into a ratio with its orbital period due to the tidal influence of the Sun during the early history of Mercury when it was molten.

Suggested reading in Ch. 9 is sections 9.1 - 9.5, pp. 303-328 (i.e., the whole chapter).

Ch. 10 Earthlike Planets: Venus and Mars

Venus is bright due to the complete coverage of clouds. It has an atmosphere of carbon dioxide and is much hotter than expected, due to a runaway greenhouse effect. The temperature on the surface is almost the same during day and night. Venus also rotates slowly, in retrograde motion. Both Venus and Mercury have almost zero orbital tilt.

Earth and Mars (and Saturn) have an orbital tilt of about 24 degrees, causing seasons.

Mars has the highest volcano in the solar system, because of its low gravity (due to Mars' small size).

Mars has a thin atmosphere of carbon dioxide. Therefore it has very little greenhouse warming and is colder than Earth. The hottest place on Mars is on the equator during its summer, at noon (like on Earth) but it only gets a few degrees above the melting temperature of water ice. Any liquid water will soon evaporate. However, there is plenty of evidence that Mars had liquid water on its surface for millions of years after it formed. Due to the lack of a magnetic field around Mars, the solar wind has eroded away the early atmosphere.

Suggested reading in Ch. 10 is the whole chapter.

As usual, look at the key terms and the summary at the end of each chapter.