Class Notes

Chaps. 16-18

The astronomical unit is the average distance from the earth to the sun. The major advantage of radio observation of the galaxy is less disturbance from cosmic dust. A nova is a star made up mostly of neutrons. Most of the supernovae we see are in other galaxies. The angle between the horizon and the North Celestial Pole is equal to the observer's latitude. The "wanderers" is the ancient name given to the planets. The two main "windows" in the earth's atmosphere that astronomers can use are visible and radio. The purpose of a telescope is to collect light and form an image. The primary reason for building large optical telescopes is to see faint objects. Radio telescopes are designed to detect sound waves given off by celestial objects. Astronomical observations made from artificial satellites are important because more forms of electromagnetic radiation are detectable. The main distinction between stars and planets is that planets are much less massive than stars. During the sun's lifetime, it will spend about 10 billion years converting hydrogen into helium. The contraction of an interstellar cloud to become a star is caused by gravitational forces. Stars in star clusters stay together because of gravity.

It is very difficult to detect planets in orbit around stars beside the sun because they will look very faint and be close to their parent stars. Pulsars were discovered by means of their emission of radio waves. A neutron star is typically the size of a city. Black holes are a name given to matter so dense that even photons can't escape. Approximately there are 100 billion stars in our galaxy.

Sunspots appear dark because they are cooler than the surrounding surface. The planets are believed to have formed as a byproduct of the formation of the sun. Spacecraft did not detect mascons on Mercury. The greenhouse effect heats a planet because infrared radiation is trapped. Ozone is important on earth because it absorbs ultraviolet radiation. Mars is easier to observe than Mercury or Venus because it is farther from the sun than the earth.The surface of Mars does not have active volcanoes. The Viking lander found ambiguous evidence for life on Mars. Strong radio radiation from Jupiter indicates the presence of a strong magnetic field. The Great Red Spot is probably a storm.

The interior of Saturn is probably most like the interior of Jupiter. Saturn's rings are partially transparent. The existence of Neptune was proposed because of its effect on Uranus's orbit. Most of the mass of the solar system is in the sun. Rings are know to exist around all the Jovian planets. Most of the asteroids orbit the sun in a belt between the orbits of Mars and Jupiter. When the moon is almost exactly lined up between the earth and the sun, the dark side of the moon faces us. This is called new moon.You go out late at night and observe a full Moon. Two days late you observe less than a full Moon. This phase of the Moon is called waning gibbous. When the Moon is new, it lies between the Earth and the Sun. Craters on the moon are due mostly to impacts. We see an eclipse of the sun when the moon's shadow falls on the surface of the earth. Eclipses of the moon occur when the moon gets into the shadow of the earth. The lunar maria are presumed younger than the highlands because they have fewer craters. The moon does not have an atmosphere because the gravity of the moon was too low to keep it.

The moon shines mostly by reflected sunlight. The surface of the moon is covered by a layer of fine-grained dust. Craters on the moon are due mostly to impacts. Mars has a rotational period closest to that of the earth. Evidence that the climate of Mars may have been different in the past is represented by the outflow channels. Planets are heated by sunlight. They re-emit this energy mostly in the infrared. Jupiter's atmosphere is mostly hydrogen and helium. The giant planets are very massive and have low density atmospheres of mostly hydrogen and helium. During the flybys of the Voyager spacecraft, the Galilean satellite that was observed to have volcanoes erupting on its surface was Io. Saturn's moon Titan is remarkable because it has a very thick and smoggy atmosphere. Saturn's rings are made of myriads of tiny particles, each in its own orbit. Members of ancient civilizations often feared comets because their appearances were unpredictable. An object that appears as a very brief streak of light visible to the a meteor. A bit of matter that enters the earth's atmosphere and survives to reach the ground is called a meteorite. The major source of the sun's energy is the fusion of hydrogen atoms to form helium. The planets are believed to have originated from material left over from the formation of the sun.

A quasar (a) has a large red shift (b) can be a strong radio emitter and (c) is probably one of millions of such objects. The densest matter found in the universe is associated with a black hole. When we see the Milky Way, we are looking at stars in the plane of the galaxy.

The sky is divided into 88 major areas called constellations. The classes of stars, O, B, A, F, G, K, and M are related to temperature and spectra. The angle between the horizon and the North Celestial Pole is equal to the observer's latitude. The stars of the constellation Orion are all located in about the same direction.

The most important cause for the difference in stellar spectra is surface temperature. The HR diagram helps us to understand the relationships between physical characteristics of stars. In the HR diagram, about 90 percentage of stars fall on the main sequence. When a star becomes a red giant, it gets much brighter. This is because it is increasing in size. It is very difficult to detect planets in orbit around stars beside the sun because they will look very faint and be close to their parent stars.

Supernova 1987A was discovered by Ian Shelton. The heavy elements (everything except hydrogen and helium) in the earth were produced by massive stars that exploded as supernovae before the solar system formed. The Crab Nebula is the remnant of a supernova seen in 1054. Pulsars were discovered by means of their emission of radio waves. Betelgeuse is a star in the constellation Orion.

Radio astronomy was discovered accidentally. Whether a star becomes a white dwarf, a neutron star, or a black hole depends on its mass. Most of the supernovae we see are in other galaxies.

The contraction of an interstellar cloud to become a star is caused by gravitational forces. A well-known planetary nebula in the constellation of Lyra is the Ring Nebula. "We are made of stardust" means some stars make elements heavier than carbon. During the night, most stars appear to move across the sky from east to west. The zodiac is selected constellation through which the Sun passes each year.