Stars and Constellations

Transcription

1 Stars and Constellations by Mary Alice Thomas Grade level 9 or Earth Science Time required Two to three 50 minute class periods Materials/Technology required Star charts for the Northern Hemisphere throughout the year Protractor Compass String Flashlight Washer Cellophane Straw (red) Tape Summary The purpose of this lesson is to have students look at archaeoastronomy and how ancient peoples viewed their world in terms of astronomy. It is also designed for students to gain an understanding of star evolution, magnitude, distances and star patterns (constellations). Students will make their own astrolabes to use on a stargazing night, so they may view those stars and constellations like early people may have done long ago. Objectives The student will be able to: 1) define a star, declination, circumpolar and constellations 2) differentiate between absolute and apparent magnitude. 3) describe the life cycle of a star. 4) explain the apparent motion of stars. 5) construct and use an astrolabe. 6) use star charts to locate stars and constellations. 7) appreciate the significance of stars to different cultures. Montana Science standards addressed 1) Students demonstrate knowledge of the composition, structures, processes and interactions of Earth s systems and other objects in space. 2) Students understand how scientific knowledge and technological developments impact society. 3) Students understand historical developments in science and technology. Assessment Evaluate students on the following basis: 1) Students will demonstrate how to use star charts properly. 2) Students will describe (oral or written) the life cycle of a star. 3) Students will explain (oral or written) what magnitude means and why some stars appear quite bright but may actually have lower absolute magnitudes. 4) Students will explain (oral or written) what constellations are and how they can be used as points of reference in the night sky. 5) Students will be assessed on their astrolabe construction and how well it measures star declination (angles) when taken in the field.

2 Background Information on Stars Apparent magnitudes of stars are assigned using a ratio scale. The ratio of brightness from one number to the next on the scale is 2.5, with brightness increasing as the number decreases. So a first magnitude star is 2.5 times brighter than a second magnitude star, and a third magnitude star is 2.5 times brighter than a fourth magnitude star. A first magnitude star is also 6.25 times (or 2.5 x 2.5) brighter than a third magnitude star. The scale is relative to the apparent magnitude of the brightest star Sirius, whose assigned apparent magnitude is Star Magnitude How Much Brighter than a Sixth Magnitude Star Times Times Times Times Times 6 Barely visible to the naked eye. Scientists measure absolute magnitude by determining what brightness (or apparent magnitude) a star would have if it were placed exactly 10 parsecs from Earth. A parsec is a distance of 3.26 light years. Ancient as well as modern stargazers, such as the Navajo, identified groups or patterns of stars (constellations) in the night sky. Constellations have often been used as landmarks to locate other stars and objects in the sky. Because Earth rotates, stars appear to move across the night sky along east-west paths, similar to our Sun. The revolution of Earth around the Sun causes different constellations to be seen at different times of the year. Background Information on Black God For the people in the Native American southwest, the sky is not something remote from Earth. Rather, it is a unifying whole, both a window into the universe and an instrument for understanding and measuring its rhythms. Indian people have been watching the sky for many generations and applying its signs to their daily lives. Examples include painted designs that receive direct sunlight at certain culturally significant times. Ceilings in caves are often decorated with paintings of stars. These are just a few of the many archeological discoveries that tell us of the close attention the heavens commanded of our ancient astronomers (archaeoastronomers). One legend from the Navajos explains how the stars were made. The story goes like this: Black God made the stars. In those days Black God s face was as dark as the nighttime sky. Later, He stomped his foot to remove the seven stars of the Pleiades from his ankle. As he did so, the stars moved upward to his face, where they are seen today on masks of

3 Black God. Black God reached into his pouch and removed a shining crystal. He positioned the crystal in the sky where it became the North Star. Black God received help from First Man who helped arrange Revolving Male (Ursa Major with the pole star). First Woman helped arrange the Revolving Female (Ursa Minor, or part of Cassiopeia). Other constellations followed: First Big One (Scorpio), Rabbit Tracks (Canis Major), Delijehe (Pleiades) and so forth. Then Coyote, the trickster, got involved. He placed the Coyote Star (Antares) in the heavens but he grew impatient with Black God s slowness in his task. Coyote grabbed Black God s pouch and flung its contents across the sky. That is why some stars have names, others do not, why some stars are arranged in neat patterns (Black God s) and others are not (Coyote s). In this story the Navajo depict two elements associated with the sky: order and chaos. Native American observers without the help of modern instruments saw a very orderly sky. To them the sky revealed balance. On a dark night, far from home and friends, the constellations could provide comfort, security and predictability. Procedure 1) Introduce students to stellar astronomy through the Navajo story of Black God. Following the story, facilitate a discussion with students about the concept of a star, constellations and how stars evolve. Lead students to understand the difference between apparent magnitude and absolute magnitude. Showing a flashlight at varying distances is a concrete means of demonstrating the difference. 2) Have students construct simple astrolabes using drinking straws, washers, string and protractors (see drawing and instructions below). Ask them to practice using their astrolabe to measure the declination of objects in the room. 3) Distribute star charts to students and allow them to examine them. Introduce students to how star charts are used and the vocabulary associated with them. 4) Choose a suitable night for a star gazing party, after students have gained an understanding of stars, constellations, star charts, astrolabes, rotation and revolution. Assist students in using the star charts and the astrolabes. Have them determine the declination and latitude of specific stars, and to compare the change in the stars positions over time. Extensions: 1) Do math activities using par secs, light years, AUs, making conversions back and forth to help understand the immensity of the universe and the distance of stars. 2) Examine H-R diagrams for inferences between surface temperature and absolute magnitude. 3) Make comparisons of the brightest stars and their distance from Earth 4) As an alternative, long-term, activity using the astrolabe and star chart, a reading of the declination could be taken of the same star, at the exact same time of evening, once per week, to see how it s position changes with the seasons Further information For further information about this activity, contact Mary Alice Thomas via electronic mail at thomas@digisys.net.

4 Astrolabe Construction and Use An astrolabe is a simple tool used to determine the angle of a star with the horizon. There is a drawing of an astrolabe below. You will construct simple astrolabes using drinking straws, nuts or washers, string, and protractors using the following procedure: 1) Tie one of end of the string to the middle of the straw and the other end to the nut or washer. 2) Tape the straw to the straight edge of the protractor so that when the straw is held level, the string hangs at the 90 mark on the protractor. Straw Paperclip or nut Choose a clear night for observation. The same site must be used for all observations. Use a compass or known local geographic features to locate north, south, east, and west. Use your star chart to locate the Big Dipper and then follow the two stars in the dipper to find the North Star in the handle of the Little Dipper. Peer through the straw of your astrolabe until you have positioned it on the North Star. Hold still, and when the weight at the end of the string has stopped swinging (as so it perpendicular to the ground), press the string against the protractor and record the reading. Subtract the number of degrees from 90 to find the angle of the star from the horizon. For example, if the string stops on 60, the star is 30 above the horizon. Record the time of each sighting and their angles, every hour for three or four hours.

5 Using Star Charts Materials Star chart for appropriate season Tape Compass Flashlight Red cellophane Procedure 1. Cover the bulb end of your flashlight with red cellophane. The red light produced by the flashlight will not reduce your night vision. 2. Examine the star chart for the appropriate season. This chart indicates the constellations and brightest stars as they appear on the dates and at the times listed. The line labeled ecliptic shows the apparent path of our Sun through the sky. 3. The horizontal and vertical lines on the chart roughly correspond to latitude and longitude lines on Earth. The horizontal lines are called declination lines, and the vertical lines are called hour circles. Determine your latitude and find the declination line that corresponds to it. Record your approximate latitude and declination on the top line of the table below. 4. To use a chart, face south and hold it overhead with the top of the chart pointing toward the north and the right side of the chart pointing west. 5. The stars located along the declination line you located above will be overhead as you look at the night sky. Names of constellations are printed in all capital letters. 6. Record your observations on the chart below. Data and Observations Latitude / Declination: degrees Date Time Season Constellation Navajo Name Major Stars