StarS and nebulae orbiting the Sun This presented problems though One such problem was that if Earth moved the stars presumed to be on a large fixed sphere should appear to observers on Earth to shift back and forth as Earth orbits the Sun once a year No such shift called parallax was seen This meant that either Copernicus was wrong or that the stars were so distant at least hundreds of times more distant than Saturn that the shift could not be detected The latter turned out to be the case The implication that the stars were so far away led some such as the Italian scholar Giordano Bruno to suggest suggest that stars were in fact like the Sun but so distant that they looked dim He believed that the stars could even have their own planets Rather than being on a sphere they were scattered through infinite space For this and mainly for various theological reasons the Roman Catholic Church burned Bruno at the stake in 1600 In 1572 the Danish astronomer Tycho Brahe saw a new star appear in the heavens only to have it fade away within weeks Ancient authorities had claimed that the stars were eternal and unchanging Starting in 1609 Galileo Galilei made observations of the heavens with telescopes His discoveries generally supported the Copernican theory theory Additionally his telescopes revealed great numbers of stars invisible to the unaided eye This undermined a popular popular belief that stars were created solely for the benefit of 12
Tycho Brahe
StarS and nebulae humans After this scientists began to think of stars as natural natural physical objects rather than as gods mystical beings or portents Isaac Newton’s work in physics in the late 17th century combined with advances in instrumentation and the study of light paved the way for great advances in the understanding of stars OBSERVING THE MOTIONS OF STARS Even casual looks at the sky a few hours apart show the stars moving westward during the night More careful observation shows that they move as if attached to a large sphere surrounding Earth The sphere’s axis of rotation passes through the North and South poles so that Polaris the North Star which lies very close to this axis appears to barely move This imaginary sphere rotates once every 23 hours and 56 minutes The 4-minute difference between this rate and the 24-hour day accumulates to 2 hours per month and a whole day in a year For this reason the positions of the constellations as seen at a certain time of night can be identified with the seasons For example Orion culminates reaches its highest point in the sky at about midnight in December but by March it does so at about 6 00 pm In June this happens at about noon so that it cannot be seen at night By September it culminates at about 6 00 am In December it is back where it started 14
the nature of StarS An observer at the Equator eventually gets to see all the stars by waiting all night or all year An observer at the North Pole sees only the same stars all the time and these stars appear to go around in horizontal circles At the South Pole a completely different set of stars is seen In the midlatitudes there are some stars that never rise some that never set and a large number that rise and set daily Australians get to see Crux the Southern Cross but never the Big Dipper Observers in the northern United States see the Big Dipper but never the Southern Cross In both countries Orion appears half the time These motions are due to Earth’s daily rotation on its axis combined with its yearly revolution around the Sun Note that the constellations maintain their shapes as the stars appear to move in lock step The individual stars actually move independently however Their very gradual gradual apparent motions will after hundreds of thousands of years make the current constellations unrecognizable Astronomers call these individual apparent motions proper motion A star’s proper motion combined with its motion toward or away from the observer is used to determine the star’s actual velocity relative to the other stars This speed can be hundreds of miles per second The distances to stars are so great however that these motions are not noticeable to the naked eye over a human lifetime 15
