Stars and Nebulae

Stars and Nebulae — page 5 — CHAPTER 1 The NaTure of STarS In the observable universe it’s estimated that there are roughly as many stars as there are grains of sand on all of the beaches on Earth There are all different types of stars of all different sizes and ages Throughout the Milky Way Galaxy and even near the Sun itself astronomers have discovered stars that are well evolved or even approaching extinction or both as well as occasional stars that must be very young or still in the process of formation Evolutionary effects on these stars are not negligible even for a middle-aged star such as the Sun More massive stars must display more spectacular effects because the rate of conversion of mass into energy is higher While the Sun produces energy at the rate of about two ergs per gram per second a more luminous main-sequence star can release energy at a rate some 1,000 times 10

Stars and Nebulae — page 6 — the nature of StarS greater Consequently effects that require billions of years to be easily recognized in the Sun might occur within a few million years in highly luminous and massive stars A supergiant star such as Antares a bright main-sequence star such as Rigel or even a more modest star such as Sirius Sirius cannot have endured as long as the Sun has endured These stars must have been formed relatively recently While roots can be traced back through Arab and Greek contributions modern astronomy started with the work of Nicolaus Copernicus in Poland in the early 16th century Copernicus concluded that the Sun not Earth was the center of the universe and that Earth was a planet Left the Sun Hotter areas of the Sun appear in bright white Right limb darkening on the disk of the Sun Mercury can be seen as a small black dot in the lower middle of the solar disk

Stars and Nebulae — page 7 — 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

Stars and Nebulae — page 8 — Tycho Brahe

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