Appendix: What You’re Looking At Early astronomers divided astronomical objects into three main types: planets, stars, and what were called mists or clouds; in Latin, nebulae. Stars had fixed positions relative to one another, but the planets moved about, and so the ancient Greeks called them wanderers, plan¯et¯es. The changing positions of the planets were hard to explain, and none of the Solar System models proposed bytheancientGreeksorRomansadequatelyexplainedthingssuchasretrograde motion. But even though they didn’t understand why the planets moved about, the Greeks and Romans both placed huge value on interpreting the movements of the planets. This is the art of astrology, a practice that has developed independently in many different cultures around the world. That stars were in some way similar to the Sun was something the ancient Greeks had guessed at, but it wasn’t until the nineteenth century and the invention of spec- troscopy that scientists could demonstrate that the light produced by the Sun was the similar to that produced by the stars. Over time different types of stars were identi- fied, and it soon became clear that the Sun was a relatively small and ordinary star by cosmic standards, even though it is vitally important to us. Nebulae posed particular problems for astronomers prior to the invention of the telescope. To the naked eye all nebulae look alike, nothing more than faint, misty patches of light. Only with the telescope was the diversity of nebulae revealed, but while they could be seen to be morphologically different, astronomers of the early modern era didn’t understand that a planetary nebula is a very different thing to something like the Great Nebula in Andromeda, what we’d today recognize as a galaxy. Indeed, some astronomers viewed nebulae more as nuisances than as objects worthy of investigation. The deep sky objects Charles Messier added to his famous catalog were only of interest to him because they might be mistaken for comets, objects of far greater importance at the time. N. Monks, Go-To Telescopes Under Suburban Skies, Patrick Moore’s Practical 225 Astronomy Series, DOI 10.1007/978-1-4419-6851-7, © Springer Science+Business Media, LLC 2010 226 Go-To Telescopes Under Suburban Skies Eventually the differences between nebulae were understood, and galaxies in par- ticular caused a profound rethink about the size of the universe. Once it became clear that these ‘spiral nebulae’ were galaxies like our own Milky Way Galaxy, astronomers had to accept that the universe was a much bigger place than they had previously thought. Just as the Sun turned out to be a very ordinary star, so the Milky Way was revealed as just one galaxy among millions. Exploring the night sky with a go-to telescope lets you condense thousands of years’ worth of scientific progress into an hour or two. As the preceding chapters have hopefully revealed, even objects that appear quite unexciting visually become much more interesting once you understand why they’re significant. Stars What Are They? Stars are essentially very large, very hot balls of gas, primarily hydrogen and helium. Their enormous mass means that their own gravity pulls them into a spherical shape, and the immense pressure and temperature this creates within them initiates nuclear fusion. Hydrogen atoms are fused together to make helium, and as this happens energy is released in the form of heat and light. Stars vary considerably in color and temperature, from around 50,000 K down to a mere 3,000 K. Hot stars emit more of their light towards the blue end of the spectrum; in the cases of very young and hot stars they may even be emitting a lot of ultraviolet light human observers cannot even see. Cooler stars emit more light towards the red end of the spectrum, and in fact many cool stars emit most of their light in the form of infrared. So although all stars emit light across the color spectrum, they emit light most strongly at certain wavelengths, depending on their temperature, and that in turn is reflected in their color. Astronomers classify stars according to their color, from hottest to coolest as spectral types O, B, A, F, G, K, and M. Of these, O-, B-, and A- type stars appear blue to white, F- and G-type stars are white to yellow, and K- and M-type stars are orange to red. Astronomers further subdivide these spectral types into ten classes, for example G0, G1, G2, G3, and so on through to G9. So a G1 star would be cooler than a G0 star, but a G9 star would be hotter than a K0 star. There are some unusual star classes as well. The most important of these from the perspective of backyard astronomers are carbon stars (sometimes called C-type stars) and Wolf-Rayet stars (sometimes called WR-type stars). Carbon stars are deep red stars that are even cooler than the orange M-type stars. Partly they’re red because they’re cool, but they’re redder than M-type stars because their atmospheres contain a lot of carbon. The heat and pressure inside a star fuses hydrogen into helium, but it can also synthesize other elements as well, including carbon and oxygen. Carbon stars are so old that they have accumulated a lot of carbon, and this comes to surface and forms patches of soot in the atmosphere. In the same way that dust in Earth’s atmosphere makes the setting Sun look redder than it does high up in the sky, this dust in the atmosphere of a carbon star scatters Appendix: What You’re Looking At 227 blue light while letting red light pass through, giving a carbon star its distinct red appearance. The patches of soot aren’t stable, and come and go on an irregular basis. In doing so the brightness of the carbon star varies, and carbon stars are variable stars, about which more will be said shortly. Wolf-Rayet stars are supergiant stars approaching the end of their life. Having used up most of their hydrogen, helium and other elements are being fused together. This process produces enormously high temperatures, and Wolf-Rayet stars are among the hottest known stars. Gamma Velorum, for example, has a surface temper- ature of 50,000 K. However, the tremendous heat and radiation produced by Wolf- Rayet stars creates winds moving up to 9 million km/hour, and needless to say this blasts off huge amounts of material into space. Indeed, what we see when observing Wolf-Rayet stars isn’t usually the star itself but the winds around it. Within a rela- tively short period of time, likely just a few million years, the fuel that powered this process is exhausted, and the star collapses into a planetary nebula or a supernova. How Big Are They? A star has to be above a certain size simply to have enough mass for nuclear reactions at its core to begin. Gravity comes from mass, and it’s gravity that squeezes the core of a star and thereby creates the incredible temperatures required for nuclear fusion to take place. At the core of a star such as the Sun the temperature will be around 15,700,000 K! The more massive a star, the hotter its core will be, and this means that nuclear reactions will proceed faster than in a less massive star. This brings us to a key thing about star sizes: the bigger the star, the shorter its life will be. Stars can be broadly divided into those stars that are ‘giants’ and those that are ‘dwarfs.’ The Sun is a dwarf star, while giants can be hundreds of times its mass. Astronomers normally divide stars into seven size categories, using Roman numbers running from I for the biggest stars (supergiants) through to VII for the smallest (white dwarfs). So, by combining the spectral type (i.e., the color of a star) with its size, you can quickly describe a star very accurately. This tends to be how professional astronomers describe stars. The Sun would be designated as a G2V star, i.e., a G- type star, hotter than a G3 star but cooler than a G1 star, and in terms of size a category V star, i.e., a dwarf. Alpha Centauri A is another G2V star, and its similarity to the Sun is one reason why it is so interesting. Sirius is an A1V star, a star similar in mass to the Sun, but very much hotter. On the other hand, the famous Garnet Star, Mu Cephei, is designated as a M2I star, a star that is not only cooler than the Sun but also much more massive. How Bright Are They? Some stars appear brighter than others because they are nearby. Both the Sun with Alpha Centauri A are G2V stars of similar size and temperature, and they both give 228 Go-To Telescopes Under Suburban Skies out about the same amount of light. But the Sun is a far brighter object from our vantage point on Earth simply because it is very much closer. Whereas Alpha Cen- tauri A is 41,300,000,000,000 km away, the Sun is 276,000 times closer at a distance of just 149,598,000 km. Put another way, stars of similar apparent brightness can be situated at very dif- ferent distances. Vega and Rigel seem equally bright when viewed from Earth, but Vega is a much closer star, just a mere 25 light years away, whereas Rigel is about 800 light years away.