Glossary: Vexillology Throughout the book, some specifi c terms relating to fl ags were used. This glossary is to help identifying and defi ning these terms. Parts of a fl ag. Image by Fonzi, Wikimedia Commons Canton : Flags are divided in four quarters. Normally, the Can- ton refers to the upper left quarter of the fl ag, closest to the fl agstaff. Defaced : Plain fl ag bearing authorized addition(s). Field : Background of the fl ag. Fly : The vertical half of the fl ag furthest away from the fl agpole. Hoist : The vertical half of the fl ag closest to the fl agpole. Obverse : Face of the fl ag with the hoist on the left and the fl y on the right. Reverse : The back of the fl ag and, usually, the mirror image of the obverse. A.G. Bordeleau, Flags of the Night Sky: When Astronomy 335 Meets National Pride, DOI 10.1007/978-1-4614-0929-8, © Springer Science+Business Media New York 2014 Glossary: Astronomy Galaxies Galaxies are huge islands containing between several hundreds of millions of stars to several hundreds of billions of them. There are three general classes: Elliptical, Spiral, and Irregular. Elliptical galaxies look like ellipses in photographs. They display smooth, featureless light distributions. Spiral galaxies consist of a fl at disk, with a bulge in the center where most of the stars are concentrated and usually two or more spiral arms formed by stars. About half of these galaxies are barred spirals, galaxies with a bar-like structure extending from the central bulge. I rregular galaxies have lower masses and no obvious shapes. Nebulae There are fi ve main types of nebulae: Dark nebulae, H II regions (diffuse nebulae), re fl ection nebulae, planetary nebulae and super- nova remnants. Dark nebulae are dusty, cold molecular clouds where stars are born and that are detected when they block the light of bright objects behind them. Our Sun was born in one such dark nebula. H II regions are clouds of ionized gas. Diffuse nebulae are clouds that are optically visible. They emit radiation from ionized gas. Re fl ection nebulae are not ionized and re fl ect the light of nearby stars. A.G. Bordeleau, Flags of the Night Sky: When Astronomy 337 Meets National Pride, DOI 10.1007/978-1-4614-0929-8, © Springer Science+Business Media New York 2014 338 Glossary: Astronomy Planetary nebulae are dense and compact nebulae formed from gaseous shells ejected from giant stars transforming into white dwarf stars. They are dense stellar remains the size of Earth and the mass of the Sun. Supernova remnants are expanding shells of gas from supergiant stars that ended their life in huge explosions. Star Clusters There are two main types of star clusters: open clusters and globu- lar clusters. Open clusters are generally thinly populated with up to a few hun- dred members, bound together by their mutual gravity. They populate the galaxy’s disk and like the sun orbit the center. Globular clusters are compact groups populated by very old stars numbering between tens of thousands to millions. These clus- ters orbit within the halos of galaxies. The Milky Way has about 150 of them orbiting its center. Types of Stars In astronomy, stars are classi fi ed according to their spectra which determine their temperature. In turn, temperature is infl uenced by the size and mass of stars. Most stars are classi fi ed using the letters O, B, A, F, G, K, and M, with regular O stars being the hot- test and regular M stars being the coolest. Most of these stars are called “main sequence” stars; they run off hydrogen and are the fi rst stellar stage. Hypergiant, supergiant, giant and sub-giant stars are ageing stars that can be found in all classes and are more lumi- nous than their main sequence counterparts. In addition, each letter has sub-classi fi cations ranging from 0 to 9 with 0 being the hottest and 9 being the coolest. Thus, a B2 star is hotter than a B7 one. A further sub-classi fi cation is the luminosity class using the Roman numbers I, II, III, IV and V. The luminosity is generally linked to the size and age of the star: A Zero, 0, is used for extreme supergiants called hypergiants, Ia for very luminous supergiant stars, Ib for less luminous supergiant Glossary: Astronomy 339 stars, II being bright giants, III for giants, IV for sub-giants and V as main sequence stars. On the main sequence, O stars are called “blue”, B stars are “blue-white”, A stars are named “white”, F stars are called “yellow- white”, G stars are “yellow”, K stars are named “orange”, and M stars are called “red”. Students use the mnemonics “Oh Be A Fine Girl Kiss Me” to help remembering the letters in the right order. O stars are very hot and extremely luminous, being bluish in color. On average, stars in this classi fi cation are more than a million times as luminous as the Sun, are more than 60 times as mas- sive with a surface temperatures above 25,000 C. Their lifetime is short, about 10 million years. B stars are very luminous and blue. They are about 1,000 times more luminous than the Sun, 10 times as massive with tem- peratures at around 20,000 C. Their lifetime is about 100 mil- lion years. A stars are white or bluish-white and are among the more com- mon naked eye stars. They are about 20 times more luminous than the Sun, twice as massive with temperatures at around 8,500 C. Their lifetime is about one billion years. F stars are white. They are about four times more luminous than the Sun, 1.5 as massive with temperatures at around 6,500 C. Their lifetime is about three billion years. G stars are the best known because the Sun is a G class star. They are yellowish-white and temperatures at around 5,700 C. Their lifetime is about 10 billion years. K stars are orange. They have about 20 % the luminosity of the Sun and 70 % of its mass with temperatures at around 4,500 C. Their lifetime is about 50 billion years. M stars are by far the most common class of stars. About 80 % of the stars in the solar neighborhood are M class. They have about 1 % the luminosity of the Sun, 20 % its mass with tem- peratures at around 3,200 C although this class also includes most giants and a few supergiants. These large stars are much brighter and, unlike their main sequence counterparts, are the only M stars visible to the naked eye. Their lifetime is about 200 billion years. 340 Glossary: Astronomy Hypergiant stars are the largest and the most luminous stars in the universe. The largest one, VY Canis Majoris, has a diameter equal to Saturn’s orbit around the Sun. Their lifespan is short, around a few million years. Supergiant stars are stars with a mass 8–12 times or greater than our Sun. They are very large stars much more luminous than our Sun. They are dying stars that span all the spectral classes. Their lives end in supernovae. Bright Giant stars are large stars that are very luminous but not quite massive enough to be considered supergiants. They too are dying stars that span all the spectral classes. Class G, K, and M giants are much more luminous than their main-sequence dwarf counterparts while class F and A giants are less so. Giant stars are stars that are a few hundred times larger and ten to several thousand times brighter than our Sun. They are also dying stars spanning all the spectral classes. Sub-giant stars are stars that are slightly brighter than main- sequence stars but not enough to be considered giant stars. Subdwarf stars are stars that are slightly dimmer than main- sequence stars. White dwarf stars are dim, dense stellar remains the size of Earth and with the mass of the Sun. Specific Types of Stars Beta Cepheids are variable stars whose pulsations of the stars’ surface cause changes in their brightness. Generally, Beta Cep- heids’ brightness varies by 0.01–0.3 magnitudes within periods of 0.1–0.6 days. Cepheid variables are very luminous variable stars whose lumi- nosity and pulsation period allows them to be used as measuring tools for Galactic and extragalactic distance scales. Delta Scuti variables are variable stars with changes in their lumi- nosity due to pulsations. Like Cepheid variables, they have been used to establish the distance of distant objects. Glossary: Astronomy 341 Neutron star is a type of stellar remnant that can result from the gravitational collapse of a massive star during a supernova. It is formed almost exclusively with neutrons, is very hot and extremely dense. Pulsar, or pulsating star, is a highly magnetized, rapidly rotating neutron star that emits pulses of electromagnetic radiation, like a lighthouse. T Tauri stars are a class of active variable stars found near molec- ular clouds and identified by their optical changes. They are the youngest visible F, G, K, M spectral type stars with a mass below 2 solar masses. They rotate rapidly and have yet to reach the main sequence of stars. .