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A Mini Stellar Glossary

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A Mini Stellar Glossary A Mini Stellar Glossary This short glossary of elementary astronomical terms associated with stars is not intended to be complete or very detailed. It is meant mostly for those of you who have no earlier experience in the subject. For the most part, we only list terms not specifically treated in the main text. An excellent overall reference to this material is Mihalas, D., & Binney, J. 1981, Galactic Astronomy, 3d ed. (San Fran- cisco: Freeman & Co.) and, on a more elementary level, B¨ohm–Vitense,E. 1989, Introduction to Stellar Astrophysics,Vol.2(Stel- lar Atmospheres) (Cambridge: Cambridge University Press). 1. Stellar populations: These are useful shorthand designations for stars sharing common properties of kinematics, location in a galaxy, and com- position. a) Population I stars have a small scale height (confined to the disk of a spiral galaxy, if that’s where you’re looking), rotate with the disk, generally have a surface composition not too different from the sun’s, and have a large range of masses since the young ones are still on the main sequence. Also look for them in any galaxy having active star-forming regions. b) Population II stars usually have a very large scale height (mostly found in the halo of a spiral galaxy), high space velocities, are poor in metals, and are of low mass since the more massive stars have already evolved. Hence Pop II stars are old stars. c) Population III: These are stars that must have been around at the time when the first stars were forming. They should have contained no metals because none were produced in the Big Bang. Thus far none have been observed either because they destroyed themselves early on or, more likely, the remaining ones accreted metal-rich material and are hence in disguise. 2. Star clusters: These are useful since they are generally composed of many stars of roughly the same composition and age. The turn-off point from the main sequence provides an age when compared to models or relative ages when one cluster is compared to another. 498 A Mini Stellar Glossary a) “O–B” associations consist of a loose cluster dominated in light by bright stars that are still associated with the interstellar gas that begat them. The cluster is not gravitationally bound and the stars are associated only because of their tender youth. b) Open clusters or galactic clusters are galactic disk (young Pop I or somewhat older Pop I) stars or stars in regions of star formation in other types of galaxies. The clusters are bound together by gravita- tion. They contain both massive and low-mass stars. The Milky Way (our galaxy) contains at least 1,200 clusters. Many others must be present but we cannot see them because of intervening dust and gas in the disk (where we reside). The Large and Small Magellanic Clouds (galaxies relatively nearby to us) contain a total of more than 6,000. They are conspicuous because of the bright massive stars. Compared to globular clusters they have fewer stars in total number. Stellar membership ranges (at least) between 10 and 200. The Pleiades is a conspicuous open cluster in the northern night sky (and see Fig 2.6). c) Globular clusters are gravitationally tightly bound and contain many low-mass Pop II stars. Their stars have surface metal contents between 1/2 and 1/200th that of the sun. They are associated with the halo of a galaxy and were formed early in the history of the galaxy. Our galaxy contains about 150 clusters with memberships of roughly 2000–106 stars. Figure 2.7 shows a HR diagram for one of them (M3). 3. Observation of stars—Photometry: Photometry refers to observing stars over one or more wavelength bands where details of the spectrum are not necessarily important. a) The magnitude scale is an astronomical scale for brightness con- structed along the same lines as the decibel scale for sound. It is logarithmic, as are all such scales, so that the mind can handle the broad dynamic range of real external stimuli. There are two main magnitude scales. (Note that in what immediately follows we assume that the stars in question are observed over the same band of wave- lengths or colors.) i. Apparent magnitude, m, is the brightness of a star as observed from earth and is thus a function of the intrinsic brightness of the star, its distance, and what is between us and the star. Unlike the decibel scale it runs backwards: the larger the magnitude, the apparently dimmer the star. An arithmetic difference of 5 in magnitude means a multiplicative factor of 100 in brightness (in, say, apparent luminosity). If bi is the apparent brightness in physical units of star i, then the rule is b 1 =2.512m2−m1 . b2 A Mini Stellar Glossary 499 A rough guide to apparent magnitudes is that a star with mag- nitude 6 is just visible to the naked eye while stars of magnitude 0 are among the brightest in the sky. ii. The absolute magnitude, M, of a star is the apparent mag- nitude the star would have if we placed the star at a standard distance of 10 parsecs (1 pc equals 3.086×1018 cm or 3.2616 light years). If absorption of light by intervening gas or dust may be ignored, then the relation between apparent and absolute magni- tude is M = m +5− 5 log d where d is the actual distance to the star in parsecs. The difference m − M is called the distance modulus. iii. The bolometric magnitude is the magnitude integrated over all wavelengths (i.e., the entire electromagnetic spectrum). Since absolute magnitudes are standardized by the common distance 10 parsecs there must be a relation between luminosity and absolute bolometric magnitude, Mbol. Using the sun as a normalization this relation for a star ()is log(L/L)=[Mbol( ) − Mbol()] /2.5 where Mbol( ) is +4.75. b) Colors of stars are a reflection of the relative dominance of various wavelengths in their spectra and hence their effective temperatures. Observational magnitudes are always quoted for some range of wave- lengths, which is standardized. An example of a standardized system is the Johnson–Morgan UBV system. The U filter lets in a band of light centered around 3,650A˚ in the ultraviolet. The B (blue) and V (visual) filters are centered around 4,400A˚ and 5,500A˚ , respectively. The magnitudes in these wavelength ranges are usually denoted by their letters. Thus the absolute magnitude of the sun in the V range of the UBV system is V = −26.7. The “color” of a star can be de- scribed by the difference in brightness in two filters. Thus, for exam- ple, and keeping in mind the backward scale for magnitudes, (B–V ) is negative for blue (hot) stars and positive for red (cool) stars. c) Bolometric correction: To obtain the bolometric magnitude you must assume an energy distribution as a function of wavelength to find the spectrum at unobserved wavelengths (as, e.g., in the far ul- traviolet for surface-based telescopes). Since the color, as well as the energy distribution, is related to the temperature, given the color you can estimate the correction to a magnitude to give the bolomet- ric magnitude. Specifically, the bolometric correction, B.C., is defined as Mbol = MV + B.C. where MV is the absolute visual magnitude (usually V ). 500 A Mini Stellar Glossary d) Color–magnitude diagram: This is the observer’s version of the Hertzsprung–Russell diagram (as in Figs. 2.6, 2.7, and more in the text). The abscissa is color (B–V , for example) and the ordinate is some magnitude (V , MV , etc.). Color is the effective temperature sur- rogate and the single magnitude plays the role of luminosity. Needless to say, there is a lot of witchcraft involved in passing back and forth between these variables. e) Time-resolved photometry is what the name implies. Snapshots are taken of the star to get magnitudes or intensities over intervals of time. The resulting times series is then used to infer dynamic prop- erties such as those seen in variable stars. 4. Observations of stars—Spectroscopy: Here the details are impor- tant and one or more spectral absorption or emission lines are used for diagnostic purposes. a) Spectral types or classes: This refers to the Henry Draper clas- sification scheme, which is based on the appearance of spectral lines of hydrogen, helium, and various metals. An “O” star shows strong HeII (second ionization) lines of helium. Since the ionization potential is high, such stars are the hottest in terms of effective temperature. Spectral type “B” stars show strong HeI plus some HI lines. The Balmer lines reach their peak in the “A” stars and begin to disap- pear in “F.” Calcium H&K lines strengthen in “G” and the “K” stars show other metallic lines. The “M” stars have strong molecular bands (particularly TiO). The effective temperatures decrease in the order O, B, A, F, G, K, M. The historical reasons for the lettering of this sequence are worth looking up in the literature. Subdivisions such as G0, G1, G2, ··· are in order of decreasing temperature. The sun is a G2 spectral class star. Our Figure 4.8 shows spectra for the nor- mal range of classes for main sequence stars (and see accompanying discussion). Our §4.7 briefly discusses the new classes L and T for dwarfs (luminosity class V below). b) Luminosity classes were introduced because stars of the same spec- tral type may have very different luminosities.
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