Physics
Stellar Spectral Classes
Stellar spectral classes are a classification system used to categorize stars based on their spectral characteristics. The system is based on the temperature of the star's surface, which is determined by the wavelengths of light that it emits. The classes range from O (hottest) to M (coolest).
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10 Key excerpts on "Stellar Spectral Classes"
No longer available |Learn more- (Author)
- 2014(Publication Date)
- University Publications(Publisher)
________________________ WORLD TECHNOLOGIES ________________________ Chapter- 9 Stellar Classification In astronomy, stellar classification is a classification of stars based on their spectral characteristics. The spectral class of a star is a designated class of a star describing the ionization of its chromosphere, what atomic excitations are most prominent in the light, giving an objective measure of the temperature in this chromosphere. Light from the star is analyzed by splitting it up by a diffraction grating, subdividing the incoming photons into a spectrum exhibiting a rainbow of colors interspersed by absorption lines, each line indicating a certain ion of a certain chemical element. The presence of a certain chemical element in such an absorption spectrum primarily indicates that the temperature conditions are suitable for a certain excitation of this element. If the star temperature has been determined by a majority of absorption lines, unusual absences or strengths of lines for a certain element may indicate an unusual chemical composition of the chromosphere. Most stars are currently classified using the letters O , B , A , F , G , K , and M (usually memorized by astrophysicists as Oh, be a fine girl/guy, kiss me), where O stars are the hottest and the letter sequence indicates successively cooler stars up to the coolest M class. According to informal tradition, O stars are called blue, B blue-white, A stars white, F stars yellow-white, G stars yellow, K stars orange, and M stars red, even though the actual star colors perceived by an observer may deviate from these colors depending on visual conditions and individual stars observed. The current non-alphabetical scheme developed from an earlier scheme using all letters from A to O ; the old letters were retained but the star classes were re-ordered in the current temperature order when the connection between the stars' class and temperatures became clear.
No longer available |Learn more- (Author)
- 2014(Publication Date)
- Learning Press(Publisher)
________________________ WORLD TECHNOLOGIES ________________________ Chapter-7 Stellar Classification In astronomy, stellar classification is a classification of stars based on their spectral characteristics. The spectral class of a star is a designated class of a star describing the ionization of its chromosphere, what atomic excitations are most prominent in the light, giving an objective measure of the temperature in this chromosphere. Light from the star is analyzed by splitting it up by a diffraction grating, subdividing the incoming photons into a spectrum exhibiting a rainbow of colors interspersed by absorption lines, each line indicating a certain ion of a certain chemical element. The presence of a certain chemical element in such an absorption spectrum primarily indicates that the temperature con-ditions are suitable for a certain excitation of this element. If the star temperature have been determined by a majority of absorption lines, unusual absences or strengths of lines for a certain element may indicate an unusual chemical composition of the chromosphere. Most stars are currently classified using the letters O , B , A , F , G , K and M (usually memorized by astrophysicists as O be a fine girl, kiss me), where O stars are the hottest and the letter sequence indicates successively cooler stars up to the coolest M class. According to an informal tradition, O stars are blue, B blue-white, A stars white, F stars yellow-white, G stars yellow, K stars orange, and M stars red, even though the actual star colors perceived by an observer may deviate from these colors depending on visual conditions and individual stars observed. This non-alphabetical scheme has been developed from an earlier scheme using all letters from A to O , but the star classes were reordered to the current one when the connection to the star's temperature became clarified, and a few star classes were omitted as duplicate of others.
eBook - PDF- James Binney, Michael Merrifield(Authors)
- 2021(Publication Date)
- Princeton University Press(Publisher)
Further-more, observers using different spectrographs and dispersions can classify on the same system simply by reobserving the standard stars with their own particular equipment. The spectral types of the MK system are essentially those of the Harvard sequence, and some of the principal spectral features characterizing each of these types are listed in Table 3.1. The luminosity classes, and the stars to which they pertain, are listed in Table 3.2. The characteristics mentioned in these lists are only illustrative; the system is defined by standard stars. The complete spectral type is specified by both the spectral type and the luminosity class of a star as determined by comparison with the standards. In §3.5 we give detailed tables of the stellar physical properties (for example, temperatures and luminosities) that have been associated with MK spectral class through astrophysical calibrations. Spectral types are subdivided into decimal subtypes, running from 0 at the hot end through 9 at the cool end: BO, Bl, B2, ..., B9; AO, Al, A2, ..., A9; FO, Fl, F2, ..., F9, and so on. The luminosity classes are usually not subdivided except for supergiants (Table 3.2). Examples of spectral types are: Sun (G2V), e Ori (BOIa), a Lyr (AOV), a Tau (K5III). Stars hotter than the Sun (types O, B, A, F) are commonly called early types, and solar-type and cooler stars (types G, K, M) are called late types. 1 Although it is explicitly two-dimensional, the MK system implies the existence of, and the need for, formal consideration of at least one more dimension in localized regions of the system in order to describe, say, weak-lined stars (for example, the 'subdwarf HD 140283 or the variable star RR Lyrae) or peculiar stars (see the following discussion). 1 These designations are archaic remnants of an obsolete scheme of stellar evolution. Although devoid of physical significance, they are universally used by astronomers.
- Britannica Educational Publishing, Erik Gregersen(Authors)
- 2009(Publication Date)
- Britannica Educational Publishing(Publisher)
Bright Star Catalogue list spectral types from the hottest to the coolest stars. These types are designated, in order of decreasing temperature, by the letters O, B, A, F, G, K, and M. This group is supplemented by R- and N-type stars (today often referred to as carbon, or C-type, stars) and S-type stars. The R-, N-, and S-type stars differ from the others in chemical composition; also, they are invariably giant or supergiant stars. With the discovery of brown dwarfs, objects that form like stars but do not shine through thermonuclear fusion, the system of stellar classification has been expanded to include spectral types L and T.The spectral sequence O through M represents stars of essentially the same chemical composition but of different temperatures and atmospheric pressures. This simple interpretation, put forward in the 1920s by the Indian astrophysicist Meghnad N. Saha, has provided the physical basis for all subsequent interpretations of stellar spectra. The spectral sequence is also a colour sequence: the O- and B-type stars are intrinsically the bluest and hottest; the M-, R-, N-, and S-type stars are the reddest and coolest.In the case of cool stars of type M, the spectra indicate the presence of familiar metals, including iron, calcium, magnesium, and also titanium oxide molecules (TiO), particularly in the red and green parts of the spectrum. In the somewhat hotter K-type stars, the TiO features disappear, and the spectrum exhibits a wealth of metallic lines. A few especially stable fragments of molecules such as cyanogen (CN) and the hydroxyl radical (OH) persist in these stars and even in G-type stars such as the Sun. The spectra of G-type stars are dominated by the characteristic lines of metals, particularly those of iron, calcium, sodium, magnesium, and titanium.The behaviour of calcium illustrates the phenomenon of thermal ionization. At low temperatures a calcium atom retains all of its electrons and radiates a spectrum characteristic of the neutral, or normal, atom; at higher temperatures collisions between atoms and electrons and the absorption of radiation both tend to detach electrons and to produce singly ionized calcium atoms. At the same time, these ions can recombine with electrons to produce neutral calcium atoms. At high temperatures or low electron pressures, or both, most of the atoms are ionized. At low temperatures and high densities, the equilibrium favours the neutral state. The concentrations of ions and neutral atoms can be computed from the temperature, the density, and the ionization potential (namely, the energy required to detach an electron from the atom).
eBook - PDF- Barbara Ryden, Bradley M. Peterson(Authors)
- 2020(Publication Date)
- Cambridge University Press(Publisher)
14.3 LUMINOSITY CLASSES The spectral types O through T are a temperature sequence. Although the photospheric temperature is the most important parameter determining a star’s spectrum, it is not the only one. In the 1930s, W. W. Morgan and Philip Keenan added an extension to the old OBAFGKM scheme by introducing the concept of luminosity classes. Empirically, the six luminosity classes, I, II, III, IV, V, and VI, correspond to different absorption line widths, with luminosity class I having the narrowest lines at a given temperature and luminosity class VI having the broadest. As an example, Figure 14.1 shows the spectra of three stars, each of spectral type A0. Spectrum (a), which has luminosity class I, has perceptibly narrower Balmer absorption lines than spectrum (c), which has luminosity class V. (In fact, the lowest spectrum is that of Vega.) In practice, it is found that the six luminosity classes correspond to stars of different radii: Luminosity Class Star Size I supergiant II bright giant III giant IV subgiant V dwarf (main sequence) VI subdwarf (a) (b) (c) FIGURE 14.1 Spectra (negative photographic images) of three A0 stars. (a) Luminosity class I. (b) Luminosity class II. (c) Luminosity class V. 344 Chapter 14 Stellar Atmospheres The majority of stars (like the Sun, Sirius A, Alpha Centauri A, Alpha Centauri B, Proxima Centauri, and Vega) are of luminosity class V. Betelgeuse is a supergiant, of luminosity class I. Arcturus and Capella are examples of giants, with luminosity class III. 8 Why should supergiants (luminosity class I) have narrower absorption lines than ordinary main sequence stars (luminosity class V) of the same surface temperature? To understand, let’s first think about what it means to look at the photosphere of a star. If you look at a star from a distance, the optical depth τ increases as you look farther into the star’s interior.
eBook - ePubSearching for the Oldest Stars
Ancient Relics from the Early Universe
- Anna Frebel(Author)
- 2015(Publication Date)
- Princeton University Press(Publisher)
Her new system later became famous as the Harvard classification scheme. She grouped the spectra into classes that are still in use today: O, B, A, F, G, K, and M—they are easily remembered by the mnemonic “Oh Be a Fine Girl/Guy, Kiss Me.” These classes are based on hydrogen-line strength, one of the most important characteristics in stellar absorption spectra. O-type stars are the hottest (with surface temperatures of up to 50,000 K), whereas M-types are the coolest stars (sometimes only 2,000 K at the surface). Figure 2.3 illustrates example spectra for the various spectral classes in the form we currently use. Figure 2.3. Examples of stellar spectra that illustrate present-day spectral types. Various absorption lines are indicated. Only specific lines are detectable in a given spectrum, depending on the surface temperature of the star. These one-dimensional spectra are obtained after data reduction and processing of the two-dimensional raw spectra (e.g., those seen in Figure 2.1). They are used for analysis. (Source : Peter Palm; reproduction of spectra from Silva and Cornell, Astrophysical Journal 2, supplemental series (1992): 865–881) Figure 2.4. Annie Jump Cannon’s notebook entries on star classifications from Saturday, November 8, 1913. Her notebooks are available in the Plate Stacks Archive (a collection of astronomical photographic plates) at the Harvard College Observatory in Cambridge, Massachusetts, USA. (Source : Anna Frebel; reproduction of one page from Annie Jump Cannon’s notebooks, Harvard College Observatory Astronomical Plate Stacks Archive) The huge collection of Cannon’s bound laboratory notebooks containing her star classifications, as well as original photographic plates of the spectra intended for classification, can still be inspected at the Harvard College Observatory in Cambridge, Massachusetts. I held some of those notebooks, marked with the initials “AJC,” in my own hands
eBook - PDF- Carlos Jaschek, Mercedes Jaschek(Authors)
- 1990(Publication Date)
- Cambridge University Press(Publisher)
This means essentially that the groupings established are bound to a certain dispersion, and resolution (and technique? See chapter 2.) Of course, the term 'stellar spectrum' is a gross exaggeration, because whereas stars radiate at all wavelengths, a 'spectrum' conventionally covers only a short interval of wavelengths. We shall use the Table 1.1. Number of stars with known parameters. Parameter Spectral type (HD) Photoelectric color (all systems) Proper motion Radial velocity Rotational velocity Radius Magnetic field Chemical composition Number 5x 10 5 1 x 10 5 3x 10 5 3x 10 4 6x 10 3 4x 10 3 1 x 10 3 2x 10 3 Stellar taxonomy 3 term 'classical region' to refer to the interval XX3500-4800. (In this book, all wavelengths are expressed in angstroms. 1 A = 0.1 nm = 10" 10 m.) In addition, if spectra in two different wavelength regions are compared, there is no logical requirement that stars identical in the 'classical region' say, should also be identical in the 'red region' or 'ultraviolet region'. With this in mind let us assume that we have arranged the spectra of N stars, taken with the same dispersion and at the same wavelength region, in m groups, each group consisting of a number n m of similar looking spectra. We can pick out any star of a given group and call it a 'typical object of the group' or, more succinctly, a 'standard'. The process of classification has thus substituted the descriptions of only m standard for the separate descrip- tions of N spectra. This operation thus facilitates further studies, since it greatly reduces the number of objects to study. For instance, it was found that about 2 x 10 5 stellar spectra classified by Harvard astronomers could be put into fewer than 10 2 groups. It always happens that a few spectra out of the N are not identical to any standard. These we termed 'abnormal spectra' and are put to one side.
eBook - PDF- Stan Owocki(Author)
- 2021(Publication Date)
- Cambridge University Press(Publisher)
Credit: NOAO/AURA/NSF. The sequence is often remembered through the mnemonic 3 “Oh, Be A Fine Gal/Guy Kiss Me.” In keeping with its status as a kind of average star, the Sun has spectral type G, just a bit cooler than type F in the middle of the sequence. In addition to the spectral classes OBAFGKM that depend on surface temperature T , spectra can also be organized in terms of luminosity classes, conventionally denoted though Roman numerals I for the biggest, brightest “supergiant” stars, to V for smaller, dimmer “dwarf” stars; in between, there are luminosity classes II (bright giants), III (giants), and IV (subgiants). In this two-parameter scheme, the Sun is classified as a G2V star. Finally, in addition to giving information on the temperature, chemical compo- sition, and other conditions of a star’s atmosphere, these absorption lines provide convenient “markers” in the star’s spectrum. As discussed in Section 9.2, this makes it possible to track small changes in the wavelength of lines that arise from the so-called Doppler effect as a star moves toward or away from us. In summary, the appearance of absorption lines in stellar spectra provides a real treasure trove of clues to the physical properties of stars. 6.4 Hertzsprung–Russell (H–R) Diagram A key diagnostic of stellar populations comes from the Hertzsprung–Russell (H–R) diagram, illustrated by the left panel of Figure 6.5. Observationally, it relates absolute magnitude (or luminosity class) on the y -axis, to color or spectral type on the x -axis; physically, it relates luminosity to temperature. For stars in the solar neighborhood with parallaxes measured by the Gaia astrometry satellite, one can readily use the associated distance to convert observed apparent magnitudes to absolute magnitudes 3 A student in one of my exams once offered an alternative mnemonic: “Oh Boy, Another F’s Gonna Kill Me.”
eBook - ePub- John S. Lewis(Author)
- 2004(Publication Date)
- Academic Press(Publisher)
6 ) − 10.7 = −26.1.The color, or spectral class, of a star can usually be estimated by photometric comparison of images of the star taken through three or more colored filters that transmit only narrow spectral intervals of light. The most commonly used filters for this purpose are ultraviolet, blue, and the center of the visible region (yellow). This is referred to as the UBV filter system. For more precision, especially with cooler stars, additional filters in the near infrared are added to the set.On the basis of the UBV photometric classification of stars a number of different color groups can be distinguished. For historical reasons, these color groups form a spectral sequence labeled with an inscrutable sequence of apparently random letters. For the spectral sequence running from violet through the visible region to red, the principal color classes are O, B, A, F, G, K, and M, and the less common classes are R, N, and S. There are endless mnemonics to assist in keeping this sequence intact and in order: my favorite is “Oscar, Bring A Fully Grown Kangaroo: My Recipe Needs Some.” (Certain other spectral classes, such as C, are often encountered in the astronomical literature but rarely seen in space.) Thus O and B stars are very strong ultraviolet emitters, blue or violet to the eye, with surface temperatures in excess of 15,000 K. A and F stars, with temperatures near 10,000 and 8000 K, respectively, may be described as white. Our Sun is a representative of the cooler yellow G stars, which have surface temperatures near 6000 K. K stars are orange in color, and M stars, with temperatures below 4000 K, are red.Given only one further type of data about these stars, their distances from us, it would be possible to construct a two-dimensional (color–luminosity) classification system for stars in which intrinsic properties alone are employed. In fact, as we have already seen, several thousand stars are close enough to the Sun so that the annual motion of the Earth around the Sun causes a measurable displacement in the position of these stars against the background of much more distant stars. Thus, by the simple expedient of comparing photographic images of these stars and their stellar backgrounds on pictures taken 6 months apart, it is possible to calculate their distances. We now can combine the two simplest measurements of the star, its apparent magnitude and its parallax, to determine the absolute magnitude of the star, M v
eBook - ePub- Charles Liu(Author)
- 2013(Publication Date)
- Visible Ink Press(Publisher)
F5V (blue-green dwarf) 11.40 Procyon B DA (white dwarf) 11.40 61 Cygni A K5V (orange dwarf) 11.40 61 Cygni B K7V (orange dwarf) 11.40 Struve 2398 A M3V (red dwarf) 11.53 Struve 2398 B M4V (red dwarf) 11.53 Groombridge 34 A M1V (red dwarf) 11.62 Groombridge 34 B M3V (red dwarf) 11.62* These stars are in the Alpha Centauri system.DESCRIBING AND MEASURING STARS
How do astronomers describe and organize the types of stars in the universe?Astronomers organize stars into types based primarily on their colors and surface temperatures. There are seven main categories of stars; they are called stellar spectral types, and each is identified by a letter—O, B, A, F, G, K, or M. Stars of Type O have the bluest colors, and the highest surface temperatures compared to other stars. On the other end, stars of Type M have the reddest colors and the lowest surface temperatures.Who helped establish the modern system of stellar spectral types?A number of astronomers, over many years, worked to create a scientifically sensible classification system for spectral types. The three astronomers who were probably most influential in establishing the OBAFGKM sequence were Williamina Flem ing (1857–1911), Antonia Maury (1866–1952), and Annie Jump Cannon (1863–1941). The order of the letters in the spectral sequence came about because there were originally more than seven types, and because the types were first identified in ways other than the colors of stars. To avoid confusion, these astronomers decided to move the order of the types around to make scientific sense, instead of creating brand-new types. Although unusual, the letter order stuck, and the system is still being used today—more than a century after its establishment.What is the spectral type of the Sun?Our Sun is a type G star, which means it is in the middle of the range when it comes to stellar colors and surface temperatures. To add more detailed information, astronomers often add an Arabic numeral and a Roman numeral after the spectral type letter for stars. In that case, the Sun’s spectral type is written G2V. The 2 means that the Sun is closer to a type F star than a type K star, and the V means that the Sun is not a giant star but rather a main-sequence star.
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