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What"s the most crucial thing to know about stars? your brightness (apparent magnitude) could top the list, however right behind would be the spectral types. Without its spectral type a star is a meaningless dot. Include a couple of letters and numbers prefer "G2V" or "B5IV-Vshnne" and the star unexpectedly gains personality and also character. To those who have the right to read that meaning, the spectral code tells in ~ a glance just what kind of object the star really is: the color, size, and luminosity contrasted to the Sun and stars that all various other types; that peculiarities, its history, and its future.

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Mass-producing stellar spectra. A slim prism was put in prior of the telescope to spread out starlight indigenous the Hyades cluster into tiny rainbows, or spectra. Spectra save on computer the "fingerprints" astronomers use to deduce myriad mainly properties. Click on this photo to see few of these stellar spectra up close.

University that Michigan department of Astronomy

The modern spectral category system is so successful that it has actually hardly been adjusted since 1943. That is based upon just two physical properties that imprint us on the spectrum that a star"s light: the star"s surface temperature and atmospheric pressure. These reveal an abundance of info that paints the star"s portrait and tells its life story.

The temperature set the star"s color and also determines its surface brightness: just how much light originates from each square meter the its surface. The atmospheric push depends ~ above the star"s surface gravity and therefore, roughly, ~ above its dimension — telling whether it is a giant, dwarf, or something in between. The size and also surface brightness consequently yield the star"s luminosity (its complete light output, or absolute magnitude) and also often that evolutionary condition (young, middle-aged, or nearing death). The luminosity (when contrasted to the star"s evident brightness in our sky) additionally gives a great idea of the star"s distance. Appended come the simple spectral type may be letters for chemical peculiarities, prolonged atmosphere, unusual surface activity, rapid rotation, or other special characteristics.

Every starwatcher demands to have a feel for spectral types.

Important things to know about Spectral varieties of Stars

Dissecting Starlight

Whether in a star"s setting or in a laboratory, absorption lines are developed when a constant rainbow of irradiate from a hot, dense object (top left) passes with a cooler, an ext rarefied gas (top center). Emissions lines, by contrast, come native an energized, rarefied gas such together in a neon light or a glowing nebula.

S&T / Steven Simpson
Our tale began in 1802, when the English experimenter william Wollaston happen a beam of sunlight with a thin slit and then with a prism. The slit provided a sharp, high-resolution watch of the acquainted rainbow spectrum, through no shade overlapping another. Once seen this way, Wollaston noticed, the Sun"s spectrum was significant by many narrow, black lines of assorted intensities. These dark lines remained at exactly the same locations in the vivid band from day to day and also year come year. Castle were later measured and cataloged by Josef von Fraunhofer, because that whom they are still dubbed "Fraunhofer lines."

Similar spectral lines showed up in laboratory experiments. Utilizing a slit and also prism, physicists found that when a solid, liquid, or dense gas is heated to glow, it emits a smooth spectrum that light v no lines: a continuum. A rarefied hot gas, top top the various other hand, glows only in details colors, or wavelengths: bright, narrow emission lines instead of a rainbow band. If a cooler sample of the exact same gas is placed in front of a glowing thing emitting a continuum, dark absorption lines appear at the wavelengths where the emissions lines would certainly be if the gas to be hot.


British amateur spectroscopist Maurice Gavin acquired this solar spectrum — a "rainbow" of sunlight v thin, dark absorb lines at many discrete wavelengths. Every chemical facet creates its very own unique collection of spectral lines.

By 1859 the situation was clear: we watch the Sun"s hot, reasonably dense surface through a cooler solar setting that imposes the dark lines.

Here, scientists realized, to be a method to carry the sunlight down right into the laboratory. Every element, every chemical compound, mirrors its own collection of spectral lines. They room as unique as fingerprints. They disclose not only which atoms and also molecules are existing but likewise many other physical conditions, beginning with temperature. And also when astronomers placed the exact same slit-and-prism an equipment (a spectroscope) on a telescope, they can even watch spectral lines in the light of stars.

It was the 19th century"s greatest huge breakthrough. Philosophers had cited the assembly of stars as an instance of something the was beyond all feasible human knowing. Currently finding the ingredient of the Sun and also stars was simply a matter of compare spectral lines checked out in a telescope with those watched in a laboratory. This wasn"t constantly simple, however it gave birth to modern astrophysics — the therapy of stars as physical objects to it is in studied and also understood, fairly than as mere clues of irradiate on the sky to be measured.

Ordering mainly Spectra


Early stellar spectra reflecting the distinctive absorb lines the astronomers offered (and usage still) to determine stars" compositions and temperatures. This collection of spectra originates from the original Henry Draper Catalogue.

Harvard university Observatory

The an initial great divide of stellar spectra was Angelo Secchi, a Jesuit monk in Rome. In the 1860s he examined the spectra of numerous stars visually in a telescope and classed lock into five main types, mostly named for bright examples. "Sirian" stars, because that instance, verified spectra choose Sirius"s, dominated by absorption lines that hydrogen atoms.

Today"s category scheme to be born at Harvard college Observatory. Beginning in 1886 under Edward C. Pickering, the observatory staff photographed and also classified hundreds of spectral types. Lock assigned them letter from A with Q, typically in alphabet order native the simplest-looking come the most complex. However soon a much more natural system came to be clear. By rearranging and merging classifications, Antonia C. Maury and also Annie J. Cannon discovered that they could fit practically all stars" spectra into one smooth, continuous sequence. The sequence suitable the stars" color temperatures, from the hottest, blue-white stars in ~ one finish to fairly cool, orange-red ones at the other.

But it was too late come reassign the letters. When the dust cleared, the rearranged succession ran O B A F G K M from warm to cool. Spectral varieties on the blue end were dubbed "early" and also those ~ above the red end "late." these terms room still offered today, though the not correct idea castle embody — the stars merely cool with period — has been useless for generations.

The sequence could be reduced even much more finely. Cannon subdivided every letter v numbers indigenous 0 come 9, so the a spectrum who appearance put it halfway in between standard G0 and also K0 stars was dubbed G5.

Using this scheme, Cannon led the category at Harvard of 325,300 spectra videotaped on wide-field photographs. The result Henry Draper Catalogue (HD) and also Henry Draper extension (HDE), published beginning in 1918, stay standard recommendations today.


Stellar spectra from warm to cool. A star"s temperature greatly determines i beg your pardon dark absorb lines appear in its spectrum. Although these spectra look prefer the actual thing, they"re synthetic — the calculation of a computer model by i get it Bell (University of Maryland) that topics a solar-type mixture that theoretical atom to different temperatures in ~ the pressures the prevail in the external layers of dwarf (main-sequence) stars.

Michael Briley, university of Wisconsin, Oshkosh

The time-honored mnemonic because that remembering the spectral sequence, developed by Henry Norris Russell when astronomy"s leadership was every male, is "Oh be A fine Girl Kiss Me." In 1995 Mercury magazine published a student"s rejoinder: "Only guys Accepting Feminism obtain Kissed Meaningfully."

The discovery in recent years of very dim, very red objects — the smallest, coolest red dwarf stars and also barely glow "brown dwarfs" — has actually led to the development of three brand-new spectral varieties past M. These room L, T, and Y, preferred from among the couple of remaining letter that could not have actually some other, confuse meaning. So now the complete spectral sequence operation O B A F G K M together T Y.

A few other spectral species don"t right the sequence however instead parallel it. Kind W or Wolf-Rayet stars are as hot and also blue as the hottest O stars however show strong emission lines, one of two people of nitrogen (WN), carbon and oxygen (WC), or neither (WR). Emissions lines indicate specifically large, special shroud of warm gas surrounding these stars. The W stars show up to have actually blown turn off their initial outer layers of hydrogen, exposing other materials beneath.

Some large stars in ~ the cool finish of the spectrum have an excess of carbon. These to be originally referred to as R and N however have been linked to type type C. "Carbon stars" can regularly be spotted in ~ a glance in a telescope by their deep red color. A bright instance in the fall sky is 19 Piscium (TX Piscium) in the Circlet that Pisces, spectral type C5. Their distinctive absorption bands (masses that overlapping spectral lines) due to the carbon compound C2, CN, and also CH darken or "blanket" the blue end of the spectrum. In various other words, a carbon star"s setting is a red filter. When seen in emission instead of absorption, these same spectral bands bright blue; the very same compounds the redden a carbon star in absorption offer comets their blue-green color in emission.

The rarely type-S stars are likewise red giants. Castle parallel kind M yet show solid bands the zirconium oxide and lanthanum oxide rather of an M star"s titanium oxide. We deserve to imagine that planets that S stars, bathed in chemically strange stellar winds, might be encrusted v gems that cubic zirconia.

Dwarfs and Giants

A Hertzsprung-Russell chart plots stars" spectral types against your intrinsic luminosities (absolute magnitudes). Many stars lie either on the hydrogen-fusing key sequence or top top the gigantic branch. A scattering the supergiants spans every spectral types. Click because that the full view, where many real stars space labeled.

Sky & Telescope / Source: James Kaler

Even in stars of the same spectral type, the absorption lines don"t constantly look alike. In some stars the lines space narrow and sharp; in others they are increased by various various effects. Chief among these effects is atmospheric pressure, which also changes the strongness ratios of details pressure-sensitive lines.

Astronomers easily realized that atmospheric pressure tells a star"s surface ar gravity and also therefore suggests its size. Small lines suggest an immense, bloated star v a weakly compressed atmosphere far native its facility of gravity. In the Henry Draper Catalogue, spectral species were prefixed v d because that dwarf, sg because that subgiant, g for giant, and also c because that supergiant.

You"ll quiet run throughout these letter from time come time, but start in 1941 castle were replaced by a more detailed scheme first published by wilhelm W. Morgan and also Philip C. Keenan. With just minor changes, this "MK" mechanism of spectral group remains the typical today. Stars are assigned come luminosity great by roman inn numerals: ns for supergiants (often subdivided into classes Ia-0, Ia, Iab, and Ib in stimulate of to decrease luminosity), II for bright giants, III for typical giants, IV for subgiants, V for dwarfs on the key sequence (defined in the illustration below), and also occasionally through for subdwarfs.

Different atoms and ions leaving their fingerprints top top starlight at different temperatures. Conditions must be fine-tuned to carry out a offered chemical "fingerprint." Hydrogen, for instance, just shows up strong in spectra near kind A — also though it renders up the mass of virtually all stars. And also hundreds the lines native "metals" (elements heavier than helium) conquer the spectra of cooler K-type stars, also though metals make up just a tiny fraction of those stars" masses.

Sky &Telescope

Thus a designation such as G2V, the Sun"s spectral type, speak both temperature and luminosity. When these amounts are plotted versus each other on a graph, the result (shown in ~ right) is called a Hertzsprung-Russell or H-R diagram. This has been a basic astrophysical device ever because it was invented about 1911.

Most stars conference in particular narrow regions of the H-R diagram follow to their masses and also ages. Stars come on what"s dubbed the main sequence soon after they are born, and this evolutionary monitor is whereby they spend most of your lives. Massive stars blaze brightly on the hot, blue finish of the key sequence. They burn up your nuclear fuel in only millions or 10s of countless years. Stars with reduced masses consist of the yellow, orange, and red dwarfs ~ above the lower-right component of the key sequence, whereby they continue to be for billions that years.

As a star starts to exhaust the hydrogen fuel in that core, it evolves far from the main sequence towards the upper right and becomes a red giant or supergiant. Stars that began with more than eight times the Sun"s mass climate evolve left and right through complicated loops on the H-R diagram together if in a frenzy to save up their power production. Climate they ultimately explode as supernovae. Less enormous giants evolve come the left and then down to come to be white dwarfs; this is the track the sun will trace with the H-R chart 8 billion year from now.

Odds and Ends

Spectra can reveal numerous other things around stars. Accordingly, small letter letters room sometimes added to the finish of a spectral type to show peculiarities. Here is a partial list:


Lastly, differences in between spectral types are far greater than distinctions in the actual chemical compositions that stars. An A star could seem come be virtually pure hydrogen, if a K star shows only trace evidence of hydrogen in a spectrum packed through lines that "metals" (the astronomer"s term because that all aspects other than hydrogen and helium). But A and also K stars room in truth made that the exact same stuff. Different atoms and also ions merely screen their spectral currently at various temperatures. Even carbon stars are made greatly of hydrogen and also helium. The true "abundances" of aspects can certainly be measured in a star. But it"s a hard job that comparing specific line strengths in a high-quality spectrum with the toughness predicted by atomic concept or measure up in the lab.

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For lot of the 20th century, the research of visible-light spectra practically was astronomy. In recent years the opening of nonvisible wavelengths and other exciting developments have distracted fist from this field. Nonetheless it continues to be the radical on which modern-day astronomy rests.