Most stars lie either on the hydrogen-fusing main sequence or on the giant branch. A scattering of supergiants spans all spectral types. Click for the full view, where many real stars are labeled. Even in stars of the same spectral type, the absorption lines don't always look alike. In some stars the lines are narrow and sharp; in others they are broadened by various different effects. Chief among these effects is atmospheric pressure, which also changes the intensity ratios of certain pressure-sensitive lines.
Astronomers quickly realized that atmospheric pressure tells a star's surface gravity and therefore suggests its size. Narrow lines indicate an immense, bloated star with a weakly compressed atmosphere far from its center of gravity. In the Henry Draper Catalogue, spectral types were prefixed with d for dwarf, sg for subgiant, g for giant, and c for supergiant. You'll still run across these letters from time to time, but beginning in they were replaced by a more detailed scheme first published by William W.
Morgan and Philip C. With only minor changes, this "MK" system of spectral classification remains the standard today. Stars are assigned to luminosity classes by Roman numerals: I for supergiants often subdivided into classes Ia-0, Ia, Iab, and Ib in order of decreasing luminosity , II for bright giants, III for normal giants, IV for subgiants, V for dwarfs on the main sequence defined in the illustration below , and occasionally VI for subdwarfs.
Different atoms and ions leave their fingerprints on starlight at different temperatures. Conditions must be fine-tuned to bring out a given chemical 'fingerprint. And hundreds of lines from 'metals' elements heavier than helium dominate the spectra of cooler K -type stars, even though metals make up just a tiny fraction of those stars' masses.
Thus a designation such as G 2V, the Sun's spectral type, tells both temperature and luminosity. When these quantities are plotted against each other on a graph, the result shown at right is called a Hertzsprung-Russell or H-R diagram. This has been a fundamental astrophysical tool ever since it was invented around Most stars gather in certain narrow regions of the H-R diagram according to their masses and ages.
Stars arrive on what's called the main sequence soon after they are born, and this evolutionary track is where they spend most of their lives. Massive stars blaze brightly on the hot, blue end of the main sequence. They burn up their nuclear fuel in only millions or tens of millions of years.
Stars with lower masses comprise the yellow, orange, and red dwarfs on the lower-right part of the main sequence, where they remain for billions of years. As a star begins to exhaust the hydrogen fuel in its core, it evolves away from the main sequence toward the upper right and becomes a red giant or supergiant.
Stars that began with more than eight times the Sun's mass then evolve left and right through complicated loops on the H-R diagram as if in a frenzy to keep up their energy production.
Then they finally explode as supernovae. Less massive giants evolve to the left and then down to become white dwarfs; this is the track the Sun will trace through the H-R diagram 8 billion years from now. Spectra can reveal many other things about stars. Accordingly, lowercase letters are sometimes added to the end of a spectral type to indicate peculiarities.
Here is a partial list:. Lastly, differences between spectral types are far greater than differences in the actual chemical compositions of stars. An A star might seem to be almost pure hydrogen, while a K star shows only trace evidence of hydrogen in a spectrum packed with lines of "metals" the astronomer's term for all elements other than hydrogen and helium.
Many people use a memory device or mnemonic to help them. Here is a common example but feel free to make up your own. The basic system of a letter to denote spectral class is further refined by adding a number from 0 to 9 following it. Each spectral class is thus broken down into ten subdivisions so that, for example, an F2 star is hotter than an F7 star. The basic characteristics of each spectral class are summarised in the following table. The four columns on the right of the table provide comparison of a star's mass, radius and luminosity power output with respect to the Sun and the main sequence lifespan for a star of that spectral class.
These factors are discussed in more detail in later sections of the site. One problem facing early attempts at classifying stellar spectra was the fact that two spectra could have the same lines present, indicating that the stars had the same effective temperature, but the lines in one star's spectrum were broader than in the other.
When the star's were plotted on an HR diagram it also became apparent that two stars could have the same effective temperature hence also colour and spectral class but vary enormously in luminosity and thus absolute magnitude. To account for this a second classification scheme of Luminosity Class was added to the original concept of Spectral Class. A simplified version of the MK system of luminosity classes is shown in the table below.
Skip to main content. Australia Telescope National Facility. Accessibility menu. Interface Adjust the interface to make it easier to use for different conditions. Interface Size. The presence of these carbon compounds will tend to absorb the blue portion of the spectrum, giving R and N type giants a distinctive red colour. R stars are those with hotter surfaces which otherwise more closely resemble K type stars.
S type stars have cooler surfaces and more closely resemble M stars. S type stars S type stars have photospheres with enhanced abundances of s -process elements. These are isotopes of elements which have been formed from the capture of a free neutron changing the isotope of the element followed by a beta decay a neutron decays into a proton and an electron, thus changing the element to one with a higher atomic number and an isotope with one less neutron.
The s -process is one of the mechanisms by which elements with atomic numbers higher than 56 Iron can be made. The s stands for slow. By way of contrast, its partner r -process for rapid takes place when there are a sufficient supply of free neutrons for additional neutrons to be acquired in the atomic nucleus before the captured neutron has a chance to beta decay.
Instead of or in addition to the usual lines of titanium, scandium, and vanadium oxides characteristic of M type giants, S type stars show heavier elements such as zirconium, yttrium, and barium.
A significant fraction of all S type stars are variable. Peculiar Stars Wolf-Rayet Stars WR Wolf-Rayet stars are similar to O type stars, but have broad emission lines of hydrogen and ionized helium, carbon, nitrogen, and oxygen with very few absorption lines. Current theory holds that these stars exist in binary systems where the companion star has stripped away the Wolf-Rayet star's outer layers. Thus the spectra observed is from the exposed stellar interior rather than the normal surface material.
The broadness of the lines also indicates that the material observed may be from high velocity gases streaming away from the star, with the range of velocities smearing out the observed lines.
T Tauri Stars T T Tauri stars are very young stars, typically found in bright or dark interstellar clouds from which they have presumably just formed. Typically T Tauri stars are irregular variable stars, with unpredictable changes in their brightness. Their spectra contains bright emission lines and a number of "forbidden lines" so-called because they are not observable in typical laboratory conditions which indicate extremely low densities. Spectral lines also show Doppler shifts with respect to the rest velocity of the star, indicating that matter is streaming out from them.
Singly ionized helium lines either in emission or absorption. Solar-type spectra. Composite spectrum; two spectral types are blended, indicating that the star is an unresolved binary. Abnormally strong "metals" elements other than hydrogen and helium for a star of a given spectral type; usually applied to A stars.
As an example of the full classification of a star, let us consider the Sun. In the modern Harvard classification scheme , our Sun is a G2V star. See also: Hertzsprung-Russell Diagram.
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