S-type star
Encyclopedia
A star with spectral type
S is a late-type giant star
(similar to class K5–M) whose spectrum displays bands from zirconium oxide in addition to titanium oxide
which is characteristically exhibited by K and M class giant stars. Other s-process
elements, for example yttrium oxide and technetium
, are also enhanced, clearly indicating neutron capture elements in the period 5
in the periodic system of chemical element
s. It may also have features from cyanogen
and lithium
. Most of these stars are long-period variables.
It has been proposed that most S stars are an intermediate stage in which asymptotic giant stars
(AGB stars) transform from ordinary M class giants to carbon star
s of class C-N. AGB stars are usually powered by fusion in a shell of hydrogen surrounding an inert core, but during "thermal pulses" helium shell fusion can come to dominate. Other S stars ("extrinsic" S stars) may be evolved, cooler versions of barium star
s, where the enhancements of carbon and s-process elements seen in the stars' spectra are relics of mass transfer between members of a binary star
system. In these systems, the presently-observed S star has not synthesized the excess carbon and s-process elements itself; that material was produced at an earlier time by its binary companion, which was at the time a carbon star. We are observing these systems long after the mass transfer event, when the companion has evolved to become a white dwarf
and is usually not directly visible.
S-type stars are generally redder than their K- or M-type counterparts of the same photospheric
temperature. The mira variable
Chi Cygni
is (when near maximum light) the sky's brightest S-type star, with a variable late type spectrum about S7 to S10, with features of zirconium, titanium and vanadium oxides, sometimes bordering to the intermediary MS type. S Ursae Majoris is another example. HR 1105
is an example of an "extrinsic" S star.
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...
S is a late-type giant star
Giant star
A giant star is a star with substantially larger radius and luminosity than a main sequence star of the same surface temperature. Typically, giant stars have radii between 10 and 100 solar radii and luminosities between 10 and 1,000 times that of the Sun. Stars still more luminous than giants are...
(similar to class K5–M) whose spectrum displays bands from zirconium oxide in addition to titanium oxide
Titanium oxide
Titanium oxide may refer to:* Titanium dioxide , TiO2* Titanium oxide , TiO, a non-stoichiometric oxide* Titanium oxide , Ti2O3* Ti3O* Ti2O* δ-TiOx...
which is characteristically exhibited by K and M class giant stars. Other s-process
S-process
The S-process or slow-neutron-capture-process is a nucleosynthesis process that occurs at relatively low neutron density and intermediate temperature conditions in stars. Under these conditions the rate of neutron capture by atomic nuclei is slow relative to the rate of radioactive beta-minus decay...
elements, for example yttrium oxide and technetium
Technetium
Technetium is the chemical element with atomic number 43 and symbol Tc. It is the lowest atomic number element without any stable isotopes; every form of it is radioactive. Nearly all technetium is produced synthetically and only minute amounts are found in nature...
, are also enhanced, clearly indicating neutron capture elements in the period 5
Period 5 element
A period 5 element is one of the chemical elements in the fifth row of the periodic table of the elements. The periodic table is laid out in rows to illustrate recurring trends in the chemical behaviour of the elements as their atomic number increases: a new row is begun when chemical behaviour...
in the periodic system of chemical element
Chemical element
A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Familiar examples of elements include carbon, oxygen, aluminum, iron, copper, gold, mercury, and lead.As of November 2011, 118 elements...
s. It may also have features from cyanogen
Cyanogen
Cyanogen is the chemical compound with the formula 2. It is a colorless, toxic gas with a pungent odor.The molecule is a pseudohalogen. Cyanogen molecules consist of two CN groups — analogous to diatomic halogen molecules, such as Cl2, but far less oxidizing...
and lithium
Lithium
Lithium is a soft, silver-white metal that belongs to the alkali metal group of chemical elements. It is represented by the symbol Li, and it has the atomic number 3. Under standard conditions it is the lightest metal and the least dense solid element. Like all alkali metals, lithium is highly...
. Most of these stars are long-period variables.
It has been proposed that most S stars are an intermediate stage in which asymptotic giant stars
Asymptotic Giant Branch
The asymptotic giant branch is the region of the Hertzsprung-Russell diagram populated by evolving low to medium-mass stars. This is a period of stellar evolution undertaken by all low to intermediate mass stars late in their lives....
(AGB stars) transform from ordinary M class giants to carbon star
Carbon star
A carbon star is a late-type star similar to a red giant whose atmosphere contains more carbon than oxygen; the two elements combine in the upper layers of the star, forming carbon monoxide, which consumes all the oxygen in the atmosphere, leaving carbon atoms free to form other carbon compounds,...
s of class C-N. AGB stars are usually powered by fusion in a shell of hydrogen surrounding an inert core, but during "thermal pulses" helium shell fusion can come to dominate. Other S stars ("extrinsic" S stars) may be evolved, cooler versions of barium star
Barium star
Barium stars are G to K class giants, whose spectra indicate an overabundance of s-process elements by the presence of singly ionized barium, Ba II, at λ 455.4nm. Barium stars also show enhanced spectral features of carbon, the bands of the molecules CH, CN and C2...
s, where the enhancements of carbon and s-process elements seen in the stars' spectra are relics of mass transfer between members of a binary star
Binary star
A binary star is a star system consisting of two stars orbiting around their common center of mass. The brighter star is called the primary and the other is its companion star, comes, or secondary...
system. In these systems, the presently-observed S star has not synthesized the excess carbon and s-process elements itself; that material was produced at an earlier time by its binary companion, which was at the time a carbon star. We are observing these systems long after the mass transfer event, when the companion has evolved to become a white dwarf
White dwarf
A white dwarf, also called a degenerate dwarf, is a small star composed mostly of electron-degenerate matter. They are very dense; a white dwarf's mass is comparable to that of the Sun and its volume is comparable to that of the Earth. Its faint luminosity comes from the emission of stored...
and is usually not directly visible.
S-type stars are generally redder than their K- or M-type counterparts of the same photospheric
Photosphere
The photosphere of an astronomical object is the region from which externally received light originates. The term itself is derived from Ancient Greek roots, φῶς, φωτός/phos, photos meaning "light" and σφαῖρα/sphaira meaning "sphere", in reference to the fact that it is a spheric surface perceived...
temperature. The mira variable
Mira variable
Mira variables , named after the star Mira, are a class of pulsating variable stars characterized by very red colors, pulsation periods longer than 100 days, and light amplitudes greater than one magnitude in infrared and 2.5 magnitude in visual...
Chi Cygni
Chi Cygni
Chi Cygni is a variable star of the Mira type in the constellation Cygnus.Chi Cygni shows one of the largest variations in magnitude known. Typically it brightens and fades from 5th to 13th magnitude. The average period of this brightness fluctuation is 407 days. Observed extremes were 3.3 and...
is (when near maximum light) the sky's brightest S-type star, with a variable late type spectrum about S7 to S10, with features of zirconium, titanium and vanadium oxides, sometimes bordering to the intermediary MS type. S Ursae Majoris is another example. HR 1105
BD Camelopardalis
BD Camelopardalis is an S star and symbiotic star in the constellation Camelopardalis. It was recognized as a spectroscopic binary star in 1922, and its orbital solution published in 1984; it has a 596-day orbital period...
is an example of an "extrinsic" S star.