Tau⁴ Serpentis, Latinized from τ⁴ Serpentis, is a

variable Variable may refer to: * Variable (computer science), a symbolic name associated with a value and whose associated value may be changed * Variable (mathematics), a symbol that represents a quantity in a mathematical expression, as used in many ...

M-type

giant star A giant star is a star with substantially larger radius and luminosity than a main sequence, main-sequence (or ''dwarf'') star of the same effective temperature, surface temperature.Giant star, entry in ''Astronomy Encyclopedia'', ed. Patrick Moo ...

in the

constellation A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object. The origins of the e ...

Serpens Serpens ( grc, , , the Serpent) is a constellation in the northern celestial hemisphere. One of the 48 constellations listed by the 2nd-century astronomer Ptolemy, it remains one of the 88 modern constellations designated by the International ...

, approximately 710

light-years A light-year, alternatively spelled light year, is a large unit of length used to express astronomical distances and is equivalent to about 9.46 trillion kilometers (), or 5.88 trillion miles ().One trillion here is taken to be 1012 ...

from the

Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...

. With a

spectral classification In astronomy, stellar classification is the classification of stars based on their spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting the ...

M5IIIa, Tau⁴ Serpentis is a cool red giant star. The spectrum varies, and some sources classify it between M4IIIe and M6IIIe. Some of its spectral lines show an inverse

P Cygni profile P Cygni (34 Cygni) is a variable star in the constellation Cygnus. The designation "P" was originally assigned by Johann Bayer in ''Uranometria'' as a nova. Located about 5,300 light-years (1,560 parsecs) from Earth, it is a hypergiant ...

, where cold infalling gas on to the star creates

redshift In physics, a redshift is an increase in the wavelength, and corresponding decrease in the frequency and photon energy, of electromagnetic radiation (such as light). The opposite change, a decrease in wavelength and simultaneous increase in f ...

ed hydrogen absorption lines next to the normal emission lines. It is classified as a semiregular late-type variable, and its magnitude varies between +5.89 and +7.07 with a period of approximately 100 days. τ⁴ is unique among the stars with the Bayer designation τ Serpentis as being the only one with no HR catalog number.

References

{{DEFAULTSORT:Tau4 Serpentis Serpens (constellation) Semiregular variable stars M-type giants Serpentis, 17 Serpentis, Tau4 139216 076423 Durchmusterung objects Asymptotic-giant-branch stars