Mu Cephei also known as Herschel's
Garnet Star, is a
red supergiantRed supergiants are supergiant stars of spectral type K or M. They are the largest stars in the universe in terms of volume, although they are not the most massive...
starA star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
in the
constellationIn modern astronomy, a constellation is an internationally defined area of the celestial sphere. These areas are grouped around asterisms, patterns formed by prominent stars within apparent proximity to one another on Earth's night sky....
CepheusCepheus is a constellation in the northern sky. It is named after Cepheus, King of Aethiopia in Greek mythology. It was one of the 48 constellations listed by the 2nd century astronomer Ptolemy, and remains one of the 88 modern constellations.-Stars:...
. It is one of the largest and most
luminous stars known in the
Milky WayThe Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...
. It appears
garnetThe garnet group includes a group of minerals that have been used since the Bronze Age as gemstones and abrasives. The name "garnet" may come from either the Middle English word gernet meaning 'dark red', or the Latin granatus , possibly a reference to the Punica granatum , a plant with red seeds...
red and is given the
spectral classIn 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 M2Ia.
History
The deep red color of Mu Cephei was noted by
William HerschelSir Frederick William Herschel, KH, FRS, German: Friedrich Wilhelm Herschel was a German-born British astronomer, technical expert, and composer. Born in Hanover, Wilhelm first followed his father into the Military Band of Hanover, but emigrated to Britain at age 19...
, who described it as "a very fine deep garnet colour, such as the periodical star
ο CetiMira also known as Omicron Ceti , is a red giant star estimated 200-400 light years away in the constellation Cetus. Mira is a binary star, consisting of the red giant Mira A along with Mira B. Mira A is also an oscillating variable star and was the first non-supernova variable star discovered,...
," and it is thus commonly known as Herschel's "Garnet Star".
Giuseppe PiazziGiuseppe Piazzi was an Italian Catholic priest of the Theatine order, mathematician, and astronomer. He was born in Ponte in Valtellina, and died in Naples. He established an observatory at Palermo, now the Osservatorio Astronomico di Palermo – Giuseppe S...
called it
Garnet sidus in his
catalogueA star catalogue, or star catalog, is an astronomical catalogue that lists stars. In astronomy, many stars are referred to simply by catalogue numbers. There are a great many different star catalogues which have been produced for different purposes over the years, and this article covers only some...
. An alternative name,
Erakis, used in
Antonín BečvářAntonín Bečvář was a Czech astronomer who was active in Slovakia. He was born in Stará Boleslav. Among his chief achievements is the foundation of the Skalnaté Pleso Observatory and the discovery of the comet C/1947 F2 .Bečvář is particularly important for his star charts: he led the compilation...
's
star catalogueA star catalogue, or star catalog, is an astronomical catalogue that lists stars. In astronomy, many stars are referred to simply by catalogue numbers. There are a great many different star catalogues which have been produced for different purposes over the years, and this article covers only some...
is probably due to confusion with
Mu DraconisMu Draconis is a binary star with a combined magnitude of 4.92m located approximately 85 light years from the Solar System, near the head of the constellation Draco. The component stars are nearly identical yellow-white stars in a tight orbit...
, which was previously called
arˈraːqis in Arabic.
In 1848, English astronomer
John Russell HindJohn Russell Hind FRS was an English astronomer.- Life and work :John Russell Hind was born in 1823 in Nottingham, the son of lace manufacturer John Hind, and was educated at Nottingham High School...
discovered that it was variable. This variability was quickly confirmed by German astronomer
Friedrich Wilhelm ArgelanderFriedrich Wilhelm August Argelander was a German astronomer. He is known for his determinations of stellar brightnesses, positions, and distances.- Life and work :...
. Almost continual records of the star's variability have been maintained since 1881.
Properties
A very luminous red supergiant, Mu Cephei is one of the largest stars visible to the naked eye, and in the entire galaxy. It is best seen from the northern hemisphere from August to January.
The distance to Mu Cephei is not very well known. The
HipparcosHipparcos was a scientific mission of the European Space Agency , launched in 1989 and operated between 1989 and 1993. It was the first space experiment devoted to precision astrometry, the accurate measurement of the positions of celestial objects on the sky...
satellite was used to measure a parallax of milliarcseconds, which corresponds to an estimated distance of about 1,612
parsecThe parsec is a unit of length used in astronomy. It is about 3.26 light-years, or just under 31 trillion kilometres ....
s. However, this value is close to the margin of error. A determination of the distance based upon a size comparison with
BetelgeuseBetelgeuse, also known by its Bayer designation Alpha Orionis , is the eighth brightest star in the night sky and second brightest star in the constellation of Orion, outshining its neighbour Rigel only rarely...
gives an estimate of . A 2005 maximum likelihood estimate of the distance gives a value of .
The star is approximately 1,650 times larger than our Sun's
solar radius, and were it placed in the Sun's position, its radius would reach between the orbits of
JupiterJupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...
and
SaturnSaturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...
. Mu Cephei could fit almost 4.5 billion Suns into its volume. Only three known stars (
VY Canis MajorisVY Canis Majoris is the largest known star and also one of the most luminous. Located in the constellation Canis Major, it is a red hypergiant, between 1800 and 2100 solar radii, 8.4–9.8 astronomical units in radius, about 3.0 billion km or 1.9 billion miles in diameter, and about 1.5 kiloparsecs ...
,
VV CepheiVV Cephei, also known as HD 208816, is an eclipsing binary star system located in the constellation Cepheus, approximately 2,400 light years from Earth.Size, mass and luminosity estimates are all considerably uncertain due to insufficient knowledge of the Cephei star system: Professor Kaler writes...
and
V838 MonocerotisV838 Monocerotis is a red variable star in the constellation Monoceros about 20,000 light years from the Sun, and possibly one of the largest known stars. The previously unknown star was observed in early 2002 experiencing a major outburst. Originally believed to be a typical nova eruption, it...
) are believed to be larger. It is so large that it could fit 6.4 quadrillion Earths in it. If Earth were a
golf ballA golf ball is a ball designed to be used in the game of golf.Under the Rules of Golf, a golf ball weighs no more than 1.620 oz , has a diameter not less than 1.680 in , and performs within specified velocity, distance, and symmetry limits...
(about 1.7 in/4.3 cm), the diameter of Mu Cephei would be greater than the length of two
Golden Gate BridgeThe Golden Gate Bridge is a suspension bridge spanning the Golden Gate, the opening of the San Francisco Bay into the Pacific Ocean. As part of both U.S. Route 101 and California State Route 1, the structure links the city of San Francisco, on the northern tip of the San Francisco Peninsula, to...
s laid end-to-end (about 3.4 mi./5.5 km).
Mu Cephei is a
variable starA star is classified as variable if its apparent magnitude as seen from Earth changes over time, whether the changes are due to variations in the star's actual luminosity, or to variations in the amount of the star's light that is blocked from reaching Earth...
and the prototype of the class of the Mu Cephei variables. Its apparent brightness varies without recognizable pattern between magnitude +3.62 and +5 in a period of 2 to 2.5 years. Mu Cephei is 38,000 times brighter than the Sun, with an absolute visible magnitude of M
v = −7.0. Combining its absolute visible brightness, its infrared radiation and correcting for its interstellar extinction gives a luminosity of around 350,000 solar luminosities (bolometric magnitude about −9.1), making it one of the most luminous stars known.
Mu Cephei is nearing death. It has begun to
fuseNuclear fusion is the process by which two or more atomic nuclei join together, or "fuse", to form a single heavier nucleus. This is usually accompanied by the release or absorption of large quantities of energy...
heliumHelium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...
into
carbonCarbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
, whereas a
main sequenceThe main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Hertzsprung and Henry Norris Russell...
star fuses
hydrogenHydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
into helium. The helium-carbon cycle shows that Mu Cephei is in the last phase of its life and may explode as a
supernovaA supernova is a stellar explosion that is more energetic than a nova. It is pronounced with the plural supernovae or supernovas. Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months...
'soon' in astronomical terms, although this might not be for some millions of years. When a supergiant star becomes a supernova it is destroyed, leaving behind a vast gaseous cloud and a small, dense remnant, which for a star as massive as Mu Cephei may be a
black holeA black hole is a region of spacetime from which nothing, not even light, can escape. The theory of general relativity predicts that a sufficiently compact mass will deform spacetime to form a black hole. Around a black hole there is a mathematically defined surface called an event horizon that...
. Mu Cephei is currently an unstable
starA star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
, showing irregular variations in light output, temperature and size.
The
photosphereThe 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...
of Mu Cephei has an estimated temperature of . It may be surrounded by a shell extending out to a distance at least equal to a 0.33 times the star's radius with a temperature of . This outer shell appears to contain molecular gases such as
COCarbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...
,
H2OWater is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
and SiO.
Emissions from the star suggest the presence of a wide ring of dust and water with outer radius four times that of the star (i.e. 2,600 Solar radii) and inner boundary twice the radius of the star (1,300 Solar radii). Placed in the position of our
SunThe Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
, its disk would span between 5.5
astronomical unitAn astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....
s (within
JupiterJupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...
's orbital zone) and 11
astronomical unitAn astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....
s (beyond
SaturnSaturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...
's orbit).
The star is surrounded by a spherical shell of ejected material that extends outward to an angular distance of 6″ with an expansion velocity of . This indicates an age of about 2000–3000 years for the shell. Closer to the star, this material shows a pronounced asymmetry, which may be shaped as a torus. The star currently has a mass loss rate of a few times 10
−7 solar massThe solar mass , , is a standard unit of mass in astronomy, used to indicate the masses of other stars and galaxies...
es per year.
External links
<<< 3. VV CepheiVV Cephei, also known as HD 208816, is an eclipsing binary star system located in the constellation Cepheus, approximately 2,400 light years from Earth.Size, mass and luminosity estimates are all considerably uncertain due to insufficient knowledge of the Cephei star system: Professor Kaler writes...
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5. V838 Monocerotis V838 Monocerotis is a red variable star in the constellation Monoceros about 20,000 light years from the Sun, and possibly one of the largest known stars. The previously unknown star was observed in early 2002 experiencing a major outburst. Originally believed to be a typical nova eruption, it...
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