Supergiant star | Wikipedia audio article


Supergiants are among the most massive and
most luminous stars. Supergiant stars occupy the top region of
the Hertzsprung–Russell diagram with absolute visual magnitudes between about −3 and −8. The temperature range of supergiant stars
spans from about 3,450 K to over 20,000 K.==Definition==
The title supergiant, as applied to a star, does not have a single concrete definition. The term giant star was first coined by Hertzsprung
when it became apparent that the majority of stars fell into two distinct regions of
the Hertzsprung–Russell diagram. One region contained larger and more luminous
stars of spectral types A to M and received the name giant. Subsequently, as they lacked any measurable
parallax, it became apparent that some of these stars were significantly larger and
more luminous than the bulk, and the term super-giant arose, quickly adopted as supergiant.===Spectral luminosity class===Supergiant stars can be identified on the
basis of their spectra, with distinctive lines sensitive to high luminosity and low surface
gravity. In 1897, Antonia C. Maury had divided stars
based on the widths of their spectral lines, with her class “c” identifying stars with
the narrowest lines. Although it was not known at the time, these
were the most luminous stars. In 1943 Morgan and Keenan formalised the definition
of spectral luminosity classes, with class I referring to supergiant stars. The same system of MK luminosity classes is
still used today, with refinements based on the increased resolution of modern spectra. Supergiants occur in every spectral class
from young blue class O supergiants to highly evolved red class M supergiants. Because they are enlarged compared to main-sequence
and giant stars of the same spectral type, they have lower surface gravities, and changes
can be observed in their line profiles. Supergiants are also evolved stars with higher
levels of heavy elements than main-sequence stars. This is the basis of the MK luminosity system
which assigns stars to luminosity classes purely from observing their spectra. In addition to the line changes due to low
surface gravity and fusion products, the most luminous stars have high mass-loss rates and
resulting clouds of expelled circumstellar materials which can produce emission lines,
P Cygni profiles, or forbidden lines. The MK system assigns stars to luminosity
classes: Ib for supergiants; Ia for luminous supergiants; and 0 (zero) or Ia+ for hypergiants. In reality there is much more of a continuum
than well defined bands for these classifications, and classifications such as Iab are used for
intermediate luminosity supergiants. Supergiant spectra are frequently annotated
to indicate spectral peculiarities, for example B2 Iae or F5 Ipec.===Evolutionary supergiants===
Supergiants can also be defined as a specific phase in the evolutionary history of certain
stars. Stars with initial masses above 8-10 M☉
quickly and smoothly initiate helium core fusion after they have exhausted their hydrogen,
and continue fusing heavier elements after helium exhaustion until they develop an iron
core, at which point the core collapses to produce a Type 2 supernova. Once these massive stars leave the main sequence,
their atmospheres inflate, and they are described as supergiants. Stars initially under 10 M☉ will never form
an iron core and in evolutionary terms do not become supergiants, although they can
reach luminosities thousands of times the sun’s. They cannot fuse carbon and heavier elements
after the helium is exhausted, so they eventually just lose their outer layers, leaving the
core of a white dwarf. The phase where these stars have both hydrogen
and helium burning shells is referred to as the asymptotic giant branch (AGB), as stars
gradually become more and more luminous class M stars. Stars of 8-10 M☉ may fuse sufficient carbon
on the AGB to produce an oxygen-neon core and an electron-capture supernova, but astrophysicists
categorise these as super-AGB stars rather than supergiants.===Categorisation of evolved stars===
There are several categories of evolved stars which are not supergiants in evolutionary
terms but may show supergiant spectral features or have luminosities comparable to supergiants. Asymptotic-giant-branch (AGB) and post-AGB
stars are highly evolved lower-mass red giants with luminosities that can be comparable to
more massive red supergiants, but because of their low mass, being in a different stage
of development (helium shell burning), and their lives ending in a different way (planetary
nebula and white dwarf rather than supernova), astrophysicists prefer to keep them separate. The dividing line becomes blurred at around
7–10 M☉ (or as high as 12 M☉ in some models) where stars start to undergo limited
fusion of elements heavier than helium. Specialists studying these stars often refer
to them as super AGB stars, since they have many properties in common with AGB such as
thermal pulsing. Others describe them as low-mass supergiants
since they start to burn elements heavier than helium and can explode as supernovae. Many post-AGB stars receive spectral types
with supergiant luminosity classes. For example, RV Tauri has an Ia (bright supergiant)
luminosity class despite being less massive than the sun. Some AGB stars also receive a supergiant luminosity
class, most notably W Virginis variables such as W Virginis itself, stars that are executing
a blue loop triggered by thermal pulsing. A very small number of Mira variables and
other late AGB stars have supergiant luminosity classes, for example α Herculis. Classical Cepheid variables typically have
supergiant luminosity classes, although only the most luminous and massive will actually
go on to develop an iron core. The majority of them are intermediate mass
stars fusing helium in their cores and will eventually transition to the asymptotic giant
branch. δ Cephei itself is an example with a luminosity
of 2,000 L☉ and a mass of 4.5 M☉. Wolf–Rayet stars are also high-mass luminous
evolved stars, hotter than most supergiants and smaller, visually less bright but often
more luminous because of their high temperatures. They have spectra dominated by helium and
other heavier elements, usually showing little or no hydrogen, which is a clue to their nature
as stars even more evolved than supergiants. Just as the AGB stars occur in almost the
same region of the HR diagram as red supergiants, Wolf–Rayet stars can occur in the same region
of the HR diagram as the hottest blue supergiants and main-sequence stars. The most massive and luminous main-sequence
stars are almost indistinguishable from the supergiants they quickly evolve into. They have almost identical temperatures and
very similar luminosities, and only the most detailed analyses can distinguish the spectral
features that show they have evolved away from the narrow early O-type main-sequence
to the nearby area of early O-type supergiants. Such early O-type supergiants share many features
with WNLh Wolf–Rayet stars and are sometimes designated as slash stars, intermediates between
the two types. Luminous blue variables (LBVs) stars occur
in the same region of the HR diagram as blue supergiants but are generally classified separately. They are evolved, expanded, massive, and luminous
stars, often hypergiants, but they have very specific spectral variability, which defies
the assignment of a standard spectral type. LBVs observed only at a particular time or
over a period of time when they are stable, may simply be designated as hot supergiants
or as candidate LBVs due to their luminosity. Hypergiants are frequently treated as a different
category of star from supergiants, although in all important respects they are just a
more luminous category of supergiant. They are evolved, expanded, massive and luminous
stars like supergiants, but at the most massive and luminous extreme, and with particular
additional properties of undergoing high mass-loss due to their extreme luminosities and instability. Generally only the more evolved supergiants
show hypergiant properties, since their instability increases after high mass-loss and some increase
in luminosity. Some B[e] stars are supergiants although other
B[e] stars are clearly not. Some researchers distinguish the B[e] objects
as separate from supergiants, while researchers prefer to define massive evolved B[e] stars
as a subgroup of supergiants. The latter has become more common with the
understanding that the B[e] phenomenon arises separately in a number of distinct types of
stars, including some that are clearly just a phase in the life of supergiants.==Properties==Supergiants have masses from 8 to 12 times
the Sun (M☉) upwards, and luminosities from about 1,000 to over a million times the Sun
(L☉). They vary greatly in radius, usually from
30 to 500, or even in excess of 1,000 solar radii (R☉). They are massive enough to begin helium-core
burning gently before the core becomes degenerate, without a flash and without the strong dredge-ups
that lower-mass stars experience. They go on to successively ignite heavier
elements, usually all the way to iron. Also because of their high masses, they are
destined to explode as supernovae. The Stefan-Boltzmann law dictates that the
relatively cool surfaces of red supergiants radiate much less energy per unit area than
those of blue supergiants; thus, for a given luminosity, red supergiants are larger than
their blue counterparts. Radiation pressure limits the largest cool
supergiants to around 1,500–2,600 R☉ and the most massive hot supergiants to around
a million L☉ (Mbol around −10). Stars near and occasionally beyond these limits
become unstable, pulsate, and experience rapid mass loss.===Surface gravity===
The supergiant luminosity class is assigned on the basis of spectral features that are
largely a measure of surface gravity, although such stars are also affected by other properties
such as microturbulence. Supergiants typically have surface gravities
of around log(g) 2.0 cgs and lower, although bright giants (luminosity class II) have statistically
very similar surface gravities to normal Ib supergiants. Cool luminous supergiants have lower surface
gravities, with the most luminous (and unstable) stars having log(g) around zero. Hotter supergiants, even the most luminous,
have surface gravities around one, due to their higher masses and smaller radii.===Temperature===
There are supergiant stars at all of the main spectral classes and across the whole range
of temperatures from mid-M class stars at around 3,000–3,450 K to the hottest O class
stars over 40,000 K. Supergiants are generally not found cooler than mid-M class. This is expected theoretically since they
would be catastrophically unstable; however, there are potential exceptions among extreme
stars such as VX Sagittarii.Although supergiants exist in every class from O to M, the majority
are spectral type B, more than at all other spectral classes combined. A much smaller grouping consists of very low-luminosity
G-type supergiants, intermediate mass stars burning helium in their cores before reaching
the asymptotic giant branch. A distinct grouping is made up of high-luminosity
supergiants at early B (B0-2) and very late O (O9.5), more common even than main sequence
stars of those spectral types.The relative numbers of blue, yellow, and red supergiants
is an indicator of the speed of stellar evolution and is used as a powerful test of models of
the evolution of massive stars.===Luminosity===
The supergiants lie more or less on a horizontal band occupying the entire upper portion of
the HR diagram, but there are some variations at different spectral types. These variations are due partly to different
methods for assigning luminosity classes at different spectral types, and partly to actual
physical differences in the stars. The bolometric luminosity of a star reflects
its total output of electromagnetic radiation at all wavelengths. For very hot and very cool stars, the bolometric
luminosity is dramatically higher than the visual luminosity, sometimes several magnitudes
or a factor of five or more. This bolometric correction is approximately
one magnitude for mid B, late K, and early M stars, increasing to three magnitudes (a
factor of 15) for O and mid M stars. All supergiants are larger and more luminous
than main sequence stars of the same temperature. This means that hot supergiants lie on a relatively
narrow band above bright main sequence stars. A B0 main sequence star has an absolute magnitude
of about −5, meaning that all B0 supergiants are significantly brighter than absolute magnitude
−5. Bolometric luminosities for even the faintest
blue supergiants are tens of thousands of times the sun (L☉). The brightest can be over a million L☉ and
are often unstable such as α Cygni variables and luminous blue variables. The very hottest supergiants with early O
spectral types occur in an extremely narrow range of luminosities above the highly luminous
early O main sequence and giant stars. They are not classified separately into normal
(Ib) and luminous (Ia) supergiants, although they commonly have other spectral type modifiers
such as “f” for nitrogen and helium emission (e.g. O2 If for HD 93129A).Yellow supergiants
can be considerably fainter than absolute magnitude −5, with some examples around
−2 (e.g. 14 Persei). With bolometric corrections around zero, they
may only be a few hundred times the luminosity of the sun. These are not massive stars, though; instead,
they are stars of intermediate mass that have particularly low surface gravities, often
due to instability such as Cepheid pulsations. These intermediate mass stars’ being classified
as supergiants during a relatively long-lasting phase of their evolution account for the large
number of low luminosity yellow supergiants. The most luminous yellow stars, the yellow
hypergiants, are amongst the visually brightest stars, with absolute magnitudes around −9,
although still less than a million L☉. There is a strong upper limit to the luminosity
of red supergiants at around half a million L☉. Stars that would be brighter than this shed
their outer layers so rapidly that they remain hot supergiants after they leave the main
sequence. The majority of red supergiants were 10-15
M☉ main sequence stars and now have luminosities below 100,000 L☉, and there are very few
bright supergiant (Ia) M class stars. The least luminous stars classified as red
supergiants are some of the brightest AGB and post-AGB stars, highly expanded and unstable
low mass stars such as the RV Tauri variables. The majority of AGB stars are given giant
or bright giant luminosity classes, but particularly unstable stars such as W Virginis variables
may be given a supergiant classification (e.g. W Virginis itself). The faintest red supergiants are around absolute
magnitude −3.===Variability===While most supergiants such as Alpha Cygni
variables, semiregular variables, and irregular variables show some degree of photometric
variability, certain types of variables amongst the supergiants are well defined. The instability strip crosses the region of
supergiants, and specifically many yellow supergiants are Classical Cepheid variables. The same region of instability extends to
include the even more luminous yellow hypergiants, an extremely rare and short-lived class of
luminous supergiant. Many R Coronae Borealis variables, although
not all, are yellow supergiants, but this variability is due to their unusual chemical
composition rather than a physical instability. Further types of variable stars such as RV
Tauri variables and PV Telescopii variables are often described as supergiants. RV Tau stars are frequently assigned spectral
types with a supergiant luminosity class on account of their low surface gravity, and
they are amongst the most luminous of the AGB and post-AGB stars, having masses similar
to the sun; likewise, the even rarer PV Tel variables are often classified as supergiants,
but have lower luminosities than supergiants and peculiar B[e] spectra extremely deficient
in hydrogen. Possibly they are also post-AGB objects or
“born-again” AGB stars. The LBVs are variable with multiple semi-regular
periods and less predictable eruptions and giant outbursts. They are usually supergiants or hypergiants,
occasionally with Wolf-Rayet spectra—extremely luminous, massive, evolved stars with expanded
outer layers, but they are so distinctive and unusual that they are often treated as
a separate category without being referred to as supergiants or given a supergiant spectral
type. Often their spectral type will be given just
as “LBV” because they have peculiar and highly variable spectral features, with temperatures
varying from about 8,000 K in outburst up to 20,000 K or more when “quiescent.”===Chemical abundances===
The abundance of various elements at the surface of supergiants is different from less luminous
stars. Supergiants are evolved stars and may have
undergone convection of fusion products to the surface. Cool supergiants show enhanced helium and
nitrogen at the surface due to convection of these fusion products to the surface during
the main sequence of very massive stars, to dredge-ups during shell burning, and to the
loss of the outer layers of the star. Helium is formed in the core and shell by
fusion of hydrogen and nitrogen which accumulates relative to carbon and oxygen during CNO cycle
fusion. At the same time, carbon and oxygen abundances
are reduced. Red supergiants can be distinguished from
luminous but less massive AGB stars by unusual chemicals at the surface, enhancement of carbon
from deep third dredge-ups, as well as carbon-13, lithium and s-process elements. Late-phase AGB stars can become highly oxygen-enriched,
producing OH masers.Hotter supergiants show differing levels of nitrogen enrichment. This may be due to different levels of mixing
on the main sequence due to rotation or because some blue supergiants are newly evolved from
the main sequence while others have previously been through a red supergiant phase. Post-red supergiant stars have a generally
higher level of nitrogen relative to carbon due to convection of CNO-processed material
to the surface and the complete loss of the outer layers. Surface enhancement of helium is also stronger
in post-red supergiants, representing more than a third of the atmosphere.==Evolution==O type main-sequence stars and the most massive
of the B type blue-white stars become supergiants. Due to their extreme masses, they have short
lifespans, between 30 million years and a few hundred thousand years. They are mainly observed in young galactic
structures such as open clusters, the arms of spiral galaxies, and in irregular galaxies. They are less abundant in spiral galaxy bulges
and are rarely observed in elliptical galaxies, or globular clusters, which are composed mainly
of old stars. Supergiants develop when massive main-sequence
stars run out of hydrogen in their cores, at which point they start to expand, just
like lower-mass stars. Unlike lower-mass stars, however, they begin
to fuse helium in the core smoothly and not long after exhausting their hydrogen. This means that they do not increase their
luminosity as dramatically as lower-mass stars, and they progress nearly horizontally across
the HR diagram to become red supergiants. Also unlike lower-mass stars, red supergiants
are massive enough to fuse elements heavier than helium, so they do not puff off their
atmospheres as planetary nebulae after a period of hydrogen and helium shell burning; instead,
they continue to burn heavier elements in their cores until they collapse. They cannot lose enough mass to form a white
dwarf, so they will leave behind a neutron star or black hole remnant, usually after
a core collapse supernova explosion. Stars more massive than about 40 M☉ cannot
expand into a red supergiant. Because they burn too quickly and lose their
outer layers too quickly, they reach the blue supergiant stage, or perhaps yellow hypergiant,
before returning to become hotter stars. The most massive stars, above about 100 M☉,
hardly move at all from their position as O main-sequence stars. These convect so efficiently that they mix
hydrogen from the surface right down to the core. They continue to fuse hydrogen until it is
almost entirely depleted throughout the star, then rapidly evolve through a series of stages
of similarly hot and luminous stars: supergiants, slash stars, WNh-, WN-, and possibly WC- or
WO-type stars. They are expected to explode as supernovae,
but it is not clear how far they evolve before this happens. The existence of these supergiants still burning
hydrogen in their cores may necessitate a slightly more complex definition of supergiant:
a massive star with increased size and luminosity due to fusion products building up, but still
with some hydrogen remaining.The first stars in the universe are thought to have been considerably
brighter and more massive than the stars in the modern universe. Part of the theorized population III of stars,
their existence is necessary to explain observations of elements other than hydrogen and helium
in quasars. Possibly larger and more luminous than any
supergiant known today, their structure was quite different, with reduced convection and
less mass loss. Their very short lives are likely to have
ended in violent photodisintegration or pair instability supernovae.==Supernova progenitors==Most type II supernova progenitors are thought
to be red supergiants, while the less common type Ib/c supernovae are produced by hotter
Wolf–Rayet stars that have completely lost more of their hydrogen atmosphere. Almost by definition, supergiants are destined
to end their lives violently. Stars large enough to start fusing elements
heavier than helium do not seem to have any way to lose enough mass to avoid catastrophic
core collapse, although some may collapse, almost without trace, into their own central
black holes. The simple “onion” models showing red supergiants
inevitably developing to an iron core and then exploding have been shown, however, to
be too simplistic. The progenitor for the unusual type II Supernova
1987A was a blue supergiant, thought to have already passed through the red supergiant
phase of its life, and this is now known to be far from an exceptional situation. Much research is now focused on how blue supergiants
can explode as a supernova and when red supergiants can survive to become hotter supergiants again.==Well known examples==Supergiants are rare and short-lived stars,
but their high luminosity means that there are many naked-eye examples, including some
of the brightest stars in the sky. Rigel, the brightest star in the constellation
Orion is a typical blue-white supergiant; Deneb is the brightest star in Cygnus, a white
supergiant; Delta Cephei is the famous prototype Cepheid variable, a yellow supergiant; and
Betelgeuse, Antares and UY Scuti are red supergiants. μ Cephei is one of the reddest stars visible
to the naked eye and one of the largest in the galaxy. Rho Cassiopeiae, a variable, yellow hypergiant,
is one of the most luminous naked-eye stars.==See also==Hypergiant
List of stars with resolved images Yellow supergiant
Planetary Nebula

Add a Comment

Your email address will not be published. Required fields are marked *