A spiral galaxy is a certain kind of galaxy originally described by Edwin
Hubble in his 1936 work The Realm of the Nebulae and, as such, forms part of the
Hubble sequence. Spiral galaxies consist of a flat, rotating disc containing
stars, gas and dust, and a central concentration of stars known as the
bulge. These are surrounded by a much fainter halo of stars, many of which
reside in globular clusters. Spiral galaxies are named for the spiral
structures that extend from the center into the galactic disc. The spiral arms
are sites of ongoing star formation and are brighter than the surrounding disc
because of the young, hot OB stars that inhabit them.
Roughly two-thirds of all spirals are observed to have an additional component
in the form of a bar-like structure, extending from the central bulge, at the
ends of which the spiral arms begin. The proportion of barred spirals relative to
their barless cousins has changed over the history of the Universe, with only
about 10% containing bars about 8 billion years ago, to roughly a quarter
2.5 billion years ago, until present, where over two-thirds of the galaxies in
the visible universe have bars. Our own Milky Way has recently been
confirmed to be a barred spiral, although the bar itself is difficult to
observe from our position within the galactic disc. The most convincing
evidence for its existence comes from a recent survey, performed by the Spitzer
Space Telescope, of stars in the galactic center.
Together with irregular galaxies, spiral galaxies make up approximately 60% of
galaxies in the local Universe. They are mostly found in low-density regions and
are rare in the centers of galaxy clusters.
Structure Spiral galaxies consist of five distinct
components: A flat, rotating disc of stars and
interstellar matter A central stellar bulge of mainly older
stars, which resembles an elliptical galaxy
A near-spherical halo of stars, including many in globular clusters
A supermassive black hole at the very center of the central bulge
A near-spherical dark matter halo The relative importance, in terms of
mass, brightness and size, of the different components varies from galaxy
to galaxy. = Spiral arms =
Spiral arms are regions of stars that extend from the center of spiral and
barred spiral galaxies. These long, thin regions resemble a spiral and thus give
spiral galaxies their name. Naturally, different classifications of spiral
galaxies have distinct arm-structures. Sc and SBc galaxies, for instance, have
very "loose" arms, whereas Sa and SBa galaxies have tightly wrapped arms.
Either way, spiral arms contain many young, blue stars, which make the arms
so bright. = Galactic bulge =
A bulge is a huge, tightly packed group of stars. The term commonly refers to
the central group of stars found in most spiral galaxies.
Using the Hubble classification, the bulge of Sa galaxies is usually composed
of Population II stars, that are old, red stars with low metal content.
Further, the bulge of Sa and SBa galaxies tends to be large. In contrast,
the bulges of Sc and SBc galaxies are much smaller and are composed of young,
blue Population I stars. Some bulges have similar properties to those of
elliptical galaxies; others simply appear as higher density centers of
disks, with properties similar to disk galaxies.
Many bulges are thought to host a supermassive black hole at their
centers. Such black holes have never been directly observed, but many
indirect proofs exist. In our own galaxy, for instance, the object called
Sagittarius A* is believed to be a supermassive black hole. There is a
tight correlation between the mass of the black hole and the velocity
dispersion of the stars in the bulge, the M-sigma relation.
= Galactic spheroid = The bulk of the stars in a spiral galaxy
are located either close to a single plane in more or less conventional
circular orbits around the center of the galaxy, or in a spheroidal galactic
bulge around the galactic core. However, some stars inhabit a spheroidal
halo or galactic spheroid, a type of galactic halo. The orbital behaviour of
these stars is disputed, but they may describe retrograde and/or highly
inclined orbits, or not move in regular orbits at all. Halo stars may be
acquired from small galaxies which fall into and merge with the spiral
galaxy—for example, the Sagittarius Dwarf Spheroidal Galaxy is in the
process of merging with the Milky Way and observations show that some stars in
the halo of the Milky Way have been acquired from it.
Unlike the galactic disc, the halo seems to be free of dust, and in further
contrast, stars in the galactic halo are of Population II, much older and with
much lower metallicity than their Population I cousins in the galactic
disc. The galactic halo also contains many globular clusters.
The motion of halo stars does bring them through the disc on occasion, and a
number of small red dwarf stars close to the Sun are thought to belong to the
galactic halo, for example Kapteyn's Star and Groombridge 1830. Due to their
irregular movement around the center of the galaxy—if they do so at all—these
stars often display unusually high proper motion.
In 2013 and 2014 papers were published presenting evidence that the spheroid is
actually a planar structure in about half of all galaxies.
= Oldest spiral galaxy = The oldest spiral galaxy on file is
BX442. At eleven billion years old, it is more than two billion years older
than any previous discovery. Researchers think the galaxy’s shape is caused by
the gravitational influence of a companion dwarf galaxy. Computer models
based on that assumption indicate that BX442's spiral structure will last about
100 million years. Origin of the spiral structure
The pioneer of studies of the rotation of the Galaxy and the formation of the
spiral arms was Bertil Lindblad in 1925. He realized that the idea of stars
arranged permanently in a spiral shape was untenable. Since the angular speed
of rotation of the galactic disk varies with distance from the centre of the
galaxy, a radial arm would quickly become curved as the galaxy rotates. The
arm would, after a few galactic rotations, become increasingly curved
and wind around the galaxy ever tighter. This is called the winding problem.
Measurements in the late 1960s showed that the orbital velocity of stars in
spiral galaxies with respect to their distance from the galactic center is
indeed higher than expected from Newtonian dynamics but still cannot
explain the stability of the spiral structure.
Since the 1960s, there have been two leading hypotheses or models for the
spiral structures of galaxies: star formation caused by density waves
in the galactic disk of the galaxy. the SSPSF model – star formation caused
by shock waves in the interstellar medium.
These different hypotheses do not have to be mutually exclusive, as they may
explain different types of spiral arms. = Density wave model =
Bertil Lindblad proposed that the arms represent regions of enhanced density
that rotate more slowly than the galaxy’s stars and gas. As gas enters a
density wave, it gets squeezed and makes new stars, some of which are short-lived
blue stars that light the arms. This idea was developed into density
wave theory by C. C. Lin and Frank Shu in 1964.
Historical theory of Lin and Shu The first acceptable theory for the
spiral structure was devised by C. C. Lin and Frank Shu in 1964, attempting to
explain the large-scale structure of spirals in terms of a small-amplitude
wave propagating with fixed angular velocity, that revolves around the
galaxy at a speed different from that of the galaxy's gas and stars. They
suggested that the spiral arms were manifestations of spiral density waves -
they assumed that the stars travel in slightly elliptical orbits, and that the
orientations of their orbits is correlated i.e. the ellipses vary in
their orientation in a smooth way with increasing distance from the galactic
center. This is illustrated in the diagram. It is clear that the elliptical
orbits come close together in certain areas to give the effect of arms. Stars
therefore do not remain forever in the position that we now see them in, but
pass through the arms as they travel in their orbits.
Star formation caused by density waves The following hypotheses exist for star
formation caused by density waves: As gas clouds move into the density
wave, the local mass density increases. Since the criteria for cloud collapse
depends on density, a higher density makes it more likely for clouds to
collapse and form stars. As the compression wave goes through, it
triggers star formation on the leading edge of the spiral arms.
As clouds get swept up by the spiral arms, they collide with one another and
drive shock waves through the gas, which in turn causes the gas to collapse and
form stars. = More young stars in spiral arms =
The arms appear brighter because there are more young stars. These massive,
bright stars also die out quickly, which would leave just the darker background
stellar distribution behind the waves, hence making the waves visible.
While stars, therefore, do not remain forever in the position that we now see
them in, they also do not follow the arms. The arms simply appear to pass
through the stars as the stars travel in their orbits.
= Alignment of spin axis with cosmic voids =
Recent results suggest that the orientation of the spin axis of spiral
galaxies is not a chance result, but instead they are preferentially aligned
along the surface of cosmic voids. That is, spiral galaxies tend to be oriented
at a high angle of inclination relative to the large-scale structure of the
surroundings. They have been described as lining up like "beads on a string,"
with their axis of rotation following the filaments around the edges of the
voids. = Gravitationally aligned orbits =
Charles Francis and Erik Anderson showed from observations of motions of over
20,000 local stars, that stars do move along spiral arms, and described how
mutual gravity between stars causes orbits to align on logarithmic spirals.
When the theory is applied to gas, collisions between gas clouds generate
the molecular clouds in which new stars form, and evolution towards grand-design
bisymmetric spirals is explained. Distribution of stars in spirals
The stars in spirals are distributed in thin disks with surface luminosity.
with being the disk scale-length; is the central value; it is useful to
define: as the size of the stellar disk, whose luminosity is
The spiral's light profiles, in terms of the coordinate , do not depend on galaxy
luminosity. Spiral nebula
"Spiral nebula" was a term used to describe galaxies with a visible spiral
structure, such as the Whirlpool Galaxy, before it was understood that these
objects existed outside our Milky Way galaxy. The question of whether such
objects were separate galaxies independent of the Milky Way, or a type
of nebula existing within our own galaxy, was the subject of the Great
Debate of 1920, between Heber Curtis of Lick Observatory and Harlow Shapley of
Mt. Wilson Observatory. Beginning in 1923, Edwin Hubble observed Cepheid
variables in several spiral nebulae, including the so-called "Andromeda
Nebula", proving that they are, in fact, entire galaxies outside our own. The
term "spiral nebula" has since fallen into disuse.
Milky Way The Milky Way was once considered an
ordinary spiral galaxy. Astronomers first began to suspect that the Milky
Way is a barred spiral galaxy in the 1990s. Their suspicions were confirmed
by the Spitzer Space Telescope observations in 2005 which showed the
galaxy's central bar to be larger than previously suspected.
Famous examples See also
Classification Other
References External links
Giudice, G.F.; Mollerach, S.; Roulet, E.. "Can EROS/MACHO be detecting the
galactic spheroid instead of the galactic halo?". Physical Review D 50:
2406–2413. arXiv:astro-ph/9312047. Bibcode:1994PhRvD..50.2406G.
doi:10.1103/PhysRevD.50.2406. Stephens, Tim. "AEGIS survey reveals new
principle governing galaxy formation and evolution". UC Santa Cruz. Retrieved 24
May 2006. Spiral Galaxies @ SEDS Messier pages
SpiralZoom.com, an educational website about Spiral Galaxies and other spiral
formations found in nature. For high school & general audience.
Spiral Structure explained GLIMPSE: the Galactic Legacy Infrared
Mid-Plane Survey Extraordinaire Merrifield, M. R. "Spiral Galaxies and
Pattern Speed". Sixty Symbols. Brady Haran for the University of Nottingham.