MILKY WAY- THE UNIVERSE CONTAINS THE SOLAR SYSTEM
Introduction
General
The Milky
Way is the galaxy that
contains the Earth. This name derives from its appearance
as a dim "milky" glowing band arching across the night sky, in which
the naked eye cannot distinguish individual stars. The term "Milky
Way" is a translation of the Classical Latin “via lactea”, from the Hellenistic Greek “galaxías kýklos” (milky circle).
The Milky Way appears like a band
because it is a disk-shaped structure being viewed from inside. The fact that
this faint band of light is made up of stars was proven in 1610 when Galileo Galilei used his telescope to resolve it into
individual stars. In the 1920s, observations by astronomer Edwin Hubble showed that the Milky Way is just one
of many galaxies.
The Milky Way is a barred spiral galaxy 100,000–120,000 light-years in diameter containing 200 - 400
billion stars. It may contain at least as many
planets, with an estimated 10 billion of those orbiting in the habitable zone of
their parent stars.
The Solar System is located within the disk, around two
thirds of the way out from the Galactic Center, on
the inner edge of a spiral-shaped concentration of gas and dust called the Orion-Cygnus
Arm. The stars in the inner ≈10,000 light-years are organized in a bulge and one or more bars. The very center
is marked by an intense radio source named Sagittarius A* which is likely to be a supermassive black hole.
The Galaxy rotates once every 15 to 50
million years. The Galaxy as a whole is moving at a velocity of 552 to
630 km per second, depending on the relative frame of reference. It is
estimated to be about 13.2 billion years old, nearly as old as the Universe. Surrounded by several smaller satellite galaxies, the Milky Way is part of the Local Group of galaxies, which forms a
subcomponent of the Virgo Supercluster.
Not counting transient events such as gamma-ray bursts, the
brightest object in the gamma-ray sky is the plane of our Milky Way Galaxy.
This glow results from a vast sea of cosmic-ray particles slamming into
interstellar gas and dust, generating gamma rays. In fact, 75% of the gamma
rays in our galaxy come from these cosmic-ray interactions. This bright
gamma-ray glow gives the GLAST science team a golden opportunity to study the
structure, composition, and dynamics of the interstellar material that pervades
our home galaxy.
Etymology and mythology
In western culture the name "Milky
Way" is derived from its appearance as a dim un-resolved "milky"
glowing band arching across the night sky. The term is a translation of the Classical Latin via lactea, in turn derived from the Hellenistic Greek γαλαξίας, short forγαλαξίας κύκλος (pr. galaktikos
kyklos, "milky circle"). The Ancient Greek γαλαξίας (galaxias), from root
γαλακτ-, γάλα (milk) + -ίας (forming adjectives), is also the root of
"galaxy", the name for our, and later all such, collections of stars.
The Milky Way "milk circle" was just one of 11 circles the Greeks identified in the sky,
others being the zodiac, the meridian, the horizon, the equator, the tropics of
Cancer and Capricorn, Arctic and Antarctic circles, and two colure circles passing through both poles.
There are many creation myths around the world which explain the
origin of the Milky Way and give it its name. In Greek myth, the
Milky Way was caused by milk spilt by Hera when
suckling Heracles.[101] It is also described as the road to mount Olympus, and the
path of ruin made by the chariot of the Sun god Helios.[102]
In Sanskrit and several other Indo-Aryan languages, the Milky Way is called Akash Ganga (Ganges of
the heavens); it is held to be sacred in the Hindu Puranas (scriptures), and the Ganges and the
Milky Way are considered to be terrestrial and celestial analogs. Kshira ( milk)
is an alternative name for the Milky Way in Hindu texts in Sanskrit.
Astronomical history
The shape of the Milky Way as deduced
from star counts by William Herschel in 1785; the Solar System was assumed near
center.
As Aristotle (384–322 BC) informs us in Meteorologica (DK 59 A80), the Greek philosophers Anaxagoras (ca. 500-428 BC) and Democritus (450-370 BC) proposed the Milky Way
might consist of distant stars. However, Aristotle himself believed
the Milky Way to be caused by "the ignition of the fiery exhalation of
some stars which were large, numerous and close together" and that the
"ignition takes place in the upper part of the atmosphere, in the
region of the world which is continuous with the heavenly motions." The Neoplatonist philosopher Olympiodorus the Younger (c.
495–570 A.D.) criticized this
view, arguing that if the Milky Way were sublunary it should appear different
at different times and places on the Earth, and that it should have parallax, which it
does not. In his view, the Milky Way was celestial. This idea would be
influential later in the Islamic world.
According to Mohaini Mohamed, the Arabian astronomer, Alhazen (965–1037 AD), refuted this by making
the first attempt at observing and measuring the Milky Way's parallax. He determined that the Milky Way has
no parallax and concluded that it must be remote from the Earth, not part of
Earth's atmosphere.
The Persian astronomer Abū Rayhān al-Bīrūnī (973–1048)
proposed that the Milky Way is "a collection of countless fragments of the
nature of nebulous stars".[110] The Andalusian astronomer Avempace (d. 1138) proposed the Milky Way to be
made up of many stars but appears to be a continuous image due to the effect of refraction in the Earth's atmosphere, citing his observation of a conjunctionof Jupiter and Mars in 1106 or
1107 as evidence.[106] Ibn Qayyim Al-Jawziyya (1292–1350)
proposed the Milky Way Galaxy to be "a myriad of tiny stars packed
together in the sphere of the fixed stars" and that these stars are larger
than planets.
According to Jamil Ragep, the Persian
astronomer Naṣīr al-Dīn al-Ṭūsī (1201,1274) in his Tadhkira writes: "The Milky Way, i.e. the
Galaxy, is made up of a very large number of small, tightly-clustered stars,
which, on account of their concentration and smallness, seem to be cloudy
patches. because of this, it was likened to milk in color."
Actual proof of the Milky Way
consisting of many stars came in 1610 when Galileo Galilei used a telescope to study the Milky Way and discovered
that it was composed of a huge number of faint stars. In a treatise in 1755, Immanuel Kant,
drawing on earlier work by Thomas Wright, speculated (correctly) that the
Milky Way might be a rotating body of a huge number of stars, held together by gravitational forces akin to the Solar System but on much
larger scales. The resulting disk of stars would be seen as a band on the sky
from our perspective inside the disk. Kant also conjectured that some of the nebulae visible
in the night sky might be separate "galaxies" themselves, similar to
our own. Kant referred to both our Galaxy and the "extragalactic nebulae"
as "island universes", a term still current up to the 1930s.
The first attempt to describe the shape of the
Milky Way and the position of the Sun within
it was carried out by William Herschel in 1785 by carefully counting the
number of stars in different regions of the visible sky. He produced a diagram
of the shape of the Galaxy with the Solar System close to the center.
In 1845, Lord Rosse constructed
a new telescope and was able to distinguish between elliptical and
spiral-shaped nebulae. He also managed to make out individual point sources in
some of these nebulae, lending credence to Kant's earlier conjecture.
later identified as
theAndromeda Galaxy.
In 1917, Heber Curtis had observed the nova S Andromedae within the "Great AndromedaNebula" (Messier object M31). Searching the
photographic record, he found 11 more novae. Curtis noticed that these novae
were, on average, 10 magnitudes fainter
than those that occurred within our Galaxy. As a result he was able to come up
with a distance estimate of 150,000 parsecs. He became a proponent of the
"island universes" hypothesis, which held that the spiral nebulae
were actually independent galaxies.[116] In 1920 the Great Debate took place between Harlow Shapley and Heber Curtis, concerning the
nature of the Milky Way, spiral nebulae, and the dimensions of the universe. To
support his claim that the Great Andromeda Nebula was an external galaxy,
Curtis noted the appearance of dark lanes resembling the dust clouds in the
Milky Way, as well as the significant Doppler shift.
The matter was conclusively settled by Edwin Hubble in the early 1920s using the Mount Wilson observatory 100 inch (2.5 m) Hooker
telescope. With the light-gathering power of
this new telescope he was able to produceastronomical photographs that resolved the outer parts of some
spiral nebulae as collections of individual stars. He was also able to identify
some Cepheid variables that he could use as a benchmark to estimate the distance to the
nebulae: proving they were far too distant to be part of the Milky Way. In
1936, Hubble produced a classification system for galaxies that is used to this
day, theHubble
sequence.
But as Large Area Telescope science team member David
Thompson of NASA Goddard, explains, "It’s not easy to understand something
when you’re in the middle of it." Adding to the complexity is the fact
that our galaxy is filled with many different types of particles and energy
sources, including protons, electrons, electromagnetic radiation, magnetic
fields, and so forth — most of which have not been accurately measured.
To study our galaxy, theorists create models of how these
different particles interact with magnetic fields in different locations and
with different strengths. Astronomers can then compare these models to actual
observations made at radio, infrared, optical, ultraviolet, and X-ray
wavelengths to see how well they match the data. The LAT will contribute vital
data that will enable theorists to constrain and improve their models.
Having an accurate model of gamma-ray production within our
galaxy is not only important in its own right, it is vital for the measurement
of localized gamma-ray sources. The sources are seen against the bright
background of the Milky Way glow. If the galaxy is not modeled correctly, then
information about other objects could be distorted. As GLAST Program Scientist
F. Rick Harnden Jr. notes, "The same gamma rays that measure galactic
structure are also a background for other observations."
Appearance
as seen from a non-light polluted area (the Black Rock Desert, Nevada).
When observing the night sky, the term
"Milky Way" is limited to the hazy band of white light some 30
degrees wide arcing across the sky[16] (although all of the stars that
can be seen with the naked eye are part of the Milky Way Galaxy). The
light in this band originates from un-resolved stars
and other material that lie within the Galactic plane. Dark regions within the
band, such as the Great Rift and
the Coalsack,
correspond to areas where light from distant stars is blocked by interstellar dust.
The Milky Way has a relatively low surface brightness. Its visibility can be greatly reduced by
background light such as light pollution or stray light from the moon. It is readily visible when the limiting magnitude is
+5.1 or better, while showing a great deal of detail at +6.1.[17] This makes the Milky Way difficult to
see from any brightly-lit urban or suburbanlocation
but very prominent when viewed from a rural area when the moon is below the horizon.
The
Milky Way passes through parts of roughly 30 constellations. The center of the Galaxy lies in the direction of the constellation Sagittarius; it is here that the Milky Way is
brightest. From Sagittarius, the hazy band of white light appears to pass
westward to theGalactic anticenter in Auriga. The band then continues westward the rest of the way
around the sky back to Sagittarius. The fact that the band divides the
night sky into two roughly equal hemispheres indicates that the Solar System lies
close to the Galactic planet. The
Galactic plane is inclined by about 60 degrees to the ecliptic (the plane of the Earth's orbit).
Relative to the celestial equator,
it passes as far north as the constellation of Cassiopeia and
as far south as the constellation of Crux, indicating the high inclination of
Earth's equatorial plane and the plane of the ecliptic relative to the Galactic plane. The
north Galactic pole is situated at right ascension 12h 49m, declination +27.4° (B1950) near beta Comae Berenices, and the south Galactic pole is near alpha Sculptoris.
Because of this high inclination, depending on the time of night and the year,
the arc of Milky Way can appear relatively low or relatively high in the sky.
For observers from about 65 degrees north to 65 degrees south on the Earth's
surface the Milky Way passes directly overhead twice a day.
shot from a dark sky location in Chile
Size and composition
The stellar disk of the Milky Way
Galaxy is approximately 100,000 light-years (30kiloparsecs) in
diameter, and is, on average, about 1,000 ly (0.3 kpc) thick. As a
guide to the relative physical scale of the Milky Way, if it were reduced
to 100 meters (110 yd) in diameter, the Solar System, including the
hypothesized Oort cloud, would
be no more than 1 millimeter (0.039 in) in width, or a grain of sand in a
sports field. The nearest star,Proxima Centauri,
would be 4.2 mm (0.17 in) distant.
The Milky Way contains at least
100 billion stars and may have up to 400 billion stars. The exact figure depends on the number of very
low-mass, or dwarf stars, which
are hard to detect, especially at distances of more than 300 ly
(90 pc) from the Sun. As a comparison, the neighboring Andromeda Galaxy contains an estimated one trillion (1012)
stars. Filling the space between the stars is a disk of gas and dust called the
interstellar medium. This disk has at least a comparable
extent in radius to the stars. while the thickness of the gas layer ranges from
hundreds of light years for the colder gas to thousands of light years for
warmer gas. Both gravitational microlensing and planetary transit observations
indicate that there may be at least as many planets bound to stars as there are
stars in the Milky Way, while microlensing measurements indicate that there are
more rogue planets not bound to host stars than there are
stars. Earth-sized planets may be more numerous than gas giants.
The disk of stars in the Milky Way does
not have a sharp edge beyond which there are no stars. Rather, the
concentration of stars drops smoothly with distance from the center of the
Galaxy. Beyond a radius of roughly 40,000 ly (12 kpc), the number of
stars per cubic parsec drops much faster with radius, for reasons that are not
understood. Surrounding the Galactic disk is a spherical Galactic Halo of stars and globular clusters that extends further outward, but is
limited in size by the orbits of two Milky Way satellites, the Large and the
Small Magellanic Clouds,
whose closest approach to the Galactic center is about
180,000 ly (55 kpc).[29] At this distance or beyond, the orbits
of most halo objects would be disrupted by the Magellanic Clouds. Hence, such
objects would likely be ejected from the vicinity of the Milky Way. The
integrated absolute visual magnitude of
the Milky Way is estimated to be -20.9.
Estimates for the mass of the Milky Way
vary, depending upon the method and data used. At the low end of the estimate
range, the mass of the Milky Way is 5.8 x1011 solar masses (MS), somewhat smaller than the Andromeda Galaxy. Measurements
using the Very Long Baseline Array in
2009 found velocities as large as 254 km/s for stars at the outer edge of
the Milky Way, higher than the previously accepted value of 220 km/s. As
the orbital velocity depends on the total mass inside the orbital radius, this
suggests that the Milky Way is more massive, roughly equaling the mass of
Andromeda Galaxy at 7x1011 MS within
50 kiloparsecs (160,000 ly) of its center. A 2010 measurement of the radial
velocity of halo stars finds the mass enclosed within 80 kiloparsecs is 7×1011 MS. Most of the mass of the
Galaxy appears to be matter of unknown form which interacts with other matter
through gravitational but not electromagnetic forces; this is dubbed dark matter. A dark matter halo is spread out relatively uniformly to
a distance beyond one hundred kiloparsecs from the Galactic Center
Structure
with two major
stellar arms and a bar.
The Galaxy consists of a bar-shaped
core region surrounded by a disk of gas, dust and
stars. The gas, dust and stars are organized in roughly logarithmic spiral arm
structures (see Spiral arms below). The mass distribution within
the Galaxy closely resembles the SBc Hubble classification, which is a spiral galaxy
with relatively loosely wound arms.[1]Astronomers
first began to suspect that the Milky Way is a barred spiral galaxy, rather than an ordinary spiral galaxy, in
the 1990s.[38] Their suspicions were confirmed by theSpitzer Space Telescope observations
in 2005[39] that showed the Galaxy's central bar
to be larger than previously suspected.
1-Galactic Center
The Sun is 8.0-8.7 kpc (26,000–28,000
ly (light years)) from the Galactic Center.
This value is estimated based upon geometric-based
methods or using selected astronomical objects that serve as standard candles,
with different techniques yielding different values within this approximate
range. In the inner few kpc (≈10,000 light-years) is a dense concentration of
mostly old stars in a roughly spheroidal shape called the bulge.
The Galactic Center
The nature of the Galaxy's bar is
actively debated, with estimates for its half-length and orientation spanning
from 1-5 kpc (3,300-16,000 ly) (short or a long bar) and 10–50 degrees relative
to the line of sight from Earth to the Galactic Center
2- Spiral arms
Beyond the gravitational influence of
the Galactic bars, astronomers generally organize the interstellar medium and
stars in the disk of the Milky Way in four spiral arms. All of these arms
contain more interstellar gas and dust than the Galactic average as well as a
high concentration of star formation, traced by H II regions and molecular clouds. Counts
of stars in near infrared light indicate that two arms contain approximately
30% more red giant stars than would be expected in the
absence of a spiral arm, while two do not contain more red giant stars than
regions outside of arms.
Maps of the Milky Way's spiral
structure are notoriously uncertain and exhibit striking differences. Some 150
years after Alexander (1852) first suggested that the Milky Way was a spiral,
there is currently no consensus on the nature of the Galaxy's spiral arms.
Perfect logarithmic spiral patterns ineptly describe features near the Sun, namely
since galaxies commonly exhibit arms that branch, merge, twist unexpectedly,
and feature a degree of irregularity. The possible scenario of the Sun within a
spur / Local arm emphasizes that point and indicates that such features are
likely not unique, and exist elsewhere in the Galaxy.
As in most spiral galaxies, each spiral
arm can be described as a logarithmic spiral. Estimates of the pitch angle of the arms
range from ≈7° to ≈25°. Until recently, there were thought to be four major
spiral arms which all start near the Galaxy's center. These are named as
follows, with the positions of the arms shown in the image at right:
The gray lines
radiating from the Sun's position (upper center) list
the three-letter
abbreviations of the corresponding constellations.
Color
|
Arm (s)
|
cyan
|
3-kpc and Perseus Arm
|
purple
|
|
green
|
|
pink
|
|
There are at least two smaller
arms or spurs, including:
|
|
orange
|
Orion-Cygnus Arm (which
contains the Sun and Solar System)
|
Two spiral arms, the Scutum–Centaurus
arm and the Carina–Sagittarius arm, have tangent points inside the Sun's orbit
around the center of the Milky Way. If these arms contain an overdensity of
stars compared to the average density of stars in the Galactic disk, it would
be detectable by counting the stars near the tangent point. Two surveys of
near-infrared light, which is sensitive primarily to red giant stars and not
affected by dust extinction, detected the predicted overabundance in the Scutum–Centaurus
arm but not in the Carina–Sagittarius arm. In
2008, Robert Benjamin of the University of Wisconsin–Whitewater used this observation to suggest that
the Milky Way possesses only two major stellar arms: the Perseus arm and the Scutum-Centaurus
arm. The rest of the arms contain excess gas but not excess stars. This would
mean that the Milky Way is similar in appearance to NGC 1365.
Outside
of the major spiral arms is the Monoceros Ring (or Outer Ring), proposed by
astronomers Brian Yanny and Heidi Jo Newberg, a
ring of gas and stars torn from other galaxies billions of years ago.
As is typical for many galaxies, the
distribution of mass in the Milky Way Galaxy is such that the orbital speed of most stars in the Galaxy does not
depend strongly on their distance from the center. Away from the central bulge
or outer rim, the typical stellar velocity is between 210 and 240 km/s.[67] Hence the orbital period of the typical star is directly
proportional only to the length of the path traveled. This is unlike the
situation within the Solar System, where two-body gravitational dynamics
dominate and different orbits are expected to have significantly different
velocities associated with them. This difference is one of the major pieces of
evidence for the existence of dark matter.
Another interesting aspect is the so-called "wind-up problem" of the
spiral arms. If the inner parts of the arms rotate faster than the outer part,
then the galaxy will wind up so much that the spiral structure will be thinned
out. But this is not what is observed in spiral galaxies; instead, astronomers
propose that the spiral pattern is a density wave emanating from the Galactic Center
Halo
The Galactic disk is surrounded by a spheroidal halo of old stars and globular clusters,
of which 90% lie within 100,000 light-years (30 kpc) of the Galactic Center Galactic Center
While the disk contains gas and dust
which obscure the view in some wavelengths, the spheroid component does not.
Active star formation takes place in the disk (especially in
the spiral arms, which represent areas of high density), but not in the halo. Open clusters also occur primarily in the disk.
Discoveries in the early 21st century
have added dimension to the knowledge of the Milky Way's structure. With the
discovery that the disk of the Andromeda Galaxy (M31) extends much further than
previously thought,[71] the possibility of the disk of the
Milky Way Galaxy extending further is apparent, and this is supported by
evidence from the 2004 discovery of the Outer Arm extension of theCygnus Arm. With
the discovery of the Sagittarius Dwarf Elliptical Galaxy came the discovery of a ribbon of
galactic debris as the polar orbit of the dwarf and its interaction with the Milky
Way tears it apart. Similarly, with the discovery of the Canis Major Dwarf Galaxy, it was found that a ring of galactic
debris from its interaction with the Milky Way encircles the Galactic disk.
On January 9, 2006, Mario Jurić and others of Princeton University announced
that the Sloan Digital Sky Survey of
the northern sky found a huge and diffuse structure (spread out across an area
around 5,000 times the size of a full moon) within the Milky Way that does not
seem to fit within current models. The collection of stars rises close to
perpendicular to the plane of the spiral arms of the Galaxy. The proposed
likely interpretation is that a dwarf galaxy is merging with the Milky Way. This
galaxy is tentatively named theVirgo Stellar Stream and
is found in the direction of Virgo about
30,000 light-years (9 kpc) away.
of the Milky Way
(center).
Gamma-ray bubbles
On November 9, 2010, Doug Finkbeiner of
the Harvard–Smithsonian Center for Astrophysics announced that he had detected two
gigantic spherical bubbles of high energy emission that are erupting to the
north and the south of the Milky Way core, using data of the Fermi Gamma-ray Space Telescope. The diameter
of each of the bubbles is about 25,000 light-years (7.7 kpc); they stretch
up to Grus and to Virgo on the
night-sky of the southern hemisphere. Their origin remains unclear, so far.
Sun's location and neighborhood
The Sun (and
therefore the Earth and
the Solar System) may
be found close to the inner rim of the Galaxy's Orion Arm, in the Local Fluff inside the Local Bubble, and
in the Gould Belt, at a
distance of 8.33 ± 0.35 kiloparsecs
(27,200 ± 1,100 ly) from the Galactic Center. The
Sun is currently 5–30 parsecs (16–98 ly) from the central plane of the Galactic
disk. The distance between the local arm and the next arm out, the Perseus Arm, is
about 6,500 light-years (2.0 kpc). The Sun, and thus the Solar System, is
found in the Galactic
habitable zone.
There are about 208 stars brighter than absolute magnitude 8.5
within 15 parsecs (49 ly) of the Sun, giving a density of 0.0147 such
stars per cubic parsec, or 0.000424 per cubic light-year (from List of nearest bright stars). On the other
hand, there are 64 known stars (of any magnitude, not counting 4 brown dwarfs)
within 5 parsecs (16 ly) of the Sun, giving a density of 0.122 stars per
cubic parsec, or 0.00352 per cubic light-year (from List of nearest stars), illustrating the fact that most stars
are less bright than absolute magnitude 8.5.
The Apex of the Sun's Way, or the solar apex, is the
direction that the Sun travels through space in the Milky Way. The general
direction of the Sun's Galactic motion is towards the star Vega near
the constellation of Hercules, at an angle of roughly 60 sky degrees to the
direction of the Galactic Center.
The Sun's orbit around the Galaxy is expected to be roughly elliptical with the
addition of perturbations due to the Galactic spiral arms and non-uniform mass
distributions. In addition, the Sun oscillates up and down relative to the
Galactic plane approximately 2.7 times per orbit. This is very similar to how a simple harmonic oscillator works with no drag force (damping)
term. These oscillations were until recently thought to coincide with mass extinction periods on Earth.[79] However, a reanalysis of the effects
of the Sun's transit through the spiral structure based on CO data has failed
to find these correlations.
It takes the Solar System about 225-250
million years to complete one orbit around the Galaxy (a Galactic year),
so the Sun is thought to have completed 18-20 orbits during its lifetime and
1/1250 of a revolution since the origin of humans.
The orbital speed of the Solar System about the center
of the Galaxy is approximately 220 km/s or 0.073% of the speed of light.
At this speed, it takes around 1,400 years for the Solar System to travel a
distance of 1 light-year, or 8 days to travel 1 AU (astronomical unit).
Environment
The
position of the Local Group within the Virgo Supercluster.
The Milky Way and the Andromeda Galaxy are a binary system of
giant spiral galaxies belonging to a group of 50 closely bound galaxies known
as the Local Group, itself
being part of the Virgo Supercluster.
Two smaller galaxies and a number of dwarf galaxies in the Local Group orbit the
Milky Way. The largest of these is the Large Magellanic Cloud with
a diameter of 20,000 light-years. It has a close companion, the Small Magellanic Cloud. The Magellanic Stream is a peculiar streamer of neutral hydrogen gas connecting these two small
galaxies. The stream is thought to have been dragged from the Magellanic Clouds
in tidal interactions with the Milky Way. Some of the dwarf galaxies orbiting the Milky Way are Canis Major Dwarf (the closest), Sagittarius Dwarf Elliptical Galaxy, Ursa Minor Dwarf, Sculptor Dwarf,Sextans Dwarf, Fornax Dwarf, and Leo I Dwarf. The
smallest Milky Way dwarf galaxies are only 500 light-years in diameter. These
include Carina Dwarf, Draco Dwarf, and Leo II Dwarf. There may still be undetected dwarf galaxies,
which are dynamically bound to the Milky Way, as well as some that have already
been absorbed by the Milky Way, such asOmega Centauri.
Observations through the Zone of Avoidance are frequently detecting new distant
and nearby galaxies. Some galaxies consisting mostly of gas and dust may also
have evaded detection so far.
In January 2006, researchers reported
that the heretofore unexplained warp in the disk of the Milky Way has now been
mapped and found to be a ripple or vibration set up by the Large and Small
Magellanic Clouds as they circle the Galaxy, causing vibrations at certain
frequencies when they pass through its edges.[89] Previously, these two galaxies, at
around 2% of the mass of the Milky Way, were considered too small to influence
the Milky Way. However, by taking into account dark matter, the
movement of these two galaxies creates a wake that influences the larger Milky
Way. Taking dark matter into account results in an approximately twentyfold
increase in mass for the galaxy. This calculation is according to a computer
model made by Martin Weinberg of the University of Massachusetts Amherst. In this model,
the dark matter is spreading out from the Galactic disk with the known gas
layer. As a result, the model predicts that the gravitational effect of the
Magellanic Clouds is amplified as they pass through the Galaxy.
Current measurements suggest the Andromeda Galaxy is approaching us at 100 to 140
kilometers per second. The Milky Way maycollide with it
in 3 to 4 billion years, depending on the importance of unknown lateral
components to the galaxies' relative motion. If they collide, individual stars
within the galaxies would not collide, but instead the two galaxies will merge
to form a single elliptical galaxy over the course of about a billion
years.
Velocity
Galaxy rotation curve for
the Milky Way. Vertical axis is speed of rotation about the Galactic Center.
Horizontal axis is distance from the Galactic
Center
In
the general sense, the absolute velocity of any object through space is not a
meaningful question according to Einstein's special theory of relativity, which declares that there is no
"preferred" inertial frame of reference in space with which to compare the
object's motion. (Motion must always be specified with respect to another
object.) This must be kept in mind when discussing the Galaxy's motion.
Astronomers believe the Milky Way is
moving at approximately 630 km per second relative to the average velocity
of galaxies taken over a large enough volume so that the expansion of the
Universe dominates over local, random motions: the local co-moving frame of reference that
moves with the Hubble flow. The
Milky Way is moving in the general direction of the Great Attractor and other galaxy clusters,
including the Shapley supercluster, behind it. The Local Group (a cluster of gravitationally bound
galaxies containing, among others, the Milky Way and the Andromeda Galaxy)
is part of asupercluster called the Local Supercluster, centered near the Virgo Cluster:
although they are moving away from each other at 967 km/s as part of the
Hubble flow, the velocity is less than would be expected given the
16.8 million pc distance due to the gravitational attraction between the
Local Group and the Virgo Cluster.
Another reference frame is provided by
the cosmic microwave background (CMB). The Milky Way is moving at 552 ± 6 km/s[10] with respect to the photons of the
CMB, toward 10.5 right ascension,
−24° declination (J2000 epoch,
near the center of Hydra). This motion is observed by satellites such as the Cosmic Background Explorer (COBE) and the Wilkinson Microwave Anisotropy Probe(WMAP) as a
dipole contribution to the CMB, as photons in equilibrium in the CMB frame get blue-shifted in the direction of the motion and red-shifted in the opposite direction.
The
Galaxy rotates about its center according to its galaxy rotation curve as
shown in the figure. The discrepancy between the observed curve (relatively
flat) and the curve based upon the known mass of the stars and gas in the Milky
Way (decaying curve) is attributed to dark matter.
References
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