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Milky Way

Contributed by: Leo Blitz

Publication year: 2014

The large disk-shaped aggregation of , gas, and dust in which the solar system is located. The term

Milky Way is used to refer to the diffuse band of light visible in the night sky emanating from the Milky Way

Galaxy. Although the two terms are frequently used interchangeably, Milky Way Galaxy, or simply the Galaxy, refers to the physical object rather than its appearance in the night sky, while Milky Way is used to refer to either.

Appearance

The Milky Way is visible in the night sky to the unaided eye as a broad diffuse band of light stretching from horizon to horizon when viewed from locations away from bright city lights ( Fig. 1 ). Nearly all of the visible light is due to individual stars, which in many directions are too numerous to be resolved without a telescope. The patchy appearance of the Milky Way is due to collections of microscopic dust particles which block the light of more distant stars. All of the stars seen by the unaided eye are part of the Milky Way Galaxy and lie relatively close to the Sun. The overall appearance is due to the Sun’s location near the midplane of the galactic disk; the diffuse band of light is seen toward directions close to the midplane where there are many more stars along the line of sight than in directions away from the plane.

Structure and contents

11 The Milky Way Galaxy contains about 2 × 10, solar masses of visible matter. Roughly 96% is in the form of stars, and about 4% is in the form of interstellar gas. The gas both inside the stars and in the is primarily hydrogen (roughly 87–90% by number of atoms) and helium (about 10%) with a small admixture of all of the heavier atoms (0–3% depending on the location within the Galaxy). The mass of dust is about 1% of the interstellar gas mass and is an insignificant fraction of the total mass of the Galaxy. Its presence, however, limits the view from the Earth in the plane of the Galaxy to a small fraction of the Galaxy’s diameter in most directions.

See also: INTERSTELLAR MATTER .

The Milky Way Galaxy contains four major structural subdivisions: the nucleus, bulge, disk, and halo. The Sun is located in the disk about halfway between the center and the indistinct outer edge of the disk of stars. The currently accepted value of the distance of the Sun from the is 8.5 kiloparsecs, although some 5 measurements suggest that the distance may be as small as 7 kpc. (A parsec is equal to 2.06 × 10, times the average distance between the Sun and the Earth, and is approximately equal to the average distance between stars in the solar neighborhood; it is also equal to 3.26 light-years.) Figure 2 is an infrared image of the Milky Way AccessScience from McGraw-Hill Education Page 2 of 9 www.accessscience.com

WIDTH:EFig. 1 Panoramic photograph of the entire Milky Way. The dark patches are nearby clouds containing interstellar dust that block out the background starlight. The photograph conveys the overall impression of the flattened distribution of stars that make up the disk. The direction of the galactic center is in the center but it is hidden by foregr ound dust. The can be seen as the thickening of the disk in the center. ( European Southern Obser vator y )

WIDTH:CFig. 2 Infrared image of the Milky Way as seen from the location of the Sun, made from the 2MASS infrared sky survey. The image shows both the bulge and the disk of the Milky Way. Absorption by dust is minimized in the infrared, which accounts for the difference between this image and Fig. 1. However, the dust remains visible as the dark band across the disk and other dark patches. The Large and Small Magellanic Clouds are visible in the lower right. The faint vertical wisp coming down from the left part of the bulge is the Sagittarius Dwarf. ( 2MASS )

made from the Sun’s position in the disk, showing the bulge and disk of the Galaxy. The infrared image minimizes the obscuring effect of dust. Figure 3 shows a that is structurally similar to the Milky

Way, giving an approximate idea of how the Milky Way would appear if viewed obliquely. See also: INFRARED

ASTRONOMY ; PARSEC .

Nucleus. The nucleus of the Milky Way is a region within a few tens of parsecs of the geometric center and is totally obscured at visible wavelengths. The nucleus is the source of very energetic activity detected by means of radio waves and infrared radiation.

At the galactic center, there is a very dense cluster of hot stars observed by means of its infrared radiation. In

1997, astronomers confirmed the existence of a with a mass of about 2.5 million times the mass of the AccessScience from McGraw-Hill Education Page 3 of 9 www.accessscience.com

WIDTH:DFig. 3 NGC 4603, a galaxy with a gross morphology similar to the Milky Way, seen obliquely. The spiral arms and the bulge are visible in the center. The bulge appears to be elongated because the galaxy also contains a bar, an extended feature containing primarily old stars that rotate collectively about the center as if they were a solid body. ( NASA )

∗ Sun at the position of an unresolved source of radio emission known as Sgr A, in the middle of the central cluster. The black hole appears to be the dynamical center of the Milky Way, and evidence for its existence appears to be unequivocal. Many spiral show evidence of black holes at their centers. At a distance of ∗ about 1.5 pc from Sgr A, is a ring of gas consisting primarily of molecular hydrogen that surrounds the central star cluster. Within this ring is a three-armed “minispiral” of ionized gas that appears to be falling into the central cluster and possibly onto the black hole ( Fig. 4 ). Molecular hydrogen gas, which is characterized by low temperatures and relatively high densities, is found in great profusion in the inner few hundred parsecs of the

Milky Way. Great arcs of gas resulting from the interaction of cosmic rays and magnetic fields have been mapped in the central tens of parsecs of the Milky Way, attesting to the energetic activity taking place there. See also:

BLACK HOLE ; COSMIC RAYS ; RADIO .

Bulge. The bulge is a spheroidal distribution of stars centered on the nucleus which extends to a distance of about

3 kpc from the center. It contains a relatively old population of stars, very nearly as old as the Milky Way itself.

The bulge can be seen by the unaided eye as a thickening of the diffuse band of light that constitutes the Milky

Way in the direction of the galactic center, toward the constellation Sagittarius. There is little gas and dust throughout most of the volume of the bulge. Direct imaging with infrared satellites has demonstrated that the bulge is actually an elongated barlike structure with a length about two to three times its width. The Milky Way is thus classified as a , a classification that includes about half of all disk-shaped galaxies. See also: . AccessScience from McGraw-Hill Education Page 4 of 9 www.accessscience.com

WIDTH:CFig. 4 Radio image of the ionized gas within about 1.5 parsecs of the center of the Galaxy. The image, made with the Very Large Array (VLA), shows what appears to be a three-armed spiral of gas. Other observations indicate that this gas may be falling into the central star cluster (not seen in this image). The position of Sgr A* is indicated by the elongated bright dot seen above the horizontal bar of emission. The dot is thought to be radio emission from hot gas falling into the black hole. ( NRAO, courtesy of N. Killeen and K.-Y. Lo )

Disk. The disk is a thin distribution of stars and gas orbiting the nucleus of the Galaxy. The disk of stars begins near the end of the bar and can be identified to about 16 kpc from the center of the Galaxy; the disk of gas can be identified to about twice this distance, about 35 kpc from the center. The thicknesses of the disks are characterized by a scale height: the distance from the midplane at which the density of gas and stars falls by a factor of e (2.718. . . ). In the solar vicinity, the scale height is different for each of the components of the disk, varying from about 75 pc for the molecular gas to about 350 pc for the lowest-mass stars. The faint, low-mass stars make up most of the mass of the disk. There is also a of stars and gas with a scale height of about

1.5 kpc for the stars and about 1 kpc for the atomic gas. The origin of these thick disks is not known. The of stars contains most of the mass and has a thickness relative to its diameter similar to that of a commercial compact disk. At this scale, the bulge would have the size of a sausage about 2.5 cm (1 in.) in length superimposed on the nucleus.

The star and gas disks are both warped; the gas disk deviates from a true plane by about 3 kpc (about 10%) in its outermost parts. The disk also becomes thicker with increasing distance from the galactic center. The thickening AccessScience from McGraw-Hill Education Page 5 of 9 www.accessscience.com

occurs because there is less to confine the gas and stars to the midplane. The reason for the warping is not yet understood, but warping is a common feature of spiral galaxies.

The disk is the location of the spiral arms that are characteristic of most disk-shaped galaxies, as well as most of the present-day . Attempts to map the location of the spiral arms are hampered by the Sun’s location in the disk, but in the parts of the Galaxy beyond the Sun’s distance from the center, several long coherent spiral arms have been identified. The inner regions appear to be more chaotic, and no agreed-upon spiral arm structure has been identified. The spiral arms are where giant clouds of molecules are primarily found. 6 These clouds are the most massive objects in the Milky Way, ranging up to about 5 × 10, solar masses; they are the sites of all present-day star formation. The Sun presumably once formed in a giant molecular cloud. See also:

MOLECULAR CLOUD .

Halo. The halo is a rarefied spheroidal distribution of stars nearly devoid of the interstellar gas and dust that surrounds the disk. The stars found in the halo are the oldest stars in the Galaxy. The stars are found individually as “field” stars as well as in globular clusters: spherical clusters of up to about a million stars with very low abundances of elements heavier than helium. The extent of the halo is not well determined, but globular clusters with distances of about 40 kpc from the center have been identified. Dynamical evidence suggests that the halo contains nonluminous matter in some unknown form, commonly referred to as dark matter (see below). The dark matter contains most of the mass of the Galaxy, dominating even that in the form of stars. See also: STAR

CLUSTERS .

Companion galaxies

The Milky Way has 11 companion galaxies clustered within about 250 kpc of its center. The largest and brightest ◦ ◦ of these, the Large and Small Magellanic Clouds, are seen as diffuse patches of light roughly 7 and 5 in diameter ◦ ◦ and separated from the plane of the Milky Way by about 30 and 40 , respectively. They are visible mainly from the Earth’s southernmost latitudes. The distance to the Magellanic Clouds is about 55 kpc, not quite twice the diameter of the stars in the disk of the Galaxy. A third small galaxy, known as the Sagittarius Dwarf, was discovered in 1994 and is the nearest galaxy to the Milky Way. It went undiscovered for such a long time because it lies close to the obscuring dust in the midplane. The Sagittarius Dwarf is seen in the general direction of the center of the Galaxy, and its distance from the Sun is about 24 kpc, about three times the distance of the Sun from the center of the Galaxy.

All three galaxies are interacting with the Milky Way. The Magellanic Stream, a narrow band of gas drawn out of the Magellanic Clouds, which stretches over much of the sky, is produced either by the tidal forces of the Milky

Way or by the ram pressure of the clouds going through a tenuous halo of galactic gas. The Sagittarius Dwarf shows evidence of being stretched apart by the strong tidal forces of the Milky Way, and is about to collide with the disk, a process that will take millions of years. See also: MAGELLANIC CLOUDS . AccessScience from McGraw-Hill Education Page 6 of 9 www.accessscience.com

Dynamics and kinematics

The stars and gas in the disk of the Milky Way rotate around the galactic center in nearly circular orbits in the plane of the disk. The deviations from circularity are generally small, and give rise to oscillations of stellar motions perpendicular to the disk, epicyclic motions in the plane of the disk, and other, more complex motions.

The overall rotation of the disk is differential rather than like a solid body in that the rotation period of the stars and the gas increases with increasing distance from the center. Large-scale deviations from circularity are associated with the spiral arms of the disk.

The barlike bulge produces large noncircular motions in the disk of gas contained within it, providing independent confirmation that the Milky Way contains a “bar” in its inner parts. A bar is a large collection of stars in the inner part of a galaxy in which the stars collectively rotate about the center as a solid body would, even though the individual stellar orbits are quite complex.

The orbits of the stars and clusters in the bulge and halo are not confined to the and are, in general, fully three-dimensional. Although the motions of most stars are too small for the orbits to be determined from their changes in position alone, orbits can be inferred from the measured radial velocities of the stars (the velocities measured along the line of sight) and from their positions in the Galaxy. The orbits of stars close to the

Sun can be observed directly from their apparent motions in the sky measured over many years. Motions of small numbers of stars as distant as the center of the Galaxy can be measured directly by special techniques.

In the outermost parts of the Milky Way, one expects that the orbital velocities of stars and gas around the galactic center should decrease in a well-determined way, as they do, for example, in the solar system.

Measurements show, however, that in the outermost regions of the disk the velocities of the gas and stars do not decrease with increasing distance from the galactic center ( Fig. 5 ). These measurements suggest that unseen matter is pulling on the visible matter in the disk and producing the anomalously large velocities. The constancy of the rotation velocity of the Milky Way is generally thought to be the most compelling evidence for large quantities of material in the Milky Way in a form other than stars, gas, and dust. This dark matter is common to most galaxies for which rotation measurements are possible.

Dark matter

1 The dark matter in the Milky Way constitutes by various estimates 2 ∕2 to 10 times the total amount of known matter in the Milky Way, and is consequently the dominant component of mass in the Galaxy. Because the kinematics of most galaxies indicate that they too contain dark matter as their largest mass constituent, the dark matter appears to be the dominant form of matter in the universe. The composition of the dark matter is currently unknown and is one of the major unsolved problems in astronomy. AccessScience from McGraw-Hill Education Page 7 of 9 www.accessscience.com

WIDTH:CFig. 5 Plot of the measurement of the rotational velocity of gas and stars about the center of the Milky Way Galaxy as a function of distance from the galactic center, showing that the rotational velocities are essentially constant. Beyond the solar distance, one would expect that the rotational velocity would decrease markedly to a value about two-thirds the value measured at the Sun’s distance if only visible matter contributed to the gravity of the Milky Way. ( Courtesy of M. Fich, L. Blitz, and A. A. Stark )

In the Milky Way, various forms for the dark matter have been ruled out. It cannot be in the form of ordinary stars or in remnants such as white dwarfs, neutron stars, or black holes that are the end products of ordinary . It cannot be in the form of gas in any form, nor can it be in the form of small, solid dust particles.

Although a small fraction of the dark matter may reside in the disk, most of it appears to reside in the halo. See also: NEUTRON STAR ; WHITE DWARF STAR .

Two possibilities that have not yet been ruled out are that the dark matter consists of small planet-sized bodies that are insufficiently luminous to be detected with present instruments, and that it consists of primordial black holes in the halo. Still other possibilities include weakly interacting massive particles (WIMPs) that are predicted by various elementary particle theories. Observations of the supernova 1987A have apparently ruled out the possibility that neutrinos (massless or very low mass neutral particles) constitute most of the dark matter in the universe. An alternative theory to newtonian gravitation has also been proposed to explain the rotation curve of the Milky Way and other galaxies, but this is generally considered to be a particularly radical explanation. See also: COSMOLOGY ; DARK MATTER ; GRAVITATION ; NEUTRINO ; SUPERNOVA ; WEAKLY INTERACTING MASSIVE PARTICLE (WIMP) .

Location

The Milky Way is part of a small group of galaxies known as the Local Group. The Local Group also contains two large spiral galaxies, the great nebula in Andromeda (M31) and M33, and 40 small irregular, spheroidal and elliptical galaxies. Some of these small galaxies have been discovered only in the last few years, and it is likely that AccessScience from McGraw-Hill Education Page 8 of 9 www.accessscience.com

the census of galaxies in the Local Group is not yet complete. About 95% of the mass in the Local Group is associated with either the Milky Way or M31. The Local Group is itself part of a large of galaxies known as the supercluster containing about 1000 known galaxies. The Local Group is an outlying collection of galaxies in the Virgo supercluster and is about 15 Mpc from its center. The Virgo supercluster is one of many such clusters in the universe and occupies no special location within it. See also: ;

GALAXY, EXTERNAL ; LOCAL GROUP ; UNIVERSE .

Formation and evolution

Inferences about the formation and evolution of the Milky Way can be drawn from a large variety of sources, including the theory and observation of the kinematics and dynamics of the gas and stars in the Milky Way, the chemical abundances of the stars, their locations in the Galaxy, and stellar evolution theory. Current evidence suggests that the ages of the oldest stars in the Milky Way are within about 10% of the age of the universe as a whole; thus parts of the Milky Way must have formed early in the history of the universe, about 12 billion years ago. See also: STELLAR EVOLUTION .

There is increasing evidence that the Milky Way formed as a result of the coalescence of small galaxies and , objects with the masses of small dwarf galaxies that are thought to have been among the first objects to form in the universe. The coalescence would have proceeded rapidly at first and then more irregularly as relatively large pieces merged to form the Milky Way. According to this picture, the formation of the Milky

Way is not yet complete, and both small galaxies (such as the Magellanic Clouds and the Sagittarius Dwarf) and starless clouds of gas are continuing to rain in on the Milky Way. Direct evidence for this picture comes from the orbit of the Sagittarius Dwarf, which shows it to be colliding with the Milky Way, and from the presence of clouds of hydrogen gas with high velocities and low abundances of heavy elements, which appear to be accreting from intergalactic space.

When the Galaxy was about two-thirds its present age, 4.6 billion years ago, the Sun and the planets formed from the interstellar medium in the disk, the Sun being a rather unremarkable star in an unremarkable location. The part of the disk that has not yet been used up in the process of star formation continues to form stars. See also:

SOLAR SYSTEM ; SUN .

The Milky Way and M31 are approaching each other at a velocity of 125 km ∕ s (78 mi ∕ s). At that velocity, the two galaxies will collide in about 5 billion years if the transverse velocity of M31 is not too large. The collision will not be violent; the distance between the stars is too vast for more than a few stars to actually collide. The likely outcome is a merged system that resembles an . See also: GALAXY FORMATION AND EVOLUTION .

Leo Blitz AccessScience from McGraw-Hill Education Page 9 of 9 www.accessscience.com

Bibliography

J. Binney and M. Merrifield, , Princeton University Press, 1998

L. Blitz et al., The centre of the Milky Way, Nature , 361:417–424, 1993 DOI: http://doi.org/10.1038/361417a0

L. Blitz and P. Teuben (eds.), Unsolved Problems of the Milky Way , Kluwer Academic Publishers, 1996

B. J. Bok and P. F. Bok, The Milky Way , 5th ed., Harvard University Press, 1981

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284:576–598, 1997

B. K. Gibson, T. S. Axelrod, and M. E. Putman (eds.), The , Astronomical Society of the Pacific, 1999

I. King, G. Gilmore, and P. C. van der Kruit, The Milky Way as a Galaxy , University Science Books, 1994

Additional Readings

D. E. Backman and M. A. Seeds, Universe: Solar System, Stars, and Galaxies , 7th ed., Cengage Learning, Boston,

MA, 2012

C. Scannapieco et al., Formation history, structure and dynamics of discs and spheroids in simulated Milky Way mass galaxies, Mon. Not. R. Astron. Soc. , 417(1):154–171, 2011

DOI: http://doi.org/10.1111/j.1365-2966.2011.19027.x

M. A. Seeds and D. E. Backman, Horizons: Exploring the Universe , 12th ed., Cengage Learning, Boston, MA, 2012

M. A. Seeds and D. E. Backman, Stars and Galaxies , 8th ed., Cengage Learning, Boston, MA, 2013

J. Shen et al., Our Milky Way as a pure-disk galaxy: A challenge for galaxy formation, Astrophys. J. Lett. ,

720(1):L72, 2010 DOI: http://doi.org/10.1088/2041-8205/720/1/L72