Milky Way

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For other uses, see Milky Way (disambiguation).

The Milky Way (a translation of the Latin Via Lactea, in turn derived from the Greek Γαλαξίας (Galaxias), sometimes referred to simply as "the Galaxy"), is a barred spiral[1] galaxy of the Local Group. Although the Milky Way is but one of billions of galaxies in the universe, the Galaxy has special significance to humanity as it is the home of the Solar System. The Greek philosopher Democritus (450 BC–370 BC) was the first known person to claim that the Milky Way consists of distant stars.

The term "milky" originates from the hazy band of white light appearing across the celestial sphere visible from Earth, which comprises stars and other material lying within the galactic plane. The galaxy appears brightest in the direction of Sagittarius, towards the galactic center. Relative to the celestial equator, the Milky Way 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 fact that the Milky Way divides the night sky into two roughly equal hemispheres indicates that the solar system lies close to the galactic plane.

The main disk of the Milky Way Galaxy is about 80,000 to 100,000 light-years in diameter, about 250,000 to 300,000 light-years in circumference, and outside the Galactic core, about 1,000 light-years in thickness. It is composed of 200[2] billion stars but this number might reach 400 billion [1] if small-mass stars predominate. As a guide to the relative physical scale of the Milky Way, if the galaxy were reduced to 130 km (80 mi) in diameter, the solar system would be a mere 2 mm (0.08 inches) in width. The Galactic Halo extends outward, but is limited in size by the orbits of the two Milky Way satellites, the Large and the Small Magellanic Clouds, whose perigalacticon is at ~180,000 light-years.[3] As detailed in the Structure section below, new discoveries indicate that the disk extends much farther than previously thought.

The Milky Way's visual absolute magnitude is −20.9[4]

The galactic center in the direction of Sagittarius. The primary stars of Sagittarius are indicated in red.

Age

It is extremely difficult to define the age at which the Milky Way formed, but the age of the oldest stars in the Galaxy is currently estimated to be about 13.6 billion years, which is nearly as old as the Universe itself.[5]

This estimate is based upon research performed in 2004 by a team of astronomers: Luca Pasquini, Piercarlo Bonifacio, Sofia Randich, Daniele Galli, and Raffaele G. Gratton. The team used the UV-Visual Echelle Spectrograph of the Very Large Telescope to measure, for the first time, the beryllium content of two stars in globular cluster NGC 6397. This allowed them to deduce the elapsed time between the rise of the first generation of stars in the entire Galaxy and the first generation of stars in the cluster, at 200 million to 300 million years. By including the estimated age of the stars in the globular cluster (13.4 ± 0.8 billion years), they estimated the age of the oldest Milky Way stars at 13.6 ± 0.8 billion years.

Structure

Observed structure of the Milky Way's spiral arms

The mass distribution within the Milky Way closely resembles the Sbc Hubble classification, which is a spiral-galaxy with relatively loosely-wound arms.[6] It was only in the 1980s that astronomers began to suspect that the Milky Way is a barred spiral[7] rather than an ordinary spiral, which observations in 2005 with the Spitzer Space Telescope have since confirmed, showing that the galaxy's central bar is larger than previously suspected.[8] This argues for a classification of type SBbc (loosely wound barred spiral). In 1970 Gérard de Vaucouleurs predicted that the Milky Way was of type SAB(rs)bc, where the "rs" indicates a broken ring structure around the core region.[9]

As of 2006, the Milky Way's mass is thought to be about 5.8×1011 M☉[10][11][12] comprising 200 to 400 billion stars. Its integrated absolute visual magnitude has been estimated to be -20.9.

The galactic disk, which bulges outward at the galactic center, has an estimated diameter of about 100,000 light-years. The distance from the Sun to the galactic center is now estimated at 26,000 ± 1400 light-years while older estimates could put our parent star as far as 35,000 light-years from the central bulge.

The galactic center harbors a compact object of very large mass (named Sagittarius A*), strongly suspected to be a supermassive black hole. Most galaxies are believed to have a supermassive black hole at their center.[13]

As is typical for many galaxies, the distribution of mass in the Milky Way is such that the orbital speed of most stars in the galaxy does not depend strongly on its distance from the center. Away from the central bulge or outer rim, the typical stellar velocity is between 210 and 240 km/s.[14] Hence the orbital period of the typical star is directly proportional only to the length of the path traveled. This is unlike in the solar system where different orbits are also expected to have significantly different velocities associated with them, and is one of the major pieces of evidence for the existence of dark matter.

The galaxy's bar is thought to be about 27,000 light-years long, running through the center of the galaxy at a 44 ± 10 degree angle to the line between our sun and the center of the galaxy. It is composed primarily of red stars, believed to be ancient. The bar is surrounded by a ring called the "5-kpc ring" that contains a large fraction of the molecular hydrogen present in the galaxy and most of the Milky Way's star formation activity. Viewed from the Andromeda Galaxy, it would be the brightest feature of the Milky Way[15]

Each spiral arm describes a logarithmic spiral (as do the arms of all spiral galaxies) with a pitch of approximately 12 degrees. There are believed to be four major spiral arms which all start at the Galaxy's center. These are named as follows, according to the image at left:

Observed and extrapolated structure of the spiral arms (click to see legend)

There are at least two smaller arms or spurs, including:

  • 11 - Orion Arm (which contains the solar system and the Sun - 12)

Outside of the major spiral arms is the Outer Ring or Monoceros Ring, a ring of stars around the Milky Way proposed by astronomers Brian Yanny and Heidi Jo Newberg, which consists of gas and stars torn from other galaxies billions of years ago.

The galactic disk is surrounded by a spheroid halo of old stars and globular clusters, whose 90% lie within 100,000 light-years [2], suggesting a stellar halo diameter of 200,000 light-years. However, a few globular clusters have been found farther, such as PAL 4 and AM1 at more than 200,000 light-years away from the galactic center. While the disk contains gas and dust obscuring 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.

Most of the mass of the Milky Way is thought to be dark matter, forming a dark matter halo of an estimated 600-3000 billion solar masses (M☉) which is concentrated towards the Galactic Center.[12]

Recent discoveries have given added dimension to our knowledge of the structure of the Milky Way. With the discovery that the disc of the Andromeda Galaxy (M31) extends much further than previously thought,[16] the possibility of the disk of the Milky Way extending further is apparent, and this is supported by evidence of the newly discovered Outer Arm extension of the Cygnus Arm.[17] With the discovery of the Sagittarius Dwarf Elliptical Galaxy came the discovery of a ribbon of galactic debris as the polar orbit of Sagittarius and its interaction with the Milky Way tears it apart. Similarly, with the discovery of the Canis Major Dwarf Galaxy, a ring of galactic debris from its interaction with the Milky Way encircles the galactic disk.

On January 9, 2006 Mario Juric and others of Princeton University announced that the Sloan Digital Sky Survey of the northern sky has 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 our current models. The collection of stars rises close to perpendicular to the plane of the spiral arms of the Milky Way. The proposed likely interpretation is that a dwarf galaxy is merging with the Milky Way. This galaxy is tentatively named the Virgo Stellar Stream and is found in the direction of Virgo about 30,000 light-years away.

The Sun's place in the Milky Way

360-degree photographic panorama of the entire galaxy, from the viewpoint of our solar system.

The Sun (and therefore the Earth and Solar System) may be found close to the inner rim of the Orion Arm, in the Local Fluff, at a distance of 7.94±0.42 kpc from the Galactic Center.[18][19][20] The distance between the local arm and the next arm out, the Perseus Arm, is about 6,500 light-years.[21] Our Sun, and thus the solar system, is found in what scientists call the galactic habitable zone.

The Apex of the Sun's Way, or the solar apex, refers to 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.

It takes the solar system about 225-250 million years to complete one orbit (a galactic year),[22] and so is thought to have completed about 20-25 orbits during its lifetime or 0.0008 orbit since the origin of humans. The orbital speed of the solar system is 217 km/s, i.e. 1 light-year in ca. 1400 years, and 1 AU in 8 days.

The Milky Way environment

NGC 7331 is often referred to as "the Milky Way's twin." This is what an observer from another galaxy might see when looking at the Milky Way.

The Milky Way and the Andromeda Galaxy are a binary system of giant spiral galaxies. Together with their companion galaxies they form the Local Group, a group of some 50 closely bound galaxies. The Local Group is part of the Virgo Supercluster.

The Milky Way is orbited by two smaller galaxies and a number of dwarf galaxies in the Local Group. 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. 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 Milky Way, causing vibrations at certain frequencies when they pass through the edges of our Galaxy. 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 Milky Way. 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 Milky Way disk with the known gas layer. As a result, the model predicts that the gravitational impact of the Magellanic Clouds is amplified as they pass through the Milky Way.

Speed through space

In the general sense, the absolute speed 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 galaxy's motion. (Motion must always be specified with respect to another object.)

With this in mind, many astronomers believe the galaxy is moving through space at approximately 600km per second relative to the observed locations of other nearby galaxies. Most recent estimates range from 130 km/s to 1,000 km/s. If indeed the Milky Way is moving at 600 km per second, we are traveling 51.84 million km per day, or more than 18.9 billion km per year. For comparison, this would mean that each year, we are traveling about 4.5 times the distance that Pluto lies from the Earth (at its closest). The Milky Way is thought to be moving in the direction of the constellation Hydra, and may someday become a close-knit member of the Virgo cluster of galaxies. Our galaxy may also collide with the Andromeda galaxy in roughly 4 billion years. See below.

Another reference frame is provided by the CMB. The Milky Way is moving at around 552 km/s[23] with respect to the photons of the cosmic microwave background. This can be observed by satellites such as COBE and WMAP as a dipole contribution to the CMB, as photons in equilibrium at the CMB frame get blue-shifted in the direction of the motion and red-shifted in the opposite direction.

Future of the Milky Way

Main article: Andromeda-Milky Way collision

Current measurements suggest the Andromeda Galaxy is approaching us at 100 to 140 kilometers per second, and that the Milky Way might collide with it in several (3-4) billion years, depending on the importance of unknown lateral components to the galaxies' relative motion. If they do collide, it is thought that our sun and the other stars of the Milky Way will probably not collide with the stars of Andromeda, but that the two galaxies will merge to form a single elliptical galaxy over the course of about a billion years.[24]

Name and myths

Main article: List of names for the Milky Way
Main article: Milky Way (mythology)

There are many creation myths around the world which explain the origin of the Milky Way and give it its name. The English phrase is a translation from Greek Γαλαξίας Galaxias which is derived from the word for milk (γάλα, gala). This is also the origin of the word galaxy. In Greek myth the Milky Way was caused by milk spilt by Hera when suckled by Heracles.

References

  1. ^ C. Alard (2001). "Another bar in the Bulge". Astronomy and Astrophysics. 379 (2): L44–L47.
  2. ^ Sanders, Robert (January 9, 2006). "Milky Way galaxy is warped and vibrating like a drum". UCBerkeley News. Retrieved 2006-05-24.
  3. ^ Connors; et al. (January 26, 2007). "N-body simulations of the Magellanic stream". Monthly Notices of the Royal Astronomical Society. Retrieved 2007-01-26. {{cite news}}: Explicit use of et al. in: |first= (help)
  4. ^ "The Local Group of Galaxies". A.A Springer. Retrieved 2007-03-14.
  5. ^ 17 August 2004 - Press release, European Southern Observatory
  6. ^ Ortwin, Gerhard (2002). "Mass distribution in our Galaxy". Space Science Reviews. 100 (1/4): 129–138. Retrieved 2007-03-14.
  7. ^ Chen, W.; Gehrels, N.; Diehl, R.; Hartmann, D. (1996). "On the spiral arm interpretation of COMPTEL ^26^Al map features". Space Science Reviews. 120: 315–316. Retrieved 2007-03-14.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  8. ^ 16 August 2005 - New Scientist article
  9. ^ López-Corredoira, M.; Cabrera-Lavers, A.; Mahoney, T. J.; Hammersley, P. L.; Garzón, F.; González-Fernández, C. (2007). "The Long Bar in the Milky Way: Corroboration of an Old Hypothesis". The Astronomical Journal. 133 (1): 154–161. Retrieved 2007-03-15.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  10. ^ Karachentsev, I. D.; Kashibadze, O. G. (2006). "Masses of the local group and of the M81 group estimated from distortions in the local velocity field". Astrophysics. 49 (1): 3–18.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  11. ^ The Physics Factbook - entry citing references about the mass of the Milky Way. URL accessed March 16, 2006.
  12. ^ a b The radial velocity dispersion profile of the Galactic halo: Constraining the density profile of the dark halo of the Milky Way, Battaglia et al. 2005, MNRAS, 364 (2005) 433 Cite error: The named reference "galmass" was defined multiple times with different content (see the help page).
  13. ^ Blandford, R.D. (1999). "Origin and evolution of massive black holes in galactic nuclei". Galaxy Dynamics, proceedings of a conference held at Rutgers University, 8-12 Aug 1998,ASP Conference Series vol. 182. {{cite conference}}: Cite has empty unknown parameter: |coauthors= (help); Unknown parameter |booktitle= ignored (|book-title= suggested) (help)
  14. ^ http://zebu.uoregon.edu/~imamura/123/lecture-2/mass.html
  15. ^ [ 23 April 2006] - http://www.bu.edu/galacticring/new_introduction.htm
  16. ^ 6 April 2005 - Ibata, R. et al, Astrophys. Journal, 634 (2005) 287-313
  17. ^ http://www.solstation.com/x-objects/gal-ring.htm
  18. ^ Reid, M. J. (1993), "The distance to the center of the Galaxy". Annual Review of Astronomy and Astrophysics, Vol. 31, p. 345-372.
  19. ^ Eisenhauer, F., et al (2003), "A Geometric Determination of the Distance to the Galactic Center" Astrophys.J. 597 L121-L124.
  20. ^ Horrobin, M. et al (2004), "First results from SPIFFI. I: The Galactic Center" (PDF). Astronomische Nachrichten, Vol. 325, p. 120-123.
  21. ^ 14 January 2000 - Press release, Canadian Galactic Plan Survey
  22. ^ http://hypertextbook.com/facts/2002/StacyLeong.shtml
  23. ^ 23 October 2006 - ApJ COBE paper
  24. ^ Wong, Janet (April 14, 2000). "Astrophysicist maps out our own galaxy's end". University of Toronto. Retrieved 2007-01-11.

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