Spiral galaxy containing our Solar System
This article is about the galaxy. For other uses, see Milky Way (disambiguation).
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Milky Way Galaxy
The Galactic Center as seen from Earth's night sky (the laser creates a guide-star for the telescope)Observation dataTypeSb, Sbc, or SB(rs)bc(barred spiral galaxy)DiameterStellar disk: 170-200 kly 
Dark matter halo: ≈1.9 ± 0.4 Mly (580 ± 120 kpc)Thickness of thin stellar disk≈2 kly (0.6 kpc)Number of stars250–500 billion [(1–4)×1011]Mass(0.8–1.5)×1012 M☉Angular momentum≈1×1067 J sSun's distance to Galactic Center25.6–27.1 kly (7.86–8.32 kpc)Sun's Galactic rotation period240 MyrSpiral pattern rotation period220–360 MyrBar pattern rotation period100–120 MyrSpeed relative to CMB rest frame552.2±5.5 km/sEscape velocity at Sun's position550 km/sDark matter density at Sun's position0.0088+0.0024−0.0018 M☉pc−3 or 0.35+0.08−0.07 GeV cm-3See also: Galaxy, List of galaxies
The Milky Way[a] is the galaxy that contains our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye. The term Milky Way is a translation of the Latin via lactea, from the Greek γαλαξίας κύκλος (galaxías kýklos, “milky circle”). From Earth, the Milky Way appears as a band because its disk-shaped structure is viewed from within. Galileo Galilei first resolved the band of light into individual stars with his telescope in 1610. Until the early 1920s, most astronomers thought that the Milky Way contained all the stars in the Universe. Following the 1920 Great Debate between the astronomers Harlow Shapley and Heber Curtis, observations by Edwin Hubble showed that the Milky Way is just one of many galaxies.
The Milky Way is a barred spiral galaxy with a visible diameter between 150,000 and 200,000 light-years (ly). It is estimated to contain 100–400 billion stars and at least that number of planets. The dark matter halo around the Milky Way may span as much as 2 million light years.
 The Solar System is located at a radius of about 27,000 light-years from the Galactic Center, on the inner edge of the Orion Arm, one of the spiral-shaped concentrations of gas and dust. The stars in the innermost 10,000 light-years form a bulge and one or more bars that radiate from the bulge.
The galactic center is an intense radio source known as Sagittarius A*, a supermassive black hole of 4.100 (± 0.034) million solar masses.
Stars and gases at a wide range of distances from the Galactic Center orbit at approximately 220 kilometers per second. The constant rotation speed contradicts the laws of Keplerian dynamics and suggests that much (about 90%) of the mass of the Milky Way is invisible to telescopes, neither emitting nor absorbing electromagnetic radiation.
This conjectural mass has been termed “dark matter”. The rotational period is about 240 million years at the radius of the Sun. The Milky Way as a whole is moving at a velocity of approximately 600 km per second with respect to extragalactic frames of reference.
The oldest stars in the Milky Way are nearly as old as the Universe itself and thus probably formed shortly after the Dark Ages of the Big Bang.
The Milky Way has several satellite galaxies and is part of the Local Group of galaxies, which form part of the Virgo Supercluster, which is itself a component of the Laniakea Supercluster.
A view of the Milky Way toward the constellation Sagittarius (including the Galactic Center), as seen from a dark site with little light pollution (the Black Rock Desert, Nevada), the bright object on the lower right is Jupiter, just above Antares
Play media A time-lapse video capturing the Milky Way arching over ALMA
The Milky Way is visible from Earth as a hazy band of white light, some 30° wide, arching across the night sky. In night sky observing, although all the individual naked-eye stars in the entire sky are part of the Milky Way, the term “Milky Way” is limited to this band of light. The light originates from the accumulation of unresolved stars and other material located in the direction of the galactic plane. Dark regions within the band, such as the Great Rift and the Coalsack, are areas where interstellar dust blocks light from distant stars. The area of sky that the Milky Way obscures is called the Zone of Avoidance.
The Milky Way has a relatively low surface brightness. Its visibility can be greatly reduced by background light, such as light pollution or moonlight. The sky needs to be darker than about 20.2 magnitude per square arcsecond in order for the Milky Way to be visible. It should be visible if the limiting magnitude is approximately +5.
1 or better and shows a great deal of detail at +6.1. This makes the Milky Way difficult to see from brightly lit urban or suburban areas, but very prominent when viewed from rural areas when the Moon is below the horizon.
[b] Maps of artificial night sky brightness show that more than one-third of Earth's population cannot see the Milky Way from their homes due to light pollution.
As viewed from Earth, the visible region of the Milky Way's galactic plane occupies an area of the sky that includes 30 constellations. The Galactic Center lies in the direction of Sagittarius, where the Milky Way is brightest.
From Sagittarius, the hazy band of white light appears to pass around to the galactic anticenter in Auriga. The band then continues the rest of the way around the sky, back to Sagittarius, dividing the sky into two roughly equal hemispheres.
The galactic plane is inclined by about 60° to the ecliptic (the plane of 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 β Comae Berenices, and the south galactic pole is near α Sculptoris.
Because of this high inclination, depending on the time of night and year, the arch of the Milky Way may appear relatively low or relatively high in the sky. For observers from latitudes approximately 65° north to 65° south, the Milky Way passes directly overhead twice a day.
The Milky Way arching at a high inclination across the night sky, (this composited panorama was taken at Paranal Observatory in northern Chile), the bright object is Jupiter in the constellation Sagittarius, and the Magellanic Clouds can be seen on the left; galactic north is downward
Size and mass
It Would Take 200,000 Years at Light Speed to Cross the Milky Way
The Milky Way's starry disk is bigger than previously thought, a new study reports. It extends to at least the inner dotted circle in this illustration, and may reach even farther out.
(Image: © R. Hurt, SSC-Caltech, NASA/JPL-Caltech)
The disk of our home galaxy – the Milky Way – is bigger than we previously thought. A new study shows it would take 200,000 years for a spaceship traveling at the speed of light to go across the entire galaxy.
Researchers made the find after analyzing the abundances of metals (heavy elements) in stars, also known as their metallicities. When looking beyond the previously assumed boundary of the Milky Way's disk, scientists were surprised to see stars with compositions resembling those of disk stars. [Amazing Photos of Our Milky Way Galaxy]
“We have shown that there is an appreciable fraction of stars with higher metallicity, characteristic of disc stars, further out than the previously assumed limit on the radius of the galaxy disc,” study co-author Carlos Allende, a researcher at Astrophysics Institute of the Canary Islands (Instituto de Astrofisica de Canarias, said in a statement.
The new study estimates the size of the Milky Way's disk at 200,000 light-years across. Past studies have suggested the Milky Way is between 100,000 light-years and 160,000 light-years across. (One light-year is the distance light travels in a year, about 6 trillion miles or 10 trillion kilometers.)
To put the find in perspective with the location of our own sun, astronomers said the newly found disk stars are about three times farther from the galaxy center than the sun. It's possible there could be even more disk stars about four times farther away, the team added in the statement.
Researchers made the find after analyzing survey data from the Apache Point Observatory Galactic Evolution Experiment (APOGEE) and the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST), which collect the spectra of stars. A star's spectrum is the breakdown of its light into different colors. By analyzing the pattern of colors, scientists learn what elements are present within the star.
It's not the first time scientists have revised a galaxy's parameters. A recent study of the Andromeda galaxy revealed that the celestial body is actually about the same mass as the Milky Way, instead of larger. This affects predictions of the two galaxies' motions as they head for an inevitable crash in 4 billion years.
The new study was published in the journal Astronomy & Astrophysics and led by Martin Lopez-Corredoira, a researcher at IAC.
How Big Is The Milky Way?
The Milky Way is our home galaxy, the spot where the Earth resides. We are not anywhere near the center — NASA says we’re roughly 165 quadrillion miles from the galaxy’s black hole, for example — which demonstrates just how darn big the galaxy is. So how big is it, and how does it measure up with other neighborhood residents?
The numbers are pretty astounding. NASA estimates the galaxy at 100,000 light-years across. Since one light year is about 9.5 x 1012km, so the diameter of the Milky Way galaxy is about 9.5 x 1017 km in diameter. The thickness of the galaxy ranges depending on how close you are to the center, but it’s tens of thousands of light-years across.
Our galaxy is part of a collection known as the Local Group. Because some of these galaxies are prominent in our sky, the names tend to be familiar.
The Milky Way is on a collision course with the most massive member of the group, called M31 or the Andromeda Galaxy. The Milky Way is the second-largest member, with M33 (the Triangulum Galaxy) the third-largest, NASA says.
Andromeda appears much brighter in the night sky due to its size and relatively closer distance. There are about 30 members of this group.
The Andromeda Galaxy will collide with the Milky Way in the future. Credit: Adam Evans
Because we are inside the Milky Way’s arms, it appears as a band of stars (or a fuzzy white band) across the Earth’s sky.
Casting a pair of binoculars or a telescope across it shows a mix of lighter areas and darker areas; the darker areas are dust that obscures any light from stars, galaxies and other bright objects behind it.
From the outside, however, astronomers say the Milky Way is a barred spiral galaxy — a galaxy that has a band of stars across its center as well as the spiral shape.
If you’re looking for the center of the galaxy, gaze at the constellation Sagittarius, which is low on the summer sky horizon for most northern hemisphere residents.
The constellation contains a massive radio source known as Sagittarius A*.
Astronomers using the Chandra space telescope discovered why this supermassive black hole is relatively weak in X-rays: it’s because hot gas is being pulled inside the nebula, and most of it (99%) gets ejected and diffused.
Which spiral arm of the Milky Way contains our sun?
Artist’s concept of our Milky Way galaxy, as seen from far galactic north (the direction of the constellation Coma Berenices) via NASA/ JPL-Caltech/ R. Hurt/ Wikimedia Commons.
Our Milky Way galaxy is the island of stars we call home. If you imagined it as a disk with spiral arms emanating from the center, our sun is about a third of the way from the center to the visible edge.
Our solar system is located between two prominent spiral arms, in what astronomers once thought was a mere bridge of stars, gas, and dust clouds. In recent decades, research advances have revealed that we live in our very own spiral arm of the galaxy, albeit a relatively minor one.
Our spiral arm is formally called the Orion-Cygnus Arm. It’s also known simply as the Orion Arm or Local Arm, and you sometimes still hear the names Orion Bridge or Orion Spur.
Th Milky Way is a spiral galaxy. In fact, the Milky Way is a barred spiral galaxy, which means it has a central bar. There’s still a lot we don’t know about the structure of our galaxy.
According to the best current knowledge, the Milky Way is about 150,000 to 200,000 light-years across, and about 2,000 light-years deep, and has 100 to 400 billion stars.
There may be four primary spiral arms emanating from its center bar with an unknown number of smaller offshoot arms.
Where, within this vast spiral structure, do our sun and its planets reside? We’re about 26,000 light-years from the center of the galaxy, on the inner edge of the Orion-Cygnus Arm.
It’s sandwiched by two primary spiral arms, the Sagittarius and Perseus Arms. The artists’ concepts above and below show the Orion-Cygnus Arm, the home spiral arm of our sun in the Milky Way galaxy.
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The Milky Way galaxy may be much bigger than we thought
It's no secret that the Milky Way is big, but new research shows that it may be much bigger than we ever imagined.
The research, described May 7 in the journal “Astronomy & Astrophysics,” indicates that our spiral galaxy's vast rotating disk of stars spans at least 170,000 light-years, and possibly up to 200,000 light-years.
It's hard to fathom just how far that is. If you could ride a light beam from one side of the disk to the other, it would take 200,000 years to span the distance. If you could drive across and averaged 60 miles an hour, it would take more than 2 trillion years. That's about 150 times greater than the age of the universe, which is estimated to be about 13.8 billion years.
The colored region is the previously known galactic disk. The new research has extended its limits much farther away: there is a probability 99.7% or 95.4% respectively that there are disk stars in the regions outside the dashed/dotted circles. Yellow dot is the position of the sun. Background Milky Way image from “A Roadmap to the Milky Way.”R. Hurt / SSC-Caltech, NASA/JPL-Caltech
For many years, astronomers believed the Milky Way's disk spanned about 100,000 light-years. Then in 2015, researchers showed that a distance of 150,000 light-years was closer to the mark.
To arrive at the new number, researchers at the Canary Islands Institute of Astrophysics and the National Astronomical Observatories of Beijing turned to a pair of star atlases and studied the chemical composition of thousands of stars in the outermost parts of the galactic plane — the plane that extends through the center of the disk. The researchers used a statistical analysis to determine that the far-flung stars are chemically similar to the stars in the galactic disk and thus should be considered part of it.
“We were able to confirm that some stars of the outermost regions in the plane belong to the disk,” Martin Lopez-Corredoira, a researcher at the institute and the first author of the article describing the research, told NBC News MACH in an email.
Spiral galaxy NGC 6744 (shown here) is believed to be similar in appearance to our own Milky Way Galaxy.Dan Goldman
The finding offers further confirmation of the disk's complex structure, Heidi Newberg, an astrophysicist at Rensselaer Polytechnic Institute and the leader of the research team that revised the size estimate of the Milky Way in 2015, told NBC News MACH in an email.
“We are still trying to understand the details of how galaxies form, how spiral structure is formed and sustained, and how elements…are created in stars and then transported through the cosmos so they can be incorporated into planets and life,” Newberg said. “Information on the extent and detailed structure of the disk help answer these questions.”
The research also gives a new sense of the sun's position within the Milky Way. Previously, it was thought that the sun orbits the center of our galaxy at about half the galactic radius.
But now we know that some stars are more than three times that distance from the galaxy's center possibly more than four times that distance — so while the sun hasn't made any surprising moves, it's much closer to the center of the galaxy than we thought.
One thing that hasn't changed is the number of stars in the Milky Way. “Although we have increased the size of the galactic stellar disk, the number of stars and the total mass of the galaxy [are] not significantly affected because the outermost disk…has a very low density of stars,” Lopez-Corredoira said in the email.
Astronomers believe the Milky Way contains about 200 billion stars.