Milky Way's Spiral Disk

Robert Hurt, GLIMPSE, SSC,
Caltech, JPL, NASA






Larger and jumbo illustration.






The central bulge has a long bar
identified with some 30 million stars
that may help to feed the central
black hole with gas and dust (more).

Within the spiral disk, the Solar system can be found less than half way out (26,000 ly) from the galactic center in Sagittarius, on the core-ward side of the 2,000 to 3,000 ly thick "Orion Spur" of one of the Milky Way's two major spiral arms. Roughly 6,000 ly separate the Orion Spur from the major Perseus arm on the outside and the minor Sagittarius-Carina arm on the inside. Sol is located 67 ly north of the galactic plane within a roughly 200-ly wide band that is rich in gas, dust, and newborn stars, particularly the associations of extremely bright, bluish, and massive O and B stars and emission nebulae (H II) that light up and define the spiral arms. The apparent voids between spiral arms are actually full of dimmer, redder, and less massive stars like Sol. From our perspective, the galactic rim is in the direction of Auriga and Taurus.

Due to the dust that blocked direct observation with optical telescopes, astronomers have found it extremely difficult to study the galaxy from the vantage of the Solar System point within the MIlky Way's disk. As a result, many specific structural features of the spiral arms are not well known, especially when compared with other spirals observed outside the Milky Way such as Andromeda (which is located a few million ly above the galactic plane of obscuring dust in the Milky Way's disk. Fortunately, astronomers have been able to use longer radio wavelengths that are not absorbed by the obscuring dust and radiowave-emitting molecules like carbon monoxide (which are concentrated in the spiral arms) to trace the spiral disk's structure. (SAO Weekly Science Update).

Thomas M. Dame and
Pat Thaddeus, SAO


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Spiral galaxies like the Milky Way and its largest neighbor,
Andromeda, have large central bulges of mostly older stars,
as well as a relatively young thin spiral disk (surrounded
by older, thick disk stars that may have come from mergers
with satellite galaxies) and a luminous halo that includes
numerous globular clusters (more).

On January 10, 2008, astronomers using the infrared Spitzer Space Telescope announced that they had found some "thin" spiral galaxies (lacking a central "bulge" or concentration of stars) which also host supermassive central black holes. These holes were detected because they were "feeding" on accreting matter and "spitting out" high energy light via bi-polar jets that ionize gas in and around galactic cores, including neon-5 which black holes (but not stars) have sufficient energy to excite such a highly ionized state. Previous observations had suggested that the more massive the central bulge in spiral galaxies, the more massive their central black holes. Their findings indicate that the development of a central bulge is not necessarily associated with the development of a supermassive black hole. Alternatively, some astronomers suggest that large concentrations of dark matter may have played vital roles early in the development of supermassive black holes (more from SSC and Seth et al, 2007).

Unknown artist, SSC,
JPL, Caltech, NASA





Larger illustration.





Some spiral galaxies do
not have a central "bulge"
(or concentration of stars)
but may still host a
supermassive central
black hole -- shown here
with bi-polar jets erupting
from an active central black
hole (more).

Most stars in the Solar neighborhood are disk stars. Often called Population I stars, the relatively younger stars of the galactic disk can be further subdivided into four distinctive groups: very young spiral arm; young thin disk; intermediate-age disk; and older thick disk and nucleus. As mentioned previously, the spiral arms include most of the galaxy's interstellar gas and dust, young stars, and stellar associations, including: O and B stars; supergiants; Cepheid variables; pre-main sequence, T-Tauri stars and Herbig-Haro objects (e.g., R Coronae Australis/CD-37 13027); and some A stars. Less than a hundred million years old, these stars are rich in metals (as rich as, but ranging up to twice, Sol's abundance) and have highly circular galactic orbits within 1,000 ly of the galactic plane. While often extremely bright when not obscured by dust, these stars probably total substantially less than one percent of all Milky Way or nearby stars.

Robert Gendler, Amanda Smith, IoA, Cambridge U.


Larger and jumbo image.


Spiral galaxies like the Milky Way and its largest neighbor,
Andromeda, have large central bulges of mostly older stars,
as well as a relatively young thin spiral disk (surrounded
by older, "thick disk" stars that may have come from mergers
with satellite galaxies) and a luminous halo that includes
numerous globular clusters (more).

Young thin disk stars lie within 1,500 ly of the galactic plane and have galactic orbits of low eccentricity. Around one billion years or more in age, they include many A and F stars, AFGK giants, some GKM main-sequence dwarfs, and white dwarfs. While they have a mean metals abundance near Sol's (1.0), some may be twice as rich. Totalling as much as nine percent of all stars in the solar neighborhood, they include Sirius2 (A0-1Vm and A2-5VII -- also DA2-5) and Vega (A0Va).

Intermediate-age disk stars include our Sun (G2V), most G and some K and M dwarfs, some subgiants and red giants, and planetary nebulae. Many are around five billion years old and have a metals content ranging from 0.5 to 1.0 of Sol's (with a mean around 0.8). These stars lie within 3,000 ly of the galactic plane, with moderately eccentric galactic orbits. For example, Sol is traveling at seven kilometers per second northward out of the plane and may eventually rise 200-250 ly above it after 15 million years, while the Alpha Centauri3 (G2V, K1V, and M5.5Ve flare star) system may eventually travel about 800 ly out with an upward velocity that is three times faster. As much as 84 percent of the stars in the solar neighborhood are included in this group.

Most thick disk and many nucleus stars are old. While many are more than eight billion years old, they are probably less than 10 billion years old. They include many K and M dwarfs, white dwarfs, some subgiants and red giants, moderately metals-poor stars, long-period variables, and RR Lyrae variables with periods less than 12 hours. Most thick disk stars lie within 5,000 ly of the galactic plane (thick disk mean of 3,500 ly) and have considerably eccentric orbits. Their metals abundance ranges from 0.2 to 0.5 of Sol's (with a thick disk mean of 0.3). Thick disk stars may comprise as much as four percent of nearby stars, including Lalande 2115 (M2.1V) which is moving perpendicular to the galactic plane at a fast velocity of 47 km/sec.

© Robert Gendler,
Astroimaging Gallery
(Used with permission)

Larger mosaic (more).


The Milky Way Galaxy may be
as large as Andromeda, another
spiral galaxy that was previously
believed to be the largest
member of the Local Group of
galaxies, which includes the
Milky Way and its satellite
galaxies. Andromeda's own
satellites include M32, at
center left, and M110, at
lower right (more from APOD).

On January 5, 2009, astronomers using the Very Long Baseline Array (VLBA) radio telescope announced that the rotation speed of the galaxy at the location of the Solar System around 26,000 light-years from the galactic center is roughly 100,000 miles per hour faster than previously estimated, at 568,000 miles (or 914,000 kilometers) per hour (CfA press release). In findings presented at the 213th American Astronomical Society meeting, the astronomers also noted that the increased rotation speed implies that the Milky Way is some 50 percent more massive than previously estimated, making it about as large as the nearby Andromeda Galaxy. The Milky Way's larger mass estimate means that its gravitational pull is greater, which increases the likelihood of collisions with neighboring galaxies.