M8 (The Lagoon Nebula) (The Hourglass Nebula) (NGC 6532, NGC 6530)

Distance: 5800 light years

Right Ascension: 18 : 03.8 (hours : minutes)
Declination: -24 : 23 (degrees : minutes)

text copyright Robert Gendler

Similar to other bright HII regions M8 exists as a thin glowing blister of excited gas on the near surface of a giant molecular cloud. The outer perimeter of M8 shows a noticeable paucity of stars betraying the subtle presence of a dark molecular cloud which permeates the region and attenuates the background stars. The large HII region dubbed the "Lagoon Nebula" (NGC 6532) is centered on the bright young cluster NGC 6530. The nebula is illuminated by several O-type giants within NGC 6530. The brightest part of the nebula is known as the "hourglass nebula" and its gases are excited primarily by two massive O-type supergiants designated Herschel 36 and 9 Sagittarii. These two stars lie to the west of NGC 6530 and are separated from it by a dark lane which divides the nebula like a black lagoon, giving it its name. Twisted funnel shaped clouds are visible close to the illuminating stars, especially Herschel 36. The twists are due to shear forces between the hot excited gases at the surface and the cold interior of the clouds. Remarkably, at least 60 B-type giants are embedded in the nebula which make the Lagoon Nebula 3 to 4 times richer in massive stars than the Orion nebula.

Detailed investigation of the stars of NGC 6530 show a distinct gradient of stellar ages, confirming that star formation in the M8 region has proceeded in a strictly sequential manner. Herschel 36, the dominant illuminating star of the HII cloud is embedded about 16 light years to the west of NGC 6530 and is thought to have formed much more recently than other members of the cluster. Star formation apparently began near the center of NGC 6530 and propagated southeast and southwest as indicated by the stellar ages which range from 0.5 to 4 million years old. The stars of NGC 6530 are only slightly older than the young Orion Nebula stars.

Conspicuous dark globules called Bok globules are visible in parts of M8 and believed to represent collapsing protostellar cores with diameters of about 10,000 astronomical units. These dark structures represent condensations of gas and dust which will later condense to form infant protostars. Recent x-ray surveys of M8 have identified up to 800 protostars within its clouds. Protostars, sometimes referred to as Young Stellar Objects (YSO's) are pre-main sequence stars. Their pre-main sequence status is due to the fact that they do not yet make energy by nuclear fusion. Their energy comes from gravitational contraction. The contraction ceases when the protostar becomes a mature main sequence star and begins to fuse its nuclear fuel.

Protostars are classified as Class 0, I, II and III depending on their evolution. The transition to the next class involves changes in the stars infrared spectrum as the dust and gas envelope surrounding the star diminishes. Class O protostars are still heavily cloaked by their parent molecular cloud. At 10,000 years old they are still relative infants by stellar standards and are only a few tens of degrees warmer than the cloud of cold molecular gas from which they formed. Paradoxically Class 0 protostars produce jets of hot plasma reaching 10 to 100 million degrees emitting powerful x-rays. The high energy outflows can cause neighboring cloud cores to collapse triggering a cascade of new stars to form. Class 1 protostars are more evolved and are about 100, 000 years old. Molecular gas around the star has condensed to form a rotating accretion disk. Class 1 protostars are still not visible at optical wavelengths but have stronger emission in the near infrared. Class 2 and 3 protostars include the Classic and Weak-lined T Tauri stars which are optically detectable and which show their characteristic variability and emission lines including x-ray emission. As the star becomes further evolved the rotating accretion disk slowly drains into the star until it becomes a mature main sequence star.