M8 – the Lagoon Nebula
The Lagoon Nebula is on Messier’s list along with several other objects. It is a huge gas cloud in which large amounts of stars are formed. A completely new cluster has already been created in its Eastern region! The gas of the Lagoon Nebula is brought to light by very massive stars that are still hidden in the mist, creating a spectacular show. You can already see this nursery of stars with binoculars or a small telescope; northeast of the “Teapot” that is formed by the brightest stars of Sagittarius.
Lagoon Nebula Facts and figures
- Other designations: NGC 6523, Sharpless 25
- Position: right ascension 18h03m37s;declination -24h23m12s
- Type: Emission Nebula with open cluster
- Magnitude: 6.0
- Diameter: 110 x 50 light-years
- Distance: 4100 light years
The Lagoon Nebula was first discovered by Hodierna in the seventeenth century. He has a long history of observations and descriptions and his work was cataloged several times in the 1800’s. Messier found Hodierna in 1764, in the records of English astronomer John Flamsteed (1646-1719), inventor of the numbering system for bright stars. A system that is still used to this day. In the catalog, Messier described both the nebula itself and the resulting open cluster: “A group that appears in the form of a mist through an ordinary three-inch telescope, but with an excellent instrument one sees nothing but a large number of small stars. Near this group is a fairly bright star surrounded by a faint glow.” However; Messier had no idea that M8 is a star forming region. It was not realzed until the early twentieth century that, that was an exact picture of stars. Its name comes from the dark band that seems to divide it in two and it was first used in 1890 by Agnes Clerke.
The red light from the Lagoon Nebula M8
The Lagoon Nebula will appear gray because our naked eyes are not sensitive enough to see its true color. But through a telescope it is invariably red. This color is caused by hydrogen gas and the spray becoming ionized by the strong light of the massive stars that are located in the mist. Nebulae, like M8, also have HII regions called HII where the mist is in an ionized hydrogen state. This hydrogen has lost its excess energy which makes it radiate and conseqeuntly makes the nebula visible. It is one of the largest of its kind in the Milky Way but in other galaxies tens of thousands of stars exist at the same time with similar forms. In the Lagoon Nebula, this number is in the hundreds.
Massive stars in Lagoon Nebula
In the eastern region of the Lagoon Nebula the NGC 6530, an open cluster that is only about 2 million years old, can be found. This cluster contains a number of massive stars that contribute to the illumination of the nebula. The brightest of these is the star Sagittarii 9, which can be seen from a great distance by the naked eye. It is a blue main sequence star of the type O4V where the hydrogen supply literally flows with great ease. It is 40 to 50 times heavier than the Sun. (Which is heavy for stars of this magnitude.) 2 million years old and already at half of its entire existence! At the other end of the dark band a clear bright spot can be detected at its center. This spot has a large hourglass shape. It is the main generator of the Lagoon Nebula hidden by the star Herschel 36 . This star is as hot and heavy as 9 Sagittarii, but possibly only a few tens of thousands of years old. To Herschel there are about 36 of these types of stars forming. Their collective energy is so intense that the Lagoon Nebula will slowly but surely be blown apart.
Within the Lagoon Nebula is dark gas and dust. Usually, these aren’t more than a few light years wide and are several times heavier than the mass of the Sun. Measurements show that these are likely cocoons in which lighter stars are formed. Sometimes a globule strays too close and is gradually blown apart. Afterwards, a “tail” of matter can be seen. If they remain untouched, stars (and possibly planets) can then be formed. The star formation process will continue for the time being at M8, but the residual gas (10% of the average nebula emission converted into new stars) will disappear from sight. On the site of the lagoon a second open cluster will appear, along with NGC 6530; a new “double open cluster”. Perhaps with a few doppelgangers of our Sun.