Lagoon Nebula (M8)
What is M8?
An emission nebula
The Lagoon would look grey to our eyes, but a long exposure photograph, like this one taken by László Francsics, shows M8 pretty in pink. That tells us it's an emission nebula. When hydrogen gas is energized, as it is here by the nebula's hot young stars, the hydrogen atoms emit a reddish light.
A stellar nursery
The New General Catalogue of Nebulae and Clusters of Stars (1888) listed the nebulous part of M8 as NGC 6523. It contains the vast amounts of gas – mainly hydrogen – needed for star formation on an epic scale.
An open star cluster NGC 6530
Star production in the nebula has been so prolific that a cluster of young stars – listed as NGC 6530 – has already formed within the last few million years. That may sound like a long time, but our Sun, for example, is only halfway through its life, and it's five billion years old.
The Hourglass Nebula
In the center of the Lagoon is an hourglass-shaped feature that John Herschel discovered and named in the 19th century. It's small compared to the whole nebula, making it hard to spot. However you can see the Hourglass Nebula in this detail of a photo taken by David Malin.
The brightness of the Hourglass is mainly attributable to the young star called Herschel 36. It has enough mass to make thirty stars like our Sun, is about seven times hotter than the Sun, and 200,000 times brighter.
Discovery and naming
With good conditions and a dark sky, M8 can be seen with the unaided eye as a fuzzy patch near the spout of the Sagittarius “Teapot”. Yet if anyone noticed it before the 17th century, there's no record of it.
The earliest known observation of the nebula was by Sicilian astronomer Giovanni Battista Hodierna (1597-1660). Over a century later, it was independently discovered by French astronomer Guillaume Le Gentil (1725-1792) who noted both the nebula and the associated star cluster. Charles Messier listed it as number 8 in his Catalogue of Nebulae and Star Clusters.
The nickname “Lagoon Nebula” was given to M8 in 1890 by Irish astronomer Agnes Clerke because of its wide, lagoon-shaped dust lane, shown clearly in this photograph by Joe Bauman.
This lagoon is not sleepy
An old orchestral piece gives a sound picture of a “Sleepy Lagoon”, but such tranquility doesn't apply to M8. It's anything but sleepy. Although star formation is a creative process, at the same time, it's violent and destructive.
The beginning: a gas cloud
Star formation begins in a gas cloud when a disturbance sets off the collapse. This can happen in many locations in an enormous cloud like the one that produced the star cluster NGC 6530. But it can also be happening in a much smaller cloud such as a Bok globule. These globules are small dark nebulae that look like holes in space – the Lagoon is peppered with them.
As the collapse of the gas proceeds, it gets denser and hotter, and begins to spin. Like a skater that spins faster when she puts her arms down, the star material spins faster as it contracts. Some of it then flattens out into a disk. The central mass becomes a protostar and the disk will provide material for planets and moons and other smaller bodies. A lot of dust still surrounds the protostar, acting like a cocoon. But protostars aren't sleepy babies. They often have a pair of powerful jets, one at each pole shooting out material at high speed.
When the protostar gets sufficiently dense and hot to sustain hydrogen fusion, a star has been born. And the infant star is a bit of a handful. Its energetic stellar winds blow at speeds of a few hundred kilometers per second. When the wind of hot gas and atomic particles hits the surrounding cloud, it generates shockwaves, creating considerable turbulence and breaking up the cloud. The star also emits intense ultraviolet radiation which not only makes the hydrogen gas glow, but also vaporizes the surrounding cloud. This is how the star escapes its cocoon and why stellar nurseries have a sculpted look to them.
Birth and death in the Lagoon
The nursery is full of stars and protostars. Young star Herschel 36 is a central one in the Lagoon, and very active. Its fierce brightness lights up the nebula near the Hourglass. We can also see in this close-up from a Hubble image that Herschel 36 has opened up its dust cocoon.
Not only are stars being born in the Lagoon, they're also dying. Massive stars like Herschel 36 are short-lived, and they literally go out with a bang. There's evidence in the nebula of the filaments associated with supernova explosions. But when a star dies in a supernova, its heavy elements are flung out to be recycled into new stars. That is the destiny of Herschel 36.
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