In the Lair of a Lagoon

Wide angle view of Trifid and Lagoon Nebulae, Sep 20, 2024.
Photo credit: Purnendu Gupta

The Lagoon nebula is a popular late-summer / early fall target for stargaers in the northern hemisphere. Perched above the tipped "teapot" of Sagittarius it disappears below the horizon quickly in the following months. By that measure, this Friday, September 20th, two days before the official start of fall, was past its prime viewing window. Yet, I chose to give it a shot.  

The result was the image above. A two-for-one wide angle view of two nebulae, the Trifid on the left and the Lagoon on the right. Light that is more than four thousand years old, gathered over 52 minutes through a narrowband filter. The image is rich in red Hydrogen-alpha light which is the signature of emission nebulae. To me, the swirling clouds around a brighter core of the Lagoon looks like the whorls in rose petals. The nearby Trifid has menacing dark streaks across its face. Unfettered by such features, the Lagoon might appear to be a calmer place.  Yet, nothing could be farther from the truth.

Same But Different

But before I go into an explanation, a few notes on what was different in my equipment approach, this time. If that is not your interest, feel free to jump to the next section, you will miss nothing. 

While this would be my first time imaging the Lagoon, I had photographed its immediate neighbor - the Trifid nebula four years ago with more modest equipment. A lot had changed since then. Instead of my stock DSLR, I would be using a dedicated astronomy camera now from the ZWO product line. The cooled color camera has much lower thermal noise and, lacking a built in IR filter like DSLRs, does not cut out the red glow of H-alpha. My old alt-az motorized mount which prohibited exposures longer than 25 seconds due to star trails was now replaced by a portable Sky Watcher AZ-Gti. Originally an alt-az mount, it is now set up in equatorial mode with a wedge. It can comfortably handle long exposures of 2 minutes or more (guided) without causing star trails. Lastly, I had replaced my long focal length 1500 mm Maksutov scope (which can sometimes crop large objects such as nearby nebulae). My new telescope, a 430 mm focal length Apertura 72 ED refractor, acquired before the solar eclipse this year, offered a wider field of view. I could, for  the first time, include two separate nebulae in one shot. Finally, this was also the first time I had set up in my front yard (as opposed to my backyard). What that meant, is I could not see the pole star as my house came in the way. And yet, due to the magic of software assisted three-point-alignment, I could align my scope to the north star even while being able to see only southwards. 

Storm Near A Teapot

The Lagoon, unlike what its watery name signifies, is no lake placid. It was first discovered around 1654, by an Italian priest-astronomer Giovanni Holdierna, a contemporary of Galileo. He used the newly invented Galilean style telescope of around 20X magnification which offered limited view of the nebula similar to average binoculars of today. In modern times, scientists have peered deep into the heart of the Lagoon with immensely more powerful tools like the Hubble space telescope and revealed some of its secrets. Beyond the shroud of gas and dust which is opaque to visible light, and revealed in infra-red, lies a an open cluster known as NGC 6530. It is a stellar nursery where clouds of hydrogen are collapsing under gravity to form new stars.       

Infrared view into the heart of the Lagoon nebula (right) revealing nascent stars hiddden by gas in the
optical view (left). Credit: Nasa, ESA and STSci   

Twisters in Tranquility

Core of the Lagoon Nebula - Labelled.
Source: Hubblesite - 1996

As the dust and gas in such a region collapses upon itself, it forms a proto star that consumes matter swirling around it in the form of an accretion disk, eventually starting nuclear fusion at the core. Newly formed stars nearby give out radiation in the form of high energy ultraviolet light. This UV light strips nearby hydrogen molecules of their lone electron and creates ionized hydrogen which glows in the infra red region as it recombines emitting a radiation at a fixed frequency. This light is known as H-alpha.  In the Lagoon, the primary source of the illuminating UV glare is the star known as Herschel 36. Thirty two times heavier and 200,000 times brighter than our sun, this million year old youngster is sculpting a fantasy landscape around it with its radiation and hurricane like stellar winds. The brightest part of the image a shape known as the "hourglass" due to obvious reasons also contains stormy twisters. Billowing clouds of gas several light years long, penetrating into nearby walls of cold dark gas and dust. According to this ESA article: "...These features are quite similar to their namesakes on Earth — they are thought to be wrapped up into their funnel-like shapes by temperature differences between the hot surface and cold interior of the clouds. The nebula is also actively forming new stars, and energetic winds from these newborns may contribute to creating the twisters" [3]

Birth Mark

Another type of transient phenomena going on such region are Herbig-Haro objects. Young stars still surrounded by their accretion discs sometimes shoot out long tendrils of accreted matter from their poles. The energized gas is pushed out into colder surrounding gas giving rise to the billowing columns and shapes that resemble the bow-shock ahead of watercraft speeding into still waters.These polar jets give rise to paired formations known as Herbig-Haro (HH) objects named after their joint discoverers. The Lagoon Nebula, has shown evidence of such HH objects in studies conducted during the 2005-2010 period [4]. 

Schematic View of
Herbig Haro (HH) Object.
Source: Wikipedia image by
Gmaxwell, Georg-Johann, Cherkash. 

Three-colour composite of the young object
Herbig-Haro 34 (HH-34),
now in the protostar stage of evolution.
By ESO, CC BY 4.0, Source: Wikimedia Commons 

Layers of the Lagoon

Giovanni Hodierna.
Source: SEDS

In the four centuries that have passed since in the deep south of Italy, the little known Giovanni Hodierna put his finger on this nebula for the first time, we have come a long way. That was a time when human knowledge of space was bound to our solar system. The geocentric model was just being challenged and Gallileo, for the first time in human history was delving into strange "stars" in orbit around Jupiter.  Born into poverty, a son of a mason or a shoemaker, Hodierna was mostly a self taught man of science trying to figure it all out. 

Today, we we use tools the likes of Hodierna could not conceive, and build on centuries of knowledge which he had no access to, advancing our curiosity into the a universe he did not know exists. And yet we unearth more questions than we have answered. There seem to be endless layers to the lagoon that is the universe. Like the Hodiernas of the world before us, in the end, our best tools remain curiosity and the ability to learn from our mistakes. As a species without any other recourse, we remain self taught, trying to figure it all out. As they say, with other fields as well as in astronomy, the more things change, the more they stay the same.  

Reference:

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1. Giant "Twisters" and Star Wisps in the Lagoon Nebula from Hubblesite
2. Messier 8 from Nasa
3. Stormy seas in Sagittarius from esahubble.org
4. Breaking waves in the stellar Lagoon from esahubble.org
5. Giovanni Battista Hodierna from messier.seds.org

Further Reading:

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• Herbig-Haro Object from Wikipedia