Star Formation Factory
The Trifid Nebula as seen by the European Southern Observatory Wide-field Imager. Click to embiggen.
One of the sky sights in the Milky Way that delights summertime stargazers in the Northern Hemisphere (and late winter-early spring gazers in the Southern Hemisphere) is the Trifid Nebula. It lies in the constellation Sagittarius and is a massive stellar factory. Professional astronomers study it to understand the whys and wherefores of star formation; amateurs just like to look at its gorgeousness. The European Southern Observatory has just released a new wide-field image of the Trifid that really lets you explore the details of this region, as seen in visible light.
Let’s take a little tour of the image. First, if you can, right-click on the image and open in a separate window.
Now, look at the bluish patch to the upper left. This is what’s known as a reflection nebula. It does what it sounds like it does — the gas in the nebula scatters light from nearby stars that were born in the nebula. The larger ones shine hotter and brighter, especially in the blue portion of the visible spectrum. Dust grains and molecules scatter blue light more efficiently than red light and that makes this part of the nebula look so very pretty and blue.
The pink-reddish area is a typical emission nebula. That differs from reflection nebula in a very important way — instead of reflecting light, the gases are heated by the hot, young nearby stars and that ultra-hot bath of radiation causes the gases to glow. They emit the red signature light of hydrogen, which is the major component of the gas.
That’s two kinds of nebula in the scene, but there’s a third type. The gases and dust that crisscross the clouds make up the third kind of nebula. They form what’s called a dark nebula, and they block out the light from the parts of the nebula that lie behind them — similar to the way a dust cloud on Earth blocks out sunlight. These aren’t dead clouds, however. The remnants of previous rounds of star birth are clumping together and coalescing under the pull of gravity from within. Eventually, the cloud gets dense and hot enough and the pressure from the coalescence triggers nuclear fusion where the clouds are the thickest — this is the formative event of a newborn star.
Finally, if you look at the lower part of the emission nebula, you can see a finger of gas poking out, pointing directly at the central star powering the Trifid. This is an example of an evaporating gaseous globule, or “EGG”. At the tip of the finger, which was photographed by Hubble, a knot of dense gas is holding out against the onslaught of radiation from the massive star.
There are star formation sites in many places in our galaxy — and of course, in other other galaxies. Astronomers study them to see how the process of star birth progresses — which, in turn, gives them insight into how our own star formed more than 4.5 billion years ago.