Chandra lays an EGG

Contributed by
Feb 15, 2007

The orbiting Chandra X-Ray Observatory pointed its Superman vision at the Eagle Nebula, home of the famous "Pillars of Creation". The image below is the Hubble image with the sources seen by Chandra superposed. With the two images together, it becomes clear that Chandra spied something interesting.

Actually, it's what it didn't see that's interesting.

The Eagle is a huge cloud of gas and dust that is busily making baby stars. The Pillars are actually one small part of a much larger nebulosity, but it was the Pillars that put Hubble on the public's map. The image was devastating, of course, with an aching beauty to it. But the Pillars were so-named because they are the very cocoons where stars are being born. The Pillars have regions of higher density called EGGs, for Evaporating Gaseous Globules. It's thought that these are pockets of gas where the star birth is taking place.

Young stars still going through the throes of birth should be highly magnetic and emit X-rays, and Chandra should see that. However, the superposed images show that no EGGs appear to be emitting X-rays:

Any X-rays sources detected by Chandra are identified in the image by circles (click the image to see a bigger version on my Flickr site). The size of the circle represents how well-determined the source is, so a smaller circle is actually a brighter detection. The red diamonds are where previous astronomers found EGGs. All the circles have been found to correspond to young stars (including the ones at the tips of the Pillars), but not ones that are embedded in EGGs. In other words, the EGGs are not glowing in X-rays, even though there are four EGGs with protostars in them that should be X-ray emitters. But they're dark.

Hmmmmm. This could mean one or more of several things. One is that the stars are so young, buried deep in their wombs, that they have not yet started emitting X-rays. That's one conclusion of the astronomers who made these observations. That's certainly possible. Another idea is that there are no stars forming there, despite the claims of previous astronomers. As the new paper says (my notes in brackets):

A most interesting aspect of M16 is the presence of "evaporating gaseous globules" (EGGs), deeply embedded infrared sources located at the edges of the pillars. Hester et al. (1996, 2004) have argued that the EGGs are a very early stage in protostellar evolution when shock waves and photoevaporation of the molecular gas in the pillars by the harsh radiation from the nearby O stars produce a dense core that could become a star. We find that none of the 73 EGGs studied in the near-infrared by McCaughrean & Andersen (2002) are X-ray sources above our detection threshold. Eleven of the EGGs have faint near-infrared point sources. Of these, seven have substellar masses and are not expected to have X-ray emission above our detection threshold. The nondetection (above our threshold) of the four EGGs with core masses in the range 0.35–1 [times that of the Sun] indicates that either (1) the EGGs do not contain YSOs [Young Stellar Objects] or (2) that at a very early stage of evolution YSOs have not yet become X-ray active.

I'm sure Jeff Hester, who wrote the paper that is the pillar of the EGG field (har har) would prefer the second explanation. But I find it interesting at the very least that we can study an object that is pretty well known (it's one of the brightest nebulae in the sky) and still find ourselves a little baffled by it. I recently reported that one astronomer thinks a supernova went off inside the nebula a few thousand years ago!

It may seem like astronomers don't know what's going on, but this sort of confusion is typical when new observations in a new wavelength are made (no one has ever made high-resolution X-ray maps of the Eagle before). It's new territory, and there are bound to be some things that are weird and hard to explain. But then someone will come along, and say "Hmmm, what if we try this..." and then it becomes more clear.

Science is the way we learn about the Universe and ourselves. It's the only way I know to make an honest appraisal by weighing the evidence, and using a little imagination to make further investigations. Then, and only then, can we learn from those results.

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