The turbulent and dynamic Swan Nebula (M17) has been imaged by NASA’s Spitzer Space Telescope, producing the clearest view yet of the star-forming region. Within the twisted cloud of gas and dust, violent stellar winds constantly blast the medium, generating flows around stars, creating vast bow shocks. A few massive stars in the centre of M17 are the main source of the relentless stellar “rivers” of gas, immersing smaller stars in the the flow, acting like stationary rocks on a riverbed…
This new observing campaign by Spitzer (an infrared telescope that has been in Earth orbit since 2003, and is expected to be operational until 2009), has imaged the M17 nebula with unprecedented clarity. Although it is a known fact that stellar winds inside star-forming regions generate dynamic features such as bow shocks, you cannot put a price on actually seeing these structures in an infrared image (pictured top). From analysis of these Spitzer results, Matt Povich of the University of Wisconsin has published a paper describing these new findings in the December 10th issue of the Astrophysical Journal.
“The stars are like rocks in a rushing river,” said Povich when describing the scene. “Powerful winds from the most massive stars at the center of the cloud produce a large flow of expanding gas. This gas then piles up with dust in front of winds from other massive stars that are pushing back against the flow.”
The Swan Nebula can be found in the constellation of Sagittarius, some 6000 light years away. It is a very active star-forming cloud where powerful stellar winds are eroding away the dust, clearing the region. Driving this mechanism are a group of massive stars exceeding 40 times the mass, and 100,000–1 million times the brightness, of the Sun. The stellar winds bullying smaller stars and blowing away the clouds of dust in the middle of the nebula have flow velocities exceeding 7.2 million km/hr (4.5 million mi/hr). To put this in perspective, the fast solar wind (the fastest component of our Sun’s two-component solar wind) reaches a maximum velocity of 2.8 million km/hr (1.7 million mi/hr); the stellar winds inside the Swan are 2.5 times more powerful.
So what’s the result of this powerful stellar wind engine in M17? A very obvious cavity is created inside the nebula, a process thought to spark the birth of new stars. This stellar nursery is fuelled by the compression of the edge of the cavity, producing bow shocks around anything that is relatively stationary (i.e. other stars). The direction of the bow shocks provide information about the direction of the stellar winds.
Povich studies another star forming region called RCW 49 in addition to M17, picking out the glowing gases generated inside the shock fronts maintained by the flow of stellar flows. Spitzer turns out to be the perfect tool to peer deep into nebulae, picking out the infrared emissions from bow shocks, mapping them.
“The gas being lit up in these star-forming regions looks very wispy and fragile, but looks can be deceiving,” co-author Robert Benjamin added. “These bow shocks serve as a reminder that stars aren’t born in quiet nurseries but in violent regions buffeted by winds more powerful than anything we see on Earth.”
Further observation campaigns like this one will ultimately help astronomers understand how stellar systems, like our Solar System, form out of the violence of stellar birth.