Deep in the large Magellanic Cloud a 325 light-year sized cosmic monster is born. Its open maw spans some 250 light years across, and from it spews massive particle winds. Held in its glowing gas jaws could be the expanding shells of old supernovae and it has even coughed forth hot X-ray emitting gas. What exactly is this wide-mouthed creature? Step inside…
In the area of open star cluster NGC1929, a complex nebula known as the N44 superbubble has been the controversial study of many telescopes and scientists over the years. This “super bubble” may have formed when one or more massive stars in the central cluster exploded as supernovae, blowing a hole through the nearby turbulent gas complex. But a lot of uncertainties of its evolution still remain. “When we look at the speed of gases in this cloud we find inconsistencies in the size of the bubble and the expected velocities of the winds from the central cluster of massive stars,” says astronomer Phillip Massey, “Supernovae, the ages of the central stars, or the orientation and shape of the cloud might explain this, but the bottom line is that there’s still lots of exciting science to be done here.”
So what’s at home in this area? Try Wolf-Rayet stars, evolved massive stars, O-type supergiants, high mass double-lined binaries, luminous blue variables and luminous B(e) stars. These are massive stars that have formed very quickly but at differing metallicities. Winds and intense radiation from hot, young, luminous stars in N44F excite and sculpt filaments and streamers of the glowing nebular gas.
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Buried in it’s mouth is an OB association of stars known as LH47. According to Wills (et al), “The IMF of the stars outside the shell shows a slightly steeper slope than inside the shell. The slope of the IMF is very similar to values found for other associations and open clusters as well as in the solar neighborhood, thus supporting the idea of an universal shape of the IMF. LH47 turns out to be a well behaved young association embedded in a molecular cloud.”
So it it possible that N44F is actually a “super shell” instead of a superbubble? According to studies done by E.A. Magnier (et al); “Superbubbles are the shells found around OB associations. Supergiant shells are the shells found around large stellar “complexes”. Both may contain hot gas, but the heating history and cooling timescales are likely to be very different. N44 and LMC-2 in the Large Magellanic Cloud are the only resolved superbubble and supergiant shell to have been observed. These two structures are generally similar, morphologically, but have very different size. There have been reports of blowouts in both structures.”
But supernovae – the death explosions of the massive short lived stars – have also likely contributed to the region’s enormous, blown-out shapes. According to Georgelin (et al): “The bubbles of unknown origin have line ratios greater than those of the H II regions and thus appear to fill the gap between thermal and nonthermal radio sources. All the bubbles or filamentary nebulae have important internal kinematical motions. The large complex nebulae have values similar to simple H II regions in their brightest parts, while the faintest parts exhibit greater dispersions and conspicuous splittings and broadenings. The ionized bubbles appear to be intermediate between classical young H II regions and supernova remnants.”
But taken an even closer look and you’ll see there’s another bubble as well. It is believed that as superbubbles expand and age their surface brightness fades. Finally it is thought that superbubbles can trigger new star formation in areas of the shells where gases condense. According to Sally Oey of the University of Michigan, X-ray-emitting gas is escaping from N44F with a temperature of about 1,000,000 Kelvin. While established massive stars may very well be key contributors to the diffuse hot gas throughout space, Oey and Massey report: “We examined the stellar population which is associated with a superbubble region in the N44. We find no evidence that an unusual stellar population gave rise to the shell morphology of the gas.”
So what exactly is going on with this cosmic monster? We know N44 contains the X-ray-brightest superbubble and the break-out structure at the southern rim of the superbubble has been confirmed by the nebular dynamics and plasma temperature variations. However, Sungeun Kim and associates also have their own take; “The total kinetic energy of the neutral and ionized gas of Shell 1 is still more than a factor of 5 lower than expected in a pressure-driven superbubble. It is possible that the central OB association was formed in a molecular cloud, and a visible superbubble was not fully developed until the ambient molecular gas had been dissociated and cleared away. This hypothesis is supported by the existence of a molecular cloud toward N44 and the fact that the apparent dynamic age of the superbubble Shell 1 is much shorter than the age of its OB association LH 47.”
Even though it is a safe160,000 light years away, the combined action of stellar winds hurling forth a storm of particles moving at about 7 million kilometers per hour and multiple supernova explosions is frightening enough. Combined with several compact star-forming regions at the rim and a central star ejecting more than a 100 million times more mass per second than our Sun is only part of what makes this “cosmic monster” both beautiful and terrifying to behold.