ASTRONOMY

Debugging nebula

Small telescopes are generally favoured for viewing objects that appear “different” in the sky. Long ago, William Herschel made a serious effort to discover objects which resembled Uranus. He found plenty of disc-like objects and called them planetary nebulae. As we improved our vision with bigger and better telescopes, these nebulae remained mysterious. What appeared as fuzzy patches through small telescopes showed exotic structures when viewed from large telescopes that resulted in interesting names like the Eskimo, Dumbbell, Helix, Cat’s Eye and so on. One of them was the ‘Bug’ nebula - many feel that it resembles a squashed bug.

Of the 2,000-and-odd planetaries known in our galaxy, not many have attracted scientists as much as the Bug nebula. Located in the constellation of Scorpius, it is estimated to be about 3,500 light years away. It was unlike the general interpretation of planetary nebulae which generally have a halo kind of an outer region, accompanied by a flow of plasma in a toroidal shape and extended lobes. These characteristics vary with individual objects. Today they are classified into circular, elliptical and ‘butterfly-like’. Bug nebula belongs to the last category.

These shapes provide clues to the history of the formation of nebula. A sun-like star ends its life by throwing out almost 80 per cent of its mass. The core left behind is extremely hot (however, its mass is much less than the Chandrashekhar limit of 1.4 solar mass) and is responsible for the colourful glow of the expanding ejecta. A decade ago, a team of British scientists studied the spectrum of the Bug nebula for some peculiar spectral lines. They suspected the central star to be extremely hot, as much as 250,000K. 

Another result in 2001 was based on the observations in different wavelengths as obtained from space observatories. Scientists were able to identify many dust particles including silicates. The structure of the nebula was probed in later years revealing knots and asymmetries in it. They were able to detect Raman scattering in the spectral lines. A dark lane almost midway along the nebula was the subject of intense study. The Hubble Space Telescope and many others were used for the study of the nebula. An approximate estimate puts the mass in the nebula to about three per cent of the solar mass.

Yet, all these were in the material surrounding the central hot source which remained elusive.
Molecular motion at speeds of 20km/s, their distinctive lobes, and knot-like structures were detected from interferometry. At the central hottest zone, the mass was estimated to be almost 50 per cent of the solar mass.
Once upon a time, there was perhaps a sun-like star there and what we see now as the nebula is the remnant of its death. If so, when did it happen? Theoretical models based on stellar evolutionary tracks yield three crucial quantities satisfactorily explaining the observations. They are the central temperature, the luminosity of the central stars and the nebular age. This was attempted for the Bug nebula. At last, a perfect fit was achieved with a temperature of 200,000K. The core is 2,000 times more luminous than the sun.  This was possible after “seeing” the central star, which is well concealed in the surrounding colourful nebulosity. When did the eruption take place? The accepted answers differ slightly, one puts it at 2,200 years ago. The other puts it at 1,900 years ago. This can be considered as a recent event in astronomical time scales. Thus the Bug nebula is young and the material in the central region is rushing out at speeds exceeding 600km/s.
These deductions make hot news. So far, it was not possible to poke into the central region of a young nebula and see the hottest star.

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