Regular solar observer & correspondent Harry Roberts reports on the NGC 1760 to 1769 group also known as the N11 complex – a nebula and hot young star cluster – in the Large Magellanic Cloud.
For southern viewers the Milky Way’s two companion galaxies, the Clouds of Magellan, are visible most of the year. Since the two ‘clouds’ are close, some 180,000ly away, even binoculars will show bewildering numbers of star clusters, nebulae, globulars and other features – particularly in the ‘Large’ cloud, the LMC, where over 100 NGC objects are plotted! So, while the ‘Clouds’ are familiar, few amateurs probe them deeply – and comet hunters avoid them!
I was recently urged (thanks EOC) to sketch the nebula ‘complex’ NGC1760 to 1769 – a grouping sited well to the NW of the LMC’s densest parts at 4h57m, -66°32’, and hence easier to find. Bino’s showed a featureless round glow at the site – but the 10in ‘scope told another story!
A nice assembly of bright and faint nebulae, adorned with star chains, surrounding some rich star clusters, was seen– the biggest contained a bright multiple star! This was a surprising site: big and showy, it filled the fov of the 14R [14mm Radian eyepiece] at 90X. At least six NGC numbers occupy this one field! A sketch ensued (Fig).
N11. It seems from published work that this part of the LMC is best described as ‘LMC Nebula N11’, or just N11 for short. And, that it is a single huge nebula – or ‘bubble’ that surrounds an OB stellar assembly that has blown a ‘hole’ in the gas cloud. But what an assembly it is!
The diameter of N11 is some 1100ly! It’s the second largest nebula in the LMC after the 30Dor or ‘Tarantula’ Nebula. The latter is almost 2000ly in diameter and is vastly more complex than N11. How does N11 compare to familiar Milky Way nebulae?
O-type Luminaries. Well-known nebula M42, the Orion Nebula, is some 80ly in diameter, less than a tenth of N11, and contains just one spectral type O star, i.e. Theta Orionis C, while N11 contains at least 80 such stars – many of which are seen in the fov (Fig). I have no data on the number of O-types in the ‘Tarantula’, but it’s likely several hundred!
In other words: the N11 nebula is bigger than anything in the Milky Way – visible to us at least – apart from the 900ly diameter Eta Car Nebula.
N11dissected. The biggest single unit of N11 is N11B on the west side (Fig). It is a bean-shaped bright nebula ‘excited’ by some twenty O-type stars in chains, at least 10 are sketched. Strong H-beta response was seen in this component. The stars of N11B are stellar assembly LH10.
N11C, the second brightest part of the complex, is round with a dark notch or two on its east side. I saw only two stars in this part, but research shows at least 10 O-types here. To the SW is N11E with two or more O stars: ‘deep’ images show six. To the northeast N11F completes the circle – a faint nebula with still more O-stars.
The ‘Hole’. Near the centre of N11 is the rich cluster of thirty type-O stars with a tight little cluster of B-types on the north side (Fig), NGC1761, it’s also termed OB assembly LH9. These O stars have blown a hole in the N11 nebula. This was not visible – due, I guess, to the brightness of the clusters members. The ‘Hole’ may be seen in bigger ‘scopes. In the centre, apparently a bright multiple star, is variable HV5499. It seemed bright on the night – it is a rare Wolf-Rayet, type WC5 or 6. It seemed very blue in colour – spectral emission bands may be seen with a bigger ‘scope.
The “Clouds of Magellan” website is a very useful link for those who want to explore them in detail.
Overall, LMC nebula N11 is a remarkable component of the LMC, one that has been studied in some depth. That such detail may be seen in a modest ‘scope in a galaxy “far, far away” is even more amazing – and that this detail is altogether much larger and more active than anything in our rather quiet patch of the Milky Way – is more surprising still! Make a visit to the clouds. Clear skies!
Harry Roberts is a Sun and Moon observer, a regular contributor to the Sydney Observatory blog and a member of the Sydney City Skywatchers