Blobs are a distinct and very rare class of compact H II regions in the Large Magellanic Cloud

One would be forgiven for thinking that “blob” is not a scientific term, but when the very rare class of compact H II were discovered by Mohammad Heydari-Malayeri, an astronomer at Paris Observatory, he called them “blobs” since no features could be distinguished with the available telescopes at the time.

In 1982 he discovered the first high excitation blob (HEB), N159-5, which lies in the ferocious star forming complex N159 (in NGC 2083). They were previously considered to be stars; he showed that they were not stars at all but are in fact compact H II regions characterized by high excitation, small size (about 5 to 10 arc-seconds in diameter), and high density where young massive stars are just hatching from their natal molecular clouds. He notes that they may be counted among the densest and most excited emission nebulae in the Magellanic Clouds and that despite their small size, HEBs are in general excited by more than one new-born massive star.

They often form adjacent to or inside the typical giant H II regions, and rarely in isolation. They are generally affected by significant amounts of local dust. Although astronomers do not yet fully understand the formation mechanisms of these objects, the one thing that seems sure is that they represent the youngest massive stars of their OB associations.

The first blob discovered, N159-5, was nicknamed the Papillon Nebula because it resembles a butterfly in images. Credit: M. Heydari-Malayeri (Paris Observatory) and NASA/ESA

It is a wondrous observation experience to know that you are looking at young massive stars just hatching from their embryonic molecular clouds and hidden from our sight within the tiny droplets of nebulosity. A dark sky, superb transparency and a UHC filter are essential if you want to have a good view of these exotic little creatures.

Without a doubt the most superb example of a HEB is N11A which resides in the superbubble N11. The other HEBs are buried in nebulosity, rendering them small bright knots within varying degrees of nebulous brightness whereas N11A (10″ in diameter in my telescope) appears to be floating unattached in a field speckled with faint stars.

The famous and conspicuous “eyes” in the Ghost Head Nebula, N160A-1, A-2, which reside in the superbubble N160 are another terrific pair to observe. Even without a filter the two blobs are impressive, appearing as two quasi-stellar knots of brightness lying in the very bright, irregularly shaped, glow of NGC 2080.

N214C is a small but gloriously bright, rounded, sharp-edged… well… blob! Indeed, the bright blob, placed as it is with NGC 2103’s nebula flowing out behind it in either direction like a bow wave gives one the lovely impression in the telescope that the nebula is flying through the galaxy… a really gorgeous impression of movement!

The exquisite little N214C. Credit: ESO

Even the HEBs that appear as nothing more than faint “stars” are fascinating. The exotic Papillon Nebula (HEB N159-5a) appears as a faint mag 15 “star” on the SE side of N159’s star-forming maelstrom. And albeit HEB N83B-1 appears as a 13th mag “star” at the southeast edge of small, bright ~30″ N83B = NGC 1748, it is a very interesting object to view. Astronomers say that N83B probably represents the most recent massive starburst in the massive N83 complex and the OB association LH 5. The young age is inferred from the fact that the burst contains not one but two adjacent high excitation blobs. N83B-1, a mere ~2″8 (0.7 pc) across is the brightest and most excited part of N83B. The second blob, N83B-2, is even more compact, with a size of only ~ 1″ (0.3 pc).

The brighter blob is N83B-1, and N83B-2 lies to its left. Credit: NASA, ESA, Mohammad Heydari-Malayeri (Observatoire de Paris, France)

In 2006, Heydari-Malayeri published the first study ever of another hitherto unknown interstellar HII component in the Magellanic Clouds – low excitation blobs (LEBs). They are lower excitation counterparts of HEBs and are excited by massive stars of lower temperature/mass, compared with those that power HEBs and typical giant H II regions. LEBs tend to be more evolved since their exciting stars have disrupted the bulk of their associated molecular clouds.

There is a noteworthy difference between the environments of HEBs and LEBs in the Magellanic Clouds. Whereas the HEBs occur mostly in the close vicinity of giant H II regions and rarely in isolation, LEBs are by contrast, isolated objects lying far from giant H II regions, and no conspicuous molecular clouds have been detected to be associated with them.

Like their HEB counterparts, a dark sky, excellent transparency, and a UHC filter are essential for viewing these tiny objects. For an exquisite little example of a LEB, N191A is the one to observe. The mag 12.7 blob responds well to the UHC filter, appearing as a ~15” brightish, silky glow, very slightly elongated N-S, and with edges that melt rapidly into the faint, roundish glow that surrounds it, and that itself rapidly melts away into the sky.

Located on the southern periphery of supergiant shell LMC 8, N191A is a beautiful little LEB lying in the small OB Association LH 23.

N90 appears as a moderately bright, hazy knot, ~15″ in diameter. N156 which lies just west of the incredibly rich superbubble LMC 2, appears as a fairly bright ~20″ slash of light elongated E-W, smooth and with even edges.  Mag 13.6 N193A lies in a small, round haze of small and unresolved stars, the rare object appearing as a tiny, brightish droplet of light.  And N197 appears as a faint but obvious, ~10″ speck of light elongated N-S.