Sand and Stars

A Wolf-Rayet Bubble in the Carina Nebula

Carina Nebula. Image credit ESA/Hubble

13 Mar 2018

The Carina Nebula is, to my eye, the supreme nebula in our skies… it is preposterously beautiful… a billowing three-dimensional cloudscape, its raggedy outer edges building up out of the surrounding sky; all those obscure dust clouds, the astonishing radiance of the central masses bifurcated by the colossal forked dust lane, rich little Trumpler 16 which contains Eta Carinae and its intriguing Homunculus, the mysterious little smudge of the Keyhole Nebula, its collection of sparkling clusters… 

… And if that’s not enough, in the north lies a dumfoundingly huge and gorgeous Wolf-Rayet Bubble! 

Wind-blown Wolf-Rayet bubbles are among the most rare, intriguing and tantalising objects to observe – the impressive nebula, ionized and illuminated from within, formed as a rare and exotic Wolf-Rayet star shed mass rapidly by means of very powerful stellar winds up to 10 million kilometres per hour.

The bubble is named WR 23… and what a beauty!  Frankly, WR bubbles stagger the imagination.

The stupendous Wolf-Rayet Bubble, WR 23

Using my 16″ f/5 Dobs, the difficulty with the bubble is to visually disentangle it from the Eta Car nebulosity! But wow, what a wonderful observing experience trying to unscramble all the nebulous turbulence thread by thread – having lots of time certainly helps. The inimitable observer, Steve Gottlieb, observed this beauty from Australia and wrote that oddly, he found it responded well to the H-beta filter… but he’s right, it did (oddly) respond well to it. From magnifications of 76x to 228x, it really was superb seeing the vast partial ring of the bubble – what a WR bubble!

With the great dark void to its east, and the way that the eastern flank of the bubble’s rim is missing, at low magnification it reminded me of one of the damaged craters on the Moon whose crushed wall has allowed the lava to flow in. And although there is a noticeably brighter patch of nebulosity just inside the bubble’s interior to the SE, the manner in which the very faint haze of the bubble’s interior also simply dissolves into the darkness towards the NE, added to the impression. Very lovely impression.

The bubble’s southern flank, running in a roughly E-W direction just north of Bochum 10, is a beautifully bright and well-defined sweep of nebulosity and very prominent. With increasing magnification, it becomes quite complex; it looks like a little scrap of silk ribbon that has been left outside, all weathered and wind-twisted and frayed and disintegrated but still retaining its beautiful glow as only silk can. Very eye-catching and the more I looked, the more I felt I was able to separate additional strands of bubble from Eta Car nebulosity, but hard to know for sure, frankly. The edges of the ribbon were well defined, although it was a rather tangly-looking stretch of nebulosity. At its eastern edge, the ribbon of nebulosity is frayed into a fine, feathery tangle of very faint nebulosity that simply fades away into the enveloping nebulosity that itself fades away into the dark void. 

On the western edge of this ribbon there is a bit of a gap in the bubble’s rim before it curves northwards in a pretty large, unevenly bright and wispy-rich arc. A couple of wisps within the arc were quite a bit brighter and stood out conspicuously. Both edges of the arc were well-defined; it was easy to see both the interior edge and the exterior of this portion of the bubble’s rim, but the edges became increasing less defined as I traced it northwards… the northern end of the arc softened into a very wispy, broad, faint and indistinct nebulosity that simply melted away with no defining edges on either side.

The northern edge of the bubble is not at all defined – it appears as a faint nebulous haze; I couldn’t distinguish anything beyond the faint hazy glow. The outer side of the hazy nebulosity was particularly indistinct; it simply faded away. However, this whole northern edge retained an impression of bubble-roundness, especially with averted vision; there is no doubting this is a bubble (once I knew it was a bubble, of course! Before it was simply a complex area of all sorts of interesting whirls and flurries of nebulosity!)

The interior of the bubble was filled with faint nebulosity, apart from the detached patch of brighter nebulosity lying in the SE corner of the bubble’s interior. Wandering averted vision gave the faint nebulosity some subtle definition and unevenness in the southern half; the northern half was very faint and even. Without an eastern edge, the way it faded away into that gloriously dark void was very attractive.

9th magnitude WR 23. DSS2 image

And then there was the WR star itself… mag 9 WR23 (HD 92809)…  lying well west of centre and looking like a tiny diamond nestled in that very faint and delicate veil of nebulosity. (Two WR imposters also lie in the nebulosity…!) It is simply dumfounding to look at that star, and then look at its vast wind-blown bubble and try to imagine what’s gone on, and continues to go on, with that astonishing star. Indeed, looking at that vast rippling bubble, the most compelling impression I got was a sense of the inconceivable forces involved. Like everything astronomical, at the very edge of the mind’s grasp. All in all… a fabulous WR bubble!

When I was cooling down from the observation with a cup of tea, I was trying to decide which is more amazing… seeing the rare and extraordinary Wolf-Rayet star responsible for the wind-blown bubble, or seeing the rare and extraordinary Wolf-Rayet star’s wind-blown bubble. Like most of those out-in-the-dark musings, I couldn’t decide.

Copyright © Susan Young 2018