Wind-blown bubbles are among the most rare, intriguing and tantalising objects to observe

Theories of wind-blown bubbles were initiated in the late 1960s, but the term ‘bubble’ was only coined in 1975 by Castor et al. Bubbles are blown by fast stellar winds from single massive stars, and they evolve along with the central stars as their formation is dependent on the evolution and mass loss history of the central massive star. During the Main Sequence phase, a massive star is surrounded by the interstellar medium. It loses mass via tenuous fast stellar wind, thus its wind-blown bubble contains interstellar material and is an interstellar bubble.

As the massive star evolves into the Red Supergiant phase (copious slow wind) or Luminous Blue Variable (copious slow or not-so-slow wind), the copious mass loss forms a small circumstellar nebula inside the cavity of the main sequence bubble.

As they race through their lifecycles, the brief Wolf-Rayet phase represents the most advanced evolutionary stage in the lives of luminous massive stars. A Wolf-Rayet star sheds mass rapidly by means of very fast stellar winds that sweep up the circumstellar material, thus its wind-blown bubble contains stellar material and is a circumstellar bubble.

The Large Magellanic Cloud offers us the rare and awe-inspiring sight of three massive, furiously hot, blue-white O-type stars whose powerful stellar winds, with velocities of thousands of kilometres per second, have blown interstellar bubbles. All three are found in the magnificent superbubble, N44.

N44F = NGC 1929 is absolutely superb – a bright and perfect ring with its central star sparkling within it. By contrast, the other two – N44J and N44M are considerably fainter.

The fascinating SNR and complex, N120, offers us the sight of a B-type star in its bubble. Not as clear-cut as N44F, it is nonetheless a beautifully round bubble with the mag 12.7 B-type star glinting in the centre.

The beautiful and bright bubble N44F = NGC 1929, lying on the NW rim of the N44, is superb – a perfect ring. Credit: NASA, ESA, Y. Nazé (University of Liège, Belgium), and Y.-H. Chu (University of Illinois, Urbana)

Wolf-Rayet stars represent a final burst of activity of the universe’s most massive stars, creating strong stellar winds, majestic nebulae, spectacular supernovae and the building blocks for future stellar and planetary systems. They shed mass rapidly by means of very powerful stellar winds up to 10 million kilometres per hour (6 million miles per hour). To put it into context, their mass is being blown away at a rate of 10 solar masses every million years, the equivalent of three Earths’ worth of material each year is launched from Wolf-Rayet stars at a speed that is about 3,500 times as fast as a bullet fired from a rifle. The ferocious winds shape the surrounding gas and dust into a bubble; the inner region forms from the current stellar wind, while outer shells are remnants of previous mass-loss episodes.

As a result of their high fuel-burning rate and their rapid shedding of outer material, Wolf-Rayet stars are incredibly short-lived, lasting as little as just a few 100,000 years. To observe this brief stage of the impending death of a gigantic star, veiled in a distinctive shroud of gas, is a rare and awe-inspiring sight. Our own galaxy offers us a handful to observe… the LMC has an incredible five visible to our telescopes, offering observations from the utterly gorgeous quintessential Wolf-Rayet bubble (N57C = NGC 2020) to those with only a portion of their bubble just-on-the-threshold-of visibility.

When observing these astounding objects, I can never quite decide which is more dumbfounding: seeing the rare and extraordinary Wolf-Rayet star responsible for a wind-blown bubble or seeing the rare and extraordinary Wolf-Rayet star’s wind-blown bubble.

N57C = NGC 2020 is the quintessential bubble, and one of the Cloud’s most magnificent sights. Credit: NASA, ESO, Hubble