Bubble Nebula (NGC 7635): A Star-Blown Shell for Hubble's 26th Year

The bubble in this picture is about seven light-years across, about 1.5 times as far as the gap between the Sun and Alpha Centauri, and it was blown by a single star. Hubble's Wide Field Camera 3 photographed it in February 2016, and NASA and ESA released the result on April 21 as the image marking 26 years since the telescope's launch aboard the space shuttle on April 24, 1990.

A star that blows its own bubble

The nebula sits in Cassiopeia, about 7,100 light-years away according to NASA's 2016 release (the ESA/Hubble release of the same picture gives about 8,000; distances to nebulae like this carry real uncertainty). William Herschel found it in 1787. The source of the bubble is a massive O-type star cataloged as BD +60°2522, some 45 times the mass of the Sun.

A star like this is so hot at its surface that gas boils away from it continuously. The resulting stellar wind leaves at more than four million miles per hour and slams into the cold interstellar gas lying ahead of it. Like a snowplow pushing a wall of snow ahead of the blade, the wind piles that gas into a thin shell, and the shell is the bright outline of the bubble.

The star is about four million years old. Its outer layers of hydrogen are mostly gone, and the core now burns helium, building it into heavier elements. In 10 to 20 million years it will probably explode as a supernova.

Why the star is off-center

A perfect bubble would have its star in the middle. This one does not. In the Hubble frame the star sits at about the 10 o'clock position, well away from the center. The shell is expanding into gas that is not uniform: on one side it has run into a dense region of cold gas, which slows its growth there, while on the other side it spreads more freely. The asymmetry pushes the bubble's center away from the star that made it.

The shell itself is thin and partly transparent. Along its edge the swept-up gas is compressed into a bright, sharp rim, while across its face the wall is faint enough for structures behind it to show through. That transparency is what lets the camera see clouds on the far side of the bubble, as the section on the pillars below describes.

That surrounding cloud is visible in the wide ground-based view above. The bubble is only the most sharply defined part of a much larger region of glowing gas lit by the same star.

How the 2016 colors were assigned

The Wide Field Camera 3 exposures were taken on February 25 and 26, 2016 under proposal 14471, through three narrowband filters that each isolate light from one gas.

Filters and assigned colors

Filter	What it records	Shown as
F502N	Doubly ionized oxygen [O III] at 502 nanometers	Blue
F656N	Hydrogen-alpha at 656 nanometers	Green
F658N	Ionized nitrogen [N II] at 658 nanometers	Red

Oxygen, at the shortest wavelength, is shown blue, and it glows brightest in the bubble close to the star, where the gas is hottest. Hydrogen-alpha is shown green and ionized nitrogen red. Those two lines sit only two nanometers apart, so the filters are very close in wavelength, but they still separate the two gases. Where hydrogen and nitrogen glow together, the green and red combine to yellow, which is the color of the cooler pillars and clouds around the bubble. The logic behind such palettes, and why they are not meant to show what an eye would see, is covered in Are the Colors Real?

The result reads as layers of temperature. The interior of the bubble around the star is blue, its rim is a mix, and the clouds outside, farther from the star and cooler, turn yellow and orange.

Pillars like the Eagle's

At the upper left of the image are dense columns of cool hydrogen laced with dust. More finger-like shapes appear almost face-on, seen through the translucent wall of the bubble behind it. The ultraviolet radiation of the central star lights them, the same process that illuminates the famous columns of the Eagle Nebula, and over time it erodes them.

The resemblance is more than visual. In both cases a hot, massive star bathes cold gas in radiation, and the gas survives only where it is dense enough to resist. The difference is the source: in the Bubble the light comes from one star sitting inside its own shell.

Three cameras, one bubble

Few objects have been photographed by Hubble with several generations of cameras, and the Bubble is one of them. The first Wide Field and Planetary Camera imaged it in September 1992, and Wide Field and Planetary Camera 2 returned to it at the end of the decade. The 2016 Wide Field Camera 3 picture is the newest of the three, and it was also a milestone for the project behind it: the 200th image release of the Hubble Heritage Project since its first picture in 1998.

Comparing the three is a way to see how much the cameras improved. The 1992 frame dates from before the December 1993 servicing mission that corrected the blur in Hubble's main mirror, and each later camera brought finer pixels and a wider field. A shell of a very different origin, the debris of a supernova, is the subject of The Red Bubble.

Questions readers ask

How big is the Bubble Nebula?

The bubble is about seven light-years across, about 1.5 times the Sun's distance from the Alpha Centauri system. It is part of a much larger region of glowing gas in Cassiopeia that is lit by the same central star.

What star creates the Bubble Nebula?

It is BD +60°2522, a massive O-type star about 45 times the mass of the Sun and about four million years old. Its wind, moving at more than four million miles per hour, sweeps up the surrounding cold gas into the shell that forms the bubble.

Why is the star not in the center of the bubble?

The shell is expanding into gas of uneven density. On one side it runs into a dense, cold cloud that slows it down, while on the other side it expands more easily. That lopsided growth leaves the star near 10 o'clock in Hubble's frame.

What do the colors in the Hubble image represent?

Blue shows oxygen, green shows hydrogen and red shows nitrogen, each recorded through its own narrowband filter. Hot oxygen glows near the star inside the bubble, while the cooler pillars look yellow because hydrogen and nitrogen light combine there.

Will the Bubble Nebula's star explode?

Probably. BD +60°2522 is one of the most massive kinds of star, its outer hydrogen is largely gone, and helium fusion has taken over in its core. It is expected to end as a supernova in roughly 10 to 20 million years.