NGC 5189: Hubble's View of a Twisted Planetary Nebula in Musca

Released a week before Christmas 2012, this Hubble picture of NGC 5189 was presented as a cosmic holiday ornament: a glass-blown bauble with a glowing ribbon wound around it. The resemblance is a happy accident. The nebula is the debris of a dying star about 2,200 light-years away in the southern constellation Musca, the Fly, and its tangled form records an uneven, possibly two-star, death.

NGC 5189 is a planetary nebula. Such nebulae are the short final chapter for stars of moderate mass, the Sun included. While burning the last fuel in its core, the star expels much of its outer envelope. The radiation of the remaining core heats that gas until it glows, and because the mass is lost unevenly in both time and direction, the glowing clouds can take on complex shapes.

Two sets of lobes

Most of NGC 5189 is knotty and filamentary, and it is lopsided in a particular way. The mass loss produced two nested structures, tilted relative to each other, each expanding from the center in a different direction. Astronomers describe this as a double bipolar, or quadrupolar, shape. In the image the bluish lobes show the effect: one pair points roughly from upper right to lower left, the other along a different axis.

One explanation is a companion. If a second star orbits the central one, its pull can change the direction in which the dying star sheds material over time, so successive outflows leave along different axes. What is left of the central star, stripped of much of its mass, is living out its last days. Planetary nebulae with simpler geometry, such as the Ring Nebula, show what a single-axis outflow looks like.

The golden band

The bright golden ribbon that twists and tilts across the image is made of many radial filaments and cometary knots. Features like these usually form through the combined action of ionizing radiation and stellar winds from the central star.

Seen close up, the knots have dense heads and fainter tails, and the tails point outward, away from the center of the nebula. The shape follows the push from the star: radiation and wind strip material off each clump and drag it outward, the same process that shapes the far larger pillars in star-forming regions.

Five filters, five colors

The Hubble Heritage Team photographed NGC 5189 with Wide Field Camera 3 in 2012, under its own program 12812, with a total exposure of seven hours. Three narrowband filters isolated the light of fluorescing oxygen, hydrogen and sulfur atoms, and two broad filters in visible and near-infrared light captured the colors of the stars.

Filters and assigned colors

Filter	What it records	Shown as
F502N	Doubly ionized oxygen [O III]	Cyan
F606W (V)	Broad visible band, for star colors	Blue
F657N	Hydrogen-alpha and ionized nitrogen [N II]	Orange
F673N	Ionized sulfur [S II]	Green
F814W (I)	Near-infrared band, for star colors	Yellow

The color mapping does not follow wavelength order. Sulfur, the longest-wavelength gas filter, is shown green, while hydrogen and nitrogen are orange and oxygen is cyan. The two star filters were given blue and yellow. The result is the warm ribbon against cool lobes that made the ornament comparison so easy. Are the Colors Real? explains when and why processors depart from the wavelength sequence. The nebula appears with the other planetary nebulae of the series in the Hubble Heritage gallery.

Reading the colors back into gas

Because each hue stands for a known filter, the picture can be read in reverse. The pale blue and cyan that fill the lobes come mostly from oxygen, which needs strong ultraviolet light to glow in this way and so traces gas still bathed in the full radiation of the central star. The orange of the ribbon and the outer tufts comes from hydrogen and nitrogen, and the green of sulfur mixes into the edges of the knots. Where orange and cyan overlap, the gas looks white.

The stars scattered across the frame keep natural-looking tints of white, yellow and pale blue because their light was recorded separately, through the two broad filters. Assigning the starlight its own filters keeps the stars from taking on the strong gas colors, so they read as points of light in front of the nebula, not as part of it.

A brief stage in a star's life

A planetary nebula does not last long. The expelled gas keeps expanding and thinning, and as the stellar remnant fades the glow eventually dies away, a short episode compared with the billions of years the star spent shining steadily. NGC 5189 is caught in that moment. Its knots and filaments will spread into the space between the stars and mix with the gas there, material from which later generations of stars and planets can form.

Questions readers ask

What kind of object is NGC 5189?

It is a planetary nebula, the expelled outer layers of a Sun-like star near the end of its life. The gas glows because the hot core left behind floods it with radiation. NGC 5189 is unusually complex, with two sets of lobes on different axes and a twisted band of knots.

Why does NGC 5189 have such an irregular shape?

The star shed its mass unevenly, producing two nested structures tilted against each other. A likely cause is a companion star orbiting the central one, which would change the direction of the outflow during the nebula-making phase. Radiation and winds then sculpted knots and filaments in the gas.

Where is NGC 5189 in the sky?

It lies in Musca, a small constellation of the far southern sky, about 2,200 light-years from Earth. Its southern position means it is best seen from the Southern Hemisphere, and it does not rise for observers at mid-northern latitudes.

Why was NGC 5189 called a holiday ornament?

The image was released on December 18, 2012, and its shape suggested a glass ornament with a glowing ribbon wrapped around it. The bluish lobes resemble the bauble and the golden band of filaments the ribbon.