The Red Rectangle Nebula, a protoplanetary nebula in Monoceros

Image Credit: H. Van Winckel (KU Leuven), M. Cohen (UC Berkeley), H. Bond (STScI), T. Gull (GSFC), ESA, NASA

The Red Rectangle Nebula (designated HD 44179) is a bipolar protoplanetary nebula of only about one third of a light-year across, located some 2,300 light-years away toward the constellation of Monoceros. The nebula is so called because of its red color and rectangular shape.

Despite their name, protoplanetary nebulae have nothing to do with planets: they are clouds of dust and gas formed from material shed by an aging central star similar in mass to our Sun. For such a star death is a long process. After billions of years, the hydrogen fuel that powers the star begins to run out. The star balloons to great size and becomes a red giant.

Eventually, however, the star collapses back on itself. This increases the temperature at its core and most of the stars material is catapulted into space, enveloping itself in clouds of gas, but the core is not yet hot enough to make the gas itself glow on its own. Instead, the gas is merely reflecting the light from the star.

But as the star continues to evolve, it becomes hot enough to emit strong ultraviolet light. At that point it will have the power needed to make the gas glow, and will become a real full-fledged planetary nebula. But before the nebula begins to shine, fierce winds of material ejected from the star will continue to shape the surrounding gas into intricate patterns. Then the star cools down and all that is left after this process is the exposed, hot and dead core, known as a white dwarf.

A protoplanetary nebula is a relatively short-lived phenomenon, so finding one is a rare opportunity for astronomers to learn more about them and to observe the beginning of the formation of planetary nebulae (hence the name protoplanetary, or preplanetary nebulae).

The Red Rectangle Nebula – a strong source of infrared radiation – is created by the outflows of gas and dust from a central dying star. The outflows are ejected in two opposing directions, producing a highly symmetric, bipolar structure in a distinctive X-shape with ladder-like rungs, what makes the nebula look like the fabled ‘Stairway to Heaven’.

A likely explanation for its symmetry is that the central star – actually a close pair of stars – is surrounded by a thick dust torus which pinches the otherwise spherical outflow into tip-touching cone shapes. Because we view the torus edge-on, the boundary edges of the cone shapes seem to form an X. The distinct rungs may have arisen in episodes of mass ejection from the star occurring every few hundred years. They could represent a series of nested, expanding structures.

The Red Rectangle Nebula is made up of gas and dust, mixed with icy dust grains and hydrocarbon molecules – potentially vital organic molecules for the formation of life. The red color of the nebula is probably produced by these hydrocarbons which are formed in the cool outflow from the dying central star. This central binary star system is completely obscured, providing no direct light.

The Red Rectangle Nebula should be transformed into a planetary nebula as its cool central star becomes a hot white dwarf over the next few thousand years.

This image is taken with the Wide Field Planetary Camera 2 onboard the Hubble Space Telescope.

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