The Boomerang Nebula, a protoplanetary nebula in Centaurus

 
The Boomerang Nebula, a protoplanetary nebula in Centaurus

Image Credit: Hubble Heritage Team, STScI/AURA, ESA, NASA

The Boomerang Nebula (also called the Bow Tie Nebula or the Centaurus Bipolar Nebula, and designated PGC 3074547) is a young bipolar protoplanetary nebula of about 2 light-years across, located some 5,000 light-years away from Earth in the constellation of Centaurus.

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 Boomerang Nebula is, with a temperature of −272 °C, only 1 °C warmer than absolute zero (the lowest limit for all temperatures). Even the −270 °C background radiation from the Big Bang is warmer than the nebula. It is the only object found so far that has a temperature lower than the background radiation.

The general bow-tie shape of the Boomerang is created by a high-speed wind of gas and dust that is blowing from the dying central star at speeds of nearly 600,000 kilometers per hour. Over the last 1,500 years, nearly one and a half times the mass of our Sun has been lost by the central star of the nebula in an ejection process known as a bipolar outflow. This is 10-100 times more than in other similar objects. The rapid expansion of the nebula has enabled it to become the coldest known region in the Universe.

This reflecting cloud of dust and gas is blown into two nearly symmetric lobes of matter. Each lobe of the nebula is nearly one light-year in length. Embedded within the diffuse gas of these smooth lobes, the Boomerang Nebula shows faint arcs and ghostly filaments. The diffuse bow-tie shape of this nebula makes it quite different from other observed planetary nebulae, which normally have lobes that look more like ‘bubbles’ blown in the gas. However, the Boomerang Nebula is so young that it may not have had time to develop these structures. Why planetary nebulae have so many different shapes is still a mystery.

Shining with light of reflected dust from the central star, the Boomerang Nebula is evolving toward the planetary nebula phase.

This image was taken with the Advanced Camera for Surveys onboard the Hubble Space Telescope in early 2005, using polarizing filters and color coded by the angle associated with the polarized light. The result traces the small dust particles responsible for polarizing and scattering the light.

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