Looking Up – Bob Eklund

HUBBLE SEES “SEARCHLIGHT BEAMS” COMING FROM NEBULA: The NASA/ESA Hubble Space Telescope has been at the cutting edge of research into what happens to stars like our Sun at the ends of their lives. One stage that stars pass through as they run out of nuclear fuel is called the preplanetary (or protoplanetary) nebula stage. A new Hubble image of the “Egg Nebula” shows one of the best views to date of this brief but dramatic phase in a star’s life.

The preplanetary nebula stage is a short period in the cycle of stellar evolution, and has nothing to do with planets. Over a few thousands of years, the hot remains of the aging star in the center of the nebula heat it up, excite the gas, and make it glow as a planetary nebula. The short lifespan of preplanetary nebulae means that there are relatively few of them in existence at any one time. Moreover, they are very dim, requiring powerful telescopes to be seen. This combination of rarity and faintness means they were only discovered comparatively recently. The Egg Nebula, the first to be discovered, was first spotted less than 40 years ago, and many aspects of this class of object still remain shrouded in mystery.

At the center of this image, and hidden in a thick cloud of dust, is the nebula’s central star. While we can’t see the star directly, four “searchlight beams” of light coming from it can be seen shining out through the nebula. It is thought that ring-shaped holes in the thick cocoon of dust, carved by jets coming from the star, let the beams of light emerge through the otherwise opaque cloud. The precise mechanism by which stellar jets produce these holes is not known for certain, but one possible explanation is that a binary star system, rather than a single star, exists at the center of the nebula.

The onion-like layered structure of the more diffuse cloud surrounding the central cocoon is caused by periodic bursts of material being ejected from the dying star. The bursts typically occur every few hundred years.

The distance to the Egg Nebula is only known very approximately, the best guess placing it at around 3,000 light-years from Earth. This in turn means that astronomers do not have any accurate figures for the size of the nebula (it may be larger and further away, or smaller but nearer).

The Hubble Space Telescope is the first major optical telescope to be placed in space. It is named for Edwin Hubble, a staff astronomer at Mount Wilson Observatory, who in the 1920s used the 100-inch Hooker Telescope—the world’s largest telescope at that time—to measure the expansion of the Universe by observing galaxies beyond our own.

The Hubble Space Telescope has a diameter almost exactly the same as Mount Wilson’s 100-inch, but its location in space—in an orbit more than 100 miles above Earth’s atmosphere, with its image distortion, rain clouds, and light pollution—places it on the “ultimate mountaintop.”

Both the Hubble Telescope’s launch in 1990 and Edwin Hubble’s work at Mount Wilson Observatory were among the most significant advances in astronomy since Galileo first pointed a telescope toward the sky in 1609. They changed forever our perspective of the universe and our place within it.


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