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Nebulae

What Are They?

Nebulae are interstellar clouds of dust, hydrogen gas, helium gas and plasma. If they are large and massive enough they are frequently places of star formation, thus generating big associations or clusters of stars. Some of the young stars are often very massive and so hot that their high energy radiation can excite the gas of the nebula (mostly hydrogen) to shine; such nebula is called emission nebula. If the stars are not hot enough, their light is reflected by the dust and can be seen as white or bluish reflection nebula. As most diffuse emission nebulae also contain dust, they typically have a reflection nebula component also.

Over the past few years the Hubble Telescope has captured some amazing images of the different types of Nebulae:

Appearing like a winged fairy-tale creature poised on a pedestal, this object is actually a billowing tower of cold gas and dust rising from a stellar nursery called the Eagle Nebula. The soaring tower is 9.5 light-years or about 90 trillion kilometres high, about twice the distance from our Sun to the next nearest star.

Stars in the Eagle Nebula are born in clouds of cold hydrogen gas that reside in chaotic neighbourhoods, where energy from young stars sculpts fantasy-like landscapes in the gas. The tower may be a giant incubator for those newborn stars. A torrent of ultraviolet light from a band of massive, hot, young stars [off the top of the image] is eroding the pillar.

The starlight also is responsible for illuminating the tower's rough surface. Ghostly streamers of gas can be seen boiling off this surface, creating the haze around the structure and highlighting its three-dimensional shape. The column is silhouetted against the background glow of more distant gas.

The edge of the dark hydrogen cloud at the top of the tower is resisting erosion, in a manner similar to that of brush among a field of prairie grass that is being swept up by fire. The fire quickly burns the grass but slows down when it encounters the dense brush. In this celestial case, thick clouds of hydrogen gas and dust have survived longer than their surroundings in the face of a blast of ultraviolet light from the hot, young stars.

Inside the gaseous tower, stars may be forming. Some of those stars may have been created by dense gas collapsing under gravity. Other stars may be forming due to pressure from gas that has been heated by the neighbouring hot stars.

The first wave of stars may have started forming before the massive star cluster began venting its scorching light. The star birth may have begun when denser regions of cold gas within the tower started collapsing under their own weight to make stars.

The bumps and fingers of material in the centre of the tower are examples of these stellar birthing areas. These regions may look small but they are roughly the size of our solar system. The fledgling stars continued to grow as they fed off the surrounding gas cloud. They abruptly stopped growing when light from the star cluster uncovered their gaseous cradles, separating them from their gas supply.

Ironically, the young cluster's intense starlight may be inducing star formation in some regions of the tower. Examples can be seen in the large, glowing clumps and finger-shaped protrusions at the top of the structure. The stars may be heating the gas at the top of the tower and creating a shock front, as seen by the bright rim of material tracing the edge of the nebula at top, left. As the heated gas expands, it acts like a battering ram, pushing against the darker cold gas. The intense pressure compresses the gas, making it easier for stars to form. This scenario may continue as the shock front moves slowly down the tower.

The dominant colours in the image were produced by gas energized by the star cluster's powerful ultraviolet light. The blue colour at the top is from glowing oxygen. The red colon in the lower region is from glowing hydrogen. The Eagle Nebula image was taken in November 2004 with the Advanced Camera for Surveys aboard the NASA/ESA Hubble Space Telescope.

The Carina nebula is one of the largest diffuse nebulae in our skies. Although it is some four times as large and even brighter than the famous Orion Nebula, the Carina Nebula is much less well known, due to its location far in the Southern Hemisphere. It was discovered by Nicolas Louis de Lacaille in 1751�52 from the Cape of Good Hope.

Within the large bright nebula is a much smaller feature, immediately surrounding Eta Carinae itself. This small nebula is known as the Homunculus Nebula (from the Latin meaning Little Man), and is believed to have been ejected in an enormous outburst in 1841 which briefly made Eta Carinae the second-brightest star in the sky.

 

The Orion Nebula Messier 42 (M42, NGC 1976) is the brightest starforming, and the brightest diffuse nebula in the sky, and also one of the brightest deepsky objects at all. Shining with the brightness of a star of 4th magnitude, it visible to the naked eye under moderately good conditions, and rewarding in telescopes of every size, from the smallest glasses to the greatest Earth-bound observatories as well as outer-space observatories like the Hubble Space Telescope. It is also a big object in the sky, extending to over 1 degree in diameter, thus covering more than four times the area of the Full Moon.

As it is so well visible to the naked eye, one may wonder why its nebulous nature was apparently not documented before the invention of the telescope. Only some Central American, Mayan folk tales may be interpreted in a way suggesting that these native Americans may have known of this nebulous object in the sky (O'Dell 2003, p. 3). However, the brightest stars within the nebula were noted early and cataloged as one bright star of about fifth magnitude: In about 130 AD, Ptolemy included it in his catalog, as did Tycho Brahe in the late 16th century, and Johann Bayer in 1603 - the latter cataloging it as Theta Orion in his Uranometria. In 1610, Galileo detected a number of faint stars when first looking at this region with his telescope, but didn't note the nebula. Some years later, on February 4, 1617, Galileo took a closer look at the main star, Theta1, and found it to be triple, at his magnification of 27 or 28x, again not perceiving the nebula.

The Horsehead Nebula (also known as Barnard 33 in bright nebula IC 434) is a dark nebula in the constellation Orion. The nebula is located just below Alnitak, the star farthest left on Orion's Belt, and is part of the much larger Orion Molecular Cloud Complex. It is approximately 1500 light years from Earth. It is one of the most identifiable nebulae because of the shape of its swirling cloud of dark dust and gases, which is similar to that of a horse's head. The shape was first noticed in 1888 by Williamina Fleming on photographic plate B2312 taken at the Harvard College Observatory.

The red glow originates from hydrogen gas predominantly behind the nebula, ionized by the nearby bright star Sigma Orionis. The darkness of the Horsehead is caused mostly by thick dust, although the lower part of the Horsehead's neck casts a shadow to the left. Streams of gas leaving the nebula are funneled by a strong magnetic field. Bright spots in the Horsehead Nebula's base are young stars just in the process of forming.

 

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