Diffuse Nebulae

Diffuse nebulae are thin conglomerations of dust and gas that if they acquire sufficient mass may begin to form stars within them. Some may form a handful of stars while others may form stars by the hundreds, creating large open clusters. Diffuse nebulae therefore are not stellar tombstones, but nurseries, where stellar lives begin. As stars begin to shine from within the dust cloud, if they are sufficiently hot and massive, their high-energy radiation will excite the gas to shine just as they do in planetary nebulae. These types of nebulae are also called emission nebulae. If local temperatures are not hot enough to excite the gas, the dust in the cloud will reflect the light of the stars, creating what is called a reflection nebula. Many emission nebulae also have an outer area associated with them that is also a reflection nebula. Diffuse nebulae are also often called H II regions because they principally consist of ionized hydrogen (hydrogen atoms that have lost their electron).

Diffuse nebulae, like planetary nebulae tend to have relatively short lifetimes because the gas and dust that it is created of will either be consumed into the newborn stars within it or be blown away by the hot stellar winds of those same new born stars. Eventually all the matter contained within the stars is blown away to space or accreted into the stars or accompanying planets (something we are discovering more and more stars have). After all the stars have moved clear of the gas and dust clouds or after the cloud has been completely dispersed, all that remains is a new open cluster of infant stars beginning their main sequence lifetimes. A typical lifetime for a diffuse nebula is about a million years, though that number can vary wildly depending upon the rate of star formation and the mass of the stars created.

The Orion Nebula (M42), visible to the unaided eye, has been known since antiquity and was observed telescopically very shortly after the invention of the telescope in the year 1610. Its nearby companion M43 was discovered by telescope in 1731, then the Omega, sometimes called "Swan" nebula (M17) in 1745. Until the 1860s all these wondrous sights were thought simply to be unresolved star clusters until the emerging technique of spectroscopy revealed their gaseous nature. Five of the six diffuse nebulae in the Messier catalog are primarily emission nebulae, though some do have reflection components. Only M78 is a pure reflection nebula. There are however many fine examples of reflection nebula that are not in Messier's catalogue. Take another look at the star field picture at the front of Chapter 12 and see the gas cloud with the outline that strongly resembles North America (and is called by coincidence the North America Nebula). This gas cloud shines primarily from light reflected from the massive nearby star Deneb, as does the Eagle Nebula slightly to the south and west (right). Another such area of nebulosity is easily imaged around Antares in Scorpius.

Just 1,600 light years away and hanging from the belt of Orion near the celestial equator, which happily allows the nebula to be viewed from anywhere on Earth, lies the brightest of the diffuse nebulae in our sky, the magnitude +4.0 Orion Nebula. Orion, located about 1,600 light-years from Earth, is a vast star-forming region that delights all who view in the smallest of binoculars to the mightiest of observatories on Earth and in space. The visible portion of the nebula extends to about 85 by 60 arc minutes and is many light years in size. It is the most easily visible part of a much larger dust and gas cloud that spans more than 10 degrees of sky, spreading over many hundreds of light years. The overall span of the complex contains several other areas famous on their own including Barnard's Loop and the Horsehead Nebula. The Orion Nebula is on its own so large, bright and rich in detail that many astronomers refer to various features by name as though they were discussing topographic features of the Moon or Mars.

M42 is a massive star-forming region and in the middle of the nebula, in an area where gases are clearly being blown away by stellar winds (called the Fish's Mouth) are four bright stars that make up a formation known as the Trapezium. Huygens found the first three members in 1656 and Abbe Picard found the fourth member in 1673. Over the next two hundred years, four more faint members of the cluster were found. The cluster members are designated "A" through "H." Today we know that both the "A" and "B" stars are both Algol-type eclipsing variables. The "A" star varies between magnitude +6.7 and +7.5 each 63 days. The "B" star varies between magnitude +7.9 and +8.5 each 6.5 days.

As the Orion Nebula graces the winter's skies so the Trifid Nebula (M20) graces the summer in the constellation Sagittarius. The nebula was so named by John Herschel because of its three-lobed appearance. Like with many nebulae, the distance to the Trifid is somewhat uncertain with published estimates ranging from 2,200 light years to over 9,000. The beauty of the Trifid Nebula lies in dichotomy of colors, a reddish emission nebula contrasting with a conspicuous blue reflection nebula that is particularly bright surrounding the north side of the emission nebula. Magnitude estimates of the nebula also vary widely. Brightness estimates range from magnitude +6.8 to +9.0. The brighter magnitude estimates are probably largely based upon the brightness of the triple star system at the center of M20. The three stars shine at a combined magnitude of +7.0. The brightness of the triplet makes the task of estimating the overall brightness of the nebula difficult.

Nearby M20 is an even larger nebula, M8, also known as the Lagoon Nebula. When Messier originally described this area, his description more resembled the nearby star cluster and in fact his stated position more closely defines the position of the cluster than the nebula, but the nebula is generally today considered to be M8. The open star cluster is designated NGC 6530. The Lagoon Nebula shines at an estimated magnitude +6.0 and estimates of its distance range from 4,800 to 6,500 light years. The nebula is very big, spanning some 90 by 40 arc minutes. If we use the commonly accepted distance of 5,200 light-years, this would imply at that distance that the nebula is about 140 by 60 light years in size. Near the center of the nebula is a bright region that is called the "Hourglass Nebula." The intrinsically bright young star Herschel 36, a spectral type O7, magnitude +9.5 celestial toddler, illuminates the Hourglass. The brightest star associated with the nebula is magnitude +5.9, spectral type O5 9 Sagittarii. Both of these nebulae are also active stellar nurseries, cranking out young stars to wander about the galaxy.

These three nebulae represent half of the only six diffuse nebulae that appear in the Messier catalogue. Today we have catalogued many thousands of diffuse nebulae throughout our galaxy and in many others. They are sights of awesome multicolored beauty and show us the very beginnings of stellar evolution. In the planetary nebulae,we have viewed the relatively peaceful ending of Sun-mass stars. Some nebulae represent the spectacular deaths that await the most massive of stars.

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