Observing Project 13D The Detail in the Great Nebula of Orion

It sprawls over more than one full square degree of the sky, equal to four full moons. It is the crucible and nursery where new stars are born. In open clusters, we have viewed the beginning of the journey of infant stars. In the cluster M16 and the adjacent Eagle Nebula, we have viewed newborns leaving the womb. Now in Orion, we can see the star creation engine in progress.

The nebula is so huge and so relatively close to Earth that we can see an amazing wealth of detail in its structure. Remember that the nebula you see is just the brightest part of a much larger cloud of gas and dust that sprawls over hundreds of light-years in size and will be forming stars for millions of years to come. Astronomers have actually mapped the detail of the nebula and given names to various features as though they were craters on the Moon or Mars. In the picture on the left you can clearly see two separate gas and dust clouds, a large main cloud at the center of the picture and a small fainter cloud at the upper right. The smaller cloud is designated M43. The two nebulae are actually the same nebula separated by a dark lane of dust. The dark nebula that separates the two forms a semicircular indentation in M42 that is called the "Fish's Mouth." From the sides of the Fish's Mouth two wing shaped features spread out to the top left and middle right of the picture that are called "wings." The extension at the right (east) side of the Fish's Mouth is called the "Sword." The bright nebulosity to the lower right of the Fish's Mouth is called the "Thrust." The fainter wing on the left (west) is named the "Sail." Emerging from the Fish's Mouth is a cluster of stars called the "Trapezium." Over the years, eight of these stars have been found. A telescope of 2 inches or more will easily show the two brightest. Two more are visible to telescopes of at least 6 inches under light-polluted skies or smaller scopes under dark skies. The others are rather faint and only visible in the largest amateur telescopes. These stars are among the youngest known in the sky and may be less than 100,000 years old.

Figure 13.4. Great Orion Nebula, image by author (with some tutoring) using Meade DSI and Celestron Super C8 Plus at f 6.3. Ten-minute exposure. Processed with Photoshop.

Even under the most heavily light-polluted skies the Orion Nebula can awe one with its beauty. Other nebulae are not quite so bright, beautifully detailed or so well placed but with the right techniques and the right tools, other nebulae will reveal their beauty to you. LPR filters are very helpful here because they will enhance the light of hydrogen-beta, which is one of the primary wavelengths that diffuse nebulae shine in. In the summertime, don't forget to turn your scope to M8 and M20 above the Sagittarius Teapot and the Eagle Nebula, associated with M16 in Serpens. Some of these emission nebulae are too far spread out to be easily viewed in telescopes but by aiming a 35-mm camera at them mounted on a clock-driven telescope can easily show large structures like the North America Nebula, which is excited to shine by light from nearby Deneb. Just aim the camera at Deneb and lock the shutter open for ten or fifteen minutes. The nebula will be easily visible in a wide field exposure. Your star atlas is full of these sprawling star-clouds so do a little advance planning and figure out when some of these beauties are visible.

Now having looked around the galaxy at beautiful stars, clusters of astonishing youth and astonishing age and spectacularly beautiful stellar tombstones and nurseries, let's now look out beyond the Milky Way and into intergalactic space.

CHAPTER FOURTEEN

Faint, Fuzzy Things. Part II: The Island Universes

Figure 14.1. M31, the Andromeda Galaxy. Image by author using a piggybacked 35-mm SLR. Exposure time is five minutes. Look carefully and you can see M32 and M110.

Figure 14.1. M31, the Andromeda Galaxy. Image by author using a piggybacked 35-mm SLR. Exposure time is five minutes. Look carefully and you can see M32 and M110.

Each October, it rises high overhead in the northern evening sky crossing the zenith for observers at 40 degrees north. The small nebulous patch had been known for many years. It was first noted in writing by the Persian astronomer Abd-al Rahman al Sufi who noted it in his Book Of Fixed Stars that was published in AD 964. Messier catalogued the nebula in 1764 and, unaware of the earlier discoveries, ascribed the discovery of the nebula to Simon Marius, who first observed it with a telescope in 1612. Marius though never claimed to have discovered it. Messier assigned the nebula the number 31 in his famous list. For many years, astronomers thought that M31 was one of the closest of the nebulae. It was William Herschel who first correctly surmised that M31 was a relatively close-by island universe. While Herschel's thoughts about the true nature of M31 were correct, he was way off on his estimates of its size and distance. Herschel believed that M31 was about 2,000 times the distance of Sirius (17,000 light years) and some 850 Sirius-distances in diameter and 155 Sirius-distances thick. It turns out he was off just a little bit in that regard. It was William Huggins, the pioneer of the science of spec-troscopy,who obtained the first definitive proof of what M31 was when he obtained spectra of the nebula in 1864. The nebula's spectra were continuous, rather than being dominated by emission lines like planetary nebula. M31's spectra looked more like those of the stars. We began to learn more about its nature in 1887 when the astronomer Issac Roberts photographed it for the first time and began to reveal its spiral structure. The final critical discovery that nailed down the first reasonable distance for M31 came in 1923 when Edwin Hubble discovered a Cepheid variable in M31. Though Hubble's estimate was incorrect by a factor of two, his discovery gave us our first glimpse of the awesome scale of intergalactic distances. Later observations beginning with the 1953 completion of the Hale Telescope at Mount Palomar provided more exact Cepheid information, pinning down the distance to M31 to about 2.9 million light years. M31 has become by far the most studied of the island universes but it was also just the beginning of our expansion into a universe far larger and more amazing than anyone could ever have envisaged.

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