M42/M43, NGC1976, Orion Nebula
The Orion Nebula (M42/M43, NGC1976/1982) This picture shows the great nebula in the constellation of Orion the Hunter. On a good clear night, from a dark site well away from the lights of modern civilization, this glowing cloud of gas and dust can be seen with the naked eye as a fuzzy patch surrounding the star Theta Orionis in the Hunter’s Sword, below Orion’s belt. It is probably the most spectacular of all the objects cataloged by Charles Messier and now called by their `M’ numbers. M42 had been known since the beginnings of recorded astronomy as a star, but it is so outstanding that it was first noted as an extended nebula in 1610, only a year after Galileo’s first use of the telescope. Detailed descriptions started appearing later in the seventeenth century, and it has been a popular target for anyone with a telescope ever since. So many details are visible in even a small telescope that M42 will more than repay the observer who makes it a frequent target, and who will find that it is hard to make a realistic sketch that can capture all of the finer features. So outstanding is this nebula that it has two numbers. M43/NGC1982 is the separated portion to the north-east (top left), surrounding an irregular variable star. Although Messier stopped at only two, other parts of the nebula in this region have received further NGC numbers. M42 is our closest example of an HII region, being composed mainly of ionized hydrogen which gives off the red glow so dominant in every picture of the nebula. Deep photographs such as this one show that it is nearly a degree across, larger than the full Moon (although the Moon is so bright that it looks much larger). The energy to keep the nebula glowing comes from the very hot young stars in a formation called the Trapezium, embedded in the brightest part of the nebula and not visible in this photograph. The nebula and the brighter stars are very young indeed by astronomical standards, at about 30000 years. Compare this to our own Sun, which is considered to be a middle-aged star at over four billion years! M42 probably contains several hundred stars younger than a million years, still bursting with the energy of youth. Stars are still being born in a dense cloud behind the nebula, but they are hidden from our view by a concentration of dust which reduces their light to only a million-millionth of its original intensity. Fortunately, astronomers have developed special cameras and other detectors which are sensitive to infra-red radiation, more popularly known as heat, which penetrates the dust and reveals to us this stellar nursery. Although M42 is mostly hydrogen, in both neutral and ionized states, with a fair quantity of dust, it does contain significant amounts of other elements, especially oxygen. The green glow of doubly-ionized oxygen is strongest near the intense ultraviolet starlight at the middle of the nebula. To the north-east (the upper left in this picture) is a feature called the Dark Bay, which is a thick cloud of neutral gas which has not yet been ionized. Location: 05 hrs 35.4 min, -05 deg. 27 min (2000). Distance: nearly 500 parsecs (1600 light-years). Size: about 66 by 60 arc minutes. Mass: about 300 solar masses. Magnitude: 4.0. Power source: O and B stars. Photograph: Bill Schoening, KPNO 4m telescope, October 1st 1973. Original Ektachrome color transparency. Minimum credit line: Bill Schoening/NOAO/AURA/NSF
Trifid, M20, NGC6514
The Trifid Nebula, M20 or NGC6514, is a familiar sight and an excellent example of an emission and reflection nebula. The red emission nebula contains a bright blue star cluster near its center: it glows red because the ultraviolet light of the stars ionizes the hydrogen gas, which then recombines and emits the characteristic red hydrogen-alpha light. Further out, when the radiation from these hot young stars becomes too weak to ionize hydrogen, the gas and dust instead glows by reflecting the original blue light. M20 is in the constellation of Sagittarius, at a poorly-known distance somewhere between 2200 and 7600 light-years. We also have a deep black and white image from the KPNO Mayall 4-m telescope. M20 is quite close on the sky to the open cluster M21, as shown in this lower resolution but wider field picture.
Minimum credit line: Todd Boroson/NOAO/AURA/NSF
Tiny Alien Planets, Monster Galaxies And Twin Suns
Hubble Snaps a Splendid Planetary Nebula
The gaseous outer layers of a Sun-like star glow in space after being expelled as the star reached the end of its life.
The Omega Nebula is a galactic Rorschach test
These stars, though still shrouded in dust and hydrogen gas, are some of the youngest and most massive stars in the galaxy. They’re part of the Omega Nebula, which for 200 years has been a sort of cosmic inkblot test.
Just about every astronomer who observes this nebula comes away with a different sense of what the thing looks like. Its first known discoverer, the 19th century astronomer John Herschel, decided the nebula looked like the Greek letter omega, giving it its first name. Since then, other astronomers have called it the Horseshoe Nebula - which admittedly would closely resemble the omega symbol - as well as the Swan Nebula, the Checkmark Nebula, and my personal favorite, the Lobster Nebula. Its official names are Messier 17 and NGC 6618, so apparently astronomers can’t even settle on a single designation for the damn thing.
Astronomers from the European Southern Observatory add:
In this particular section of the nebula, the newest stars on the scene - dazzlingly bright and shining blue-white - light up the whole ensemble. The nebula’s smoky-looking ribbons of dust stand in silhouette against the glowing gas. The dominant reddish colours of this portion of the cloud-like expanse, arise from hydrogen gas, glowing under the influence of the intense ultraviolet rays from the hot young stars. The nebula is located about 6500 light-years away in the constellation of Sagittarius (The Archer). A popular target of astronomers, this illuminated gas and dust field ranks as one of the youngest and most active stellar nurseries for massive stars in the Milky Way.
Photograph courtesy NASA/Andrew Fruchter (STScI)