Nebulae (the plural of nebula) are clouds of gas or dust that lie between the stars. Actually they can produce stars or be produced by stars. The winter sky contains two of the most important nebulae to be found in our skies. One is huge while the other though small is growing.
The Great Orion Nebula, M42, surrounds the middle star in Orion's sword and surrounds a small clutch of 4 stars called the Trapezium which were born out of it's gasses The key to getting the best views of the nebula is to use the lowest available magnification for the telescope. By the way the lowest usable power for a telescope is 3 times the telescope's aperture (width of the primary lens or mirror) in inches. At this power the exit pupil of the eyepiece is 7 mm, the width of the human eye's dark adapted pupil giving the brightest images. It is possible to see color in the brightest part of the nebula. It photographs as being blood red, but to the eye it appears a ghostly green, the bright red of hydrogen, being outside the human's range of night vision.
M42 is the brightest part of a nebula that nearly covers the area of Orion behindits bright stars. It is where the Hubble Space Telescope has detected cocoons of gas and dust from which new stars will soon emerge. M43, slightly north, is another extension of the same cloud of gas.
The M42-M43 complex lies about 1,700 light years away and is about 40 light years in diameter. It is the closest emission nebula to us. It is called an emission nebula because it shines by its atoms emitting light, energized by the ultraviolet light from the stars of the Trapezium. In a spectrograph the light of the nebula is emitted as lines at a few wavelengths or frequencies indicating the composition and state of the atoms within it. The red hydrogen line called hydrogen alpha photographs well, but our eyes see the greenish hydrogen beta line along with two lines of Oxygen III, ionized oxygen atoms with two electrons missing.
There is another nebula as important to astronomers as the Great Orion Nebula in the winter sky. Called the Crab Nebula or M1, it signifies the opposite end of a stars life, its death. Number 1 on Charles Messier's list of objects that weren't comets was first seen in AD. 1054 as a new star or nova shining for more than a year. The nebula has been expanding now for 944 years to its present 6 light year diameter.
It is small and located about one degree northwest of the star Zeta (z) Tauri, the tip of the eastern of Taurus' horns. M1 is sometimes listed as a planetary nebula, though it is not the product of the relatively gentle gas expansion from a low mass star at the end of life. The Crab is a supernova remnant. The expanding shell of a star that literally blew up. The star is still there, by the way. What's left of this giant star is crushed to a 30 mile diameter spinning 30 times a second. It is so dense that the protons and electrons of the original star are crushed together by its extreme gravitational field into neutrons, hence the name for this type of star, neutron star. Radio and optical pulses have been detected from the central star.
The energy for the light of this nebula comes from a different source than M42. Besides visible light the Crab is an X-ray source. It actually emits 100 times more energy as X-rays as visible light. The central star contains a huge magnetic field that is rotating with it. Electrons tangling with this magnetic field emit what is called synchrotron radiation, after the radiation produced by second generation cyclotrons physicists used to accelerated subatomic particles.
On top of all that the Crab is a radio source. As an constant source of radio and X-rays, it is an important calibration standard for these two radiations.
Two great nebulae in one sky. What a deal!
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