Orion Belt Region including the Flame Nebula and the Horsehead Nebula and M42/43(Jon Talbot)
The Horsehead Nebula
Alternative Nomenclature: Barnard 33
Right Ascension: 05h 40m 59s
Declination: -02º27′ 30”
Magnitude: dark nebula
The Flame Nebula
Alternative Nomenclature: NGC 2024, Sharpless 2-277
Right Ascension: 05h 41m 54s
Declination: -01º51′ 0”
Alternative Nomenclature: includes Barnard 33
Right Ascension: 05h40m59s
Declination: -02º27′ 30”
The Horsehead Nebula was first recorded on a photographic plate by Scottish astronomer Williamina Fleming in 1888, using the Harvard College Observatory. In describing the bright nebula IC 434 that surrounds the Horsehead, she described the nebula as having:
“a semicircular indentation 5 minutes in diameter 30 minutes south of Zeta Orionis.”
Williamina Fleming was the first member of what became known as human computers for the Harvard College Observatory. Working under the guidance of Edward Charles Pickering, an all-women team of women undertook the difficult tasks of analyzing and cataloging astronomical data. The Harvard human computers group of women, sometimes referred to as “Pickering’s Harem”, would also include Henrietta Swan Leavitt, Antonia Maury, and Annie Jump Cannon.
Some controversy surrounds the initial discovery of the Horsehead Nebula. Upon reviewing Fleming’s notes, William Henry Pickering, who had taken the photograph plate in which the Horsehead appeared, speculated that the spot was dark obscuring matter. Yet all subsequent articles and books denied both Williamina Fleming and W. H. Pickering credit. The compiler of the first Index Catalogue, J.L.E. Dreyer, eliminated Fleming’s name from the list of objects then discovered by Harvard, attributing them all instead merely to “Pickering”. This caused astronomers using the Index Catalog to assume the director of Harvard College Observatory, Edward Charles Pickering, to be the discoverer. By the release of the second Index Catalogue by Dreyer in 1908, Fleming, W. H. Pickering, and others at Harvard were recognized for later object discoveries, but not for the earlier discoveries of IC 434 and the Horsehead Nebula. The rightful credit for the discovery has now been established.
Located some 1,500 light years away, the Horsehead Nebula is a challenging object to observe visually.
The area where IC 434 and the Horsehead Nebula is perhaps the easiest to find Cosmic Duet while the most difficult to see visually. Locate the Belt of Orion, and locate Alnitak, the belt star on the left. And look slightly below Alnitak. The Flame Nebula will be on the opposite side of Alnitak.
The challenge is visually observing the Horsehead Nebula. It is basically a photographic object, but the Horsehead has been seen visually by amateur astronomers. However, it is a tough object to see. Although telescopes of at least 8-to-10 inches aperture have been used to observe faint hints of the Horsehead, apertures of 17 inches or greater are often necessary to see it successfully . Extremely dark, transparent skies are a must. A hydrogen-beta nebula filter is necessary to provide an increase in contrast. With all that in hand, the Horsehead Nebula will still be a challenge to view.
The observability of the Horsehead can change dramatically in the space of hours and also from individual to individual. It is a very difficult target in any scope under 16 inches in aperture. Observing the Flame Nebula, IC434, and the Horsehead Nebula simultaneously visually is especially difficult, and remains ideally suited as a photographic or astro-imaging target.
The ease with which it can be seen are dependent on many things:
- The darkness of the skies. The darker, the better.
- The transparency of the skies. Low humidity is a must. No high cirrus clouds.
- Clean optics in the scope are a must.
- A well baffled scope is a must. This has a major effect on contrast.
- High grade optics yield better contrast than low grade optics, making it marginally easier to see.
- Use an eyepiece that yields a suitable exit pupil. 3mm to 5mm is ideal as this effects contrast and target luminosity
- Use a high quality eyepiece with good light throughput and contrast.
- Use a nebula filter. A Hydrogen-Beta filter helps enormously and a narrowband or UHC filter helps a lot.
- And most importantly, the sensitivity to red light are the individual observers eyes. A very significant portion of the light emitting from the background emission nebula IC434 is at the red end of the spectrum and observers with eyes that are less sensitive to red light will simply not see it irrespective of the conditions, because they cannot see the background emission nebula very well.
If and when the Horsehead is spotted, it is not in the upright position often seen in photographs. Since IC 434 is oriented downwards and slightly trending to the leftward direction, the Horsehead profile will be on its side, as if the horse was looking in the direction of Alnitak.
As with M42, Barnard 33, the Horsehead Nebula is part of the Orion Molecular Cloud Complex. It is one of the most identifiable nebulae because of the shape of its swirling cloud of dark dust and gases.
On the other side of the star Alnitak resides the Flame Nebula, NGC 2024. The gas that is energized by Alnitak to energize NGC 2024 and the nearby IC 434 is a part of the Orion Molecular Cloud Complex. At the center of the Flame Nebula is a cluster of newly formed stars. X-ray observations by the Chandra X-Ray Observatory show that 86% of the Flame Nebula’s 800 stars have circumstellar discs, indicating the presence dense gas and dust and the early formation of planetesimals leading to possible planet formation.
The glow of the Flame Nebula is as a result for the now-familiar forbidden transition and the hydrogen-alpha recombination line radiation, as described in the Orion Nebula section. Visually, a large aperture and an O III filter will reveal the Flame Nebula. But like its neighbor the Horsehead Nebula, the Flame Nebula is a more satisfying astro-imaging target.
My own experience of viewing the Horsehead Nebula and Flame Nebula was a the top of a ten foot ladder through an eyepiece on a 24” Obsession Dobsonian at the 2014 Winter Star Party (yes, the same telescope in which I viewed Thor’s Helmet from last month’s column.). A 20mm Nagler eyepiece and a 2” Lumicon Hydrogen-Beta filter on a dark, cloudless, transparent night facilitated the observation. Even with all the technology of large aperture, 82°AFOV eyepiece and Hydrogen-Beta filtering, viewing the Horsehead Nebula was not easy. The tendency for most backyard astronomers is look for patches of light and illumination. Viewing a dark nebula such as the Horsehead requires a mental adjustment and looking for where the light isn’t. I glimpsed it, but without a doubt in my mind, the Horsehead Nebula is at its best as a photograph. The choice of the Nagler 20mm eyepiece hindered the light transmission because of the light absorption of its nine element design. But the 24 inches of aperture from the Obsession Dobsonian helped overcome that shortcoming. However, balancing oneself on top of a ten foot ladder to peer through an eyepiece does require a little internal fortitude!