梯形恒星

我们对M42中心的最直观印象就是明亮，而且还有几颗灼热的恒星。参考下图Stellarium演示，东南的是θ2，西北的是θ1。今天我们研究的是θ1。

把θ1放大，可以勉强看到四颗星。

这就是我们常说的猎户座大星云梯形恒星了，我们把视场倒转一下：四颗星分别是ABCD。然而，Stellarium没有显示更暗弱的恒星，也就是EFGHI。

小口径的望远镜很容易看到ABCD。其他恒星在长曝光图片里看的很轻松，不过实际上E和F都是11等星，需要6寸的望远镜才能分辨。推荐使用125倍以上的倍率观看(建议200x-300x)。

这个区域也叫惠更斯区域，里面这些恒星几乎都是变星。所有这些都属于梯形星所在的星团，该星团目前大部分都隐藏起来，但是有一天将被认为是天空中最好的星团之一。猎户座星云中确实埋藏着数千颗恒星。视觉上只有几个是可见的，每一个都是巨大的挑战。直到猎户座的灰尘被更多的恒星消耗掉并被恒星风吹走，才有可能显示出未来业余爱好者会流口水的星团。

仔细搜索附近密集的发光气体中正在散发出来的其他恒星。最简单的是LQ Ori，其次是V1399，MT和LV。LV之所以棘手，是因为它嵌在一个微弱的星云结中。短暂地看到了位于V1399以东的V494。其他都很容易。

http://www.skyandtelescope.com/observing/star-trapping-in-orions-trapezium/

https://www.cloudynights.com/topic/604862-finally-saw-the-e-star-in-trapezium/

Our destination tonight will be the bright inner core of the Orion Nebula (circled) centered on the Trapezium multiple star.
Rawastrodata / Wikipedia CC BY-SA 3.0

When you want to catch an animal, you choose the right trap, bait, and location. Amateur astronomers also make choices on how to best bag their stellar quarry, choosing the appropriate time, telescope, magnification, and sighting technique.

If we’re lucky, the sky will clear when we have the night free. Seeing might even be good, yielding the sweet and steady views so essential for making out delicate details like a spiral arm or planetary nebula’s central star. One of the great ironies of astronomy is that we gaze at enormous objects millions or even billions of miles across that are easily reduced to mush depending on how the wind blows.

Last week, the wind blew like crazy, but star images still glared motionless as mannequins. Times like this call for special projects, and one of my favorite over the years has been to explore the innermost sanctum of the Orion Nebula at high magnification. You’ll sometimes hear it referred to as the Huygenian Region after the 17th-century astronomer Christian Huygens, who first studied it in detail.

The Theta-1 (θ1) Orionis multiple star is better known as the Trapezium. Its four bright stars are easily visible in a 3-inch telescope. A 6-inch will show two additional stars, E and F, both 11th magnitude. G and H require a large amateur instrument.
Jerry Lodriguss with additions by the author

Orion’s a different beast at 200× and up. The clash of dark and bright nebulosity in the heart of the nebula is pure turmoil, as if the observer were cast into an oncoming thunderstorm. This is especially true where the finger-like dark nebula dubbed the Fish’s Mouth spreads partway over the glowing nest of stellar eggs called the Trapezium. Using 357× and averted vision, what appeared to be opaque dark nebula at lower magnification gave way to ragged clouds and dense black knots silhouetted against the brighter background nebula in glorious 3D.

On nights of average seeing, four bright stars in the shape of a trapezoid give the bright quadruple star its name. But when the air settles down, a 6-inch scope magnifying 125× or higher will bring the fainter components E and F into view, turning the quartet into a sextet. Keep watch on components A and B at the narrow end of the figure. They’re both eclipsing binaries: V1016 Ori ranges from magnitude 6.7 to 7.7 over a period of 65 days and BM Ori from 7.9 to 8.7 every 6.5 days. With its much shorter period, it should be relatively easy to catch BM at both maximum and minimum sometime this season.

Intense ultraviolet light from the massive, young stars of the Trapezium excites gases such as hydrogen and oxygen to glow within the heart of the nebula.
Jerry Lodriguss

On occasion, I’ve noticed additional fainter stars buried within the dabs and folds of the central nebula. Although some are listed as 10th and 11th magnitude, they appear much fainter because they necessarily compete with the glowing gases. That gives them the same fragile, barely-there appearance of the central stars of some well-known planetary nebulae such as the Owl (M97), the Ring (M57), and the Cat’s Eye (NGC 6543). Just as those planetary stars tantalize and test the limits of our vision, so do these tiny sparks within the nebula. I was able to discern five of these in the immediate vicinity of the Trapezium and an additional half-dozen or more a bit further afield.

This color composite mosaic image of the central part of the Orion Nebula is based on 81 images obtained with the infrared multi-mode ISAAC instrument on the ESO Very Large Telescope (VLT). The famous Trapezium stars are seen near the center of a rich star cluster still well-hidden from visual observers of the 21st century by dust. One day, the cluster will make for a magnificent sight as the nebula fades.
ESO / M.McCaughrean et al. (AIP)

All these suns belong to the Trapezium Cluster, which is mostly hidden for now, but will one day be considered one of the finest clusters in the sky. There are literally thousands of stars buried within the Orion Nebula. Only a few are visible visually and each makes for an excellent challenge while giving an observer a chance to visualize the Great Nebula’s future. Until Orion’s dust is consumed in the making of more stars and blown away in their stellar winds, only infrared light reveals the cluster future amateurs will drool over.

Nearly every single star within the nebula’s confines bears a variable star designation. Most belong to the IN class (“I” for irregular, and “N” for association with a nebula). Many of the stars in the nebula are 300,000 years old or younger and still accreting material from their surroundings. Differences in the accretion rate can set off stellar flares that temporarily boost a star’s brightness. Young stars are typically fast rotators, generating strong magnetic fields that create large and active starspots that can cause a star’s light to vary with its rotation.

Almost every star in the green, glowing core of the nebula centered on the Trapezium is a variable star (click to enlarge). Newborns like these often undergo irregular changes in their brightness before they settle into stable lives on the main sequence. I’ve added the approximate location of the dark nebula called the Fish’s Mouth to the map.
Bob King, Source: Chris Marriott’s SkyMap

Sometimes the very dust that the star engorges to grow can rob it of light, causing the newborn to briefly fade. If you regularly study the appearance of these several stars in the Huygenian Region, don’t be surprised if one or another becomes either easier or more difficult to see. Growing up, whether human or star, has its bumps!

A side benefit in seeking these newborns is getting to know the billows and pillows of glowing nebulosity that encircle the Trapezium. I’ve always believed that if you name something, you’ll come to know it, so I’ve included several of my own fanciful names for various parts of the inner region. Names serve as guideposts, places to stop and get to know. They help us see more clearly an otherwise complex nebula that might be perceived as an assortment of shapeless blobs.

To make my observations of the Huygenian Region, I used a 15-inch (37-cm) reflector at 286× and 357× and sketched the result using Photoshop. I labeled all the stars I could discern in good seeing. Most are variable stars such as V1399 Orionis. The “P” stands for astronomer P. Parengo. Nicknames are my own. South is up.
Bob King

In excellent seeing using my 15-inch reflector at 286× and 357×, I began at the Trapezium for a most satisfying look at all six stars, each appearing crisp and white-hot. Then I carefully searched for additional stars eking through the dense cushions of glowing gas nearby. The easiest was LQ Ori (see sketch above) followed by V1399, MT, and LV. LV was tricky because it’s embedded in a tiny knot of nebulosity — a cool sight. V494, located just east of V1399 was seen fleetingly. All the others were fairly easy.

Once you’ve poured over Trapezium area, follow the Fish’s Mouth east into the deep darkness of what can only be described as the “black lagoon” of nebulosity separating M42 from the smaller nebula M43. I absolutely love rooting around here. The longer you look, letting your eyes adapt to the dusty darkness, the more texture the “darkness” will divulge. One of my favorite objects is an extremely faint star near the center of the lagoon just east of the Fish’s Mouth. Here the dark gas seems to absorb all light and appear blacker than the blackest night. But deep inside flickers a single tiny light, the proverbial candle in the dark. Don’t miss it.

Here it is in all its chaotic magnificence! The Trapezium shines just left of center.
NASA / ESA / Hubble Space Telescope

Larger scopes are best for deep exploration of the eye of the nebula, but even a 6-inch on a good night using a magnification of 150× or higher, concentration, and averted vision can show much more than you’d expect. Don’t be afraid to go the highest magnification the seeing will allow. Sometimes amateurs, including myself, are too conservative when it comes to magnification. Go nuts. Like the Orion Nebula, you only live once.