One of the most well known and easily identifiable constellations is Orion. According to one of the stories of Greek mythology, Orion was a supernatural hunter who threatened to kill every creature on Earth. Angered by this, the Earth goddess Gaia sent a scorpion to slay the boastful Orion. Appearing in the sky at opposite times of year, it is said that the two continue to pursue one another to this day. Orion is sometimes portrayed with a raised club whilst defending himself with a shield against the attacking Taurus, the adjacent constellation of the bull. Other illustrations have him holding a bow and hunting a hare with his canine companions. The sword he carries is depicted hanging from his belt, which is represented by three bright stars arranged in a nearly straight line that are arguably the constellation’s most recognizable feature.
Other cultures have a completely different take on this part of the sky. For example, in the Southern Hemisphere (where Orion appears inverted compared to what is observed from the Northern Hemisphere), one of the Australian Aboriginal stories recognizes the constellation as a large canoe with the bright stars Betelgeuse and Rigel marking the canoe ends. The bright belt stars denote three brothers fishing, and the stars of the sword represent a caught fish being reeled in. According to the Yolngu story, the brothers caught a king-fish — a forbidden act. Their violation angered the Sun-woman, who blew them up into the sky as punishment.
With the seven brightest stars of the constellation in such close proximity to one another and all holding rank in the list of the 100 brightest stars, the diversity of stories and mythologies revolving around this portion of the sky is not surprising. These elaborate narratives were also born from what people could see with just their bare eyes. Now let’s take a closer look at one portion of Orion to explore what went unseen as these creative stories were developed over the centuries. One can only imagine how the tales would have changed if the telescope had been invented a millennium or two earlier.
The Orion Nebula
To the eye, Orion’s sword appears to most as three moderately bright stars, which are sometimes difficult to pick out in light-polluted skies. But even a small pair of binoculars or a very small telescope will reveal there is much more going on here. The first thing one would notice with either of these implements is the central star appears somewhat fuzzy. That fuzz is real — it’s the Orion Nebula. The nebula appears somewhat like a faint cloud because it is; however, this cloud is well over 10 light-years wide and is still in the process of forming stars. It glows thanks to incredibly hot, bright stars at its heart. A modest telescope with a bit of magnification will reveal a trapezoid of stars, the Trapezium, in the brightest part of the nebula, the Huygens Region. These are the hot, bright stars responsible for making the nebula visible to us with the brightest of the four, Theta Ori C, doing the most of the work. These stars emit a tremendous amount of ultraviolet light that ionizes the gas of the nebula, causing it to fluoresce. As most of the gas is hydrogen, long-exposure photographs will pick up the distinctive reddish-pink glow characteristic of hydrogen. Unless you have a very large telescope, very sensitive eyes, and dark sky conditions, don’t expect to notice any sort of a reddish tint. With enough aperture, however, you might detect a faint green hue emanating from the Huygens region. This is light produced by oxygen atoms in the nebula, and you have a better chance of detecting this light because human eyes are more perceptive to the green part of the visible spectrum, including the low-light sensors of our eyes (the “rods”) used when looking at faint objects in a telescope eyepiece.
Larger telescopes with wide fields of view are required in order to see the fainter and much larger bowl structure of the nebula. The bowl shape is real — the intense radiation pressure and stellar winds are pushing gas and dust away, hollowing out the nebula. At the top-left (northeast) portion of the Orion Nebula is a lobe of gas delineated from the main part of the nebula by a bar of dust. Though this is really part of the whole Orion Nebula, it is usually catalogued separately as de Mairan’s Nebula after French astronomer Jean-Jacques d’Ortous de Mairan who discovered it in the early 18th century. French astronomer and noted comet hunter Charles Messier catalogued the Orion Nebula and de Mairan’s Nebula as the 42nd and 43rd objects, respectively, in his list of objects not to be confused with comets.
The Running Man
Above the Orion Nebula in the “top star” of the sword is another fun treasure, but it’s going to require a long-exposure image to reveal it. Known as the Running Man Nebula, this fainter region is a combination of reflection and emission nebulae. At first glance, one notices the distinctive blue color of the nebula, and this hue can also be picked up faintly in the outer regions of the Great Orion Nebula. The blue color is due to the presence of dust, and the extremely small size of the dust particles allows them to preferentially scatter bluer colors of light. Thus, these blue areas are known as reflection nebulae. The slight pink hue interspersed among the blue of the Running Man Nebula is also real — that is another example of an emission nebula like the Orion Nebula. This glow is due to irradiated hydrogen gas giving off its distinctive color. But, there’s more! Notice that there are regions just below the Running Man Nebula and to the upper left of the Great Orion Nebula that look like dark, dusty clouds. In fact, that is what they are. Dust clouds are particularly good at blocking visible light, so they are often referred to as dark nebulae. If you popped a hot star in front of a dark nebula, the dark nebula would scatter blue light and be considered a reflection nebula.
Unintended Features
There are also some artifacts in the image that astronomers and astrophotographers often have to deal with when trying to obtain data or just take a pretty photo. To the right of the Orion Nebula is a prominent streak, which one might think is a satellite trail or even a meteor as it vaporized in our atmosphere; however, it is neither. Take a look at the brighter stars in the image. Many, especially the brightest, have four prominent diffraction spikes. These spikes are not real parts of stars but are an effect produced thanks to the wave properties of light. As light enters the telescope, it deflects ever so slightly as it passes the four fin-like supports, known as spider vanes, that hold the smaller secondary mirror of the telescope in front of the larger primary mirror. Other reflecting telescopes, such as the Hubble Space Telescope, produce the same artifacts on point sources in their images. The James Webb Space Telescope even produces eight diffraction spikes thanks to the hexagonal shape of its primary mirror and the three secondary mirror supports. The brighter the star in the image, the larger and more pronounced its diffraction spike. The prominent streak in the featured image is actually part of the diffraction spike of Hatysa, the brightest star at the bottom of the image.
There are also several faint lines running horizontally across the entire image between Hatysa and the Orion Nebula. These are satellites that moved through the field of view during multiple exposures. These particular satellites aren’t part of the SpaceX constellation that have been in the news for causing issues with astronomical observations. These satellites are geostationary satellites about 22,000 miles away, much farther than those of SpaceX. The farther out a satellite orbits, the slower it moves in its larger orbit. If at the right distance, the satellite will take the same amount of time to orbit Earth as it takes Earth to rotate. As a result, the satellite seems to hover over the same position on the Earth (somewhere above Earth’s equator). As it happens, the Orion Nebula region crosses the celestial equator, the projection of Earth’s equator on the sky, so it is not uncommon to catch geostationary satellites (and sometimes other satellites) during imaging runs.
Fun fact: Though it looks like the satellites were zipping through the telescope’s field of view, the streaking is due to the telescope carefully following the Orion Nebula as it moved across the sky thanks to Earth’s rotation. If these satellites were easy enough to observe with just the naked eye, they would appear as stars that did NOT move over the course of the night (here is a great example). If you ever wanted to spy one of these for yourself, just follow the Orion Nebula with a small telescope. Eventually you will see one or more faint satellites moving through the field of view. If you turn off the telescope’s tracking motor, the satellites will suddenly appear to stop while the Orion Nebula begins to move.