The Orion constellation's red glow, as captured in a stunning deep space photo by astrophotographer Miguel Claro from the Dark Sky Alqueva Reserve in Portugal, is caused by hydrogen alpha emission from ionized hydrogen gas within the Orion Molecular Cloud Complex. This intricate reddish cloud, spanning about 1,600 light-years away, includes Barnard's Loop, a massive nebula encircling Orion's Belt, along with iconic features like the Orion Nebula (M42), Horsehead Nebula, and Witch Head Nebula. Claro’s image, taken with a Samyang 35mm f1.8 lens and Poseidon C Pro setup, highlights the vibrant hydrogen emissions, creating a striking contrast against the starry backdrop. The photo, challenging to capture due to weather and Orion’s early setting, showcases one of the most colorful regions of the night sky.
Here's a breakdown of the full details:
1. The Red Glow: Ionized Hydrogen (Hydrogen-Alpha Emission)
- Hydrogen Abundance: Hydrogen is the most abundant element in the universe, and vast clouds of it exist in regions where stars are born.
- Ionization: Within nebulae like the Orion Nebula, hot, young, massive stars (especially the O-type and B-type stars of the Trapezium Cluster at the heart of the Orion Nebula) emit powerful ultraviolet radiation. This energetic radiation strips electrons from hydrogen atoms, a process called ionization, creating a plasma of free electrons and protons.
- Recombination and Emission: When a free electron recombines with a hydrogen proton, it often settles into a high energy level around the nucleus. As the electron cascades down to lower energy levels, it emits photons of light at specific wavelengths. One of the most prominent of these emissions from hydrogen is the hydrogen-alpha (Hα) line, which has a wavelength of 656.3 nanometers and falls within the red part of the visible spectrum.
- The Dominant Color: This intense Hα emission is what gives the vast hydrogen clouds in Orion their characteristic reddish hue.
2. Other Colors and Features in Orion:
While red is dominant, other colors are also present:
- Blue-Violet: Some regions, particularly around very hot, blue-white stars, appear blue-violet. This is primarily due to reflection nebulae, where interstellar dust scatters the blue light from nearby stars more efficiently than other colors (similar to why our sky is blue).
- Green (less common in photos, but significant historically): Historically, astronomers were puzzled by a green tint observed in the Orion Nebula. It was eventually determined to be caused by a "forbidden transition" in doubly ionized oxygen. This specific emission requires the extremely low-density and quiescent conditions found in deep space, making it difficult to reproduce in a lab.
3. Key Nebulae and Structures in Orion that Glow Red:
- Orion Nebula (M42): The most famous and brightest part of Orion's glowing regions, located in Orion's "sword." It's a massive stellar nursery where thousands of new stars are forming. Its reddish glow is a prime example of hydrogen-alpha emission.
- Barnard's Loop (Sh2-276): A vast, intricate semi-circular nebula that encircles much of the Orion constellation, including Orion's Belt. This enormous structure, also composed of glowing hydrogen, contributes significantly to the overall reddish appearance of the constellation in wide-field deep space photos. It is believed to be the remnant of an ancient supernova explosion or a "superbubble" created by the stellar winds and supernova explosions of massive stars.
- Horsehead Nebula (Barnard 33): While known for its dark, horse-head shape silhouetted against a brighter background, the background itself is an emission nebula that glows red due to ionized hydrogen.
- Flame Nebula (NGC 2024): Another prominent emission nebula in Orion, glowing red from excited hydrogen gas.
4. Astrophotography and the Red Glow:
- Long Exposure: Capturing the faint, diffuse red glow of these nebulae requires long-exposure photography, often spanning many hours of cumulative exposure time. This allows the camera sensor to collect enough photons to reveal the subtle colors that are often invisible to the naked eye.
- Specialized Filters: Astrophotographers often use specialized filters, particularly "hydrogen-alpha (Hα) filters," which are designed to isolate the specific red wavelength emitted by ionized hydrogen. This helps to enhance the red glow and bring out details within these vast gas clouds.
- Camera Modifications: Some DSLR cameras are modified by removing or replacing their internal infrared (IR) filter, which normally blocks some of the Hα light, further enhancing the camera's sensitivity to the red emissions from nebulae.
In essence, the gorgeous red glow seen in deep space photos of the Orion constellation is a direct visual testament to the ongoing processes of star formation, where powerful radiation from newly born stars energizes vast clouds of hydrogen gas, causing them to emit light at specific, beautiful red wavelengths.