The super cool starry sky "palette" in the hands of the cosmic painter, you may have never seen it before

Author | Feng Ziyang

Review | Dong Chenhui

Editor | Zhao Jingyuan

In the early summer when it is clear and moonless, if we go to a place far away from the light pollution of the city and look southeast after dark, we can see the rising summer Milky Way slowly rising from the horizon. In Scorpio, west of the center of the Milky Way, there is an orange-yellow bright star with an apparent magnitude of about 1. This is the brightest star in the direction of Scorpio - Antares. It is surrounded by a large interstellar cloud complex composed of emission nebulae, reflection nebulae, and dark nebulae. We call it the ρ Ophiuchi cloud complex. Because of its colorful colors, it looks like a paint palette, presenting a beautiful scene of black, red, blue, yellow and other colors intertwined, so it is affectionately called the "space color palette" by astronomy enthusiasts. In the summer starry sky, it is undoubtedly one of the most popular observation targets, and countless astronomy enthusiasts are fascinated by its beautiful colors.

Stellarium color palette. Source: Stellarium

The cosmic color disk is one of the most colorful regions in the night sky. It was first discovered by American astronomer Edward Barnard in 1883. It is a star-forming region composed of dense gas and dust. The right ascension of its central coordinates is 16h28m06s, and the declination is -24°32'30”. It is a deep sky object in the summer sky in the northern hemisphere. It is about 460 light-years away from us and is one of the closest star-forming regions to us. In addition to the Rho Ophiuchi complex and the cosmic color disk, it also has other names, such as the Ophiuchus molecular cloud, Integral 691, and XSS J16271-2423.

A star map of the sky near the stellar color disk. Source: Roberto Mura (CC BY-SA 3.0)

The sky area near the stellar color disk includes many deep sky objects, among which the yellow part is cataloged as IC 4606, which is a reflection nebula located in the space around Antares (α Scorpii). It is illuminated by Antares, so it is also called the Antares reflection nebula. Antares is a binary star system, which includes a red supergiant (Antares A) and a blue main sequence star (Antares B). Red supergiants like Antares A, which are at the end of their lives, will expand outward, the surface temperature will decrease (hence the red-yellow color), and the brightness will increase sharply. IC 4606 reflects the light of Antares and appears yellow. There is a globular cluster NGC 6144 with a very low star density in the sky area near IC 4606. Although the projection of this cluster on the celestial sphere is relatively close to that of IC 4606, they are very far apart in space and there is no physical connection. There is another globular cluster M4 (also known as the Crab Cluster) to the right of IC 4606. There is no physical connection between the two. M4 is the first globular cluster in human history to be resolved into single stars. Some of its structures can be distinguished using a small telescope.

Antares binary star system. Source: SpaceEngine

The blue part of the cosmic color disk is composed of IC 4604 (LBN 1111), IC 4603 (LBN 1109), and IC 4605, which are reflection nebulae like IC 4606. This part not only contains more than half of the mass of the cosmic color disk, but is also the most active region of star formation in the cosmic color disk, giving birth to many stars. In the core of IC 4604, you can see the star that illuminates it, Rho Ophiuchi (which is the origin of the name "Rho Ophiuchi complex"). Rho Ophiuchi is a multiple star system consisting of five stars, all of which are blue main sequence or subgiants, which is why IC 4604 appears blue.

There are a large number of red emission nebulae near Antares (τ Scorpii) and Antares (σ Scorpii), such as Sh2-9 (Gum 65) around Antares. The high-energy ultraviolet radiation in the starlight excites the hydrogen atoms in the gas cloud, causing their electrons to separate from the nuclei. When the electrons and hydrogen nuclei recombine, they release lower energy, thus producing this reddish emission nebula. However, Sh2-9 also reflects the faint blue light emitted by Antares, so it is also a reflection nebula.

Globular cluster M4 and emission/reflection nebula Sh2-9. Source: Wikipedia

Next, let's turn our attention to the dark nebulae B44 and B45 in the northeast of the space color disk, which are like a pair of wings inserted in the space color disk. This is where the cold hydrogen clouds are very dense, and it is also the "cradle" of future new star formation. They block the visible light emitted by the stars hidden behind them. These dust clouds extend eastward into the Milky Way, beyond the range of the space color disk, to another dark nebula called the "Pipe Nebula".

The Pipe Nebula. Credit: ESO

The cosmic color disk is a natural specimen for studying stellar evolution. In March and April 2023, the Webb Space Telescope photographed IC 4603 in the cosmic color disk, showing unprecedented details of this huge nebula complex. This small area of ​​the sky in the image below has a field of view of 6.4 arc minutes and is about 390 light-years away from us. It has nurtured about 50 young stars, most of which are similar in mass to the sun or smaller. The darkest areas in the image are the most densely dusty, and thick dust wraps around the protostars that are still forming. The huge bipolar molecular hydrogen jets (red bar-shaped gas clouds) located above and to the right of the image dominate the image. This happens when stars first break through the dust envelope. They will eject a pair of jets in opposite directions into space, just like a newborn baby stretching its arms to the world for the first time. In addition, a binary star system numbered S1 Ophiuchi depicts a yellow glowing "cave" in the lower half of the image. The mass of the primary star of this binary star system is about 4.1 times that of the sun, making it the only star in the image with a significantly larger mass than the sun. The images from the Webb Space Telescope have once again refreshed our understanding of the complex structure of this nebula and are of great significance for the study of stellar evolution.

IC 4603 as imaged by the Webb Space Telescope. Credit: ESA

For astronomy enthusiasts, especially those who like starry sky photography, the starry sky color wheel has always been one of the most popular subjects. So what should we pay attention to when shooting the starry sky color wheel? It is best to use an astronomical modification when choosing a camera. The cameras we usually use usually add an infrared cutoff filter in front of the photosensitive element. This filter needs to be removed to enable the camera to more efficiently receive the infrared radiation of the starry sky color wheel, so that the photos show more details and colors. In addition, in the early preparation, we also need equipment such as large aperture lenses, tripods and equatorial mounts. This is to offset the rotation of the earth and avoid the dragging of star points to achieve long exposure and improve the signal-to-noise ratio; if these equipment are not available, short exposure methods can also be used to shoot. The 300 rule can be used to determine the exposure time of a single image [that is, the exposure time (in seconds) ≤ 300 ÷ the focal length of the lens (in millimeters)] and superimposed in the later stage. If conditions permit, you can also use a telescope with a larger field of view to shoot. If a wide-angle lens is used, you can also capture the magnificent scene of the starry sky color wheel strolling on the edge of the Milky Way.

A gorgeous star color palette. Source: Wikipedia

The shooting location needs to be a place away from light pollution. As long as the atmospheric conditions are good, after a long exposure time, we can capture the brilliant colors of the cosmic color disk. In early June in the northern hemisphere, the cosmic color disk can be found in the low sky in the southeast before about 21:00; it rises to the upper zenith at midnight, which is the best time to shoot it; and after 4:00 a.m. the next day, the cosmic color disk will fall below the horizon in the southwest.

When we gaze at the stellar palette, we are not only appreciating the beautiful sight, but also witnessing the birth and evolution of stars in the universe. This magical sky carries billions of years of cosmic history and endless mysteries, making us feel deeply insignificant in the vast universe. Through photography, we can capture the splendor and magnificence of the stellar palette, and let this beauty remain in our lenses and memories forever. May our camera lenses record those twinkling and beautiful moments, dance with the universe, and feel the miracle of life.

References

1. Rho Ophiuchi cloud complex, Wikipedia, https://en.wikipedia.org/wiki/Rho_Ophiuchi_cloud_complex

2. August 2019 Feature – Rho Ophiuchi Cloud Complex, Little Rock University, https://ualr.edu/tv/2019/08/08/august-2019-feature-rho_ophiuchi_cloud_complex/

3. Antares, Wikipedia, https://en.wikipedia.org/wiki/Antares

4. Webb Celebrates First Year of Science With Close-up on Birth of Sun-like Stars, Webb Space Telescope, https://webbtelescope.org/contents/news-releases/2023/news-2023-128?news=true

5. Rho Ophiuchi is filled with glorious color, EarthSky, https://earthsky.org/clusters-nebulae-galaxies/rho-ophiuchi-cloud-complex-antares-m4-dark-nebula/

6. Deep Space Exploration/Huang Dandan: Space Color Diagram, China Photography News, Issue 72, 2021, Page 7

7. The night sky is full of amazing "color palettes!" How to enjoy and photograph them? Quick search guide, Starlink CSVA, https://mp.weixin.qq.com/s/H6OTfMGcxXOfGRULOY4jXw

8. H II nebula, AstroFotoBlog, https://www.astrofotoblog.eu/?cat=49

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