Why did the gorgeous nebula overturn the "palette"? Revealing the magical "Hubble color"!

Author | Zhou Yihao

Audit | Huang Jian

Editor | Wang Qiyuan

The Carina Nebula, a composite of images taken by the Hubble Space Telescope and the Blanco Telescope (Image courtesy of NASA)

I believe that when you see the above picture for the first time, you will be shocked by the blend of soft and warm orange and cold green. It is like an overturned color palette, magnificent and mysterious, making you feel as if you are in a starship in "Star Trek".

Bayer filter in a color camera (Image from Wikipedia)

So how did this gorgeous image come about? To answer this question, let's first follow Xiaoxing to learn about the principle of digital cameras taking color photos. The camera's sensor can only sense the intensity of light, just like drawing only black and white outlines, but the color camera's sensor is covered with a layer of small filters, each of which only allows red, green or blue light to pass through. In this way, each pixel only records the brightness information of one color. After that, the camera will use a unique algorithm, just like "filling in" a black and white painting, to deduce the complete red (R), green (G), and blue (B) information of each pixel based on the colors of the neighboring pixels, and combine them to generate a color picture.

The Wide Field Camera WFPC2 on the Hubble Space Telescope (Image courtesy of NASM)

The Hubble Space Telescope is different. The wide-field cameras WFPC2 and WFC3 on board can collect light signals of different wavelengths with different filters. These wavelengths include visible light visible to the human eye, as well as ultraviolet and infrared light that the human eye cannot perceive. Each time it takes a picture, the Hubble Space Telescope selects a specific filter to isolate a certain wavelength of light, such as the spectral line emitted by hydrogen atoms (Hα line) or the spectral line emitted by oxygen atoms (OIII line). In this way, each image captured by the camera is actually a grayscale photo of a specific wavelength, recording the intensity distribution of light at that wavelength. This "monochrome" photo allows scientists to more accurately study the physical properties of different celestial bodies, such as stellar temperature, gas composition, and dust distribution.

Color synthesis of the Bubble Nebula photo (Image courtesy of NASA)

The raw data from the Hubble Space Telescope is grayscale, and to create color images, it is processed through a technique called false color. Specifically, different wavelengths of image data are assigned specific colors, such as red for infrared, blue for ultraviolet, and green for certain visible wavelengths. These "colored" layers are then stacked to create a color image that is rich in information. Although these false colors are not the true colors of celestial objects, they can help identify key features. For example, in the above Hubble Space Telescope image of the Bubble Nebula, red corresponds to nitrogen, blue to oxygen, and green to hydrogen.

The Spider Nebula photographed using the SHO filter (Photo: Zhou Yihao)

The Hubble palette in astronomical photography usually refers to the pseudo-color images synthesized by the SHO narrow-band filter. The three filters of sulfur (SII, wavelength 672.4 nanometers), hydrogen (Hα, wavelength 656.28 nanometers) and oxygen (OIII, wavelength 500.7 nanometers) are used to take monochrome pictures, and then they are given red, green and blue channels to synthesize colorful nebula images. Because the synthesized images are similar to the colors of the Hubble Space Telescope narrow-band photos, they are named "Hubble Palette".

Comparison of the SHO color (left) and RGB color (right) of the Christmas Tree Nebula (Photo: Zhou Yihao)

The use of color here is not just for "looking good", but also to highlight the originally dim or difficult to distinguish details, and allow us to transcend the limitations of the band and see the rich and colorful colors in the depths of the universe. When seeing these color-enhanced images, everyone can feel more layers and mystery behind the "sea of ​​stars", inspiring a stronger desire for exploration and imagination. In contrast, if we only rely on images that are consistent with the colors visible to our naked eyes, although they are closer to the world we can capture, they often lack the presentation of dark structures or special energy radiation. We may only see a bright or dark cloud, but ignore the infrared or ultraviolet details that contain information.

The emergence of Hubble colors perfectly combines astronomical exploration with artistic aesthetics, making scientific research vivid and tangible, and bringing the wonders of the universe into the public eye. Through these colorful and detailed images, people can intuitively appreciate the multi-layered structure of nebulae, the interactions between galaxies, and the vastness and complexity of the deep universe. The curiosity and amazement brought about by these visual impacts are precisely the most moving power of popular science.

References

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[8] The photos of the nebula are so beautiful! Are these colors real?, Beijing Planetarium, https://www.bjp.org.cn/twwd/wyjytwsy/4028c1367a595bb8017a5bec0c97000a.shtml.

[9] The Meaning of Light and Color, HubbleSite, https://hubblesite.org/contents/articles/the-meaning-of-light-and-color

[10] Molly Wakeling. How to take Hubble-like photos: Filters and post-processing guide, https://www.astronomy.com/observing/observing-basics-image-like-hubble/

[11] Mamenchisaurus keeper, why is the nebula I see black and white, but the one they photoshopped is in color?, WeChat public account "Guoker", https://mp.weixin.qq.com/s/R8Y3fnt5ucxnJat3XUQomg

For more questions about Hubble color, please leave a message in the comment area, Xiaoxing will answer them for you one by one~

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