Learn To Read False-Color NASA/ESA Images

Learn To Read False-Color NASA/ESA Images
Image (left) courtesy of NASACXC/SAO (X-ray); NASA/STScI (optical); NSF/NRAO/AUI/VLA (radio) under Public Domain and Image (right) courtesy of NASA/STSci under Pubic Domain

Can you identify which of the two shown above is a false-color image? Can you identify the different colors associated with the bands of the electromagnetic spectrum (i.e., Radio, Infrared, Visual, Ultra Violet, X-ray, Gamma Ray) in both images? Spoiler Alert: Here’s the answer – Right image is true color that is also in the background of the left image; the left image also has false color red for radio band and blue for X-ray band superimposed. Read on to learn how to read false color images like that shown above for Cygnus A.

Dr. MMM has taught astronomy to over 10,000 higher education students over 15 years at a Carnegie Level R1 type University in the USA. Dr. MMM continues to teach more than 10 classes of astronomy and physics each year at different colleges. She contributes to several astronomy textbooks used worldwide and writes her own solo textbook Astronomy Essentials. Read on to learn how to identify if an image is false color and how to read false color images.

In this post, you will see links to a video glossary of astronomy vocabulary that are not only defined at the YouTube channel AstroPictionary but also explained. Astronomy has one of the largest vocabularies in the sciences and sometime words are re-used with different meanings; an analogy would be English vocabulary words there, their, and they’re. See the video glossary and subscribe as we constantly add more to the site. Enjoy!

NASA, ESA, and Images

National Aeronautics and Space Administration (NASA) and European Space Agency (ESA), for example, publish images, drawings, sketches, animations, etc. that, in turn, are promoted to you by TV news, online news, blogs, press releases, etc. Often, these types of pictures are not well described, leaving you – the observer – to decipher cold/hot colors or different wavelength bands of the EM spectrum on your own.

NASA/ESA images taken with a camera like the one in your cell phone are mostly in optical band or in V band of the EM spectrum. These images try to replicate what you eyes might see. For example, the image of Cygnus A on right at the top of this post is taken in mostly optical or in V band, which we might refer to as true color images. NASA/ESA images taken with a camera that is that is not like the one in your cell phone are often false-color. For example, the image of Cygnus A on the left at the top of this post is false color.

Caption: Image courtesy of NASA/SDO/AIA under Public Domain

The image of the Sun shown at left is sometimes confused by students as true color, as the Sun sometimes appears orange when viewed near the horizon of Earth during sunrise or sunset. This image has been taken from NASA’s Solar Dynamic Observatory (SDO) using the Atmospheric Imaging Assembly (AIA) Extreme Ultraviolet (EUV) 304 Angstroms channel (i.e., A1A 304). Because most human eyes cannot see in UV wavelengths, this image is not true color; this image is false color. Read on to learn how to identify whether an image is true or false color.

Identifying Image as True or False Color

Identifying false color images is easier when the celestial object is well known to the observer. However, identification of true or false color is much more difficult for unfamiliar images. Let’s look at some examples:

Identification Using Known Celestial Objects

caption: Image of Earth (true color, left and false color added, right) courtesy of NASA/JHUAPL under Public Domain

Identifying whether an object is true or false color is easier when the subject is well known to the viewer or reader. For example, many people will be able to identify the above left image of Earth as true color; the colors seen in the image on the left are true or similar to colors that human eyes would see for land, oceans, clouds, etc. The camera used to take such a true color image would be like that used in a cell phone camera, which mostly takes images in the visual band of the electromagnetic (EM) spectrum.

If an image is taken in a wavelength band other than optical or V band, then the image may have a false color added for clarity. The AstroPictionary video below explains false color images using Io, a Galilean moon of Jupiter as an example.

Another example is the images of Earth shown above. Both images have been taken by NASA’s MESSENGER spacecraft, mostly in the V band, with the right image also having near infrared (NIR) false colors added. Notice that the above right image has the Amazon forest in South America colored red (see center of the image). As most humans cannot see NIR wavelengths with their eyes, false colors are added to help a viewer locate NIR regions in the V band image. As green leafy plants contain chlorophyll, which is highly reflective to NIR wavelengths, regions on Earth that reflect significant amounts of NIR wavelengths are colored red in the image; as the Amazon jungle contains a significant amount of green leafy plants, those portions of the Amazon jungle that highly reflect NIR wavelengths is colored red in the upper right image of Earth.

Identification Using Not Well Known Celestial Objects

When celestial objects are less well known, identification of an image as false or true color is more difficult. The below left image is Uranus seen in true color; the below right image is false color. Both images were taken by a camera on the NASA Voyager 2 spacecraft.

The left image of Uranus has been taken through blue, green, and orange filters and processed to reflect what a cell phone camera, or your eyes, might see. The dark shadow along the rim of Uranus from the 12 o-clock to 6 o’clock positions indicate night, whereas the rest of the planet is bathed in sunlight. As shown in the left image, Uranus appears pale green-blue to human eyes.

Caption: Image courtesy of NASA/JPL under Public Domain

The right image of Uranus has a false color to reveal the southern polar region; the northern polar region lies on the far side, that is, the side hidden from view in both images. If only the right image is shown on a newscast on TV or shown in an online news article, the celestial object may be difficult to name by sight. The newscaster or article may or may not identify the celestial object as being false colored but should identify the object as Uranus.

To determine whether an image has been false colored, read the image credit, if one has been given.

  • A person’s name or persons’ names may be listed; follow any link to any website that may list types of used such as a camera, video, or a personal ground-based telescope; astrophotographers and cameras mostly operate in the V band of the EM spectrum; most ground-based personal telescopes view in V band filters; all V band images are true color.
  • A person’s affiliation may be listed such as NASA or ESA, followed by persons involved or telescope and instruments used in taking the image; follow any links to identify whether a camera was used, which is true color; if a telescope is used, research further to find which band of the EM spectrum as any band other than V band is likely false color.
  • Read any captions and look for keywords: Keyword camera or astrophotographer indicates a true color image; keyword false color identifies the image as false color; keyword Hubble, Chandra, Spitzer, VLA, or Compton, for example, are NASA and/or ESA telescopes that mostly operate in the V band, X-ray band, infrared band, radio band, and gamma ray bands, respectively; image from these telescopes are false color, except for Hubble V band (i.e., real color) images.

For example, the caption to the Uranus images above lists the right image as false color and the left image as natural, which is another way of saying real image.

How To Read a False Color Image

After having identified an image as false color, read the caption to the image to gain further insight into the color coding. If no description is given, usually bright colors like white indicate much non V band wavelengths is detected whereas black indicates no detection. For example, the image shown below is in V band or true color; however, the image shows a TV screen displaying an image taken from a thermographic camera that operates in the infrared band. Images taken with thermographic cameras show heat wave emitting from an object, which in this case is a human holding a camera. Notice that the human is seen in shades of white, gray, and black; white indicates significant amounts of heat being emitted such as the human’s skin; gray tones indicate less amounts of heat being emitted such as that coming from the torso, which is covered by clothing; black indicates no heat emission such as the human’s eye glasses, camera lens, and watch. If you get a chance, touch your bare skin and compare that with touching the face of your watch; the watch will feel cooler; this temperature difference is picked up in thermographic images: Notice the person’s very white head compared to the less white arms that may indicate that this person being scanned at the Ioannini Greece airport may have a fever from from covid-19, swine flu, or other infection.

caption: image courtesy of Etan Tal under CC BY 3.0

Let’s return to the false color image of Uranus to better understand details that are revealed by false colors and extreme contrast. Portions of Uranus that are seen as violet in true color have been falsely colored blue in the false color image; similarly, portions seen as orange in true color have been falsely colored green, and portions of Uranus reflecting UV light are falsely colored red.

The composite of colors reveals a bulls eye around the southern polar region of Uranus surrounded by concentric circles; the bulls eye and nearest two concentric circles indicate much UV reflected light that decreases with increasing radius from the polar axis; the outer yellow and blue concentric circles nearest the equatorial region of Uranus indicate little reflected UV light. As shown by the white rim from the 12 to 6 o’clock positions, the night time side of Uranus does not reflect much UV light; what is colored white in the false color image is shown as black in the real image; white is used for contrast to the blackness of space.

Returning to the image shown at the top of this post regarding Cygnus A, the V band real image (repeated below, left) shows space as calm. However, when looking at Cygnus A in X-ray (below, right) or radio (below, center), many more events are revealed, indicating that Cygnus A is not calm!

Caption: V band image (upper left) courtesy of NASA/STSci under Public Domain, X-ray image (right) courtesy of NASA/CXC/SAO under Pubic Domain, and radio band image (bottom) courtesy of VLA under CC BY-SA 3.0

Visible light from Cygnus A has been captured by the Hubble Space Telescope and DSS ground-based telescope (see above left) in true color and we see shades of yellow. The center of the field of view (FOV) of the visible light and X-ray images contains the core of the galaxy. The X-ray image (see above right) is false color in shades of blue and white, with white indicating significant significant amounts of X-rays, blue indicating much X-ray emission, but not as much as white, and black indicating none. As the X-ray image shows, a hot gas bubble surrounding the core of the galaxy is emitting much X-rays. The bottom panel shows the radio band image taken using the ground based VLA telescopes. The center of the FOV shows the galaxy core with two jets streaming outward that collide into two large lobes at the upper right and lower left of the image. Like the X-ray image, the radio band image is false color with much radio emission in red, some in green, much less in blue, and black indicates none.

Once you learn to identify true and false color images and learn how to read them, you can identify hot and cold regions. The blackness of space in the above images reveals no emission of V band, X-ray band, or radio band light; space is cold. Higher energy regions of a wavelength band may involve hotter temperatures; for example, the white and blue areas in the X-ray image is a hot gas compared to the cold temperature and blackness of space in the image. A second example is the radio band image of the lobes, with red regions having a higher intensity and hotter region than compared to the green colored or blue colored regions.

You have to analyze the false color images first before determining the hot and cold regions in the image. For example, the blue, white, black X-ray image of Cygnus A has white and blue as hot and black as cold, but the radio image has red as hot, blue as cooler, and black as cold.

In summary, first determine if the image is true or false color. Second, analyze the image to determine which colors represent high energy or shorter wavelengths of a particular EM wavelength band and which represent low energy or longer wavelengths of that bad. In true color, red is cooler than violet in the ROYGBIV spectrum. However, a false color image could have red as cool and violet as hot or vice versa, depending on the artist. Third, as a check and balance, determine if a high energy region is also a hotter region than a low energy and cooler region. If colors, wavelengths, energies, and temperature analyses agree, you have successfully analyzed the image regarding these parameters. You pass!

Caption: Image courtesy of Arno/Coen under CC BY-SA 3.0

Now let’s test your knowledge gained: In the image of lemurs in a tree on Earth, a) is the image true or false color if the EM band shown is infrared? b) Which object or part of object emits the most infrared light and which emits the least? Include in your analysis the tail of the lemurs. c) Which color represents hot in the image and which represents cold? Which object is hottest and which is coldest? d) does orange represent a hot, cold, or in-between temperature? e) Which two colors represent the highest energies emitted in the infrared band and which two colors represent the lowest? Answers are given below.

Scroll down to see answers.

Answers: a) false b) back and yellow color parts of tail emits most; ground emits least c) white is hot, followed by yellow, orange, red, violet, blue and black is coldest; lemur back is hottest and ground is coldest d) in-between e) white and yellow are highest energies and violet and black are lowest

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About the author

Michele M. Montgomery earned a B.S. Degree in Nuclear/Mechanical Engineering from the Pennsylvania State University, an M.S. Degree in Physics from The University of Alabama with a concentration in Solar Physics, and a Ph.D Degree in Physics from Florida Institute of Technology with a concentration in close binary star systems. She joined the faculty at The University of Central Florida Physics Department in 2004 where she regularly taught astronomy, astrophysics, and cosmology. In 2006, she noticed that a large, urban college nearby to UCF did not teach astronomy at one of their largest campuses. She began teaching astronomy at this East Campus of Valencia College, a college that has more than 60,000 students; she still teaches four courses of astronomy each fall, spring, and summer semesters. The astronomy program atValencia College East has grown significantly with several more faculty added who teach astronomy.

By 2019, Dr. Montgomery has taught astronomy to more than 10,000 college and university students, both online and face-to-face. Many of her students have gone on to take her astrobiology, astrophysics, and space physics courses. 

By 2016, Dr. Montgomery had co-authored several astronomy texts and quiz/exam banks. Her work appears in several domestic and international astronomy text books (e.g., Horizons by Cengage, Universe by Cengage, Foundations of Astronomy by Cengage) that are used both at the higher education as well as at the high school levels. Starting in Fall 2019, Dr. Montgomery switched gears to authoring digital textbooks and research full time, while still teaching 12 courses of astronomy and up to eight conceptual, algebra, and/or calculus-based physics courses each year. Her research interests are numerical simulations using Smoothed Particle Hydrodynamics of close binary star systems. She also regularly is granted telescope time on the NASA's Kepler space telescope for observing eclipsing binary star systems. She has also observed using Gemini South, Keck, and Kitt Peak ground-based telescopes. Her major teaching areas are Astronomy, Astrobiology, Astrophysics, Cosmology, Space Weather/Space Physics.