Astronomers can look at images of celestial objects like planets, nebulae and galaxies and immediately see their significance, but for the average person seeing them on the Evening News the pictures are, of course, beautiful but at the same time, generally mysterious. My recent book The Hidden Universe takes 68 images you might have encountered, shows you how they were created, and what interesting story lurks beneith their gorgeous countenences. This blog will describe one of these.
By now you have seen the spectacular images provided by the Webb Space Telescope (JWST), and boy are they breath-taking even for an astronomer! I have been waiting all my professional life for HD-quality images of objects in the mid-infrared spectrum (wavelength: 1 to 28 microns), and JWST has not dissappointed. NASA’s Spitzer Space Telescope also produced high-quality infrared images to be sure (wavelength: 3 to 160 microns), but JWST is a much-larger telescope (50-times larger than Spitzer) with decades-more-modern sensors!
Selecting Your Color Pallet
The first thing to remember when looking at JWST images at wavelengths between 0.6 and 28-microns is that they are not colorized the way your eye would see the light. Human eyes end their color sensitivity to visible light at around 0.6-microns, so the idea that something seen by JWST would ‘look green or red’ is completely incorrect. JWST, in fact many astronomical images used in research, are colorized to help the astronomer detect differences in how objects emit light at different wavelengths.
Our brain has evolved our color vision to be a sophisticated pattern-recognizer, so all you have to do is put your data into ‘RGB’ form, and BANG! you can use the image processing power of the wet-ware in your brain to quickly survey a new subject.
The particular color pallet an astronomer uses is all about their artistic sensibility and what they want to emphasize, so it is not unreasonable for a bright red color to be used to represent hot dust and bright blue colors for cooler dust, etc. Ironically, in the electromagnetic spectrum, red wavelengths are cooler objects and blue wavelengths are hotter objects, but humans feel that blue is a cooler color than red, so our interpretation of color is backwards! For astronomers, you can even colorize the speeds of gas clouds (ie Doppler shift) so that blue colors represent clouds moving towards you and red clouds are clouds moving away from you, irrespective of the wavelength being used.
Image Analysis 101.
To analyze JWST images, let’s start with an image that has become an astronomical and even world-wide ‘classic’: The Pillars of Creation.
Vital Statistics: The ‘Pillars’ are actually a small part of the Eagle Nebula, also called Messier 16. This is a beautiful star-forming region in the constellation Serpens located 7,000 light years from Earth. With a size of about 60 light years, the region contains 8,000 stars of which the brightest of these, HD-168076, is 80 times the mass of our sun and only a few million years old. Its intense ultraviolet light illuminates the nebular gas and causes the atoms to emit the colorful patina of light that we see in optical photographs. Here is what the entire nebula looks like with ‘true color’ RGB filters in the visible spectrum:
This three-color composite optical image was obtained with the Wide-Field Imager camera on the MPG/ESO 2.2-meter telescope at the La Silla Observatory. At the center of the nebula you can see the Pillars silhouetted against the bright nebular gases, which makes these ‘dark nebulae’ really stand out. The cluster of bright stars to the upper right is called NGC 6611, which the Pillars are pointing at, and are the home to the massive and hot stars that illuminate the Pillars.
Ionizing Radiation: The intense ultraviolet radiation from the massive stars in the cluster at the upper-right is actively eating away at the dense interstellar cloud that gave birth to them at the lower-left. These are what astronomers classify as O and B-type stars or just ‘OB’ stars. Each is more than 5 times as massive as our sun, and with surface temperatures above 20,000o C they emit most of their light at ultraviolet wavelengths. Astronomers can detect this ionized gas at radio-wavelengths too. They are called ‘HII’ (H-two) regions because HI is the symbol for ordinary ‘neutral’ hydrogen and ‘II’ means that the neutral hydrogen atoms have lost one electron to make them ionized, hence ‘HII’. This ultraviolet radiation streaming through the hydrogen HI gas to ionize it into HII results in many unusual wind-swept shapes including what astronomers call ‘elephant trunks’ such as the Pillars. Their shapes act like ‘wind socks’ and point towards the main source of the ionizing gas flow, which is expanding outwards from the OB star cluster at speeds up to 10 km/s….. that’s about 22,000 mph. At this pace, the ionzation front can travel 4 light years in 100,000 years, which is enough to cross the space of most star-forming regions.
What you also notice is that the optical image shows the Pillars as dark and obscurring the background nebula which we call ‘dark nebulae’, while the JWST image shows the Pillars being very bright as ’emission nebulae’. The bright emission nebula produced by the stars is also missing in the JWST image, and instead you see thousands of stars in the background. The reason this happens is your first exposure to astrophysics!
Here is a side-by-side comparison of the Hubble (left) and Webb images (right).
Interstellar Dust: Interstellar gas clouds would be entirely invisible were it not for the fact that for about every trillion atoms of gas you have one dust grain. These dust grains are about 1-micron in diameter and were formed in the cool atmospheres of ancient red super giant stars like rain condensing out of a storm cloud on Earth. The dust grains do two things. First they scatter light at optical wavelengths, just like the dust in our atmosphere does, making the sky (or a nebula) appear blue. Secondly, they are very cold, but even so they emit their own ‘heat radiation’ in the infrared spectrum. If a cloud contains lots of dust grains, it will obscure the light from background stars, but at the same time emit infrared radiation making them appear bright at infrared wavelengths. This is what we see in the Pillars.
These linear Pillar clouds are about 2 to 4 light years in length and are rich in dust grains, so they block optical light in the famous Hubble images making them look dark, but emit their own radiation making them look bright at infrared wavelengths in the JWST images. For the first time, astronomers can study just how lumpy the surfaces of these interstellar clouds are by looking at the infrared light they are emitting directly. Taking a closer look at the Pillars gives even more information.
Star-forming regions: Zoom in a little more, until you see red dots springing into view. There are dozens of them. Each of those red dots covers an area larger than our solar system. The finger-like protrusions are also larger than our solar system, and are made visible by the shadows of evaporating gaseous globules (EGGs), which shield the gas behind them from intense UV flux. EGGs are themselves incubators of new stars. The stars then emerge from the EGGs, which then are evaporated.
So there you have it! One picture rendered into ‘a thousand words’. To be sure, this one image will easily form the basis for someone’s PhD thesis because it is so rich in detail missing from even the best Hubble images. When combined with spectroscopic data from JWST and data from radio astronomy, we will have a detailed understanding of just ONE star-forming region in our Milky Way. We are definitely living in exciting times for astronomical research!
If you want to see more astronomical images analyzed this way, have a look at my new book ‘The Hidden Universe’ available at Amazon.
Check back here for my next blog!