A color you’ll never find in a star is responsible for the universal color of star-forming regions.
If you look through a telescope’s eyepiece, distant galaxies always appears white.
Spiral-shaped galaxies, so long as no new matter routinely falls into them, were long thought to remain static in size and extent over time. Through an eyepiece, a human being will see only the dominant, white color of the starlight averaged together. (NASA, ESA and W. Harris — McMaster University, Ontario, Canada)
But with advanced cameras that pick up individual photons, some regions show a different color: pink.
This visible-light image composite of the Orion Nebula was created by the Hubble Space Telescope team back in 2004–2006. The colorations presented here are scientifically accurate. (NASA, ESA, M. Robberto (Space Telescope Science Institute/ESA) and the Hubble Space Telescope Orion Treasury Project Team)
In our own galaxy, it’s the overwhelming color of star-forming regions like the Orion Nebula.
A young, star-forming region found within our own Milky Way. Note how the material around the stars gets ionized, and over time becomes transparent to all forms of light. Until that happens, however, the surrounding gas absorbs the radiation, emitting light of its own of a variety of wavelengths. (NASA, ESA, and the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration; Acknowledgment: R. O’Connell (University of Virginia) and the WFC3 Scientific Oversight Committee)
In some galaxies, the pink color can dominate a telescope’s entire field-of-view.
The starburst galaxy Henize 2–10, located 30 million light years away. When an entire galaxy forms stars, it undergoes a starburst, turning pink where the most active new star-formation occurs. (X-ray (NASA/CXC/Virginia/A.Reines et al); Radio (NRAO/AUI/NSF); Optical (NASA/STScI))
This isn’t some optical illusion or a false-color image; these regions and galaxies truly appear pink.
The star forming region 30 Doradus, in the Tarantula Nebula in one of the Milky Way’s satellite galaxies, contains the largest, highest-mass stars known to humanity. The largest, R136a1, is approximately 260 times the Sun’s mass; the light from these hot, new, bright stars is predominantly blue, however. (NASA, ESA, and E. Sabbi (ESA/STScI); Acknowledgment: R. O’Connell (University of Virginia) and the Wide Field Camera 3 Science Oversight Committee)
At first glance, it’s surprising, since there are no pink stars, and the majority of young starlight is preferentially blue.
Two of the sky’s more famous residents share the stage with a lesser-known neighbour in this enormous three gigapixel image from ESO’s VLT Survey Telescope (VST). On the right lies the faint, glowing cloud of gas called Sharpless 2–54, the iconic Eagle Nebula (Messier 16) is in the centre, and the Omega Nebula (Messier 17) to the left. Note the pinkish color of all three; these are all star-forming regions. (ESO)
But once you realize that it isn’t just stars, but gas, that can make light, the mystery solves itself.
Younger populations of stars contain short-lived objects that are hotter and bluer, and emit more ultraviolet, ionizing radiation. The net effect means that many hydrogen atoms surrounding these stars become ionized. (Spazturtle/Wikimedia Commons)
New star-forming regions produce lots of ultraviolet light, which ionizes atoms by kicking electrons off of their nuclei.
When free electrons recombine with hydrogen nuclei, the electrons cascade down the energy levels, emitting photons as they go. The n=3 to n=2 transition, known as Balmer alpha, is the strongest visible-light line, and is red in color. (Brighterorange & Enoch Lau/Wikimdia Commons)
These electrons then find other nuclei, creating neutral atoms again, eventually cascading down through its energy levels.
Of all the possible energy-level transitions in the Hydrogen atom, only four lines are visible, with the brightest and strongest being the red line at 656.3 nanometers. (NASA)
Hydrogen is the most common element in the Universe, and the strongest visible light-emitting transition is at 656.3 nanometers.
A portion of the galactic plane, with star forming regions highlighted in pink due to the emission of hydrogen atoms. (Y. Beletsky (LCO)/ESO)
The combination of this red emission line — known as the Balmer alpha (or Hα) line — with white starlight adds up to pink.
The Whirlpool Galaxy (M51) appears pink along its spiral arms due to a large amount of star formation that’s occurring. In this particular case, a nearby galaxy gravitationally interacting with the Whirlpool galaxy is triggering this star formation, but all spirals rich in gas exhibit some level of new star birth. (NASA, ESA, S. Beckwith (STScI), and The Hubble Heritage Team STScI / AURA))
Red and white make pink, explaining the color of star-forming regions.
The dark swaths that permeate spiral galaxies are neutral clouds of gas and dust, and block visible and ultraviolet light. However, when gravitational collapse triggers the formation of new stars, these regions will light up in pinks and blues as they either ionize or reflect starlight, respectively. (NASA, ESA, and the LEGUS team)
Mostly Mute Monday tells the astronomical story of an image, object, or phenomenon in visuals, pictures, and no more than 200 words. Talk less, smile more. Ethan Siegel is the author of
Beyond the Galaxy and Treknology. You can pre-order his third book, currently in development: the Encyclopaedia Cosmologica.