Galaxies appear in a range of colors, from blue to red, when viewed through a telescope. However, some galaxies have a distinct purple hue to them. The purple color is the result of the combination of wavelengths of light emitted by stars and other celestial objects within the galaxy.
What causes the purple color in galaxies?
There are two main factors that contribute to galaxies looking purple:
- The presence of young, hot stars that emit short wavelengths of ultraviolet light
- The abundance of ionized hydrogen gas that gives off a reddish glow
When the violet light from hot stars mixes with the red light emitted by hydrogen gas, it creates the impression of a purple color to our eyes.
Ultraviolet light from young stars
Stars come in a range of temperatures and colors. The hottest, youngest stars tend to be bluish-white or violet in color. This is because they have surface temperatures of 10,000 degrees Celsius or more. At these extreme temperatures, the peaks of their emitted light spectra are in the ultraviolet part of the electromagnetic spectrum.
Although our eyes cannot see ultraviolet wavelengths, telescopes can detect them. When ultraviolet light from young stars mixes into other visible wavelengths of light, it lends a violet/purple tinge to the overall color we perceive.
Red glow of ionized hydrogen
In addition to hot young stars, galaxies contain large amounts of hydrogen gas spread throughout. This gas is found in between stars and also in vast nebulae clouds where new stars are formed. The hydrogen is ionized, meaning the electrons have been stripped from the hydrogen atoms. This ionized gas glows with a distinctive red color when the electrons reunite with the proton nuclei of the hydrogen atoms.
This red glow combining with the ultraviolet and blue light from stars is what gives many galaxies a purple-ish tint. The red hue is especially dominant in specific regions of active star formation where hydrogen gas is plentiful.
Differences in purple coloration
Not all galaxies that appear purple have the exact same shade and intensity of purple color. There are a few key factors that cause variation:
- Age of stellar population – Younger stars give off more UV light
- Gas density – More hydrogen gas means a stronger red glow
- Chemical composition – More helium results in bluer color
- Dust content – Dust scatters light, affecting color
Let’s take a look at some examples to see how these factors influence purple color in galaxies:
Starburst galaxies
Starburst galaxies are undergoing rapid star formation across large areas, lighting up these regions with hot young stars. The prodigious UV light from all these short-lived massive stars gives starburst galaxies a violet/blue tinge. When combined with the ubiquitous glow of ionized hydrogen gas, the overall color is a vivid purplish-blue. Examples of starburst galaxies include M82 and NGC 3310.
Elliptical galaxies
Elliptical galaxies contain older stellar populations and less active star formation. The peak radiation from old stars is in the yellow/red part of the spectrum. However, some ellipticals still contain enough hotter stars and ionized gas to take on a purplish appearance, albeit a more subdued purple than starburst galaxies. Examples include NGC 474 and NGC 3379.
Irregular galaxies
Irregular galaxies have a chaotic structure and active star formation in sporadic localized clusters. The scattered ultraviolet light from youthful stellar nurseries combined with nebular emission from gas creates blotchy patches of purple across these galaxies. The overall purple color is less uniform than in spirals or ellipticals. The Large Magellanic Cloud and NGC 4449 are good examples.
Why don’t all galaxies look purple?
While some shade of purple is relatively common for galaxies, not all galaxies exhibit this coloration. Here are some reasons why a galaxy might not appear purple:
- Older stellar population – Lacks hot stars to emit UV light
- Low gas content – Insufficient nebular emission in red wavelengths
- High metallicity – More blue and white stars
- Dust obscuration – Dust blocks and scatters light
- Viewing orientation – Edge or face-on view alters color
For example, lenticular and elliptical galaxies tend to be made up of older reddish stars and have very little dust or gas. Without those components, the violet hue is missing and these galaxies just appear yellowish or orange instead of purple.
Notable purple-colored galaxies
Here are some of the most dramatic and visually striking purple-colored galaxies visible from Earth:
| Galaxy name | Type | Distinctive features |
|---|---|---|
| NGC 7552 | Spiral galaxy | Bright violet disk with red center |
| M83 | Barred spiral | Vibrant purple arms studded with young star clusters |
| Mayall’s Object | Ring galaxy | Collision with galaxy gave ring purple hue |
| M101 | Spiral galaxy | Spiral arms tinged deep violet color |
| Minkowski’s Object | Peculiar galaxy | Streamers of gas glow purple against background |
Conclusion
In summary, the purple coloration observed in many galaxies is due to a combination of ultraviolet light from hot young stars and reddish glow from clouds of ionized hydrogen gas. The specific shade of purple can vary based on the ages of stars, gas content, chemical composition, dust levels, and viewing orientation. While not all galaxies appear purple, this hue is common in spiral, elliptical, and irregular galaxies that have sufficient star formation activity and nebular gas emission occurring.
The attractive violet/purple color in galaxies is a direct window into their stellar nurseries, chemical enrichment history, and structural evolution. Galaxies exhibit a rainbow of cosmic colors, but purple is one of the most visually striking and informative about the underlying astrophysics at play.
