“My dream concept is that I have a camera and I am trying to photograph what is essentially invisible. And every once in a while I get a glimpse of her and I grab that picture.”
- Leonard Nimoy, in an interview with Anne A. Simpkinson, 2001
Astronomers may not be searching for the same kind of invisible essence to which Nimoy refers in his photography. However, science does push us to extend beyond what our eyes can see in order to interpret the nature of the physical universe.
For hundreds of thousands of years, humans have had just our eyes to detect a certain type of light that shines down abundantly on us from our sun. It just so happens that we can also see some of the other “suns” in our galaxy on a clear night. Our vision of the cosmos expanded when Galileo began recording astronomical observations made with a telescope a little over four hundred years ago. Since then, astronomers have been building ever bigger telescopes with which to peer further and deeper.
However, it was only in the 20th century that astronomers made a concerted effort to look at the “invisible universe,” or study the universe with light that our eyes cannot detect. This new animation from the European Space Agency celebrates this quest for the invisible as we look at our nearest big galaxy, Andromeda, or Messier 31, in microwave, infrared, optical, ultraviolet, and x-ray light.
All light is essentially the same thing. We call it electromagnetic radiation, and that covers the radio frequencies of your cell phones, the x-rays at the dentist’s office, and the light shining from your computer monitor. However, looking at a galaxy such as Andromeda is each part of the spectrum shows us vastly different processes.
As we zoom into the Andromeda Galaxy from a full-sky image of the sky in microwaves, we can already get a hint of its disk shape. The familiar visible image appears as a close-up, where we can see the blue-tinged disk of stars with spiral waves throughout, and a yellow-ish bulge of older stars in the center of the galaxy.
Slowly, the infrared observations come into view. Since we do not see infrared, we have to choose a color with which to represent it. The galaxy looks flat now, with sharper spiral arms and not much activity in the center. The infrared light is not from stars, but is the glow of dust particles within the busy traffic-jams that are the star-forming spiral structures of such a galaxy. In particular, we are looking at cool dust, which tends not to be in the even busier central bulge of the galaxy.
Then, we swing on over to the x-ray light, where it no longer looks like a spiral galaxy, but a cluster of points of various sizes. Many of these represent the endpoints of the lives of massive stars that have long since exploded and left behind cold-dead remnants. Well, almost dead, as interactions with their environment can cause these remnants, such as neutron stars and black holes, to give off lots of energy in the form of these x-rays. X-ray sources in Andromeda have even been seen to vary in brightness over the span of several years.
Finally, we get to the light that gives you a tan (or a sunburn), the ultraviolet. The most massive, young, and short-lived stars give off plenty of ultraviolet light, so again we see these sites of recent star formation. Altogether, these fantastic images in visible and invisible light help astronomers to complete the picture of star formation, life, and death in a galaxy. Maybe somewhere out there, an alien civilization can see our Milky Way Galaxy in the invisible.
Confession: I muted the original video and turned up the Star Trek theme song to full volume. Yeah, I’m a geek.
Image: Seeing Andromeda in different wavelengths. Credit: infrared: ESA/Herschel/PACS/SPIRE/J. Fritz, U. Gent; X-ray: ESA/XMM-Newton/EPIC/W. Pietsch, MPE; optical: R. Gendler