NGC 55, a nearby edge-on dwarf spiral galaxy, seen by Hubble Space Telescope. Credit: NASA, ESA, R. de Jong (Leibniz-Institut fur Astrophysik Potsdam [AIP]), and G. Illingworth (University of California – Santa Cruz)
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NGC 55, a nearby edge-on dwarf spiral galaxy, seen by Hubble Space Telescope. Credit: NASA, ESA, R. de Jong (Leibniz-Institut fur Astrophysik Potsdam [AIP]), and G. Illingworth (University of California – Santa Cruz); Processing: Gladys Kober (NASA/Catholic University of America)

Extragalactic stars in my eyes

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Sep 17, 2021, 9:00 AM EDT

For centuries, astronomers scratched their heads and wondered what the hell these weird “spiral nebulae” in the sky were.

They showed up pretty much everywhere they looked. Some were small, some big, some were amorphous blobs, some showed clear spiral or other structure. Astronomers knew that some nebulae — Latin for “fog” — were gaseous by the end of the 19th century, but the true nature of their diversity remained elusive. Some astronomers speculated that some of these were separate galaxies like our own Milky Way, while others thought they were gaseous clouds inside our Milky Way, which comprised the entire Universe.

This changed in the 1920s when, among other things, a team of astronomers (including Edwin Hubble) found variable stars in the “Andromeda Nebula” that clinched its distance as being outside the Milky Way — thus earning being renamed to the Andromeda Galaxy — around the same time that Vesto Slipher (such a great name) found that more distant galaxies were moving away from us, leading to the understanding that the Universe is expanding.

This all happened because we made our technology better, and we could probe these distant objects and understand them better. One of the biggest breakthroughs was that bigger telescopes could have cameras mounted on them, and we could see that some of these nebulae were made of stars. Their great distance meant these stars all blurred together in to a fog (hence nebulae), but with enough resolution could be seen for what they truly were.

Now, a century later, seeing stars in other galaxies is trivial. It’s no less magical, but when you take a telescope, put it in space, and name it after Hubble, stars in other galaxies become easy pickings. Like with the very nearby galaxy NGC 55:


NGC 55 (also called Caldwell 72; click here for a full-size image nearly 4,000 pixels on a side) is a member of the Local Group, our small suburban neighborhood of a few dozen galaxies including Andromeda, the Milky Way, and lots of tiny galaxies called dwarfs. This one is a little over 6 million light years away, so twice as far from us as Andromeda, but far smaller. In fact it’s similar to a dwarf galaxy that orbits the Milky Way called the Large Magellanic Cloud (or LMC), one that is mostly irregular in shape but seems to be trying to form a spiral structure.

NGC 55 is flattened, and we see it nearly edge on, so it looks like a long smear or cigar-like shape. The Hubble image reveals it to be loaded with stars, as well as its own collection of nebulae. The colors in this image are a little strange; what’s displayed as blue is actually red light, and what you see as red is near-infrared light, just outside the range of the human eye’s ability to detect. That’s why the gas clouds look blue here; they’re actually quite red, glowing with the light characteristic of warm hydrogen.

The images were taken as part of a project to look at nearby edge-on spiral galaxies (NGC 55 counts because it appears to be a spiral, even though it resembles the LMC). These galaxies are loaded with opaque dust which can block us from seeing their internal structure, so the project took snapshots of lots of galaxies in these two colors to see just how much light is blocked on very small scales; infrared light gets through dust better than visible light.

This type of work is important because understanding the internal structure of other galaxies helps us understand their behavior, their history, and their evolution. Also, the Milky Way is a spiral, so understanding how other ones behave helps us understand our own home.

But that’s not why I’m writing this, not the point I want to make. Instead, seeing this image reminded of something I haven’t thought about in years...

For several years I worked on a camera designed and built to be placed on Hubble called the Space Telescope Imaging Spectrograph, or STIS. I got the job pretty much right after I got my PhD working on older Hubble images, and it was a dream job. Obviously.

One of the upgrades Hubble got at the same time STIS was placed on board by Shuttle astronauts in 1997 was a new solid state data recorder, allowing the telescope to store data much more quickly than the old tape recorder (!!) it used before. That meant that when one camera was being used to observe something in the sky, other cameras could be turned on and take data too. They pointed nearby to some other part of the sky, allowing what is essentially free data to be returned. This was called the Parallels Program.

NGC 55, a nearly edge-on dwarf spiral galaxy, seen using the MPG/ESO 2.2-meter telescope in La Silla, Chile. This image is closer to true color, with gas clouds showing as red. Credit: ESO

Those data were always immediately made public, so any astronomer could grab them and take a look. The team I was a part of would do this automatically when STIS parallels came in, and would archive them. As a dork and amateur astronomer my whole life I was fascinated by this idea, and wrote software that would allow me to peruse the parallels easily on my monitor. I would come in to work every morning and check them first thing, just to see what could be seen.

Most of the time they were mostly blank, showing just a handful of foreground stars and the occasional fuzzy blob of a very distant galaxy. But sometimes we struck gold. When someone used one of the new cameras to look at a nearby galaxy, we’d be seeing inside it too, because the galaxy is big and the cameras all pointed near the same spot in the sky.

Whenever we observed the Andromeda Galaxy, or the Magellanic Clouds (there’s a Small one, too), or Triangulum, or Centaurus A, we’d get an image just loaded with stars, crowding each other and filling the images edge to edge.

To someone outside the field it would look fairly unimpressive, just a black field with a few thousand slightly fuzzy dots spread across it, like someone had spilled beads on black velvet.

But to me, these were magic. I could casually sit in my office chair, sipping coffee, and look at individual stars in galaxies millions of light years away. Astronomers from a century ago would have cheerfully sawed off one of their arms to have this privilege.

It got to the point where I could ID the galaxies by the stars in them; the Magellanic Clouds are close by in relative terms, under 200,000 light years, so the stars were bright. The other galaxies’ stars had subtle differences in brightness and distribution that I learned to recognize with unerring accuracy. It was almost a game for me to see which was which.

After a few years though, once the camera was safely on board and working well, it came time to move on. I left doing research and moved on to astronomy education, and eventually writing for a living.

I do sometimes miss looking at parallels, seeing these amazing and grand structures in space that once so baffled the best scientists on our planet, creating consternation and debates and arguments until we developed the technology to solve these issues.

Seeing NGC 55 in such detail in a press release that appeared in my email brought all that back to me, and for a few minutes I was able to revel in my own personal memories of galactic astronomical discovery.

So, to the thousands of people who made Hubble Space Telescope a reality, and the thousands of astronomers who came before so that astronomers today might peer a little further and with more clarity: Thank you.