At 33 years in orbit, one might wonder if it’s still possible to be surprised by images from the Hubble Space Telescope. After all this time, hasn’t the orbiting observatory seen just about everything? Well, there’s certainly a lot of sky out there! Plus, Hubble actually only sees a very small portion of it at any given moment. In fact, even after decades of observations, it turns out that Hubble has only observed just about 1 percent of the entire night sky. (For context, see Marissa Wuw’s article: “See nearly all of Hubble’s observations in one amazing composite photo.”) Given that context, it’s no wonder then that we continue to be amazed by what Hubble beams back to Earth—and this year’s anniversary image of the reflection nebula NGC 1333 is no exception. In today’s post, we’ll take a closer look at NGC 1333, including why we selected this target, the challenges we had observing it, and the image processing techniques that bring this scene to life.
What Does a Reflection Nebula Reflect?
NGC 1333 is known as a reflection nebula and is actually confined to the top portion of the Hubble image, where the bright star is embedded in blue clouds. This is a favorite target of amateur astrophotographers because of the large size of the object and the color contrasts within the nebula. With that understanding, and its apt naming, we decided to take a look with Hubble for the observatory’s 33rd birthday.
As the name implies, a reflection nebula is a collection of gas and dust situated near a bright star or stars that reflect some of that bright starlight. In the blue portion at the top, those clouds are reflecting starlight similar to how wispy clouds surrounding a bright full moon might reflect and redirect some of the moon’s light. This process is very different from an emission nebula, where gas is ionized by intense radiation from nearby stars and begins to glow of its own accord as a result. This process more accurately describes the glowing red clouds at the bottom of the image in the region known as HH 12. (Side note: the “HH” here stands for Herbig-Haro object, named after George Herbig and Guillermo Haro who were the first astronomers to study these objects in detail.) The Hubble image only tells part of the story though. NGC 1333 is actually located within a much larger dust complex where new stars are in the process of forming.
Some of those newly forming stars emit jets that impact their environment in different ways, creating shock waves and shock-heated emission, and ionizing gas in the path of the jet. It is the combined view of all of these interactions that defines the look of this nebula and is what we were most eager to capture with Hubble. This image is also a prime example of how observing the universe in multiple wavelengths of light opens new windows in the physical processes that shape the appearance of these objects. Compare the wide-field image from Figure 1 above with this view of the same region taken with the Spitzer Space Telescope, which viewed the universe in infrared wavelengths before it was decommissioned in early 2020. In infrared, the light from those deeply embedded stars can penetrate the thick veil of dust, allowing us a glimpse into their formation processes, providing much needed context about the dynamics of this region. (Another side note: It’s likely that the James Webb Space Telescope will eventually take a look at this region. For now, we can only imagine the depth and detail that Webb will reveal!)
Observations and Challenges
Planning to observe this object with Hubble came with some complications. NGC 1333 is part of a fairly large complex in the sky, spanning a large area nearly a degree across. (The full moon is about half a degree on the sky.) Hubble needed to build a mosaic of the region, observing in multiple positions. We had to stitch those images together.
Part of what makes this region so aesthetically alluring is the delicate layer of filamentary dust and gas that obscures most of the stars in the area at optical wavelengths. It’s exactly this dust that makes it difficult to observe, since Hubble’s fine guidance sensors need a selection of guide stars to use as an anchor in the sky so Hubble can hold its mirror and cameras steady on one spot continuously while it observes. In an area where there are very few guide stars, the likelihood that Hubble will not be able to lock onto that area of the sky increases. This is no fault of the observatory, some areas are just difficult to observe. This happened with 3 out of 33 of the observations in this mosaic which required revisiting the affected tiles. When Hubble can’t find a guide star, the observatory does its best to hold onto the target, but with no anchors, it inevitably drifts while observing, causing stars to streak across the image. The data are not wasted. They’re available in the archive for further study but would not make for a good image.
With the help of mission schedulers, we were eventually able to tweak the observing parameters to make it work. Thankfully, our team planned ahead for this scenario and built some extra time into the schedule to allow for repeat observations.
These mosaic observations were designed to maximize the field of view by minimizing the overlap between tiles. This technique allows for a larger field of view in the same number of observations, but requires us to align multiple frames, which becomes more difficult and requires star alignment with an external star catalog such as the GAIA catalog. The lack of stars in this particular region while hauntingly beautiful also creates a situation where it is very difficult to find stars to match to a catalog to precisely align the frames. Getting those frames aligned required a lot of trial and error, but our colleagues Jennifer Mack and Varun Bajaj from Hubble’s Wide Field Camera 3 (WFC3) instrument team did a herculean job of aligning and combining multiple observations into a cohesive field of view for each filter that could be composited to form the color image you see at the top of this post.
Slicing up the Light
One consideration of every Hubble observation is deciding which filters to use. For NGC 1333, we wanted to include a selection of wide band filters to roughly cover the wavelengths of human vision to create a true color image in red, green, and blue. Additionally, we observed with a narrow band filter to isolate the light of hydrogen and nitrogen. The narrow band data were also given special treatment by using a technique known as continuum subtraction. This technique is used to hone in on precisely the light of interest here – ionized hydrogen and nitrogen. These are the elements that glow brightly in Herbig-Haro objects as a result of shock heating. The narrow band image was assigned a reddish orange hue and when blended with the broad band images, it really highlights the glowing red region at the bottom of the image.
Head in the Clouds
Once the alignment issues were handled, we began the work of assigning colors by wavelength, cleaning up image artifacts, and preparing the final color composite image. As noted above, we observed NGC 1333 with a selection of broad band color filters to create a true-color red, green, blue image along with the narrow band image isolating hydrogen and nitrogen. As is typically the case with astronomical images, colors are assigned according to wavelength with the shortest wavelengths set to the blue color channel, the medium wavelengths to green, and the longest wavelengths to red. This relatively simple approach to color yields a surprisingly rich color image with subtle shades of blues gently fading to oranges and reds as the density of dark clouds increases toward the bottom of the image. The image takes on an almost three-dimensional quality the longer you look at it when you start to see faint wisps illuminated by nearby stars along with stars more deeply embedded in dust. The addition of the narrow band image has little effect on the overall color of the image, but a dramatic effect on highlighting the HH objects scattered throughout. I highly recommend downloading the full-resolution image and getting lost in its details!
Speaking of details, processing an image like this one from Hubble does require quite an attention to detail. As mentioned above, the data suffered from some of the usual artifacts like cosmic ray noise, ghosts, glints, dragon’s breath, and charge bleed, all of which had to be cleaned up as described in our image artifacts post.
Color calibration was also an important consideration. The data were all scaled to each other so that the overall flux from each filter was roughly equal. From there, color calibration was handled by measuring the colors of the few stars in the field and making sure they match up with the expected colors of those stars from stellar catalogs. Since there is no real neutral sky background to speak of in this particular image, the black level reflects the fact that the darkest regions of the image are actually dense clouds and not sky background, thus taking on a dark-brownish appearance.
Thirty-Three Years and Counting!
Celebrating Hubble’s anniversary is something we all look forward to every year! The process of selecting a target and devising an observing strategy can be fraught with complications, as can actually executing the observations, but with the proper planning and perseverance, we always come away with another iconic Hubble image that demonstrates the strength and staying power of this unique and wonderful observatory. I very much look forward to seeing what the future brings for Hubble, and how Hubble data can be combined with data from Webb to expand our understanding of the cosmos. Looking even further into the future, we have the Nancy Grace Roman Space Telescope to look forward to, which will provide us with a field of view equivalent to 100 Hubble observations in a single observation! Roman would certainly make quick work of NGC 1333 and this ability to cover such large areas of the sky will be a huge asset to astrophysics in the years to come. The future of astronomy is bright!