NGC 1333: Hubble's 33rd Anniversary Image of a Dusty Star Nursery

After 33 years in orbit, Hubble had made roughly 1.6 million observations of nearly 52,000 targets, and still the telescope had looked at only about 1 percent of the sky. Its anniversary image for 2023 was a part of that unexplored 99 percent: NGC 1333, a star-forming region about 960 light-years away in the Perseus molecular cloud, released on April 20, 2023. The picture is a tall strip of dust and light, blue at the top, nearly black at the bottom, and it was one of the harder anniversary targets to observe.

A reflection nebula above a glowing keyhole

Strictly speaking, NGC 1333 is the blue part at the top, where a bright star sits in clouds of dust. It is a reflection nebula: the fine dust, essentially soot, scatters the star's light, much as thin clouds around a full Moon catch and redirect its glow. The dust scatters blue light most strongly, which sets the color. The object is large on the sky and full of color contrast, which has made it a favorite of amateur astrophotographers.

The red glow at the bottom works differently. That region, known as HH 12, is gas that shines by itself: ionized hydrogen lit up where narrow jets from newly forming stars outside the frame slam into the cloud. HH stands for Herbig-Haro, after George Herbig and Guillermo Haro, the first astronomers to study such objects in detail. The young stars that launch the jets are surrounded by disks that may someday form planets, and magnetic fields channel their outflows into twin beams.

Between the two, a super-hot star shines through filaments of dust like the Sun through broken cloud, and a diagonal string of fainter stars looks reddish because the dust lets more of their red light through. Several hundred stars are forming inside the dark cloud, most of them hidden. Infrared telescopes see much deeper here; the Spitzer Space Telescope, retired in early 2020, mapped the embedded stars of the wider complex, and the James Webb Space Telescope was expected to look at the region eventually. Peering Through the Dust explains why infrared sees through. A cloud like this one is also a model for the birthplace of the Sun and planets 4.6 billion years ago.

A mosaic and missing guide stars

The wider complex spans nearly a degree of sky, about twice the width of the full Moon, so Hubble covered its chosen field with a mosaic of 2 by 3 tiles. With four filters, that came to 24 separate images in the final composite.

The dust that makes the region beautiful also makes it hard to observe. To hold steady during an exposure, Hubble's Fine Guidance Sensors lock onto guide stars, and dust hides most of the stars here. Without a lock the telescope does its best but slowly drifts, and the stars in the frame smear into streaks. That happened in 3 of 33 observations in this program, and the affected tiles had to be repeated. Extra time had been built into the schedule for exactly that, and with help from the mission schedulers, adjusted observing parameters eventually made it work. The drifted frames are not wasted; they remain in the archive for research.

The mosaic was designed with minimal overlap between tiles to cover the most sky with the fewest observations. The cost is alignment: the frames have to be tied to an external catalog such as Gaia, and this field has very few stars to match. Combining the tiles into one clean image per filter took a long process of trial and error by specialists on Hubble's Wide Field Camera 3 instrument team.

Filters and the hydrogen layer

The filters were chosen to roughly cover the range of human vision in blue, green and red, with broadband filters near 475, 606 and 814 nanometers, plus a narrowband F657N filter for hydrogen and nitrogen. Those two elements glow brightly in Herbig-Haro objects heated by shocks.

The narrowband data got special treatment with continuum subtraction. Removing the broad starlight and dust glow underneath leaves only the light of ionized hydrogen and nitrogen, so HH 12 stands out far more clearly than in the raw frame. Tinted a reddish orange and blended with the broadband images, the layer changes the overall color very little but makes the Herbig-Haro objects pop wherever they appear.

Cleaning and color calibration

The data had the usual collection of artifacts: cosmic rays, ghosts, glints, a dragon's breath glow and charge bleed. Around the bright star at the top, a bleed ran toward the left, a tight donut sat just to its upper right, and a dragon's breath streak extended to its right; elsewhere, nearby bright stars left glints and donuts. All were cleaned with the methods in Are Hubble Images Real? Image Artifacts Explained.

Color calibration needed care. The data from each filter were scaled so that their total brightness was roughly equal, then balanced against the handful of stars visible in the frame, whose measured colors were matched to the colors expected from stellar catalogs. With no true empty sky in the frame, the darkest parts are dense dust, not space, so the black level was left a dark brown. Colors follow the usual wavelength order, from blue for the shortest filter to red for the longest; Are the Colors Real? covers that convention.

Questions readers ask

What is NGC 1333?

A reflection nebula and star-forming region in the Perseus molecular cloud, about 960 light-years away. Several hundred stars are forming in its dark cloud. Hubble photographed it for its 33rd anniversary, and the image was released on April 20, 2023.

What is a Herbig-Haro object?

A patch of glowing gas created where a jet from a newly forming star strikes the surrounding cloud and heats it. The name honors George Herbig and Guillermo Haro. In the NGC 1333 image, the red region HH 12 at the bottom is one.

Why did some NGC 1333 observations fail?

Hubble needs guide stars to stay locked on a target, and the thick dust in this region hides most stars. In 3 of 33 observations the telescope could not lock on, drifted, and recorded streaked stars. Those tiles were observed again.

How much of the sky has Hubble observed?

Only about 1 percent, even after decades of observing, because each Hubble frame covers a tiny area. Survey telescopes with wider fields, such as the planned Nancy Grace Roman Space Telescope, with a field equal to about 100 Hubble observations, will cover far more sky at once.