This Hubble image captures globular star cluster (NGC 6541) that is roughly 22,000 light-years from Earth.  A globular cluster is a spherical collection of stars that orbits a galactic core. They are very tightly bound by gravity, which gives them their spherical shapes,and relatively high density of stars toward their centers.  The cluster is bright enough that backyard stargazers in the Southern Hemisphere can spot it with binoculars, though certainly not in this detail. (NASA, ESA, and G. Piotto (Università degli Studi di Padova)

For almost 30 years now, the Hubble Space Telescope has transformed how we see the cosmos.  In terms of scientific output as well as making visible the splendors of the sky above us, the Hubble has been arguably the most consequential telescope ever to peer into space.

To commemorate 30 years of Hubble science and images, NASA and the European Space Agency have released 30 previously unpublished images of galaxies, star clusters and nebula from what is known as the Caldwell catalogue,  a collection compiled by British amateur astronomer and science communicator Sir Patrick Caldwell-Moore.

These images have been taken by Hubble throughout its time in space and used for scientific research or for engineering tests, but NASA had not fully processed the images for public release until now.

At the end of a difficult year, they offer the glitter, the grandeur and the cosmic marvel  that the Hubble provides so well and that perhaps people could use right now.

This Hubble image captures a small region on the edge of the inky Coalsack Nebula.  A nebula is an enormous cloud of dust and gas occupying the space between stars and acting as a nursery for new stars.  Coalsack is a “dark nebula” which completely blocks out visible wavelengths of light from objects behind it. The image was made  using Hubble’s Advanced Camera for Surveys in both visible and infrared wavelengths.  (NASA, ESA, and R. Sahai of NASA’s Jet Propulsion Laboratory)

The Hubble famously entered into Earth orbit and began its mission with the calamitous discovery of a near-fatal mistake — the main mirror had been ground incorrectly and could not accomplish much viewing.  The telescope was about 340 miles from Earth and never before had NASA undertaken a mission to repair a spacecraft that far away.

But in 1993 seven astronauts flew to the Hubble on the space shuttle Endeavour, spent five days repairing it and the rest is history.  In all, astronauts visited the observatory five times to fix it, maintain it, and improve it.  The Hubble is now considerably more capable than it was in its early day, and is expected to last for another decade or two.

Today, Hubble has two primary cameras to capture images of the cosmos. Called the Advanced Camera for Surveys and the Wide Field Camera 3, they work together to provide wide-field imaging over a broad range of wavelengths.

The telescope’s visible-light observations allow for the viewing of  cosmic objects in the wavelengths of light we see with our own eyes, but in a much greater level of detail.

Infrared observations extend our vision, detecting lower-energy light than our eyes can see and peering through shrouds of dust to image some of the faintest and farthest objects yet discovered. Hubble’s ultraviolet vision extends our view in the opposite direction, opening a window on the evolving universe and allowing us to glimpse some of the more violent events in the cosmos.

NGC 5248 is a spiral galaxy located in the constellation Boötes, and it is notable for the ring structure around its center. These nuclear rings are characterized by “hot spots” of starburst activity. Starburst regions are sites where stars form at a much higher rate than usual. At a distance of 59 million light-years, the starburst regions in Caldwell 45 are actually some of the nearest to Earth and are less visually obstructed than many others .( NASA, ESA, J. Lee of the California Institute of Technology), and A. Filippenko of University of California – Berkeley)

The original Caldwell catalogue is a collection  of 109 star clusters, nebulae, and galaxies that can be observed by amateur astronomers.

The list was compiled by Moore to complement the much older Messier catalogue, assembled by French comet-hunter Charles Messier in the 1700s and 1800s and including 110 relatively bright but fuzzy objects. Those smudgy spots in the sky identified by Messier have since been identified as distant galaxies, star clusters, and nebulae.  The Messier catalogue has been revised over the years and includes deep-sky objects that can be viewed in detail using larger telescopes but are also bright enough to be seen through a small telescope.

While the Messier catalogue is used widely by amateur astronomers, Moore knew that Messier’s list was not compiled for that purpose and excluded many of the sky’s brightest deep-sky objects. The  Messier catalogue was actually compiled as a list of known objects that might be confused with comets.

The Caldwell catalogue highlights galaxies, star clusters, and nebulae that are not included in Messier’s catalogue but that are also bright enough to be seen by amateur astronomers. He intentionally avoided including any of the Messier objects in his catalogue, hoping to expand his fellow amateur astronomers’ cosmic horizons.

In addition, the Caldwell objects are split between the northern and southern hemisphere skies, while Messier’s only covered the northern skies.

Moore (1923-2012) used his other surname – Caldwell – to identify xxx of the list, since the “M” of  “Moore” was already used in the Messier catalogue.

Galaxies come in a variety of different structures and the nature of these structures drive the evolution of that galaxy. One such structure is the galactic bulge in spiral galaxies, such as Caldwell 40 (or NGC 3626.) This bulge is a densely packed region of stars that make up the heart of a spiral galaxy. Most galactic bulges host supermassive black holes, with the masses of the black hole and the bulge typically linked — bigger bulges harbor more monstrous black holes. (NASA, ESA, and P. Erwin (Max Planck Institute for Extraterrestrial Physics)

Moore assembled his catalogue pre-Hubble during the 1980s, with 46 star clusters, 35 galaxies, and 28 nebulae — 109 objects in all.   From nearby clouds of gas and dust that are left over from dying stars to remote galaxies that formed billions of years ago, the Caldwell catalogue is a celestial smorgasbord.

While the Hubble Space Telescope has not taken images of every object in the Caldwell catalogue, it has observed 98 of them as of August 2020. Processed images for 87 Caldwell objects are displayed here, including the 30 new ones. More images may be added to Hubble’s catalog of Caldwell objects in the future.


Hubble has allowed astronomers to view galaxies of all shapes and sizes from nearly every angle. When a galaxy is seen edge-on, the perspective reveals a surprising slice of the universe. Caldwell 43, also known as the “Little Sombrero,” is one such galaxy. Caldwell 43 features a bright central bulge, a thin disk full of dust, and a glowing halo of gas and stars that sprawls out into space. Also known as NGC 7814, the roughly 80,000-light-year-wide galaxy is billions of years old. (ESA/Hubble & NASA, with acknowledgment to Josh Barrington)

Because of the Hubble’s detailed field of view, some of its pictures do not capture the entirety of a Caldwell object, sometimes instead zooming in on clusters of young stars in the arms of a spiral galaxy, stars on the outskirts of a cluster, or the zombie star at the heart of a nebula. But in other cases, a mosaic of Hubble observations are assembled to create a complete or nearly complete portrait of the celestial marvel.

Caldwell 83, or NGC 4945, is a barred spiral galaxy (like our Milky Way) that appears edge-on using a ground-based telescope (right.) The composite image of the galaxy’s center (left) comes combines Hubble observations taken in visible and infrared light.
Caldwell 83 interests astronomers because it is the source of the first water megamaser ever found. Masers produce amplified microwave radiation similar to lasers, which amplify visible light. This emission has been observed from a variety of molecular sources throughout the universe, including water molecules, and megamasers can be up to 100 million times brighter.
(Ground-based image: European Southern Observatory; Hubble image: NASA, ESA, and H. Falcke of the Max Planck Institute for Radio Astronomy)

The innumerable objects of the cosmos as presented by Hubble in visible light (and then color coded) are especially evocative and compelling, capturing the shapes and colors of nebulae and galaxies. But  there are downsides to seeing in visible light alone which are illustrated here:  it makes a glorious image of a nebula but cannot see through the thick clouds of dust.

The James Webb Space Telescope, scheduled to launch next year, will have a larger mirror than Hubble and will have capabilities Hubble does not.  It sees only in infrared wavelengths, which allow for much deeper and vision that can see through nebula.  Because of that (and more), it will be  scientific advance on Hubble. But it will not provides images quite as dramatic as those  captured by the Hubble’s visible light capacity.

The Hubble Space Telescope produced these visible-light (left) and near-infrared composite images of these fingers of gas in the Eagle Nebula, also known as the Pillars of Creation. The visible image is color-coded for chemical composition: blue is doubly ionized oxygen, green is ionized hydrogen, and reddish orange is ionized sulfur. The infrared image cuts through much of the gas and reveals stars inside.
(NASA / ESA / Hubble Heritage Team of STScI / AURA)

Like many avid stargazers, Sir Patrick Moore developed an interest in astronomy at a very young age, which blossomed into a lifelong obsession. Educated at home because of a heart condition, he discovered the world of astronomy at six years old when he read a late 19th century tome on the solar system.

Sir Patrick Moore was an English amateur astronomer, writer, television host and creator of the Caldwell catalogue.
(South Downs Planetarium)

At 11 he became the youngest-ever member of the British Astronomical Association. He published his first paper, on small craters in the Mare Crisium on the moon, at the age of 13.  At 14 he took over the running of a small observatory near his house.

He became a prolific amateur astronomer and writer, gaining prominence as the presenter of the BBC documentary series The Sky at Night for over 40 years and earning knighthood in 2001 for “services to the popularization of science and to broadcasting.”

The Caldwell Catalogue was first published in Sky & Telescope in 1995.

The original Caldwell catalogue, with images of galaxies, nebula and star clusters that amateur astronomers have used for decades to guide their nighttime viewing. (Wikipedia Commons).

While revolutionizing our understanding and appreciation of the structure and beauty of the cosmos, the Hubble has also been a scientific bonanza.

It has:

  • Played a significant role in determining that the universe is 13.7 billion years old.
  • Provided the information that has led scientists to conclude that nearly all galaxies harbor supermassive black holes.
  • Has helped scientists determine the process of how planets are born.
  • Detected the first organic molecule discovered on a planet outside our solar system.
  • Identified a distant supernova that suggests the universe only recently began speeding up.
  • Found four moons around dwarf planet Pluto, geysers on the moon Europa and so much more.

The image below of an “Einstein Ring” is a very recent example of the kind of science that the Hubble makes possible.  The galaxy cluster, named GAL-CLUS-022058s or “Molten Ring,” is not in the Caldwell catalogue.  But it was too wonderful to leave out.

The narrow galaxy curving around its spherical companion in this image is a example of a strange and rare phenomenon, one first theorized to occur by Albert Einstein in his General Theory of Relativity.  Now called an Einstein ring,  it is the result of gravitational lensing, which causes light shining from far away to be bent and pulled by the gravity of an object between its source and the observer. In this case, the light from the background galaxy has been distorted into the curve we see by the gravity of the galaxy cluster sitting in front of it, This Hubble image of captures the largest and one of the most complete Einstein rings ever discovered. (ESA/Hubble & NASA, S. Jha with acknowledgment to L. Shatz.)