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Starts With A Bang

This Is The Single Most Important Image In Astronomy’s History

One peek into a small part of the sky, one giant leap back in time. This small patch of sky represents less than 1/100,000,000th of the volume of the Universe, but reveals nearly 1,000 galaxies that had never been seen before. This small fraction of the original Hubble Deep Field image is a huge part of how we learned what our Universe looks like. (R. WILLIAMS (STSCI), THE HUBBLE DEEP FIELD TEAM AND NASA/ESA)

But the upcoming James Webb Space Telescope compels us to add, “so far.”


Beginning with its 1990 launch, NASA’s Hubble Space Telescope revolutionized our conception of the Universe.

This photo of the Hubble Space telescope being deployed, on April 25. 1990, was taken by the IMAX Cargo Bay Camera (ICBC) mounted aboard the space shuttle Discovery. It has been operational for 30 years, and has not been serviced since 2009. With a 2.4-meter diameter mirror, it gathers as much light in 1 minute as a 160-mm (6.3″) telescope would require 3 hours and 45 minutes to gather. (NASA/SMITHSONIAN INSTITUTION/LOCKHEED CORPORATION)

Its first servicing mission, in 1993, served two incredible purposes.

Astronaut Jeffrey Hoffman removes Wide Field and Planetary Camera 1 (WFPC 1) during change-out operations during the first Hubble servicing mission. This 1993 servicing mission was crucial on a number of fronts, but the replacement of WFPC 1 with the new and improved WFPC2 would make a tremendous difference from 1993–2009. (NASA)

One was to fix Hubble’s flawed primary mirror: an unimpeachable success.

The before-and-after difference between Hubble’s original view (left) with the mirror flaws, and the corrected images (right) after the proper optics were applied. The first servicing mission, in 1993, brought the true power of Hubble to the forefront of astronomy, where it’s remained ever since. (NASA / STSCI)

The second purpose was a phenomenal instrument upgrade, including the WFPC2 camera.

The Wide Field and Planetary Camera 2 (WFPC2) was Hubble’s workhorse camera for many years. It recorded images through a selection of 48 colour filters covering a spectral range from far-ultraviolet to visible and near-infrared wavelengths. The ‘heart’ of WFPC2 consisted of an L-shaped trio of wide-field sensors and a smaller, high resolution (Planetary) Camera placed at the square’s remaining corner. (NASA)

Controversially, a high-risk proposal was selected with discretionary time: the Hubble Deep Field.

Looking back from the present day, we can see a ‘pencil beam’ view of the distant Universe. But a huge number of galaxies are still undiscovered, owing to the limitations of how we’re capable of looking. Hubble has taken us remarkably far, but there’s still farther to go. (NASA, ESA, AND A. FEILD (STSCI))

The plan was to repeatedly image the same “blank” area of sky.

The blank region of sky, shown in the yellow L-shaped box, was the region chosen to be the observing location of the original Hubble Deep Field image. With no known stars or galaxies within it, in a region devoid of gas, dust, or known matter of any type, this was the ideal location to stare into the abyss of the empty Universe. (NASA / DIGITAL SKY SURVEY, STSCI)

If nothing novel appeared, it would be the biggest waste of premier telescope time in history.

The original Hubble Deep Field image, for the first time, revealed some of the faintest, most distant galaxies ever seen. Only with a multiwavelength, long-exposure view of the ultra-distant Universe could we hope to reveal these never-before-seen objects. (R. WILLIAMS (STSCI), THE HUBBLE DEEP FIELD TEAM AND NASA)

Instead, it revealed a glimpse of the Universe unlike any other.

A small portion of the original Hubble Deep Field, showcasing hundreds of previously unseen galaxies. Every separate point of light in the image is its own galaxy, with billions of stars occupying each one, and with them coming in a variety of shapes, ages, and each with a different star-formation history. (R. WILLIAMS (STSCI), THE HUBBLE DEEP FIELD TEAM AND NASA)

Across cosmic time and at distances never seen before, galaxies were everywhere.

The streaks and arcs present in Abell 370, a distant galaxy cluster some 5–6 billion light-years away, are some of the strongest evidence for gravitational lensing and dark matter that we have. The lensed galaxies are even more distant, with some of them making up the most distant galaxies ever seen. The Frontier Fields program searches for lensed galaxies by deeply imaging galaxy clusters. (NASA, ESA/HUBBLE, HST FRONTIER FIELDS)

Revealing the unknown Universe through long-exposure, deep imaging subsequently became routine.

This sea of galaxies is the complete, original COSMOS field from the Hubble Space Telescope’s Advanced Camera for Surveys (ACS). The full mosaic is a composite of 575 separate ACS images, where each ACS image is about one-tenth the diameter of the full Moon. The jagged edges of the outline are due to the separate images that make up the survey field. (ANTON KOEKEMOER (STSCI) AND NICK SCOVILLE (CALTECH))

The eXtreme Deep Field, with 23 cumulative days of data, provides today’s deepest views.

The Hubble eXtreme Deep Field (XDF) may have observed a region of sky just 1/32,000,000th of the total, but was able to uncover a whopping 5,500 galaxies within it: an estimated 10% of the total number of galaxies actually contained in this pencil-beam-style slice. The remaining 90% of galaxies are either too faint or too red or too obscured for Hubble to reveal. (HUDF09 AND HXDF12 TEAMS / E. SIEGEL (PROCESSING))

Overall, approximately ~2 trillion galaxies should be contained within our observable Universe.

This tiny slice of the eXtreme Deep Field illustrates an important concept: if we count the number of galaxies in this image and extrapolate how many such similar images we’d need to cover the entire sky, we can get an estimate for how many galaxies would be revealed over the entire sky to Hubble’s eyes. That number, of about 170 billion, is too small by approximately a factor of 10. The actual number of ~2 trillion galaxies is significantly larger. (NASA, ESA, H. TEPLITZ AND M. RAFELSKI (IPAC/CALTECH), A. KOEKEMOER (STSCI), R. WINDHORST (ARIZONA STATE UNIVERSITY), AND Z. LEVAY (STSCI))

But Hubble, even at today’s limits, can only reveal about 10% of them.

Galaxies identified in the eXtreme Deep Field image can be broken up into nearby, distant, and ultra-distant components, with Hubble only revealing the galaxies it’s capable of seeing in its wavelength ranges and at its optical limits. The dropoff in the number of galaxies seen at very great distances may indicate the limitations of our observatories, rather than the non-existence of faint, small, low-brightness galaxies at great distances. (NASA, ESA, AND Z. LEVAY, F. SUMMERS (STSCI))

With James Webb scheduled to launch on December 18, 2021, that should change again.

The James Webb Space Telescope vs. Hubble in size (main) and vs. an array of other telescopes (inset) in terms of wavelength and sensitivity. Its power is truly unprecedented, and it ought to reveal, particularly with deep imaging, faint and distant galaxies that are well beyond the current limits of Hubble. (NASA / JWST)

Webb will observe its first “deep field” in 2022.

This simulated image represents what the James Webb Space Telescope ought to see, as compared with the previous (earlier, actual) Hubble image. With the COSMOS-Webb field expected to come in at 0.6 square degrees, it should reveal approximately 500,000 galaxies in the near-infrared, uncovering details that no observatory to date has been able to see. (JADES COLLABORATION FOR THE NIRCAM SIMULATION)

Viewing faint, distant galaxies beyond Hubble’s limits, new revolutions certainly lie ahead.

The COSMOS-Webb survey will map 0.6 square degrees of the sky — about the area of three full Moons — using the James Webb Space Telescope’s Near Infrared Camera (NIRCam) instrument, while simultaneously mapping a smaller 0.2 square degrees with the Mid Infrared Instrument (MIRI). (JEYHAN KARTALTEPE (RIT); CAITLIN CASEY (UT AUSTIN); AND ANTON KOEKEMOER (STSCI) GRAPHIC DESIGN CREDIT: ALYSSA PAGAN (STSCI))

Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words. Talk less; smile more.

Starts With A Bang is written by Ethan Siegel, Ph.D., author of Beyond The Galaxy, and Treknology: The Science of Star Trek from Tricorders to Warp Drive.


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