What’s behind every square degree in outer space?

The canopy of the night sky is both immense and awe-inspiring.

Although extended objects, like the plane of the Milky Way and a few distant galaxies beyond our own, are identifiable with the naked eye, there are only a few thousand stars that can be seen and resolved with the naked eye. Depending on your eyesight and the darkness conditions, most humans can see between 6000 and 9000 stars if you could see the entire sky at once.

All told, cumulatively, the full sky contains 41,253 square degrees.

In this photo of the night sky over the Very Large Telescope at Paranal, an arm and hand is shown for angular scale. The circled pinkie nail takes up about one square degree on the sky, while the angular separation between the pinkie and index finger is about 12 degrees: the amount that the Moon appears to shift in the sky from night-to-night.

If you hold your hand at arm’s length, your pinkie finger’s nail covers about 1 square degree.

Artist’s logarithmic scale conception of the observable universe. The Solar System gives way to the Milky Way, which gives way to nearby galaxies which then give way to the large-scale structure and the hot, dense plasma of the Big Bang at the outskirts. Each line-of-sight that we can observe contains all of these epochs, but the quest for the most distant observed object will not be complete until we’ve mapped out the entire Universe.

Behind every single square degree, there’s a portion of the Universe that unveils its entire story.

Gaia’s all-sky view of our Milky Way Galaxy and neighboring galaxies. The maps show the total brightness and color of stars (top), the total density of stars (middle), and the interstellar dust that fills the galaxy (bottom). Note how, on average, there are approximately ~10 million stars in each square degree, but that some regions, like the galactic plane or the galactic center, have stellar densities well above the overall average.

Nearby, we first intercept the stars of the Milky Way: an average of ~10 million in each square degree.

Although some regions of space are rich in nearby galaxies while others are relatively poor, each proverbial slice of the sky allows us to grab objects of all different distances so long as our observations are sensitive enough to reveal them. The nearest, brightest objects are the easiest to resolve, but the entire cosmic story is told across the entire sky, and must be observed deeply and across many wavelengths in order to truly reveal the full extent of what’s out there.

Beyond our home galaxy, there are many others extending back across time and space.

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, but when we extrapolate over the entire observable Universe, we expect to obtain a total of ~2 trillion galaxies.

Our deepest view of the Universe, the Hubble eXtreme Deep field, covers just 1/32,000,000th of the sky.

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. It’s important to remember that the light we see is only the light arriving right now, after journeying through the vast expanse of space.

It revealed 5500 galaxies, distributed throughout our Universe’s cosmic history.

Galaxies comparable to the present-day Milky Way are numerous, but younger galaxies that are Milky Way-like are inherently smaller, bluer, and richer in gas in general than the galaxies we see today. Fewer galaxies have disks and spiral shapes as we look farther back in time. Over time, many smaller galaxies become gravitationally bound together, resulting in mergers but also in groups and clusters containing large numbers of galaxies overall.

We can see how galaxies, stars, and the elements inside grow and evolve with time.

The full Moon takes up approximately 0.2 square degrees on the sky, meaning that approximately five of them are required to fill one square degree of space. The Hubble eXtreme Deep Field, however, is much smaller, and it would take approximately 776 of them to cover one square degree on the sky. That tiny region of sky had 5500 galaxies within it, despite covering just 1-32,000,000th of the sky.

It takes 776 such deep fields, stitched together, to fill up just one square degree.

The COSMOS-Web survey (renamed from COSMOS-Webb, as it will survey a portion of the cosmic web) 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). It will doubtlessly reveal many faint and distant galaxies that were unobservable to Hubble, and should help enlighten us as to how the Universe grew up. Spectroscopic follow-up on many of the most distant galaxy candidates within this field should help us understand whether the early JWST results truly indicate an anomaly in how quickly galaxies grow up in the Universe.

The Universe contains approximately 50 million galaxies in each square degree.

This animation showcases a portion of the Hubble eXtreme Deep Field, with 23 days of cumulative data, and a simulated view of what scientists expected JWST might see when it viewed this region. This simulation predates JWST’s launch, and has since been spectacularly superseded by actual JWST data.

Fainter, redder, and more distant galaxies are certain to be revealed by future observatories.

Along every line-of-sight, there are distant background objects like galaxies and quasars whose light inevitably passes through intervening gas clouds. When this occurs, the elemental contents, the densities and ratios of various elements, and the temperature of the matter inside can all be inferred. Younger, less processed gas clouds reveal how the Universe’s composition has evolved with time.

Earlier views reveal a hotter, more pristine and uniform Universe.

At any epoch in our cosmic history, any observer will experience a uniform “bath” of omnidirectional radiation that originated back at the Big Bang. Note that the CMB isn’t just a surface that comes from one point, but rather is a bath of radiation that exists everywhere at once. As each new year passes, the CMB cools down further by about 0.2 nanokelvin, and in several billion years, will become so redshifted that it will possess radio, rather than microwave, frequencies.

Each successive “pencil beam” helps us understand how our Universe evolved and grew up over cosmic time.

This view of about 0.15 square degrees of space reveals many regions with large numbers of galaxies clustered together across cosmic time in clumps and filaments, with large gaps, or voids, separating them. This region of space is known as the ECDFS, as it images the same portion of the sky imaged previously by the Extended Chandra Deep Field South: a pioneering X-ray view of the same space.

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