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Starts With A Bang

What’s behind every square degree in outer space?

Even a tiny sliver of the Universe can reveal the cosmic story of what's out there and how it came to be the way it is today.
every square degree
Every time we look out at the night sky, we're seeing a specific field-of-view, which means we're probing a certain amount of area on the sky. Objects that are nearby, at intermediate distances, and even at very great distances can all appear in the same image, and just how faint and far back we see is limited only by the quality of our observations.
(Credit: NASA/ESA/A. Feild (STScI))
Key Takeaways
  • A square degree isn't much real estate in space, corresponding to the size of your pinkie finger's nail when held at arm's length.
  • The Sun and the full Moon are about half a degree each in diameter, and it would take more than 40,000 square degrees to cover the entire sky.
  • Nevertheless, there are millions of Milky Way stars and even greater numbers of galaxies contained in every "pencil beam" of space, and each one teaches us how the Universe grew up.

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

The Milky Way, as seen at La Silla observatory, is a stunning, awe-inspiring sight to anyone, and offers a spectacular view of a great many stars in our galaxy. Although there are definitely regions, like toward our galactic center, that are denser in stars than others, the average “square degree” on the sky contains ~10 million stars from the Milky Way.
(Credit: ESO/Håkon Dahle)

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.
(Credit: ESO/Y. Beletsky; modifications: E. Siegel)

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

hot big bang
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.
(Credit: Pablo Carlos Budassi; Unmismoobjetivo/Wikimedia Commons)

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.
(Credit: ESA/Gaia/DPAC)

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.
(Credit: ESO/INAF-VST/OmegaCAM. Acknowledgement: OmegaCen/Astro-WISE/Kapteyn Institute)

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

unreachable
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.
(Credit: HUDF09 and HUDF12 teams; Processing: E. Siegel)

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

Fewer galaxies are seen nearby and at great distances than at intermediate ones, but that’s due to a combination of both evolution and observational limitations. Over time, galaxies mergers, grow, and evolve, but the most distant, faint galaxies remain beyond Hubble’s capabilities to observe. Future observatories, both on the ground and in space, will reveal what Hubble, at present, has been unable to show us.
(Credit: NASA/ESA)

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

Galaxies comparable to the present-day Milky Way are numerous throughout cosmic time, having grown in mass and with more evolved structure at present. Younger, galaxies are inherently smaller, bluer, more chaotic, richer in gas, and have lower densities of heavy elements than their modern-day counterparts.
(Credit: NASA, ESA, P. van Dokkum (Yale U.), S. Patel (Leiden U.), and the 3-D-HST Team)

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.
(Credit: NASA; ESA; and Z. Levay, STScI; Moon Credit: T. Rector; I. Dell’Antonio/NOAO/AURA/NSF)

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.
(Credit: Jeyhan Kartaltepe (RIT); Caitlin Casey (UT Austin); and Anton Koekemoer (STScI) Graphic Design Credit: Alyssa Pagan (STScI))

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

james webb hubble
A portion of the Hubble eXtreme Deep Field that’s been imaged for 23 total days, as contrasted with the simulated view expected by James Webb in the infrared. With large-area mosaics such as COSMOS-Web and PANORAMIC, the latter of which takes advantage of pure parallel observing, upcoming, we should not only shatter the cosmic record for most distant galaxy, but should learn about what the earliest luminous objects in the Universe looked like.
(Credit: NASA/ESA and Hubble/HUDF team; JADES collaboration for the NIRCam simulation)

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.
(Credit: ESO)

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

universe temperature
At any epoch in our cosmic history, any observer will experience a uniform “bath” of omnidirectional radiation that originated back at the Big Bang. Today, from our perspective, it’s just 2.725 K above absolute zero, and hence is observed as the cosmic microwave background, peaking in microwave frequencies. At great cosmic distances, as we look back in time, that temperature was hotter dependent on the redshift of the observed, distant object.
(Credit: Earth: NASA/BlueEarth; Milky Way: ESO/S. Brunier; CMB: NASA/WMAP)

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.
(Credit: NASA/Spitzer/S-CANDELS; Ashby et al. (2015); Kai Noeske)

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

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