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Starts With A Bang

Remember why we look up

Looking up at the night sky gives us a glimpse of the Universe beyond our terrestrial concerns. Here’s what’s out there.
While the stars, galaxies, and Milky Way are familiar sights in the night sky, they are joined here by the faint zodiacal light that arises from light (mostly direct sunlight) reflecting off of Solar System dust particles. Profusely present in the inner Solar System, zodiacal dust is fundamentally limiting when we collect faint observations of the distant Universe.
ESO/B. Tafreshi (
Key Takeaways
  • Ever since the first human beings looked up at the night sky, it has been a source of wonder, inspiration, and awe.
  • Today, most of humanity only has access to a heavily polluted sky rather than the pristine views our ancestors enjoyed, but that sense of awe and wonder still remains.
  • Here’s a reminder of what’s out there, beyond the confines of this Earth, that calls to our sense of adventure, exploration, and our urge to know the unknown.

The night sky, familiar to all of us, holds a sense of wonder unlike anything else.

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.
Credit: ESO/Håkon Dahle

For countless generations, we’ve gazed upon the heavenly abyss with awe.

The effects of light pollution on what a naked-eye observer can see in the night sky. The artificial light produced by objects on the ground can wash out the naturally occurring objects in the night sky, rendering many objects unable to be seen. Light pollution can wash out all but the brightest meteors during a meteor shower.
(Credit: Stellarium Labs)

Today, light pollution and satellite contamination have stolen those pristine views from many of us.

new satellites
This image of Venus and the Pleiades shows the tracks of Starlink satellites. The reflective surfaces of the satellites, coupled with the fact that they are orbiting around Earth, mean that astronomical observations that require very long exposures capture “tracks” of the satellites in their images.
Credit: T. Hansen/IAU OAE/Creative Commons Attribution

Even so, each glimpse of what lies beyond Earth compels us to look farther.

Milky Way center
This spectacular composite image, which combines X-ray, infrared, and optical light from NASA’s great observatories, was our best view of what’s going on in the galactic center as of 2009. Over the past ~15 years, however, we’ve taken data that has revealed novel features that, at present, have yet to be fully explained. Perpendicular to the plane of the galaxy, features rise above and below indicating the transport of energy and gas in a chimney-like fashion.
Credit: NASA/JPL-Caltech/ESA/CXC/STScI

Out there, among the stars, galaxies, and beyond, lies the answers to the greatest mysteries of all.

m81 group
This multiwavelength view of the two largest, brightest galaxies in the M81 group shows stars, plasmas, and neutral hydrogen gas. The gas bridge connecting these two galaxies infalls onto both members, triggering the formation of new stars. If each star were shrunk down to be a grain of sand, this group would be 36 million km away, but the two galaxies would be separated only by a little over 400,000 km: the Earth-Moon distance. The galaxies comprising the M81 group will likely be the very last ones to recede from our reach in our dark energy-dominated Universe.
Credit: R. Gendler, R. Croman, R. Colombari; Acknowledgement: R. Jay GaBany; VLA Data: E. de Block (ASTRON)

Each glittering point of light is its own stellar system, containing planets, moons, and possibly even life.

51 Eri b was discovered in 2014 by the Gemini Planet Imager. At 2 Jupiter masses, it is the coolest and lowest mass imaged exoplanet to date, and orbits only 12 Astronomical Units from its parent star. To image beings on the surface of this world would require a telescope with billions of times our present best resolution.
(Credit: Jason Wang (Caltech)/Gemini Planet Imager Exoplanet Survey)

We’ve seen individual planets forming, and may soon discover extrasolar biological activity.

A sample of 20 protoplanetary disks around young, infant stars, as measured by the Disk Substructures at High Angular Resolution Project: DSHARP. Observations such as these taught us that protoplanetary disks form primarily in a single plane and tend to support the core accretion scenario of planet formation. The disk structures are seen in both infrared and millimeter/sub-millimeter wavelengths. We have recently learned that gaps begin to form in protoplanetary disks after ~1-2 million years, with younger disks displaying no such substructure.
Credit: S.M. Andrews et al., ApJL, 2018

Within the Milky Way galaxy, alone, there are billions of potentially inhabited worlds.

5000 exoplanets
This diagram shows the discovery of the first 5000+ exoplanets we know of and where they’re located on the sky. Circles show location and size of orbit, while their color indicates the detection method. Note that the clustering features are dependent on where we’ve been looking, not necessarily on where planets are preferentially found. No planets have been found within globular clusters, including the long-imaged 47 Tucanae and Omega Centauri.
(Credit: NASA/JPL-Caltech)

Even beyond that, there are anywhere from 6 to 20 trillion galaxies within the observable Universe.

how many galaxies
Various long-exposure campaigns, like the Hubble eXtreme Deep Field (XDF) shown here, have revealed thousands of galaxies in a volume of the Universe that represents a fraction of a millionth of the sky. This image contains 5,500 galaxies, but takes up just 1-32,000,000th of the total sky. But even with all the power of Hubble, and all the magnification of gravitational lensing, there are still galaxies out there beyond what we are capable of seeing.
(Credit: NASA/ESA/H. Teplitz and M. Rafelski (IPAC/Caltech), A. Koekemoer (STScI), R. Windhorst (ASU), and Z. Levay (STScI))

We can’t see farther because the Universe is a finite age: it’s been a mere 13.8 billion years since the hot Big Bang.

The size of our visible Universe (yellow), along with the amount we can reach (magenta) if we left, today, on a journey at the speed of light. The limit of the visible Universe is 46.1 billion light-years, as that’s the limit of how far away an object that emitted light that would just be reaching us today would be after expanding away from us for 13.8 billion years. Anything that occurs, right now, within a radius of 18 billion light-years of us will eventually reach and affect us; anything beyond that point will not. Each year, another ~20 million stars cross the threshold from being reachable to being unreachable.
Credit: Andrew Z. Colvin and Frederic Michel, Wikimedia Commons; Annotations: E. Siegel

Beyond those limits, the story of our ultimate origin remains obscure.

The quantum fluctuations inherent to space, stretched across the Universe during cosmic inflation, gave rise to the density fluctuations imprinted in the cosmic microwave background, which in turn gave rise to the stars, galaxies, and other large-scale structures in the Universe today. This is the best picture we have of how the entire Universe behaves, where inflation precedes and sets up the Big Bang. Unfortunately, we can only access the information contained inside our cosmic horizon, which is all part of the same fraction of one region where inflation ended some 13.8 billion years ago.
Credit: E. Siegel; ESA/Planck and the DOE/NASA/NSF Interagency Task Force on CMB research

The Universe provides hints, but the necessary information to draw definitive conclusions eludes us.

From a pre-existing state, inflation predicts that a series of universes will be spawned as inflation continues, with each one being completely disconnected from every other one, separated by more inflating space. One of these “bubbles,” where inflation ended, gave birth to our Universe some 13.8 billion years ago, with a very low entropy density, but without ever violating the 2nd law of thermodynamics.
Credit: Nicolle Rager Fuller

Each glimpse into the great cosmic abyss continues our neverending pursuit of knowledge.

jwst change science
This image, of a dusty region of the Large Magellanic Cloud, was taken with JWST’s MIRI instrument at a wavelength of 7.7 microns. By measuring the Universe at unprecedented wavelengths, depths, sensitivities and resolutions, JWST can reveal details that have never been revealed before. From dust to stars to black holes and even to potential biosignatures, its capabilities could show us a Universe we never even expected to find.

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


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