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Credit: NASA, ESA, CSA, STScI, Janice Lee (STScI), Thomas Williams (Oxford), PHANGS Team
On a clear night with dark skies, you can catch a glimpse of the Milky Way, a luminous arc stretching across the horizon. To the unaided eye, its translucent clouds are simply beautiful splashes in the night, their grandeur inspiring hundreds of creation myths. But a telescope reveals subtle structures within and outside the Milky Way that show how our own galaxy and others formed, explain their evolution through cosmic time, and even hint at their fate.
A galaxy is a massive collection of gas, dust, and huge numbers of stars and their solar systems, all held together by their mutual gravity. Our own galaxy exists within a neighborhood of closely gathered galaxies known as the Local Group. In the vastness of space, galaxies are arranged in filaments, clusters, and sheets, which form superstructures on a still larger scale.
Estimates of the number of galaxies in the universe range from 200 billion to 2 trillion. Dwarf galaxies may contain less than a thousand stars. At the other extreme of mass, supergiant galaxies can encompass a hundred trillion stars in orbit around the galactic center. The Milky Way also has a supermassive black hole at its core: Sagittarius A* (pronounced “A-star”).
The Milky Way above the Atacama Large Millimeter/submillimeter Array, above the European Southern Observatory in the Chilean Andes, a collaborator in the multi-facility Event Horizon Telescope. Inset: Sagittarius A* as seen through the Event Horizon Telescope.
Credit: ESO/José Francisco Salgado, EHT Collaboration
With respect to structure, there are three types of galaxies: spiral, elliptical, and irregular. Scientists also classify galaxies according to the activity at their core, such as a supermassive black hole. Around 10% of known galaxies are active, which means their luminous centers shine hundreds or thousands of times brighter than the combined light of their stars. These active galactic nuclei or quasars are what happens when a supermassive black hole devours a large amount of matter. Jets of high-energy photons blast from the quasar’s rotational poles, reaching tens or hundreds of light years into intergalactic space.
The Milky Way is not considered an active galaxy—at least, not right now—but according to NASA it likely experienced a “burst of activity” within the past few million years.
Spiral Galaxies
By far the most common type of galaxy, spiral galaxies have a bright, dense core of stars collected in a central globe, from which two or more spiral arms extend in a wide, flat disc. Most have a supermassive black hole at their center. (Sagittarius A* accounts for some 4.3 million solar masses, a measurement that won the 2020 Nobel Prize for Physics.)
Spiral galaxies often have roughly spherical halos, composed of gas, dust, old stars, star clusters, and dark matter—invisible material that does not emit or reflect light but still has a gravitational pull on other matter. Clusters of stars form in stellar nurseries, regions rich in gas and dust found within a spiral galaxy’s arms, while older stars can be found throughout the disk and halo.
Spiral galaxies account for the vast majority of galaxies, outnumbering other types by no less than two to one; the Hubble project gives a conservative figure of 60%, while the Sloan Digital Sky Survey’s higher-end population estimate identified as many as 77% of all galaxies as spiral galaxies.
Spiral galaxies, as seen by the JWST. Glowing dust appears in shades of red and orange. Older stars are blue. Diffraction spikes indicate that these galaxies may have central active supermassive black holes.
Credit: NASA, ESA, CSA, STScI, Janice Lee and Elizabeth Wheatley (STScI), Thomas Williams (Oxford), PHANGS Team
About two-thirds of spiral galaxies are barred spirals, in which the central disk is stretched out into a ribbon. The Milky Way belongs to this class, as well as our nearest galactic neighbor, the Andromeda Galaxy.
Elliptical Galaxies
Older and scarcer than spirals, elliptical galaxies have shapes ranging from long, narrow ovals to spheres. Unlike spirals, elliptical galaxies don’t have much ambient gas and dust, and they show little discernible organization or structure. Member stars orbit around the galactic core in random directions, and little of the gas needed to form new stars remains.
Honorable Mention: Globular Clusters
Globular clusters are often mentioned in the same breath as galaxies, so they deserve a nod. A globular cluster can contain anywhere from tens of thousands to “many millions” of member stars, and globular clusters are found within nearly all galaxies. The Milky Way has no fewer than 157, but more globular clusters are thought to exist within the innermost regions of the galaxy which are shrouded in an obscuring veil of gas and dust.
Lenticular Galaxies
Lenticular galaxies are a kind of cross between spirals and ellipticals. They have the central bulge and disk common to spiral galaxies, but lenticular galaxies don’t have arms. However, like ellipticals, lenticular galaxies have older stellar populations and not much ongoing star formation.
There are two leading theories about how lenticular galaxies evolved. One suggests these galaxies are late-in-life snapshots of older spirals whose arms have mostly faded or dispersed. The other proposes that they are what remains after the mergers of spiral galaxies, à la the fate that will eventually befall the Milky Way and Andromeda.
In September 2024, the James Webb Space Telescope captured this collision in progress between an elliptical galaxy and a special type of spiral galaxy called a Seyfert galaxy, collectively known as Arp 107.
Credit: NASA, ESA, CSA, STScI
Irregular Galaxies
A surprising number of galaxies don’t quite fit the spiral/elliptical archetypes. These strange and contrarious sky objects may have shapes from teardrops to needles, hooks, and rings. They aren’t a special, exotic type of galaxy. Instead, it’s thought that many irregular galaxies might be spiral galaxies seen edge-on, distorted by gravitational lensing, or ancient elliptical galaxies disrupted by greater tidal forces.
On a Cosmic Scale
It’s easy to feel small, looking up at the Backbone of Night. Our Solar System alone is an easy three light years wide, and it’s barely a rounding error in comparison with the galaxy as a whole. The Orion arm of the Milky Way we reside in is a thousand light years thick from bottom to top, and almost a hundred thousand light years wide from edge to edge.
Webb’s First Deep Field (2022) captured a galaxy cluster, SMACS 0723, in truly unprecedented detail. This image contains thousands of galaxies.
Credit: NASA, ESA, CSA, and STScI
Galaxies are truly gigantic, but they’re dwarfed by the larger-scale structures to which they belong. The Milky Way galaxy, Andromeda, and all our respective satellites form a cluster known as the Local Group. The Local Group and hundreds of others like it form the Virgo Supercluster, which is itself one of four “lobes” of the Laniakea Supercluster: a cosmic structure of mind-boggling size whose common gravitational center is known as the Great Attractor. With its combined mass of 10¹⁵ M☉, our Virgo Supercluster is perhaps 0.1% of the total mass of the complex—and yet, the Great Attractor has a gravitational pull that suggests a mass ten times the mass of the entire Virgo Supercluster.
Laniakea and its neighboring superclusters are constituent parts of the Pisces-Cetus Supercluster Complex, a gigantic filament almost a billion light years long. Galactic filaments are the largest known structures in the universe. The largest, known as the Hercules-Corona Borealis Great Wall, is thought to be ten billion light years long.
Looking that far out means looking back in time. The most distant galaxies are often also the oldest, so the light we see today shows us a picture of the galaxy as it existed when the light was emitted, billions of years in the past. The very oldest appear to us today as they did just a few hundred million years after the Big Bang. For example, astronomers used the JWST to identify CEERS J141946.36+525632.8 or “Maisie’s Galaxy” formed about 13.2 billion years ago, when the universe was just 390 million years old. Another distant galaxy named GLASS-Z12 has a light travel distance or lookback time of 13.6 billion years, which means it had formed no less than 13.6 billion years ago: well within the cosmic Dark Ages, before reionization of the Universe was complete. But due to the expansion of the universe, its current proper distance is some 33.2 billion light-years.
Redshift
The speed of light is a kind of universal speed limit, and if the universe began 13.8 billion years ago, then you’d think the visible universe could only be 13.8 billion years wide at most. But there’s more to the story. Galaxies like GLASS-Z12 can be so far away because the universe itself is expanding. Scientists first discovered this due to a phenomenon called Doppler shifting, or redshift. Light waves from approaching objects are compressed, which makes them seem shorter and thus “bluer” than they are. Objects moving away from an observer will lengthen and shift toward the red instead. The Z12 in GLASS-Z12’s name describes its Doppler shift or redshift.
In calculating the size of the visible universe, astronomers including Edwin Hubble and Georges Lemaître discovered that light from distant galaxies was redshifted, with an increase in wavelength corresponding to their distance from Earth. If everything was expanding, though, where had it all started? From that central question arose the idea of the Big Bang. Hubble, meanwhile, lent his name to a mathematical law describing the expansion of the universe—and the space telescope that substantiated his work.
Galaxies near and far are a favorite target for Hubble’s spiritual and technological successor, the James Webb Space Telescope, built to investigate (among other things) how galaxies form, grow, merge, and eventually fade. Since its 2021 debut, the JWST has set and broken record after record for the biggest, oldest, and most distant galaxies ever discovered. Another of Webb’s key objectives: to check and refine astronomers’ models of dark matter and the Hubble constant, both of which are key to the expansion of the universe. Cold dark matter may be invisible by normal methods, but it still has a gravitational pull. Invisible or not, if it’s there, the JWST will find it.