Sign up for the Starts With a Bang newsletter
Travel the universe with Dr. Ethan Siegel as he answers the biggest questions of all
Here in the Milky Way, our own galaxy’s structure remains obscure.
The European Space Agency’s space-based Gaia mission has mapped out the three-dimensional positions and locations of more than one billion stars in our Milky Way galaxy: the most of all-time. Looking toward the center of the Milky Way, Gaia reveals both light-blocking and luminous features that are scientifically and visually fascinating. Being confined to observing the Milky Way from within it, there are many features of our own galaxy that remain unknown.
Credit: ESA/Gaia/DPAC
From within our home galaxy, even multiwavelength observations are limited.
This updated Radio/X-ray composite of the galactic center, featuring data from both MeerKAT and Chandra, showcases the new information that can be gleaned from stitching together multiple wavelengths of light. In the future, improved observations and superior observatories may help us solve the scientific mysteries of the origin of a variety of features within the Milky Way, including lobes, bubbles, and sprites.
External galaxies, however, teach us many relevant lessons.
The spiral galaxy UGC 12158, with its arms, bar, and spurs, as well as its low, quiet rate of star formation and hint of a central bulge, may be the single most analogous galaxy for our Milky Way yet discovered. It is neither gravitationally interacting nor merging with any nearby neighbor galaxies, and so the star-formation occurring inside is driven primarily by the density waves occurring within the spiral arms in the galactic disk.
The largest galaxy on the sky is Andromeda: 2.5 million light-years away.
This full-scale view of the Andromeda Galaxy, M31, showcases its star-forming regions lining its spiral arms, its dust lanes, and its central, gas-poor region. Unlike the Milky Way, Andromeda lacks a prominent central bar. This image is a fairly close approximation of what human eyes would see if they could make out these details in Andromeda.
Edwin Hubble observed individual stars within it in 1923, proving Andromeda’s extragalactic nature.
Hubble’s discovery of a Cepheid variable in the Andromeda galaxy, M31, opened up the Universe to us, giving us the observational evidence we needed for galaxies beyond the Milky Way and leading us, in short order, to the discovery of the expanding Universe.
Nearly a century later, the Hubble Space Telescope began surveying it.
This low-resolution view is actually the zoomed-out version of the original PHAT: the Panchromatic Hubble Andromeda Treasury, from 2015, which imaged more than a third of the Andromeda Galaxy and contained more than 100 million stars, individually, that could be resolved within it.
In 2015, the Panchromatic Hubble Andromeda Treasury was completed.
This zoomed-in view grabs only a fraction of the full Panchromatic Hubble Andromeda Treasury mosaic, showcasing millions of stars in the galactic plane of Andromeda, along with dust lanes and newborn (blue) stars and star clusters. Andromeda, although quiet, is still forming new stars today, much like our own Milky Way.
394 hours of Hubble observations were required to construct it, imaging over 100 million stars individually.
This vertically-oriented view of the plane of the Andromeda galaxy is the largest photomosaic ever assembled from Hubble images. Appearing as large as the diameter of six full Moons, it took some ~600 separate Hubble observations to construct this image. All told, over 200 million individual stars have been spotted in the full-resolution 2.5 gigapixel image.
Now, in 2025, the full galactic mosaic of Andromeda is complete.
Over 200 million stars are found inside this 2.5 gigapixel image, composed from ~600 separate observations.
This view of a portion of Andromeda, near the edge of the disk, showcases large populations of dust sweeping around the galaxy, spanning tens of thousands of light-years in physical extent. The dust effectively blocks the light from stars behind it, while the bluer regions highlight regions devoid of dust: where the stars shine through unimpeded.
We’ve learned, from apparent star color, where Andromeda’s dust lanes are.
Dust lanes aren’t just present as optical, light-blocking material, but also as regions where stars are reddened relative to “normal,” dust-free regions. Here, the top and left of the image shows a dust-rich portion of Andromeda, while the lower-right region shows a dust-sparse or even dust-free region. In between them, a bright bevy of blue stars highlight a region of new star-formation, which is still taking place as far as we can tell.
We’ve spotted newborn star clusters: evidence of modern-day star-formation.
These observations of young, blue stars in Andromeda highlight the ongoing new star formation that’s happening there: within the dust lanes that line the galaxy’s spiral arms. The individual blue stars represent sites of recent star-formation, as the hottest, bluest, shortest-lived stars shine brightly here.
We’ve learned Andromeda’s tilt: just 13° away from being perfectly edge-on.
Here. one prominent dust lane sweeps across the inner disk of Andromeda, blocking the light from background stars and reddening it severely, but also providing key information about the galactic tilt of Andromeda based on the geometry of the resultant reddening. Andromeda, once thought to be tilted by as much as ~30 degrees relative to our perspective, is now known to be inclined at a mere ~13 degrees relative to a perfectly edge-on perspective.
Younger stars line the arms, but are absent from the galactic core.
At left, infrared views of Andromeda are shown from NASA’s WISE satellite, while at right, ultraviolet views of Andromeda from GALEX are illuminated. Young stars from the latest Hubble Andromeda study, of below 500 million years (HeB) and below 200 million years (MS) are also shown. WISE highlights warm dust that Hubble is insensitive to, while GALEX highlights dense, older stellar populations that the Hubble data does not reveal.
Older stars, meanwhile, are more uniformly distributed.
These four stellar density maps of Andromeda, shown here, show different types of stars that are proxies for different ages of stellar population. At left, (RGB) stars 2 billion years or older are shown, while further to the right (AGB) stars between 0.8 billion and 2.0 billion years are highlighted. Next to it, (HeB) stars of up to 500 million years old are shown, and finally, at right, (MS) stars of no more than 200 million years are shown. Note the total lack of the youngest stars in the galactic center.
Within Andromeda, Hubble only resolves stars intrinsically brighter than the Sun.
This view of the central region of Andromeda allows one to “pan down” from the edge of the galaxy’s disk through several dust lanes, new star-forming regions, and down to the galactic innards and core of Andromeda. At the bottom of the image, the galactic nucleus, which contains no new stars younger than ~200 million years, is highlighted as the brightest, most stellar-rich region of the galaxy of all.
Recent star-forming events were likely triggered by M32: a stripped satellite galaxy.
The elliptical galaxy adjacent to Andromeda, Messier 32 (M32), has properties that are very similar to what we expect a stripped galactic core that one was gas-rich and that gave rise to stellar streams around a galaxy would look like. Although this hypothesis is not yet proven, the latest Hubble data about Andromeda and its satellites supports this picture quite strongly.
Major new star-formation episodes will ensue when “Milkdromeda” forms.
A series of stills showing a visualization of the Milky Way-Andromeda merger and how the sky will appear different from Earth as it happens. This merger will begin occurring roughly 4 billion years in the future, with a huge burst of star formation leading to a depleted, gas-poor, more evolved galaxy ~7 billion years from now. Despite the enormous scales and numbers of stars involved, only approximately 1-in-100 billion stars will collide or merge during this event. The final form of the galaxy, despite the illustration here, is more likely to be a gas-rich, disk-possessing galaxy than the elliptical one shown, as only a small percentage of major mergers lead to a red-and-dead, gas-free elliptical final state.
Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words.
Sign up for the Starts With a Bang newsletter
Travel the universe with Dr. Ethan Siegel as he answers the biggest questions of all
