Our galactic home in the cosmos — the Milky Way — is merely one among the observable Universe’s two trillion.
From our own perspective on Earth, we’ve identified the presence of spiral arms.
By viewing the Milky Way in infrared wavelengths of light, we can see through the galactic dust and view the distribution of stars and star-forming regions behind them. As revealed by the 2 micron all-sky survey (2MASS), the densest collections of galactic dust can be seen tracing out our spiral arms. (TWO MICRON ALL SKY SURVEY/IPAC/CALTECH & UMASS)
However, being stuck within the Milky Way itself, we exclusively view it edge-on.
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. However, even with all the features that observatories like this can identify in our Milky Way, much remains obscure to our eyes due to our limited perspective. (ESA/GAIA/DPAC, CC BY-SA 3.0 IGO)
Even our best spaceborne views leave much ambiguity in our galaxy’s overall structure.
The grand spiral galaxy Messier 51, also known as the Whirlpool galaxy, has sweeping, extended spiral arms, most probably owing to its gravitational interactions with the nearby neighboring galaxy shown at right. Galaxies such as this often have large waves of star formation occurring along their spiral arms, but only ~10% of spirals exhibit this grand structure. (NASA, ESA, S. BECKWITH (STSCI) AND THE HUBBLE HERITAGE TEAM (STSCI/AURA))
We’re not a grand spiral galaxy, as we’re lacking extended outer arms.
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. However, unlike the Milky Way, Andromeda lacks a prominent central bar. (ROBERT GENDLER; SUBARU (NAOJ)/HSC COLLABORATION/KAVLI IPMU/HUBBLE SPACE TELESCOPE/LOCAL GROUP SURVEY, KITT PEAK NATIONAL OBSERVATORY (NOAO), DIGITIZED SKY SURVEY (OLIVER CZERNETZ))
Nor are we similar to Andromeda, our nearest large neighbor, which lacks a central bar.
The enormous bar at the core of galaxy NGC 1300 spans many tens of thousands of light-years, nearly the full width of the galaxy. While many spiral galaxies contain large, prominent bars such as this one, our Milky Way’s central bar is far more modest, extending only about a third of the way out to the Sun’s position. (NASA, ESA, AND THE HUBBLE HERITAGE TEAM STSCI/AURA))
While one-third of spiral galaxies have bars, ours is smaller than many, like NGC 1300’s.
The galaxy NGC 2775, shown here, displays one of best known examples of flocculent spiral arms, where the arms have wound up many times over in the outskirts of this galaxy. The inner, central region is highly symmetrical and devoid of dust, explaining its yellow color, while the outer arms continue to create waves of new star formation. (NASA / ESA / HUBBLE / J. LEE / PHANGS-HST TEAM / JUDY SCHMIDT, WWW.GECKZILLA.COM)
The outer arms are neither irregular nor tightly wound; we’re not “flocculent.”
Many spiral galaxies, such as the Sombrero galaxy (M104), possess both spiral features and also a large central bulge. For comparison, the Milky Way possesses only a small central bulge, but it is still a significant feature in describing our galaxy. (NASA AND THE HUBBLE HERITAGE TEAM (STSCI/AURA))
Additionally, the Milky Way possesses a small but significant central bulge.
The Southern Pinwheel Galaxy, Messier 83, displays many features common to our Milky Way, including spiral arms, a central bar, as well as spurs and minor arms. However, without a better perspective on what our Milky Way looks like, we cannot be certain that this galaxy is analogous to our own. (ESO)
We also exhibit major arms, minor arms, and spurs, with the Orion spur home to our Sun.
The Milky Way has two major arms, called the Perseus Arm and the Scutum-Centaurus Arm. There are also two minor arms, and two smaller ‘spurs’. Earth, its sun, and the rest of our solar system, is embedded within the Orion spur. While the general features of the Milky Way are thought to match this picture, the finer details of the galaxy are largely unknown. (NASA/JPL-CALTECH/R. HURT (SSC/CALTECH))
While many galaxies copiously form stars, the Milky Way is relatively quiet.
A young, star-forming region found within our own Milky Way. Note how the material around the stars gets ionized, and over time becomes transparent to all forms of light. Star-forming regions in the Milky Way are few in number and small in nature, particularly in comparison to the more active galaxies in our Universe. (NASA, ESA, AND THE HUBBLE HERITAGE (STSCI/AURA)-ESA/HUBBLE COLLABORATION; ACKNOWLEDGMENT: R. O’CONNELL (UNIVERSITY OF VIRGINIA) AND THE WFC3 SCIENTIFIC OVERSIGHT COMMITTEE)
Only within the arms themselves are new stars primarily formed.
The spiral arms of galaxy NGC 6384 are where new stars primarily form in this galaxy. Under normal circumstances, spiral arms in the disk of a spiral galaxy are where the majority of new stars form. (ESA/HUBBLE & NASA)
It’s as though the Milky Way is a large, barred spiral galaxy with a small, elliptical center.
The spiral galaxy NGC 1084 has no central bar, but exhibits enormous levels of star formation, as well as an unusually large supernova rate for spiral galaxies. This galaxy, despite numerous superficial properties in common with our Milky Way, cannot be a very good analogue. (NASA, ESA, AND S. SMARTT (QUEEN’S UNIVERSITY BELFAST); ACKNOWLEDGEMENT: BRIAN CAMPBELL)
Many similar galaxies are known, but nobody knows exactly which one is most like our Milky Way.
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. (ESA/HUBBLE & NASA)