Our galactic home in the cosmos – the Milky Way – is just one of two trillion observable in the Universe.
From our own vantage point on Earth, we have identified the presence of spiral arms.
By observing the Milky Way in infrared wavelengths of light, we can see through the galactic dust and see 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 our spiral arms.
However, being stuck in the Milky Way itself, we see it exclusively from the side.
The European Space Agency’s Gaia space mission has mapped the positions and three-dimensional locations of more than a billion stars in our Milky Way galaxy: 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.
Even our best views from space leave a lot of ambiguity in the overall structure of our galaxy.
The large spiral galaxy Messier 51, also known as the Whirlpool Galaxy, has extended and extended spiral arms, most likely due to its gravitational interactions with the neighboring galaxy shown to the right. Galaxies like this often have large waves of star formation occurring along their spiral arms, but only ~10% of spirals exhibit this large structure.
We are not a large spiral galaxy because we lack extended outer arms.
This large-scale view of the Andromeda Galaxy, M31, shows its star-forming regions bordering its spiral arms, its dust lanes, and its gas-poor central region. However, unlike the Milky Way, Andromeda does not have a prominent central bar.
We are also not like Andromeda, our nearest large neighbor, which has no central bar.
The huge bar at the heart of galaxy NGC 1300 spans several tens of thousands of light-years, almost the entire width of the galaxy. While many spiral galaxies contain large, prominent bars like this, our Milky Way’s central bar is much more modest, extending only about a third of the way to the position of the Sun.
While a third of spiral galaxies have bars, ours is smaller than many, like that of NGC 1300.
The galaxy NGC 2775, shown here, features one of the best-known examples of flocculant spiral arms, where the arms have wrapped around the outskirts of this galaxy several times. The inner and central region is highly symmetrical and devoid of dust, which explains its yellow color, while the outer arms continue to create waves of new star formation.
The outer arms are neither irregular nor tightly coiled; we are not “flocculants”.
Many spiral galaxies, such as the Sombrero Galaxy (M104), have both spiral features and also a large central bulge. For comparison, the Milky Way has only a small central bulge, but it’s still an important feature in describing our galaxy.
Additionally, the Milky Way has a small but significant central bulge.
The southern pinwheel galaxy, Messier 83, shares many features common to our Milky Way, including spiral arms, a central bar, as well as spurs and minor arms. However, it is only about half the diameter of the Milky Way. Without a better perspective on what our Milky Way looks like, we can’t be certain that this galaxy is analogous to our own.
We also display major arms, minor arms and spurs, with the spur of Orion housing our Sun.
The Milky Way has two main arms, called the Perseus arm and the Scutum-Centaurus arm. There are also two minor arms and two smaller “spurs”. The Earth, its sun and the rest of our solar system are integrated into the spur of Orion. While the general features of the Milky Way are thought to match this image, the finer details of the galaxy, especially once we look within a few thousand light-years of our own location, are largely unknown.
While many galaxies copiously form stars, the Milky Way is relatively quiet.
This wide-field ground image of the Eagle Nebula shows the star-forming region in all its glory, with new stars, reflection and emission nebulae, and dusty features all present. Note how the material around the stars ionizes and over time becomes transparent to all forms of light. Star-forming regions in the Milky Way are few and small, especially compared to the more active galaxies in our Universe.
It is only in the arms themselves that new stars mainly form.
The spiral arms of galaxy NGC 6384 are where new stars mostly form in this galaxy. Under normal circumstances, the spiral arms of a spiral galaxy’s disk are where the majority of new stars form. With many characteristics in common with our own Milky Way, NGC 6384 is one of the best candidates for a near-twin of the Milky Way.
It is as if the Milky Way was a large barred spiral galaxy with a small elliptical-like center.
The spiral galaxy NGC 772 has no central bar, but exhibits huge levels of star formation and an unbalanced dust distribution: evidence of large populations of bright stars on the far side of the dusty galaxy. This galaxy, despite having many surface properties in common with our Milky Way, cannot be a very good analog.
Many similar galaxies are known, but no one knows exactly which resembles our Milky Way the most.
The spiral galaxy UGC 12158, with its arms, helm and spurs, along with its quiet, low rate of star formation and hint of a central bulge, is perhaps the most Milky Way-like galaxy ever discovered. .
Mostly Mute Monday tells an astronomical story in pictures, visuals and no more than 200 words. Talk less; smile more.
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