How to tell which edge of a galaxy is tipped towards you

Spirals, initially recorded as faint, fuzzy objects with no discernible structure through more primitive telescopes, were clearly observed since the mid-1800s to be prevalent in the night sky. We now understand that spirals are galaxies like our own Milky Way, with central dust-rich planes and spiral arms that can be more easily identified depending on the tilt of the galaxy.

The spiral galaxy NGC 772 has no central bar, but exhibits enormous levels of star formation and a lopsided dust distribution: evidence of large populations of bright stars on the far side of the dusty galaxy. Large spiral and elliptical galaxies, although they’re not nearly as abundant as low-mass dwarf galaxies, house the majority of stars that have ever formed throughout the Universe’s history.

By identifying both the spiral (disk-like) and elliptical (halo-like) components of the Sombrero galaxy, one can subtract the elliptical portion of the data out from the optical image, leaving only the disk-like component. This view, created with Hubble data, reveals our best optical views of the disk-like portion alone. Although the Sombrero galaxy is seen nearly edge-on, like most spiral galaxies, it’s tipped at an angle with respect to us.

A map of star density in the Milky Way and surrounding sky, clearly showing the Milky Way, large and small Magellanic Clouds, and if you look more closely, NGC 104 to the left of the SMC, NGC 6205 slightly above and to the left of the galactic core, and NGC 7078 slightly below. All told, the Milky Way contains some 200-400 billion stars over its disk-like extent. There are a great many galaxies to be discovered, but within about 10 degrees above and below the galactic plane, visible light is a lousy tool for revealing them.

By viewing the Milky Way in infrared wavelengths of light, we can see through large amounts of 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, but the center of the plane of the Milky Way is where the dust is densest. Infrared and visible light views both showcase this, but in vastly different ways.

The Southern Pinwheel Galaxy, Messier 83, displays many features common to our Milky Way, including a multi-armed spiral structure and a central bar, as well as spurs and minor arms, plus a central bulge of stars; the stellar density is greatest near the center, and drops the farther away one travels. The pink regions showcase transitions in hydrogen atoms driven by ultraviolet light: produced by new stars. The Southern Pinwheel galaxy is one of the closest and brightest barred spiral galaxies at a distance of just 15 million light-years, with a similar diameter (118,000 light-years) to our own Milky Way.

This Hubble image of the Bullseye galaxy, LEDA 1313424, showcases many of its low surface-brightness structures: red inner rings, where gas has been evacuated, blue, star-rich outermore rings, and a large, faded, still-expanding ring. Overall, the galaxy, because of its distended structure, spans 250,000 light-years: more than double the diameter of our Milky Way. Although this is a highly unusual galaxy, it still has the highest density of stars towards its center, dropping off at greater distances from the galactic center.

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.