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

All throughout the Universe, spiral galaxies are extremely common.

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.

Along with elliptical galaxies, most of the Universe’s stars reside inside them.

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.

Most observed spirals appear neither edge-on nor face-on, but tipped: inclined at an angle.

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.

Remarkably, just by a visual inspection, you can conclude — with confidence — which edge of the galaxy is closest.

A mesmerizing spiral galaxy with a bright core, surrounded by countless stars and distant galaxies, gracefully tipped toward the edge of the cosmos against a dark background.

Unlocking the answer requires putting just two pieces of key information together.

Located approximately 44 million light-years away, galaxy NGC 5866, also known as Messier 102, is a practically perfectly edge-on spiral galaxy sometimes colloquially called the Spindle galaxy. Its dust lane, although slightly warped by what observations suggest is a recent interaction with a companion galaxy, almost perfectly bisects the plane of the galaxy itself.

First, recognize that spiral galaxies are dustiest in their central galactic planes.

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.

We can observe this directly by examining spiral galaxies seen edge-on, including our own.

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.

Second, understand that spiral galaxies have more stars near their centers than their outskirts.

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 is most clearly revealed by examining face-on spiral galaxies.

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 toward its center, dropping off at greater distances from the galactic center.

When a galaxy appears tipped, or inclined, starlight that’s behind the dust gets reddened and blocked.

If you model a galaxy as a thick, uniform disk of stars with a layer of dust through the center of the disk, you’ll see the stars in front of the dust layer as normal, but the stars behind the dust layer as being reddened or even as having their light blocked entirely. However, in this simple, unrealistic model, all regions of the galaxy would be reddened by the same amount.

However, stars are densest toward any galaxy’s 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.

Therefore, different regions of the galaxy will appear dust-obscured by different amounts.

A slightly more refined model of a spiral galaxy has its stars distributed in a thick disk, but where the stellar density is greatest toward the center and drops off toward the outskirts. Then, a layer of dust in the central galactic plane will redden more stars in the “top part” of the image, where more stars are behind the dust layer, while reddening fewer stars in the “bottom part” of the image, as most of the stars along that line-of-sight are in front of the dust layer.

The galactic edge farthest from us will have few stars blocked and reddened by dust.

This image shows the Sunflower Galaxy, Messier 63, as seen with the Hubble Space Telescope. The Sunflower appears much dustier on the “bottom” of this image as compared with the top, as the galaxy is tipped toward us at about the 7 o’clock position, with the 1 o’clock position being maximally tipped away from us.

Contrariwise, the nearest edge will appear dustiest, with emitted starlight most greatly obscured along that line-of-sight.

Galaxy Messier 106 is neither perfectly edge-on nor face-on, but is rather inclined with respect to our line-of-sight. It may not be immediately clear that the closest part of this galaxy to us is at the 5 o’clock position, but the maximal effects of abundant dust in that direction, coupled with a minimal effect in the opposing direction, allows us to draw that conclusion.

Just by identifying the dustiest edge of a spiral galaxy, you’ll know that’s the closest part to us.

Perhaps the most famous dust-obscured galaxy is the Black Eye galaxy, known as either Messier 64 or NGC 4826. This semi-dark appearance is due to a thick layer of central dust in the disk of the spiral galaxy, which blocks starlight maximally where it is tipped toward us, at about the 3 o’clock position, and is least dusty in the opposite (9 o’clock) position due to being tipped away from us in that direction.

Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words.