This image shows the dusty galaxy NGC 4526, the galaxy that famously hosted a bright supernova back in 1994. It has a central dust disk in the middle of the galaxy, and the fact that it appears dusty on one side moreso than the other enables us to learn which edge is closest to us. (Hint: it’s the dustiest edge.) (Credit: ESA/Hubble & NASA, Processing: Judy Schmidt)
When we see spiral galaxies, some are face-on, others are edge-on, but most are tipped at an angle. But which side is closest to us?
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. (Credit: ESO/P. Grosbøl)
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. (Credit: International Gemini Observatory/NOIRLab/NSF/AURA; Image processing: T.A. Rector (University of Alaska Anchorage), J. Miller (Gemini Observatory/NSF’s NOIRLab), M. Zamani & D. de Martin)
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. (Credit: Vicent Peris (OAUV / PTeam), MAST, STScI, AURA, NASA)
Remarkably, just by a visual inspection, you can conclude — with confidence — which edge of the galaxy is closest.
Like many inclined or tilted spiral galaxies, NGC 7331, shown in the foreground of this image, exhibits a much dustier display on one side compared to the other. The prominence of this “one-sided” dustiness gives us major clues as to the galaxy’s tilt with respect to our perspective. (Credit: Fort Lewis College Observatory)
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. (Credit: NASA, ESA, and The Hubble Heritage Team (STScI/AURA))
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. (Credit: ESA/Gaia)
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. (Credit: 2MASS/IPAC/Caltech & UMass)
Second, understand that spiral galaxies have more stars near their centers than their outskirts.
