The question of how astronomers can take a picture of the Milky Way when the Earth is inside the galaxy is a common paradox. The Milky Way is a barred spiral galaxy shaped like a thin, rotating disk of stars, gas, and dust. Since we are embedded within this structure, we do not have the luxury of a distant vantage point to snap a single portrait. The solution to this problem is not a single photograph, but a synthesis of perspective, distance measurements, and the use of the entire electromagnetic spectrum.
The View from Within Our Galactic Home
The most immediate clue to the galaxy’s shape is the hazy band of light that stretches across the night sky in dark locations. This visual phenomenon occurs because we are situated within the flat, disk-like plane of the galaxy itself. When we look out along this plane, we are looking through the greatest concentration of billions of distant stars, which blend together into a continuous, milky glow.
Our solar system is not at the galactic center, but instead resides in a minor concentration of stars called the Orion Spur, which is located between two larger spiral arms. This position is roughly 26,000 light-years away from the galactic core. Looking toward the center of the galaxy in the direction of the constellation Sagittarius reveals the brightest and densest part of the visible band. Conversely, looking perpendicular to the galactic plane, above or below the disk, yields a view of far fewer stars, which confirms the flat geometry of our galactic home.
Mapping the Galaxy’s Structure in Three Dimensions
To construct a picture of the galaxy’s external appearance, astronomers determine the three-dimensional locations of stars and gas clouds. For objects within a few thousand light-years of Earth, the distance is measured using a geometric method called stellar parallax, which tracks a star’s apparent shift against the background as the Earth orbits the Sun. For greater distances, astronomers rely on “standard candles,” which are objects with a known intrinsic brightness.
A standard candle is the Cepheid variable star, which pulsates at a rate directly related to its true luminosity. By comparing this known intrinsic brightness to the star’s fainter observed brightness, a precise distance can be calculated, allowing astronomers to map their locations across the galaxy. Recent studies using thousands of these Cepheids have revealed that the Milky Way’s outer disk is not perfectly flat, but is instead warped and twisted, resembling an “S” shape. The positions and velocities of these stars, combined with radio observations of hydrogen gas that traces the spiral arms, provide the structural framework for the galaxy’s overall map.
Seeing Through the Cosmic Dust with Different Wavelengths
A primary obstacle to viewing the galactic core is the thick layer of interstellar dust that fills the spiral arms and blocks visible light. This dust absorbs and scatters optical wavelengths, preventing us from seeing more than a few thousand light-years into the disk. To overcome this obscuration, astronomers use telescopes that observe the universe in different parts of the electromagnetic spectrum.
Infrared (IR) light, which has a longer wavelength than visible light, is able to pass through the dense dust clouds largely unimpeded. Near-infrared observations penetrate the dust to reveal the stars in the crowded galactic center and the central bar structure. Radio waves are also invaluable for mapping the cold gas distribution, which outlines the spiral arms and star-forming regions that are invisible in optical light. Each non-visible wavelength provides a different layer of data, allowing scientists to piece together a complete picture of the galaxy’s components.
Assembling the Milky Way Image
The images of the Milky Way showing a spiral structure are not single photographs taken by a spacecraft, as no human-made probe has traveled far enough to capture such a view. Instead, these representations are composite visualizations and computer models built from the data collected by various telescopes. Astronomers combine the measured three-dimensional positions of stars, the mapped gas clouds, and the calculated structure of the spiral arms to generate a structural model.
The final visualizations translate non-visible data, such as infrared and radio emissions, into colors that the human eye can perceive, creating a false-color image. These renderings synthesize observational data from multiple wavelengths with artistic interpretation, resulting in portraits that illustrate what our galaxy would look like from the outside. This process transforms billions of individual data points into a cohesive image of our galactic home.