The Milky Way: Understanding and Observing Our Home Galaxy

The Milky Way is our cosmic home, a barred spiral galaxy containing hundreds of billions of stars, stretching 100,000 light-years across. Photo: Hristo Fidanov / Pexels

Our Address in the Cosmos

On a clear, moonless night from a truly dark location, you can see it: a soft, luminous band stretching from horizon to horizon, brighter and wider in some places, dimmer and narrower in others, split by dark rifts of obscuring dust. This is the Milky Way, the combined light of hundreds of billions of stars that make up our home galaxy, seen from our vantage point deep within its disk.

The ancient Greeks called it “galaxias kyklos,” the milky circle, and the name stuck. For most of human history, nobody understood what it was. It took the invention of the telescope for Galileo to resolve the diffuse glow into countless individual stars, and it took centuries more for astronomers to determine that the Milky Way is actually a vast island of stars, one galaxy among billions in the observable universe.

Structure of the Milky Way

The Milky Way is a barred spiral galaxy, roughly 100,000 light-years in diameter and about 1,000 light-years thick in the disk. It contains an estimated 100 to 400 billion stars, along with vast quantities of gas, dust, and dark matter.

The Major Components

The Central Bar and Bulge: At the center of the Milky Way is a dense, elongated structure called the bar, surrounded by a spheroidal bulge of older, reddish stars. The bar is estimated to be about 27,000 light-years long. This central region is crowded with stars and dominated by the supermassive black hole Sagittarius A*, which has a mass of approximately 4 million times that of the Sun.

The Disk: The thin disk contains most of the Milky Way’s gas, dust, and younger stars, organized into spiral arms. Star formation occurs primarily in the disk, particularly along the spiral arms where gas and dust are compressed by density waves. The disk also contains a thicker component of older stars.

The Spiral Arms: The Milky Way has two major spiral arms (the Perseus Arm and the Scutum-Centaurus Arm) along with several smaller arms and spurs. Our solar system resides in a minor arm called the Orion-Cygnus Arm (sometimes called the Orion Spur), located between the Perseus and Sagittarius arms, roughly 26,000 light-years from the galactic center.

The Halo: Surrounding the disk is a roughly spherical halo containing old stars, globular clusters, and a significant amount of dark matter. The halo extends far beyond the visible disk, with some estimates suggesting it reaches 300,000 light-years or more in diameter.

Sagittarius A*: The Monster at the Center

At the very heart of our galaxy lurks Sagittarius A* (pronounced “Sagittarius A-star”), a supermassive black hole with a mass of about 4 million Suns compressed into a region smaller than Mercury’s orbit. In May 2022, the Event Horizon Telescope collaboration released the first direct image of Sagittarius A*, showing the glowing ring of superheated gas orbiting just outside the event horizon.

We cannot see Sagittarius A* with optical telescopes because it is hidden behind 26,000 light-years of gas and dust. It is studied primarily at radio, infrared, and X-ray wavelengths. Stars near the galactic center orbit Sagittarius A* at tremendous speeds; the star S2 completes a full orbit in just 16 years, reaching velocities of nearly 3% the speed of light at its closest approach. Tracking these stellar orbits was the work that earned Reinhard Genzel and Andrea Ghez the 2020 Nobel Prize in Physics.

What You See When You Look at the Milky Way

When you gaze at the Milky Way band in the sky, you are looking along the plane of the galactic disk. The concentration of stars along this plane creates the diffuse glow. The fact that it forms a complete band circling the sky confirms that we are inside the disk, not above or below it.

The brightest and widest section of the Milky Way lies in the direction of Sagittarius and Scorpius, because this is the direction toward the galactic center where stars are most densely packed. The dark rifts that split the Milky Way, most prominent in Cygnus (called the Great Rift), are clouds of interstellar dust that absorb the light of stars behind them.

From a dark sky site with a Bortle scale rating of 3 or better, the Milky Way is a breathtaking sight. You can see structure within it: brighter knots that are star clouds, dark lanes that are dust clouds, and the subtle glow of emission nebulae like the Lagoon Nebula and the Swan Nebula embedded within it.

Best Times for Observing the Milky Way Core

The galactic core, the most dramatic and photogenic section of the Milky Way, is best visible during specific months depending on your hemisphere.

Northern Hemisphere

• Late April to May: The core rises in the pre-dawn hours (2:00 AM to 4:00 AM). Early risers get the first views of the season.
• June to August: Prime Milky Way season. The core is visible for much of the night, reaching its highest point around midnight in July. This is when most Milky Way photography happens.
• September to early October: The core is visible in the evening after sunset, setting in the southwest by midnight. Last chance until the following spring.

Essential Conditions

• Dark skies: The most important factor. You need Bortle 4 or darker to see the Milky Way well, and Bortle 2-3 to see it in its full glory. Use lightpollutionmap.info to find dark sites.
• No Moon: Plan around the new Moon. Check our Moon photography guide for lunar phase tips. Use astronomy apps to track lunar phases and find the best nights. Even a quarter Moon significantly washes out the Milky Way.
• Clear, dry air: Humidity and haze scatter light and reduce contrast. Nights after a cold front passes often provide the best transparency.
• Altitude: Higher elevations mean less atmosphere to look through. Mountain sites produce the best views.

Photographing the Milky Way

Milky Way photography is one of the most popular and rewarding forms of astrophotography, and it is accessible to anyone with a camera that has manual exposure control — see our astrophotography on a budget guide.

Camera Settings

• Lens: 14mm to 24mm on full-frame (10-18mm on crop sensor). Wider is better. Aim for f/2.8 or faster.
• ISO: 3200 to 6400, depending on your camera’s noise performance.
• Exposure: Use the 500 Rule (500 divided by focal length = max seconds before star trailing). For a 14mm lens: 500/14 = about 35 seconds. I usually start at 20-25 seconds and adjust from there.
• Focus: Manual focus set to infinity. Use live view zoomed in on a bright star to nail focus precisely.
• Format: Always shoot RAW for maximum post-processing flexibility.

Composition Is Everything

A Milky Way image without an interesting foreground is just a picture of stars. The best Milky Way photographs include a compelling earthly element that gives the image scale, depth, and emotional resonance. A lone tree, a mountain silhouette, a calm lake with reflections, a winding road, an old barn, or a person silhouetted against the sky all work beautifully.

Plan your composition before you arrive. Use apps like PhotoPills or Stellarium to preview where the Milky Way core will be positioned relative to your foreground at different times. Arrive with some twilight remaining to scout and frame your shot, then wait for full darkness.

With a Star Tracker

If you have a star tracker, you can take longer exposures at lower ISO (and for even longer exposures, consider autoguiding), capturing dramatically more color and detail in the Milky Way. Shoot 60-second to 3-minute exposures of the sky at ISO 800-1600, then blend with an untracked foreground exposure in post-processing. This technique produces images with the clean, detailed skies of tracked photography and the sharp foregrounds of stationary photography.

Satellite Galaxies and the Future

The Milky Way is not alone. It has at least 50 known satellite galaxies — many now imaged in unprecedented detail by the James Webb Space Telescope, smaller galaxies gravitationally bound to it. For deep sky viewing, explore our Messier catalog guide. The two largest are the Large Magellanic Cloud and the Small Magellanic Cloud, visible to the naked eye from the Southern Hemisphere. The LMC, at about 160,000 light-years away, contains the Tarantula Nebula, the most active star-forming region in the Local Group.

The Milky Way is also on a collision course with the Andromeda Galaxy (M31). The two galaxies are approaching each other at about 110 kilometers per second and will begin merging in approximately 4 to 5 billion years. The eventual result will be a large elliptical galaxy that astronomers have informally named “Milkdromeda.” Individual stars are so widely spaced that stellar collisions will be extremely rare, but the gravitational disruption will reshape both galaxies dramatically.

For the most detailed three-dimensional map of the Milky Way, explore the ESA Gaia mission data. But that is billions of years away. Tonight, if the sky is clear and dark, the Milky Way is waiting for you. Drive away from the city lights, let your eyes adapt for 20 minutes, and look up. What you will see is not just light. It is the combined radiance of hundreds of billions of stars, our cosmic neighborhood, seen from our singular vantage point on a small rocky world orbiting one ordinary star in the Orion-Cygnus Arm. There is nothing else quite like it.