Milky Way

How did we discover the shape of the Milky Way?

No photograph shows the Milky Way from above. The Solar System lies inside the galactic disc, so every telescope looks outward from within it. Astronomers therefore measure not only the directions of stars and gas, but also their distances and motions. Plotting those measurements on the same map reveals a long bar at the centre and spiral arms winding around it.

The pale band across the night sky is our view of the Milky Way’s disc from inside it. The name Milky Way comes from the band’s milky appearance, like a pale path across the sky. Looking sideways through the star-filled disc, countless points of light overlap to create this band. The band alone, however, cannot tell us which stars are nearby and which lie tens of thousands of light-years farther away. Interstellar dust also dims and reddens distant stars, making the galactic centre and the far side of the disc especially difficult to see in visible light.

• About 26,600 light-years Distance from the Sun to the galactic centre calculated in a 2019 study
• About 200 sites High-mass star-forming regions whose distances were measured directly with radio observations
• More than 33 million Stars in Gaia DR3 with both positions and three-dimensional velocities available
• Two arms and four arms Two stand out in the distribution of older stars, while four are clearest in gas and young stars

How do we measure the distances to nearby stars?

Knowing that a star appears toward Sagittarius does not give us one point on a map. It gives us a line extending from the Sun in that direction. We must determine where along that line the star lies before converting the observation into coordinates for a face-on view of the Galaxy. For nearby stars, astronomers use parallax: as Earth orbits the Sun, a star appears to shift slightly against the background. The smaller the shift, the farther away the star is.

Gaia observed the same objects repeatedly to measure this tiny angle and their direction of motion across the sky. Its DR3 release contains data for more than 33 million stars whose positions and three-dimensional motions can be studied together. Researchers selected about 580,000 hot, young stars and 988 clusters less than 100 million years old to trace star-forming regions within roughly 13,000 to 16,000 light-years of the Sun. This is not a map showing every star; it uses a selected population of young stars to mark the spiral arms.

Gaia observed about two billion objects, but that does not mean distances and motions are known equally well for all two billion. The available information and its uncertainty depend on an object’s brightness and distance, how crowded its surroundings are, and how much dust blocks the view. Spacecraft observations ended in 2025, but work to process and release the collected data continues.

How do we trace gas beyond the dust?

Radio emission at a wavelength of 21 centimetres from cold hydrogen, along with radio emission from carbon monoxide molecules, passes through dust better than visible light. Measuring how much the wavelength has lengthened or shortened reveals whether the gas is moving toward or away from us. Astronomers can then apply a model of galactic rotation to estimate the gas’s distance and mark long concentrations of gas as possible spiral arms.

Yet the same velocity can sometimes correspond to two different distances. Gas does not always follow perfectly circular paths, either; the gravity of the central bar and spiral arms can push it sideways. Radio observations can reach gas on the far side of the Galaxy, but a distance calculated from rotation is not always unique. Direct distance measurements made by other methods are therefore needed as reference points.

Radio waves show the direction and velocity of gas hidden behind dust. Trigonometric parallax directly measures the distance to stars forming with that gas. Both measurements are needed to place a spiral arm on the map.

Can we measure the distances to very remote star-forming regions?

In clouds where massive stars are forming, certain molecules can emit extremely intense radio waves concentrated near a single wavelength. A small region producing this emission is called a maser. Observing a maser simultaneously with widely separated radio telescopes makes it possible to measure minute changes in position. The difference seen when Earth is on opposite sides of the Sun yields a trigonometric parallax, allowing distance to be calculated without a model of galactic rotation.

By 2019, the BeSSeL and VERA teams had collected parallaxes and proper motions for about 200 such high-mass star-forming regions. Connecting the points revealed long bands associated with the Sagittarius–Carina, Perseus, Scutum–Centaurus and Outer arms, and a model with four major gas and star-forming arms described the data well. The same analysis placed the Sun 8.15±0.15 kiloparsecs, or about 26,600 light-years, from the galactic centre.

This does not mean every part of every spiral arm has been measured continuously. Masers are bright only in particular star-forming regions, and measurements remain sparse in the southern sky and on the far side of the Galaxy. Researchers connect separated points with mathematical spiral models. A smooth arm on a map can therefore contain both directly measured stretches and sections calculated between measurements.

1. Measure the direction Record where in the sky the starlight or radio emission comes from.
2. Measure the distance Use Gaia stellar parallax and maser parallax to locate the source along that direction.
3. Measure the motion Combine movement across the sky with velocity toward or away from us.
4. Compare different tracers Map older stars, young stars, clusters, hydrogen and molecular clouds separately.
5. Calculate between measurements Derive the shapes of the bar, arms and disc while marking unseen regions and uncertainty.

How did we discover the bar at the centre of the Milky Way?

Dust in front of the galactic centre blocks visible light, but infrared observations reveal many more stars there. Astronomers select red-clump stars, whose intrinsic brightnesses are similar, and estimate their distances from how bright they appear. Their mapped positions show that the central stars do not form only a round concentration; they make a long structure tilted to one side. The motions of stars and gas also cannot be explained by a circular disc that is identical in every direction.

Models that account for both positions and motions place the central bar at an angle of about 25 to 30 degrees to the line from the Sun to the galactic centre, extending roughly 16,000 light-years from the centre. The orbit of a star such as S2 near the centre provides an independent check on the centre’s position and distance. We call the Milky Way a barred spiral galaxy because separate observations of its central stars, disc gas and spiral arms join into one coherent structure.

Does the Milky Way have two spiral arms or four?

Counts of older disc stars in Spitzer infrared observations make the Scutum–Centaurus and Perseus arms especially prominent. Maps of hydrogen, molecular clouds, young stars and masers instead show four long star-forming arms. Even within one galaxy, the places where older stars are densest need not trace exactly the same lines as compressed gas where stars are just beginning to form.

The region containing the Sun was once drawn mainly as a short “Orion Spur.” Maps of young stars made with masers and Gaia now show the Local Arm as a long segment extending for at least about 26,000 light-years. There is no official boundary separating an “arm” from a “spur.” Names depend on length, the amount of star formation and how a feature appears to connect to other arms, so studies can draw slightly different arm counts and boundaries.

How much of the Milky Way map has been measured directly?

We do not need to leave the Galaxy to identify its bar, disc and spiral structure. But this knowledge does not come from one telescope or one image. Stellar parallax densely maps the nearby disc, radio waves locate gas behind dust, maser parallaxes anchor distances in remote arms, and infrared observations together with stellar motions expose the central bar.

As new data arrive, ideas about how arms connect, how long the bar is, or whether to call the Local Arm an arm or a spur may change. That does not mean earlier maps were drawn without evidence. Stating which stars and gas were used, and which distances were measured directly, lets us distinguish the confirmed parts of a Milky Way map from the parts calculated between them.

Sources

• The Astrophysical Journal — Trigonometric Parallaxes of High-Mass Star-Forming Regions
• Astronomy & Astrophysics — Gaia DR3: Mapping the Asymmetric Disc of the Milky Way
• Astronomy & Astrophysics — Gaia DR3: Chemical Cartography of the Milky Way
• ESA — Gaia Mission and Data-Release Status
• Research in Astronomy and Astrophysics — The Bar and Spiral Arms in the Milky Way
• Science Advances — The Local Spiral Structure of the Milky Way
• NASA Science — Why the Milky Way Has Its Name