Like early explorers mapping the continents of our globe, astronomers are busy charting the spiral structure of our galaxy, the Milky Way. Using infrared images from NASA's Spitzer Space Telescope, scientists have discovered that the Milky Way's elegant spiral structure is dominated by just two arms wrapping off the ends of a central bar of stars. Previously, our galaxy was thought to possess four major arms.

The annotated artist's concept illustrates the new view of the Milky Way. The galaxy's two major arms (Scutum-Centaurus and Perseus) can be seen attached to the ends of a thick central bar, while the two now-demoted minor arms (Norma and Sagittarius) are less distinct and located between the major arms.

The Milky Way Galaxy

Like early explorers mapping the continents of our globe, astronomers are busy charting the spiral structure of our galaxy, the Milky Way. Using infrared images from NASA's Spitzer Space Telescope, scientists have discovered that the Milky Way's elegant spiral structure is dominated by just two arms wrapping off the ends of a central bar of stars. Previously, our galaxy was thought to possess four major arms.

The annotated artist's concept illustrates the new view of the Milky Way. The galaxy's two major arms (Scutum-Centaurus and Perseus) can be seen attached to the ends of a thick central bar, while the two now-demoted minor arms (Norma and Sagittarius) are less distinct and located between the major arms.

Our Galaxy

The Milky Way Galaxy

The major arms consist of the highest densities of both young and old stars; the minor arms are primarily filled with gas and pockets of star-forming activity.

The artist's concept also includes a new spiral arm, called the "Far-3 kiloparsec arm," discovered via a radio-telescope survey of gas in the Milky Way. This arm is shorter than the two major arms and lies along the bar of the galaxy.

Our Sun lies near a small, partial arm called the Orion Arm, or Orion Spur, located between the Sagittarius and Perseus arms.

We now know that the Milky Way resides within the Local Group of galaxies, made up of over 30 galaxies including Andromeda, Triangulum and Leo I to name but a few. It turns out that it's pretty good to know who your neighbors are, as they may be closer than you think. The Milky Way is currently hurtling towards Andromeda at 250,000mph (400,000 km/h). Though there is no need to worry just yet, this crash of cosmic proportions is not due for another 4 billion years. 

NASA and other space entities have been observing distant galaxy collisions for decades now to get a sense of what we might be facing when Andromeda and the Milky Way collide. The short story is there is little to worry about; the longer tale is the process is an interesting one as it shows how galaxies may evolve.

For example, observations of a three-way galactic collision in 2022 using the famed Hubble Space Telescope gave some intriguing insights. The largest of the group, as it got into a tight orbit with the other two, snagged some material with its relatively stronger gravity. This created an intriguing streak of gas, dust and other materials flowing into the larger galaxy, visible even from Earth.

While the arms of the Milky Way will surely be ripped up by this process, individual stars are relatively safe as the spaces between them are quite large. In other words, don't look for star collisions, as they will be practically non-existent. Starbirth, however, will accelerate due to the amount of gas being pumped into our galaxy, causing our galaxy to brighten and for its population to expand in the coming millions of years following the collision.

Our own solar system, therefore, should be relatively safe due to the low risk of star collision. That said, we may find ourselves thrown into a completely different path around the new galactic center as the merger pushes through. 

One practical effect is that the constellations we observe from Earth may change as star orbits alter or new stars are added into the mix; that said, the collision is happening so far in the future that the constellations we see today may be altered in any case, due to natural starbirth and star death outside of the collision. This Milky Way timelapse shows how the night sky will shift over time.

Studying the Milky Way used to be notoriously difficult. Astronomers sometimes compare the effort to attempting to describe the size and structure of a forest while being lost in the middle of it. From our position on Earth, we simply lack an overview. But two ground-breaking space telescopes launched since the 1990s have helped usher in the golden age of Milky Way research. Major strides have been made, especially since the 2013 launch of the European Space Agency's (ESA) Gaia mission. 

Telescopes enabled astronomers to distinguish the basic shape and structure of some of the closest galaxies before they knew they were looking at galaxies. But reconstructing the shape and structure of our own galactic home was slow and tedious. The process involved building catalogs of stars, charting their positions in the sky and determining how far from Earth they are.