Tracing the spiral arms of the Milky Way

Our galaxy, the Milky Way, is thought to be a huge spiral galaxy like many such galaxies we see across the Universe. One of the tasks that modern-day astronomers are trying to achieve is to map the size and structure of our galaxy so we can better understand how it formed and how it will evolve in the future.

The spiral galaxy Messier 100 - similar to our own Milky Way? (Credit: ESO)

One of the key tasks in such work is to map the spiral arms of our galaxy. This is important because spiral arms are thought to be where the majority of dense gas is found in galaxies, and therefore where the majority of star formation takes place. Spiral arms aren't fixed objects though, the stars in our galaxy actually move in and out of the spiral arms as they orbit within our galaxy. Spiral arms are actually thought to be density waves that rotate around our galaxy, independently of the stars in our galaxy, just like waves in the ocean move independently of the water in them.

Spiral arm model of the Milky Way with four arms.
The Sun is located towards the top of this image.
(Credit: Georgelin & Georgelin 1976)

Identifying spiral arms is easy when you're outside of a galaxy and looking at it face on, but its much harder when you're embedded within the galaxy and all you can see is the plane of our galaxy. We can't directly see the spiral arms of our galaxy, but we can trace their presence by looking for signposts that identify them. Signposts such as giant molecular clouds, star forming regions, and bright young stars are all indicators of where spiral arms are found.

The Milky Way was first identified as a spiral galaxy thanks to the work of William Morgan from Yerkes Observatory who showed that the distribution of bright and hot OB stars, which are known to be very young objects, appear to be distributed in spiral arms. Morgan identified three spiral arms, which he labelled the Perseus, Orion and Sagittarius arms.

Later studies that attempted to discern the spiral structure of the Milky Way used the radio emission from hydrogen gas to trace its structure, but it can be tricky to determine the distance to such gas, making it hard to reveal the 3-dimensional structure.

A major breakthrough came in the 1970s when scientists combined radio measurements of hydrogen gas with optical measurements of the distances to the young stars associated with the gas. This work lead to a model made up of four spiral arms called the Norma, Scutum-Centaurus, Sagittarius and Perseus arms. While many researchers debated the distances to the various star forming regions used for this model (and therefore the exact structure and number of spiral arms the model predicted), this picture was for over 30 years the standard model of the spiral structure of the Milky Way.

The model changed again in 2008 thanks to data from NASA's infrared Spitzer Space Telescope, which allowed astronomers to count the number of stars all the way across our galaxy. The number of stars they counted suggested that there weren't four spiral arms, but only two, with a number of smaller spiral arms lying in between them.

Artist's conception of our new view of the Milky Way's structure thanks to results from the Spitzer Space Telescope.
The Sun's position is marked towards the bottom of this image.
(Credit: NASA)

This new model suggests that the Perseus and Scutum-Centaurus arms are the two major arms, while the Norma and Sagittarius arms are actually relatively minor arms. The two major arms connect up with the inner Galactic Bar, which dominates the central part of our Milky Way and may also play a role in the origin of the spiral arms.

Recently a flurry of results have taken this work even further with suggestions of a new and distant spiral arm that wraps completely around one side of the galactic centre, while other researchers have started using the distribution of star clusters to trace the structure of the Milky Way. Further improvements in the model of our galaxy's structure have come thanks to improved distance estimates for many of the stars and clusters in our galaxy, allowing the exact size and extent of the galaxy to be better determined.

Upcoming missions such as the Gaia observatory that will determine the distances to a billion stars across our galaxy will dramatically improve our understanding of our galaxy's size and shape. The motions that the Gaia spacecraft will measure will allow astronomers to study the orbits of these stars as well, improving our understanding of our galaxy from a purely structural model to a more advanced dynamical model.