Andromeda Project

M31 and PHAT

The Andromeda galaxy is the closest spiral galaxy to our own Milky Way. For a hundred years, Andromeda (also known by its Messier Catalog identifier, M31) has played an important role in shaping our view of the Universe. In the early 1920's, Edwin Hubble's observations of Andromeda confirmed for the first time that galaxies lie outside of the Milky Way, and that Andromeda must contain billions of stars. Today, Andromeda is a template for understanding how spiral galaxies form and evolve.

The Panchromatic Hubble Andromeda Treasury (PHAT) survey (survey webpage here) opens a new window on Andromeda. This four-year Hubble Space Telescope (HST) project has recently finished mapping one-third of Andromeda's spiral disk at six wavelengths ranging from the near-infrared to the ultraviolet (Dalcanton+ 2012). The HST images have exquisite resolution, allowing PHAT to reveal more than 100 million stars in Andromeda. This beautiful data set is the heart of the Andromeda Project.

Hunting for Stellar Clusters

Star clusters are collections of hundreds to millions of stars that were born at the same time from the same cloud of gas. This shared origin makes star clusters unique tools for understanding how stars form and evolve. Additionally, they are useful for studying the major chapters in the history of galaxies. But before Andromeda's star clusters can unlock these secrets, we need the help of Citizen Scientists to find the clusters. Not just the big bright ones, but the small faint ones as well. This is the goal of the Andromeda Project.

Star clusters vary greatly in terms of mass, size, age, and local environment. As a result, star clusters can appear quite different from one another depending on the properties of the clusters and where they are located in the galaxy. This makes the process of identifying clusters tricky and difficult to automate. From the first year of PHAT data, a team of eight astronomers searched through each image, manually identifying star clusters by eye. Using less than 1/5th the total PHAT survey area, we cataloged about 600 star clusters (Johnson+ 2012). With the Andromeda Project, we hope that you will help us find the thousands of star clusters hiding in the rest of the survey!

Because the appearances of star clusters vary so much, it is important for us to learn what kinds of clusters we can actually see. For this reason, we have inserted realistic synthetic clusters with known ages, masses, and sizes into some of the PHAT images. By identifying both real and synthetic clusters, we will learn what types of clusters are undetectable in Andromeda. This information is critical for understanding the age and mass distributions of the clusters by allowing us to determine whether certain populations of clusters do not exist or if they are simply avoiding detection.

After you help us to find these star clusters, we will use several techniques to determine the age and mass of these objects. In some clusters, we can resolve individual stars that allow us to determine the age, mass, and other aspects of the object. In other clusters, where individual stars are too faint or too close together, we can use the summed light from a cluster to determine its properties (Fouesneau+ 2014). With these ages and masses in hand, we can use these clusters to study a host of interesting topics: rapid and rare stages of stellar evolution, the structure and scale of star formation, the evolution of cluster populations, and how Andromeda has changed over billions of years.

Distant Galaxies Peeking Through Andromeda's Stars

Star clusters are not the only interesting things in the PHAT images. Although Andromeda's stars dominate the images, parts of the galaxy are transparent enough that we can see distant galaxies that lie far beyond Andromeda. These "background galaxies" are excellent tools for studying the structure of Andromeda's interstellar gas and dust because they can reveal details about these dark dusty structures at small size scales. The background galaxies can also masquerade as stellar clusters, so we need your help to carefully sort these objects.