Fast Facts About Moving Groups of Stars in the Milky Way


Beta Pictoris


Stars are born in nurseries with a collection of siblings that range in mass but share chemical compositions and dynamic histories.  As time passes, the tightly bound nursery that they were born inside dissipates with increased gravitational interactions from outside sources (passing giant molecular clouds or stars).  One form of evolved stellar nurseries is called a “moving group”.  In this article I will review four fast facts about these loosely bound collections of co-moving, co-evolving stars.

Jackie Faherty, astrophysicist at the American Museum of Natural History and Editor at The Daily Galaxy.

Fast fact #1 A moving group is so named because it’s a loosely bound collection of stars that is co-moving through the Galaxy 

Sometimes when we look up at the sky we catch a really obvious over density of stars.  Something that strikes the eye immediately as a pattern.  One such example is the Pleiades star cluster which has several strikingly blue and beautiful bright stars (often referred to as the seven sisters even though there are far more than seven of them) that is easily spotted in the northern sky.  Moving groups are not quite that.  They wouldn’t strike your eye when you look up.  Whereas the Pleiades contains hundreds of stars surrounded by a wispy blue nebulosity occupying no more than 1-2 degrees of the sky; moving groups are spread across tens to more than a hundred degrees of the sky and are more loosely bound. Some moving groups the Sun is actually passing through therefore we can look in either the northern or southern hemisphere and find members.  The AB Doradus and Beta Pictoris moving groups are examples of loosely bound collections of stars that the Sun just seems to be passing through.    

Fast fact #2 Moving groups contain relatively young stars with common compositions

The star formation process begins with a giant molecular cloud that collapses and fragments pieces that ignite into young stars.  The number of stars that can form varies and will be dependent on the size of the natal environment.  After enough time passes, the stars –once tightly packed together — will begin to move apart.  Interactions with large structures in the Galaxy (other giant molecular clouds or stars for instance) will help break apart the cluster, separating original siblings until they find a path through the Galaxy on their own.  A moving group may very well be a fragment of a dissolving more dense open cluster.  They may also be the result of a less dense fragmentation of a giant molecular cloud.  Either way, moving groups are an evolved state of star formation.  The stars within them have a similar composition.  Meaning that the abundances of certain molecules, and the ratios of certain elements, will be consistent across all the stars in a given group.  We often say that these collections of stars share a metallicity.  So if you were to take the light of a given star, pass it through a prism and then measure how much Iron was present, you would find a consistent number across all the stars that you looked at.  Moreover, given that moving groups by their very nature still contain stars that are bound, they span a relatively young age (see the fast facts on young stars to understand what “young” really means).  Notable nearby moving groups within 300 light years of the Sun range in age from 10 Myr to just over 500 Myr.  

Fast fact #3 The most famous moving group in the sky is the Ursa Major moving group


Big Dipper


Arguably one of the most famous moving groups in the nighttime sky is the Ursa Major moving group.  The name “Ursa Major” might not ring a bell to you but the asterism within it called “The Big Dipper” should.  This is one of the most notable arrangements of stars that humans have ever mapped.   Interestingly, the vast majority of constellations are composed of stars at different distances that have absolutely nothing to do with each other.  However within Ursa Major and specifically within the Big Dipper, the stars have everything to do with each other.  Five of the seven stars — called Merak, Phecda. Megrez, Alioth, and Mizar — are co-moving and co-evolving at a common distance of ~80 light years from the Sun.  Dubhe — the cornerstone star of the dipper– and Alkaid — the far star in the handle of the dipper — are the only two not a part of the moving group. If you were to move time forward or backwards rapidly so you could see stars on their trajectories around the Galaxy, you would note that the Big Dipper holds together as a grouping of stars.  In total there are just 15 stars which define the core of the Ursa Major moving group.  Our best estimate of the age of these stars is ~400 Myr.

Fast fact #4 Moving groups are close and young so they are the targeting ground for directly-imaged planets

Within 300 light years of the Sun there are more than 20 of these loosely bound moving groups.  Given that our solar system is passing through some of them, there can be members as close as a handful of light years away.  This means that the closest, young stars to the Sun are moving group members.  Campaigns are underway to characterize these close young stars as they might harbor gas giant planets that we can directly image while they are still hot from their birth.  Corongraphs or adaptive optics surveys on large telescopes such as NASA’s Keck, the Very Large Telescope (VLT) or Gemini Observatory have found success at directly imaging massive Jupiter-like gas giant planets that are orbiting moving group stars such as Beta Pictoris (of the Beta Pictoris moving group), HR8799 (of the Columba moving group), and AB Pictoris (of the Tucana Horologium moving group).

Image credit: Beta Pictoris, Carnegie Institution, processed by University of Arizona and JPL, courtesy of NASA. Image from the 2.5-meter telescope at the Carnegie Institution’s Las Campanas Observatory in Chile.

Jackie Faherty, astrophysicist, Senior Scientist with the American Museum of Natural History. Jackie was formerly a NASA Hubble Fellow at the Carnegie Institution for Science. Aside from a love of scientific research, she is a passionate educator and can often be found giving public lectures in the Hayden Planetarium. Her research team has won multiple grants from NASA, NSF, and the Heising Simons foundation to support projects focused on characterizing planet-like objects. She has also co-founded the popular citizen science project entitled Backyard Worlds: Planet 9 which invites the general public to help scan the solar neighborhood for previously missed cold worlds. A Google Scholar, Faherty has over 100 peer reviewed articles in astrophysical journals and has been an invited speaker at universities and conferences across the globe. Jackie received the 2020 Vera Rubin Early Career Prize from the American Astronomical Society, an award that recognizes scientists who have made an impact in the field of dynamical astronomy and the 2021 Robert H Goddard Award for science accomplishments.

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