A Giant Group of Siblings to the Big Dipper stars?

If you are new on this blog, welcome! In the past I have focused this website mostly for other astrophysics researchers, but I decided to try and bring our exciting science to a larger audience. This is great timing as I have recently started a new position as scientific advisor for the Montreal Planétarium Rio Tinto Alcan | Espace pour la Vie—I am hoping to keep you informed with our discoveries with similar blog posts in the future! While I’m at it, I would also encourage you to read the blogs of my colleagues André Granchamps and Marc Jobin.

In this post, I describe a new discovery from our team, in collaboration with Jackie Faherty and Mark Popinchalk from the American Natural Museum of History (AMNH) in New York city. We think we might have found a giant group of “sibling” stars that were born together with stars in the Big Dipper asterism in the Ursa Major constellation. The scientific paper describing this discovery is available here.

This blog post is also available in French here.

The birth of stars takes place in large clouds of cold gas that are denser than ordinary interstellar matter. The physical conditions inside these clouds are such that complex molecules naturally form in them—some have even been reported to contain ethanol! For this reason, we call them molecular clouds.

When part of such a molecular cloud gets compressed, whether it was caused by a collision with another cloud, a nearby star going supernova, turbulence or else, an event of star formation can be triggered. This happens because once the gas reaches a critical density, the gravitational pull that a small part of the gas exerts on other parts around it can result in a runaway effect where gravitational attraction makes the gas denser, allowing it to attract even more gas, etc., until this leads to a pressure and temperature so high that nuclear reactions start happening, converting hydrogen into helium, and liberating a tremendous amount of energy in the process: a star is born.

Matthew Bate from the University of Exeter created computer simulations that show this process in incredible detail. In the video below, we can see how the gas density goes up during star formation, as they indicated gas density with the color and brightness of the gas in that simulation:

If you pay attention, you will notice that one star already went supernova before the end of the simulation, at the lower left of the video frame. Viewing the same simulation, but this time with color and brightness tracking temperature, makes the birth of each star much clearer, because the temperature of cold gas suddenly goes up in a drastic way when nuclear reactions begin:

These star-forming events usually lead to the birth of many stars at once like this simulation shows. This whole process also takes place very rapidly in terms of astrophysical time scales, well within a million years. The result is therefore a group of stars with about the same age (we call them “coeval stars” in technical jargon), and that formed from the same molecular cloud of gas. Because they formed from the same gas, all stars in this population also have the same composition of chemical elements, something we can measure with our instruments. Sometimes, these groups are relatively sparse and diffuse, with only a couple hundred stars in them. In other situations, they can be made of thousands of stars and can be much more compact—this is the case of the well-known Pleiades cluster, and a myriad of other open clusters in our galaxy, the Milky Way.

It is useful to realize that all stars near us, and molecular clouds as well, are actually on gigantic orbits around the Galaxy, with typical orbital periods (or “galactic years”) of about 250 million years. The Sun is also on such an orbit around the Galactic center, as nicely shown by this animation constructed by the team at AMNH:

Because most stars in our galactic neighborhood are on galactic orbits with different shapes and inclinations, their motions seem random when we view them from Earth. They also look like straight lines when we only wait for a few years, because the curvature of a 250 million years-long orbit is extremely subtle on such a short, human timescale. This is demonstrated nicely by this video below, in which my colleague Jackie Faherty from AMNH shows the 3D trajectories of stars in our neighborhood as measured by the recent Gaia spacecraft, launched by the European Space Agency.

When a molecular cloud “breaks down” into a group of fresh new stars, its orbit around the Milky Way is not affected. This means that all the stars born from it are initially on a Galactic orbit similar to their parent cloud, and viewed from the Earth, they all seem to be moving in the same direction. This does not remain the case forever, however; stars continually pass close to other molecular clouds, stellar clusters, or other stars along their orbits, and those gradually tug them in random directions and slowly changes their orbits.

Because every one of these sibling stars will encounter slightly different perturbations over time, their orbits will be affected in different ways, and they will eventually end up with completely different orbits and velocities across the Milky Way. In this sense, our galaxy is a bit like a big mixer that slowly scatters away all the members or star clusters. Typically, it takes at least a couple orbits around the Galaxy before these clusters get completely mixed beyond our ability to recover them—clusters that initially contain more stars are also more robust and will typically survive more orbits. The Sun, which is about 4.6 billion years old, has long lost its siblings, and they are now somewhere in our galaxy, with motions completely different. Because of this, we are unable to tell which stars in our galaxy were actually born from the same cloud as the Sun.

One of the best examples we have of this process of cluster “evaporation” is the Ursa Major cluster, an ensemble of only a dozen massive stars, located very close to us (only about 80 light years away from the Sun), and that are all moving in the same direction. Six of these stars—Alioth, Mizar, Merak, Phecda, Megrez, and Alcor,—are part of the well-known Big Dipper asterism.

To give you an idea of how close to the Sun this is, I recommend watching this video that shows all known stars within that distance (80 light years). The Ursa Major group really is the immediate backyard if the Sun!

This population of stars that make up Ursa Major is, however, quite weird: they are all relatively massive stars, from about 1.5 times to 2.5 times the mass of the Sun. Such massive stars normally only make up a small fraction of the stars that form inside a molecular cloud. For every star like that, we would normally expect to find about 20 more siblings that are much smaller (with masses of about 0.1 to 0.5 times that of the Sun), as well as a couple others with masses in between!

The thing is, there are no such small stars around the Ursa Major cluster, and we did look! In two of my most recent scientific papers (here and here), I scanned the neighborhood of the Sun for members of many groups of stars like Ursa Major, and I found many of them, but barely anything associated with Ursa Major. We know from various techniques of astrophysics that the stars in Ursa Major are about 400 million years old, meaning that they already had time to go around their galactic orbit more than once. Because smaller stars are easier to pull away from the cluster, the Ursa Major association we see today might just be a remnant of what was once a glorious, massive cluster that got completely torn apart by many small interactions with other clusters, clouds and stars it encountered in its past orbit.

This is actually not a new idea, and papers dating back from 1992 even identified about 40 stars in the Sun’s neighborhood that move in a somewhat similar way to Ursa Major stars, and could therefore be related. Still, their relation to Ursa Major is not yet confirmed, and we should expect Ursa Major to have contained many more than these 40 additional stars when it formed.

The Gaia telescope, which Jackie talked about in the video above, is revolutionizing our knowledge of stars in our neighborhood, especially how they move around over time. The Gaia spacecraft measured the precise distance and motions of more than a billion stars! This allowed our teams and others to make many recent discoveries about entirely new groups of stars—see, for example, this recent science paper that Jackie led, or this one that I led during my first postdoc at Carnegie Institution for Science in Washington, D.C. A team of Western Washington University recently published a really cool study where they identified a whopping 1,640 groups of stars that seem to be moving together, many of which were previously unknown. Marina Kounkel, the first author of that study, also created this cool 3D interactive figure that allows you to fly around and look at these structures. I also made this video where you can see all of the groups they uncovered:

When I read this study, I decided to look at the details of these newly identified groups of stars, and began comparing them to the well-known groups of stars in our immediate neighborhood. This led me to realize something interesting: a couple of the structures that Marina Kounkel and her collaborator discovered, totaling 1,600 stars, are moving in a very unusual direction, and that direction is similar to the direction where Ursa Major stars also move!

The 1,600 stars moving like Ursa Major stars also seem to be aged about 400 Myr, and even more interesting, they are distributed along two large tails that extend up to a whopping 900 light years from the Sun! If you are wondering, only two of these 1,600 stars are relatively easily visible to the naked eye (the stars 30 Cep and HD 3856), and they are nowhere near Ursa Major on the night sky (30 Cep is in the Cepheus constellation, and HD 3856 is in Cassiopeia).

Our previous searches for missing Ursa Major members did not recover these extended structures, because we were focusing on stars too close to the Sun, and also because their velocities have started being mixed around more than we expected while the cluster is getting ripped apart. Here’s a 3D view of the Ursa Major group I made. After zooming in on the classical Ursa Major core members, I turned on these gigantic new structures that I think may be related to Ursa Major.

You can notice that there is a gap between the core of Ursa Major and the two tails; this is probably not real, and instead I think it is a consequence of the computer algorithms that the Western Washington University team used to search for groups of stars. If you go back to the video above where I showed all the groups they uncovered, you will see that they found almost nothing within almost 200 light years from the Sun—exactly where the gap happens—so if I’m right about this, we should be able to find some more stars related to Ursa Major between the Sun and the inner edge of the two large tails, and thus close the gap.

You might be wondering why this large structure hasn’t been noticed before, and that’s a reasonable question. The reason why it is so hard to find large and diffuse groups of stars is simply related to the vast number of other, unrelated, stars that they are hidden amongst. For example, if I only show the brightest 10% of all stars within about 700 light years from the Sun (the region that includes the possible Ursa Major tails), and slowly move toward the Sun, you will get an idea of how many stars there are in our neighborhood:

We just published our result in the Research Notes of the American Astronomical Society journals, but a lot of work remains to be done. We are still missing the detailed velocity of some stars in this large, new structure, and there are many stars in there, even some white dwarfs, that can be studied in greater details to test our hypothesis that they could constitute the missing population of the Ursa Major cluster.

Something else we need to elucidate is why the smaller stars in Ursa Major seem to have been spread so far from its core. Similar extended structures (in technical jargon, “tidal tails”) have recently been uncovered around other similar clusters, like the ~600-million-years-old Coma Berenices open cluster (discovered in 2019 with Gaia). However, in this example, the extended structure only spans about 130 light years. Below, I made another video demonstrating how the tidal tails of Coma Berenices are much smaller than those we might have just found around Ursa Major.

If these structures are really the tidal tails of Ursa Major, this discovery opens up a few possibilities: are there similarly gigantic tidal tails around Coma Ber, and we just haven’t found them yet? Maybe they also consist of stars with velocities that are more mixed around than we expected. Otherwise, could the larger tails of Ursa Major be an indication of a collision between it and another cluster, having caused Ursa Major to get ripped apart more violently? If that happened, then it will be interesting to study the past trajectory of Ursa Major around the Milky Way and try to understand what it might have collided with.

I would like to thank Marc Jobin and André Granchamps for their comments.