This site uses cookies to offer you a better browsing experience. Read about how we use cookies.
In this multi-age astronomy activity, students explore the dynamic relationships between galaxies using tactile 3D models of the Sagittarius stellar stream, the Local Group of galaxies, and generic spiral and elliptical galaxies. Using these 3D models that were developped in the A Touch of the Universe" project, students explore how galaxies interact, evolve, and form structures in space. Through tactile exploration, guided observation, and mathematical reasoning, students gain insight into the dynamics of the Milky Way’s environment and the cosmic forces shaping it.The activity is designed to be inclusive and hands-on, making complex cosmic phenomena accessible to all learners, including those who are visually impaired.
Image: the two 3d models of an elliptical galaxy (left) and a spiral galaxy (right).
Ages 10-12:
Ages 12-16:
Ages 16-19+:
Large galaxies have dozens of dwarf galaxies orbiting around them called satellite galaxies because they revolve around a larger host galaxy.
Satellite galaxies are under the influence of the intense gravitational force of the host galaxy, which in some cases causes them to gradually dismember due to the tides it induces: the host galaxy attracts gas, dust and, above all, stars from the satellites as they revolve around it, with stellar arcs appearing along their trajectories in space. These tidal forces are of the same kind as those that produce tides on Earth's oceans. The resulting streams are technically called, therefore, "tidal stellar streams".
One of the Milky Way's satellite galaxies is the Sagittarius Dwarf, a small galaxy with the appearance of an elongated ball and about 10,000 light years in diameter, captured by our own about 4 or 5 billion years ago. Its orbit around the Milky Way is polar (i.e. it moves perpendicular to the disk of our galaxy passing through the north and south galactic poles), approaching as close as 50,000 light years to the center of our galaxy.
The tidal forces exerted by the Milky Way on Sagittarius are causing its gradual dismemberment. In the sky we can see arc-shaped structures that are the trails of stars that are being torn away from the dwarf galaxy. This stellar stream is not the only one that has been identified in our Galaxy, but it is the most prominent and best studied, in particular by ESA's Gaia satellite.
Because the stream is inside the Milky Way halo, the Sagittarius stars are gradually becoming part of the Milky Way, as a result of the collision between both galaxies.
Sagittarius Dwarf, the Milky Way and a few more other galaxies are the members of a galaxy group called the Local Group.
Image: Artistic impression of Local Group and nearest galaxies (the photos of galaxies are not to scale). Credits: Antonio Ciccolella - Own work; Link to wikipedia page
Galaxy groups are gatherings of at most about 50 galaxies bound together by the force of gravity, confined to a region of space a few million light years across.
Our galaxy, the Milky Way, is bound by gravity to other nearby galaxies, forming the Local Group. This collection of galaxies has a diameter of about 10 million light years. It is part of the Virgo cluster, which in turn is within the larger Laniakea supercluster.
Andromeda (also known as M31) is the largest galaxy in the Local Group. It is estimated to have a diameter of 200,000 light years and is home to 1 trillion (1012) stars. The Milky Way is the second largest galaxy in the Local Group, with an estimated diameter of about 100,000 light years. It has about 100 billion (100x109) stars.
Both Andromeda and the Milky Way are spiral-type galaxies. Very close to both of them we find dwarf spheroidal and irregular galaxies, which are their satellites. Among the satellite galaxies of the Milky Way, the Large Magellanic Cloud, the Small Magellanic Cloud and the Sagittarius Dwarf stand out as the largest.
The Milky Way and Andromeda are approaching each other at the radial speed of about 110 kilometers per second, so they could collide in about 7-8 billion years (Sawala, 2025). The uncertainties in the tangential velocity of Andromeda and the role played by other galaxies, like M33, make it very difficult for astronomers to predict with certainty this collision.
Because the space between stars is quite large, when two galaxies collide, we do not expect stars directly crashing against other stars - they will just change their locations as a consequence of the re-distribution of matter in the newly formed galaxy. Collisions of galaxies are better described as mergers: stars, gas and dust from both galaxies will mix up, and the rise of regions with large contents of dust and gas will induce the formation of new generations of stars in them.
The final fate of the Local Group could be to become an enormous single elliptical galaxy, tens of billions of years from now.
To better illustrate the intricacy of the Sagittarius stellar stream and the Local Group of galaxies, and to make them accessible also to visually impaired publics, we will use two 3D models developed as part of the "A Touch of the Universe" 3D astronomy model collection.
In this 3D model, we easily identify the Milky Way at the center, with its characteristic spiral arms around the bulge, or nucleus. Attached to the disk is an elongated bulge: it is the Sagittarius spheroidal dwarf galaxy, which is currently very close to the disk of our galaxy.
Starting from Sagittarius and perpendicular to the galactic disk, we can follow the structure of the stellar stream formed by the trail of stars that the Milky Way has stolen from Sagittarius. The small bulges represent stars, and the lattice in which they are found can be interpreted as the gas and dust they have dragged with them.
The presence of loops in the stream indicates that Sagittarius has completed several orbits around the Milky Way, roughly always in the same plane of space passing through the galaxy’s north and south poles, as expected from the laws of Physics.
In the model, the size of the Milky Way’s disk has been exaggerated to be able to connect it to the stellar stream, which in reality is not attached to our galaxy's disk.
The model has been created by Emilio Terol (OAUV), Alberto Fernández Soto (IFCA) and David Martínez Delgado (CEFCA) using real data obtained by the European satellite Gaia, which has measured the positions of one billion stars with great precision.
In this 3D model, the brightest galaxies in the Local Group are shown. Their relative positions are accurate, according to the astronomical data available.
The galaxies are "suspended" in the dark matter lattice in which the Local Group is immersed. This halo is not shown in its entirety: we have left gaps so that all the galaxies inside it can be accessed with your hands.
The shape of each galaxy reflects its actual morphology: spiral, spheroidal or irregular.
The largest spiral galaxy is Andromeda. Attached to it in an almost perpendicular position, we find another spiral galaxy: the Triangulum galaxy, the third largest in the Local Group.
Above and below the Andromeda disk we find satellite galaxies, represented by spheres, since most are dwarf galaxies of the spheroidal type. A little further away we also see some irregular ones.
Near the center of the model, we find our Milky Way Galaxy, with its spiral shape. Attached to it are its satellite dwarf galaxies and the two Magellanic Clouds, which barely stand out from the other satellite spheroidal galaxies, but which we can identify by their spiral and irregular shapes and because their ends are sharp to touch. The spiral one is the Large Magellanic Cloud; the irregular one is the Small Magellanic Cloud. The Sagittarius dwarf galaxy is also very close to our Milky Way.
The model has been created by Emilio Terol (OAUV) and Alberto Fernández Soto (IFCA) using real data on distances and positions of the galaxies of the Local Group.
Getting ready:
Activity (ages 10-12):
Activity (ages 12-16):
Activity (ages 16-19+):
All education levels: Check if the student is correctly answering the questions and mathematical problems proposed throughout the activity.
Natural Sciences:
Technology:
Physics & Earth Sciences:
Mathematics:
Physics and Astronomy:
