How do you differentiate a galaxy from a mere group of stars? It's easy, isn't it? A galaxy is a large collection of millions or billions of stars, while a star heap only has about a thousand. Well, this type of thinking will not be a ph.D. In astronomy! Seriously, the border between the galaxy and the star cluster is not always clear. A typical example, UMA3/U1.
It is easy to distinguish galaxies like Andromeda and the Milky Way. They are large, gravitatively bound and dominated by dark matter. It is also easy to distinguish star clusters like the Pleiades. They are loosely bound star groups without dark matter. But for a kind of small dwarf galaxy, which is known as the ultra-loved dwarf (UFDS), the dividing line becomes blurred.
UFDS are dominated by dark matter. The mass of the Milky Way is, for example, about 85% dark matter. Ultrafaine dwarf galaxy can have a thousand times more dark matter than shining matter. That's why they are so weak. Since Ufds often contain some of the oldest stars in the universe, astronomers love to study them for references to the origins of galaxies. That brings us to Uma3/U1.
Even his name tells us that there is a problem. If the object is actually a dwarf galaxy, its name Ursa Major III should be, since it is a satellite enaxy in the Ursa Major constellation. If it is an old star cluster, it should be called unions 1, as it was discovered by the ultraviolet near the optical infrared -optical north survey (unions). If it is a galaxy, it is the smallest and dark material galaxy that has been discovered so far. If it is a star cluster, it is the oldest star cluster, which has been discovered so far so far, at the age of around 11 billion years.
UMA3/U1 is almost tiny. There are only 20 light years, contains only about 60 stars and has a visible mass of only 16 suns. In comparison, the Pleies has about the same diameter, but contains more than 1,000 stars and 800 solar masses. The real question for UMA3/U1 is whether it is dominated by dark matter.
In a recently carried out study, the team examined several tests to distinguish star clusters and dwarf galaxies. Their first approach was to look at the dynamics of the visible stars, provided that it is a star cluster. Based on their known applications, the team simulated how long it would take for the stars to be free, a process that is known as evaporation. Based on its simulations, the cluster could survive another 2-3 billion years. This is a good fraction of the estimated age of 11 billion years, which indicates that U1 is simply a stable star cluster.
The second test that the team applies for is what is called a mass function. This is a diagram of the way in which the mass of the cluster varies with the distance. If it is a cluster, the mass should be more evenly distributed, but if it is a galaxy, stars should be clapped towards the middle. Here the data is less conclusive. The distribution of visible stars is a decent agreement with the cluster model, but for a galaxy, the central stars would be mainly white dwarfs and neutron stars, which are too weak to distinguish them with current observations.
Overall, the evidence for UMA3/U1 tends to be a star cluster, but the team notes that further observations of other UFDs are finally necessary. Fortunately, upcoming telescopes such as Vera Rubin Observatory will discover many other weak dwarfs in time.
Reference: Devlin, Scot, Holger Baumgardt and Sarah M. Sweet. “Uma3/U1 Realate: Stern Cluster or the smallest known galaxy?” Monthly announcements by the Royal Astronomical Society (2025): Staf572.
