Satellite galaxies of the Milky Way

Our big galaxy has many small friends. These are called satellite galaxies. There are 61 small galaxies near us. You can see two with just your eyes. They are the Large and Small Magellanic Clouds. One small galaxy is called Sagittarius. It is very large for a small friend. The big galaxy is pulling on it. This pulls gas away from the small galaxy. This makes the small galaxy lose its gas. Some small galaxies are still far away. They still have lots of gas left. It is amazing how galaxies move together.

Our Milky Way galaxy has many small neighbors. We call these satellite galaxies. Scientists have found 61 of them. Most are too faint to see. You can see two with just your eyes. These are the Large and Small Magellanic Clouds. People have seen them since prehistory.

Some small galaxies orbit our big galaxy. The largest one is the Sagittarius Dwarf Spheroidal Galaxy. It is about one twentieth the size of the Milky Way. This small galaxy is being pulled apart. It is being consumed by the Milky Way. It will pass through our galaxy in 100 million years. This leaves behind a trail of stars called the Sagittarius Stream.

Gravity changes these small galaxies too. The Milky Way has a halo of hot gas. This gas strips cold gas away from the satellites. This happens to galaxies that orbit close to our center. Galaxies that stay far away keep much more of their gas. Some small galaxies may even orbit other satellite galaxies. The Local Group is the big cluster where we all live.

Our Milky Way galaxy is not alone in space. It has many smaller neighbors called satellite galaxies. These small galaxies are part of the Milky Way subgroup. This group is part of a much larger cluster called the Local Group. Scientists have confirmed 61 small galaxies near our own. Not all of them orbit the Milky Way directly. Some might actually orbit other small satellite galaxies instead.

Gravity changes how these small galaxies look and act. The Milky Way has a huge halo of hot gas. When a satellite galaxy orbits close to our center, it hits this gas. This interaction strips cold gas away from the smaller galaxy. Because of this, satellites near the center have very little hydrogen gas. Galaxies that orbit much further away can keep plenty of gas.

Humans have known about some neighbors for a very long time. The Large and Small Magellanic Clouds are visible to the naked eye. People have observed them since prehistoric times. In 2006, the Hubble Space Telescope took new measurements. These measurements suggest the Magellanic Clouds move very fast. They might be moving too fast to stay in orbit around us.

Many different types of small galaxies exist in our neighborhood. The Sagittarius Dwarf Spheroidal Galaxy is the largest orbiting satellite. It is about one twentieth the diameter of the Milky Way. This galaxy is being consumed by our own galaxy right now. It will pass through the Milky Way in 100 million years. This leaves a trail of stars called the Sagittarius Stream.

Scientists continue to find new neighbors using modern tools. They discovered the Antlia galaxy in 2018. They found the Crater II galaxy in 2016. Some very small ones, like Leo K, were only found in 2024. These discoveries help us map the Local Group. Each new name adds to our map of the sky.

The Milky Way is not an isolated system of stars. It is surrounded by many smaller neighbors known as satellite galaxies. These galaxies are gravitationally bound to the Milky Way. This collection of neighbors is called the Milky Way subgroup. The subgroup is itself part of a much larger collection called the Local Group. Scientists have confirmed 61 small galaxies within this neighborhood. Some of these satellites might orbit the Milky Way directly. Others may actually orbit different satellite galaxies instead of our own.

Gravity and gas play a major role in how these galaxies look. The Milky Way is surrounded by a massive dark matter halo. Inside this halo, there is also a dense halo of hot gas. When a satellite galaxy orbits close to the center, it interacts with this hot gas. This interaction causes a process where the Milky Way strips cold gas away from the satellite. Because of this, satellites that orbit near the edge of the disc are often depleted in hydrogen gas. However, satellites that orbit at much greater distances can still hold onto large amounts of gas.

Not all satellite galaxies are the same. They are categorized by their shapes and how they appear. Some are classified as dwarf spheroidal galaxies, often shortened to dSph. Others are irregular galaxies, labeled as Irr. Some are dwarf elliptical galaxies, or dE. The Sagittarius Dwarf Spheroidal Galaxy is a notable example. It is the largest orbiting satellite of the Milky Way. Its diameter is roughly one twentieth that of the Milky Way itself. This galaxy is currently being consumed by the Milky Way. It is expected to pass through our galaxy within the next 100 million years.

Humanity has been aware of some of these neighbors for millennia. The Large and Small Magellanic Clouds are the only satellites visible to the naked eye. People have observed these two clouds since prehistoric times. In 2006, the Hubble Space Telescope provided new data about them. These measurements suggest the Magellanic Clouds move at very high speeds. Because of this speed, they may actually be moving too fast to be in a stable orbit around the Milky Way.

Modern astronomy has expanded our map of these neighbors significantly. Many galaxies were discovered very recently using advanced technology. For example, the Antlia galaxy was confirmed in 2018. The Crater II galaxy was identified in 2016. Even more recent discoveries include Leo K and Leo M, which were found in 2024. These new findings show that our understanding of the Local Group is still growing. Each new discovery helps scientists understand the complex gravitational dance of our cosmic neighborhood.

Astronomers also study the remains of galaxies that have already been affected by the Milky Way. When a galaxy is pulled apart by gravity, it can leave behind structures. The Sagittarius Stream is a long trail of stars in a polar orbit. This stream was leeched from the Sagittarius Dwarf galaxy. Another example is the Virgo Stellar Stream. Scientists believe this stream was once an orbiting dwarf galaxy. Over time, the Milky Way's gravity completely distended it, stretching it into a stream of stars.

Understanding satellite galaxies helps us learn about the larger universe. They provide clues about how galaxies form and change over time. By studying the gas levels and orbits of these 61 confirmed neighbors, we learn about the dark matter halo. We also see how gravity can shape the very structure of space. The relationship between the Milky Way and its satellites is a small part of the much larger Local Group. This group shows us how gravity organizes matter on a massive scale.