Galactic tide

Big groups of stars pull on each other.

This pull can stretch them out. It can even pull stars away. This makes long tails of stars.

These tails can look curved. Two galaxies can even crash into each other. This can pull gas and stars out of their homes. This pull can also affect small groups of stars. It can pull stars from a small group to a big one. It can even move icy rocks in our own space.

These rocks can then become comets. It is a big dance in space.

Galaxies have a strong pull. This is called gravity. A galactic tide happens when this pull stretches things.

This often happens when two galaxies pass near each other. The pull can stretch the galaxies into long shapes. These shapes are called tidal tails. They are made of stars and gas.

The Mice Galaxies are a good example of this. Another example is the Antennae Galaxies.

Small galaxies can also feel this pull. A big galaxy can strip stars away from a small one. This is called tidal stripping. The small galaxy M32 is near the Andromeda Galaxy. Andromeda may have stripped the arms from M32.

Even our own space feels this. Far away from the Sun, there is a shell of icy rocks. We call this the Oort cloud.

The pull from our galaxy can stretch this cloud. This pull can nudge the icy rocks. Some rocks fall toward the Sun. They become comets. Up to 90% of comets may come from this way.

A galactic tide is a special kind of pull. It happens because of a galaxy's gravity. This force can stretch objects in space.

It is not just about how strong gravity is. It depends on the change in gravity across an area. This is called a gradient. Most of these effects happen near a galaxy. They can change how galaxies and stars look. They can even move icy rocks in our own space.

When two big galaxies pass near each other, they feel huge forces. They rarely hit each other head-on. Instead, the pull stretches them along an axis. This axis points toward the other galaxy. As they orbit, parts of the galaxies get pulled away. These parts are sheared off by the galaxy's own rotation. They fly into space as long, curved shapes called tidal tails.

Sometimes, a bridge of material forms between the two galaxies. This happens if one galaxy is much larger than the other.

Small satellite galaxies also feel these forces. A large galaxy can use its gravity to strip stars away. This is called tidal stripping.

The dwarf galaxy M32 is a satellite of the Andromeda Galaxy. It may have lost its spiral arms this way. The pull can also squeeze gas clouds inside a small galaxy. This squeezing can cause many new stars to form. If a small galaxy stays near a big one too long, it may break apart completely. It can turn into a long stream of stars and gas.

We can see these effects in famous places in space. The Mice Galaxies show clear tidal tails from a collision. The Antennae Galaxies are another great example of these long tails.

In our own neighborhood, the Milky Way affects the Solar System. Far from the Sun, there is a shell of icy rocks called the Oort cloud.

The galaxy's gravity can stretch this cloud into a new shape. It pulls the cloud toward the center of the galaxy.

These tides can change how things move in the Solar System. The Sun's gravity is very weak at the edge of the Oort cloud. Because of this, the galactic tide can nudge icy rocks. These rocks are made of rock and ice. When they fall toward the Sun, they become comets. This happens because of the heat from the Sun. In fact, up to 90% of comets may come from this process. The galactic tide is a very important part of how our space works.

A galactic tide is a specific tidal force felt by objects within a galaxy's gravitational field. This force is not determined by the total strength of gravity alone. Instead, it depends on the gradient of the gravitational field. A gradient is the rate at which the strength of gravity changes over a certain distance. Because of this, tidal effects are usually limited to the immediate surroundings of a galaxy. These forces can reshape entire galaxies, strip material from smaller neighbors, and even influence the movement of icy objects in our own Solar System.

When two large galaxies pass near each other or undergo a collision, they experience massive tidal forces. These galaxies rarely collide head-on. Instead, the tidal forces distort each galaxy along an axis. This axis points roughly toward and away from the other galaxy, which is known as the perturber. As the galaxies briefly orbit one another, the distorted regions are pulled away from the main galactic bodies. The galaxy's own differential rotation then shears these regions. This process flings the material into intergalactic space, creating long, curved structures called tidal tails.

The appearance of these structures depends on the mass of the interacting galaxies. If the perturbed galaxy is equal to or smaller than its partner, a large tidal tail forms. However, if one galaxy is significantly more massive than the other, the results look different. The trailing arm remains relatively minor. In this case, the leading arm becomes more prominent and is often called a tidal bridge. These bridges can be difficult to see. They might be absorbed by a merging galaxy or obscured if one galaxy is in the foreground. In rare cases, a tidal loop can form where a tail joins its parent galaxy at both ends.

Satellite galaxies are also highly susceptible to these forces because they exist in the immediate vicinity of large galaxies. A large host galaxy can exert enough force to cause tidal stripping. This is a process where stars and gas are torn from the extremities of a satellite galaxy. For example, the dwarf galaxy M32 is a satellite of the Andromeda Galaxy. It may have lost its spiral arms due to tidal stripping. The gravitational pull can also knead and compress the interstellar gas clouds inside a small satellite. This compression can induce high rates of star formation within the remaining core.

The scale of the satellite galaxy also changes how the debris looks. If a satellite is very small, the tidal debris tails are likely to be symmetric. These tails follow a similar orbit and trace the path of the satellite. However, if a satellite is larger than one ten-thousandth the mass of its host, its own gravity may interfere. This can break the symmetry and accelerate the tails in different directions. Over many orbits, a dwarf satellite may be completely disrupted. This can result in a tidal stream of stars and gas that wraps around the larger host galaxy. Some scientists suggest that the extended discs of gas around galaxies like Andromeda are actually the results of such complete disruptions.

Tidal effects also reach into the very edges of our own Solar System. While a star's gravity usually dominates its own local system, that gravity becomes weak at great distances. In these outer reaches, the galactic tide can become significant. A major area of interest is the Oort cloud, a theoretical shell of icy objects surrounding the Solar System. The Oort cloud may be over a light-year in radius. Across such a massive distance, the gradient of the Milky Way's gravitational field plays a noticeable role.

The galactic tide can physically deform the Oort cloud. It can stretch the cloud toward the galactic center while compressing it along the other two axes. This is similar to how the Moon affects Earth's oceans. Because the Sun's gravity is so weak at these distances, these small galactic perturbations can dislodge planetesimals. These objects are composed of a mixture of rock and ice. When they are nudged from their orbits and move toward the inner Solar System, solar radiation turns them into comets. It is estimated that up to 90% of all comets originating from an Oort cloud may be the result of the galactic tide.