There are a number of satellite galaxies that orbit our galaxy, the Milky Way. Examples of these satellite galaxies include the Large Magellanic Cloud and the smaller Small Magellanic Cloud. Satellite galaxies are small galaxies that orbit round larger galaxies. They are close but they are not the closest galaxy to us, the Milky Way.
The nearest galaxy is the Canis Major Dwarf Galaxy and it is believed that we have already collided with it. The Canis Major Dwarf Galaxy is closer to us than Sagittarius A, the supermassive black hole that is at the centre of the galaxy. It gets its name from being located in the constellation Canis Major.
The most galaxy of substantial size is the Sagittarius Dwarf Spheroidal Galaxy which has a diameter of about 10,000 light years. That has a radius of about tenth of our galaxy.
The nearest galaxy that is larger than our own is the Andromeda Galaxy (M31) which is twice the radius of our galaxy. It is over 2.5 million light years from us. It is moving towards us at a rate of 300 km/s and due to collide in four billion years time. The resulting merger will create a new galaxy known as Milkomeda. By the time the collision occurs, our Sun will probably have gone supernova and our human race will have hopefully move to another planet.
Before the Milkomeda event, we will have probably collided with some of those satellite galaxies. It won't be the first time that we've consumed other galaxies. The star Methuselah Star, the oldest known star is believed to have originated from another galaxy which was consumed by ours. Part of the reason for the theory is that the star is different to other stars in the galaxy.
The resulting collision will make our galaxy resemble something like the Tadpole Galaxy. The Tadpole Galaxy was created by the collision of two galaxies millions of years ago. The tail consists of stars that have been thrown out of the galaxy.
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