The can cause some problems for the . It takes about 250 million (a quarter of a ) years for the to go once round the Galaxy. On its way it may pass through several of , dust and new . These could cause problems, for example by cutting down the sunlight reaching the Earth. happen every 250 million years or so. One theory links them with the Sun’s orbit round the Galaxy, but this is not generally accepted.
As we will see in this story, from time to time the Earth has probably been struck by which have many different animals and plants. These meteorites may have come from outside the , for example from nearby stars as the Sun passed through a spiral arm of the Galaxy.
We still have a lot to learn about the Galaxy in which we live, and the dangers it has for on Earth.
The Milky Way did not form alone. It is the second largest galaxy in the small of galaxies to which it belongs. It was discovered in 1994 that there is a small galaxy which is in the process of colliding with the Milky Way. It is called the Sagittarius Dwarf Elliptical Galaxy (SagDEG). It lies on the other side of the Milky Way from the Earth, so it is completely hidden from view by the clouds of gas and dust in the plane of the Galaxy.
Reconstructed image built from images by NASA, ESA and the Hubble Heritage Team (STScI/AURA)
Some astronomers speculate that repeated collisions with SagDEG might have created the Milky Way’s spiral arms, could have caused a warp in the disc of the Milky Way or otherwise affected our history.
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