A giant cloud of hydrogen gas is speeding toward a collision with our Milky Way Galaxy, and when it hits — in less than 40 million years — it may set off a spectacular burst of stellar fireworks. The cloud, called Smith’s Cloud, after the astronomer who discovered it in 1963, contains enough hydrogen to make a million stars like the Sun. Eleven thousand light-years long and 2,500 light-years wide, it is only 8,000 light-years from our Galaxy’s disk. It is careening toward our Galaxy at more than 150 miles per second, aimed to strike the Milky Way’s disk at an angle of about 45 degrees. Don’t worry! It will hit 30,000 light years away from Earth.