Astronomers spot massive halo of gas surrounding Andromeda Galaxy

The ring stretches a million light-years past the galaxy, and could result in the merger of Andromeda and the Milky Way in just 4 billion years.
By Sarah Webb | May 10, 2015
Scientists have spotted a massive halo of gas encircling the Andromeda Galaxy, stretching a full million light-years outside the galaxy.

The research, published in the Astrophysical Journal, estimates that this huge invisible ring of gas has as much mass as half the stars in the entire galaxy, according to a Register report.

The Andromeda Galaxy is huge, and is 25 percent larger than our own Milky Way, containing a trillion stars.

Scientists discovered the ring with the help of the Hubble Space Telescope from NASA, and they believe hte ring is about 100 times the diameter of the moon.

To spot this invisible ring, astronomers had to use the bright light of objects in the background -- such as quasars, which are bright bursts of energy that come from matter falling into supermassive black holes and being spewed back out -- that interact with the gas, allowing scientists to measure it.

The gas from the halo absorbs some of the light from the quasar, making it appear slightly darker. The scientists were able to measure the change in brightness and tell how much gas there is between Earth and the quasar.

Such halos have been known to exist, but Andromeda's halo is the largest ever to be spotted so close to Earth.

The Milky Way could have a similar halo, which means that the two galaxies could collide in about 4 billion years and former a massive elliptical galaxy.

The Andromeda Galaxy is located about 2.5 million light-years from Earth, making it the closest major galaxy, although not the nearest galaxy overall. It is the largest galaxy in what has been called the Local Group, which also includes our Milky Way, the Triangulum Galaxy, and about 44 other galaxies that are smaller.

Andromeda has about 1 trillion stars, by far the most in the Local Group. In comparison, the Milky Way has an estimate 200 to 400 billion stars.

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