In a recent study, using NASA’s Hubble Space Telescope, scientists mapped the supermassive halo of gas surrounding the Andromeda galaxy.

According to Hubble observations, galactic halos are more massive and complicated than scientists thought.

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These large spherical halos of gas surround all galaxies. They reach vastly farther than the main population of stars, a bit like a galactic atmosphere.

Andromeda’s nearly invisible halo of diffuse plasma extends 1.3 million light-years from the galaxy—about halfway to our Milky Way—and as far as 2 million light-years in some directions. This means the halos of Andromeda and the Milky Way are merging together.

Andromeda’s halo would appear to be over 100 times the apparent diameter of the full Moon. That’s of course if our eyes could see it.

Andromeda is our closest large galactic neighbor.

The Milky Way and Andromeda are extremely close, on the cosmic scale, and they are getting ever-closer. Andromeda is hurtling towards our galaxy at an estimated rate of around 250,000 miles per hour. That’s 2,000 times faster than a fastball, according to NASA estimates.

In fact, Our Milky Way Galaxy will have a head-on collision with Andromeda, triggered by their gravitational force. Eventually, they will merge in around 4.5 billion years.

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The researchers also found Andromeda’s halo has a layered structure, with two main nested and distinct shells of gas. This is the most comprehensive study of a halo surrounding a galaxy and its vastly important.

“Understanding the huge halos of gas surrounding galaxies is immensely important,” explained co-investigator Samantha Berek of Yale University in New Haven, Connecticut. “This reservoir of gas contains fuel for future star formation within the galaxy, as well as outflows from events such as supernovae. It’s full of clues regarding the past and future evolution of the galaxy, and we’re finally able to study it in great detail in our closest galactic neighbor.”

“We find the inner shell that extends to about a half-million light-years is far more complex and dynamic,” explained study leader Nicolas Lehner of the University of Notre Dame in Indiana. “The outer shell is smoother and hotter. This difference is a likely result from the impact of supernova activity in the galaxy’s disk more directly affecting the inner halo.”

We cannot see the halo with the naked eyes. Andromeda’s halo comprises of highly ionized gas that does not emit radiation.

In order to observe the halo, the researchers examined the light form 43 quasars located far beyond Andromeda. They then observed how the light from the quasars gets absorbed by the Andromeda halo, and how that absorption changes in different regions.

“This is truly a unique experiment because only with Andromeda do we have information on its halo along not only one or two sightlines, but over 40,” Nicolas Lehner said in a statement.

“This is groundbreaking for capturing the complexity of a galaxy halo beyond our own Milky Way.”

The researchers published their results on Thursday in The Astrophysical Journal.

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