What do you think of this fact?
Picture the full Moon. Now picture something about six times as wide, hanging in the night sky, and almost nobody noticing it. That’s the Andromeda Galaxy.
Too faint to see in full
Andromeda, also known as M31, is about 2.5 million light-years away. It’s the Milky Way’s nearest large galactic neighbor. According to NASA, it appears about 6 times the diameter of the full Moon. ESA/Hubble also describes it as several times the size of the full Moon, “although much fainter.”
The catch is brightness. Astronomy magazine explains that with the naked eye, you only see a fuzzy smudge about a quarter of a degree wide, roughly half the width of the full Moon. The faint outer spiral arms only show up in long-exposure photos. Then the galaxy grows to a bit more than 3 degrees across.
And then there’s the halo
Even that isn’t the whole thing. In 2015, a team led by Nicolas Lehner of the University of Notre Dame used the Hubble Space Telescope to map a halo of thin, hot gas around Andromeda. They couldn’t photograph it directly. Instead, they looked at the light of 18 quasars far behind the galaxy and measured how much of it the gas absorbed.
The result: the halo stretches about one million light-years from Andromeda, roughly halfway to the Milky Way. According to NASA, if you could see it with your eyes, it would span 100 times the diameter of the full Moon.
Bonus fact: A later study used 43 quasars. According to Astronomy magazine, if you could see Andromeda’s full halo, it would stretch more than 60 degrees, over a third of the way across the sky.
That faint smudge is just the visible core of something gigantic, and its gas already reaches halfway to us.