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I saw this question on Quizlet which said:

What is the difference between the Milky Way and the Milky Way Galaxy?

And the answer was:

The Milky Way is a fairly narrow band of faint diffuse light around the celestial sphere. The Milky Way Galaxy is a spiral galaxy of about 100 billion stars.

But isn't the term "Milky Way" means the Milky Way Galaxy? Why is there a difference? If there is a difference, could one include graphs that show the difference between the two?

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – Connor Garcia
    Sep 12 at 22:52
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I think that distinction is wrong, or at least not commonly accepted.

We live in a disk-shaped galaxy, which is interchangeably called "the Milky Way", "the Milky Way Galaxy", or "the Galaxy" (capitalized to differentiate it from other galaxies).

Viewed from inside, it looks to us as a narrow, diffuse band of light because we see more stars along the disk than away from the disk. That phenomenon is called "the Milky Way", and has indeed been called that for thousands of years.

But even calling the phenomenon "the Milky Way Galaxy" would not offend any astronomers, I think. Although you could argue that everything you see on the sky is part of the Galaxy, excluding a handful of fuzzy blobs.

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    $\begingroup$ Just because we can only see part of something doesn't mean the part we can see shouldn't be identified with the whole. After all, we only see one side of the moon! $\endgroup$ Sep 9 at 22:55
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    $\begingroup$ @curiousdannii No, I agree, but in this case, the term "the Milky Way" has been used long before people realized it was "a whole". So the original meaning of the term described a phenomenon that was visible on the sky, just as a nebula or a planet or a shooting star. $\endgroup$
    – pela
    Sep 10 at 8:07
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    $\begingroup$ @curiousdannii The difference is that we are inside the galaxy. Any star you see in the sky is part of the Milky Way galaxy. If you are trying to describe the glowing band you can see across the sky on a dark night and say 'That is the Milky Way Galaxy', that would exclude all the other stars that are also part of the galaxy. That is like pointing to Jupiter and saying 'That is the solar system'. There is a deeper explanation for why the Milky Way looks like it does, and you are not talking about the entire galaxy when you are indicating the visible phenomenon. $\endgroup$ Sep 11 at 4:56
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Milky Way vs Milky Way Galaxy

I recommend recognizing and honoring the distinction!

The two words being interchangeable is a narrow view that only one well versed in Astronomy can have, and doesn't fit the reality of how ordinary people view it, being the circa 1010 people who have seen the Milky Way but never having been formally taught about galaxies.

The Milky Way

Anyone can gaze at the sky on a dark night and know what the Milky Way is without any connection or reference to the concept of a galaxy or even that the Earth orbits the Sun. The Milky Way is what anybody who looks up on a dark night sees, regardless of science.

It's simply that milky pattern up there.

The Milky Way Galaxy

The Milky Way Galaxy is what scientists deduce from observations.

It is an abstraction, a model, that fits observation.

Of course if I had to bet five dollars I'd say that it is real and we're in it, but that's because I'm a scientist and/or believe in science.

But no matter what anybody believes, anyone who looks will see that stuff up there.

What at least sighted folks can agree on is that The Milky Way is up there, and what scientists and lay people can tend to agree upon is that it is our galaxy.

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    $\begingroup$ Very nice explanation! +1 $\endgroup$
    – pela
    Sep 10 at 8:14
  • $\begingroup$ not "it is our galaxy", but "it is most of our galaxy". Because Jupiter is part of our galaxy but not part of the Milky Way we can see! Being out by the edge, most of it is in the same direction from us. $\endgroup$
    – user253751
    Sep 10 at 10:34
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    $\begingroup$ Generally a good answer. Unfortunately, a large proportion of today's population has never been able to look up on a dark night and see the Milky Way, due to light pollution. $\endgroup$
    – LarsH
    Sep 10 at 12:45
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    $\begingroup$ @user253751 that's like saying "That's not the wall, that's one side of the wall." $\endgroup$
    – uhoh
    Sep 10 at 16:20
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    $\begingroup$ @uhoh There are parts of the Milky Way Galaxy which are in the completely opposite direction. $\endgroup$
    – user253751
    Sep 10 at 16:33
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All the various answers are making the same correct point in different ways, but I still can't resist saying this:

  1. When you are talking about galaxies, and you want to specify our own, you can say either "the Milky Way" or "the Milky Way Galaxy." They are both fine for talking about the whole galaxy as one among many, and Quizlet is wrong to suggest otherwise.

  2. The pale stripe you can see in the sky is normally just called "the Milky Way." It would be unusual to call it "the Milky Way Galaxy" because it is only a part of the galaxy, consisting of the far away stars in the disk. Also, historically, that stripe was often called just "the Galaxy." As names for features in the sky, using both terms feels redundant. But, when you are talking about different galaxies, saying "Milky Way Galaxy" is not redundant.

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  • $\begingroup$ +1 I think this is the best and most concise answer. $\endgroup$
    – uhoh
    Sep 11 at 23:42
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A lot of people vaguely call The Milky Way Galaxy simply the Milky Way, but I don't approve of such vague speech which blurs the distinction between two separate things.

The Milky Way is a faint band of light in the night sky, which can been seen where the background sky is dark enough.

The Milky Way is thus an appearance, or a sighting, or an illusion.

Just like the sky is more or less an illusion. There is a relatively thin layer of atmosphere over the surface of the Earth, and above that an almost total vacuum extending to infinity which is called outer space.

In the daytime, atmospheric molecules reflect and scatter sunlight, so the blu elight coming from every direction makes it look like there is a solid dome above us which is called the sky.

At night, there is no reflected sunlight, and the atmosphere is almost totally transparent, and we can see through it to the vacuum of outer space, which is black except where light from distant shining objects reaches Earth.

Most of the shining objects seen at night are stars, which have greater or lesser absolute luminosities and which are at nearer or farther distances from Earth, and thus have greater or lesser apaprent brightness as seen from Earth.

There are a few thousand individual stars that humans can see in the night sky without telescopes, amd millions of others that can bee seen with binoculars and telescopes. And the light of many millions of distant stars is blended together to make the light of the Mikly Way.

The Milky Way Galaxy is a galaxy of billions of stars. It has a galactic disc where the stars are packed closer together than in its halo region. The Sun and the Earth are within the galactic disc. We see scattered individual stars when we look away from the galactic disc. When we look though the plane of the galactic disc we see the light of star after star after star after star blended together in a pale light, the Milky Way.

So the Milky Way is an optical appearance, like the blue sky or a refection in a mirror. It is sort of an illusion.

But the Milky Way Galaxy is a physical object and group of smaller objects, and a physical place and group of smaller places. The Milky Way Galaxy is certainly not an illusion.

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    $\begingroup$ The term "Milky Way" — i.e. without "Galaxy" — is definitely also used for the Galaxy, not just for the faint band of light. So I think your sentence "The Milky Way is thus an appearance, or a sighting, or an illusion" sounds a bit… disturbing… $\endgroup$
    – pela
    Sep 10 at 8:10
  • $\begingroup$ @pela People use a lot of illogical expressions, like calling a grand uncle, brother of a grandparent, a great uncle. I don't encourage the inaccurate expression of calling the Mikly Way Galaxy "the Milky Way" And I added a paragraph saying so to my answer. $\endgroup$ Sep 10 at 17:28
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    $\begingroup$ Well good luck trying to change the minds of 10,000 astronomers… $\endgroup$
    – pela
    Sep 10 at 22:34

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