Is the Milky Way the same as 'The Galaxy'?

[Lukeblackhill]

Good Morning,

I've got a doubt originated from a statement of Berkeley Physics Course, Vol.1 - Mechanics, pg. 107, where in a footnote we read,

"The systems are called galaxies, that one which contains our own sun is known as the Galaxy. The Milky Way is part of our galaxy".

I've always considered the Milky Way as our entire galaxy, and have never heard about "The Galaxy". Could you please explain me if that is either an old classification (the book is from the 1960's) or if I'm not well informed about the matter?

Thank you!
Cheers,
Luke.

 

[Borg]

Good Morning,

I've got a doubt originated from a statement of Berkeley Physics Course, Vol.1 - Mechanics, pg. 107, where in a footnote we read,

"The systems are called galaxies, that one which contains our own sun is known as the Galaxy. The Milky Way is part of our galaxy".

I've always considered the Milky Way as our entire galaxy, and have never heard about "The Galaxy". Could you please explain me if that is either an old classification (the book is from the 1960's) or if I'm not well informed about the matter?

Thank you!
Cheers,
Luke.

Apparently this refers to the difference between The Galaxy itself (or The Milky Way Galaxy) and what we can see from Earth (The Milky Way). I had heard both of these phrases in the past but I also didn't realize the difference.
From the first footnote on Wikipedia's Milky Way article.

Jay M. Pasachoff in his textbook Astronomy: From the Earth to the Universestates the term Milky Way should refer exclusively to the band of light that the galaxy forms in the night sky, while the galaxy should receive the full name Milky Way Galaxy. See:

• Pasachoff, Jay M. (1994). Astronomy: From the Earth to the Universe. Harcourt School. p. 500. ISBN 0-03-001667-3.

 

[CygnusX-1]

Wikipedia is a questionable resource in this case.

The Milky Way Galaxy is often called just "the Milky Way." Take a look at any issue of Sky and Telescope. For example, from the April 2018 issue:

Page 17: "Galaxies very near the Milky Way have a redshift of nearly 0"
Page 24: "The Milky Way is far bigger and brighter than most other galaxies"

It's clear that in both sentences, "the Milky Way" refers to the entirety of our Galaxy, contrary to what the ridiculous Wikipedia footnote says.

"The Milky Way" can also refer to the band of light that threads through constellations in and near the Galactic plane. Presumably this is what the sentence "the Milky Way is part of our Galaxy" means, although it would be helpful to know what preceded the quote to know for sure.

And here is the definition of "Milky Way" from Ken Croswell's book The Alchemy of the Heavens: Searching for Meaning in the Milky Way.

Milky Way. 1. Our Galaxy. 2. The band of light that stretches across the sky during summer and winter, produced by innumerable stars in the plane of the Galaxy.

From The Alchemy of the Heavens by Ken Croswell, page 277.

 

[Janus]

The word "galaxy" is actually from the Greek for "milky". In respect to the Milky way and our galaxy, they were once thought to be one in the same. In other words, the band of light we see at night was thought to be the entire extent of the galaxy* we are in and we were near the center of it. It wasn't until we realized that we weren't taking dust and gas that was blocking our view into account that we came to the conclusion that the galaxy was much larger than we supposed, and that we were not near the center. But by this time the terms "Milky way" and "galaxy" were so intertwined that the name Milky way galaxy became the norm.

*and the entire universe itself, in fact. Objects that we now know as being other galaxies were once thought to be smaller nebula contained within our galaxy. Even after it came to be known that they were themselves large collections of stars far away and outside our galaxy, they were sometimes called "island universes".

 

[mathman]

I feel that trying to distinguish between "milky way" and "milky way galaxy" is a trivial quibble.

 

[stefan r]

Good Morning,

I've got a doubt originated from a statement of Berkeley Physics Course, Vol.1 - Mechanics, pg. 107, where in a footnote we read,

"The systems are called galaxies, that one which contains our own sun is known as the Galaxy. The Milky Way is part of our galaxy".

I've always considered the Milky Way as our entire galaxy, and have never heard about "The Galaxy". Could you please explain me if that is either an old classification (the book is from the 1960's) or if I'm not well informed about the matter?

Thank you!
Cheers,
Luke.

Suppose we are sitting on a couch in suburbia and limit our perspective what can be seen from the couch. Houses, yards, and a street can be observed by looking through the big window. We cannot take a picture of the Home House because we only see the interior. The condition of the roof of Home House can only be inferred by things like the lack of rain or water damage. On other houses we see the roof and siding. The drywall is a part of Home House. Because our language developed on Couch we wonder if we have neighbors living in the "Andromeda Dry Wall". All of our photos of Andromeda are exterior photos and they are taken through glass window panes. Any real photo of Home are interior photos.

 

[Ken G]

The word "galaxy" is actually from the Greek for "milky".

That's interesting, I had not realized the term "Milky Way Galaxy" was so redundant to anyone who knows both English and ancient Greek!

 

[sophiecentaur]

I had a sort of parallel conversation with my four year old granddaughter today. She was totally flummoxed when England was said to be a country but, when we looked out of the window and saw fields and crops, that was 'the country'. Where all the houses are is called 'the town'.
We ended the conversation in much shorter time than Seven Posts because it was wasting good colouring in time.
If you have seen a picture of "The Milky Way" (or even the Milky Way itself) then you would be aware that Polaris is not part of it (being nearly 90 degrees away from it). I think we would agree that Polaris is part of our Galaxy. So where is the problem? We are dealing with Usage of words - that's all.

 

[Ken G]

The problem is that if you go to a professional meeting, and someone starts talking about "the Milky Way," it's vastly more likely they are talking about our whole galaxy than just the swath of light. It was recommended not to do that, to always say "Milky Way galaxy," but people just aren't going to add the extra word. You pretty much always have to go from context, there's not much point in sticking to unwavering definitions. "Star" and "planet" aren't much better-- I'm not sure why we even bother to try to define these terms, people are never going to stick to those definitions in practice.

 

[sophiecentaur]

We have to remember that, when the Milky Way group of stars was named - and until quite recently - we didn't even know we were part of a Galaxy. We thought that the Universe consisted of what was in 'our - later- known-as ' Galaxy. Many Fuzzy objects (e.g. Andromeda Galaxy) that could be seen were assumed to be nearby Nebulae. There was no obvious existing name that could be assigned to our galaxy than Milky Way. Perhaps 'Our Galaxy' would have been the obvious name(??).

 

[Lukeblackhill]

Wikipedia is a questionable resource in this case.

The Milky Way Galaxy is often called just "the Milky Way." Take a look at any issue of Sky and Telescope. For example, from the April 2018 issue:

Page 17: "Galaxies very near the Milky Way have a redshift of nearly 0"
Page 24: "The Milky Way is far bigger and brighter than most other galaxies"

It's clear that in both sentences, "the Milky Way" refers to the entirety of our Galaxy, contrary to what the ridiculous Wikipedia footnote says.

"The Milky Way" can also refer to the band of light that threads through constellations in and near the Galactic plane. Presumably this is what the sentence "the Milky Way is part of our Galaxy" means, although it would be helpful to know what preceded the quote to know for sure.

And here is the definition of "Milky Way" from Ken Croswell's book The Alchemy of the Heavens: Searching for Meaning in the Milky Way.

Milky Way. 1. Our Galaxy. 2. The band of light that stretches across the sky during summer and winter, produced by innumerable stars in the plane of the Galaxy.

From The Alchemy of the Heavens by Ken Croswell, page 277.

Thank you @CygnusX-1 , your answer was enriching.

 

[JMz]

From just the current issue of the premier professional publication in this domain, The Astrophysical Journal (http://iopscience.iop.org/issue/0004-637X/857/1):
1. From one abstract: "...encounters between two identical Milky Way-like galaxies...".
2. From another: "GMCs [Giant molecular clouds] ... appear qualitatively and quantitatively similar to those in the Milky Way disk..." and "GMC mass spectrum is similar to those in the inner disks of spiral galaxies (including the Milky Way)."
3. And even an article title: "A Near-infrared RR Lyrae Census along the Southern Galactic Plane: The Milky Way's Stellar Fossil Brought to Light", whose abstract discusses "an ancient Galactic spheroid or classical bulge component residing in the central Milky Way."http://iopscience.iop.org/article/10.3847/1538-4357/aab4fa

So even among professionals in their most formal usage, Milky Way always refers to the whole galaxy. (And, yes, in a delightful bit of etymology -- note the similarity to "lactose" or milk sugar -- the phrase and the word once meant the same thing... and still do! :-)

 

[sophiecentaur]

So even among professionals in their most formal usage, Milky Way always refers to the whole galaxy.

Absolutely. But when you go out on a clear night, what do you call that faint band of light that stretches across the sky? That is the Milky Way that the earlier Astronomers saw and named. It does actually contain the majority of the stars in our galaxy so the name would certainly fit when considering our galaxy among others. There's no real conflict is there?

 

[JMz]

Hmm. When I go out on a clear night, I refer to that band as "the Milky Way" in the same way as (with binoculars) I refer to a certain fuzzy patch as "the Andromeda Galaxy", even though I can't see most of M31, both because it's too faint for my eyes and because a good part of it is obscured by its own dust -- just as the MW's dust does.

(Not sure what you mean by, "contain the majority of the stars in our galaxy", since we see only ~ 5000 stars on a good night. Even, say, Hubble probably sees less than a billion, ~ 0.1% of the whole. Due, again, to dust.)

 

[sophiecentaur]

(Not sure what you mean by, "contain the majority of the stars in our galaxy",

Sloppy, perhaps but I meant that the main disc (which is what we are looking at when we see the MW) contains most of the stars and (as when we see the Andromeda Galaxy) would be what the Astronomers in Andromeda would see of us, I suppose.

 

[JMz]

Sloppy, perhaps but I meant that the main disc (which is what we are looking at when we see the MW) contains most of the stars and (as when we see the Andromeda Galaxy) would be what the Astronomers in Andromeda would see of us, I suppose.

Ah: Yes, the MW as a whole extends "mostly" in the same directions as the visible band. (And, yes, by coincidence, the MW has about the same angle as seen from M31 as M31 has from here.)

 

[Ken G]

Absolutely. But when you go out on a clear night, what do you call that faint band of light that stretches across the sky?

We tend to think we need precise definitions to avoid this kind of ambiguity, but actually, ambiguity serves us better than precision. Ambiguity allows us to understand what we are talking about from the context, which is dynamic and flexible, rather than static and unyielding. For example, "stellar astronomers" study white dwarfs, and normally regard them as stars because they share interesting connections with the objects more formally defined as stars, but white dwarfs are not officially stars. It doesn't matter what is official, we learn more from context. Similarly, when you hear "Milky Way," it works better to just understand what is meant from the context, than to bother to distinguish two different phrases.

 

[JMz]

@Ken G, I'm curious what this means:

white dwarfs are not officially stars

That is, you seem to be relying on an authority here, one that can make things "official". Who is that -- maybe the IAU?

 

[Chronos]

Given the IAU is the official source for all things astronomical, it seems a good place to start. While they have managed to define just about everything else, the therm 'Milky Way' has apparently eluded coinage. This appears to be as close as it gets: "The Earth rotates at half a kilometre a second at the equator, and is moving around the Sun at 29 kilometres a second; our Sun is also moving through space at about 19 kilometres each second and is orbiting the centre of the Milky Way (our galaxy) at about 215 kilometres a second." from https://www.iau.org/public/themes/place_in_cosmos/. Obviously, the IAU was aware of the potential for confusion and made it clear the term 'MIlky Way' means our galaxy.

 

[JMz]

Given the IAU is the official source for all things astronomical, it seems a good place to start. While they have managed to define just about everything else, the therm 'Milky Way' has apparently eluded coinage. This appears to be as close as it gets: "The Earth rotates at half a kilometre a second at the equator, and is moving around the Sun at 29 kilometres a second; our Sun is also moving through space at about 19 kilometres each second and is orbiting the centre of the Milky Way (our galaxy) at about 215 kilometres a second." from https://www.iau.org/public/themes/place_in_cosmos/. Obviously, the IAU was aware of the potential for confusion and made it clear the term 'MIlky Way' means our galaxy.

Sure (though for actual professional usage, I'd probably give the nod personally to ApJ over the IAU -- but that's just me). But I was curious about the source behind @Ken G's wording, that's all.

 

[Ken G]

@Ken G, I'm curious what this means:

That is, you seem to be relying on an authority here, one that can make things "official". Who is that -- maybe the IAU?

In the past I had thought I had encountered a consistent effort to define "star" as an object undergoing fusion, in various sources such as textbooks and course websites, and I may have even thought the IAU took that position. However, at the moment, searches like "scientific definition of a star" are not coming up with much to say that there is any "official" definition of a star at all. If the IAU has a definition for star, it is sure hard to find. So I must confess, my current position is that there simply does not exist an official definition of a star, and that everyone more or less uses their own meaning. In particular, some places appear to take pains to not use the language "white dwarf star," while that language is in use elsewhere. It sounds like it's just another context-related term whose actual definition is intentionally vague to accommodate many purposes. Although it might sound odd that such a crucial astronomy term has no precise definition, that state of affairs actually serves the need better, because it allows "stellar astronomers" to study white dwarfs without needing to feel they are straying outside of "stellar astronomy." What this says about planetary astronomers, I won't say.

 

[stefan r]

@Ken G, I'm curious what this means:

That is, you seem to be relying on an authority here, one that can make things "official". Who is that -- maybe the IAU?

Webster could be "official" for English usage:

1 a : a natural luminous body visible in the sky especially at night
b : a self-luminous gaseous spheroidal celestial body of great mass which produces energy by means of nuclear fusion reactions

I'm wondering if someone should write to webster and tell them it is a plasma.

American heritage looks better to me:

from The American Heritage® Dictionary of the English Language, 4th Edition

• n. A self-luminous celestial body consisting of a mass of gas held together by its own gravity in which the energy generated by nuclear reactions in the interior is balanced by the outflow of energy to the surface, and the inward-directed gravitational forces are balanced by the outward-directed gas and radiation pressures.
• n. Any of the celestial bodies visible at night from Earth as relatively stationary, usually twinkling points of light.
• n. Something regarded as resembling such a celestial body.

 

[Ken G]

Yeah, each of the dictionaries says something different, and also offer multiple definitions as dictionaries do, so it's not clear which is supposed to be the scientific meaning. Meanwhile, the scientific meaning is often allowed to revert to the dictionaries. We are left with questions, how spherical does it need to be? (Some stars have an equatorial radius up to 50% wider than their polar radius.) Does it have to be undergoing fusion? (Not in the profession, no, but in many sources, yes.) If there is anything one could call a "scientific definition of a star" that is widely in use, I haven't seen it-- each person is more or less free to say what they mean when they use the word, and in particular, some would say a white dwarf is a star, some would say it isn't, and it hardly matters either way because white dwarfs are still going to be studied by people who regard their expertise as being in "stellar astronomy," and the "Milky Way" is going to be regarded as a galaxy by specialists in "extra-galactic astronomy."

 

[JMz]

Yeah, each of the dictionaries says something different, and also offer multiple definitions as dictionaries do, so it's not clear which is supposed to be the scientific meaning. Meanwhile, the scientific meaning is often allowed to revert to the dictionaries. We are left with questions, how spherical does it need to be? (Some stars have an equatorial radius up to 50% wider than their polar radius.) Does it have to be undergoing fusion? (Not in the profession, no, but in many sources, yes.) If there is anything one could call a "scientific definition of a star" that is widely in use, I haven't seen it-- each person is more or less free to say what they mean when they use the word, and in particular, some would say a white dwarf is a star, some would say it isn't, and it hardly matters either way because white dwarfs are still going to be studied by people who regard their expertise as being in "stellar astronomy," and the "Milky Way" is going to be regarded as a galaxy by specialists in "extra-galactic astronomy."

Concur.

Popular-usage dictionaries should be not confused with authoritative sources for professional usage. In professional circles, if a dictionary says that planet is defined by certain properties that Pluto has, and the IAU says it's not, the IAU wins. And this is PF: For words that have special meanings in the professional community, we generally go with professional usage, not popular usage, and professional authorities, not popular ones.

 

[CygnusX-1]

White dwarfs definitely ARE stars. Nuclear fusion is NOT a requirement for starhood. A star is a self-luminous object in space, nothing more.

Likewise, T Tauri stars are also stars, even though they are not engaging in nuclear fusion.

As for Pluto, some astronomers consider it a planet and others don't.

 

[Ken G]

Here's another one-- how many stars are in a binary? Obviously, if you study them as physical objects, you'd say two, but to an observer, its a binary star, not a binary stars! It's like asking if a hotdog is a sandwich, these questions are really all about context.