B Milky Way is "in the top percentile of all the galaxies that exist"

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Summary
Our Galaxy is far, bigger, brighter, and more massive than most others
Our Galaxy is far bigger, brighter, and more massive than most others: Knowable Magazine

Check out the amazing video, which shows all the many--more than 50!--satellite galaxies of the Milky Way burst into existence as they are discovered, year after year.
 

davenn

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Thanks, interesting article :smile:
Didn't know the Milky Way has so many satellite galaxies


Dave
 

Klystron

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Yes, interesting article and video. Also interesting to see how discovery of numerous satellite galaxies inform cosmology, particularly theories of galaxy formation and the history of our own galaxy.
 
The numbers quoted are slightly misleading when expressed in this way. Yes the Milky way is more massive than all dwarf galaxies; but it is run-of the-mill for spiral galaxies and less massive than average for elipticals.
 

davenn

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The numbers quoted are slightly misleading when expressed in this way. Yes the Milky way is more massive than all dwarf galaxies; but it is run-of the-mill for spiral galaxies and less massive than average for elipticals.
Yes, agreed, I was considering writing something similar. I would go as far as "quite misleading".
Even the Andromeda Galaxy of our local group is larger than our Milky Way Galaxy


Dave
 

pinball1970

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Summary: Our Galaxy is far, bigger, brighter, and more massive than most others

Our Galaxy is far bigger, brighter, and more massive than most others: Knowable Magazine

Check out the amazing video, which shows all the many--more than 50!--satellite galaxies of the Milky Way burst into existence as they are discovered, year after year.
You quoted sky and telescope from April last year that said the same. Did it go on to qualify regarding dwarf galaxies?
It was in the thread galaxy Vs milky way
 
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No, it's not misleading at all.

First, even compared with other spiral galaxies, the Milky Way is above average. M33 and the Large Magellanic Cloud--which many astronomers think would be a spiral were it not for the interference of the Small Magellanic Cloud--are both lesser galaxies than the Milky Way. Take a look at all of the nearest spirals (e.g., all within 20 million light-years) and you'll see that the Milky Way's absolute visual magnitude is brighter than most, even if Andromeda is an exception.

Second, in order to be a spiral, a galaxy must spin fast (as discussed in the April 2019 Sky & Telescope) and thus be fairly massive--more massive than the average galaxy.

Third, dwarf galaxies ARE galaxies too.

Fourth, most elliptical galaxies are smaller than the Milky Way. They range from dwarfs like M32 to behemoths like M87. The latter are incredibly rare.
 
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For example, Table 1 of McCall (2014) lists 59 luminous galaxies nearby, plus the Milky Way. Most are spirals; some are ellipticals; at least one is an irregular. The fifth column gives the absolute visual magnitude of each galaxy.

According to his data, only 2 of these 59 galaxies are more luminous than the Milky Way: Andromeda and the Sombrero.

But even this vastly understates the Milky Way's superiority, because McCall's table excludes all the faint galaxies. Even the Large Magellanic Cloud is not in that table, nor is any Local Group galaxy except the top three (Andromeda, the Milky Way, M33). And the Local Group currently has more than 100 known galaxies.

Repeat:

The Milky Way Galaxy is far bigger, brighter, and more massive than most others.
The Milky Way Galaxy is bigger, brighter, and more massive than most other spirals.
The Milky Way Galaxy is bigger, brighter, and more massive than most ellipticals.
The Milky Way Galaxy is smaller, fainter, and less massive than....the few galaxies that are bigger, brighter, and more massive than it is!

Reference: McCall (2014): https://ui.adsabs.harvard.edu/abs/2014MNRAS.440..405M/abstract
--
 

davenn

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No, it's not misleading at all.
Yes it is on an all galaxy scale. It isn't misleading when comparing members of our local
group, because most ~ 95% or more are dwarf galaxies.

There's many galaxies out there that make Andromeda and the Milky Way in particular look
like dwarf galaxies.



First, even compared with other spiral galaxies, the Milky Way is above average. M33 and the Large Magellanic Cloud--

ohhh come on !
you cannot even begin to compare those two.
M33 is the 3rd largest in the local group; Andromeda, Milky Way then M33

LMC is small in comparison ....
Based on readily visible stars and a mass of approximately 10 billion solar masses, the diameter of the LMC is about 14,000 light-years (4.3 kpc), making it roughly one one-hundredth as massive as the Milky Way
Yes, it is the 4th largest in the local group but it is just on the top end of a dwarf galaxy rating
 
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The Knowable article is NOT misleading. It says that the Milky Way is far bigger, brighter, and more massive than MOST other galaxies. It does NOT say that the Milky Way is far bigger, brighter, and more massive than ALL other galaxies.

If you don't understand this, read the third, fourth, and fifth paragraphs of that article again. Only nearby, within a million light-years, can we hope to see all the galaxies that exist, but even here, as the article mentions, we are probably missing at least half the galaxies that exist. All the missing galaxies are much fainter than the Milky Way.

As for M33 and the Large Magellanic Cloud, here are the numbers:

M33 stellar mass: 7.6 billion solar masses
LMC stellar mass: 2.7 billion solar masses

So the Large Magellanic Cloud is roughly a third as luminous as M33.
 

davenn

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As for M33 and the Large Magellanic Cloud, here are the numbers:

M33 stellar mass: 7.6 billion solar masses
LMC stellar mass: 2.7 billion solar masses

So the Large Magellanic Cloud is roughly a third as luminous as M33.

dunno where you got those from but it disagrees with what I have read and posted
 

Ken G

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I would say that whether or not something is "misleading" depends on what point it is trying to make. The article's point is that there are lots and lots of very tiny galaxies, which we did not know about in the past. This is certainly true. It goes on to say that since there are so many tiny galaxies, this means that the Milky Way is in the "top percentile" by size, counting all things that get called "galaxies." That's also true, so one could just leave it at that-- the article is true.

But of course, the rub is-- what counts as a "galaxy"? We'd have the same situation if we looked at "planets" and said that the Earth is in the top percentile of planet size. Would that be misleading? It depends on what you mean by a "planet." If you count all the "dwarf planets" as "planets", then yes, the Earth is huge. If you don't, then the Earth is average at best. So it's all in what comparison is being made. The Milky Way is huge when compared to a dwarf galaxy, and there are lots and lots of dwarf galaxies. It is fairly average when compared to other galaxies like it, which is kind of obvious. So there's not much more to say-- the article is about the existence of lots and lots of tiny galaxies, just like there are lots and lots of rubble orbiting the Sun. What that says about our galaxy, or our Earth, really depends on what group you put it in with-- and why you would put it in that group. The latter is all about what comparison you are trying to draw.

Personally, what I would do is count stars, and ask, are most stars in dwarf galaxies, which makes our situation seem unusual, or are most stars in large galaxies like ours, or are most stars in giant ellipticals? That would be an interesting comparison to make, and would answer whether we should regard our own galaxy as large or small. The weakness I see in the article is that it does not answer that basic question, and indeed, I don't know what the answer is.
 
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Actually, the last two paragraphs of the Knowable article do answer that question: most stars are in giant galaxies like the Milky Way.
 
A Dunning-Kruger demonstration? Allow me to proceed.

The interesting thing is we have a fairly good view of one galaxy, and are making sweeping claims about all the others, many of which are permanently out of sight.

The other interesting thing is, the word Galaxy might arguably include these orbiting galaxy fragments. In the same way the Oort Cloud is part of the defined Solar System. It's still early, 12 or 13 billion years. Don't be too hasty. Let the scientists do their jobs.
 

Ken G

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Actually, the last two paragraphs of the Knowable article do answer that question: most stars are in giant galaxies like the Milky Way.
Ah, OK, then that is indeed a useful point made in the article. However, I think it also makes it clear that our galaxy should be regarded as quite normal-- if it is the kind of galaxy that most stars are in. It says we should regard the tiny galaxies, though certainly very numerous, as not terribly significant in the grand scheme, much as we look at asteroids and dwarf planets in our own solar system (since most of the mass is in the larger planets).
 
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I understand your point of view but respectfully disagree with it.

All galaxies, whether large or small, are galaxies, just as all dogs, whether large or small, are dogs. Suppose, for the sake of argument, that most canine mass resides in big dogs. No one would say that a big dog such as an Irish setter or a golden retriever is an average dog. Likewise, no one should say that a giant galaxy such as the Milky Way or Andromeda or the Whirlpool or M87 is an average galaxy just because most stars reside in giant galaxies. Indeed, the word giant conveys the correct fact that these galaxies are much bigger than the average galaxy.

In the solar system, we rightly distinguish between planets on the one hand and asteroids and comets on the other. Although the definition of planet is controversial, let's restrict ourselves to the classical nine. In terms of mass and size, Earth is perfectly average: four planets are larger and four are smaller. As it happens, 99.6% of the solar system's planetary mass is in the four giant planets. But no one would call Jupiter, Saturn, Uranus, and Neptune "average" planets. Instead, we call them giant planets just as we call the Milky Way and its peers giant galaxies.
 

Ken G

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But the question is, why do we call dwarf galaxies galaxies, but we don't call dwarf planets planets? I see no reason at all, merely an accident of history. It is this kind of completely arbitrary distinction that masks the things that actually matter. What matters in a universe is stars, and what matters in a solar system is planets that are large enough to be of some significance. So when we look what is a typical galaxy, and what is a typical planet, we should ask for what is typical among what matters, not among the arbitrary nomenclature that has seemed to evolve for no particular reason whatsoever. There will always be lots and lots of little things, be they lakes, animals, planets, or galaxies. (How small does a lake need to be before we call it a pond? Who cares? What matters is where the water is.) The little ones always vastly outnumber the bigger ones, with almost everything we put a name to-- but when it is only the big ones that matter, we should never regard those big ones as atypical, because they are quite typical-- for the ones that matter.
 

Vanadium 50

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But the question is, why do we call dwarf galaxies galaxies, but we don't call dwarf planets planets?
Because that makes Michael Brown sad.
 
Didn't know there were so many new satellites to the Milky Way, that graphic of their discoveries is really great.

"The Milky Way is in the top percentile of all the galaxies that exist"

Isn't this really just a natural result from the fact that galactic distances (on average) are very large. So naturally which ever galaxy you are in in the universe would appear to be one that had had the chance to accrete more of it's satellites. For example, if i were at our currently measured time since the big bang, but in whatever the furthest galaxy measured so far from this location is, would i not probably observe it to be above average in size, mass and luminosity, compared with the statistics i take from my sky surveys?

I guess what i'm saying is, isn't it kind of misleading to readers to present the distribution of galaxies in this way. I know they sort of cover this at the end of the article, but i don't think they made it clear enough as to what the significance (if it really is significant) of these statistics is. Surely it's more meaningful to gauge galactic vital statistics at given distance ranges and thus, at different time periods since the big bang? Then you can generate meaningful questions like "Why does this particular galaxy appear to be further behind in its evolutionary processes than galaxies of a similar age?"

I do like the sign off though.
But chances are, so can they.
 
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The numbers quoted are slightly misleading when expressed in this way. Yes the Milky way is more massive than all dwarf galaxies; but it is run-of the-mill for spiral galaxies and less massive than average for elipticals.
Similarly, the Sun is in the top 10% (and probably top 1%) of objects in hydrostatic equilibrium.
 
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Ken G

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All that is true, but it is also true that the Milky Way is "run-of-the-mill" for a galaxy that a randomly chosen star finds itself in. And if you queried all the intelligent species in the universe about their galaxy, the majority would say "ours is a lot like yours." So the majority can take that to mean their own galaxy is unusual by the same argument as in the press release, but it's probably more natural for the majority to take that fact to mean their galaxy is quite normal-- for a galaxy that intelligent life finds itself in.

What I find so ironic is how the anthropic principle is often invoked to solve the "fine tuning problem" in our universe, but here we have essentially the exact opposite argument being used to help us feel special about our galaxy.
 

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