back to article You've been dying to know. Here's the answer: The Milky Way tips the cosmic scales at '1.5tr' times mass of the Sun

NASA and the European Space Agency have teamed up to attempt to answer what seems like an impossible question: what is the mass of the Milky Way? The topic has been bugging scientists for a while, and it's a useful figure to know because it's needed in simulations of galaxies and of our galaxy within the universe. Previous …

  1. Winkypop Silver badge

    Does this Oort Cloud look big on me?

    Can someone convert this figure into London Busses for me?

    1. Anonymous Coward
      Anonymous Coward

      Re: Does this Oort Cloud look big on me?

      London Buses or not, it's still a heck of a lot of "stuff".

      Where did it all come from, and where is it going to go?

      1. Spoonsinger

        Re: Does this Oort Cloud look big on me?

        So just a quick google

        Sun apparently 1.989 × 10^30 Kg

        London Bus 12650kg

        So? 1.989 × 1030kg / 12650kg = 1.572332e+26 London buses per solar mass, therefore galaxy should be circa 2.358498 x 10^38 London buses.

        Before coffee and will bow to more demonstrably accurate numbers.

        1. teknopaul Silver badge

          Re: Does this Oort Cloud look big on me?

          Way off. Its friday there are 3500kg of dunks on each London bus.

      2. Killing Time

        Re: Does this Oort Cloud look big on me?

        'Where did it all come from, and where is it going to go?'

        A single point and away from each other apparently.....

    2. NightFox
      Black Helicopters

      Re: Does this Oort Cloud look big on me?

      Remember to include the one on the moon.

  2. Anonymous Coward
    Anonymous Coward

    London Bus

    According to wiki the classic London bus is 7.5 tonnes, and the new-fangled one 12.5 tonnes, so rounding to 10 tonnes to keep the maths easy, I reckon there are 2 x 10^26 busses per sun.

    Assuming we are defining a trillion as 10^12, then approx 2 x 10^38 buses in the galaxy.

    Which is one heck of a traffic jam, especially when none are going to where you want to go.

    1. Jedit

      "approx 2 x 10^38 buses in the galaxy"

      Which is proof that the Big Bang theory is correct: you wait forever for a bus then 2 x 10^38 of them come along at once.

      1. Crypto Monad

        Re: "approx 2 x 10^38 buses in the galaxy"

        2 * 10^38? That's almost exactly 2^128.

        Coincidence?? I think not!

        At last, evidence the that the universe is a digital simulation.

    2. The First Dave Silver badge

      Re: London Bus

      Is that with, or without, passengers? And fuel. And tyres.

    3. Valerion

      Re: London Bus

      Which is one heck of a traffic jam, especially when none are going to where you want to go.

      Soon the earth will be demolished by Vogons to make for a hyperspace bus lane.

  3. Duncan Macdonald Silver badge

    MOND ?

    The assumed mass for the Milky Way depends on dark matter. If the MOND theory is correct then instead of dark matter there is a departure from 1/R^2 gravitational field at low field strengths. This would leave the Milky Way with a much smaller mass (around 200 billion solar masses).

    As dark matter has yet to be proven, the paper should start with the big warning that it depends on the dark matter theory being correct.

    (MOND theory - see

    1. tfb Silver badge

      Re: MOND ?

      Dark matter obviously hasn't been observed directly, but the bullet cluster is a lot of nails in the coffinof MOND theories.

    2. Cuddles Silver badge

      Re: MOND ?

      "If the MOND theory is correct then instead of dark matter"

      I'll ignore the many, many arguments pointing out how MOND is obviously not correct, and simply quote from the very Wiki page you linked:

      "The most serious problem facing Milgrom's law is that it cannot completely eliminate the need for dark matter"

      If MOND is correct, dark matter still exists. It never ceases to baffle me why anti-science types constantly insist on bringing up crackpot theories which don't even fix the supposed problems they're complaining about.

      1. tfb Silver badge

        Re: MOND ?

        I think MOND is a perfectly reasonable hypothesis. It seems to me to be a lot more radical than the hypothesis that there is dark matter, because if it turns out to be true then GR must be in, at least, big trouble. As best I can tell MOND also turns out to fail experimentally pretty badly.

        So, like you, I'm left wondering why it keeps coming up. Obviously there is still room for doubt (I have read articles by people with good credentials explaining why the bullet cluster might actually not kill MOND), but I don't think that's why. I think it's because MOND has become a secret poster child for the whole anti-science conspiracy-theory thing that is slowly killing us all: if MOND is correct then a huge chunk of modern physics falls, and, of course, modern physics is a giant conspiracy by liberal elites to suppress the truth and keep the white working class poor I mean look at that Einstein what was his family background eh people with big noses knowwhatImean its a conspiracy I tell you they don't want us to know the truth it's just the same as the climate change they lie to us about nothing wrong with coal they should be rounded up and put in camps the lot of them the germans had it right you know although of course they never did what people say you know people where's the proof [...].

        So I think that's why.

      2. tfb Silver badge

        Re: MOND ?

        Inspired by your comment, I looked at the Wikipedia page for MOND, and then followed that to the Tensor-vector-scalar gravity page, because I was once a GR person so I'm obviously interested in that.

        And, yuck. I mean, seriously, that makes me itch. 'Let's just add in some extra fields, some of which don't take part in the dynamics and a great mass of extra parameters to hold the thing together and call it a theory'. GR is this beautiful jewel-like thing which you can never really forget once you've seen it because it's just so pretty and right, and this is ... what, I don't even know, some kind of machine someone has bolted together from bits and some of the bits have imperial threads and some have metric and they've just used a really big spanner to force the thing together in some horrible, cross-threaded way and oil is leaking out of it and it's also on fire in various places. If physics works like that then God is mad in a psychopathic-axe-murderer kind of way.

        1. Long John Brass Silver badge

          Re: MOND ?

          The problem is that ugly truths often molest beautiful theories. Not trying to comment about MOND in any way; Just pointing out that the universe is under no obligation to work in a neat pleasing manner.

          "There is always an easy solution to every human problem--neat, plausible, and wrong."

      3. Justthefacts

        Re: MOND ?

        The probability is now strong that *neither* MOND nor dark matter can be correct.

        MOND never pretended to be *the truth*. It is ad hoc, so it can match problematic experimental observations in many ways. The naive version can’t be the truth, because it doesn’t integrate into our other understanding of the world.

        But dark matter in its naive form is also almost disproved: to fully match detailed observation, it has to clump in a specific way, which requires Cold Dark Matter, not Hot. That tells us its minimum mass. MACHOs are disproved experimentally. WIMPs (weakly interacting massive particles) are the only working assumption left, and huge resources have gone into searching parameter space. 98% of where they could have been, we’ve already checked via various high energy physics experiments and found they *definitely* aren’t. The myth is “it might be completely non-interacting except gravity”. No. It can’t. And that is *highly* constraining.

        There are still some places to look, but.....if you are searching for your keys in the house, and the only place left to look is in the loft.....possibly the keys aren’t in the house?

        Basically, we *know* that GR is not the full truth about gravity, because it doesn’t segue into Quantum Gravity. So, it is perfectly reasonable to think that is a good place to investigate. See Entropic Gravity. Not all modified gravity is the ad hoc MOND formula. Also, most (all?) self-consistent theories of how it could work will have quantum field equivalent perspective - i.e. there is a “dark matter particle” way of looking at it, just it’s a lousy way to think about the problem.

        1. Kyle Roberts

          Re: MOND ?

          "Dark Matter" is the new Flogiston!

        2. tfb Silver badge

          Re: MOND ?

          It's not the case that we've looked at 98% of the places WIMPs might be: we've looked at 98% of the places WIMPs predicted by supersymmetry might be: I don't keep up, but supersymmetry is either dead or nearly so because of this.

          I'm not claiming GR is correct: obviously it's not. I am claiming that fundamental theories tend to be pretty, and that TeVaS is anything but.

  4. ColonelDare

    Mass or weight?

    Well done Reg. I was surprised by the number of headlines I saw from others such as:

    "Hubble and Gaia accurately weigh the Milky Way" [] reported verbatim by the Guardian, et al.

    Shirley this should be - as you put it - about Mass. A Massive mistake maybe?

  5. Nick Kew Silver badge

    No wonder chocolates make you fat!

    I'll get me coat. It's the one with the buttons flown off.

    1. Fruit and Nutcase Silver badge
  6. Blockchain commentard Silver badge

    What about all the exploding stars, converting mass into light? Or black holes. Have we started measuring how much they've eaten?

    I reckon it's just a quiet Friday in some astronomy newsroom.

    1. eldakka Silver badge

      IANAP/C (IANA Physicist/Cosmologist).

      What about all the exploding stars, converting mass into light?


      This means that m=E/c2.

      This means that converting matter, i.e. concentrated energy, into light (energy) doesn't change total mass, as energy has mass. This is why particle physicists use the eV (electron volt) for both saying how much energy a collision has, as well as how much mass is involved, as they are the same thing, just in different forms.

      Or black holes.

      It would be more accurate to say black holes accumulate matter, rather than 'eating' or 'swallowing' or even 'annihilating' as people sometimes say. If a black hole accumulates 10 solar masses of matter (or energy) the black hole will increase in mass by 10 solar masses. Therefore the mass from the matter being swallowed by the black hole is not lost, the black hole now contains that mass.

      Therefore neither converting matter to energy (explosions) nor matter being accumulated by black holes will change the mass of a galaxy (while the black hole and energy remain within the galaxy and don't get ejected from the galaxy).

  7. Marco van de Voort

    Aren't planets dark?

    Planets are "dark" too, they only reflect sunlight. Except Earth maybe.

    1. tfb Silver badge

      Re: Aren't planets dark?

      They are relatively dark, but I think they're not candidates for dark matter any more. Collectively these things are known as MACHOs (massive compact halo objects), which includes planets not attached to stars, black holes and various sorts of very dim stars or failed stars. You look for these things by gravitational lensing ('gravitational microlensing') effects as they pass in front of luminous objects. And we've done that and we don't find nearly enough of them to account for the missing mass.

      So, this is a good, testable, idea, people looked, and they weren't there.

      1. Geoff May (no relation)

        Re: Aren't planets dark?

        @ftb, Thank you, I had always wondered about that.

  8. Mystic Megabyte Silver badge

    Google news

    Yesterday in the Science section every second story was a snippet from the Daily Express. {Hang on I'll nip over and fetch one or two}

    "NASA Moon landing SHOCK: What did Apollo 11 discover during lost two minutes of SILENCE?"

    "'Killer asteroid' will 'inevitably' hit earth causing EXTINCTION – 'start planet search'"

    and so on; usually about killer asteroids. I must say that I never realised how scientifically informed their readership was :)

  9. DougS Silver badge

    Supermassive black hole at the center of the galaxy

    I thought it was on the order of a few BILLION solar masses, when did it get down to a mere 4 to 5 million? Has it been on a diet of low carb matter?

    1. tfb Silver badge

      Re: Supermassive black hole at the center of the galaxy

      I don't think Sagittarius A* has ever been thought to be billions of solar masses. There are certainly SMBHs with masses in that range, but not ours.

  10. Horridbloke

    I can't be the only one...

    ... who read that article in the voice of Eric Idle.

    1. Michael H.F. Wilkinson Silver badge
      Thumb Up

      Re: I can't be the only one...


  11. Luiz Abdala

    Insert "Mama so Fat" jokes here.

    I willl see myself out.

  12. adnim Silver badge

    what is the mass of the Milky Way?

    43g for the UK bar and 58.12 g for the US version all though they used to weigh more. Like all food products, especially chocolate bars they have been getting lighter for years. Brexit will increase the price AND make them lighter.

  13. M.V. Lipvig

    The Milky Way weighs exactly, down to the last milligram, exactly one galaxy.

    If anyone wants to fund my work in devising a galaxy/kilogram app, please send bitcoin to my wallet, 3usts63si6c8xic85ditc73zitx8rxix57xix57cy85t8r7uxrx7t75x74x74xitx.


  14. Sleep deprived

    One solar mass is a whopping 1.989 x 10^^30 kilograms

    When was that? Every second, the Sun converts 4 million tons of matter into energy that escapes from it.

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