Milky Way Weighs 1.5 Trillion Suns and Most is Dark Matter

NASA’s Hubble Space Telescope and the European Space Agency’s Gaia satellite have more precisely weighed the Milky Way galaxy.

Above – On the left is a Hubble Space Telescope image of a portion of the globular star cluster NGC 5466. On the right, Hubble images taken ten years apart were compared to clock the cluster’s velocity. A grid in the background helps to illustrate the stellar motion in the foreground cluster (located 52,000 light-years away). Notice that background galaxies (top right of center, bottom left of center) do not appear to move because they are so much farther away, many millions of light-years.
Credits: NASA, ESA and S.T. Sohn and J. DePasquale (STScI)

The Milky Way weighs in at about 1.5 trillion solar masses (one solar mass is the mass of our Sun), according to the latest measurements. Only a tiny percentage of this is attributed to the approximately 200 billion stars in the Milky Way and includes a 4-million-solar-mass supermassive black hole at the center. Most of the rest of the mass is locked up in dark matter, an invisible and mysterious substance that acts like scaffolding throughout the universe and keeps the stars in their galaxies.

Old estimates for our galaxy’s mass ranged between 500 billion to 3 trillion solar masses. The improved measurement is near the middle of this range.

The lightest galaxies are around a billion solar masses, while the heaviest are 30,000 times more massive.

The Hubble and Gaia observations are complementary. Gaia was exclusively designed to create a precise three-dimensional map of astronomical objects throughout the Milky Way and track their motions. It made exacting all-sky measurements that include many globular clusters. Hubble has a smaller field of view, but it can measure fainter stars and therefore reach more distant clusters. The new study augmented Gaia measurements for 34 globular clusters out to 65,000 light-years, with Hubble measurements of 12 clusters out to 130,000 light-years that were obtained from images taken over a 10-year period.

When the Gaia and Hubble measurements are combined as anchor points, like pins on a map, astronomers can estimate the distribution of the Milky Way’s mass out to nearly 1 million light-years from Earth.


Written By Brian Wang

18 thoughts on “Milky Way Weighs 1.5 Trillion Suns and Most is Dark Matter”

  1. Actually, my milligram calculation was of by a factor of 1,000!:

    ⅓ milligram
    = ¹⁄₃₀₀₀ gram
    = ¹⁄₃₀₀₀₀₀₀ kg so therefore

    E = ½mv²
    E = 0.5 × ¹⁄₃₀₀₀₀₀₀ × 180,000,000²
    E = 5,400,000,000 J … ÷ 4.186×10⁹ J/Ton
    E = 1.3 tons of TNT equivalent

    A lot less. A much more “handleable kaboom”
    Just saying,
    GoatGuy ✓

  2. Hope that five extra megatons was my memory and not my math; it was a fair number of years back.

    Of course, if that 2.5kg is antimatter, things get a lot more exciting.

    Although, even if we could make antimatter affordably, and in those kind of quantities, and avoid accidents, it would probably be a bit irresponsible to be accelerating it up to relativistic speeds and putting it out somewhere that we might not recover it.

    But then, come to think of it, anything we accelerated to relativistic speeds would put something of a moral onus on us to ensure it was either slowed back down, impacted into something unimportant, or on a course that would never ever hit anything.

  3. Yah…

    E = ½mv²
    E = 0.5 × 2.5 × ( 60% of 300,000,000 m/s )²
    E = 4.06×10¹⁶ J ÷ 4.186×10¹⁵ J/MTon 
    E = 9.7 megatons TNT

    Big. Big bada boom. 

    The trick is to get it to vaporize a throw-away material, turning itself and that into plasma; using high intensity magnetics, fan out the plasma … then catch a bit more downwind … and so on, until the immense streak-of-plasma dissipates thermally. 

    Mostly though, you’d prefer not hitting anything at all, bigger than a grain of table salt. 

    Even that is sobering: 

    E = ½mv² … m = ⅓ milligram (salt grain)
    E = 0.5 × 0.0003333 × 180,000,000²
    E = 5,400,000,000,000 J ÷ 4,186,000,000,000 J/kton
    E = 1.3 kilotons

    As I said… a wee mote of space stuff is quite enough to do serious damage to one’s front end. 

    Just saying,
    GoatGuy ✓

  4. That might do it. I think I once worked out that 2.5kg of anything, impacting at 60% of c, would release about 15 megatons. Not record breaking, but substantial all the same.

  5. I agree on probably being bound by the speed of light. I’m actually starting to think of it more as the speed of space-time and that the reason for this limit is that, being made of the stuff, we can’t perceive when it is stretched either way. So, in a sense, we can’t exceed the speed of light for much the same reason that we cannot climb a ladder faster than one rung per rung, or age faster than one second per second, or run further than one mile per mile, and so on.
    My own tentative conclusion is that it may be freakishly unlikely for everything to come together and we are freaky rare, but it doesn’t stop there.
    There is also a growing collection of information that seems to be indicating that, in a 13.8 billion year old universe, we might not only be incredibly rare, but incredibly early . . .

    And, frankly, that doesn’t surprise me if it is true. I had already been thinking on that. Further, I keep coming back to the question of how another intelligent, tool-using, civilization-building species could develop on Earth with us already here in our 7+ billions. It couldn’t. Not without a huge assist from us.

    In the game of galactic expansion, if you are not first, you probably never exist at all.

  6. But, but, but… Jim! It very likely IS settled! 

    We’re just far too dumb and ignorant to be “talked to” by the resident aliens. I mean, its not like humankind tries to communicate with ant colonies or aphid infestations. 

    Though it edges on crack-pottery, I have come to accept the idea that its our combination of fruit-fly like lifespans, our only–200-years of science limiting our communications and/or detection tech, our geocentrism and ultimately the briefness of our civilization’s existence that has so far failed to reveal the teeming but FAR slower moving (in time, not speed) alien civilizations around us. 

    We’ll catch up, eventually.
     Just not in a Star Trek or Star Wars dramatic cinema timeframe. 

    I also think that the Laws of Physics are likely to be shown to be pretty darn resilient, and that things like “time warps” and “spacetime wormholes” aren’t tenable, and that ALL interstellar spacefaring civilizations will simply be bound by “c”. 

    Which is good at slowing down expansion rate AND at diluting alien presence on a century of millennial timescale. 

    Just saying,

  7. Actually, a decent technological answer is the idea of keeping disks of water ice (snow) out in front of the ‘ship’ by a few velocity-hours distance, in a chain. Something hits one, it blasts through as a plasma; the plasma cone expands on its way to the next one, which again blasts thru, but to a much wider and heavier cone. And so on. Until the “scoops at the rear” capture the stuff, condense it, and start reprocessing for robotic transport up front, to repair the sacrificial damage. GoatGuy

  8. Pretty easy, actually. 

    Between you and the far horizon (here on Planet Dirt) may be not billions but trillions-to-quadrillions of itsy-bitsy aerosol particles. The net effect of ’em is that blue-white haze that increases with distance. The haze that makes mountains and more distant mountains in turn ever more washed out. 

    If you were whizzing along at nearly the speed of light, minus the air, the intervening aerosols would be a disaster. 

    Such is Dr. Pat’s thought. 

    Now, imagined to be “only” at a density of a billion walnut-sized chunks per cubic light year, one’s space ship isn’t very likely to run into a chunk, even if it makes The Kessel Run in 12 parsecs (LOL) a hundred times. Cross section of the Millennium Falcon is about 250 m² (from Fandom sites), so the interception probability is only 10⁻¹² per trip to intercept a hazelnut sized rock at near light speed, at a density of 1,000,000,000,000,000,000 walnuts per cubic lightyear. (double LOL)

    Just saying. 
    Dr. Pat’s conjecture is not a bad one. 
    But the density of hazelnuts is an unknown.
    Triple LOL


  9. There’s a quite-new theory floating around in the physics community about the possibility that the “missing matter” is actually composed of light itself. Light and neutrinos. 

    In a way, there’s a sense to it: if the one invariant we can count on is “for every action there is an equal and opposite reaction”, then since light carries momentum (real, measurable), and is deflected (“bent”) by gravity, then the inertial change must actually exert an equal-and-opposite force on the thing doing the bending. The gravitational point.

    Anyway, the theory goes on to posit that with the light-has-mass idea, with a sufficiently low mass per photon (well below and within the upper limit of theory), the “missing dark matter” would no longer be missing. 

    Its an intriguing idea that has a lot of physics “esthetics” to it. Its here, its everywhere, it can’t be directly seen. And it mathematically works. Good stuff.

    Just saying,

  10. With the Bullet Cluster observation you have the gravitational lens observation showing two centers of mass that does not agree with the Xray emissions. So I can see where some would say it must be Dark Matter.

    But, If dark matter is spread homogeneously thru space we should see its effect in the solar system and we don’t. The other thing is what would keep Dark Matter from collapsing into black holes. Unless, DM has its own version of nuclear and EM forces like regular matter.

  11. Some people think maybe the 1/r component only operate at a distant greater than some minimum distance. Of course this sound like a kluge but so does dark matter.

    One thing I have consider is that due to the expansion of the universe that matter at a distance has a slightly higher mass. And at the end of the universe that the mass must be infinite. The end of the universe and the beginning of time must seem like a singularity. I don’t know how this would affect the galaxy. I personally think the effect would be small but I can’t be sure. And I don’t have the chops to set up the equations much less to solve them.

  12. I thought the Bullet Cluster observation had ruled out such a modified gravity as the explanation.

  13. If a practical fusion reactor is developed the tech civilization can settle all those rocks in their Oort cloud & then settle the next stars Oort cloud. Your ‘solution’ doesn’t stop interstellar expansion, it merely slows it, & not by enough to explain why the Galaxy isn’t settled.

  14. Of course, the result depends on 1/r square rule for the force of gravity. Maybe instead of dark matter there may be a 1/r component to the gravity force.

  15. Dark matter is such a fun subject. Even the sky isn’t the limit on speculation. Suppose it’s matter/waves left over from previous universes that already experienced their heat deaths? This stuff might still deform our space-time, but would be on the wrong wavelength to interact with anything from our Big Bang. It also wouldn’t interact with any of the other previous universe’s leftovers, so there wouldn’t be any clumping.

    Of course, this would require an almost infinite number of previous universes to have existed, but why would this be a barrier? No one says those earlier universes had to have as much dark matter as ours. Maybe our universe exists at a privileged point, where it has just enough dark matter to lead to us, while universes much earlier did not have enough and, by extension, later universes will have too much.

    This then begs the question of what we mean when we say “earlier” or “later,” since any time on that scale would have nothing to do with what we consider to be space-time.

  16. Actually, a really cautious interstellar ship might have some options.

    Be redundant, one of many identical ones (tricky with humans on board but not insurmountable, depending on how they are stored)

    Or it could have thousands, or even millions, of networked probes flying from light minutes to light months ahead and to the sides, ready to report back at the speed of light if they detect any obstacles, or if some of their brethren are destroyed by hitting something (basically the same thing).

    How many of them would be needed, and how far out these probes would need to be, would depend on how effective their detectors are, how maneuverable the main ship or ships are, how crowded the interstellar medium really is (we know it is not a lot), and how much risk we are willing to accept.

  17. It’s a nice thought, but if it’s a dense enough cloud to be a problem, how come we can see out without attenuation?

  18. Fermi Paradox solution = there is just so much stuff out there (rocks) that any attempt to travel between the stars is doomed.

    Tech civilizations can arise, but they can’t really move beyond their local Oort Cloud.

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