≡ Menu

A Walk in the Galaxy

On my walk this morning, I was musing about the ongoing AAS meeting in Long Beach when I found myself having one of those epiphanies that seem to open a window into the heart of things. The day was unusually warm but gusts of wind tossed the trees and low clouds laced with rain scudded past. And suddenly I was no longer walking along a quiet street but became aware that I walking a planet within a star system, within a cloud of stars, and that by being made up of elements from those stars, I was in some sense an expression of that universe as it observed itself.

It’s hardly an original notion, but the sense of it was palpable, an almost physical awareness that translated something known factually into something experienced. It was spurred by the recent news that the Milky Way is fifty percent more massive than we thought, maybe the twin of the Andromeda Galaxy. Increasing our sense of scale adds to the grandeur. The punch of the Fermi Paradox comes from the sheer size of galaxies — we wonder about other civilizations because we understand that there are hundreds of billions of stars around us in this celestial city, surely many of them not all that different from our own.


I felt awash in immensity as I walked the windy streets. For if it’s breathtaking to think that the Milky Way may hold hundred of billions of stars — 400 billion is a figure often cited — how much more so to consider it as massive as three trillion suns?

Image: The Andromeda Galaxy, perhaps a twin of our own. We can see Andromeda from without, but determining the structure of the galaxy we move through ourselves is a continuing challenge. Credit: NASA.

A mass of three trillion suns is what the work of Mark Reid (Harvard-Smithsonian Center for Astrophysics) and colleagues suggests. The team of astronomers used the Very Long Baseline Array to look at star-forming regions across the galaxy, zeroing in on natural radio emissions that are amplified by gas molecules in certain areas. These ‘cosmic masers’ are markers that can be studied by the VLBA through parallax, measuring the apparent shift of the objects against the cosmic background. The work is particularly telling because it relies on few assumptions:

“The new VLBA observations of the Milky Way are producing highly-accurate direct measurements of distances and motions,” said Karl Menten of the Max Planck Institute for Radio Astronomy in Germany, a member of the team. “These measurements use the traditional surveyor’s method of triangulation and do not depend on any assumptions based on other properties, such as brightness, unlike earlier studies.”

Using these direct measurements, the astronomers could determine that most of the star-forming regions move around the galactic center not in a circular but an elliptical orbit. They were also able to determine that the Milky Way probably has four spiral arms of gas and dust that are continually producing stars, with young stars dominating two of these. But most striking was the team’s finding that our Solar System is rotating much faster around the galactic core than previously believed, in the neighborhood of 965,000 kilometers per hour.

The higher speed of our movement around the center demonstrates the need to up our estimates of the galaxy’s mass by the fifty percent mentioned earlier. And it brings home a crucial fact: Although we live within it, we still must contend with our limited knowledge of the Milky Way’s size and structure. And if we are going to start talking about a serious upgrade to the mass of the Milky Way, as the evidence that Reid reported at the AAS session on Monday suggests, we have to remember that we still cannot see ourselves whole, the way we can examine, say, Andromeda.

Walking in the early morning winds, I reflected on all this, and on the fact that seeing something complete from within is notoriously tricky, whether that something be as vast as a swarm of stars or as miniature as a human relationship. Identifying the markers, like Reid’s cosmic masers, is crucial, but even then, the sample of markers has to be extended to broaden the picture. As we work out the science, the sense of awe inspired by our newly massive galaxy creates a freshening humility that enriches everyday experience.

More on Reid’s work in this National Radio Astronomy Observatory news release. Both the New York Times and Science News also pick up on the story.

Comments on this entry are closed.

  • Adam January 7, 2009, 17:24

    The amount of dark matter required to generate the rotation curve of the Galaxy has changed, but not the amount of luminous matter (i.e. stars) because it’s not any brighter according to this new work. And I am not so sure about the 3 trillion solar masses figure being bandied about because a recent study downgraded the masses of both M31 and the MW – thus 50% more is probably more like 1.8 trillion. But that’s a quibble. It’s an amazing work that Reid has done, being able to get parallax across tens of thousands of light-years with the VLBA.

    We can extrapolate: Our Galaxy will be a well-mapped place by the time we can travel through it.

  • Pat Galea January 7, 2009, 18:17

    “an almost physical awareness that translated something known factually into something experienced.”

    It’s weird when that happens. A moment of clarity that makes you say “Oh! I *get* it!” And yet if you try to explain to someone else what you now understand, you’ll use exactly the same words that you’d have used before. So they’ll be left wondering what exactly it is that you now know that you didn’t two hours ago. I think it’s that you understand it at a deeper level, as if you’ve gone down one layer in the protocol stack.

    I believe this is exactly what Heinlein meant by the word “grok”.

    Regarding the calculation of the galaxy’s mass, is it possible that the composition of dark matter, if the theory is correct, could have some bearing here? From the way the calculation is being done, it sounds like some detail of the attributes of dark matter could have an effect on the figure we end up with for the galactic mass.

    If it sounds like I’m being vague about what I mean, it’s because I’m being vague about what I mean because I don’t really know what I’m asking. It just feels like there’s something to be considered here.

  • Administrator January 7, 2009, 19:13

    Pat, you home in on exactly the right point re dark matter, which plays a great role in determining how galaxies form in the first place. As Adam points out, the luminous matter (stars) we see has not changed in brightness, but the amount of dark matter involved in constraining the galaxy’s rotation is now updated. I’m still interested in the question of how many stars we estimate for the entire galaxy, a number which varies hugely depending on what you read.

    By the way, great call re ‘grok,’ which is indeed the precise word in the context of my piece this morning.

  • Mark January 8, 2009, 0:40

    You mention the Fermi paradox. Well, I’ve figured out why we can’t find any other civilizations in our galaxy.

    The Milky Way, like every other galaxy, is in reality a generation ship. Each ship having been equipped at the start of the voyage with exactly one planet containing life. Our destination is that big drain in the cosmos you guys reported on a few weeks back. You know, the one toward which all galaxies appear to be headed. Who knows what we’ll find when we get to the drain.

    In the meantime, the ship is ours to explore and enjoy.

  • Paul Hughes January 8, 2009, 1:27

    Wow, thanks for sharing about your wonderful experience this morning. I only discovered your blog a few weeks ago, so I still don’t know your name, but I wanted to thank you for what has become my favorite piece of reading each morning. Your interest/angle is very much my own, and it’s refreshing to see someone like you write with such eloquence. This is the type of story I like to blog about, but you do it so much better!

    1.8 trillion. 3 trillion. Either way, a major upgrade to the number of stars. A trillion is a much more enjoyable (and astonishing) number to throw around than four hundred billion.

  • Adam January 8, 2009, 6:27

    Hi Paul

    AFAIK the only estimate that has been inferred from bright matter in the MW is about 100 billion stars. But for some strange reason that figure seems to get inflated by commentators… 200, 300, 400 billion, then a trillion…

    The integrated luminosity of the Galaxy is ~30 billion Suns, but, of course, the “average” star is not as bright as the Sun.

  • Administrator January 8, 2009, 9:20

    Paul Hughes, thanks so much for this comment. Glad to have you with us. I’m Paul Gilster — you can read something about the background of this site by clicking the ‘About Centauri Dreams’ link on the home page. And I look forward to further comments from you. Be sure to read Adam’s comments above about the number of stars — we’re actually talking about an increase in mass, most of which seems to be dark matter, rather than the number of stars. But the sheer immensity of the galaxy continues to inspire awe.

  • James M. Essig January 8, 2009, 23:06

    Hi Folks;

    Indeed, the Milky Way is huge, with perhaps 400 billion or more stars. As I was looking at the beautiful photograph Paul included at the beginning of this thread tonight, I began to think about the possibility of a sort of time travel that relies on nothing other than special relativity and classical electrodynamics.

    This psuedo-time travel involves nothing other than using telescopes to look back in time in a manner that we now do ubiquitously as a civilization whenever we look at distant galaxies or even at stars in our own galaxy.

    I bring up this obvious fact in this thread which discusses the Milky Way in all of its richness and grandure because as we go back to the Moon, establish permanent outposts on the Moon, including telescopic research stations, we may likely be capable and hopefully will set of optical VLBI telescopes that might have photon collection areas of 1,000s of square kilometers if not 10s of thousands, and an effective apeture hundreds of kilometers accoss.

    These monstrousities, coupled with ultra sensitive atomic clocks for data packet time recording and collation may enable us to actually see the tree lines on terrestrial planets as far away as perhaps a few thousand light years. The resolution would certainly enable us to see buildings and other civil infrastructure on these distant worlds as they existed thousands of years ago in an act of a sort of optical information effectly backward time travel.

    VLBIs spanning the diameter of our solar system and comprised of ultraprecisely controlled membranous reflectors with perhaps a photon collection area as great as a whopping one trillion to 100 trillion square kilometers might enable us to see the tree lines on planets 10 billion lightyears distant as well as civil infrastructure on these planets as such existed 10 billion years ago.

    Alternatively, we might be able to photograph bodily ETI person’s faces in nearby galaxies as they went about their dailly activities on their home planets millions if not tens of millions of years ago.

    The real point I am trying to emphasize is that we can effectively travel in an indirect sense to cosmically remote spatial temporal locations in the past simply by using optical telescopy.

    Right this very century, even before we develop interstellar manned mission programs to physically reach for the stars, we can take a picture “Walk in the Galaxy” and look back in time to see the civil history of ETI worlds as is progressed as much as 70,000 years ago. I can imagine that any ETI peoples many thousand year old histories that we manage to map out would be proud of our accomplishments.

    I will have more to say on general topic in the comming days.



  • Pat Galea January 9, 2009, 13:03

    Hi Jim.

    I find VLBI fascinating. It’s like getting a huge lens on the cheap; dirt cheap!

    I can see that two telescopes separated by a long distance is the bare minimum for a VLBI array, and that adding more telescopes will improve things.

    How many telescopes do you need before the VLBI gets close to acting like one genuinely big lens? I’ve done some back-of-envelope calculations for phased-array radars in the distant past, but I don’t know how the numbers scale with the shorter wavelengths of light.


  • James M. Essig January 9, 2009, 18:10

    Hi Pat Galea:

    Thanks for asking the above question.

    You need a minimum of two seperated lenses to obtain the VLBI configuration. However, you must also have an adequate capture area in oder to collect enough photons to produce an image. A small number of photons will give you a rough outline portrait of an ETI structure while a large number will give you a high resolution image.

    Inorder to show the reader how a 100 trillion square kilometer precisely controlled membranous reflector might capture enough photons to image a feature the size of an Egyption Pyramid at a distance of 10 billion light years, consider that in broad star light, each square meter of the object may give off 1,000 Watts of reflected star light and blackbody emmissions ptotons combined. We will make a conservative estimate that the average photon energy is about 1 eV and so a thousands watts of emmitted optical photons would result in about [( 1.60217646 x 10 EXP – 19) EXP – 1](10 EXP 3) photons emmitted per second per square meter. This is roughly 10 EXP 22 (photons/second) per sqaure meter. Let us further assume that all of these photons make it into space or at least, most of them do.

    Next we calculate the photon flux per square meter per second at a distance of 10 billion lightyears which is equal to 10 EXP 23 kilometers or 10 EXP 26 meters. We will assume that all of the emmitted photons fall within a sqaure that is (10 EXP26 meters) EXP 2 in area or within about 10 EXP 52 sqaure meters.

    Thus, the photon flux per square meter per second at 10 billion lightyears is (10 EXP – 52)(10 EXP 22) photons per sqaure meter per second. This is 10 EXP – 30 photons (meter EXP – 2)(second EXP -1).

    Now we had originally assummed that the membranous reflector has a capture area of 100 trillion square kilometers which is equal to 10 EXP 20 square meters.

    Now we assume that the Pyramid sized structure has an area of 10 EXP 5 sqaure meters. The number of photons collected by the reflector per second from the pyramid will then be equal to (10 EXP -30)(10 EXP 20)(10 EXP 5) or 10 EXP – 5 photons per second. In one year, about 300 photons will be collected assumming 1 eV photons. In 100 years, about 30,000 photons will be collected which should permit imaging of the basic structure of the pyramid. If we assume that the pyramid emmits IR photons at a black body emmision rate of 1,000 Watts/(meter EXP 2) and that these photons have an energy of 10 EXP – 1 eV, then the losses due to the absorption and inaccuracies in the assumed actual geometric distribution of the emmitted and reflected visible light and IR photons, still permits a conservative estimate.

    What could we image with a collector perched outside of our solar system with a monolithic reflector having a diameter of 10 billion kilometers or a surface area of [3/(pi)](10 EXP 20) square kilometers. This bad boy could collect about 7 or 8 photons per second while imaging our ETI friends’ pyramid at 10 billion lightyears. This amounts to about 200 million photons/year; enough to get a good detailed image.

    We could image the face of our ETI friend from a distance of 1 million LYs with about 700 photons/second. If the ETI stood still or sat motionless for a paultry 1 minute, we would obtain about 42,000 imaging photons, enough to make a good detailed color sketch of the ETIs facial features. This would be fantastic!

    The caveats are several here. Perhaps the most significant are the collection of an adequate number of imaging photons, prescisely enough controlled reflectors which would be difficult given quantum mechanical scale fluctuations for micrometer square differential area elements of the reflector, the filtering out of extrenous noise (in short the signal to noise ration would be miniscule), and the actual assembly of such a huge structure.

    This rather lengthy posting was meant to convey some of the posible limitations of a VLBI as apposed to a monolithic reflector of the same distance span.



  • philw1776 January 9, 2009, 20:12

    One side ‘benefit’ of this result is that because of the increased mass and Newton’s Laws (which MUST be obeyed) we’ll get to see the collision between us and Andromeda a billion years or more sooner!

  • Adam January 10, 2009, 1:59

    As much as I like M31 – not that I can ever see it in the Southern Hemisphere – I really don’t want it in my backyard.

    Another interesting factoid is that at 300,000 ly – the rough limit for the Milky Way’s halo – the escape velocity is roughly 10 times the escape velocity from Earth’s orbit around the Sun and not much different to the escape velocity at the Sun’s radial distance from the Galactic Core. There’s a lot of Milky Way mass we can’t see… now even more than we previously knew.

  • Pat Galea January 10, 2009, 11:06

    Hi James.

    Thanks for that very comprehensive reply. That was exactly what I was looking for!

  • Ronald January 11, 2009, 17:01

    James, others,

    ref. to the elaborate post above concerning VLBI and light capture area, I have the following question, also related to the recent thread about the Sun’s gravitational focus:
    could sufficient light collecting area also be a problem, i.e. a limiting factor, at the Sun’s gravitational focus at 550 AU (where magnification thanks to this effect is about 10^8) or does this mechanism work differently?

  • James M. Essig January 11, 2009, 21:05

    Hi Ronald;

    Thanks for the above question.

    I think the gravitational focus of sun light at 550 AU is mainly due to the gravitational lensing out side the Sun but nonthelass, at a small radial distance from the Sun’s center. At a distance of 550 AU, I do not think that one would collect significantly more photons than one could collect by simply using a monolithic reflector with a diameter of 10 billion kilomters, even if the 10 billion km diameter collector was positioned at 550 AU. Most of the Sun’s resolving power if I am not mistaken comes from the gravitational field within a few solar radii from the Sun’s surface or within about 0.03 AU of the Sun’s surface.

    Now, for smaller secondary optics placed at 550 AU, serious benifit could be obtained at 550 AU, but I do not think that one could get good inages of ETI faces and pyramid sized structures at 1 million LY and 10 billion LY in a timely manner unless one used a monolithic reflector about 10 billion or so km in diameter. Still, for sufficiently still ETI such as those that might be resting for a long time at a distance of 1 million LY, and for several decades time exposure of 10 billion light year away stationary infrastructure , one should be able to image ETI faces and pyramid sized structures given a few weeks collection time and a few decades collection time, respectively, with only a 100 trillion square kilometers of reflector collection area distributed in the form of a VLBI.

    Note however, that in order to sucessfully generate such images, extraordinarilly precise object tracking would be required as well sufficient computational computing power to collect the photographic data points or photon based pixels.

    The Sun’s gravitational field relatively near its surface acts like a single lense instead of a VLBI which uses multiple discreet lenses located at a significant distance from each other. Note howver, that I can imagine the possibility of using several smaller lenses or reflectors seperated in a VLBI at 550 AU in order to permit a higher resolution than would be possible if the lenses were simply located at any arbitrary position within or solar system at the same distance from each other as they would be located at 550 AU. There would, however, be some imperfactions in the basic image at the focus of 550 AU based on the spherical assymetry of the Sun’s mass distribution as well as from the mass distribition fluctuations due to the roiling turbulance near the Sun’s surface.



  • ljk January 12, 2009, 14:03

    The Milky Way Transit Authority: