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The View from Outside the Galaxy

The Russian Federal Space Agency (Roscosmos) has recently released a video (viewable here on YouTube) showing how a number of celestial objects might look if they were substantially closer to Earth than they are. The image of the Andromeda galaxy and its trillion stars projected against an apparent Earthscape is below. Unfortunately, this seems to be an astronomical image inserted into a view that purports to show what we would see in visible light. What we would actually see if we were standing in such a location is much different. After all, astronomical images are teased out of lengthy exposures in carefully chosen wavelengths.


In reality the Andromeda galaxy is gigantic even when viewed from 2.5 million light years, but I doubt the average person has any idea where it is in the sky. Although considerably wider than the Moon as seen from Earth, M31 is visually faint, a fact that reminds us of the importance of photographs and charged coupled devices (CCDs) in light gathering as we probe the universe. The human eye is a very limited instrument. As it happens, I’m currently reading Alan Hirshfeld’s Starlight Detectives: How Astronomers, Inventors, and Eccentrics Discovered the Modern Universe (2014), which examines the transition between sketches of visual observations and the steady advances in photography in the 19th Century that brought so much more of the galaxy into view.

About a year ago I looked at Poul Anderson’s novel World Without Stars, which tells of a starship crew dispatched to a planet far outside the Milky Way, a place where from 200,000 light years, galaxy-rise is a rather muted event (see The Milky Way from a Distance). Anderson depicts a galaxy filling 22 degrees along its major axis, but one that appears ‘ghostly pale across seventy thousand parsecs.’ In the same issue of Analog (June, 1966) in which the novel was originally serialized, John Campbell published Anderson’s letter describing the calculations that went into that description, and explaining why the galaxy from outside would be so dim.

One point came up which may interest you. Though the galaxy would be a huge object in the sky, covering some 20⁰ of arc, it would not be bright. In fact, I make its luminosity, as far as this planet is concerned, somewhere between 1% and 0.1% of the total sky-glow (stars, zodiacal light, and permanent aurora) on a clear moonless Earth night. Sure, there are a lot of stars there — but they’re an awfully long ways off!

But it’s not just the distance. Galaxies, as UC-Santa Cruz astrophysicist Greg Laughlin has explained on his systemic site, are mostly empty space, or as he puts it, “To zeroth, to first, to second approximation, a galaxy is nothing at all.”

Building the Galactic Map

Interesting, then, to think about recent work on mapping the galaxy. The WISE mission (Wide-Field Infrared Survey Explorer) produced the data used in this effort to improve our understanding of the Milky Way’s spiral arm structure. Precisely because we don’t have the kind of overview of the galactic disk referred to above, we’re trying to sense the shape of our galaxy from within a disk obscured by dust and seen from our vantage two-thirds of the way out from the galaxy’s center. The new work supports a four-arm model for the Milky Way.

To derive this result, Denilso Camargo (Federal University of Rio Grande do Sul, Brazil) and team worked with WISE data revealing over 400 embedded star clusters, stellar nurseries of the kind that form in the dust- and gas-packed spiral arms, where most stars originate. Usefully for the purposes of the study, young clusters like these have not had the chance to drift out of the arms, thus providing a powerful tool for visualizing spiral arm structure.

In the paper, Camargo and team investigated 18 embedded clusters, seven of which are newly discovered in the WISE images — this complements a list of 437 new clusters Camargo recently published. The work supports the hypothesis that embedded clusters are predominantly found in the galactic thin disk and along its spiral arms. The Perseus and Scutum-Centaurus arms are the most prominent, while the Sagittarius and Outer arms show fewer stars but appear to have the same amount of gas as the other two arms.


Image: This illustration shows where WISE data revealed clusters of young stars shrouded in dust, called embedded clusters, which are known to reside in spiral arms. The bars represent uncertainties in the data. The nearly 100 clusters shown here were found in the arms called Perseus, Sagittarius-Carina, and Outer — three of the galaxy’s four proposed primary arms. Credit: NASA/JPL-Caltech/Federal University of Rio Grande do Sul.

Our Sun is in a minor arm called Orion Cygnus, marked on the image. You can see what a mapping challenge it is to work out, from within the disk itself, what kind of spiral arm structure exists. What we wouldn’t give for a perspective like the one above…

Supernovae in the Deep

Backing out to see the galaxy whole would take us far into intergalactic space, where we’re also learning about three supernovae found between galaxies in large galactic clusters. Melissa Graham, a UC-Berkeley postdoc, is the leader of a study of these objects. She also turns out to be a science fiction fan, one whose reference to the intergalactic deep isn’t the Anderson story I cited above, but Iain Banks’ novel Against a Dark Background (Orbit, 2009). There, the planet Golter lies a million light years from the nearest star.

We can imagine Anderson’s pale galaxy in an otherwise starless night sky, and Banks’ as well. Planets around the three supernovae in Graham’s study would have been destroyed in the event, but if there were any before, their night skies would likewise have been depleted of stars. “It would have been a fairly dark background indeed,” Golter adds, “populated only by the occasional faint and fuzzy blobs of the nearest and brightest cluster galaxies.”

Graham’s work using Hubble Space Telescope imaging confirms the earlier discovery (at the Canada-France-Hawaii Telescope on Mauna Kea) of the three supernovae and shows that they reside in a population of solitary stars in regions where the density of stars is about a million times less than we see from Earth. Gravitational interactions in massive galactic clusters can sometimes fling as many as 15 percent of a single galaxy’s stars out of the main disk, though the stars remain gravitationally bound within the cluster itself. Stars like these are going to be all but invisible unless they explode as Type Ia supernovae. Their explosions thus prove useful as a way to study the broader population of intracluster stars.

And this is intriguing: A fourth exploding star was found by the same observatory, one that may well be inside a globular cluster. If this is the case, we have an unusual event, the first time that a supernova has been found inside a globular cluster (GC). From the paper:

We have shown that the SN Ia in Abell 399 was very likely hosted by a faint red point-like source that has a magnitude and color consistent with both dwarf red sequence galaxies and red GCs. Our statistical analysis of the expected surface densities has shown that a dwarf galaxy is less likely at that location than a GC, due to the presence of a nearby elliptical galaxy. We have demonstrated that the rate enhancements in dwarfs or GCs implied by this new faint host are plausible under current observational constraints, and we do not reject either hypothesis.


Image: One of the four supernovae (top, 2009) may be part of a dwarf galaxy or globular cluster visible on the 2013 HST image (bottom). Credit: Melissa Graham, CFHT and HST.

The supernovae paper is Graham et al., “Confirmation of Hostless Type Ia Supernovae Using Hubble Space Telescope Imaging,” accepted at The Astrophysical Journal (preprint). The paper on galactic mapping is Camargo et al., “Tracing the Galactic spiral structure with embedded clusters,” Monthly Notices of the Royal Society Vol. 450, Issue 4 (20 May, 2015), pp. 4150-4160 (full text).


Comments on this entry are closed.

  • Marshall Eubanks June 5, 2015, 16:59

    I have seen M31 with the naked eye from my back yard – it is worth seeing, but it is very faint (something like a much dimmer Pleiades).

    I recently got in a big on-line argument with a noted science fiction writer about simulations about the future of M31 and the Milky Way – basically, he liked the pictures, and I felt that they considerably overstated the brightness these views would have to the human eye. (We, of course, both chose to ignore the very small likelihood of the human eye existing unchanged for the next 7 billion years, but you have to draw the line somewhere.)


  • David Ivory June 6, 2015, 12:09

    I’ve mentioned previously here that those of us living in the southern hemisphere commonly see the Magellanic Clouds out of the corner of our eyes but they almost disappear when you try to look at them directly. Very dim indeed .

  • Christopher Phoenix June 7, 2015, 3:54

    Indeed, the Roscosmos video spreads misconceptions as all they did was project astrophotographs of various galaxies and nebula on the night sky. Those images are the result of hours of light-gathering time by large telescopes using CCDs and even more hours of post-processing to produce a pretty image. Through these techniques, we can see details the human eye could never see. But that means these images in no way represent what we would actually see.

    As Paul mentions, M31 (the Andromeda Galaxy) is rather large in our own skies- several times the diameter of the full moon- but it is rather faint. The problem with seeing most deep-sky objects is not that they are small, but that they are very dim and hard to spot.

    I wonder if the folks who put this video together are aware of this.

    So our Milky Way is a four-armed semi-barred spiral, then? It would be interesting to see a galaxy photograph that best approximates what astronomers currently envision our own home galaxy looks like- a “twin” of the Milky Way, so to speak!

    On Melissa Graham’s work- these intergalactic “free” supernova are quite interesting. With up to 15% of the stars in some galaxies getting ejected in galactic collisions, a percentage of all stars that host or will host alien civilizations must wind up extragalactic orphans. For such civilizations, space travel beyond their own planetary system will be all-out impossible, even with technology right out of science fiction.

    I think not of Ian Banks or Poul Andersons’ fiction but the home planet of ravenous Coeurl in A.E. van Vogt’s Space Beagle stories. On the long (even with FTL) voyage to M31, the “great nebula in Andromeda”, the dauntless crew stop by a planet out beyond the rim of our galaxy orbiting a nameless star with no other planets. The inhabitants our long dead and only Coeurls (a deadly variety of alien) remain.

    A scientist realizes that the inhabitants of this planet were trapped by cosmic geography. Human space travel developed by a series of leaps to ever remoter planetary destinations- the Moon, Mars, Jupiter and beyond- followed by the leap to the nearest star and the eventual discovery of FTL travel. The civilization on the Coeurl’s planet had nowhere to go unless they were up to the intergalactic leap to the nearest island galaxy from the get go.

    Perhaps all our dreams of interstellar exploration are the prerogative of people who inhabit privileged positions in the cosmos. A hypothetical civilization around an extragalactic orphan star would face such a vast gulf of space separating them from the rest of the stellar universe that it is unlikely they would develop deep space travel beyond interplanetary hops. It would be an unimaginably claustrophobic situation (to me, at least).

  • Robert June 8, 2015, 13:27

    I have often wondered about what the views of the Hubble pictures would really look like and have always thought that there is no vantage point that would give the same view to human eyes but since the Milky Way can be seen with the naked eye I should think if one were looking from a planet about the same distance above the plane but centered on the galaxy it should be quite large and clearly visible. Sure, not quite as bright as the pictures we see but since there would be less dust in our line of sight it should be somewhat brighter and a glorious sight to behold.

  • Alex Tolley June 8, 2015, 15:29

    I am always amused that even an alien world requires that we see with human eyes, it is so anthropocentric. Even on earth, a nocturnal primate like a Tarsier will see a much brighter object. Our instruments offer hugely improved imaging, that even in the 20th century, the photographic images became the definitive images, rather than the faint naked eye ones. This has become so much accepted, that now SciFi movies have starships flying by solid looking dust clouds and nebula that are depicted in the iconic Hubble images. One can only imagine the eyes that will peer at M31 from a lonely world in galactic space, and what their instruments will train them to expect as reality.

  • ljk June 9, 2015, 10:18

    I cannot stand the over-nebulafication of the Universe as depicted these days, even in the new Cosmos. Of course this “science” documentary also depicted the Voyager probe zipping through space at the edge of the Sol system as if it were constantly running through a fog. Oy.


    Is it just because humans like pretty colorful clouds, or are they fearful of the infinite blackness barely broken by the pointy light of the stars?

  • Robert June 9, 2015, 11:51

    I wonder what such dim celestial objects such as the M31 or the Magellanic clouds would look under dark sky conditions with night vision goggles? Perhaps in the future someone will develope such goggles that translate the light into the correct frequencies to give true color for astronomy buffs.

  • Christopher Phoenix June 9, 2015, 19:59

    @ljk I have not seen much of the new Cosmos, but from what I have seen and the reviews of trusted friends it is a disappointment. Particularly as it cravenly avoided the topic of evolution show as not to ruffle the feathers of Creationists/(un)Intelligent Design people.

    A popular exposition of science that avoids discussing a major theory of science because the facts as they stand conflict with the emotional beliefs of some people could hardly stray further from the core spirit and values of science. The philosophy of science is to see and model nature as it IS- not as we wish it to be. No set of ideas or beliefs about the physical universe sacrosanct so as not to be beyond questioning. If scientists coddled ideas we would still have adherents to the geocentric cosmos. Instead the wisely chose to discard a those theories that do not fit the facts.

    This, of course, only applies to religion where it treads on science’s toes- faiths where the physical universe must fit in with a personal mythology, like the four elephants on the turtle or Young Earth Creationism.

    Maybe they will next avoid mentioning that the Earth is round so as not to upset the Flat Earth Society. Or avoid the topic of space travel so the Moon Landing Hoax people aren’t bothered. *ironic grin*

    I guess the “Nebulafication” issues you raise are similar- it all comes down to giving people what they expect to see. A sort of cosmic coconut effect. I think that the true crystalline beauty of space would be far, far more interesting to show people.

    @Robert Mmm… I’m afraid such goggles are fantasy, real night vision makes everything jittery and green, not much good for sky-gazing. The colors you see in photographs of nebula and the details of galaxies are brought out by processing hours and hours of light gathering time with CCDs at the focus of large telescopes. No goggles could do this in real time.

    If you want increase the light gathering power of your eyes, I suggest the traditional tool of sleepless astronomy buffs- the humble Newtonian reflector. *big grin* Even with my tiny refractor the Orion Nebula is a fascinating sight. Even though it is faint and shows only the faintest hint of green, it is awe-inspiring.

    Space is beautiful to the human eye. Don’t let yourself be spoiled by pretty images from Hubble to the point where the real thing disappoints. :-)

  • Marshall Eubanks June 10, 2015, 1:54

    Christopher Phoenix – I didn’t think the new Cosmos pulled any punches. Episode 1 had a big section on Giordano Bruno, and Episode 2 was entirely about evolution.

  • ljk June 10, 2015, 11:12

    As much as I found the new Cosmos to be wanting in several key areas compared to the original by Carl Sagan, they did address evolution pretty directly, I will give them that much. Hey, it’s only the early 21st Century, after all.


    However, whereas Sagan was very good at persuading his listeners without making them feel forced to agree or like a bunch of idiots for not knowing something, I found the new Cosmos to often be rather pushy about its agendas.

    It also tended to manipulate its instances of history lessons. Examples include making Robert Hooke look and act like some kind of monstrous freak in the parts about Sir Isaac Newton. Yes, Newton was a genius, but he was also a big jerk and Hooke had many brilliant ideas which Newton and his sycophants often downplayed or ignored because Hooke had the nerve not to sufficiently kiss the feet of His Greatness.



    Among the biggest offenders was the very first animated segment from the first episode portraying Giordano Bruno. This defrocked Dominican monk has often been used as a champion of science over religion, when the truth is Bruno was an eccentric mystic who didn’t know when to shut up when it came to disagreeing with everyone who disagreed with him. Which was a really stupid thing to do in post Reformation Italy.

    If Bruno had not happened to agree with the Copernican heliocentric view of the Universe (because it fit HIS worldview), chances are Bruno would be little more than a historic footnote as one of many cranks and heretics who took on the Roman Catholic Church and lost.

    The new Cosmos made the whole situation rather black and white, with Church officials looking almost literally like Disney villains jumping on poor but righteous Bruno for believing that Earth was not the center of existence and that alien life existed. Earlier scholars such as Nicolas of Cusa had brought up such ideas and did not get into trouble. In reality the Church was far more concerned about Bruno’s views on Christianity, but that is not the impression one would get if they only had this Cosmos to go on.

    This excellent article goes into detail on the subject:


    I know it is certainly better that the new Cosmos existed and will hopefully inspire a new generation of thinkers and doers, but that does not mean it is excused from proper criticism or cannot be improved upon. I hope the next iteration of Cosmos takes to heart the examples of Sagan’s first effort and does not just ride on his coattails.

  • Alex Tolley June 10, 2015, 11:42

    Re: Cosmos – There are se4veral places where the script pulls no punches. IIRC, the controversy was that a tv affiliate station in Kansas{?) did not screen a part of one of the episodes.

    Cosmos is now streaming on Netflix and can be binge watched and without adverts. I found some of the graphics very annoying and I still liked the original series better, but. IMO, it was a good effort.

  • ljk June 10, 2015, 11:42

    Christopher Phoenix said on June 9, 2015 at 19:59:

    “I guess the “Nebulafication” issues you raise are similar- it all comes down to giving people what they expect to see. A sort of cosmic coconut effect. I think that the true crystalline beauty of space would be far, far more interesting to show people.”

    In an ideal world, science documentaries are supposed to be about showing the audience reality as it is, not as they might perceive or wish it to be. They can disagree with the portrayal if they want to, but science has an obligation to be as accurate as possible.

    “We must understand the Cosmos as it is and not confuse how it is with how we wish it to be.” ― Carl Sagan, Cosmos (1980)

  • Robert June 10, 2015, 12:52

    “Space is beautiful to the human eye. Don’t let yourself be spoiled by pretty images from Hubble to the point where the real thing disappoints. :-)”

    Thanks. I often peruse my large coffee table book ‘Galaxies’ by Timothy Ferris. The pictures are large enough to let my mind drift among the beautiful islands floating in space. I find that my thoughts often go beyond the science to contemplate what the Psalmist said “The heavens declare the glory of God;
    and the firmament sheweth his handywork.”

    Psalm 19:1 KJV

  • Paul Gilster June 10, 2015, 13:00

    Ferris’ gorgeous book on galaxies is a personal favorite, an astonishing visual feast.

  • David Herne June 11, 2015, 23:03

    David Ivory – go somewhere truly dark and the Magellanic Clouds stand up to direct scrutiny. They almost appear as ‘clouds’ in the sky. Beautiful. I recommend viewing the Milky Way from outback Australia to all who are interested.

  • Rob Henry June 12, 2015, 19:09

    Christopher Phoenix, I believe that the original Cosmos was the wiser in not addressing evolution to non scientists, as it is a topic that is very hard to convey without mathematics. An unbiased account of its historic development would have Mendel providing the greater part of its foundation (added to latter by Fischer et al in the 1940s), but the compensatory Darwin providing much of the original interest. However, if you wrote the script that way you would have creationists jumping up and down and claiming victory.

    Sagan liked his pretty pictures without oversimplification or propaganda, and so do I.

  • ljk June 14, 2015, 22:32

    Carl Sagan did address evolution directly in the second episode of the original Cosmos series. He even said evolution is a fact straight out.

    “One Voice in the Cosmic Fugue”:


    This episode includes many famous features, such as the story of the heike crab of Japan, computer animation of how DNA works, the evolution of Earth life from the first microbes to humans, and the possible life in the atmosphere of a Jovian planet with massive floaters, sinkers, and hunters.

    A review of this episode from The Planetary Society: