Why are Andromeda and the Milky Way on a Collision Course Rather than Moving Away from Each Other?

Question: If the universe is expanding and all galaxies are moving away from each other, how is it possible that the Andromeda and milky way galaxies are on a collision course?  – Johnny

Answer: It is correct that on the largest scales that the universe is expanding such that all galaxies are moving away from each other.  On smaller scales, though, there are so-called “peculiar motions” of galaxies, where one galaxy is found to be moving toward another galaxy due to local gravitational effects in the vicinity of the two galaxies.

Jeff Mangum

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What is the Size of the Giant Galaxy IC1101?

Question: I’ve been looking at information about the largest known galaxy, IC 1101. Most sources say it is ~6 million light years in diameter but I can’t find any primary sources for this size estimate.

I was looking through research papers and the closest I could find to a direct measurement was 600kpc (so only about 2 million light years). There are lots of other figures mentioned around the internet but they just seem to have been pulled out of thin air!

I even tried measuring it myself using an image from Chandra, but I don’t think the usual trigonometry calculations work when objects are so far away and redshifted (I got a much smaller 150,000 light years as a result!)

I suppose my question is, how big is IC 1101 really, and how do we know?

Many thanks,


Answer: You can get a pretty complete listing of the information available on just about any galaxy from the NASA Extragalactic Database (NED) at http://ned.ipac.caltech.edu/.  You can use the object name search to look up the information for IC1101, where it lists in its “basic data” section that IC1101 has a major axis diameter of 1.2 arcminutes, which for a distance of 328 Mpc yields a major axis diameter of about 114 kpc.  Note, though, that this is likely a size based on optical measurements of just the stars in this galaxy.  I was not able to find any references to actual measurements of this galaxy which confirm the claimed sizes in the 6 million light year (about 2 million parsec) range.  Note, though, that the actual diameter, which would include matter that is “dark”, could easily be 10 times larger than its optical size.  So, since you have clearly done a bit more digging into the research literature than I have, your 2 million light year estimate is probably a pretty good estimate of this galaxy’s size.


Jeff Mangum

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The Kerfuffle Over Non-Cosmological Quasar Redshifts

Question:  Some years ago I read an article that said (if I recall correctly) that there were quasars that seemed to be associated with galaxies (maybe in the center), but the quasar’s much larger red shifts implied that their distance was far more than the associated galaxies’ distances.  Has this ever been resolved?  – Bill

Answer: The research that you are referring to was done mainly by two astronomers, Halton Arp and Geoffrey Burbidge.  They proposed, based on observations of seemingly associated nearby galaxies and purportedly distant quasars, that quasars were simply ejected matter from these galaxies.  In fact, once large surveys of galaxies (such as the Sloan Digital Sky Survey), became available it was possible to better test this apparent correlation.  In summary, Arp and Burbidge were wrong, their assertion due in fact to what astronomers call a “selection effect”.  If you are interested in more details on this now historical discussion see the Galactic Interactions blog post on the subject.

Jeff Mangum

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How Do Astronomers Study How Things Evolve?

Question:  We know that stars and galaxies we see are just fossil light as they were millions or billions of years ago. Is it possible to extrapolate the changes that we see today in those galaxies to determine their current state?  – Vinod

Answer: In a way, yes.  Since, as you point out, we see what amounts to the “fossil light” from stars and galaxies in the universe, we can piece-together how things evolve with time by sampling various times within this fossil record to study the evolution of these stars and galaxies.  Note also that the timescales for the evolution of objects in the universe are, with few exceptions, much longer than a human lifetime, or even the total historical record of scientific measurements.  This means that astronomers must study the evolution of just about every object in the universe by sampling its evolutionary state at different times in the cosmological record.

Jeff Mangum

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Why Do Stars Form Preferentially in Spiral Arms?

Question:  Why most of the star forming regions/open clusters are in the periphery of galaxies(in spiral arms)?  – Vinod

Answer:  Star forming regions are concentrated in parts of galaxies that contain high concentrations of the material from which stars are made: gas and dust.  Depending upon the type of galaxy and the kinds of gravitational interactions it might experience, these concentrations of gas and dust can be “pushed” to the point where they collapse to create stars.  The spiral arms in spiral galaxies are one type of environment where gravity is pushing gas and dust to form stars more efficiently than in other parts of a spiral galaxy.  This is why you see more star forming regions and collections of young stars (open clusters) in spiral arms than in other parts of a spiral galaxy.

Jeff Mangum


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What is the Universe Expanding Into?

Question:  Thank you for helping so many people. Is there an educated guess by professionals about the difference between what the universe is expanding into and the universe itself?  – Jay

Answer:  This is going to sound very paradoxical, but the answer is that the universe is already infinitely big, so it is in fact not expanding anything.  What is actually happening is that the space between everything in the universe is getting stretched, which results is out seeing all galaxies in the universe, which are not under the influence of local gravity like a cluster of galaxies, moving away from all other galaxies.  Now, if you want a much more in-depth explanation of this rather odd fact, check out the Curious About Astronomy page answer to this question by my colleague Dave Rothstein.

Jeff Mangum

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What if a Galaxy had Only One Star?

Question:  What if a galaxy only has 1 sun and 1 planet an it is in the exact middle?  – Lance

Answer:  Galaxies are defined as collections of stars (possibly with planets), gas, and dust.  A single sun with a planet would not really qualify as being a “galaxy”.  What you are really describing would be considered a free-floating star and planet, which somehow ended-up outside the galaxy from which it formed.  To my knowledge we have never seen such an object.

Jeff Mangum

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Can Gravitationally-Lensed Objects be Studied at Any Wavelength?

Question:  As per definition, Gravitational lensing refers to bending of light rays from a distant source around a massive object (Galaxy cluster) which tends to magnify the background light source. If visible light rays bend around those massive objects then X-rays, Gamma rays, UV and IR rays which forms a part of electromagnetic spectrum must also bend around those massive objects. If true, Are there any initiative to detect those distant lensed invisible objects using the space observatories(Chandra X-ray, Spitzer IR etc.)?  – Vinod

Answer:  Yes.  In fact, just a few days ago there was a press release announcing the detection of the spin of a black hole using the gravitationally-lensed x-ray emission from a black hole in a distant quasar.  Gravitational lensing has also been used to study galaxies using the Spitzer Space Telescope.  In fact, as their is no wavelength dependence to the gravitational lens effect it is possible to study gravitationally-lensed objects at all wavelengths.

Jeff Mangum

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Top Achievements for the NRAO Very Large Array

Question:  I’m embarrassed to ask this, as I feel after working here for so long I should already know the answer. But here it is: What are the top 3 discoveries/achievements of the VLA? And why?  — Doug

Answer:  As you can imagine, an actually ranking of the Very Large Array (VLA) achievements would be open to debate.  Kind of “beauty in the eye of the beholder” problem.  Dave Finley, NRAO Public Information Officer, provided the following links listing what NRAO believes are the VLA’s top scientific achievements:

Jeff Mangum

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Are Quasars and Radio Galaxies the Same Thing?

Question:  I’m interested in the differences and similarities between radio galaxies and quasars. Different sources say different things. Are radio galaxies just quasars with the “jets” pointed away from us, or are there other differences between them?  – Steve

Answer:  It turns out that even though quasars, which is short for “quasi-stellar radio source”, were first discovered as sources of radio emission that appeared to be point-like (like a “star”) in the optical, only about 10% of the known quasars today are also sources of radio emission.  You can think of radio galaxies and quasars as just different kinds of galaxies.  There are lots of radio galaxies, some of which are also quasars.  There are also lots of quasars, only a fraction of which are also sources of radio emission.

Jeff Mangum


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