How Long Does it Take the Sun to Orbit the Center of our Galaxy?

Question: How long does it take our sun to complete a full orbit around the super blackhole at the center of the Milky Way?  — Dustin

Answer: We can calculate this value by noting that the Sun is about 8 kilo-parsecs, or about 2.5×10^(17) km, from the center of our galaxy and travels at a speed of about 225 km/sec around the center of the galaxy.  Assuming that the Sun’s orbit about the center of the galaxy is circular, we know that the circumference of that circular orbit is 2*pi*r, where r is the distance from our Sun to the galactic center.  Since distance = rate * time, we know that time = (2*pi*r)/rate = (2*pi*2.5×10^(17) km)/220 km/sec =~ 225 million years.

Jeff Mangum

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The Eventual Collision of the Andromeda and Milky Way Galaxies

Question: If I understand the various articles I’ve read and programs I’ve seen on TV and the internet.  It has been determined that the Andromeda and Milky Way galaxies are on a collision course and are destined to “collide” in about 4 billion years. My question is this. If the collision will happen, how can it?  Given the observations thus far that all of the galaxies are receding from each other due to the expansion of space-time itself.  — Guy

Answer: You are right in that on the large scale all galaxies are expanding away from each other.  On smaller scales, though, there are what are called “peculiar motions” where galaxies can have motions relative to each other that are due to local gravitational attraction.  The Andromeda and Milky Way galaxies have peculiar motions whereby they are moving toward each other, which results in the prediction that they will eventually collide.

Jeff Mangum

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How Do Galaxies Stay Intact?

Question: Do black holes spin? What makes galaxies stay semi organised while precessing around it? I don’t know how else to word that. Physics is hard.  —  Nathan

Answer: Yes, it is believed that black holes do spin.  The stars in galaxies stay organized over long periods of time while the galaxy rotates due to the mutual gravitational attraction of the stars, gas, and dust in galaxies.  That gravitational force balances any forces that might pull a galaxy apart, making the galaxy maintain a stable configuration over many millions of years.

Jeff Mangum


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How Can Gas Particles Stick Together to Ultimately Form Stars and Planets?

Question: There is a lot of acceptance that when the gases of the universe began to join and spin because of the impact of those collisions. That would mean that all particles hit in the same direction, like using your hand to spin a basketball on your finger. When one questions this I just get blank stares as if I’m suggesting some sort of heresy. But, apart from the difficulty I have with the Big Bang being possible on it’s own (Quantum Physics suggests you need an outside force) the answers I read about the formation of planets and gravity seems absurd yet I can’t find anyone to explain it without theory and conjecture. Can someone help me. Just to make it clear what I’m asking, please help me understand how gas particles flying in every possible direction as can form into a rotating ball that turns into rock because physics itself suggests that is impossible. I use the theory that is used for the formation of planets as….

“…there was a massive cloud of hydrogen gas left over from the Big Bang. Some event, like a nearby supernova explosion triggered a gravitational collapse of the cloud, causing the hydrogen atoms to attach to one another through mutual gravity. Each individual hydrogen atom had its own momentum, and so when the atoms collected together into larger and larger clumps of gas, the conservation of momentum across all the particles set these clumps of gas spinning.”

Now that is a lot of conjecture – considering collisions from every possible direction…let alone how those hydrogen gas particles, once collected together formed everything we have on and in the planet.

— Brent

Answer: I think that the slight misunderstanding in your logic is the equate “sticking” with the gravitational attraction between particles such as atoms, molecules, and dust particles, which ultimately results in the formation of more massive objects like asteroids, comets, planets, and stars.  It is not necessary for objects to collide and “stick” to each other immediately.  A stable cloud of massive particles that is affected by a nearby event that “pushes” on it, such as a nearby supernova, will potentially be pushed in such a way that gravity causes objects to slowly move closer to each other and ultimately coalesce.  This slow collapse of massive objects toward other massive objects ultimately builds on itself, collecting larger and larger massive objects.  This ultimately is a mechanism for forming objects like planets, stars, and galaxies.

Jeff Mangum

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Inconsistency Between the Age and Diameter of the Universe?

Question: I have read that the diameter of the universe is 96 billion light years.  How can that be if the universe is a mere 14 billion years old?  Am I to conclude that the 96 billion figure is some extrapolation based on rapid inflation?  — Carlton

Answer: One of my physicist colleagues, Frank Heile from Stanford University, has provided an excellent answer to this question.  First of all, the 13.8 billion light years is derived from the radius of a sphere of the Cosmic Microwave Background (CMB) radiation that is being observed by the Wilkinson Microwave Anisotropy Probe (WMAP) and Planck satellites.  These satellites have mapped the structures which are precursors to galaxies and clusters of galaxies.  Now, the radiation from the CMB measured by the WMAP and Planck satellites has, in the meantime, continued to expand so that the structures measured in the CMB signal would be 46.5 billion light years from us at this time.  Second, the 93 billion light year diameter estimate refers to the “observable” universe.

Now, if we waited another 46.5 – 13.8 = 32.7 billion years, we should actually be able to see the light emitted right now from those superclusters of galaxies which formed from the CMB structures.  The light is already on its way towards us but it will take a while to reach us since it will have to come from a sphere with a diameter of 93 billion light years.  This is the explanation for the difference; the “observable” universe is larger than we can see it today.

This is only, at best, a theoretical estimate of the diameter of the universe, though.  We now know that due to dark energy the expansion of the universe is accelerating.  This acceleration will not allow us to see those superclusters which are now 46.5 billion light years from us.  In fact, if we wait the requisite 32.7 billion years those superclusters will be receding from us at a rate that is greater than the speed of light.  We just will not be able to observe those galaxies and clusters of galaxies which have formed at the edge of the universe.

Jeff Mangum

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What Radio Astronomy Can Be Done at S-Band?

Question: At work, we have a 14 foot dish with an S-Band receiver.  Are there any interesting radio astronomy frequencies around S-Band?  — Frank

Answer: The radio astronomy S-band runs from about 2655 to 3352.5 MHz.  Its use in radio astronomy is for continuum measurements from sources of synchrotron and free-free emission, such as supernova remnants and regions where stars are forming.  Note, though, that there are numerous communications services which operate at S-band, so you might find it hard to detect astronomical signals if your receiver is not tuned to the radio astronomy allocated frequency range.

Jeff Mangum

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What is the Large Scale Structure of the Universe?

Question: What does the structure of the universe look like at the largest scales?

  1. Galaxy cluster are distributed evenly throughout space with no large gaps
  2. There are many more galaxies and clusters in some directions (up and down the milky way’s disk) and very few galaxies in other direction
  3. Linear or wall like distributions of galaxies, clusters, and superclusters surrounding relatively empty regions- like soap bubbles
  4. Galaxies and cluster are very thiny spread out in the near distance, and are more closely packed at greater distances from the milky way

— Jared

Answer: I think that option 3 comes closest to the actual distribution of galaxies in the universe.  The large-scale structure of the Universe is made up of filaments and voids.  When we look closely at the filaments, we find that they can be broken down into superclusters, clusters, galaxy groups, and finally into galaxies.

Jeff Mangum

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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  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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