Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Stuff I always wondered about astrophysics

  1. Sep 29, 2009 #1
    I've numbered them so you can just include it in your reply instead of screwing around with
    Last edited: Sep 29, 2009
  2. jcsd
  3. Sep 30, 2009 #2

    I'm overwhelmed! Come back with your top 1-3 questions, and maybe we can explore them a few at a time.

  4. Sep 30, 2009 #3


    User Avatar
    Science Advisor
    Homework Helper

    Fusion doesn't occur throughout a star, it happens only in the core or in a thin shell around the core. The rest of the star heats up to a few 1000 deg.

    It takes a lot longer than a few decades for photons to make their way out of star - for the sun it's around 30-40,000 years

    Neutron stars don't fuse anything. We detect them by the radio jets from their magnetic field or occasionally from X rays produced by material in their solar system crashing into the surface.
    They are blackbodies and are created at around 1M degrees, cooling by radiation fairly slowly.
  5. Sep 30, 2009 #4


    User Avatar
    Gold Member

    To follow on with mgb_phys, when stars are born in dust-clouds like we see in the Orion complex (M42) it not only takes a very long time for the proto-star to develop, but the star continues to accrete from the dust and grow, until the star is so potent that the radiation pressure from the star becomes strong enough to push away (evacuate) the dust and gases from its environs and become visible to us.

    Stars forming in such environs can be more easily detected in the longer wavelengths (deep infra-red), but their visibility in (human) visible light is limited by local systematics. SDSS's survey has been studied in great detail and there are probably a lot of proto-stars, mixed in with the brown dwarfs that can mimic quasars.

    Observational astronomy, astrophysics, and stellar evolution (all wonderful fields) often have to give up observing hours on huge telescopes with wonderful new sensors as researchers strive for "the faintest", "the most distant" etc. Pushing the envelope is admirable, but there are local galaxies that have insufficient spectroscopy and that situation could be resolved in short order. Spectroscopy of very faint objects requires long, long hours of photon-collection to get anything resembling usable data.

    Are we looking too deeply, squandering our resources on too many large instruments pursuing too many similar goals? I think we may be. I would love to see at least one large instrument each in the northern and southern hemispheres dedicated to mapping our local universe, in the hopes of illuminating some of the mysteries that confront us regarding redshift anomalies in interacting galaxies, redshift distribution in tight and loose clusters, etc. We don't know everything about our universe, and it is presumptuous to pretend that we do.

    Please remember that a prime directive of the Hubble Space Telescope was to make observations that would help determine how Hubble's distance/redshift relation could be refined. There have been lots of papers written about those observations, with seemingly warring camps led by Sandage and Freedman. Are there other possibilities outside of this conventional (and artificial) dichotomy? We ought to consider this.

    Edit: BTW, there are some pretty wide variances in the constants proposed by both groups, and if you wish to consider secondary distance indicators, things still don't get that much tighter.
    Last edited: Sep 30, 2009
  6. Sep 30, 2009 #5


    User Avatar
    Science Advisor
    Homework Helper

    5, Yes a novae is generally just material falling onto a star (typicaly a white dwarf)
    A SN is an entire star blowing itself apart. Other than they appeared to observers as a new star they aren't linked.

    6/7 - The SN light curve is pretty fast, material collapses into the core at relativistic speeds. The rise certainly less than hours observationally

    8, Novae aren't collapses. On a supernovae only the inner core of the star collapses. The core reaches almost neutron star densities before rebounding - ie. 10s of km

    9 -- Don't know, there are different types of SN depending on the source star.

    10 For small SN (typeI) energy the energy comes form the decay of isotopes formed by fusion in the collapse. For large (typeII) stars a lot of the energy comes from just the gravitational collapse of 100solar masses crashing into a point at 25% c !

    11 -- Most of the mass of the core probably ens up in the S radius, but most f the star is thrown out at 0.1C so never gets a chance.

    12 Depends on the mass of the star, supernova remnents are found around most stars but a lot of the mass is too far out to really be considered as orbiting the black hole.

    13 The black hole will rotate at high speed just from conservation of angular momentum. The neutron stars formed by a smaller collapses start their life spinning at 10,000 Hz. Non-spherical rotating black holes are a bit out of my field!

    14 The envelope is pretty opaque, most of the light is from the envolope not the core.
    Hardly anything falls into a black hole because of angular momentum it tends to form a disk around it.

    15 Type I/II SN are different just because of the mechanisms of energy formation. Decay vs gravitational

    16 Nope it's pretty much neutrons. Some of the neutrons on the surface might be in crystals rather than purely degenerate - there is a whole field of NS chemistry,

    17 Degeneracy?

    18 Same place that photons come from when an electron moves in a magnetic field.

    19 In theory quite a lot. Models suggest about 2:1 don't know if anyone has ever measured this in the wild

    20 The magnetic axis is almost the rotation axis. The jets come out very near the poles, we only see their rotation because of precesion.
  7. Oct 1, 2009 #6


    User Avatar
    Science Advisor
    Gold Member

    1. Faye, what the heck are you talking about? Proto stars ignite at their leisure [millions of years].
    2. See stellar fusion 101.
    4. What process do you have in mind that accounts for neutron star emissions?
  8. Oct 1, 2009 #7
    Never mind then.

    (Geez, it's a good thing that I only asked the astrophysics questions, and not all my questions about astronomy.)

    No offense, but I want to know ALL of those things, and I've wanted to know them since college, when I learned about all about all these interesting things.

    But between all the Hyashi tracks and nuclear chemistry and stuff, I don't think they told us any of those particular things when I was an astronomy major.

    I did ask many, many questions then, but I couldn't ask anything in class if the professor was telling us about something else, and I didn't want to ask for free individual lessons at his office. It would be gluttonous, like eating all the burgers at mcdonalds, and when it closes, following the guy home and asking him to grill more in his backyard.

    I'm like a homeless person begging for food. I don't mean to intrude or imply that you OWE me anything, it's just that no matter how much I eat, it just makes me more hungry.

    Last edited: Oct 1, 2009
  9. Oct 1, 2009 #8
    Wow, I knew about the core of course, but I didn't know that it happened in a shell around the core.

    Why doesn't the fusion happen deeper too, where there's even more pressure?


    this is an example of how eating makes me more hungry.
  10. Oct 1, 2009 #9
    Isn't there a moment when fusion begins? And when it does, doesn't that make a huge immediate difference in the appearance of the star?

    Well I don't think they do it for the Guiness Book of World Records. Amateurs do, like http://blogs.myspace.com/index.cfm?...ndId=150103974&blogId=511606888&swapped=true" specifically so she would be the youngest.

    I'm sure your're correct, but I'm curious why spectroscopy of local galaxies is important. Is it just for survey reasons, for completeness? Surely you're not looking for something new in them. Being local, their supernovae don't tell us anything about the Hubble constant.

    I can see galaxies' positions being important to cosmology, and I can also understand if what you're really studying is the absorption spectra of nearby gas in the light path. But why is the compositions of a billion stars added together not just like the spectra of all the stars in other galaxies?

    Maybe they're trying to map stellar populations.
    Not working in the field, my opinion is like being a backseat driver. Nevertheless, my opinion is that we are not.

    When I was a little kid, the largest scope was the 200", and I was actually sad because (I was told) you can't make telescopes larger than that (MMT and adaptive optics were still in the future), and that meant that there would never be any really new discoveries, like galielo and the moons.

    But you need new, more sensitive instruments to discover the new stuff. And dedicating them to filling the blank spaces in the Next General Calalogue doesn't seem, to me, to be the best use of new, unique capability.
    Hmm, so what we're missing is the dynamic stuff; it's not just about not pictures of new phenomena. That makes sense.

    Woh, I should read more than just sky and telescope. What could they possibly disagree on, much less vehemently? Is the value of the Hubble constant still in question? Is the positive cosmological constant not accepted by everyone (I hope)?

    Alan Sandage is still alive! Cool! At least SOMEBODY'S still left.

    I'd like to know more about the dichotomy, please. I don't even know what the issue in contention involves. Cosmology? Galactic evolution? Calibrating the Hubble constant?

    "Constants", as in more than one? What?

    Why not? If you've got redshift, supernovae, cepheids, and probably new stuff I've never even heard of, don't they all agree? Which one is different than the others?


    I always hate it when people apologize to me for asking so many questions, but now I know how they feel.
    Last edited by a moderator: Apr 24, 2017
  11. Oct 1, 2009 #10
    I'm not "talking about" anything. I'm asking how long it takes for stars to switch on.

    Well that's the answer to my question then. Thanks

    I was drunk that day

    If I knew that, I wouldn't have asked the question, as that's just a rephrasing of it (but someone else answered it: pure blackbody)


  12. Oct 1, 2009 #11


    User Avatar
    Science Advisor
    Homework Helper

    Metalicity distribtuion of our own and local galaxies might tell us how galaxies form and why there are galaxies.
    This might be more interesting than reducing the uncertainty in H from 5% to 4%

    It's pretty much nailed down now at around 75. For years there was a row between Sandage (H=50) and some others ,including Wendy Freeman, for H=100.
    Freeman's research with Hubble has pretty much nailed it at 72 - which ironically is close to what Sandage originally picked.
    That top scientists are not generally very conciliatory by nature and the fact they worked in the same building didn't help matters.
  13. Oct 1, 2009 #12


    User Avatar
    Gold Member

    There are other distance indicators. One of my collaborators has written extensively about some of them, including some interesting trends that seem to correlate with galaxy morphology.

    http://www.citebase.org/abstract?id=oai:arXiv.org:astro-ph/0503432 [Broken]
    http://www.citebase.org/abstract?id=oai:arXiv.org:astro-ph/0310284 [Broken]
    http://www.citebase.org/abstract?id=oai:arXiv.org:astro-ph/0503440 [Broken]
    http://www.citebase.org/abstract?id=oai:arXiv.org:astro-ph/0402001 [Broken]
    http://www.citebase.org/abstract?id=oai:arXiv.org:astro-ph/0408348 [Broken]
    Last edited by a moderator: May 4, 2017
  14. Oct 1, 2009 #13
    Woh, THANKS! Too bad it's just abstracts, but they do tell me all kinds of new ways to measure distance.

    Now I gotta look up "Tully-Fisher relationship". That will lead to something else, which will lead to...

    It's going to be another Wikinight.
    Last edited by a moderator: May 4, 2017
  15. Oct 1, 2009 #14


    User Avatar
    Gold Member

    It's not just abstracts, Faye. You can link to the full PDF documents from CiteBase. Just look at the abstract pages to see.

    Dave's papers are very well-researched, and like all papers that get published in well-respected journals, they are refereed and vetted. Our paper on M51-type galaxy associations was published in Astrophysics and Space Science - a subscription-only Springer journal. The editor urged us to post it on Arxiv as soon as the final version was accepted for publication, so there is a free version available on-line.


    We have accomplished the data-collection and most of the analysis for our second paper, and are in the process of writing that one. Like Dave's papers, it won't get many cites because it will address a very unpopular subject - the possibility that some astronomical bodies posses intrinsic redshifts in addition to the redshift-distance relation discovered by Hubble. There are still puzzles in the Universe, and observational astronomy holds the key. The cosmology tail is wagging the astronomy dog these days, IMO, and has been since Gamow and the ascendance of the Big Bang Theory.

    Some observational astronomers are concerned about this situation. Among them is the outspoken Michael Disney. BTW, I agree with him.
  16. Oct 9, 2009 #15
    35. If the monoliths have mass then they have gravity. Thus, with enough of them the added gravitational force will be enough to compress the hydrogen and initiate fusion, just as happens in the real universe.
  17. Oct 10, 2009 #16
    I'm sorry Frank, I think you missed it.

    They won't do fusion unless they're made of hydrogen. You may be thinking of them forming a black hole.

  18. Oct 10, 2009 #17
    Woh! I don't know if you're Ari or Skip, but your paper was fascinating! I've wondered how the little stub on the Whirlpool Galaxy got there ever since I saw it at the end of Outer Limits each week as the credits rolled over it. But I didn't realize there were others like that.

    I think that if there is a mechanism for "discordant redshifts", it ought to be in all the newspapers. That means that our primary distance metric is unreliable.

    Note that the difference between BC and BNC is very very much an opinion, though the way you showed how to weed out HII regions was cool!

    Also, you don't suggest a mechanism for discordant redshifts (yes I know that wasn't the purpose). But what is the mechanism? The only thing I can think of is if the galaxy is nearly edge-on and the "addendum" galaxy is approaching us along with the spiral arm it's attached to. But that effect would be seen in every edge-on spiral (and as I remember, it is).

    Is one approaching and one receding because it's a collision? Or what? And why is the position angle distribution so skewed? One would think it would be completely random.

  19. Oct 11, 2009 #18


    User Avatar
    Science Advisor
    Gold Member

    There is no discordant redshift, that is a myth according to the vast majority of astrophysicists. There are plenty of up and coming youngsters who would have jumped on that bandwagon if it had wheels. Physicists are not dogmatic fools.
  20. Oct 11, 2009 #19


    User Avatar
    Gold Member

    Interacting galaxies are actually quite common. We decided to concentrate on pairs that fit the M51 paradigm (large spiral with smaller companion at the end of one arm) because bridged systems are generally considered to be interacting and not chance projections of background galaxies with foreground spirals.

    We can leave that to the theorists. Our concentration is on observation, data collection, and analysis. Our second paper will address the statistical probability of chance projection vs interaction in such pairs. We will approach this cleanly, without regard to secondary evidence for interaction, such as arm asymmetry, tidal distortions, or enhanced star formation.

    If you will take the time to learn how to navigate the SDSS data, you'll see that many HII regions were targeted as possible QSO's and that spectroscopy was required to sort them out. We managed to do this by studying differences in blue, red, and infrared survey plates, but it sure would be nice to get new spectroscopy on relatively nearby galaxies to firm up the results.

    As for a mechanism for a redshift differential, that is a matter for theorists. We observe and constrain, and that is the role of astronomers, since astronomy is a purely observational science. Trends in the data show that about 1/3 of the small companion galaxies in bridged systems are blueshifted WRT to the large host galaxy, and those differentials are small - well within the range commonly accepted for the peculiar motions of gravitationally-bound companions. The remaining companions are redshifted WRT their hosts, and the differentials can be quite large, despite secondary evidence for interaction.

    This result is robust, and not really unexpected, given the trends in other observations. The difference with our survey is that we attempt to be as inclusive and complete as possible. There have been other studies of M51-type galaxy associations, and we reference the most prominent of those in our paper. We believe that our study is the most comprehensive and the most tightly-constrained treatment of this class of galaxy associations. For the thoughts of a professional observational astronomer on redshift asymmetry:

    (emphasis mine) You may be wondering how such trends can be possible. After all, if we think redshift is entirely due to cosmological expansion or peculiar motion (with some smaller component due to gravitation) the only way such a trend could appear is if the smaller companion galaxies are preferentially rushing away from us. We have no reason to believe that the Earth is at any special or privileged place in the universe, so that's out.

    Observational astronomers are well-aware of such trends in the spectroscopic data. The trends cannot be accommodated by current cosmological models, so they are left unaddressed at a minimum, and sometimes derided - often by people with no appreciation for the rigors of observational astronomy. It's easy to engage in nay-saying - it's a lot tougher to actually do the work and come up with a rational defensible argument against the trends. For an example of how precise spectroscopy can be, just visit NASA's NED website and start looking up objects. You'll see their redshifts expressed in many reference frames and often to an impressive number of significant digits. Spectroscopy is precision science. Trends in such data cannot be ignored without grave danger to the integrity of the fields that rely upon it.
    Last edited: Oct 11, 2009
  21. Oct 11, 2009 #20
    Why did you omit the last sentence from the paragraph on redshift asymmetries, turbo-1? If anyone wants to read the complete entry at Bill Keel's website, http://www.astr.ua.edu/keel/galaxies/arp.html" [Broken] I note that that site was last updated in 2003 and thus only contains references prior to then. Has any work on redshift asymmetries, or indeed galaxy pairs, been done using the large SDSS or 2dFGRS databases?
    Last edited by a moderator: May 4, 2017
  22. Oct 11, 2009 #21


    User Avatar
    Gold Member

    The last line in that particular paragraph got into how spectra are analyzed, and did not address trends in the data. I pulled that quote from a text entry, and not his AU page with images.

    You can quote the whole of Bill Keel's thoughts on this situation, if you wish. It won't change the fact that there are redshift anomalies in binary pairs and galaxy clusters. If extra entities are required to "explain" such anomalies, then we have a situation that needs to be explored. If you believe that Dr. Keel is wrong, along with other observational astronomers, it is incumbent on you to make your case. My collaborators and I have invested years of work to explore redshifts in interacting galaxy associations. You are welcome to our data if you'd like to construct a refutation.

    It is available on-line, and I would be happy to link you up with the raw data so you can construct your own statistical model.
    Last edited by a moderator: May 4, 2017
  23. Oct 12, 2009 #22
    Not true. The last sentence is, and I quote:

    "Detailed study of how one measures redshifts from optical spectra in complicated velocity fields shows that some of this effect comes from the different weightings of continuum and emission-line radiation (Keel 1996 ApJS 106, 27)."

    Which seems to run counter to your claim (emboldened by me):

    Which led me to believe you were being a little dishonest by removing that last sentence, which points to at least one detailed study where such a trend is addressed and is partially accounted for, in order to strengthen your argument that people are burying their heads in the sand with respect to these trends.
  24. Oct 15, 2009 #23
    turbo-l: Like Dave's papers, it won't get many cites because it will address a very unpopular subject - the possibility that some astronomical bodies posses intrinsic redshifts in addition to the redshift-distance relation discovered by Hubble.

    I don't think that subject is unpopular. I'd be very, very surprised if astronomical bodies *didn't* possess non-cosmological redshifts. However, right now I don't think that whatever intrinsic redshifts there are are large enough to require a paradigm shift.

    Also cosmology is having it big right now because there is massive amounts of cosmological data coming down from the skies.

    One final thing is that I had one conversation with a cosmologist in which he was of the opinion that Hubble just got lucky in the same way that Clyde Tombaugh did. He said that if you take Hubble's original data and then use the most modern measurements, you get a scatter diagram with no obvious redshift correlation.

    The other thing is that it's not an observationist versus theoretician split. Pretty much all of the observationalists that I know have been rather scathing in their opinions of non-cosmological redshift papers.
  25. Oct 15, 2009 #24
    There are two big and easy explanations

    1) closer galaxies tend to be brighter galaxies which means that you have massive statistical biases
    2) closer galaxies are much younger galaxies and it's known that galaxies evolve radically

    Right now, if you see any sort of redshift bias, most people I know would start with the easy explanations. The trouble with attributing them to non-standard cosmology is that you have a lot of independent evidence for the standard cosmology.

    Last edited by a moderator: Dec 4, 2009
  26. Oct 15, 2009 #25
    Also no theorist that I know of thinks that redshifts are *entirely* cosmological. There are some pretty clear evidence of non-cosmological redshift in the form of "fingers of god". The belief is that any sort of intrinsic redshift is not large enough to invalidate cosmological models and that any trends can be explained by

    1) statistical biases, or
    2) galactic evolution
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook