Talk:Pulsar

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Merge Radio pulsar & X-ray pulsar into Pulsar[edit]

The following discussion is closed. Please do not modify it. Subsequent comments should be made in a new section.

User:Quarl has suggested merging both accretion powered pulsar and rotation powered pulsar into the general pulsar article on the Astronomy WikiProject talk page.

70.51.8.30 05:21, 3 March 2007 (UTC)

Survey[edit]

Discussion[edit]

  • We have smaller articles on types of pulsars than either X-ray or radio pulsar, they can be expanded to a fair degree, I would keep them separate and make pulsar into a better overview of pulsars in general. It should overview most types of pulsars, including magnetars, AXP, SGR, short duration GRB, millisecond pulsars, etc. 70.51.8.30 05:27, 3 March 2007 (UTC)

I agree, I don't see a reason by those two should be selected out in particular for merging. Why not also merge magnetars since their mechanism of action is different? Rather each of the individual branch articles should be improved and the pulsar article should be grown with material common of all pulsars - pulsar origins, physics involved, observation methods, history,etc. – Meowist 20:28, 8 July 2007 (UTC)


The above discussion is closed. Please do not modify it. Subsequent comments should be made in a new section.


Now a new type of pulsar that emits gamma ray radiation has been discovered as well.

http://www.universetoday.com/2008/10/17/fermi-telescope-makes-first-big-discovery-gamma-ray-pulsar/ -- Chupon (talk) 14:30, 21 October 2008 (UTC)

"Religious interpretations"[edit]

I took out this:

Some modern Islamic scholars believe that the Qur'an has mentioned the pulsars: "[I swear] by Heaven and the Tariq! And what will convey to you what the Tariq is? The Star Piercing [the darkness]! " (Qur'an, 86:1-3). The Arabic word "Tariq," comes from the root "tarq," whose basic meaning is that of striking hard enough to produce a sound, or hitting. Bearing in mind the word's possible meaning as "beating," "striking hard," our attention may be being drawn to this scientific fact.

Because it's not referenced. The footnote posing as a reference for it read this:

This is not the only scientific fact mentioned in the Quran; many other scientific facts in the variance science fields (e.g. Astronomy, biology, medicine,…etc), which were discovered in the 19th and 20th centuries, are mentioned in the Quran. Harun Yahya. "miracles of the quran". 

A website authored by one person is not "some modern Islamic scholars". The claim that people believe that the Qur'an is unequivocally referring to scientific principles for which the ancients had no scientific measures is quite extraordinary, and requires extraordinary citation. As it stands, this reads as one man's theory, if not outright promotion.

I see Harun Yahya already has an article; his theories are more appropriately discussed in that article rather than adding backreferences to every other "scientific fact" supposedly appearing in the Qur'an. 82.95.254.249 13:11, 23 June 2007 (UTC)

I deleted a similar block:

Strangely enough the Quran, Muslims' holy book revealed more than 1400 years ago, speaks about pulsars in chapter 86. The original Arabic word used for these stars is 'Tariq'. The verses go on describing the Tariq as the piercing star. Those who translated the word Tariq into other languages did not translate it accurately but most native speakers of Arabic know the meaning of Tariq as 'someone or something knocking on a door or striking with a hammer'.[citation needed]

WTF? It's back there again! Well, I'm going to delete it now. If it appears for a fourth time, I will request semi-protection. --116.14.34.220 (talk) 14:51, 15 June 2009 (UTC)
;^#) Especially the connection between "Tariq" and "pulsar" seems to be requiring extraordinary proof, or else that tariq stuff requires extraordinarily violent deletion, being a kind of hallucinating double UFO-logy. ... said: Rursus (mbor) 12:33, 14 August 2009 (UTC)

I removed a similar addition to the miscellaneous facts portion of the article today. 66.91.241.126 (talk) 20:30, 18 June 2009 (UTC)

More scientific details needed[edit]

A few examples of things that should probably be in this article. I've searched the web, and can't find them, so anyone with knowledge, please contribute:

  • Radiated spectrum: are they broadband pulses, or narrowband? A few articles I found suggested narrowband, but none made any general conclusions.
    • If they're narrowband, are their peak frequencies scattered all over the spectrum? Or do pulsars tend to radiate within a small range? Also, if narrowband, does the peak frequency remain constant over time?
    • It may also be instructive to include a plot of intensity vs. frequency for an example pulsar.
  • Pulse duration vs. non-radiating period. The article mentions periods of 1.5ms to 8s, but for how much of that time are they "pulsing"? I realize that that probably depends on distance from the Earth, so maybe a better metric is the solid angle size of the radiated cone. Is this about the same size for most pulsars, or does it vary greatly?
  • Is there any noticeable precession of the radiation (or rotation) axis?

The above would help describe what a pulsar is/does, not just the history behind their discover. Thanks to any who can contribute. —Ryan McDaniel 22:17, 6 July 2007 (UTC)

Just flagged this as "attention=yes" on the Physics template; this article needs attention I can't give it. If this is not a proper use of that attribute, I apologize. —Ryan McDaniel 21:14, 11 July 2007 (UTC)

Discovery of pulsars before Jocelyn[edit]

Apparently, the US Air Force detected pulsars a couple of months before Jocelyn Bell discovered them; see [1]. Unfortunately, I can't find a better reference for it than that, hence why I'm commenting here rather than adding it to the article. Mike Peel 10:46, 19 August 2007 (UTC)

Patrick1982 added some interesting new text [2] about Charles Schisler's discovery of pulsars pre-dating that of Bell & Hewish. While there is nothing to doubt Schisler's claim, his result is not unique. Several other groups had discovered pulsars beforehand, but either had not realized what they had found, or did not follow up on it - see for example [3]. I think a discussion of Schisler's and other preceding work (not the least of which would be Minkowski's identification [4] of the stellar remnant in the Crab Nebula in 1942) has a place somewhere in this article, but certainly the main credit for the discovery should go to those who followed through with their discovery and who were in a position to publish their result. Let's discuss this further on the talk page before changing the main article. Tubbs334 11:36, 7 September 2007 (UTC)

I can understand your reasons for reverting back to the previous version. However in my opinion it is wrong to just leave this new information out of the article, treating it like it "did not happen". I think that my formulation did not weaken the credit atribution towards Bell & Hewish. In fact, I did not even change that paragraph, I simply added a new one.
But perhaps it would be best to add a small paragraph at the bottom of the discovery section, or perhaps even at the end of Subsequent history, as an 'additional footnote in history'. I think a publication in Nature, and the subsequent media coverage that was given to it (it was in every newspaper), justifies this. And this can be extended/reformulated to include other pre-discovery's. --Patrick1982 23:16, 8 September 2007 (UTC)
Patrick1982, I agree with you 100%. This should be in the article, and I am not concerned about protecting Bell & Hewish's legacy. But it shouldn't be presented as "we used to say that Bell discovered pulsars but the correct answer is now Schisler", since the full situation is more complex. I think a subsequent "footnote" is a good way to do it, as you propose. Tubbs334 14:12, 9 September 2007 (UTC)

To resuscitate this discussion—I removed a sentence placed at the start of the discovery section noting that the optical counterpart of the Crab Pulsar was discovered in 1899. While these archival data of pulsars are interesting, the coverage in the literature clearly supports the fact that Jocelyn Bell Burnell's observations mark the discovery of pulsars. Some coverage of archival data might be added, but it should not be in the first post-lead sentence. James McBride (talk) 21:50, 1 October 2012 (UTC)

Time standard?[edit]

Shouldn't something be said in this article about how pulsars have been proposed for use as an independent check (or alternative time standard) on terrestrial atomic clocks? (See Barycentric Coordinate Time.) I would say it myself if I knew how, but I was trying to do some initial research and found nothing in this article; I think there used to be something about this in one of the various articles about various sorts of pulsars, but haven't found it yet, and really, this is the article that I think it ought to be mentioned in. arkuat (talk) 07:32, 21 February 2008 (UTC)

Visual?[edit]

For X years ago, where X is at most 50 years, only a couple of pulsars where visually identified. What's the current status? I.e.: could the article relate to visual identification. I know for sure that the Crab pulsar is visually identified, but more? ... said: Rursus (mbor) 13:00, 14 August 2009 (UTC)

Maybe, we should consider merging the too small Optical pulsar into this article? ... said: Rursus (mbor) 13:07, 14 August 2009 (UTC)

how many neutron stars are pulsars?[edit]

What neither this article or "Neutron Stars" makes clear is WHAT PROPORTION of neutron stars are pulsars. Reading the ambiguous text on the "lighthouse effect", readers could well conclude that ALL neutron stars are pulsars, and that we don't see more of them pulse only because the beam from most of them does not sweep across the Earth. It amazes me how such obvious questions do not cross the minds of those who write this stuff. I gather that most neutron stars are NOT pulsars, though why some are and some are not finds no answer here. The following quote from WP article "Neutron Star" is about as close as we get.

If the surface temperature exceeds 106 kelvin (as in the case of a young pulsar), the surface should be fluid instead of the solid phase observed in cooler neutron stars (temperature <106 kelvin)[10]. Myles325a (talk) 04:37, 26 September 2009 (UTC)

This is a difficult question to answer properly, as there are many neutron stars observed by other means (e.g. X-rays), which we (scientists) don't know if they are radio pulsars or not. And there are other objects which appear to be radio pulsars at some times, and not to be radio pulsars at other times--some are magnetars (http://adsabs.harvard.edu/abs/2006Natur.442..892C), and some intermittently accrete in binary systems (http://adsabs.harvard.edu/abs/2009Sci...324.1411A). However, one can make a simple estimate; as the average pulsar lasts probably ~30 million years (see http://www.cv.nrao.edu/course/astr534/Pulsars.html, Fig. 4), but neutron stars have been formed throughout the age of the universe (13.6 billion years), thus of order ~500 times more neutron stars must exist than pulsars--probably more than ~1000 times more, as the star formation rate was higher earlier in our Galaxy's history. The fraction of neutron stars that are spun up in binary systems to become millisecond radio pulsars is quite small, but they radiate as pulsars for billions of years; the numbers for millisecond pulsars may be similar to the number of pulsars, though that isn't certain. Craigheinke (talk) 19:05, 29 November 2009 (UTC)

CTA 1[edit]

I can't find any evidence of X-ray emission changes in CTA 1, either in the referenced Atkinson article, or in the science literature (see http://www.iop.org/EJ/article/0004-637X/612/1/398/60375.html). So I took that out, and rephrased its notability to be its pulsations only being seen in the gamma-ray. There's a good chance that X-ray pulsations will be detected in the near future, since the existing X-ray pulsation constraints are weak and further observations are being performed. Craigheinke (talk) 19:14, 29 November 2009 (UTC)


PSR J0108-1431 contradiction[edit]

Some wiki-styled sources refer to the pulsar as being 280 light years away, however http://chandra.harvard.edu/photo/2009/j0108/ gives an estimate of 770 ly. Can someone doublecheck? —Preceding unsigned comment added by 216.239.79.193 (talk) 04:40, 1 December 2009 (UTC)

The recent VLBI parallax measurements by Deller et al give a distance of 240 parsecs, which is about 770 ly. However, the original discovery paper by Tauris et al gives a distance of < 100 parsecs, which is presumably the source of the 280 lightyear figure. Scog (talk) 08:44, 8 December 2009 (UTC)

Pulsar wind?[edit]

This article has no explanation of what the pulsar wind is, and pulsar wind simply redirects back here. 99.55.136.210 (talk) 04:47, 1 March 2010 (UTC)

I was going to make this exact same comment. Who decided to redirect Pulsar Wind to this article without actually giving an explanation of it? Very frustrating! --InvaderXan (talk) 13:22, 15 July 2010 (UTC)

Again myself too, I was redirected here searching pulsar wind, and there was no explanation of what it actually is. Infringement153 (talk) 09:09, 6 December 2010 (UTC)

While this is an ancient section, the objection is still accurate. There is a link to pulsar wind, but no article save the redirect to the pulsar article. There is also no mention of what a pulsar wind is, its mechanisms of operation, etc. If there is no objection, I'll remove the link to the non-existent article.Wzrd1 (talk) 00:52, 9 November 2013 (UTC)
Even better, I'll change the link to go to the article Pulsar wind nebula, which explains pulsar wind briefly.Wzrd1 (talk) 04:28, 10 November 2013 (UTC)

How Is It Possible?[edit]

How is it possible for Neutron Stars (which are supposedly electrically neutral) to have charged particles around them and emit radio pulses? Shouldn't the super strong gravitational field pull the particles into the star and neutralize them? I know that everything with a surface temperature different from it's immediate environment radiates; what is a pulsar's surface temperature?JeepAssembler (talk) 20:48, 7 August 2010 (UTC)JeepAssemblerJeepAssembler (talk) 20:48, 7 August 2010 (UTC)

Well, if you look at the equation of state of a neutron star, you see that the star isn't simply a solid ball of neutrons. Yes, the inside of the star is made up entirely of neutrons (most likely in a superfluid state), but the crust of the star is not. At the surface of the star it is mostly composed of iron (Fe). As you move into the star, the ever increasing pressure pushes more and more of the electrons into the nuclei of the atoms, thus forming increasingly absurdly heavy isotopes of the 'lighter' element, until the pressure becomes so high that all the electrons are pushed into the nuclei, and the entire mass becomes homogeneously neutron material. In short, this means that at the surface of the neutron star there are atoms with magnetically charged components, which can be acted on by the extremely strong induced electric field (specifically the component of it perpendicular to the magnetic field). This electric field is strong enough to pull charged particles out of the surface (against the strong gravitational field), and accelerate them to very high speeds in a comparatively short distance (as shown by Goldreich & Julian (1969)). The models of how these particles are accelerated etc., predict that their speeds will be ultra-relativistic (i.e., super close to the speed of light) after only a few millimeters of motion (as little as 3mm or so). This doesn't happen everywhere, however, only over the polar regions of the pulsars (magnetic poles, not rotational poles). The magnetosphere of the pulsar is divided into two regions, the closed region and the open region. The boundary between these two regions is delimited by the last field line that closes inside the so-called light cylinder. Basically any field line closer to the magnetic pole than this line would have to close at a distance farther from the pulsar than the distance at which something would need to move at the speed of light just to co-rotate with the pulsar (like a geostationary satellite staying over a certain spot on the Earth's surface). Because all the field lines in the closed region are, well, closed inside the light cylinder, no charged particles can escape this region, and the charged particle plasma nullifies the accelerating electric field in this region. Thus, the closed region consists of a cloud of charged particles co-rotating with the neutron star. In the open region the particles can escape through the light cylinder, meaning that there is a continual stream of charged particles flowing out from the neutron star surface at the magnetic polar caps. It's radiation from these particles that we see, and it is this geometry (open and closed regions) which is responsible for the beamed-ness of the emission. About the surface temperature of pulsars: Well this is a slightly more involved topic, but the basic fact is that the polar caps are much hotter than the rest of the neutron stars surface, and are the sources of some of the radiation we see in x-ray pulsars (the so-called thermal x-ray component). I'm not going to describe how the heating actually happens, because that's way more detail than I want to go into here, but what I've put down should be interesting enough. Tjips (talk) 13:37, 19 November 2012 (UTC)

How is it possible for a pulsar's magnetic axis to be at a high angle from the axis of rotation? SEppley (talk) 18:33, 29 April 2012 (UTC)

For 46 years scientists have been perfecting this incomplete model of the Pulsar oscillating source. Did anyone once stop to ask why the 'rotating lighthouse' idea still continues? I seems perfectly inane for intelligent humans to be discussing a mechanical principle left over from 200 years ago when modern engineers can do flashing sources far more simply. In fact the first radio transmitters used the simple tuned spark gap which utilises the non-linear ionised air (plasma) as the feedback loop. No moving parts and ultimately very efficient to have an electrical resonance, hence most modern lighthouses convert to strobes or flashing LEDs. Two charged stars, a bit of plasma atmosphere and resonant lightning does the trick. Find an x-ray machine that uses gravity rather than electricity, I think not!

The trouble is Cosmologists and Particle Physicists have been patching up their wonderful theories for decades. No-one can prove them wrong, although you have to laugh at the list of entities they have cooked up! I recommend a read of this page for more straight talking.Cigarshaped (talk) 21:04, 19 January 2014 (UTC)

That's all we have until we can go out there and visit the things up close. And by then, expect to have your cherished assumptions dashed asunder. Kortoso (talk) 21:42, 21 October 2014 (UTC)

Pulsar life history[edit]

Please tell what the life cycle of a pulsar is. Article tells how they form. Since they keep giving off energy, what's the long term fate of them? —Preceding unsigned comment added by 72.83.70.206 (talk) 22:43, 1 May 2011 (UTC)

It depends on the system; binaries will obviously evolve differently from lone pulsars. However, for all cases, they will eventually cool off (i.e. run out of energy) since they are no longer engaged in nuclear fusion, and as they do their emitted energy drops over time until it becomes undetectable. siafu (talk) 23:34, 1 May 2011 (UTC)

Pulsar Wind[edit]

This article could be improved by adding a section explaining Pulsar Winds. The only article with information regarding it, is the 'pulsar wind nebula' page. A pulsar wind is a significant aspect of the Pulsars, hence adding a section on it should be prioritized on further expansions of the article.


Pulsar wind nebula - page: http://en.wikipedia.org/wiki/Pulsar_wind_nebula — Preceding unsigned comment added by Fredrikdn (talkcontribs) 14:12, 29 July 2012 (UTC)

New research[edit]

'The scientists identified a pulsar that is able to dramatically change the way in which it shines. In just a few seconds, the star can quiet its radio waves while at the same time it makes its X-ray emissions much brighter. The research “challenges all proposed pulsar emission theories,” the team writes in the Jan. 25 edition of the journal Science and reopens a decades-old debate about how these stars work." ' From http://www.sciencedaily.com/releases/2013/01/130124183444.htm. -- Jo3sampl (talk) 19:33, 25 January 2013 (UTC)

Seems anthropomorphic to me. Kortoso (talk) 21:43, 21 October 2014 (UTC)

Significant pulsars[edit]

The section on significant pulsars section has been tagged as "trivia" for awhile. It was just removed today (though I restored it) with the comment "no criterion for 'significance'". I think there are pretty clearly two criteria that encompass the majority of listed pulsars: "firsts" and record holders of various types. There are probably a couple of pulsars that don't really belong (and perhaps I'll just go ahead and trim the list a little after this), but for the most part it seems to be a fairly selectively populated list, and thus isn't truly a trivia section. I think the content and layout of the section could be improved, but disagree completely with removing it entirely. James McBride (talk) 04:54, 17 April 2013 (UTC)

pioneer plaque[edit]

To root this article in something practical, how about talking about how the earth's position compared to 14 pulsars was the method used on the Pioneer plaques to describe our location to "aliens"? I think a pic of the plaque would be a good graphic here as well. It would show a newby one reason they are so important. Pb8bije6a7b6a3w (talk) 15:54, 14 May 2013 (UTC)

Isn't this already in Pulsar#Applications? Reatlas (talk) 14:46, 15 May 2013 (UTC)

Discovery[edit]

The sentence "It was not until a second pulsating source was discovered in a different part of the sky that the 'LGM hypothesis' was entirely abandoned" makes it sound like the little green men theory was disproven because there was more than one pulsar discovered. I'm pretty sure there's more science to it than that, but the cite given is not available to ordinary earthlings. Can someone summarize the science that tells us that pulsars are natural phenomena? Kortoso (talk) 17:22, 11 December 2013 (UTC)

pulsar/neutron star connection[edit]

The organization and clarity of this article still leave much to be desired. I hope my recent edit clarifies the evidence connecting neutron stars and pulsars, since that issue seemed to underlie the recent edits by User:Cigarshaped. I reverted these edits, because I am not aware of any credible source that argues for an alternative to rotating neutron stars as the source of pulsar emission, and no such source was provided in the edits of User:Cigarshaped. I would like to avoid any edit warring, so if there is more that could be done to improve this area of the article, suggestions are welcome, and I can try to improve the article further. James McBride (talk) 06:14, 20 January 2014 (UTC)

I don't have time for Revert Wars, however, I would ask what is necessary to provide a credible source? There is an extremely promising proposal which follows on from "magnetospheric disk-field-aligned-current transmission line system as the origin of the observed radiation".[1] Healy and Peratt were approaching the pulsar emission from an telecommunication/ electrical engineering POV. Anthony Peratt is a world expert on plasma and knows its emissions well. Since 99.999% of visible universe is plasma Hannes Alfven it suprises me that not more research explores astrophysical phenomena in plasma terms.

The characteristic form of pulsars strangely mimics the signature of lightning strikes. Surely an electrical discharge is a far more plausible explanation for a regular em pulse, particularly when the frequency exceeds 600Hz. I'll say it again. The earliest radio transmitters employed tuned spark gaps in all their noisy glory. Students easily knock up a relaxation oscillator with a pair of capacitors (stars) and a non-linear resistor (plasma). Why should Nature be any different. Modern lighthouse operators are binning their rotating mirrors in favour of strobes or flashing LEDs. What is more efficient? What makes practical sense?

I recommend a read of this page for more straight talking. Cigarshaped (talk) 18:55, 20 January 2014 (UTC)

The article that you cite by Healy & Peratt is a credible source. In my first comment, I should have linked to WP:Reliability, which provides criteria for sources that are considered reliable. The link to holoscience.com does not meet the criteria for reliability, as it is self-published.
I will not claim to understand all details of the Healy & Peratt article, but their model is still clearly based on a system with a rotating neutron star at its center. Their model draws upon ideas related to transmission lines, and they even provide a simple demonstration of the core ideas of their model using a transmission line system they built, but these ideas are being applied to the conditions of a plasma surrounding a rotating neutron star.
Theoretical efforts to understand pulsars are certainly not ignoring plasma physics. In fact, the theoretical astrophysicists that I can think of that work on pulsars describe themselves as studying plasma astrophysics. They are trying to understand in greater detail how a plasma behaves when there is rapidly rotating powerful magnetic dipole at the center of the plasma. As far as I know, the only such object that has been proposed that can spin as rapidly as necessary to excite the surrounding plasma and produce pulsar pulses is a neutron star, or very similar stars in which neutrons have split up into their fundamental particle constituents. I've just scanned the astronomy literature for a model that does not fit this description, and was unable to find anything. Though that doesn't mean such a model doesn't exist, I think that reframing the article in order to cast doubt on the consensus in the astrophysics community that pulsars are powered by neutron stars would place undue weight on alternative models.
As for terrestrial origins of pulsars, the observations firmly rule out that possibility. Telescopes in different parts of the Earth at different times are able to observe identical pulsation properties from objects at the same celestial coordinates. There is no way to reproduce this with lightning strikes. James McBride (talk) 19:43, 20 January 2014 (UTC)

It's a bit unfortunate that I could not read the full paper as I would have been more reluctant to quote anything with moving parts at its centre. It is also extremely disturbing that an entire disciplne has failed to come up with a much simpler alternative such as the plasma discharge model. My reference to terrestrial lightning was meant to lead on to cosmic lightning on a stellar scale. As plasma is scalable to 28 orders of magnitude [[5]], any laboratory or terrestrial phenomema can be transferred to the star (or even galactic) situation.

"I think that reframing the article in order to cast doubt on the consensus in the astrophysics community that pulsars are powered by neutron stars would place undue weight on alternative models." Well if you don't give them a kick, they will continue to waste valuable time and resources on this fruitless mechanical hypothesis. Meanwhile the world heads for various crises, totally unprepared for the effects of a galactic mutation of our solar configuration. Our only hope is to start respecting the electrical nature of this delicate universe. I am convinced that our ignorance is being deliberately orchestrated, in order that we will be completely helpless when something serious happens. Anyway you are in a powerful position to influence the next generation of scientists. Breed morons indoctrinated with Einsteinian/mathematical thinking OR free-thinking engineering-based researchers. Here's [[6]] a possible hope.Cigarshaped (talk) 21:44, 20 January 2014 (UTC)

Crab Pulsar GIF/video slow-down rate[edit]

Hello! In looking up pulsars, I saw the GIF Crab_Lucky_video2.gif[7] (used in both the Pulsar and Crab Pulsar articles) caption simply saying that the GIF's animation speed was slowed down to illustrate the Crab Pulsar's pulses. The articles state that the Crab Pulsar's rotation period is 33 milliseconds, which is 1980 rotations per minute. What I can't seem to find, either on Wikipedia or via Googling other sources, is roughly how much the GIF is slowed down. The GIF appears to be about 1.5 seconds long to me, using Firefox. I think it would be both interesting and informative to have the caption state how much it was slowed down. I don't think '33 milliseconds' would be easy to grasp for the average person, while (for example, if my guess and math are right) 'slowed down by 45 times/4500%' would be more illustrative. Thanks! - Lafin T. Jack 23:34, 27 March 2014 (UTC)

  1. ^ "Radiation Properties of Pulsar Magnetospheres: Observation, Theory, and Experiment". The SAO/NASA Astrophysics Data System.