# Density and Neutron Stars: How does this work? **URL:** \ **Category:** Factual Questions\ **Created:** [November 14, 2003, 2:12pm UTC](https://boards.straightdope.com/t/density-and-neutron-stars-how-does-this-work/213241 "2003-11-14T14:12:52Z")\ **Posts on this page:** 13\ **Page:** 1
**Author:** ![Phlosphr](https://avatars.discourse-cdn.com/v4/letter/p/c68b51/32.png) [@Phlosphr](https://boards.straightdope.com/u/Phlosphr)\ **Post date:** [November 14, 2003, 2:12pm UTC](https://boards.straightdope.com/t/density-and-neutron-stars-how-does-this-work/213241/1 "2003-11-14T14:12:52Z")
I’m trying to understand something here that is well outside my area of expertise. The little factoid came across my desk this morning that a neutron star is so dense that on earth a teaspoonful would weigh nearly a billion tons…? Q’s: What cosmic occurrences or lack there of create neutron stars? Because of their overwhelming gravitational pull how close could a prob get to one? What is the closest one to us? And are there hubble photo’s of neutrons stars? Finally, the _material_ the make up the stars…the dense material, what exactly is it? ---
**Author:** ![Kinthalis](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/kinthalis/32/16084_2.png) [@Kinthalis](https://boards.straightdope.com/u/Kinthalis)\ **Post date:** [November 14, 2003, 2:20pm UTC](https://boards.straightdope.com/t/density-and-neutron-stars-how-does-this-work/213241/2 "2003-11-14T14:20:55Z")
IIRC neutron stars are mostly made of… that’s right, neutrons! 😉 I would say that the dense stuff is, in essence, neutron soup. ---
**Author:** ![MC\_Master\_of\_Ceremonies](https://avatars.discourse-cdn.com/v4/letter/m/bb73d2/32.png) [@MC\_Master\_of\_Ceremonies](https://boards.straightdope.com/u/MC_Master_of_Ceremonies)\ **Post date:** [November 14, 2003, 2:52pm UTC](https://boards.straightdope.com/t/density-and-neutron-stars-how-does-this-work/213241/3 "2003-11-14T14:52:14Z")
When a star runs out of fuel it will throw off most of it’s mass leaving really just it’s core, if this remianing mass is is inbetween ~1.2 solar masses and ~3 solar masses the degenaracy pressure caused by electrons being repulsed by atomic protons will be less than the attraction due to gravitation and the protons and electrons (plus an antineutrino) will combine via the process of reverse beta decay to form neutrons. Neutron stars are made out of neutrons thugh IIRC they probaly have a ‘crust’ of protons and electrons. You can get as close as you like to a neutron star as long as you don’t mind being crushed, with a few gross approximations and back of the envelope calculations, I’d say don’t go closer than approx. 2 x 10[sup]9[/sup] m to a small neutron star. ---
**Author:** ![toadspittle](https://avatars.discourse-cdn.com/v4/letter/t/c0e974/32.png) [@toadspittle](https://boards.straightdope.com/u/toadspittle)\ **Post date:** [November 14, 2003, 4:37pm UTC](https://boards.straightdope.com/t/density-and-neutron-stars-how-does-this-work/213241/4 "2003-11-14T16:37:41Z")
Good intro. here: [http://www.astro.umd.edu/~miller/nstar.html](http://www.astro.umd.edu/~miller/nstar.html) > [@](#): > > So, like, how do we get neutron stars? > > Neutron stars are believed to form in supernovae such as the one that formed the Crab Nebula (or check out this cool X-ray image of the nebula, from the Chandra X-ray Observatory). The stars that eventually become neutron stars are thought to start out with about 15 to 30 times the mass of our sun. These numbers are probably going to change as supernova simulations become more precise, but it appears that for initial masses much less than 15 solar masses the star becomes a white dwarf, whereas for initial masses a lot higher than 30 solar masses you get a black hole instead (this may have happened with Supernova 1987A, although detection of neutrinos in the first few seconds of the supernova suggests that at least initially it was a neutron star). In any case, the basic idea is that when the central part of the star fuses its way to iron, it can’t go any farther because at low pressures iron 56 has the highest binding energy per nucleon of any element, so fusion or fission of iron 56 requires an energy input. Thus, the iron core just accumulates until it gets to about 1.4 solar masses (the “Chandrasekhar mass”), at which point the electron degeneracy pressure that had been supporting it against gravity gives up the ghost and collapses inward. > > At the very high pressures involved in this collapse, it is energetically favorable to combine protons and electrons to form neutrons plus neutrinos. The neutrinos escape after scattering a bit and helping the supernova happen, and the neutrons settle down to become a neutron star, with neutron degeneracy managing to oppose gravity. Since the supernova rate is around 1 per 30 years, and because most supernovae probably make neutron stars instead of black holes, in the 10 billion year lifetime of the galaxy there have probably been 10^8 to 10^9 neutron stars formed. One other way, maybe, of forming neutron stars is to have a white dwarf accrete enough mass to push over the Chandrasekhar mass, causing a collapse. This is speculative, though, so I won’t talk about it further. Essentially, the material in the star wants to go to its lowest energy state (like all good particles). So as fusion progresses, iron, the last basic product of fusion, starts accumulating like trash in the core of the star. But it’s a LOT of iron. So much that it wants to collapse in on itself completely. What stops it (and all ordinary matter, for that matter)? Electrons repel each other. So that keeps things stable for a while. Eventually, though, even that force isn’t enough to prevent collapse, and the protons and electrons collapse together to form a lower-energy mush of neutrons. What keeps the neutrons from collapsing further? The basic fact that particles don’t like to occupy the exact same space/state. At this point, you have a neutron star that is about the size of a city, but unbelievably massive. If you had a bit more mass than this, to overcome even the neutrons’ desire to stay away from each other, then the whole thing will collapse into a black hole. ---
**Author:** ![ricksummon](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ricksummon/32/3333_2.png) [@ricksummon](https://boards.straightdope.com/u/ricksummon)\ **Post date:** [November 14, 2003, 5:53pm UTC](https://boards.straightdope.com/t/density-and-neutron-stars-how-does-this-work/213241/5 "2003-11-14T17:53:29Z")
There’s also a type of star between neutron stars and black holes called a “strange star”. Instead of being composed of neutrons, it’s composed entirely of strange quarks. Astronomers have even found possible candidates which were once thought to be neutron stars. ---
**Author:** ![bughunter](https://avatars.discourse-cdn.com/v4/letter/b/aeb1de/32.png) [@bughunter](https://boards.straightdope.com/u/bughunter)\ **Post date:** [November 14, 2003, 6:20pm UTC](https://boards.straightdope.com/t/density-and-neutron-stars-how-does-this-work/213241/6 "2003-11-14T18:20:27Z")
Good layman’s explanations already posted, so I’ll just add: Read Larry Niven’s short story “Neutron Star.” It’s a classic “hard SF” short describing an explorer’s trip to the eponymous astronomical phenomenon, and the wierdnesses that inhabit its vicinity. ---
**Author:** ![eburacum45](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eburacum45/32/8690_2.png) [@eburacum45](https://boards.straightdope.com/u/eburacum45)\ **Post date:** [November 14, 2003, 8:00pm UTC](https://boards.straightdope.com/t/density-and-neutron-stars-how-does-this-work/213241/7 "2003-11-14T20:00:05Z")
[A somewhat boring Hubble image of a neutron star](http://hubblesite.org/newscenter/newsdesk/archive/releases/1997/32/); Aslightly more interesting type of neutron star, young and rotating, with jets of energy from the magnetic poles- [a pulsar](http://www.seds.org/messier/more/m001_h2.html) and a dramatic high energy magnetar- ([a graphic image only](http://www.cnn.com/TECH/space/9809/29/cosmic.blast/)) * * * SF worldbuilding at [http://www.orionsarm.com/main.html](http://www.orionsarm.com/main.html) ---
**Author:** ![aerodave](https://avatars.discourse-cdn.com/v4/letter/a/f08c70/32.png) [@aerodave](https://boards.straightdope.com/u/aerodave)\ **Post date:** [November 14, 2003, 11:58pm UTC](https://boards.straightdope.com/t/density-and-neutron-stars-how-does-this-work/213241/8 "2003-11-14T23:58:24Z")
> [@](#): > > \*Originally posted by toadspittle \* > [http://www.astro.umd.edu/~miller/nstar.html](http://www.astro.umd.edu/~miller/nstar.html) > > \*\*The stars that eventually become neutron stars are thought to start out with about 15 to 30 times the mass of our sun. \*\* Is that right? Neutron stars are between 1.4 and 3.0 solar masses. Less, and they’d be white dwarves; more, and they’d be black holes. So does a star really lose 90% of it’s orignial mass during a supernova? Does the matter comprising the neutron star only account for 10%? And are there really that many stars with 15 to 30 times the mass of the sun? ---
**Author:** ![MC\_Master\_of\_Ceremonies](https://avatars.discourse-cdn.com/v4/letter/m/bb73d2/32.png) [@MC\_Master\_of\_Ceremonies](https://boards.straightdope.com/u/MC_Master_of_Ceremonies)\ **Post date:** [November 15, 2003, 12:10am UTC](https://boards.straightdope.com/t/density-and-neutron-stars-how-does-this-work/213241/9 "2003-11-15T00:10:12Z")
Yes, for a neutron star it’s a balance between having enough mass to overcome electron degeneracy and having not enough to collapse further, though in theory neutron stars can be bigger than 3.0 solar masses (4.5?), but realistically this is not possible. As I said earlier a neutron star is basically made out of the core of a star and all the other matter, which makes up most of the mass is thrown off. Though 15-30 SM would put a star on the largish size, ther are certainly no shortage of stars in this range, esp. when you consider how many stars there actually are in total. ---
**Author:** ![toadspittle](https://avatars.discourse-cdn.com/v4/letter/t/c0e974/32.png) [@toadspittle](https://boards.straightdope.com/u/toadspittle)\ **Post date:** [November 17, 2003, 5:00pm UTC](https://boards.straightdope.com/t/density-and-neutron-stars-how-does-this-work/213241/10 "2003-11-17T17:00:58Z")
eburacum45 or others, do you know if that star paired with the crab pulsar in the photos is actually a companion star (or just “in the way” of the photo)? Is the crab pulsar in a binary system? ---
**Author:** ![Bryan\_Ekers](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bryan_ekers/32/183_2.png) [@Bryan\_Ekers](https://boards.straightdope.com/u/Bryan_Ekers)\ **Post date:** [November 17, 2003, 5:32pm UTC](https://boards.straightdope.com/t/density-and-neutron-stars-how-does-this-work/213241/11 "2003-11-17T17:32:46Z")
The limit of ~1.44 solar masses has a name: the Chandrasekhar limit, I mention for the sake of completeness. ---
**Author:** ![Grey](https://avatars.discourse-cdn.com/v4/letter/g/b782af/32.png) [@Grey](https://boards.straightdope.com/u/Grey)\ **Post date:** [November 17, 2003, 6:08pm UTC](https://boards.straightdope.com/t/density-and-neutron-stars-how-does-this-work/213241/12 "2003-11-17T18:08:23Z")
Believe the Crab Pulsar is solitary. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [November 17, 2003, 10:08pm UTC](https://boards.straightdope.com/t/density-and-neutron-stars-how-does-this-work/213241/13 "2003-11-17T22:08:40Z")
I’ve never heard that the Crab pulsar is binary, and if it were, we would certainly know it (a companion would need to be included in models of the pulsar timing). As for quark stars (aka strange stars), they’re a strong theoretical possibility, and there are a few candidate objects, but they’re still pretty far from proven. At the very least, it looks like there’s a very narrow range of initial masses which will produce one (rather than a neutron star or black hole). And they’re not composed entirely of strange quarks (that would give them a friggin’ huge charge). If I recall correctly, they’re composed of equal parts of up, down, and strange quarks (as opposed to the more typical 2:1 down:up for neutrons).