13 ms·

Neutron stars may be bigger than expected, measurement of lead nucleus suggests

161 points by furcyd 5y ago

jvanderbot 5y ago
Obligatory: Bigger in size, but not mass, meaning less dense overall, based on results showing the thickness of the atomic nuclei of lead isotopes. (which includes the radius of the thick shell of neutrons around the nucleus, which was wider than expected).
srl 5y ago
Relatedly, NICER (an X-Ray telescope) launched in 2017, and has been observing pulsars to get radius measurements since then. See for instance https://arxiv.org/abs/1912.05705 https://arxiv.org/abs/1912.05705. (I think there's a more recent measurement but I can't find it on arXiv.) I don't have a good understanding of the relationships between all the different methods, but I do know that NICER's measurements have systematically given larger radii (albeit with huge error bars) than other methods do.
cletus 5y ago
Let me add some perspective here. This only became apparent to me fairly recently and it blows my mind. It's something I hadn't thought of before. Black holes are relatively "simple". I mean that you can completely define a black hole with 3 properties, one of which isn't really relevant. The two most relevant properties are spin and mass. The last is electric charge. The reason this is somewhat irrelevant is because electric repulsion is about 60 orders of magnitude more than gravitational attraction so it's not expected black holes have a significant charge. Neutron stars OTOH are arguably the most complex objects in the Universe. Why? Because you're dealing with gravity, electric charges, nuclear forces and QCD such that there's no equation of state for describing dense nuclear matter. Probably my favourite variant of neutron stars is the extremely rare magnetar [1], the most powerful we've found has a magnetic field 100 trillion times that of Earth's [2]. This field is so strong it would flatten atoms and rip electrons from your body. So as much difficulty as we have of describing the nucleus of an atom (as referenced in this post), imagine a whole star of that stuff. [1]: https://en.wikipedia.org/wiki/Magnetar#:~:text=A%20magnetar%20is%20a%20type,X%2Drays%20and%20gamma%20rays https://en.wikipedia.org/wiki/Magnetar#:~:text=A%20magnetar%.... [2]: https://www.space.com/magnetar-eruptions-sculptor-galaxy#:~:text=The%20most%20powerful%20magnets%20in,13 https://www.space.com/magnetar-eruptions-sculptor-galaxy#:~:....
- ohazi 5y ago
  Neutron stars seem like what you would see in a black hole if the interesting parts weren't shrouded by an event horizon. Although I suppose the event horizon does give you jets. But yeah, city-sized objects with the density of an atomic nucleus. Pretty mind-bending! (literally, I guess?)
  - ramraj07 5y ago
    The “how to build a black hole” video from PBS space time says that’s how you do it - you start with a neutron star and keep adding more mass to it, which paradoxically makes it smaller until it becomes smaller than its swarzchild radius, becoming a black hole. Blew my mind and anyone else’s when I share the video with them! https://youtu.be/xx4562gesw0 https://youtu.be/xx4562gesw0
    - cletus 5y ago
      That depends on what size of black hole you want to make. It works for large black holes (you need sufficient mass for the gravitational forces to be sufficient) but those black holes aren’t that useful. Smaller black holes are harder to construct but way more useful.
      - willis936 5y ago
        It's simple, just deflate spacetime!
      - elseweather 5y ago
        I'm curious what useful applications we have for black holes of any size
        jakeogh 5y ago
        It's assumed that little ones are the most efficient mass to energy converters, via Hawking radiation.
        5y ago
excalibur 5y ago
We're talking about stars here. The difference between 10 km and 14 km is pretty negligible at this scale, these are both specks.
- ben_w 5y ago
  If they neutron stars are bigger at any given mass, that implies black holes have different minimum sizes, which has implications for other things in cosmology.
  - at_a_remove 5y ago
    Ah, no. Neutron stars have some kind of average density. That's ... not applicable to black holes in any real sense. Their sizes would not change and are pre-determined by a relatively simple formula in the case of non-rotating black holes without any significant charge.
    - treeman79 5y ago
      Well they gave a real density size and shape, it just doesn’t matter. Inside could be shaped like a dolphin a potted plant or a whale. Outside of horizon we couldn’t tell from outside.
    - ben_w 5y ago
      I’m not suggesting otherwise. I’m saying that “neutron stars have lower density than previously thought” means “neutron stars can be more massive before reaching constraint of Schwarzschild radius for given mass”.
  - DougMerritt 5y ago
    As cletus said above, black holes have only three externally-observable properties: mass, angular momentum, and charge; see for instance [1]. Note that radius/size is not one of those three. The radius of a black hole derives from its mass under general relativity, and no amount of change found in the density nor size of neutron stars will change that. You're right that new understanding of neutron stars has implications for other things in cosmology, though. [1] https://phys.org/news/2020-12-black-holes-gain-powers-fast.html https://phys.org/news/2020-12-black-holes-gain-powers-fast.h...
dsp_person 5y ago
Really enjoyed Dragon's Egg about life on a neutron star. > The adults of the star's most intelligent species, called cheela (no flexion for gender or number), have about the same mass as an adult human. However, the extreme gravity of Dragon's Egg compresses the cheela to the volume of a sesame seed,[2] but with a flattened shape about 0.5 millimeters (0.020 inches) high and about 5 millimeters (0.20 inches) in diameter. Their eyes are 0.1 millimeters (0.0039 inches) wide. Such minute eyes can see clearly only in ultraviolet and, in good light, the longest wavelengths of the X-ray band > By humans' standards, a "day" on Dragon's Egg is about 0.2 seconds https://en.wikipedia.org/wiki/Dragon%27s_Egg https://en.wikipedia.org/wiki/Dragon%27s_Egg
- farrelle25 5y ago
  Yes I thought "Dragons Egg" was a classic. I remember reading it as a child and getting interested in Physics. I read it's an example of the "hard" science fiction genre - I think the author Robert Forward was actually a physicist...
  - gizmo686 5y ago
    Since I happen to have Dragons Egg on my desk, from the "About the Author" section: > Dr. Robert L. Forward is a senior scientist at the Hughes Research Labs in Malibu, California. Dr. Forward is one of the pioneers in the field of gravitational astronomy, participating at Maryland University in the construction of te first antenna for detection of gravitational radiation from supernovas, black holes and neutron stars. (The antenna now resides in the Smithsonian Museumm.) At Hughes, Dr. Forward constructed the first laser gravity antenna and invented the rotating gravitational mass sensor. ... Fun fact, the start of Dragons Egg (copyright 1980) is April 2020
- Causality1 5y ago
  A classic indeed. I seem to recall that jumping off a high cliff or from a hovering vehicle was deadly to them because without the crushing gravity their bodies exploded into a cloud of "regular" matter.
carabiner 5y ago
A spoonful weighs a ton.
- jcadam 5y ago
  wouldn't that break the spoon?
- crdotson 5y ago
  But Ensign Rock likes to eat it.
ThePhysicist 5y ago
As a physicist I still find it fascinating that you can study some of the smallest things in the universe and learn something about some of the largest things in it.
- mabbo 5y ago
  There's a plot point in one of my favourite books, 'The First Fifteen Lives of Harry August' [0], that one of the main characters is working on the ultimate physics experiment. In it, a single atom will be probed with a "quantum mirror", and from that all of the rest of physics can be deduced. A bit silly, but it's the same idea here. From studying something very small and close by in exquisite detail, we learn something fundamental about things everywhere else in the universe. We can deduce incredible things from a few electrons bouncing off an atom in a funny way. [0] Claire North's best novel so far, imho, and I cannot believe it didn't win more awards.

161 points by furcyd 5y ago

Obligatory: Bigger in size, but not mass, meaning less dense overall, based on results showing the thickness of the atomic nuclei of lead isotopes. (which includes the radius of the thick shell of neutrons around the nucleus, which was wider than expected).

srl 5y ago

Relatedly, NICER (an X-Ray telescope) launched in 2017, and has been observing pulsars to get radius measurements since then. See for instance https://arxiv.org/abs/1912.05705 https://arxiv.org/abs/1912.05705. (I think there's a more recent measurement but I can't find it on arXiv.) I don't have a good understanding of the relationships between all the different methods, but I do know that NICER's measurements have systematically given larger radii (albeit with huge error bars) than other methods do.

cletus 5y ago

Let me add some perspective here. This only became apparent to me fairly recently and it blows my mind. It's something I hadn't thought of before. Black holes are relatively "simple". I mean that you can completely define a black hole with 3 properties, one of which isn't really relevant. The two most relevant properties are spin and mass. The last is electric charge. The reason this is somewhat irrelevant is because electric repulsion is about 60 orders of magnitude more than gravitational attraction so it's not expected black holes have a significant charge. Neutron stars OTOH are arguably the most complex objects in the Universe. Why? Because you're dealing with gravity, electric charges, nuclear forces and QCD such that there's no equation of state for describing dense nuclear matter. Probably my favourite variant of neutron stars is the extremely rare magnetar [1], the most powerful we've found has a magnetic field 100 trillion times that of Earth's [2]. This field is so strong it would flatten atoms and rip electrons from your body. So as much difficulty as we have of describing the nucleus of an atom (as referenced in this post), imagine a whole star of that stuff. [1]: https://en.wikipedia.org/wiki/Magnetar#:~:text=A%20magnetar%20is%20a%20type,X%2Drays%20and%20gamma%20rays https://en.wikipedia.org/wiki/Magnetar#:~:text=A%20magnetar%.... [2]: https://www.space.com/magnetar-eruptions-sculptor-galaxy#:~:text=The%20most%20powerful%20magnets%20in,13 https://www.space.com/magnetar-eruptions-sculptor-galaxy#:~:....

ohazi 5y ago
Neutron stars seem like what you would see in a black hole if the interesting parts weren't shrouded by an event horizon. Although I suppose the event horizon does give you jets. But yeah, city-sized objects with the density of an atomic nucleus. Pretty mind-bending! (literally, I guess?)
- ramraj07 5y ago
  The “how to build a black hole” video from PBS space time says that’s how you do it - you start with a neutron star and keep adding more mass to it, which paradoxically makes it smaller until it becomes smaller than its swarzchild radius, becoming a black hole. Blew my mind and anyone else’s when I share the video with them! https://youtu.be/xx4562gesw0 https://youtu.be/xx4562gesw0
  - cletus 5y ago
    That depends on what size of black hole you want to make. It works for large black holes (you need sufficient mass for the gravitational forces to be sufficient) but those black holes aren’t that useful. Smaller black holes are harder to construct but way more useful.
    - willis936 5y ago
      It's simple, just deflate spacetime!
    - elseweather 5y ago
      I'm curious what useful applications we have for black holes of any size
      - jakeogh 5y ago
        It's assumed that little ones are the most efficient mass to energy converters, via Hawking radiation.
      - 5y ago

excalibur 5y ago

We're talking about stars here. The difference between 10 km and 14 km is pretty negligible at this scale, these are both specks.

ben_w 5y ago
If they neutron stars are bigger at any given mass, that implies black holes have different minimum sizes, which has implications for other things in cosmology.
- at_a_remove 5y ago
  Ah, no. Neutron stars have some kind of average density. That's ... not applicable to black holes in any real sense. Their sizes would not change and are pre-determined by a relatively simple formula in the case of non-rotating black holes without any significant charge.
  - treeman79 5y ago
    Well they gave a real density size and shape, it just doesn’t matter. Inside could be shaped like a dolphin a potted plant or a whale. Outside of horizon we couldn’t tell from outside.
  - ben_w 5y ago
    I’m not suggesting otherwise. I’m saying that “neutron stars have lower density than previously thought” means “neutron stars can be more massive before reaching constraint of Schwarzschild radius for given mass”.
- DougMerritt 5y ago
  As cletus said above, black holes have only three externally-observable properties: mass, angular momentum, and charge; see for instance [1]. Note that radius/size is not one of those three. The radius of a black hole derives from its mass under general relativity, and no amount of change found in the density nor size of neutron stars will change that. You're right that new understanding of neutron stars has implications for other things in cosmology, though. [1] https://phys.org/news/2020-12-black-holes-gain-powers-fast.html https://phys.org/news/2020-12-black-holes-gain-powers-fast.h...

dsp_person 5y ago

Really enjoyed Dragon's Egg about life on a neutron star. > The adults of the star's most intelligent species, called cheela (no flexion for gender or number), have about the same mass as an adult human. However, the extreme gravity of Dragon's Egg compresses the cheela to the volume of a sesame seed,[2] but with a flattened shape about 0.5 millimeters (0.020 inches) high and about 5 millimeters (0.20 inches) in diameter. Their eyes are 0.1 millimeters (0.0039 inches) wide. Such minute eyes can see clearly only in ultraviolet and, in good light, the longest wavelengths of the X-ray band > By humans' standards, a "day" on Dragon's Egg is about 0.2 seconds https://en.wikipedia.org/wiki/Dragon%27s_Egg https://en.wikipedia.org/wiki/Dragon%27s_Egg

farrelle25 5y ago
Yes I thought "Dragons Egg" was a classic. I remember reading it as a child and getting interested in Physics. I read it's an example of the "hard" science fiction genre - I think the author Robert Forward was actually a physicist...
- gizmo686 5y ago
  Since I happen to have Dragons Egg on my desk, from the "About the Author" section: > Dr. Robert L. Forward is a senior scientist at the Hughes Research Labs in Malibu, California. Dr. Forward is one of the pioneers in the field of gravitational astronomy, participating at Maryland University in the construction of te first antenna for detection of gravitational radiation from supernovas, black holes and neutron stars. (The antenna now resides in the Smithsonian Museumm.) At Hughes, Dr. Forward constructed the first laser gravity antenna and invented the rotating gravitational mass sensor. ... Fun fact, the start of Dragons Egg (copyright 1980) is April 2020
Causality1 5y ago
A classic indeed. I seem to recall that jumping off a high cliff or from a hovering vehicle was deadly to them because without the crushing gravity their bodies exploded into a cloud of "regular" matter.

carabiner 5y ago

A spoonful weighs a ton.

jcadam 5y ago
wouldn't that break the spoon?
crdotson 5y ago
But Ensign Rock likes to eat it.

ThePhysicist 5y ago

As a physicist I still find it fascinating that you can study some of the smallest things in the universe and learn something about some of the largest things in it.

mabbo 5y ago
There's a plot point in one of my favourite books, 'The First Fifteen Lives of Harry August' [0], that one of the main characters is working on the ultimate physics experiment. In it, a single atom will be probed with a "quantum mirror", and from that all of the rest of physics can be deduced. A bit silly, but it's the same idea here. From studying something very small and close by in exquisite detail, we learn something fundamental about things everywhere else in the universe. We can deduce incredible things from a few electrons bouncing off an atom in a funny way. [0] Claire North's best novel so far, imho, and I cannot believe it didn't win more awards.