Neutron Star vs Black hole

In the following article it is stated:

https://skyandtelescope.org/astronomy-news/measuring-neutron-stars-exactly/

"We're seeing the gas whipping around just outside the neutron star's surface," said Edward Cackett (University of Michigan), in a press release. "And since the inner part of the disk obviously can't orbit any closer than the neutron star's surface, these measurements give us a maximum size of the neutron star's diameter. The neutron stars can be no larger than 18 to 20.5 miles across [29 to 33 kilometers], results that [confirm] other types of measurements."
Both groups examined spectral lines of X-ray emission from iron atoms in superhot gas that's orbiting at about 40% of light speed just above the neutron stars' surfaces.

Therefore, we see the disc of a hot gas which spins around the Neutron star at almost 40% of the speed of light. (or 0.4 c).
Based on that observation we understand that The neutron stars can be no larger than 18 to 20.5 miles across

Hence, this measurement represents the maximal size of the Neutron star, but what about the minimal size?
Why do we assume that it represents the real diameter of the Neutron star?

Let's compare that observation to a BH.
https://en.wikipedia.org/wiki/Messier_87

The visible material around the black hole rotates mostly clockwise with respect to the observer, which due to the direction of the axis of rotation causes the bottom part of the emission region to have a component of velocity toward the observer.[86] The rotation parameter was estimated at �=0.9±0.1 , corresponding to a rotation speed ≈ 0.4 c.[87]

https://en.wikipedia.org/wiki/Messier_87#/media/File:Black_hole_-_Messier_87_crop_max_res.jpg

It was a surprise for me to discover that the hot gas disc around both objects spins exactly at 0.4 c.

I wonder why that Hot gas disc is called an accretion disc when it spins around a BH at 0.4c, but there is no specific name for a hot gas disc when it spins around the Neutron star at also 0.4c?

Why the hot gas disc can give us an indication for the Neutron star' diameter but no indication for a BH' diameter?

Why do we assume that at the same mass, the diameter of a Neutron star is significantly bigger than a BH?

davLev said:

Why do we assume that at the same mass, the diameter of a Neutron star is significantly bigger than a BH?

You may have heard of Gravitational Lensing. That is when light is passing close enough to a large mass that the light is re-directed, much as a glass lens or a prism bends light.

If the same mass is concentrated to a small point, the gravitational field stays pretty much the same at a distance... however, with the gravity field concentrated to a small surface, the field intensity at the surface is very high.

A Black Hole is named that because the gravitational force at its surface is so strong that any light (emitted or passing close-by) can not escape.

See:
https://www.google.com/search?&q=schwarzchild+limit

Hope this helps.
Cheers,
Tom

Tom.G said:

You may have heard of Gravitational Lensing. That is when light is passing close enough to a large mass that the light is re-directed, much as a glass lens or a prism bends light.

If the same mass is concentrated to a small point, the gravitational field stays pretty much the same at a distance... however, with the gravity field concentrated to a small surface, the field intensity at the surface is very high.

Thanks Tom

Where the idea of a small surface came from?

Gravitational lensing works for BH and also for galaxy:
https://facts.net/nature/universe/9-mind-blowing-facts-about-gravitational-lensing/

"Gravitational lensing is a captivating phenomenon in the Universe that has fascinated scientists and space enthusiasts for decades. It occurs when the path of light from a distant object is distorted by the gravitational field of a massive object, such as a galaxy or a black hole."

The diameter of a galaxy could be more than 100,000 LY.
Hence, do you agree that there is no need for a small surface in order to get the Gravitational lensing phenomenon?

Tom.G said:

A Black Hole is named that because the gravitational force at its surface is so strong that any light (emitted or passing close-by) can not escape.

We can't see a BH and we can't see a Neutron star (assuming that there is no hot gas around it).
Therefore, why do you claim that the Gravitational lensing can work for the BH but not for the same mass Neutron star?
Would you kindly direct me to the relevant article?

davLev said:

Why do we assume that it represents the real diameter of the Neutron star?

Why do you think anyone is making that assumption?

I put in a HELP request to get some real experts here to help you with your study.

My background is mainly electronics and I've only looked at the astrophysics out of curiosity many years ago.

Cheers,
Tom

davLev said:

It was a surprise for me to discover that the hot gas disc around both objects spins exactly at 0.4 c.

Nobody said "exactly". Both measurements are approximate. Both quotes that you give explicitly say so.

davLev said:

I wonder why that Hot gas disc is called an accretion disc when it spins around a BH at 0.4c, but there is no specific name for a hot gas disc when it spins around the Neutron star at also 0.4c?

The term "accretion disc" was adopted for hot gas around black holes because it is falling into the hole, increasing the hole's mass. Unless there is a constant source of more gas, the hot gas around a black hole eventually disappears as it has all fallen into the hole.

From what I understand, this doesn't happen with neutron stars, at least not to anything like the same extent. Some of the hot gas might accrete onto the star, but because the star has an actual surface and is not vacuum like a black hole, the gas can't just fall in.

davLev said:

Why the hot gas disc can give us an indication for the Neutron star' diameter but no indication for a BH' diameter?

We can calculate the area of a black hole's horizon exactly if we know its mass and its spin rate. We can't do that for neutron stars because, unlike black holes, not all neutron stars with the same mass and spin rate are identical.

davLev said:

Why do we assume that at the same mass, the diameter of a Neutron star is significantly bigger than a BH?

We don't assume this. We know it from the laws of general relativity. A neutron star is in hydrostatic equilibrium, supporting itself against its own gravity by internal pressure. Any object like that cannot have a radius smaller than 9/8 of the Schwarzschild radius of a black hole with the same mass. (And in fact our models of neutron stars predict that they will be a fair bit larger than this minimum size.) So there is a finite size gap between black holes and neutron stars.

davLev said:

why do you claim that the Gravitational lensing can work for the BH but not for the same mass Neutron star?

I'm not sure @Tom.G was saying that. Gravitational lensing does work for neutron stars.

davLev said:

Hence, this measurement represents the maximal size of the Neutron star, but what about the minimal size?
Why do we assume that it represents the real diameter of the Neutron star?

We don't.

davLev said:

It was a surprise for me to discover that the hot gas disc around both objects spins exactly at 0.4 c.

It's not a coincidence. For a given mass you have an innermost stable circular orbit(technically more if the object rotates but let's ignore that here). Everything closer to the central object will hit the central object or escape quickly, so the innermost (fastest) matter we see comes from that orbit. It doesn't depend on the size or type of the central object, as long as it's smaller than the radius of that orbit.

The fact that we see it for that neutron star tells us the neutron star has to be smaller. This is useful information because neutron stars are complex objects that we can't model well.
The fact that we see it for M87 tells us that the black hole has to be smaller. This doesn't get any attention because we can calculate the radius from general relativity anyway.

mfb said:

The fact that we see it for that neutron star tells us the neutron star has to be smaller. This is useful information because neutron stars are complex objects that we can't model well.
The fact that we see it for M87 tells us that the black hole has to be smaller. This doesn't get any attention because we can calculate the radius from general relativity anyway.

Thanks for your message.
M87 is a Black hole.
So, how do we know if the object is BH or neutron star (assuming that they have the same mass)?
In the following article there is a nice description for those objects:

https://flexbooks.ck12.org/cbook/ck...y/lesson/neutron-stars-and-black-holes-ms-es/

Neutron Stars​

After a supernova explosion, the star's core is left over. This material is extremely dense. What happens next depends on the core's mass. The core might be less than about four times the mass of the sun. In this case, the star will become a neutron star. A neutron star (Figure below) is made almost entirely of neutrons. A neutron star has more mass than the sun; yet, it is only a few kilometers in diameter.

Black Holes​

The core remaining after a supernova could be more than about five times the mass of the sun. In this case, the core collapses to become a black hole. Black holes are unimaginably dense. Not even light can escape their gravity (Figure below)! This is why they are black. We can't see black holes.

It is stated: We can't see black holes.
Do they mean that we can see a Neutron star core?
If we can't see, then why are we so sure that after the supernova explosion, the star's core that is left is a Neutron star (at any mass)?
Why we can't just assume that we get a BH?
In other words, how do we know if the object is neutron star or BH if we can't see the core of none of them?

davLev said:

how do we know if the object is BH or neutron star

All types of compact objects other than black holes (white dwarfs, neutron stars, as well as "quark stars" and other hypothesized such objects) have a maximum mass. Any compact object more massive than the largest such maximum mass (which is not more than 3 solar masses; we don't know the exact value but we know it's not more than that) must be a black hole.

davLev said:

why are we so sure that after the supernova explosion, the star's core that is left is a Neutron star (at any mass)?

We aren't; it can only be a neutron star if its mass is less than the maximum mass for neutron stars.

If you can determine that the speed of a distance object is steady, then you can determine that the orbit is roughly circular. If you know its circular and can measure its speed and period, then you can calculate the mass of the object being orbited (the star, NS, or BH).

When that object is a BH, you can also immediately calculate its diameter. So using the orbit as a "maximum diameter" is not very useful for black holes.

PeterDonis said:

We aren't; it can only be a neutron star if its mass is less than the maximum mass for neutron stars.

Thanks

In the following article it is stated that the maximal size of the Neutron star is 2.16 Solar mass.

https://www.sciencedaily.com/releases/2018/01/180116093650.htm

Whilst most neutron stars have a mass of around 1.4 times that of the sun, massive examples are also known, such as the pulsar PSR J0348+0432 with 2.01 solar masses.

The density of these stars is enormous, as if the entire Himalayas were compressed into a beer mug.

Together with his students Elias Most and Lukas Weih, Professor Luciano Rezzolla, physicist, senior fellow at the Frankfurt Institute for Advanced Studies (FIAS) and professor of Theoretical Astrophysics at Goethe University Frankfurt, has now solved the problem that had remained unanswered for 40 years: With an accuracy of a few percent, the maximum mass of non-rotating neutron stars cannot exceed 2.16 solar masses.

It is also stated that:

However, there are indications that a neutron star with a maximum mass would collapse to a black hole if even just a single neutron were added.

Therefore, if the star is very massive, then after its Supernova, why we can't expect to get a Black Hole instead of Neutron star?

So, how do we know that after the supernova we should get a Neutron star?

In the following article it is stated that:

https://en.wikipedia.org/wiki/Magnetar
A magnetar is a type of neutron star with an extremely powerful magnetic field (~109 to 1011 T, ~1013 to 1015 G).[1] The magnetic-field decay powers the emission of high-energy electromagnetic radiation, particularly X-rays and gamma rays.[2]

However, how do we know that in the core of any magnetar/pulsar there is a neutron star?
What kind of observation convince us that magnetar/pulsar must be based on a neutron star and not on Black hole?

davLev said:

Therefore, if the star is very massive, then after its Supernova, why we can't expect to get a Black Hole instead of Neutron star?

We do expect that. Smaller ones leave neutron stars, larger ones leave black holes.

davLev said:

What kind of observation convince us that magnetar/pulsar must be based on a neutron star and not on Black hole?

Black holes can only have magnetic fields if they're electrically charged. We simply don't see macroscopic charged matter out there - it attracts charges of the opposite sign and neutralises itself very rapidly.

davLev said:

In the following article it is stated that the maximal size of the Neutron star is 2.16 Solar mass.

The article does not specify whether this limit is for non-rotating neutron stars, but I suspect it is. Rotating neutron stars will have a somewhat larger maximum mass limit that depends on the rate of rotation. Most neutron stars that we observe are rotating pretty rapidly.

Ibix said:

Black holes can only have magnetic fields if they're electrically charged.

More precisely, black holes surrounded by vacuum can only have magnetic fields if they are electrically charged.

Rotating black holes surrounded by accreting matter, however, can have magnetic fields anchored in the matter.

davLev said:

So, how do we know if the object is BH or neutron star (assuming that they have the same mass)?

If it's close enough, we look for radiation emitted by a neutron star. If that doesn't exist, it's a black hole.

For objects far away we measure the mass - if it's light it has to be a neutron star, if it's heavy it has to be a black hole, in between we can't be sure.

mfb said:

if it's light it has to be a neutron star

To be clear, we believe this not because there is a theoretical minimum mass for a black hole (there isn't), but because we are only considering the possibility of black holes forming from collapse of massive stars, and such a collapse process will only form a black hole if there is too much mass for a neutron star.

mfb said:

If it's close enough, we look for radiation emitted by a neutron star. If that doesn't exist, it's a black hole.

What do you think about M87?

https://link.springer.com/chapter/10.1007/978-94-010-3311-4_16
This paper reports upon an observation of hard X-radiation from the direction of M87.

It is a BH and it has radiation.

Ibix said:

Black holes can only have magnetic fields if they're electrically charged.

PeterDonis said:

More precisely, black holes surrounded by vacuum can only have magnetic fields if they are electrically charged.

I don't understand why black holes surrounded by vacuum can only have magnetic fields if they are electrically charged, while Neutron star have magnetic fields even if isn't electrically charged?

We must understand the difference of the internal structure between BH to Neutron star.
Therefore, let's start with the supernova:

https://en.wikipedia.org/wiki/Supernova

A supernova (pl.: supernovae or supernovas) is a powerful and luminous explosion of a star. A supernova occurs during the last evolutionary stages of a massive star, or when a white dwarf is triggered into runaway nuclear fusion. The original object, called the progenitor, either collapses to a neutron star or black hole, or is completely destroyed to form a diffuse nebula.

Neutron star -
We start the process with a star:

https://www.mccc.edu/~dornemam/Planet_Walk/Sun/the_sun.htm

The Sun is a typical star, and is also the star that is nearest to the Earth. It is composed of a mixture of 73% hydrogen, 25% helium, and 2% other elements by weight.

Hydrogen contains one proton and no neutrons - 73%

Helium contains two protons and two neutrons - 25%

Let's ignore the other elements - 2%.

So, out of 98 atoms in the Sun, there are:

Protons - 73 * 1 + 25 * 2 = 123
Neutrons - 25* 2 = 50.

For each neutron there are about 123/50 = 2.5 protons.

As there are much more protons than neutrons in a star, then why do we call it Neutron star instead of Proton star?

Neutron: Composition - 1 Up quark, 2 Down quarks + gluons
Proton: Composition - 2 Up quark, 1 Down quarks + gluons

Do we know how all of those protons are immediately converted to Neutrons during the supernova impact?

Black hole -
We assume that a black hole could be created directly by the supernova (If the star is very massive) or it could be created from a Neutrons star.
https://www.sciencedaily.com/releases/2018/01/180116093650.htm
However, there are indications that a neutron star with a maximum mass would collapse to a black hole if even just a single neutron were added.

Let's try to verify what could be the outcome if we add a single Neutron to a neutron star with a maximum mass?
Do you agree that in this case the internal gravity pressure / temp is so strong/high that it breaks the neutron/proton (Hardon) and take out the gluons which holds the 3 quarks together?

Therefore, we get quarks and gluons.
However, can those elements exist at the core of the BH while they aren't bound together in the hardon?
Do you agree that quarks are particles?
Therefore, technically they could survive in the core and their mass would count.
Never the les, the Gluons isn't a particle.

Therefore, it can't exist outside the hardon.
So, what is the outcome of the gluons once it is ejected from the Hardon?
Any idea how the gluons could keep its mass outside the Hardon?

Please be aware that the gluons contribute 99% of the Hardon mass.

If the gluons can't keep its mass outside the Hardon, then do you agree that its mass must be converted into energy (based on Einstein formula).

If that is correct, then once the Neutron star collapse, 99% of its mass (all the broken gluons) should immediately transformed into severe energy while only 1% (all the quarks) could exist.

Is it realistic?
Did we ever observed a Neutron star as it collapse?

Could it be that we miss something in our theories about BHs and Neutron stars?
It is stated:
The original object, called the progenitor, either collapses to a neutron star or black hole,
So, could it be that The original object, called the progenitor, should always collapse into dense quark core (call it as you wish)?
However, the key obstacle for this idea is:

Could this dense quark core have magnetic fields even if isn't electrically charged?

mfb said:

we look for radiation emitted by a neutron star. If that doesn't exist, it's a black hole.

You have to be careful here. Black holes can have accretion disks that emit radiation.

davLev said:

It is a BH and it has radiation.

See above.

davLev said:

We must understand the difference of the internal structure between BH to Neutron star.

Yes, indeed, and you apparently do not.

A neutron star is an object made of matter whose structure is determined by hydrostatic equilibrium under its own gravity.

A black hole is not made of matter; it is vacuum. It has no "internal structure". It is pure spacetime geometry. When an object collapses to a black hole, the matter in the object falls below the horizon and, according to classical GR (which is what we are using here), hits the singularity at ##r = 0## and is destroyed, leaving nothing behind but spacetime geometry.

The rest of your post ignores this crucial distinction.

davLev said:

Could it be that we miss something in our theories about BHs and Neutron stars?

No. You just need to improve your understanding of them.

davLev said:

Let's try to verify what could be the outcome if we add a single Neutron to a neutron star with a maximum mass?
Do you agree that in this case the internal gravity pressure / temp is so strong/high that it breaks the neutron/proton (Hardon) and take out the gluons which holds the 3 quarks together?

No. That's not what happens.

What happens is that the neutron star, if it accretes enough mass to go above the maximum mass for a neutron star, is unable to maintain hydrostatic equilibrium and starts collapsing. This has nothing whatever to do with "breaking" neutrons or "taking out gluons". It's just a loss of hydrostatic equilibrium leading to collapse.

davLev said:

dense quark core

Please give a reference for this.

PeterDonis said:

What happens is that the neutron star, if it accretes enough mass to go above the maximum mass for a neutron star, is unable to maintain hydrostatic equilibrium and starts collapsing. This has nothing whatever to do with "breaking" neutrons or "taking out gluons". It's just a loss of hydrostatic equilibrium leading to collapse.

Please see the following:

https://www.space.com/supermassive-black-holes-surprising-link-gluon

Supermassive black holes share a surprising link with subatomic gluon 'color glass walls'​

CGCs are created when atomic nuclei are accelerated to near the speed of light and then slammed together. After these collisions, conducted at facilities such as the Relativistic Heavy Ion Collider in Upton, New York, the CGCs "melt" to form a nearly perfect liquid of quarks and gluons.

the team found that gluons appear to organize themselves in a way that conforms to a universal limit on the amount of entropy, or disorder, that can exist in a system. The enormous conglomerates of gravitons that make up black holes are also believed to arrange themselves according to this limit.

This mathematical similarity suggests there is a correspondence between how black holes are born, reach thermal equilibrium with their environment and even how they may eventually decay, and how walls of gluons collide in nuclear collisions at near-light speeds.

In both systems, the bosons are arranged in the most energy- and size-efficient configuration. This creates a high degree of order that is typical of both CGCs and black holes, with both systems packing in the maximum amount of quantum information possible about their constituent bosons , including their spatial distribution, velocity and collective forces.

Do you confirm that in any star there must be significantly more protons than neutrons?
If so, Would you kindly explain why a Neutron star doesn't called proton star?

PeterDonis said:

You have to be careful here. Black holes can have accretion disks that emit radiation.

Please be aware that Neutron stars also have disks:

https://skyandtelescope.org/astronomy-news/measuring-neutron-stars-exactly/

"We're seeing the gas whipping around just outside the neutron star's surface," said Edward Cackett (University of Michigan), in a press release. "And since the inner part of the disk obviously can't orbit any closer than the neutron star's surface, these measurements give us a maximum size of the neutron star's diameter. The neutron stars can be no larger than 18 to 20.5 miles across [29 to 33 kilometers], results that [confirm] other types of measurements."

So why we can't claim that this Neutron star' disc emits radiation?

Black hole or neutron star?​

https://www.psu.edu/news/research/story/black-hole-or-neutron-star/

For decades, astronomers have been puzzled by a gap in mass that lies between neutron stars and black holes: the heaviest known neutron star is no more than 2.5 times the mass of our sun, or 2.5 solar masses, and the lightest known black hole is about 5 solar masses. The question remained: Does anything lie in this so-called mass gap?

Now, in a new study from the National Science Foundation's Laser Interferometer Gravitational-Wave Observatory (LIGO) and the European Virgo detector, scientists have announced the discovery of an object of 2.6 solar masses, placing it firmly in the mass gap.

"We've been waiting decades to solve this mystery," said Vicky Kalogera, a professor at Northwestern University. "We don't know if this object is the heaviest known neutron star, or the lightest known black hole, but either way it breaks a record."

So, could it be that we miss something in our understanding about Neutron star and BH?

PeterDonis said:

Please give a reference for this.

A quark star may have just been discovered​

https://www.advancedsciencenews.com/a-quark-star-may-have-just-been-discovered/

a recent discovery may point toward the existence of a new type of star: the quark star.

The hypothesized existence of such objects was first expressed in 1965 by Soviet physicists Dmitri Ivanenko and Dmitri Kurdgelaidze. The idea is based on the assumption that at extreme pressures in the collapsed cores of stars, neutrons become so compressed that they breakdown into the elementary particles that make them up, called quarks. Under this intense gravity and strong interaction, though free, quarks clump together, forming a quark star.

davLev said:

Please see the following

That source is based on a speculative hypothesis about quantum gravity. It is not saying that black holes are made of matter the way neutron stars are.

davLev said:

Please be aware that Neutron stars also have disks

That's true, they can. I never said otherwise.

davLev said:

why we can't claim that this Neutron star' disc emits radiation?

I never said we can't. I was pointing out (to @mfb, not you) that since black holes can have accretion disks that can emit radiation, we cannot say that if we detect radiation, the object cannot be a black hole. Nobody has said that if we detect radiation, the object cannot be a neutron star.

davLev said:

could it be that we miss something in our understanding about Neutron star and BH?

Not the part of that understanding that you are asking about, no. The "mass gap" referred to in the source you reference is not actually a problem at all; it's just a consequence of the fact that our observations are incomplete.

davLev said:

A quark star may have just been discovered

Nothing said in this article makes "quark stars" any less speculative than they were before. It's just a pop science article which, as is typical for pop science articles, overstates what the research it is describing actually shows.

Btw, so far you have not given a single reference that is an actual peer-reviewed paper. Nor, apparently, are you familiar with any of the literature that discusses things like the structure of neutron stars, such as Shapiro & Teukolsky. You cannot expect to learn actual science at the level you are trying to discuss it from pop science articles and middle school textbooks.

Would you kindly answer the following questions:

1. Do you confirm that in stars there must be more protons than Neutrons?

davLev said:

https://www.mccc.edu/~dornemam/Planet_Walk/Sun/the_sun.htm
The Sun is a typical star, and is also the star that is nearest to the Earth. It is composed of a mixture of 73% hydrogen, 25% helium, and 2% other elements by weight.
Hydrogen contains one proton and no neutrons - 73%
Helium contains two protons and two neutrons - 25%
Let's ignore the other elements - 2%.
So, out of 98 atoms in the Sun, there are:
Protons - 73 * 1 + 25 * 2 = 123
Neutrons - 25* 2 = 50.
For each neutron there are about 123/50 = 2.5 protons.

2. As there are much more protons than neutrons in a star, then why after the supernova we get a star which is mainly based on Neutrons and therefore it is called Neutron star? Why not Proton star?

3. How all/most of the protons in this star could instantly be transformed into Neutrons during the supernova impact?

4. Could it be that in Neutron star there are some protons and quarks. If so, at what ratio with regards to neutrons?

5. What kind of observations do we have that convinced us that Neutron star is mainly based on Neutrons?

davLev said:

Do you confirm that in stars there must be more protons than Neutrons?

Generally one would expect that to be true as long as there is significant hydrogen in the star. But "star" is a more general term than you appear to think; see below.

davLev said:

Would you kindly explain why a Neutron star doesn't called proton star?

Um, because it's made of neutrons? The word "star" doesn't just mean "star like the Sun".

davLev said:

What kind of observations do we have that convinced us that Neutron star is mainly based on Neutrons?

The observations of their masses and sizes (as well as other data like pulsar timings) and our theoretical knowledge about possible states of matter for objects with those properties. For example, I have already referenced Shapiro & Teukolsky, a textbook which discusses this in great detail.

davLev said:

How all the protons in the star could instantly be transformed into Neutrons during the supernova impact?

They aren't "instantly" transformed. They are transformed during the process of collapse from an ordinary star to a neutron star. The density range during which this "neutronization" takes place (mostly by inverse beta decay) is discussed in detail in Shapiro & Teukolsky.

davLev said:

Could it be that in Neutron star there are some protons and quarks. If so, at what ratio with regards to neutrons?

Our neutron star models generally have a very thin layer of white dwarf-type matter at the surface, which would contain a roughly equal number of protons and neutrons. However, this layer is only a very small fraction (I think well under one percent) of the total mass of the neutron star.

Neutrons (and protons) are made of quarks, so of course there are quarks everywhere in a neutron star. If by "quarks" you mean some other state of quarks besides neutrons or protons, there are speculations along these lines, but no good evidence for any of them that I'm aware of.

Thanks for your prompt reply!