What are neutron stars, anyway?

[Faultline]

To be more specific, my question is, "Is it safe to say a neutron star isn't really a star anymore because it isn't undergoing a fusion reaction any longer?"

I mean, what does a neutron star have in common with our sun?

[JohnW]

Great. Months of argument about the definition of "planet" and now we have to do the same for "star"...

I suppose we could call them "post-stellar remnants" or some other mouthful, but "star" is just easier. By the way, wouldn't the same argument apply to white dwarfs? If I remember correctly, they also have no fusion, and are slowly cooling down.

[Hamlet]

To be more specific, my question is, "Is it safe to say a neutron star isn't really a star anymore because it isn't undergoing a fusion reaction any longer?"

I suppose it's debatable. We could call it a compact object or just a stellar remnant, but I don't thinks that's really necessary.

I mean, what does a neutron star have in common with our sun?

They appear to be quite different beasts now, but they shared a similar evolutionary history. They both were born in a collapsing cloud of gas and dust. The both went throught a tumultuous adolescense before settling down on the Main Sequence. The neutron star progenitor, being more massive, was hotter and more luminous than our sun but the underlying reaction powering both was hydrogen fusion.

Perhaps this is akin to asking what does a living person and a cadaver have in common?

[Joff]

Perhaps this is akin to asking what does a living person and a cadaver have in common?

That seems harsh... Since a neutron star is still emitting radiation, perhaps we could call it a retired star. By analogy, not generating new income, but living off its savings from when it was.

[Hamlet]

That seems harsh... Since a neutron star is still emitting radiation, perhaps we could call it a retired star. By analogy, not generating new income, but living off its savings from when it was.

It wasn't meant to be harsh. I was just drawing an analogy between fusion/no fusion in a star/remnant and metabolism/no metabolism in a person/cadaver.

[Jorge]

Simple Question...
If a human ages and dieds... his bones are still human bones right?

So why can't a aged / dead (well more or less) star, just be a star?

[Robert A.]

"That seems harsh... Since a neutron star is still emitting radiation, perhaps we could call it a retired star"

Well, to be precise, a cadaver is still emitting radiation, too. All objects emit blackbody radiation. It's just that the neutron star is much hotter than your average cadaver.

I like the analogy: In animals, respiration = living, otherwise dead; in stars, nuclear fusion = living, otherwise dead.

Another analogy: A neutron star is to a star what a hot ember is to a fire.

By the way, the fusion process in the high mass stars that eventually become neutron stars (called the CNO cycle) is not the same as the fusion process in low mass stars like our Sun (called the proton-proton chain). The higher mass leads to higher temperatures at the core, which allows a different, much faster, nuclear process. It involves Carbon, Nitrogen, and Oxygen as intermediate products - hence the name. It is also a major reason why high mass stars burn their fuel so much faster than low mass stars.

[Joff]

It wasn't meant to be harsh. I was just drawing an analogy between fusion/no fusion in a star/remnant and metabolism/no metabolism in a person/cadaver.

I forgot to grin . I still prefer fusion = income to fusion = metabolism, although like all analogies it mustn't be pushed too far.

[Igor]

For those of us who forgot- inside a neutron star the pressure due to gravity is
so great that electrons are crushed into the protons, creating neutrons. That is
why it is called a neutron star

Igor

[Faultline]

For those of us who forgot- inside a neutron star the pressure due to gravity is
so great that electrons are crushed into the protons, creating neutrons. That is
why it is called a neutron star

Igor

I know that.

I want to know if there was a good reason to call it a 'star' anymore.

I like some of the answers I got here.

[Ilya]

I mean, what does a neutron star have in common with our sun?

Mass.

[Faultline]

Mass.

Ah! Humor! Arr-Arr!

By the way, nanu-nanu!

Good one.

[The Bad Astronomer]

Actually, a neutron star is more massive than the Sun.

It's called a neutron star out of habit more than anything else, I suppose. Some people say a star is anything that is massive that once sustained fusion in its core. Some brown dwarfs fuse deuterium for a little while, but it stops. Are they stars?

I think anytime you try to box an object in with some definition, you're making a mistake. I was in on the brown dwarf/planet naming woes some years ago. When you give something a name, it boxes in your mind. Many people who thought of BDs as planets missed their stellar qualities, and people who thought of them as stars missed their planetlike qualities.

Don't let your own arbitrary definitions cloud your view. Let the Universe talk to you. It knows what it's saying, and it doesn't care what you think.

[Jorge]

Fair point... maybe we should give multiple labels?

Main Class: Star, sub class: neurtron/no fusing Object

[Donnie B.]

Of course, some neutron stars do undergo periodic outbursts of fusion -- if they draw in material from a companion star.

So if you made fusion the criterion, these guys would be changing from not-star to star and back again from time to time. Kind of like some old Hollywood denizens...

[Jorge]

Main Class: Star, sub class: neurtron/Ocasional fusing Object

better?

[Faultline]

I think I like the BA's answer better. I had a hunch it was just a carryover from their former life as a living star. I just wanted to know if there really was any scientific reason to still call them a neutron *star*.

I'm satisfied. Thanks for your support.