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Ask an Astrophysicist: Neutron Stars

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Neutron Stars

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Library of Past Questions and Answers

    Pulsars

Who discovered the first pulsar? (Submitted February 27, 1998)

The Question

Who discovered the first pulsar?

The Answer

In 1967 Jocelyn Bell discovered the first pulsar. Because she was a graduate student at the time, her advisor (Anthony Hewish) was given a share of the 1974 Nobel Prize instead of her.

They had no idea what these signals were, so they were dubbed little green men (LGM) as a reference to extraterrestrial life. Soon after, Thomas Gold showed that a spinning neutron star could make the same pulses.

If you are still interested, there have been a number of very good articles about this discovery. At any rate, you must read:

http://www.bigear.org/vol1no1/burnell.htm

This reproduces an article originally presented as an after-dinner speech at one of the Texas Symposia by Jocelyn Bell Burnell --- it's a very personal & entertaining account of the discovery.

Sincerely,

Jonathan Keohane & Koji Mukai
for Ask an Astrophysicist

Question ID: 980227a

What is the difference between pulsars and neutron stars? (Submitted January 25, 1998)

The Question

What is the difference between pulsars and neutron stars? If a pulsar is a neutron star, is every neutron star a pulsar?

The Answer

A pulsar is a rotating neutron star that can produce radiation by spinning its powerful magnetic field through space. (There are also 'accreting pulsars' which funnel matter from a companion star onto their magnetic polar caps as they rotate.)

A neutron star uses up a lot of its rotational energy moving its magnetic field around this way, and and so it gradually slows down. When it slows down enough, it no longer radiates very much energy, and so it is no longer considered a pulsar. This usually happens within a few million years. If a neutron star had only a weak magnetic field, it would also not be a pulsar.

Most neutron stars in the Universe are old enough and tired enough that they are no longer pulsars. A recent paper estimates a thousand million old neutron stars in our Galaxy, even though the number of known pulsars is about a thousand.

http://xxx.lanl.gov/abs/astro-ph/9802032

David Palmer
for Ask an Astrophysicist

Question ID: 980125a

Why do we study pulsars? (Submitted November 12, 1997)

The Question

Why do we study pulsars?

The Answer

We study pulsars for the same reason physicists used to study other worthless phenomena, such as why steam expands, or what lightning is made of--because we are curious. We are curious about the properties of nuclear matter that make up the neutron stars of the pulsars. We are curious about the relativistic effects of extreme electromagnetic and gravitational fields moving through space at high speeds. We are curious about everything.

In the past, curiosity has always paid off, e.g., with the steam-engine age and the electronic age. Although pulsars have led to new insights on timekeeping (pulsars are the most accurate clocks known) and the forces that cause earthquakes (by using pulsars to determine how radiotelescopes move, you can map the motions of Earth's crust), and may someday lead to totally unpredictable advances, that is not the reason why people do it.

People study pulsars because it's fun to discover things that nobody has ever known before.

David Palmer
for Ask an Astrophysicist

Question ID: 971112c

Can you answer my many questions about pulsars? (Submitted April 24, 1997)

The Question

I'm researching about some information about pulsars, I've some questions to ask.

1. Is it right that the pulsars were identified as pulsating radio sources?

2. I know that they emit x-ray and gamma-ray, but do they emit ultraviolet, radio waves, and bursts of visible light?

3. Up to how many pulses per sec. does a high-frequency pulsar emit? How about the low-frequency pulsars?

4. I know that pulsars are due to the neutron star rotation, but do they rotate around their axis?

5. Is it possible to find their rapid periodical changes on other bands of electromagnetic spectrum?

6. Is it thru that all pulsars are slowing down very slightly and the period of pulsation increases gradually? If it is, is that indicates a slow yet steady loss of energy due to the radiation of energy into space?

7. The old pulsars, are they strong x-ray emitters?

8. How does their radiation affect the nearby gas clouds? Is it by ionizing and heating them?

9. About supernova remnants, do they radiate for millions of years, can we measure them by our telescopes?

10. What are nebulae? Are they the same as supernova? What kind of elements are abundant in these nebulae?

11. Is it right that the most known pulsars are found in our galaxy's disk? Are they rare in halo of the galaxy, do other galaxies show the same distributions of pulsars?

12. We can't see all the supernova explosions, but the ones which are visible to us, is it because of their beam of rays is directed to us?

13. I read somewhere that neutron stars are peculiar, what is that mean?

14. Redshift means that the subject is going away from us, but dose the Sun gives us any redshift at all, but it's not getting away from us?

15. What kind of mass do the neutron stars have? Can they be more than 5 times the mass of the sun?

16. How can we measure the period of the neutron star's orbiting around their companions?

17. Do you think there might be unknown neutron star in vicinity of the solar system that we might some day reach it?

18. Is there any possibility to land on these neutron stars? Do we get crushed by their gravity if we approach them?

19. About the Large Magellanic Cloud, that famous supernova explosion, about how many pulses per sec. did that pulsar leave behind?

The Answer

You ask some interesting questions.

1. Pulsars were first discovered as radio sources.

2. Yes, pulsars have been found that emit radiation in all these bands.

3. The fastest known pulsar (PSR 1937+21) emits 641 pulses per second. Some X-ray pulsars have periods tens of minutes long.

4. Are you mixing up orbital motion and spin? It is the rotation of the neutron star about its axis that causes the pulsation.

5. Pulsars have been seen in the radio, optical, X-ray and gamma-ray bands.

6. This is true of isolated pulsars. However, if the pulsar is in a binary system, it may accrete matter from its companion. It is possible for this to cause the neutron star to spin faster.

7. Pulsars slow down as they get older and the amount of radiation they emit decreases. However, if the pulsar is in a binary system and is accreting matter from its companion star it can be made to spin faster. Pulsars that have been spun up this way are called "recycled pulsars" and they can be strong X-ray emitters.

8. The most obvious effect on its surroundings is caused by the supernova explosion itself. Later, the radiation emitted by the pulsar will ionize and heat nearby matter. This effect is most important in systems consisting of a neutron star orbiting another star.

9. Yes, they are visible in the optical band as well as radio and X-rays. After a few hundred thousand years the remnant will have merged with the interstellar medium and will not be detectable any more.

10. Nebulae are clouds of gas and dust. Some nebulae are formed by supernova explosions. In this case the nebula is called a supernova remnant. They are mostly hydrogen and helium. If the material in the nebula has been processed in the interior of a star other elements up to iron will be present. Elements heavier than iron are only formed in supernova explosions.

11. Yes, most pulsars are found in the plane of the galaxy. They are also found in globular clusters in the halo of the galaxy. The only other galaxies in which pulsars have been detected in significant numbers are the large and small Magellanic Clouds. Because these are irregular galaxies and the Milky Way is a spiral it is difficult to compare pulsar distributions.

12. A supernova explosion is not beamed. It is visible from all directions. However, there are other reasons why it may not be seen by us. If the supernova occurs in a distant galaxy it may be too faint to be noticed. A supernova explosion on the other side of the galaxy may be hidden behind clouds of gas and dust.

13. This probably refers to the fact that neutron stars are made up of extremely dense matter. Their gravitational fields are so intense that the nuclei of atoms are squeezed together and protons combine with electrons to form neutrons. Hence the name neutron star.

You may also be thinking of 'strange stars'. These theoretical objects are similar to neutron star, except that they include particles which have 'strange quarks' in them. There are six types of quarks--Up, Down, Strange, Charm, Top, and Bottom (the physicists who name these things are somewhat lacking in gravitas). Neutrons and protons are made up of Up and Down quarks. Strange, charm, top, and bottom quarks only become important under extreme conditions because they quickly decay to up and down quarks.

14. The earth's orbit is almost circular so we are pretty much the same distance from the Sun all the time. However, when we measure spectral lines from the Sun we find both red- and blue-shifted lines. If you look with the right instruments, you can use this redshift to see parts of the Sun's surface moving up towards us, and other parts moving down.

15. Neutron stars tend to be formed at about 1.4 solar masses, but more material can fall onto them after that. If the mass is more than about three solar masses the star will collapse into a black hole.

16. We can use the Doppler effect to measure the orbital motion of a pulsar. This is the same effect you observe when an emergency vehicle passes you with its siren on. As it is approaching you the siren is higher pitched, and as it recedes from you it is lower-pitched. In the same way if the pulsar is coming towards us the pulses appear closer together. If it is moving away from us they appear further apart. If we plot the pulse frequency against time we find a pattern which repeats itself every orbit. From this we can measure the orbital period.

17. There has been some speculation that there might be a neutron star orbiting the Sun but there is not much evidence.

18. There is a Science Fiction book called 'Dragon's Egg' by Robert Forward, which is about a visit to a neutron star. If you were to land on a neutron star, the gravity (about 500 billion times as intense as Earth's) would immediately flatten you to a thin film an atom thick. Even if you went into orbit around a neutron star, the difference in gravity between your head and your feet would be enough to pull you apart (this difference in gravity is what causes tides). Robert Forward (who is a physicist as well as a science fiction writer) figured out how to live in orbit without being ripped apart.

19. I don't think a pulsar has been detected in the remnant of SN1987a yet. There was a report of a high frequency pulsar but it turned out to be a signal from a video camera the astronomers were using to aim their telescope.

You can find more information about pulsars at http://heasarc.gsfc.nasa.gov/science/objects/pulsars2.html

Damian Audley, David Palmer, and Karen Smale
for the Ask an Astrophysicist team.

Question ID: 970424d

Why is a pulsar rotation axis and magnetic field not aligned? (Submitted November 28, 2006)

The Question

Why are the magnetic field of a pulsar and the axis of rotation not aligned with each other? Doesn't a magnetic field get created because of the spinning matter that alignes itself with the axis? Why does it differ? My astronomy teacher didn't know the answer to this.

The Answer

It's an interesting question. How magnetic field is created and sustained in a normal star (like our Sun) is not fully understood yet. But one thing is clear: the magnetic field does not depend solely on the global spinning motion of the matter in a star. Rather, it depends on various other typess of motion. This is called dynamo effect. You can see
http://solarscience.msfc.nasa.gov/dynamo.shtml
for a description of this effect in our Sun.

Now, when a star (much more massive than our Sun) collapses to a neutron star (pulsar), its magnetic field lines also become denser, and hence the magnetic field becomes stronger. In the process of collapse, magnetic field may get affected by turbulent motions of matter. Therefore, there is no reason why magnetic axis of a neutron star (pulsar) has to be aligned with the spin axis. Note that even earth's magnetic axis is not aligned with its spin axis.

Hope this helps,
Sudip & Koji
for "Ask an Astrophysicist"

Question ID: 061128a

    Magnetars

Are magnetars and pulsars related? (Submitted June 09, 2010)

The Question

Is it true that magnetars are not unique objects, but are just a phase in the lifespan of pulsars?

The Answer


Thanks for your question! Both magnetars and pulsars are special types of neutron stars. A magnetar is a highly magnetized neutron star and a pulsar is a rapidly-rotating neutron star with a electromagnetic jet that sweeps across the sky as the star spins. We have observations of both types of objects, and they certainly may be unique. However, there was some recent work by scientists at McGill University and NASA that showed observations of a single neutron star that seemed to exhibit both magnetar-like and pulsar-like behavior. What we don't know is if this is a very special object or if magnetars and pulsars are always related.

Here is a link to a press release on the discovery: http://www.mcgill.ca/newsroom/news/item/?item_id=29230

Hope that Helps!
-Ira & Kevin
for "Ask an Astrophysicist"

Question ID: 100608a

How does a Magnetar form? (Submitted September 30, 1998)

The Question

I'm 15, but I can understand the answer in scientific terms.

How does a neutron star evolve to be a magnetar? Is it from the gravity of the matters in the neutron star? Does a magnetar form from a supernova, or before the star explodes?

The Answer

Hello,

This is an excellent question, and one which is at the frontier of current research. First of all, nobody knows much about magnetars since there existence was only recently suggested and the observational evidence for them is even newer. As far as I know the most likely scenario for their formation is as the remnant from a supernova. This is the ultimate origin of all neutron stars, and magnetars may be just the neutron stars which are formed with the strongest magnetic fields. An interesting observational fact is that all known magnetars appear to be rotating quite slowly by neutron star standards, about once every 8 seconds or so. This can be understood if they are created with a much more rapid rotation rate, say once every few milliseconds, because their strong magnetic fields are expected to cause them to spin down very rapidly by magnetic dipole radiation.

I don't know if there are other scenarios for magnetar formation which are equally likely.

I hope this helps,

Tim Kallman for Ask an Astrophysicist


Question ID: 980930a

Is a magnetar's mass greater than a normal neutron stars' mass? (Submitted February 30, 2010)

The Question

Is there any certainty of a magnetar's mass? And that they are larger than neutron stars, just like how a smaller white dwarf is more massive than a larger white dwarf? Except with magnetars and normal neutron stars, a difference of 1 or 2 miles in diameter?

The Answer

Thank you for your message. We've consulted Dr. Gavriil, a local expert on magnetars. According to him:

Actually, we don't know why magnetars have such extraordinarily strong magnetic fields, their defining characteristic. It is true that one hypothesis states that they come from relatively massive progenitors, and hence are more massive than the average neutron stars. This is a reasonable theory, but there are others, too.

Since none of the known magnetars are in binary star systems, we have not been able to measure their masses or radii.

Best wishes,

Koji & Georgia
for "Ask an Astrophysicist"

Question ID: 100530a

    Neutron Stars (General)

How far away is the nearest neutron star? (Submitted November 24, 1997)

The Question

Where is the closest neutron star?

The Answer

The neutron star which we currently rank as "nearest to Earth" is a radio pulsar called J0108-1431. It is within 100 parsecs of the Sun. A parsec is a unit of distance in astronomy; it is equal to 3.26 light years, or 3.1 x 1018 cm.

Regards,
Laura Whitlock

for the Ask an Astrophysicist Team

Question ID: 971124f

How do you know the approximate weight of a neutron star? (Submitted October 15, 1998)

The Question

How do you know the approximate weight of a neutron star?

The Answer

Using physics, we can calculate how large a star's iron core can be before it collapses to a neutron star, and how large a neutron star can be before it collapses to a black hole. We expect the masses of neutron stars to be within that range.

To find the weight of a specific neutron star, and see whether we got the physics right, we look at neutron stars in orbit around other stars and see how fast they move and in how large an orbit. Using that information and the laws of gravitation we can calculate the neutron star's mass.

http://imagine.gsfc.nasa.gov/ask_astro/stars.html#970609f

David M. Palmer
for Ask an Astrophysicist

Question ID: 981015b

Is there a mass limit for neutron stars like there is for white dwarfs? (Submitted June 07, 2001)

The Question

The Chandrasekhar Limit states that the maximum mass for a white dwarf star is 1.4 Solar masses. I am wondering whether we know the maximum mass for a neutron star, at which point more mass will create a black hole?

The Answer

Thanks for the question. The currently popular maximum neutron star mass is about 3-3.2 Solar masses. (However, being a scientific subject, there are some researchers who contest this limit on a few arguments.) Neutron stars are created when normal stars of 4-8 Solar masses undergo supernova explosions and throw off most of their outer material. The remaining protons and electrons collapse and are fused together to form a dense ball of neutrons. For some more information of neutron stars, see http://imagine.gsfc.nasa.gov/science/objects/pulsars1.html.

Cheers,
Scott & Laura
for Ask an Astrophysicist

Question ID: 010607a

What happens when a neutron star gets too massive? (Submitted September 21, 2010)

The Question

I've read somewhere that some pulsar's are speeding up. They do this by 'eating' another star in a binary system. What would happen if you kept adding matter? Would it eventually go faster than the speed of light (which is not possible), or will it collapse into a black hole?

The Answer

Yes, you are right! Very fast rotating pulsars, known as millisecond pulsars (because their rotation period is about 1 millisecond! Imagine that-- the star, whose radius is the size of Manhattan is spinning all the way around in one thousandth of one second!) are thought to be neutron stars that have accreted surrounding material (possibly its binary companion star) and ended up spinning faster as more material have fallen onto the surface. However, if the star accretes more mass than a critical mass, known as the Tolman-Oppenheimer-Volkoff limit, the star will collapse into a singularity, a black hole. This is because the pressure within the star can't support the star's own mass against gravity. Just to clarify, it is the mass limit that causes the neutron star to collapse, not that the rotation speed approaches the speed of light. The rotation speed can't reach the speed of light because it takes infinite energy for any material with mass to travel the speed of light.

Cheers!
-Antara & Kevin
for "Ask an Astrophysicist"

Question ID: 100921a

How long does a neutron star live? (Submitted April 06, 2000)

The Question

Could you tell me how long a neutron star 'lives'? Different sources are all conflicting.

The Answer

If you're asking how long a neutron star can actually be detected as a pulsar, the answer is that in the most recent catalog of pulsars (pulsars are rotating neutron stars), the oldest ones are more than 10,000,000,000 years old (although the large majority of pulsars is between 100,000 and 300,000,000 years old.

Now we can only date the neutron stars for which we measure a period and a period derivative, which means that there may be a large population of older "silent" neutron stars

I hope this answers your question.

Ilana Harrus for

the "Ask an Astrophysicist" team.

Question ID: 000406a

What is the "neutron star" state of matter? (Submitted October 06, 1996)

The Question

I have recently learned of the discovery of a new state of matter called the "neutron star" state, but I am unable to find any information regarding it, only that it is a new state of matter. If you have any information concerning this topic, please e-mail it to me. Your efforts would be greatly appreciated. Thank you.

The Answer

Thank you for your question regarding the neutron star state of matter. Yes, there is this state of matter, but unfortunately it is only found in the corpses of dead massive stars that have undergone a "supernova" explosion.

For information regarding neutron stars and supernovae on the world wide web you can look at Imagine the Universe! under the Basic and Advanced High-Energy Astrophysics sections. Or, any text book on astronomy will describe this as well.

Here is an additional explanation:

Neutron stars are compact objects that are created in from the core of massive stars during supernova explosions. Due to its massive weight, the core of the star collapses, and crushes together every proton with a corresponding electron turning each electron-proton pair into a neutron. The neutrons, however, can often stop the collapse and remain as a neutron star.

Neutron stars are fascinating objects, because they are the most dense objects known. They are only about 10 miles in diameter, yet they are more massive than the sun. One sugar cube of neutron star material weighs about 100 million tons, which is about as much as a mountain.

Like their less massive counterparts, white dwarfs, the heaver a neutron star gets the smaller it gets. Imagine if a 10 pound bag of flour was smaller than a 5 pound bag.

Neutron stars can be observed occasionally as an extremely small and hot star within a supernova remnant. However, they are more likely to be seen when they are a pulsar or part of an X-ray Binary.

Neutron stars are also speculated to be involved in other high energy phenomena that we still do not understand. Neutron stars are fascinating objects that are involved in many high energy phenomena.

Question ID: 961006

What is the news about a possible Strange Quark Star? (Submitted April 12, 2002)

The Question

Recently I read an article about a new type of star. They call it a strange quark star. It is like a neutron star (composed of neutrons) except it is made of quarks. The article said that there was one detected 400 light-yrs away. Is this actually true? do you have any other information on the subject?

The Answer

This is quite an interesting discovery. I may be misrepresenting it slightly, as I haven't read the whole paper, but I would say the big discovery here is that the star in question appears to be as massive as a neutron star but smaller in diameter than a neutron star can be.

Possible answers are that the diameter is not as small as we think, neutron stars can in fact be much smaller (which would mean we have a big misunderstanding of the equation of state of neutron matter), or that it's a new form of matter. If the last, quark matter seems an obvious candidate.

This is all very new, and may turn out to be incorrect. There aren't any theories yet about precisely how a quark star would form. However, it's certainly safe to say that there will be many very soon. Stay tuned!

Hope that helps.

-Kevin Boyce and Martin Still
for "Ask an Astrophysicist"

Question ID: 020412a

How can neutron stars have a magnetic field if they are made out of neutrons? (Submitted January 14, 1998)

The Question

If neutron stars consist of only neutrons, how is it possible that they have a magnetic field? I am just a hobbyist, but I thought that the presence of electrons is necessary for a magnetic field.

The Answer

This is a good question. The answer, I think, is that neutron stars are not pure neutrons. If they were, the neutrons would decay. A small (10%) fraction of electrons and protons are present which provide a rate of neutron formation via inverse beta processes which balance the neutron decay. These protons are highly degenerate and superconducting, so the magnetic field is frozen into the neutron star. The origin of the field is probably the parent star, although various schemes have been suggested for generating fields in neutron stars.

I hope this helps,

Tim Kallman
for the Ask an Astrophysicist team

Question ID: 980114b

Is there evidence for the superfluid interior structure of neutron stars? (Submitted February 13, 1997)

The Question

I am doing research on the interior structure of neutron stars. I was wondering what evidence there is for the superfluid theories, and where I can find more information about the properties that would be associated with such theories.

The Answer

The best evidence for superfluid cores in neutron stars is "glitches" observed in the evolution of the neutron star spin period. Normally, the neutron star spin period increases as time goes on. However, for some neutron stars, e.g. the Vela pulsar and the Crab pulsar, the spin period will suddenly decrease. These glitches can be understood as resulting from the superfluid core.

There are vortices in the superfluid core which move outward in response to the normal spin down. However, when they reach the boundary between the core and the crust, the vortices get pinned to the crust. This pinning keeps the crust and the core rotating together. However, a frictional drag is set up between the core and the crust, and when the stress becomes great enough, the vortices "unpin" from crust. This results in a momentary decrease in the spin period, which is observed as a "glitch".

This is at least one mechanism. There may be other explanations of exactly what happens, but they all depend on the presence of a superfluid core. For more info, you might try looking at sections 10.9-10.11 of "Black Holes, Neutrons Stars, and White Dwarfs" by Stuart Shapiro and Saul Teukolsky (1983). (This book is written at the advanced undergraduate and 1st year graduate level). In the professional astronomy literature, you might also look at Alpar, M.A. et al, "Giant Glitches and Pinned Vorticity in the Vela and Other Pulsars" in the Astrophysical Journal Letters, Vol 249, p. L29 (1981). Finally, if you're interested in more recent work, I'd suggest you use the NASA Astrophysics Data System:

http://adsabs.harvard.edu/

(Despite its name, it actually is a search engine for the professional astronomical literature.)

I've made an assumption here that you're doing research at the college level or higher. If you're in high school, my suggestions on where to look for further info may not be as helpful, but you might try them anyway. More reasonable possibilities for the high school level would be back issues of Scientific American or Sky and Telescope.

I hope this helps.

Jim Lochner
for Imagine the Universe!
(with help from Dr. Alice Harding)

Question ID: 970213


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