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Characteristics of Pulsars

  1. Nov 12, 2015 #1
    Dear PF Forum,
    I've read the current Featured Thread in November 12nd, Lost in the Milky Way scenario. It's a good thread.
    But I want know what pulsar is.

    A. Does pulsar(s) have unique frequency? Given the number of pulsar in our galaxy and the vast range of pulsar frequency, so it's unlikely that there are two pulsars that have a similar frequency in our galaxy.

    B. The implication of question A is
    Does pulsar(s) frequency remain stable for at least million of years?

    C. Is there a formula to create pulsar frequency?
    I mean:
    For a star say 1 solar mass that have the ratio of hydrogen/helium and perhaps carbon combinations the star will behave the same as our sun. And given some times the star will explode as our sun.
    Or there is a limit for neutron star mass, if I'm not mistaken it's between 1.5 solar mass to 3 solar mass.
    So what about pulsar frequency, is that really random generated?

    D. In Voyager plate, there is a picture about pulsar, I just realized that now. Because the only thing that I can recall from that plate that I first read it when I was in junior high is the naked man/woman picture. Can't remember the 3rd rock from the star, the pulsar and the 2 hydrogen circles.
    The scientists that designed it (Sagan? Drake?) carved some pulsars with their frequency. (as they say, because I don't even know what is it all about if I found that plaque in my backyard). So what I want to know is:
    Supposed an alien civilization encounter that plate, how will they know the frequency of the pulsars given just the length radius of the picture. (If they somehow can decipher that it's about pulsars)

    Thank you very much.
  2. jcsd
  3. Nov 12, 2015 #2
    a) Yes, each pulsar is unique, just like the stars that formed them.
    b) Yes, they are gigantic objects, they have so much angular momentum it'd be very difficult to change their pulses.
    c) Not sure
    d) The pulsars are labeled with a universal time unit: the spin-flip time of an electron in a hydrogen atom.
  4. Nov 12, 2015 #3
    I know my next question may be off topic. Please feel free to move/delete it.
    So for a star say 25 solar mass when it's formed from nebula cloud and somehow it has random elements composition, say 5% oxygen, 5% sillicon but it's still in main sequence.
    So, when this star is in sillicon burning stage, will it have the same composition for its element as other 25 solar mass star in sillicon burning stage which started with 95% hydrogen?

    What is the angular momentum for a star? In layman term, if our sun becomes a white dwarf will it rotate 1/100th it is now because its radius will be as small as earth radius, even if mars or jupiter still exist? (considering they survive the red giant sun)

    I read in Wiki, our sun hydrogen ratio is 75%. Does someone know what is the composition of hydrogen in our sun 4.5 billions years ago? I read that Jupiter, too has 74% of its mass in hydrogen, in mole it's 92%. So, the composition of hydrogen of our sun doesn't change much since it's created?
    This is just out of curiousity.

    Thanks for any replies.
  5. Nov 12, 2015 #4


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    The frequencies of the EM pulses are not constant over time. Pulsars generally start out emitting at high frequencies just after they are created, and then depending on their surroundings, the frequencies gradually decline as the pulsar's rotation slows, due to the emission of tremendous amounts of energy in the EM beams.

    There is a frequency below which a pulsar shuts off and no longer radiates EM pulses. It is estimated that it takes 10-100 million years after their creation for pulsars to reach shut off frequency.

    I think what you meant to say here is that pulsars are massive objects.

    Physically, pulsars are quite small. Being neutron stars, their diameters are typically less than 25 km, but each star can contain as much as 2 solar masses of degenerate matter.

  6. Nov 12, 2015 #5


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    Pulsars do spin down, increasing their period. For example, the Crab Nebula pulsar has present period of 0.331 sec and a spin down rate of 4.22e-13 sec/sec or 0.00013 sec/yr. Just 5 centuries ago, it was spinning at roughly twice as fast.
  7. Nov 12, 2015 #6
    I read that in Pioneer 10, there's a plate that maps the pulsars surrounding our sun (Sol). So if the pulsars loss frequency, how will the alien civilization will find our star?
    So it will reach Alpha Centauri in ten of thousands of year (IF it is heading toward Proxima Centauri) and...
    So how can the alien find Sun? Or there's something in the plate that indicates WHEN the plate is sent?
    Add: IF there's alien in Alpha Centauri
    My point is, is there a mechanism in Pioneer plate that TELLS when the plate was sent?
    I read that there's some 21 cm Hydrogen wave or something that can encode the picture, the creature who sent this (in this case human), the relative size of human compared to the spacecraft, the location of earth relative to the sun and the location of the sun. Is there time information in Pioneer plate?
  8. Nov 12, 2015 #7


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    This article explains the information on the Pioneer plaque:

  9. Nov 16, 2015 #8


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    It is assumed aliens would be familiar with pulsar spin down rates and can use the pulsar periods given on the pioneer plaque to estimate the time of launch from earth
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