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How do they figure out how far away a star is?

  1. Jan 2, 2004 #1
    Ok, let's say you see a bright light in the night sky. How far away is it?

    Well, it's bright; so it could either be so bright because:
    1) it's so close to Earth
    2) it's simply very bright

    If a star is dim, how do we know if that is because it is far away or if it's simply dim by its nature.

    What I'm basically asking is: How do we figure out how far away a star is?
  2. jcsd
  3. Jan 2, 2004 #2


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    There is the mass-luminosity relationship. The more massive a star the more luminous it is. Also, luminousity is related to the spectral type of the star. By examining the spectrum of the star you can determine its luminosity, and from that and the visual magnitude of the star you can get a fair estimate of its distance.

    For close stars you can use heliocentric parallax.
  4. Jan 2, 2004 #3


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    many methods

    The most certain method, which can be used for stars within ~3,000 to 10,000 light-years, is parallax*. The most extensive catalogue of accurate parallaxes is from the Hipparcos mission:


    In the main catalogue (Hipparcos), there are accurate parallaxes of ~100,000 stars; in the supplementary catalogue (Tycho) there are somewhat less accurate parallaxes of ~2.5 million stars.

    Hipparcos firmly establised the lowest rung on the distance ladder. Successive rungs - using standard candles such as Cepheids and Type Ia supernovae - take the distance scale out to ~10 billion light-years.

    The dim-bright/close-far problem affected the distance determination of quasars for at least a decade, and GRBs (gamma-ray bursts) until this year; it's still unsolved for 'short' GRBs.

    *basically, if you look at the same object from different positions, it will appear to be in a different direction; the further away it is, the smaller the difference in directions. The longest baseline is the diameter of the Earth's orbit around the Sun; the most accurate measurements of direction difference (Hipparcos) are ~0.5 mas (milli-arcsecond)
  5. Jan 2, 2004 #4

    You may enjoy reading The Cosmological Distance Ladder by Rowan-Robinson. Therein he discusses at least six methods for determining the distance to extrasolar luminous objects.
  6. Jan 3, 2004 #5


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    This is a very good summary.

    Last edited: Jan 3, 2004
  7. Jan 3, 2004 #6
    Question: Wouldn't luminosity be reduced if there is a cloud between us and the star? If so, how do you then judge the type and distance?
  8. Jan 3, 2004 #7
    The GMU link was written by a professor of mine at George Mason University. A quiet gentleman who also taught at UVA.
  9. Jan 3, 2004 #8


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    By UVA you mean University of Virginia?

    the campus designed by Thomas Jefferson and his house Monticello overlooks it so he could watch construction thru a telescope he had in his study

    where is George mason U?

    Maybe you know when and how the first measurement of the distance to a star was made.

    Huygens measured the distance to Sirius by a very bad and strange method and got an answer that was way off. but at least he tried,
    and then some German---I think his name was Bessel?---did it right and the year was around 1836,7,8 dont remember exactly.

    For each conscious species (if any exist besides us) there is a moment in its history when the distance to a star is first measured.

    At that moment the whole species walks thru a door. I dont want to bother to say this more precisely but maybe you know what I mean Loren Booda. For us (evolved fish or monkeys or whatever we are) that door is in December 1838 in Koenigsberg
    and the star is 61 Cygni
    and the distance is 11 lightyears

    Koenigsberg is on the Baltic and is now owned by Russia and
    is called Kaliningrad
  10. Jan 4, 2004 #9
    why? because we are in awe of the breadth and immenseness of our universe?

    i thought what you said was pretty cool, could you explain it a little more, marcus? (ie why, when we first measure the distance of a star, do we "walk through a door" and such?)
  11. Jan 4, 2004 #10


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    my perspective on this is just one person's narrow point of view
    I tend to think of things in terms of the earth getting ready to spread its life to other stars
    it is a very long process (in human lifetimes) and an important step along the way is the first time we realize how far away those potential destinations are

    Timothy Ferris (a good science journalist) has this book called
    "Coming of Age in the Milky Way".
    Maybe this is one of the things you go thru in adolesence---beginning to realize how big the world is and what your place in it is

    this 1838 first accurate measurement of distance to a star came right about the same time as Darwin was figuring out how earth life has evolved and how new species of life arise. The key quotes from Darwin's notebooks are dated around that year, if I remember correctly.

    I think of this as a moment of growing consciousness where we realize more exactly who we are and what our job is.

    But that is just one person's take on it---a way I personally have of seeing events in history.

    My favorite person is Aristarchus who around 250 BC got an estimate (or anyway a crude lowerbound) on how far the sun is, and after that Kepler (because of what he had to go thru to get the Aristarchus sun-centered system to fit the data---it was almost funny how stubborn he had to be to finally get it---in the meantime Copernicus had re-promoted the heliocentric model first glimpsed by Aristarchus, so it was being called Copernican).
    It is like living on the shore of an ocean and seeing islands out along the horizon.

    Eventually, someone asks how far they are.
  12. Jan 4, 2004 #11


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    BTW brum are you comfortable now with the idea of measuring distance to a nearby star by parallax?

    nobody here in this thread explained it, tho maybe it was explained in some of the links given by the other posters.

    The whole thing depends on knowing the distance to the sun!!!
    That was what Aristarchus went after in 250BC, and people were refining in Kepler's time 1600s and then in 1700s.

    If you know the distance to the sun you can parlay that info up a notch and get a handle on the distance to a nearby star.
    In 6 months the nearby star will appear to shift a tiny fraction of a degree relative to the background of more distant stars.

    the tiny angle it shifts by (or half that angle) is called its parallax angle

    most stars are far enough away that they have no detectable parallax,
    only nearby ones have a measurable parallax, and the nearer they are the bigger it is.

    if the parallax angle happened to be a millionth of a radian (which is roughly a 20,000th of a degree) then the distance to the star would be a million times our distance from the sun.

    so it comes down to knowing our distance from the sun and being able to measure fine angles that one star moves by against a background of more distant ones.

    someone else may have covered that already in that thread, ask if not clear on it,

    there is a "ladder" of different methods of parlaying distance info up notch by notch to farther and farther distances, somebody gave a link for this
  13. Jan 4, 2004 #12


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  14. Jan 4, 2004 #13

    University of Virginia. T. J. was an astronomer too, eh?

    George Mason University is in Fairfax, Virginia, outside of Washington DC.

    Koenigsburg is best known to science for its "seven bridges" problem (and Bessel for Bessel functions?) Friedrich Bessel first effectively measured distances by parallax in the 1830's.
  15. Jan 5, 2004 #14


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  16. Jan 6, 2004 #15


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    radar astronomy

    For objects in the inner solar system (and even out to Saturn and Titan), the most accurate method of determining distance is radar. However, except for stuff out to about the Moon, we need to know roughly where the object is first before we can use radar to determine its distance more accurately.

    Radar astronomy has its limitations; e.g. as the received (return) radar signal falls off at ~r4, smaller objects become undetectable faster than they do through (optical) telescopes.

    For the Moon, there's laser ranging, using the retroreflectors left there by the Apollo astronauts. This can give distances an OOM or two more accurate than radar.

    Does anyone know if Uranus has been 'detected' with Earth-based radar?
  17. Jan 6, 2004 #16
    Never mind, I just figured it out.

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