I was just watching a lecture from Carl Sagan. He talked about figuring out the distance to the stars; it got me interested in learning more about the subject.

As far as I know, the Inverse square law and parallax can be used. Can anyone expand on these? Specifically with regards to what I could do to measure the distance from Earth to Proxima Centauri.

  • $\begingroup$ In order for you to use the inverse square law you must know the distance first (unless you use what's known as a standard candle). $\endgroup$
    – astromax
    Nov 13, 2013 at 12:56
  • $\begingroup$ For Proxima Centauri, just use parallax. Record the position of Proxima Centauri (against the "fixed" stars further away from it) 6 months apart, and use the angular distance and the diameter of the Earth's orbit (about 186 million miles) to find the distance. $\endgroup$
    – user21
    Nov 17, 2013 at 7:01
  • $\begingroup$ As I have outlined in the comments below, the accepted answer here is barely relevant to standard techniques of determining distance to stars in astronomy. Relevant info can be found instead, e.g. in this reference: en.wikipedia.org/wiki/Spectroscopic_parallax $\endgroup$ Nov 21, 2013 at 14:23
  • $\begingroup$ @barrycarter It's almost that simple, but not quite - see below. $\endgroup$
    – ProfRob
    Apr 5, 2015 at 19:43

3 Answers 3


The currently accepted answer is not relevant for finding the distance to a star like Proxima Centauri.

Here's how parallax works. You measure the position of a star in a field of stars that are (presumably) much further way. You do this twice, separated by 6 months. You then calculate the angle that the star has moved against its background stars. This angle forms part of a large triangle, with a base that is equal to the diameter of the Earth's orbit around the Sun. Trigonometry then tells you what the distance is as a multiple of the distance from the Earth to the Sun. [In practice you perform many measurements with any separation in time and combine them all.]

The "parallax angle" is actually half this angular displacement, and a star is said to be 1 parsec away if the parallax angle is 1 second of arc. So 1pc is 1 AU/$\tan (\theta) = 3.08\times10^{16}$ m. The larger the parallax, the closer the star.

The Gaia satellite is currently mapping the entire sky and will estimate tiny parallaxes with precisions of $10^{-5}$ to $10^{-4}$ arcseconds (depending on target brightness) for about a billion stars.

Parallax - as illustrated at http://www.bbc.co.uk/schools/gcsebitesize/science/21c/earth_universe/earth_stars_galaxiesrev4.shtml

Parallax, taken from the BBC "bitesize website"

Now in reality, it is a bit more difficult than this because stars also have a "proper motion" across the sky due to their motion in our Galaxy relative to the Sun. This means you have to do more than two measurements to separate out this component of motion on the sky. In the case of Proxima Centauri the motion against the background stars due to proper motion is larger than the parallax. But the two components can clearly be seen and separated (see below). It is (half) the amplitude of the curved motion in the picture below that corresponds to the parallax. The proper motion is just the constant linear trend with respect to the background stars.

HST images of the path of Proxima Centauri against background stars. The green curve shows the measured and predicted path of the star against the background field over the next few years.

HST images of Proxima Centauri

Parallax measurements work best for nearby stars, because the parallax angle is larger. For more distant stars or those without a parallax measurement, there are a battery of techniques. For isolated stars, the most common is to attempt to establish what type of star it is, either from its colour(s) or preferably from a spectrum that can reveal its temperature and gravity. From this one can estimate what the absolute luminosity of the object is and then from its observed brightness one can calculate the distance. This is known as a photometric parallax or spectroscopic parallax.


One way to find distance to a collection of stars is to hope for an RRLyrae in the bunch. Since RRLyrae are standard candles, you can use the inverse square law to extract the distance.


  • 6
    $\begingroup$ What do you do if there is no RRLyrae? $\endgroup$
    – axwcode
    Nov 13, 2013 at 20:08
  • 1
    $\begingroup$ What do you do when you don't have RRLyrae and you're beyond the distance parallax can be used? Hope for some other type of variable star or supernovae in order to use as a standard candle, I'd say. Beyond that I'm not entirely sure. Anything too local will not be expanding with the universe in a predictable enough way in order to relate its redshift to distance. All stars we hope to see are unfortunately local (in our own Milky Way galaxy; save for supernovae). $\endgroup$
    – astromax
    Nov 13, 2013 at 21:17
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    $\begingroup$ Hmm - not sure why a correct answer was down-voted. You could have simply said that there are more common techniques. What would have been even better is to come up with an answer of your own. $\endgroup$
    – astromax
    Nov 21, 2013 at 3:49
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    $\begingroup$ @astromax, sorry for downvoting your answer, I don't mean any bad. However, I stress it, it is not a correct answer to the question, and is almost irrelevant. Standard technique is as I outlined before, and parallax comes as a second most common method. What you are talking about here is more suitable for determining the distances to galaxies and stellar clusters. $\endgroup$ Nov 21, 2013 at 14:20
  • 1
    $\begingroup$ I personally reserve downvoting for incorrect answers - not relevant answers which are incomplete or not necessarily the best answer. $\endgroup$
    – astromax
    Nov 21, 2013 at 15:07

For close objects, the parallax method works perfectly. Though for higher distances, the Standard candles, as mentioned before, are used. The brightness of RR Lyrae, Supernovae type Ia, could be calculated, therefore, with the amount of light we get from these objects, we can estimate the distance. For even farther objects, the redshift method is used to calculate the distance, where a given line transition with a given frequency (iron emission for instance) is measured, and the shift in frequency, caused by the expansion of the universe (a phenomenon described mathematically) gives us a hint for the distance of the object.


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