Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Measuring Distance

  1. Mar 4, 2008 #1

    This may be a dumb question, but I by no means have any background in Astronomy, I'm pretty much regulated to Science Channel, History Channel, etc. I wish I could dive into all the fields on these forums but unfortunately I was not blessed with the gifts of understanding mathematics or physics beyond a basic level.

    My question is how do you measure the distance of stars when it's light reaches us? I understand we can see that some planets are closer to us with depth perception, but what happens when we lose track of that depth perception?

    I guess I'm just wondering how we know if something is 5 million light years from us vs. 7 million light years?

    If I were to flash a light beam at you in pitch black from 10 feet vs. 100 feet, that light I assume would be smaller and dimmer from 100 feet, so you could assume that it was farther. But if I used a brigher flashlight at 100 ft and a much weaker, dimmer flashlight at 10 feet wouldn't it appear to be the same? If all stars vary in brightness (which I assumed is true) how do we standardize distance?

    Thanks for the input.
  2. jcsd
  3. Mar 4, 2008 #2


    User Avatar
    Science Advisor

    This is a very good question. Distance measures are very important in astronomy and cosmology at a whole range of scales. To get you started have a look at the http://en.wikipedia.org/wiki/Distance_ladder" [Broken] article.

    The most fundamental distance measure is Parallax, which can be used on stars relatively close to us in our own galaxy only. This is the first distance measure to understand and pretty much everything else is calibrated in the end from parallax.

    Anything that you don't understand or need more info about just ask away.
    Last edited by a moderator: May 3, 2017
  4. Mar 5, 2008 #3


    User Avatar

    Staff: Mentor

    Good thought. It turns out that stars all follow brightness patterns based on their size and age, which can be determined by their color. This provides a rough ruler for the measurement you suggest. Some very specific types of stars, such as certain types of variable stars and certain types of supernovas have very specific brightness, which makes them very accurate rulers for that type of measurement.

    Over much longer distances, doppler shift is used, under the assumption that the Hubble's theory on expansion is valid (recession speed varies with distance).
  5. Mar 5, 2008 #4


    User Avatar
    Science Advisor
    Gold Member

    Cepheids and SN Ia's are powerful [backed by solid physics] independent confirmations of the Hubble flow. They are not model dependent [cosmologically] as LCDM opponents sometimes suggest. In fact, they are the foundation for the LCDM model.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook