Astronomy - Measure distance to other galaxies

  1. I have understand that the Parallax method can be used to measure the distance to stars, but what happens when the parallax angle gets to small to be measures accurate?

    What kind of methods are used to measure distance to star that are like 5000 light years away, or to Superclusters that are maybe 1 000 000 000 light years away?
     
  2. jcsd
  3. Drakkith

    Staff: Mentor

    For close by objects, such as the Andromeda Galaxy, we can see a type of star called a Cepheid variable. These have variations in brightness that occur in cycles. These cycles vary themselves, however it was found that the brightness of the star was directly related to the time between cycles. Meaning that the dimmer stars are on a different cycle than the brighter stars. By measuring how bright these stars are and their cycles, we know about how far away they are.

    Supernova are another way we can judge distance. A type 1a supernova is the result of a White Dwarf star accumulating matter on it's surface until the extra mass results in the collapse of the star in a supernova. Type 1a supernovas are all very similar, as they all result from an almost identical set of circumstances, so again we can judge distance based on the light emitted from the supernova.
     
  4. ...and using this presumedly accurate method to gauge extremely distant galaxies comes back with measurements showing that the expansion of the universe is accelerating. Uh oh, better come up with something to sell this to people with. Let's call it...[cue ominous music] "Dark Energy!"
    Lol
     
  5. Drakkith gave an excellent answer for moderate distances.

    At larger distances (i.e. larger than millions of light-years) cosmological effects (e.g. Hubble's Law) are essentially the only method of determining distance. Because the universe is expanding everywhere, the farther away something is---the faster its moving away (this applies to the average motion at very large scales). Thus if you measure how fast something is moving away (via the Doppler Effect) you can estimate how far away it is.
     
  6. If you're bemused by such lofty concepts, feel free to ask questions to clarify your understanding.
     
  7. DaveC426913

    DaveC426913 16,439
    Gold Member

    What laypeople often fail to realize is that no one method is used to calculate distances. Many methods, built on many decades of research cause us to arrive at these numbers. One method (say, parallax) will provide an upper and lower bound to distances and/or a margin for error. Another independent method (say, Cepheids) provide another set of bounds. The two methods combine, resulting in an intersection of limits and of margins of error, the outcome being much smaller than any one method alone.

    Do this with multiple methods, with a large data set, and do it with objects very close, where the margins of error are small, and you can generate some pretty reliable techniques.


    Do not judge unless you know what you're talking about.
     
  8. Here are the two major papers on the topic

    http://adsabs.harvard.edu/abs/1998AJ....116.1009R

    http://adsabs.harvard.edu/abs/2004ApJ...607..665R

    They are pretty good examples of scientific reasoning. They do a pretty good job of going through all of the possible observational errors and then explaining why none of them will work. At this point dark energy is weirder, but any of the other explanations are even weirder.

    Also people are looking for other ways of measuring extragalactic distances. The wikipedia article lists seven.
     
  9. The other thing is that people use different methods with different physics to calculate the same number. If you end up with roughly the same number, you get a nice warm feeling that you may have something close to the real number.

    It's also the situation that people are always trying to think of new things.
     
  10. Drakkith

    Staff: Mentor

    Doreen do you speak english as a first language? It is very difficult to understand you.
     
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