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About the size, brightness and distance of celestial objects

  1. Nov 17, 2014 #1
    Can someone please explain to me in simple words the process of calculation to measure the size and the distance of a star (or a galaxy for that matter)? What are the measured values?

    I have read about what I think could be two "relatively easy" measurable variables such as “apparent size” and brightness. Still, it is not clear to me how to differentiate a very bright and distant star from a dim and near one. Also, I have no idea about how the size on the object can be calculated.
  2. jcsd
  3. Nov 17, 2014 #2


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    Hi tonyxon22,

    There's a number of different ways applicable to different distances. Wikipedia has got a good breakdown of them here:

    Perhaps you can look at that and come back if something needs clarifying.

    Sizes of individual stars are rarely measured directly, and instread are inferred from stellar evolution models.
  4. Nov 17, 2014 #3
    Well that is actually very useful! Thank you!

    I'm not even close to understand it with detail, but at least some of the basic ideas stuck to my mind. I will have to dig in with more attention and see the related articles to try to complete the puzzle.
    Of course I would like clarifications in many things I don't understand. But I'll take my time to read a little more and come back with solid questions.

    For the moment, there is something that I am curious about. When they refer to the "Parallax" there is a comparation of the position of the body against something they call a more distant background. In a related article, it mensions an object (star) at a known distance.

    I wonder if there are actually celestial bodies that can be used as a reference point, and in such cases, how do we get to know the real distances to that objects? For what I understand so far, most of the techniques rely on comparison to other bodies. So if the first reference point is not correct, all the other measures made in relation to that would be also wrong.

    Is the moon a suitable point for comparison, for example? I have a hard time imagining to use the moon as a reference since it is never in the same place in relation to the background (?). I mean, it periodically gets in the same place in relation to the earth, but at that time the earth has moved away. Also, as the distances of apparent movement in the sky that are to be measured are extremely short, so using the Moon seems rather unprecise right away (at least for me, hehe)
  5. Nov 17, 2014 #4


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    See the section "direct measurement" in the article. The first step is the astronomical unit (historically, the size of Earth was used to measure it, now we have direct measurements), then parallax, and nearly everything else is measured relative to this.
  6. Nov 17, 2014 #5


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    As far as the "background stars" go, knowledge of their distance is not necessary. All they need to be, is so far away that they don't seem to move as you switch positions on the baseline. The only distance needed is that of the baseline.
  7. Nov 18, 2014 #6

    Ken G

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    You are getting good answers already, all I'll add is that your difficulty understanding how distant large and bright objects are distinguished from more nearby objects that are less large and less bright is perfectly understandable-- it is one of the greatest difficulties in all of astronomy. It is always a really important distinction and is usually very hard to make, and this problem has been the cause of many an error in the history of understanding our cosmos. Even today we face this problem, when using type Ia supernovae to infer the acceleration of the expansion of the universe. If we have something wrong about how we tell more distant and more intrinsically bright supernovae from closer less bright ones, we could reach wrong conclusions about the expansion of the universe. The best way is to use as many independent determinations as we can think of, and seek consistency, and hope we are not missing something, even as we redouble our efforts to look for something we could be missing.
  8. Nov 18, 2014 #7
    Yes, I also think I have got some very good answers. I thank you all for that. Now it’s a matter of actually digging deeply in the Wikipedia article and I think most of my initial doubts could be cleared out.

    As the matter of fact, regarding that distinction between bright and distant objects against less bright and near ones, I think that using the “Parallax” could be very useful. The farther the object, the smaller the “Parallax” (I keep on putting that word between quotes because I don’t know if it is exactly considered a particular technique rather than just a visual effect). And as this might be pretty obvious for most of you, I had never thought about this simple idea.

    I myself, as a very -very very- amateur astronomer (with just a couple of 20x80 astronomy binoculars) would dare to compare two objects of the same apparent brightness (same apparent brightness according to me, anyway) and try to compare their “Parallax” to kind of scale the relation between their distance and brightness. For that of course I will need to come with a good idea of a comparison point first… but one step at the time hehe…
  9. Nov 18, 2014 #8


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    Unfortunately, you're unlikely to ever measure any stellar parallax. The effect is so tiny, that even for large ground-based telescopes it is swamped by atmospheric blurring. The closest star's parallax is less than 1 arcsecond.
    Typically, atmospheric effects limit the resolution to about that much. Which is, as rule of thumb, the resolution of a 13 cm diametre telescope. Even as you build larger telescopes, the image you see will be limited in its resolution to about 1 arcseconds. You need a really, really clear sky, and/or special optics to get below that. Clear sky is the main reason for building observatories at high altitudes, and why Hubble telescope is so great a tool.

    Have a look at these pages:
    There's some good, comprehensive and amateur-oriented treatment of the subject in there. The first page talks about resolution, the second about the issues with atmosphere (called "seeing").
    The whole site is a great resource, by the way.

    However, you could try using parallax to find distances to some landmarks here on Earth. That's pretty much what triangulation is all about.
  10. Nov 18, 2014 #9
    So for a mortal like me with no access to the Hubble Space Telescope it will be nearly impossible to try to see a Parallax? Hehehe.. that’s what I suspected. That’s the joy of being so innocent and unfamiliar with this subjects… One tends to think that everything is possible with homemade artifacts… Kind of a MacGyver of physics haha..
  11. Nov 18, 2014 #10


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    Finding the central position can be done with a precision much better than the width of the spot in the camera, but it still needs some clever methods to be visible.

    You can see a parallax-like effect at the outer planets: if they are roughly in opposition to the sun, their apparent motion against the stars is dominated by the motion of earth.
  12. Nov 24, 2014 #11
    ping tonyxon22 - The fundamentals of parallax and triangulation do not require Hubble. Once you have established a "ruler" divided into say centimeters, there is no limit to how many centimeters can be stacked to measure distances in meters, kilometers, etc.
    Take a look at how accurate such measurements could be even in 200BC.

  13. Nov 26, 2014 #12
    You can experience parallax yourself. Pick an object a few feet away from you, close one eye, and point at the object. While continuing to point at the object, open the closed eye and shut the other eye. You will see that you are no longer pointing at the object. Parallax is the difference in the apparent position of an object viewed along two different lines of sight.

    When using parallax to determine the distance of stars, you measure the star's location in relationship with the other background stars, then wait six months until the Earth is as far from its original position as possible, and then make similar measurements of the star's location again. The difference between the first observation and the second observation determines the parallax. For example, the binary Alpha Centauri system 4.37 light years away has a parallax of 0.7471 ± 0.00012 arcseconds.

    Parallax is the most accurate means of measuring the distance to a star. However, because the distance between stars is so vast, parallax is only useful for measuring distances of less than 1,000 light years.

    If a star is more than 1,000 light years away, other methods for determining distances are used. Such as Cepheid Variable stars which pulsate at regular intervals. It turns out that there is a relationship between the rate at which a Cepheid Variable star pulsates and the star's absolute magnitude. Once the absolute magnitude of a star has been determined, it is a simple matter of measuring the apparent magnitude of the star and then determining its distance. Using Cepheid Variable stars in this manner the distance of up to almost 3 million light years can be measured.

    However, it should be noted that there are many different types of Cepheid Variable stars and they have to be absolutely certain they know which type of Cepheid Variable they are measuring. Furthermore, the distance being measured is to the Cepheid Variable star, not the object whose distance they are actually trying to measure.

    Once you get beyond ~3 million light years there are only two ways to measure cosmological distances: Type Ia Supernovae, and Red Shift.

    A Type Ia supernovae is the result of a white dwarf in close enough proximity to its companion star that there is a transfer of mass from the companion star to the white dwarf. When the mass of the white dwarf exceeds the Chandrasekhar limit (~2.765 × 1030 kg), it explodes in a supernovae. Since mass is known, the absolute magnitude of the supernovae can be determined (Mv = −19.3).

    Again, there is a caveat. It was recently determined (February 2013) that there is a new type of supernovae, a Type Iax. These new supernovae are exactly the same as Type Ia supernovae, except that they explode before reaching the Chandrasekhar limit, and therefore the absolute magnitude is much less than a Type Ia supernovae. It is estimated that between 18% and 48% of all Type Ia supernovae observations made before February 2013 were misclassified and should actually be Type Iax supernovae. Which means that the distance to the supernovae is actually much closer than was originally thought.

    The least accurate means of determining distance is by using Red Shift. Think of the Doppler Effect, but instead of pertaining to sound it pertains to light. The further away an object is, the faster it is traveling away from us, and that shifts the light we receive from the object toward the infrared end of the electromagnetic spectrum. If a light-emitting object is moving rapidly toward us, then the light we receive from the object will be shifted toward the ultraviolet end of the spectrum, or Blue Shifted.

    Gravitational lensing can significantly effect the light being emitted by a distant galaxy, making it appear closer, or further away, than it is in reality. Furthermore, there is no way to accurately determine the absolute magnitude of an object by using red shift alone.
    Last edited: Nov 26, 2014
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