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B Does Modern Astronomy have a problem?

  1. Oct 24, 2017 #1
    Modern astronomers use parallax to determine the distance to a star. Parallax is based on the stars of the stellar universe that are stationary where the change in the angular position of a stationary star is measured after the observed on the earth propagates the distance of the earth's orbital diameter (six months) but the distance to a 4.22 light year (4 x 10^16 meters) star is more than 10^7 times larger than the earth's orbital diameter (1.4 x 10^10 m); consequently, the parallax reference distance of the earth's orbital distance is to short of a distance to produce a change in the angular position of a stationary 4.22 ly star using the Hubble that has a resolution of .1 arcsec.
     
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  3. Oct 24, 2017 #2

    jbriggs444

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  4. Oct 24, 2017 #3

    jim mcnamara

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    Lookup Cepheid variable stars and read. Then come back and tell us how those stars are used to find distances between galaxies. Or follow @jbriggs444 nice, useful link.

    The point is there are multiple methods for getting astronomical distances, not just parallax. So do you see why your question has a problem with scope of the issue? And you might also see why we want to see discussion based on existing Science, not someone's conjecture. I can see why you asked the question and it is presented reasonably clearly, but it looks like personal theory at first glance.

    Thanks.
     
  5. Oct 24, 2017 #4

    phyzguy

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    As others have pointed out, parallax is only one means of measuring distance. But even with parallax, astronomers can do much better than you have posted. By averaging many observations, the error can be much reduced. The Gaia satellite is able to measure stellar positions to about 10-20 micro-arc seconds, or on the order of 0.000001 seconds. This is sufficient to measure the parallax of most stars in our galaxy.
     
  6. Oct 24, 2017 #5

    Drakkith

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    Note that the resolution of a telescope (such as the HST) usually refers to its ability to distinguish between two objects (or the difference in position of a single object over time) in terms of their angular separation. In other words, if the angular separation of the two objects is smaller than the resolution of the telescope, they will appear to be a single object. The Rayleigh Criterion is one way to state the resolution of a telescope. But we can usually distinguish between objects even more closely spaced. How closely spaced the objects can be before we can no longer distinguish between them depends on a number of factors, but, in general, there is no hard cutoff. The more time and resources we have, the better we can distinguish between them. Thus the stated resolution of 0.1 arcseconds for the HST is not a hard limit. It depends on the wavelengths used, the amount of time we can devote to imaging the target, the density of background/foreground objects, and many other factors.

    Critically, and my main point, is that we can drastically increase our ability to determine the position of an object between successive points in time by simply increasing the amount of time we image that object. This is because we can use various image processing techniques (some of which are quite standard even in normal imaging) to determine the likely position of an object even to sub-pixel levels. In other words, a star can move less than 1 pixel in any direction and we can tell that it has moved just by measuring the small change in the distribution of light on all of the pixels falling under the image of the star. However, the more accurate we want to get, the more time it takes.
     
  7. Oct 29, 2017 #6

    ohwilleke

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    To answer the thread title, rather than just the initial post, Modern Astronomy is in a golden age.

    Never in the history of astronomy have we been able to observe such a large portion of the EM spectrum, plus gravitational waves, plus neutrino flux, plus other "cosmic rays" (which misleadingly consist mostly of very fast moving fermions).

    Never in the history of astronomy have we have such extremely precise measurements. For example, the last round of Planck measurements comes close to the theoretical maximum precision with which the cosmic microwave background can be measured with solar system based observations.

    Never in the history of astronomy have we had so many space telescopes that are free of the distortions of clouds and the atmosphere.

    Never in the history of astronomy have we had so much nearly instantaneous coordination between observatories spread all over the Earth and into Earth orbit, from Antarctica to Hawaii to the Caribbean to every other continent and major island on Earth.

    Never in the history of astronomy have we had so much computational power to analyze massive amounts of astronomy data and to do such powerful simulations of theoretical expectations to compare to the data.

    Never in the history of astronomy have we been able to build on the foundation of direct and precise observation of the laws of Nature at temperatures comparable to the hottest parts of stars in making our models.

    Never in the history of astronomy have astronomers had access to the entire global record from China to the Middle East to the Andes of ancient astronomy observations back to graphic depictions that predate literacy from the Younger Dryas era.

    On arXiv, new experimental results in astronomy leave all other disciplines in the dust in terms of sheer volume of new papers.

    We also have the excellent combination of some very good theory (e.g. the neutron star merger's properties were almost completely described prior to observation with conventional theory which the observations merely affirmed), while simultaneously having big unsolved problems (e.g. the nature of dark matter and dark energy phenomena) that new observations can and will cast meaningful light upon.

    We have immense global cooperation in the field (as demonstrated by the surge in international collaborations, such as the thousand plus investigators from all over the world in the recent neutron star merger observations), while not having any one or two sets of investigators totally control the governance of the discipline.
     
    Last edited: Oct 29, 2017
  8. Oct 29, 2017 #7

    sophiecentaur

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    The introduction to 'resolution' in my Optics Course used the Rayleigh Criterion in a very rigid way. That criterion was very much based on practical, visual observations and one can easily do a lot better. All you are looking for is a finite dip in the brightness pattern between the two images. Resolving two 'point' sources has no limit as long as you can perform the measurement over long enough time - which corresponds to low pass filtering (/integrating out the noise).
    The great thing about the sampling of an astronomical image (with pixels) is that there are no repeating spatial patterns so the risk of aliasing (Nyquist criterion problem for under sampling) is much less and the diffraction pattern of all stars that you are looking at is the same. That's a big help for image processing.
     
    Last edited: Oct 29, 2017
  9. Oct 31, 2017 #8
    Astronomy changes when people learn:

    Distance to nearest stars
    Parallax ellipses over the course of a year can theoretically be seen. It is very difficult to measure parallax to stars for several reasons. The ellipses are very small, and stars have proper motion. Another problem, the "aberration of light", caused by the earth's speed relative to the
    speed of light (about 18.5 miles per second), causes a larger ellipse than the parallax ellipse. The nutation of the earth is also involved. After these effects have been accounted for, parallax gives the best measurement. It is limited to about 150 light years.

    Size of the Milky Way Galaxy
    Certain stars such as Delta Cephei have varying luminosity with a period that is measured in days. These are called "Cepheid variables". The American astronomer Henrietta Swan Leavitt studied these in the Magellanic clouds. She noted that the brighter the cepheid variable, the longer it's period. The American astronomer Harlow Shapley used Cepheids to find the relative distance to globular clusters (from the relative period of the Cepheids, the relative luminosity was assumed). The clusters seemed to be in a spherical arrangement. This was assumed to be the center of the galaxy. He measured the average proper motion (transverse) of Cepheids in each cluster. The radial velocity (toward or away from us) should be the same as the transverse velocity. The radial velocity was determined from the spectral shift of the light (Doppler shift toward the red or violet). With transverse velocity and proper motion known, the distance can be calculated. Shapely came up with 50,000 light-years. He had not accounted for dust in the Galaxy which reddens and reduces the light. The figure was revised downward to 30,000 light-years. Jan Oort determined the general nature of the rotation of the Galaxy, and from that calculated the direction and distance of the center without using globular clusters. He got 30,000 light-years in the same direction as the clusters. Oort's calculations gave an estimate of the sun orbiting the galaxy once every 230 million years, and of 100 billion stars in the Galaxy.

    Diameters of giant stars

    Even the nearest stars seem no more than a point of light in the best Modern telescopes. The German-American physicist Albert Michelson invented a device called a light interferometer. It produces interference patterns in light rays, and made it possible to measure the very small angle between the light coming from one side of a star and the light coming from the other side. From the angle, and the distance of the star, the diameter can be calculated.

    Distance to nearest galaxy
    Sometimes stars suddenly brighten to thousands of times their normal brightness. These are called Novae. They dim down again after a few days to a few months. A Nova (S Andromedae) appeared in Andromeda in 1885 (Andromeda was thought to be a nebula at the time). In 1901, a nova was seen in the Milky Way Galaxy (Nova Persei). It's distance was measured by parallax to be 100 light-years. On the assumption that all novae have the same luminosity, the distance to Andromeda was calculated at 1600 light-years. The American astronomer Heber Curtis found and studied many novae in Andromeda. They were all far dimmer than S Andromedae had been. In 1917, a 100 inch (mirror size) telescope was installed on Mt. Wilson. The American astronomer Edwin Hubble used it to look at Andromeda. He could make out individual stars on the outskirts. Eventually, he found Cepheid variables in it, and showed that it was a galaxy. He calculated its distance at 800,000 light-years. By 1950, it was known that this was wrong. The Milky Way appeared to be larger than any other, even though it had the shape of an intermediate sized galaxy. Also, Andromeda had globular clusters like the Milky Way, and they appeared to be smaller. The Cepheid variables were classified into two types, and Baade showed that one type in the Milky Way had been compared to the other type in Andromeda. The distance to Andromeda was revised to 2.3 million light-years. It was decided that S Andromedae was in a different class than most Novae. These are called Supernovae.

    Distance to farther galaxies
    For galaxies too far away to see Cepheids, Hubble made use of any stars he could see, by assuming these were supergiants that were bright like S Doradus. If no stars could be seen, he used the relative brightness of the galaxy as a whole. Clusters of galaxies are compared to farther clusters by the relative brightness of the average of the galaxies in the cluster.
     
  10. Oct 31, 2017 #9

    Andy Resnick

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    Minor correction- a Young interferometer is used for this, not a Michaelson interferometer. It's possible to combine these (I suspect modern interferometric telescopes use a combination to maintain alignment), but the essential measurement is one of spatial coherence (Young), not temporal coherence (Michaelson).
     
  11. Nov 1, 2017 #10

    mfb

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    Gaia can get ~10% precision at a distance of 15000 light years for bright stars. Your number is a factor 100 too low.
    Take a not so near star then: Image of Antares
     
  12. Nov 1, 2017 #11
    Of course modern astronomy has problems, but not of the kind proposed by the OP. Science thrives on problems. If there were no problems there would be little need for scientists of any description. A problem is another word for "here's something we don't yet understand". Let's celebrate all astronomy's current problems. When they are resoved they will doubtless reveal even more wonders. (And new problems.)
     
  13. Nov 1, 2017 #12

    sophiecentaur

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    The general public have a problem, I think. On the one hand they love to knock the Scientists about topics that the Media choose to discuss and yet they are impressed with (and totally reliant on) the successes of Science in their everyday lives. They have a half full cup and a half empty one at the same time.
     
  14. Nov 1, 2017 #13
    My post was from a document I wrote several years ago from books that were written before that. Modern telescopes have improved, but your link was a reconstructed view. It was impressive though.
     
  15. Nov 1, 2017 #14

    mfb

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    Hipparcos had some precise parallax measurements at 1000 light years already - 25 years ago. If you use sources older than 25 years, it is not surprising that they are outdated.
    Everything you don’t see with your own eye is “reconstructed” in some way. If the star would appear as a point-source you couldn’t reconstruct such an image.
     
  16. Nov 5, 2017 #15
    I am in complete agreement about all of that. With the caveat that since everything is so new, mistakes will be made.

    Einstein wanted a static and everlasting universe so he invented the "Cosmological Constant." Hubble calculated the age of the universe at ~2 billion years because he used Cepheid variables as his "Standard Candle." We created "Dark Energy" in the 1990s to explain an accelerating universe because we mistakenly used Type Ia SNe as our "Standard Candle."

    Mistakes are not bad things, if we learn from them.
     
  17. Nov 5, 2017 #16
    Check your premise, you are using the wrong number.
    No Nobel prize, yet :-) .
     
    Last edited: Nov 5, 2017
  18. Nov 5, 2017 #17
    Please avoid feet, furlongs, etc. when discussing astronomy,
    nor any of the other length units listed here and here :smile: .
     
    Last edited: Nov 5, 2017
  19. Nov 5, 2017 #18
    FYI, Google has a Cubit to Parsec converter. :wideeyed:
     
  20. Nov 5, 2017 #19

    ohwilleke

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    The speed of light is 299,792,458 m/s which is 186,000 miles per second, about 10,000 times faster than the number you cite.
     
  21. Nov 5, 2017 #20

    mfb

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    18.5 miles per second is Earth's orbital speed, and while it is 10,000 times slower than the speed of light, it has to be taken into account for parallax measurements.
     
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