- 1695 - Measuring Astronomical Distances. This review illustrates different methods to measure distances back to the Big Bang.
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--------------------------- 1695 - Measuring Astronomical Distances
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- For nearly 25 years the Hubble Space telescope has been staring at the heavens. Staring is the right word. Hubble is looking at the same spot in the sky for up to 140 orbits. Each orbit takes 90 minutes. The long time exposure is used to see as far back in the Universe as we possibly can.
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- The latest Hubble images are using gravitational-lensing, the light bending properties of gravity from 6 remote clusters of galaxies. The goal is to study even more remote galaxies that are in the same line of sight. These far off galaxies would be too faint to observe without magnification and brightening created by the foreground clusters’ gravity.
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- What astronomers hope to learn from these deep field studies: When did the lights of the Universe first turn on? How many galaxies formed in the first few million years after the Big Bang? How quickly did they form?
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- So far astronomers have determined that the number of galaxies drops off dramatically beyond a Redshift of 8.5, corresponding to 600 million years after the Big Bang. The most distant galaxy found to date had a Redshift of 9.8, a time only 490 million years after it all started.
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- The enormous mass of a galaxy cluster bends the light, analogous to a magnifying lens. If Earth happens to be in the line of sight astronomers can image distant galaxies that would other wise be undetectable.
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- One way to learn the distances to these remote galaxies is to use the light spectrum. By measuring how much the light spectrum has shifted towards the red end of the spectrum astronomers an calculate how much the space has stretched in expansion during its long journey to Earth. Knowing the Redshift and Hubble’s constant for space expansion we can determine the distance the light has traveled:
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---------------------- Redshift = (wavelength observed - wavelength actual) / actual wavelength.
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-------------------- Distance = receding velocity / Hubble’s Constant
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------------------ Hubble’s Constant = 71 kilometers / second/ mega parsec
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------------------- Hubble’s Constant = 47,000 miles per hour / million lightyears
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--------------- Redshift ------------------ Travel Time ---------- Time after Big Bang
--------------- ------------------ lightyears ---------- million years
------------------ 1 ------------------- 7.7 ---------------- 5,930 -------------
------------------ 5 ------------------- 12.5 ---------------- 1,190 -------------
------------------ 8 ------------------- 13.0 ---------------- 650 -------------
------------------ 10 ------------------- 13.2 ---------------- 480 -------------
------------------ 15 ------------------- 13.4 ---------------- 270 -------------
------------------ 20 ------------------- 13.5 ---------------- 180 -------------
------------------ 50 ------------------- 13.6 ---------------- 47 -------------
------------------ 100 ------------------ 13.65 ---------------- 17 -------------
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- Gravitational lensing can magnify feeble radiation from tenfold to hundredfold.
The first galaxies found at a Redshift of 9.8 were a mere 500 lightyears across. Which is 500 times smaller than our Milky Way Galaxy. The belief is that these small galaxies merged in the early Universe to become the large galaxies we see today.
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- Where but in astronomy can you witness the past. We are getting very close to the Beginning. Electromagnetic waves may not be the detection method used to see the farthest in the past. Gravity waves, or neutrinos may be the detection medium used to accomplish this.
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- Or, we may use the boy scout method to measure astronomical distances. The further something is away from us the smaller it appears. A popular visual is to look down the distance of a straight railroad tracks. The distance between the tracks appears to get smaller and smaller.
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- One Example: Galaxy NGC415, called the “ Eye of Sauron” hosts a massive Blackhole at its center. The radiation for the accretion disk orbiting the Blackhole creates shockwaves that form hot dust rings around the Blackhole. The hot dust rings emit infrared radiation.
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- To measure the physical size of this dust ring astronomers measure the time delay from the emission of a flash of radiation from the accretion disk close to the Blackhole to the more distant ring of infrared emissions. The distance or physical size of the radius of the ring is the time delay divided by the speed of light.
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--------------------- Physical Size / angular size = 2*pi* Radius / 360 degrees
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- Using an interferometer of several telescopes achieved the resolving power of an 85 meter telescope. This magnification allowed the measurement of the angular size of the diameter of the ring.
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- The distance from the telescope to the galaxy is the radius of another bigger circle., having the circumference = 2 *pi*R
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- To illustrate this math we will measure the distance to the moon. We know the angular size of the Full Moon in the sky is about 0.5 degrees. The physical diameter of the Moon is 3,476 kilometers ( 2,160 miles).
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---------------- 3,476 kilometers / 0.524 = 2 * pi* R / 360
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--------------- R = 380,000 kilometers
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---------------- R = 236,000 miles
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- The distance calculated for the Galaxy was 19 mega parsecs or 62 million lightyears.
But, that is from Earth to the Galaxy. Using the Redshift method we were measuring back 13,000 million lightyears. A distance 200 times further than our boy scout method.
Stay tuned, to see what distances the gravity wave and neutrino methods will produce.
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RSVP, with comments, suggestions, corrections. Index of reviews available ---
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