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----------------- 1760 - The Birth of Galaxies?
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- The Hubble Space Telescope can look at the same spot in the sky for one million seconds. The area of the image is only 1 arc-minutes on a side. This would be like looking through a soda straw at the same spot in the dark sky for 12 days running.
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- The image recorded is a time exposure that captures 10,000 faint galaxies. The most distant of these galaxies look more ragged, probably the result of frequent collisions with other galaxies. In the early Universe everything was closer together.
- Looking into distance is looking backwards in time because the light reaching us today left the image millions, even billions of years ago. The telescopes today can see galaxies 12 billion lightyears away. So these images are when those galaxies were only 1 billion years old. They are totally different today although we have no way to see that until the light reaches us a billion years from now. What we are looking at is a much younger universe.
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- When the light waves travel through space they get stretched out to wider wavelengths because space is expanding. The longer the wavelength of light the further distance it has traveled. Longer wavelengths are called the “ Redshift” because the spectrum moves toward the red end of the light spectrum. The larger the Redshift the earlier history we are viewing.
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- Astronomers would like to look backwards in time to see the formation of the very first galaxies. The furthest back in time is viewed using “gravitational lensing“. This is where a cluster of galaxies , with their immense gravity, bend light passing by like light focused in a giant magnifying glass. Some galaxies viewed this way have Redshifts between 8 and 10, corresponding to only 500 million years after the Big Bang, looking back 13.2 billion years in time. At that time the Universe was only 4% of its current age.
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- Galaxies come in all shapes and sizes. Elliptical galaxies ( those oblong, or spherical shaped ) have mainly ancient red stars. Spiral galaxies ( disk shaped) have an abundance of young, short-lived, blue stars. Elliptical galaxies must have transformed gas into stars at an earlier stage. Spiral galaxies must have formed stars continuously at a more moderate pace. In the early Universe galaxies were closer together and galaxy collisions must have occurred frequently.
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- The discovery of Dark Matter requires that most of a galaxy’s mass lies outside its visible form and inside a Dark Matter halo. It is believed that Dark Matter is mainly responsible for gravitationally drawing the gas together to fuel star formation and eventually galaxies.
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- Surveys of galaxies over various time periods have concluded that the greatest star forming period occurred at Redshifts of 2 to 3, about 5 billion years after the Big Bang, (ABB). A Redshift of 1 is 7 billion ABB. A Redshift to 7 is 1 billion ABB. Star formation has been on the decline for the last 8 billion years.
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- Galaxies appear to form new stars until the reach some critical mass threshold. Then, star formation ends and the mature galaxy becomes a red elliptical galaxy. Lower mass galaxies make stars at a much slower rate and never reach this threshold.
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- Why do the larger galaxies stop growing ?
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- The larger the galaxy the larger the blackhole at the galaxy center. Active Blackholes have X-ray emissions and energetic jets that may expel gas out of the galaxy shutting star formation down.
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- Astronomers believe that galaxies first formed when the Universe was only 5% of its current age ( 700,000 ABB ). Some 300,000 years ABB hydrogen gas first formed. Dark Matter clouds collapsed this gas to form the first stars. These first stars contained only hydrogen and helium. These early stars were believed to be massive and short lived. They would have been formed at Redshifts between 20 and 50. Small galaxies would be formed between Redshifts 10 to 20. Small galaxies would be only 1,000 lightyears across. (The Milky Way Galaxy is 120,000 lightyears across.) All this light from these distant galaxies would be Redshifted into the infrared wavelengths when it reaches us.
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- We are closing in on it , but , when and how the first galaxies formed remains unclear.
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- How exactly do galaxies regulate star formation?
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- What exactly causes a galaxy to reach a threshold and stop star formation ? Does ever big galaxy have a massive blackhole at the center?
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- Quasars are the result of active blackholes in distant galaxies. Astronomers have cataloged over 13,000 Quasars. The energy released by a single Quasar is several thousand times greater than the entire energy output of the Milky Way Galaxy. Understanding the part that blackholes play in galaxy formation and star formation in general is a challenge remaining. Stay tuned, these is more to learn.
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