Physical Science 8th grade (1).pdf

Figure 179 hubble discovered that the redshift speed

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Figure 17.9: Hubble discovered that the redshift (speed) of galaxies increased proportionally to their distance. The expanding universe Sirius is moving away from Earth In the 1890s, astronomers began to use spectroscopy to study the stars and other objects in space. One of the first stars they studied, Sirius, had spectral lines in the same pattern as the spectrum for hydrogen. However, these lines did not have the exact same measurements as those for hydrogen. Instead, they were shifted toward the red end of the visible spectrum. This was a puzzle at first, until scientists realized that a red-shifted spectrum meant Sirius was moving away from Earth. They could even determine how fast Sirius was moving away by measuring the amount that the lines had shifted toward red (Figure 17.8). Redshift Redshift is caused by relative motion that increases the distance from the source to the observer. The faster the source of light is moving away from the observer, the greater the redshift. The opposite (blueshift) happens when an object is moving toward the observer. A star moving toward Earth would show a spectrum for hydrogen that was shifted toward the blue end of the scale. Discovery of the expanding universe In the late 1920s, Hubble began to measure the distance and redshift of galaxies. Much to his surprise, he discovered that the farther away a galaxy was, the faster it was moving away from Earth. By the early 1930s, he had enough evidence to prove that galaxies were moving away from each other with a speed proportional to the distance between them. This concept came to be known as the expanding universe . The Big Bang theory The expanding universe was a great surprise to scientists. Before Hubble’s discovery, people believed the universe had existed in its same form for all time. The fact that the universe was expanding implies the universe must have been smaller in the past than it is today. In fact, it implies that the universe must have had a beginning . Astronomers today believe the universe exploded outward from a single point smaller than an atom into the vast expanse of galaxies and space we see today. This idea is known as the Big Bang theory.
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361 17.1 G ALAXIES C HAPTER 17: G ALAXIES AND THE U NIVERSE The Big Bang theory The Big Bang theory The Big Bang theory says the universe began as a huge explosion between 10 billion and 20 billion years ago. According to this theory, all matter and energy started in a space smaller than the nucleus of an atom. Suddenly, (no one knows why) a huge explosion occurred that sent everything that makes up the universe spraying out in all directions. In its first moments of existence, the universe was an extremely hot fireball of pure energy that began to expand rapidly. Protons and neutrons form at 4 minutes As the universe expanded, it cooled down as its energy spread out over a larger volume. About four minutes after the Big Bang, the universe had cooled enough that protons and neutrons could stick together to form the nuclei of atoms. Because atoms were still flying around with high energy, heavy nuclei were smashed apart. Only one helium atom
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