Answer:
<em>1. The proportion of the universe's galaxies that can be seen from Earth is </em><em>approximately 10 per cent</em><em> (option a).</em>
<em>2. The model of galaxy formation in which sections of gas clouds collapse into small pieces which repeatedly combine into larger pieces under the influence of gravity is called the </em><em><u>SZ</u></em><em> </em><em>model or the </em><em><u>bottom-up</u></em><em> </em><em>model.</em>
<em>3. Most stars are not destroyed in a galaxy collision because </em><em>galaxies move too slowly for that kind of damage</em><em> (option c).
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<em>4. It is estimated that the galaxies in the universe have evolved since about </em><em>13.8 billion years ago, just after the universe first formed </em><em>(option b).</em>
Explanation:
1. What we can see of the universe is due to distant stellar objects, whose light has reached us. Current estimates indicate that there may be as many as 2 billion galaxies in the entire universe, many of which are not visible.
Using the "Hubble" space telescope would only allow the observation of 200 billion galaxies and study their composition, which represents about 10 per cent of existing galaxies.
In the future, through the use of more powerful and higher resolution telescopes, it would be possible to observe galaxies that cannot be studied so far.
2. Leonard Searle and Robert Zinn proposed, in 1978, a model for the formation of galaxies, in which small units -globular clusters- associated with each other to progressively form larger components.
Globular clusters are the product of the collapse that occurred before galaxies began to form, and are capable of uniting under the effect of gravity to form larger clusters and thus form the galaxy. The <u>name "bottom up" is due to the fact that small elements form larger ones</u>. Likewise, <u>the name "SZ" results from the initials of the scientists who proposed it</u>.
3. Galaxy collisions are a form of gravity-dependent interaction between galaxies, with one having an effect on the other.
The result of the collision of two galaxies, in general, is the fusion of both, where the larger one is capable of absorbing the smaller one .without this involving the destruction of stars- since the movement of the colliding galaxies is very slow.
4. The formation of galaxies has a calculated time of about 13.8 trillion years, about 500 or 600 million years after the Big Bang.
It is believed that the origin of galaxies reside in the gravitational instability induced primitive fluctuations, leading to the formation of dense units from the scattered gases. This condensation of gases is called clusters or superclusters, and galaxies may have formed from them.
