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1/ How do scientists determine the elements present in different stars?

KEY WORD for the main idea: Analysis of the starlight.

Scientists can identify what elements present in different stars by analysing the starlight.

Note: The "starlight" = spectral lines. Astronomers can can identify what kinds of stuff are in stars from the lines they find in star's spectrum. From spectral lines astronomers can determine not only the element, but the temperature and density of that element in the star.

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2/ What happens to a star when the fuel for its nuclear fusion reactions is exhausted?

KEY WORD for the main idea: generate light from energy

A star generates energy through nuclear fusion; this energy is released a light and heat.

Note: The energy source for all stars is nuclear fusion. Stars spend most of their lives repetitively compressing two hydrogen atoms into a single helium atom - plus a lot of energy, which is released as light and heat. A normal star that has exhausted the nuclear fuel in its interior, its outer layers have expanded, and it has become a red giant.

3/ Why aren't stars and galaxies spread out evenly across the universe?

KEY WORD for the main idea: Gravitational attraction

Stars and Galaxies aren't evenly distributed uniformly throughout the universe, because of the existance of gravity.

Note: Gravity, as one of the fundamental forces of the universe, is the powerful force that glues our universe together. This is the one that causes every object in the universe to attract every other object. One of the most noticeable effects of gravity in the solar system is the orbit of the planets.

Explanation:

Hope this answer can help you. Have a nice day !