Earth Science Practice Test: Stars and the Universe

In the core, extreme temperature and pressure force hydrogen nuclei together to form helium, releasing energy. This is nuclear fusion, not chemical burning or fission.

Hotter objects give off more light at shorter wavelengths, so the hottest stars look blue or blue-white. The Sun, a yellow star, is in the middle, and red stars are the coolest.

A light-year is the distance light travels in one year, about 9.5 trillion km. Despite the word year, it is a unit of distance used for the huge gaps between stars.

The Sun does not have enough mass to explode as a supernova. When its core runs out of hydrogen, it will expand into a red giant, shed its outer layers and leave a hot, dense core called a white dwarf.

Very massive stars burn fuel quickly and end in a supernova explosion. The collapsed core becomes a neutron star, or a black hole if enough mass remains. Low-mass stars like the Sun end as white dwarfs.

Stars spend most of their lives fusing hydrogen in their cores, and stars in that stage form the main sequence band. The Sun is a main-sequence star. Giants and white dwarfs are later stages.

A redshift means the light source is moving away. Since distant galaxies are moving away and the farthest move fastest, space itself is expanding, which is key evidence for the Big Bang theory.

The Milky Way has spiral arms around a central bar of stars, making it a barred spiral galaxy. Our solar system lies in one of its arms, far from the center.

Stars form when parts of a nebula, a cloud of mostly hydrogen gas and dust, collapse under their own gravity and heat up until fusion begins. A group of galaxies is a galaxy cluster.

Apparent brightness follows the inverse-square law: doubling the distance spreads the light over 2² = 4 times the area, so the star looks 1/4 as bright. One-half would ignore the square.