Gravity pulls a cold cloud of gas and dust together, and its center heats up over millions of years. What begins once that center is hot and dense enough?

A student is putting these in order from smallest to largest: galaxy, solar system, star, universe. Which order is correct?

The stars, gas and dust of a galaxy stay bound together as one system for billions of years instead of drifting apart. What holds them together?

Light from almost every distant galaxy arrives stretched toward longer, redder wavelengths, and the shift is greater for galaxies that are farther away. What does this show?

Detectors find a faint microwave glow coming from every direction in the sky at almost exactly the same temperature. What do astronomers conclude it is?

Astronomers on Earth see almost every distant galaxy moving away from the Milky Way. What would an astronomer living in a distant galaxy see?

According to the Big Bang model, what was the universe like about 13.8 billion years ago?

Surveys of gas across the universe find it is about 75 percent hydrogen and 25 percent helium by mass, with very little of anything else. How does this support the Big Bang model?

As the universe expands, the space between distant galaxies stretches.

Light from nearly every distant galaxy arrives stretched toward longer, redder wavelengths, and the stretch is larger for galaxies that are farther away. Which claim does this pattern best support?

A faint microwave glow reaches Earth from every direction at almost the same intensity, matching radiation from something near 2.7 K, and it is not concentrated along the band of the Milky Way. Which claim does it best support?

The oldest stars astronomers can measure hold almost no elements heavier than helium, yet about a quarter of their mass is helium, the same proportion found across the universe. Which claim does this best support?

Because light takes time to travel, galaxies billions of light-years away are seen as they were long ago. Those distant galaxies are smaller, more irregular and forming stars far faster than nearby ones. Which claim does this best support?

The oldest globular star clusters in the Milky Way are dated at roughly 13 billion years, and no star anywhere has been measured as older than the time since the expansion began. Which claim does this best support?

The "afterglow" of the energy from the origin of the universe that we see today.

Which statement is true?

What does the Big Bang theory suggest?

Which two chemical elements were formed during Big Bang Nucleosynthesis?

What theory proposed that new matter is continuously created to keep the density of the expanding universe constant?

Which uncrewed space probe launched in 1977 became the first human-made object to enter interstellar space?