Lesson One: Big Bang And Solar System Creation

Explanation
The correct answer is 13.7 billion years ago. This is the estimated age of the universe according to the Big Bang theory, which suggests that the universe began expanding from a hot and dense state approximately 13.7 billion years ago. This estimation is based on various scientific observations and measurements, such as the cosmic microwave background radiation and the redshift of galaxies.

Explanation
Our Sun began to form approximately 4.5 billion years ago. This is based on scientific evidence and the age of the solar system. The formation of the Sun is believed to have occurred through the gravitational collapse of a giant molecular cloud. This process eventually led to the ignition of nuclear fusion in the core of the protostar, marking the birth of our Sun.

Explanation
Hydrogen is the most abundant material in the known Universe. This is because hydrogen is the primary building block of stars and makes up about 75% of the elemental mass in the Universe. It is found in large quantities in stars, gas clouds, and even in interstellar space. Additionally, hydrogen is the main component of water and many organic compounds, making it essential for life as we know it. Therefore, due to its widespread presence and importance in the Universe, hydrogen is considered the most abundant material.

Explanation
Our Sun is located in the Milky Way galaxy, which is a barred spiral galaxy. The Milky Way is home to billions of stars, including our own Sun, as well as other celestial objects such as planets, asteroids, and comets. It is named "Milky Way" due to its appearance as a hazy band of light in the night sky, caused by the combined glow of countless stars and other astronomical objects.

Explanation
There are an estimated 125 billion galaxies in the visible universe. This estimate is based on observations made by telescopes and other instruments that can detect and study distant galaxies. The number of galaxies in the universe is constantly being refined as new observations and data become available, but the current estimate is around 125 billion.