A nebula is a cloud of gas and dust in space. It is the birthplace of stars.

There are four main types of nebulae: emission nebulae, reflection nebulae, dark nebulae, and planetary nebulae.

An emission nebula is a nebula that emits its own light. This light is caused by the ionization of the gas in the nebula.

A reflection nebula is a nebula that reflects light from nearby stars. The light from the stars is scattered by the dust in the nebula, which causes the nebula to appear to glow.

A dark nebula is a nebula that blocks light from background stars. This is because the dust in the nebula is so thick that it absorbs the light from the stars.

A planetary nebula is a nebula that is formed from the remnants of a star. When a star dies, it sheds its outer layers of gas and dust. This material forms a planetary nebula.

The ultimate fate of the universe is still unknown, but scientists believe that the universe will eventually stop expanding and begin to contract. This is because the universe is filled with dark energy, which is a mysterious force that is causing the universe to expand at an accelerating rate. However, dark energy is also causing the universe to become more and more empty, which will eventually cause the expansion of the universe to slow down and eventually stop.

The Big Bang theory is the prevailing cosmological model for the universe's origin and evolution. It states that the universe began as a singularity, a point of infinite density and temperature, about 13.8 billion years ago. This singularity then expanded and cooled, forming the universe as we know it today.

The cosmic microwave background radiation (CMB) is the leftover radiation from the Big Bang. It is a faint glow of light that fills the entire universe. The CMB is the strongest evidence for the Big Bang theory.

Dark matter is a type of matter that does not emit or reflect light. It is thought to make up about 27% of the universe. Dark matter is invisible to telescopes, but its presence can be inferred from its gravitational effects on visible matter.

Dark energy is a type of energy that is causing the universe to expand at an accelerating rate. It is thought to make up about 68% of the universe. Dark energy is a mystery to scientists, and its nature is one of the biggest unsolved problems in physics.

The Sun is a main-sequence star, which means that it is fusing hydrogen into helium in its core. When the Sun runs out of hydrogen, it will become a red giant. The Sun will then shed its outer layers of gas and dust, forming a planetary nebula. The core of the Sun will then collapse to form a white dwarf.

Stars that are more massive than the Sun will eventually become black holes. When a massive star runs out of fuel, it will collapse under its own gravity. This collapse will create a supernova, which is a powerful explosion that can be seen from across the universe. The core of the star will then collapse to form a black hole.

Stars that are less massive than the Sun will eventually become white dwarfs. When a low-mass star runs out of fuel, it will collapse under its own gravity. This collapse will create a planetary nebula, which is a beautiful shell of gas and dust. The core of the star will then collapse to form a white dwarf.