The mass of an object directly affects its gravitational pull. The more massive an object, the stronger its gravitational pull.
Stars, Planets, and Moons
Stars: Massive, luminous balls of plasma held together by their own gravity. They generate energy through nuclear fusion in their cores.
Planets: Celestial bodies that orbit a star. They are smaller than stars and do not produce their own light.
Moons: Natural satellites that orbit a planet. They are generally smaller than planets.
Definitions
(Definitions of stars, planets, and moons are provided above.)
Fusion and Gravity in the Sun
The Sun's energy is produced by nuclear fusion in its core. In this process, hydrogen nuclei (protons) fuse to form helium, releasing a tremendous amount of energy.
The Sun's gravity pulls all its mass inward, while the outward explosive force of fusion counteracts this inward pull.
The balance between gravity and fusion creates a stable state in the Sun.
Plasma
Plasma is a state of matter in which atoms are ionized, resulting in a soup of freely moving charged particles (ions and electrons).
The Sun's core consists of plasma due to the extreme temperature and pressure.
Star Formation
Stars form within giant clouds of gas and dust called nebulae.
Gravity pulls the gas and dust into a swirling vortex.
As the material is compressed, the temperature increases.
The cloud forms a giant spinning disc.
Gravity crushes the gas at the center of the disc into a super-dense, super-hot ball, forming a protostar.
Jets of gas burst out from the center of the protostar.
Life Cycle of Small Stars (Less than 8 times the mass of the Sun)
Stable Star:
Hydrogen fusion occurs in the core, generating energy.
Red Giant:
Hydrogen fuel in the core is exhausted, and hydrogen fusion stops.
Gravity pulls the helium inwards, causing the core to become denser and hotter.
Helium fusion begins, forming carbon and oxygen.
The outer layers of the star expand.
Planetary Nebula:
Helium fusion ceases.
The outer layers of the star drift away, forming a planetary nebula.
White Dwarf:
The remaining core, composed of carbon and oxygen, slowly cools and becomes a white dwarf.
Life Cycle of Large Stars
Small Star: A star with less than 8 times the mass of the Sun is a small star.
Large Star: A star with more than 8 times the mass of the Sun is a large star.
Stars with less than 8 times the mass like the Sun expand into a red giants when their hydrogen runs low. The outer hydrogen layers drift away, forming a planetary nebula. The remaining core becomes a white dwarf, which cools slowly over time.
Stars 8 to 20 times more massive than the Sun expand into red supergiants which eject most of their mass in catastrophic explosions, known as supernovas that leave behind neutron stars.
Stars more than 20 times more massive than the Sun expand into red supergiants which eject most of their mass in catastrophic explosions, known as supernovas that leave behind black holes.
Evidence for an Expanding Universe
Redshift: The light from nearly all stars is redshifted, meaning the wavelengths are longer than expected.
Redshift indicates that the stars are moving away from us.
The further away a star or galaxy is, the more its light is redshifted, indicating it's moving away faster.
This provides evidence that the universe is expanding.
Redshift Explained
Redshift occurs when the atoms that produced the light are moving away from the observer.
Elements emit the same patterns of light regardless of their location in the universe.
Scientists can detect the pattern of hydrogen in the light from distant stars and galaxies, but the wavelengths are stretched (redshifted).
The amount of redshift is proportional to the distance and velocity of the object.
Big Bang Theory
The Big Bang theory states that the universe originated from an extremely hot, dense state approximately 13.7 billion years ago.
If the universe is expanding, then in the past, everything must have been closer together.
Extrapolating back in time, everything in the observable universe was contained in a single point.
The Big Bang refers to the rapid expansion of space from this single point.
Expansion Rates and the Nature of Space
The Big Bang represents the beginning of the universe, including space itself.
In the first second after the Big Bang, space underwent a period of extremely rapid expansion called inflation.
Subatomic particles formed, and eventually joined together to form the first atoms around 370,000 years later.
Millions of years later the first stars formed, when hydrogen gas was pulled into giant balls by gravity.
The universe has continued to expand since the Big Bang.