The Universe of Galaxies
A galaxy is a gravitationally bound system of stars, stellar remnants, interstellar gas, dust, and dark matter.
Types of Galaxies
Spiral Galaxies: Characterized by a central bulge, a flat rotating disk, and spiral arms. The arms are sites of active, new star formation, making them appear blue. Our Milky Way is a spiral galaxy.
Elliptical Galaxies: Smooth, featureless, ellipsoidal (egg-shaped) collections of stars. They contain mostly old, red stars and have very little gas and dust, so there is little new star formation. They are thought to be formed by the mergers of other galaxies.
Irregular Galaxies: Have no distinct or regular shape. They are often chaotic in appearance and are typically sites of intense star formation.
Our Home: The Milky Way
It is a barred spiral galaxy.
It contains 100-400 billion stars.
Our solar system is located in a minor spiral arm (the Orion Arm), about 27,000 light-years from the galactic center.
At the center is a supermassive black hole named Sagittarius A*.
Cosmology: The Study of the Universe
Cosmology is the study of the origin, evolution, and eventual fate of the universe.
The Expanding Universe
In the 1920s, Edwin Hubble made a landmark discovery. He observed that almost all galaxies are moving away from us, and the farther away a galaxy is, the faster it is moving away.
This relationship is known as Hubble's Law (v = H₀d), where v is the recessional velocity, d is the distance, and H₀ is the Hubble constant.
This is the primary evidence that the universe is expanding. It's not that galaxies are flying through space, but that space itself is stretching, carrying the galaxies along with it.
The Big Bang Theory
This is the leading scientific theory for how the universe began. It states that the universe began as an extremely hot, dense point that has been expanding and cooling ever since.
Key Evidence:
1.The expanding universe (Hubble's Law): If the universe is expanding now, it must have been smaller and denser in the past.
2.The Cosmic Microwave Background (CMB): This is the faint, uniform thermal radiation (a 'glow') that fills the entire universe. It is the leftover heat from the Big Bang, a 'fossil' of the early universe from about 380,000 years after the initial expansion.
3.Abundance of light elements: The theory accurately predicts the observed abundances of hydrogen, helium, and lithium in the universe, which were forged in the first few minutes after the Big Bang.