Cosmology Part1

🌌 Evidence for the Big Bang & Universe Expansion

🔭 Hubble–Lemaître Law

• The farther away a galaxy is, the faster it moves away from us.

• This is observed through redshifted spectral lines, showing the universe is expanding.

• Equation:

  v=H0×Rv = H_0 \times Rv=H0​×R

  Where:

  • vvv = speed of recession

  • RRR = distance

  • H0H_0H0​ = Hubble constant (~72 km/s/Mpc)

🌌 Cosmic Microwave Background (CMB) Radiation

• Detected at ~2.725 K, this faint glow fills the universe.

• It’s the cooled remnant of the hot, dense state after the Big Bang.

• COBE satellite confirmed the blackbody spectrum of the CMB (Nobel-winning).

🧪 Primordial Element Abundance

• Universe is mainly hydrogen (H) and helium (He)—matches Big Bang predictions.

• Stars today still reflect these ratios, shown in their absorption spectra.

🔬 Absorption Spectra

• When light from stars passes through cooler gas, dark absorption lines appear.

• These lines correspond to specific elements and help determine:

  • Star composition

  • Star velocity (via redshift or blueshift)

🌠 Star Life Cycle, Classification, and Evolution

💫 Hertzsprung–Russell (H–R) Diagram

• Plots luminosity vs surface temperature.

• Major zones:

  • Main Sequence: diagonal band

  • Giants & Supergiants: upper right

  • White Dwarfs: lower left

🌟 Star Types by Temperature and Mass

🔁 Stellar Evolution Phases

1. Main Sequence – Hydrogen fusion

2. Red/Blue Giant – Outer layers expand after hydrogen runs out

3. End Stage – Depends on mass:

   • White Dwarf (Sun’s fate)

   • Neutron Star

   • Black Hole

🌟 Variable Stars and Supernovae

• Cepheid Variable: Pulsating star; brightness period relates to luminosity (used to measure cosmic distances).

• Type Ia Supernova:

  • Occurs when a white dwarf exceeds the Chandrasekhar limit (~1.4 solar masses).

  • Used as a "standard candle" for measuring distant galaxies.

🌈 Wien’s Law & Stefan–Boltzmann Law

• Wien’s Law: Hotter objects emit shorter wavelengths.

• Stefan–Boltzmann Law: Total energy emitted:

  E∝T4E \propto T^4E∝T4

🧬 Spectra and Star Chemistry

• Stars emit absorption lines based on their chemical makeup.

• Hydrogen and helium dominate most stellar atmospheres.

• Helps classify stars (O → M) and measure motion via redshift/blueshift.

🪐 Other Notables

• Proplyds: Protoplanetary disks around new stars (e.g. Orion Nebula).

• Open Clusters: Young stars from the same gas cloud (e.g. Pleiades).

• EGGs (Evaporating Gaseous Globules): Dense gas regions in nebulae protected from UV rays, part of star formation.

• Photoevaporation: UV rays strip outer layers, revealing star-forming material.

🌌 Evidence for the Big Bang – Explained

🔭 Hubble–Lemaître Law

What it means:
This law says that galaxies are moving away from us, and the farther they are, the faster they're going. This is powerful evidence that the universe is expanding.
How we know:
By looking at the light from distant galaxies, we see a redshift—their light stretches out, showing they’re moving away.

🌊 Cosmic Microwave Background Radiation (CMB)

What it is:
A faint glow of radiation leftover from the early universe, shortly after the Big Bang.
Why it matters:
It’s like a “baby picture” of the universe, showing that the universe was once hot and dense. Detected at about 2.725 K, it matches what we expect if the Big Bang happened.

🧪 Primordial Element Abundance

What it is:
Most of the universe is made of hydrogen and helium, which formed shortly after the Big Bang.
Why it matters:
These proportions match the predictions from Big Bang theory and wouldn’t make sense if the universe had always existed in its current state.

🌈 Absorption Spectra

What it is:
When light passes through gas, certain wavelengths get absorbed, leaving dark lines in the spectrum.
Why it matters:
These lines tell us what stars are made of, and how fast they’re moving.
Fun fact: These same principles are used to study atmospheres of planets!

🌠 Star Life Cycle & Stellar Concepts – Explained

💫 Hertzsprung–Russell (H–R) Diagram

What it is:
A graph that shows the relationship between a star’s brightness (luminosity) and temperature (or color).
Why it matters:
It helps scientists classify stars and understand how they evolve over time.

⭐ Main Sequence Stars

What they are:
Stars that are in a stable stage, fusing hydrogen into helium in their cores.
Example: The Sun is a main sequence star.
Why it matters: This is the longest and most stable phase of a star’s life.

🌟 Red Giant

What it is:
A later stage of a star’s life. The star swells up and cools as it runs out of hydrogen fuel.
Why it matters: It tells us the star is nearing the end of its life.

🌌 White Dwarf

What it is:
The hot, dense leftover core of a low-mass star (like the Sun) after it has shed its outer layers.
Why it matters: It’s the final stage for many stars and doesn’t undergo fusion anymore.

🟤 Brown Dwarf

What it is:
A “failed star” — too big to be a planet, too small to start fusion like real stars.
Mass: About 15 to 80 times the mass of Jupiter.

🔵 Blue Giant

What it is:
A hot, massive star that burns through its fuel quickly and lives a short life.
Why it matters: These stars often end their lives in dramatic supernova explosions.

💡 Stellar Properties & Tools

🌡 Stefan–Boltzmann Law

What it says:
The energy a star radiates increases with the fourth power of its temperature.
Formula:

E∝T4E \propto T^4E∝T4

Meaning: Hotter stars are a lot brighter than cooler ones.

📏 Wien’s Law

What it says:
Hotter objects emit light at shorter wavelengths.
Application:
Helps scientists figure out a star’s temperature by its color (e.g. blue stars are hotter than red stars).

🌈 Star Color and Temperature

• Cool stars glow red or orange (emit infrared).

• Hot stars glow blue or violet (emit ultraviolet).

• The Sun appears white because it emits across the visible spectrum, peaking in green but blending all colors.

🔁 Stellar Evolution

What happens over time:
Stars change as they use up fuel:

• Hydrogen gets turned into helium

• When hydrogen runs out, the core contracts, and outer layers expand (Red Giant)

• Low-mass stars become white dwarfs; massive stars might become neutron stars or black holes

🌟 Special Star Types

🌟 Cepheid Variables

What they are:
Stars that pulsate in brightness with a regular pattern.
Why they matter:
Their brightness pattern is linked to their true luminosity, making them excellent distance indicators in space.

💥 Type Ia Supernova

What it is:
An explosion of a white dwarf star after it gains too much mass from a companion.
Why it matters:
Always explodes with the same brightness, making it a “standard candle” to measure vast distances in the universe.

💣 Chandrasekhar Limit

What it is:
The maximum mass (~1.4 times the Sun’s mass) a white dwarf can have before it collapses.
Why it matters:
Crossing this limit leads to a supernova.

🌌 Star Formation & Nebulae

🌫 Proplyds

What they are:
Protoplanetary disks — swirling disks of gas and dust around young stars.
Where they’re found:
Famous in the Orion Nebula.

🌌 Open Clusters

What they are:
Loose groups of young stars formed from the same gas cloud.
Example: The Pleiades.

☁ Evaporating Gaseous Globules (EGGs)

What they are:
Dense pockets of gas (about 100 AU across) found in nebulae.
Why they matter:
They protect young stars from radiation and help in star formation.
Bonus: Over time, UV radiation can cause photoevaporation, carving shapes like the “Pillars of Creation.”

🧨 Final Star Fates

After using up their fuel, stars can become:

• White Dwarf – Small to medium stars (like our Sun)

• Neutron Star – Very dense core leftover after a massive star explodes

• Black Hole – When the remaining core collapses under its own gravity