Galaxies

Star - Gas - Star Cycle

1. Stars form from molecular clouds, nuclear fusion creates new, heavier elements.

2. Dying low-mass stars shed gas through stellar winds and planetary nebulae, while High-mass stars explode as supernovae, releasing heavy elements and gas into space, Return material to ISM

3. Hot gas cools over time → Atomic hydrogen forms → Molecules combine → Molecular clouds develop.

4. Gas in molecular clouds collapses under gravity to form new stars. New generation contains more heavy elements.

Spiral Arms

waves of star formation in spiral galaxies. Density waves that move through the galaxy, compressing gas clouds as they pass (not fixed group of stars) Compression triggers star formation. young, bright stars and star-forming regions are mainly found in the spiral arms.

What is a Galaxy

A galaxy is a massive, gravitationally bound system composed of stars, gas, dust, stellar remnants, and dark matter.

Spiral Galaxy Features

(Sa, Sb, Sc): Disk with arms, central bulge, gas & star formation

Barred Spiral Galaxy Features

(SBa, SBb, SBc): Spiral with central bar (straight line passing through central bulge), gas & star formation

Lenticular Galaxy Features

(S0): larger central Bulge + disk, no arms, little star formation, higher bulge-to-disk ratio, contain older stellar populations

Eliptical Galaxy Features

(E0-E7): Smooth shape, high concentration of old, red stars and very little gas and dust. less structured than spiral galaxies (no arms).

Irregular Galaxy Features

No symmetry/defined shape, young stars, significant amounts of gas and dust leads to intense star formation activity.

Evolution of Universe Structure

The universe started smoother than it is today, gravity of dark matter pulls mass into denser regions over time.

Galaxies& stars form in clumps, gravitational effects attract more matter, and growing structures. clusters form at the intersections of supercluster filaments.

Growth of Large-Scale Structure

Galaxies flow toward the densest regions of space.

It results from gravitational instability acting on early density fluctuations. The size and strength of structures depend on the cosmology (e.g., amount of dark matter, dark energy).

Critical Universe

density = critical - flat geometry

Lines remain parallel, object actual size seen

Expansion slows but never stops, infinite

Closed Universe

density > critical - spherical geometry

Parallel lines converge, object appears larger

Expansion eventually halts and reverses

Open Universe

density < critical - saddle-shaped geometry

Parallel lines diverge, object appears smaller

Expands forever at a decelerating rate

Cosmic Microwave Background (CMB)

Leftover radiation from the Big Bang

Blackbody spectrum, seen at 2.73 K, peak at 1.2 mm λ

Originated as visible light, stretched by redshift (×1000)

Released 380,000 years after Big Bang when universe cooled below 3000 K

Marks the time when atoms formed and universe became transparent

Appears mostly smooth with tiny temperature fluctuations, which are density variations, seeds of galaxies, Angular size of fluctuations shows universe is flat → near critical density

Dark Energy

close the gap in energy density for a flat universe, ~70% of the universe

Opposes gravity, causing accelerating expansion of the universe, property of space itself - expansion increases its effect, more space means more dark energy

Explains results from Type Ia Supernovae observations

Planck Era

Very start era, least known about

The uncertainty principle from quantum mechanics

the period where current physics theories break down.

quantum gravitational effects dominated (quantum theory of gravity)

Grand Unified Theory Era (GUT)

When universe cooled just below 10³² K, The four fundamental forces unify into one: gravity, strong nuclear , weak nuclear, electromagnetic, as they behave identically under extreme temperautres

At T > 10²⁹ K, strong, weak, and electromagnetic forces follow GUT laws

When the universe cooled below this energy, the forces "split" or symmetry broke, and each force began to act separately

Particle Era

After electro-weak unification, universe full of quarks and protons

As temperature falls, quarks combine into protons and neutrons (quark-hadron transition) & their antiparticles

These particles and antiparticles are in thermal equilibrium with photons here

Once the temperature drops enough, photons don't have enough energy to make more particles

What are all the Eras of the universe

Planck, Grand Universe Theory (GUT), Electroweak, Particle, Nucleosynthesis, Nuclei, Atoms, Galaxies era

Lookback time

linked to redshift, how far back in time we see when observing distant objects, distant objects show the early universe.

Early universe was smaller, denser, and radiation-dominated; later became matter-dominated.

Radiation density decreases faster than matter density as the universe expands.

Dark energy now dominates the universe's expansion.

Seyferts

a type of AGN in some spiral galaxies with very bright, active nuclei. (5%)

Their nuclei emit strong emission lines in their spectra.

Broad emission lines indicate fast-moving gas near the nucleus

Narrow emission lines show slower-moving gas or obscured regions.

Active Galactic Nuclei (AGN)

extremely bright and energetic centres seen in some galaxies. Powered by accretion of material onto a supermassive black hole (millions to billions of solar masses).

Emission features: often show strong emission lines from gas near the black hole.

AGNs influence their host galaxies by releasing energy and sometimes driving powerful jets or winds.

Quasars

Most luminous type of Seyfert AGN

Look like blue point-like stars in images due to brightness.

Found at large distances (high redshift) → early universe.

Huge luminosity powered by rapid accretion onto a supermassive black hole, still visibile from veyr far distances

Bulge

Area of galaxies central supermassive black hole, older stars; little gas or dust

Disk

area around the galaxy centre where gas and dust are located (ie. Spiral arms)

is a flat circle because of the rotation (conservation of angular momentum)

Halo

a sphere located around the centre of the galaxy including everything that is gravitationally bound to the galaxy

Globular CLusters

Dense, spherical collections of old stars found in the halo of galaxies

Contain some of the oldest stars in the universe (typically >10 billion years old).

low metallicity indicating early formation before many generations of stars enriched the galaxy

Held together by their own gravity, forming a compact, spherical shape.

Open Clusters

Loose small groups of stars that formed from the same giant molecular cloud.

Found within the galactic disk, along spiral arms

Contain younger stars with higher metallicity

Less gravitationally bound, so they tend to disperse over time due to interactions with other stars and the galactic environment

Galactic Coordinates

Longitude (L): 0 to 360 degrees, with 0 pointing towards galactic centre, increases along the direction of Milky Way's rotation

Latitude (b): -90 to 90 degrees, (down v up) 0 degrees lies in the galactic plane/disk

Cosmological Principle

the universe is homogeneous and isotropic on a large scale, ie, it looks the same in every direction and from every location

any observor at any point would see that the universe is expanding away from them

Surface of last scattering

furtherest lookback time that can be seen in any direction (spherical), equal to the distance travelled by light in ~13.8 billion yrs.

shows the time in the early universe 380,000 yrs after the big bang once the universe have become transparent