Astronomy & Cosmology

The Universe
and Its Structures

From the big picture to the solar system — a structured journey through space and time.

Chapter 01 🌌

The Universe

The observable universe is everything we could theoretically see — limited by the speed of light and the age of the universe. The universe itself, however, could be infinitely large.

13.8
Billion Years
Age of the universe
93 billion
Light-Years Diameter
Observable universe
2.7 K
Cosmic Background Radiation
Temperature of space
67–74
km/s per Megaparsec
Hubble constant — measured value disputed depending on method
10⁸⁰
Atoms
In the observable universe
~0
Total Energy
Positive + negative energy

Composition of the universe:

68%
27%
5%
Dark Energy — 68%
Dark Matter — 27%
Normal Matter — 5%

The universe began with the Big Bang 13.8 billion years ago from an extremely hot, dense state. Since then it has been expanding — and this expansion is accelerating, driven by the mysterious dark energy.

→ Explore the Topics Overview
Chapter 02 🌀

Galaxies

Galaxies are gigantic collections of stars, gas, dust, and dark matter, held together by gravity. Estimates range from several hundred billion to around two trillion galaxies in the observable universe, depending on the counting method.

Hundreds of billions
Galaxies (estimate)
In the observable universe — counting method disputed
100,000
Light-Years
Diameter of the Milky Way
200–400 billion
Stars
In the Milky Way
2.537 million
Light-Years
Distance to Andromeda
~2 million
Light-Years Diameter (Halo)
IC 1101 — one of the largest known galaxies
4.3 million
Solar Masses
Sgr A* — central black hole

Galaxy types according to Hubble classification:

Spiral Galaxies
Flat disk with spiral arms. Contains active star-forming regions. Our Milky Way is a barred spiral galaxy (SBbc).
Elliptical Galaxies
Older, round to elliptical structures. Little star formation. Often the largest galaxies (e.g., IC 1101).
Irregular Galaxies
No defined shape, often formed through collisions. The Magellanic Clouds are well-known examples.
→ More on Galaxies
Chapter 03 ✨

Cosmic Bodies

A galaxy contains far more than just stars. From collapsing giant stars to glowing gas clouds — here are all the cosmic objects that populate a galaxy.

Main-Sequence Star
Fuses hydrogen into helium in its core. Our Sun is an example — type G, average lifespan ~10 billion years.
Red Giant
Aging stage of medium-sized stars. The outer shell expands massively, the core collapses. The Sun will become one in ~5 billion years.
Supernova
Catastrophic explosion of massive stars at the end of their life. Briefly brighter than the entire galaxy. Produces heavy elements.
Neutron Star
Remnant of a supernova. Extremely dense — a teaspoonful weighs billions of tons. Only ~22–26 km in diameter.
Pulsar
Rotating neutron star that emits focused beams of radiation. Precision of an atomic clock.
White Dwarf
Remnant of Sun-like stars. Earth-sized, extremely dense. Cools slowly over billions of years.
Black Hole
Object with gravity so strong that not even light can escape. Forms from massive stars or at the centers of galaxies.
Quasar
Active galactic nucleus surrounding a supermassive black hole. The brightest continuously shining objects in the universe.
Emission Nebula
Vast clouds of gas and dust, ionized by nearby stars. Birthplace of new stars. The Orion Nebula is the best-known example.
Planetary Nebula
Shed outer shell of a dying star. Despite the name, not a planet — the term arose from a visually similar appearance.
Globular Cluster
Spherically symmetric collection of 100,000 to several million old stars. Orbits galaxies as satellites.
Open Cluster
Loose group of young stars of the same formation age. The Pleiades are the best-known example — visible to the naked eye.
→ Stellar Evolution in Detail
Chapter 04 ☀️

The Solar System

Our solar system formed about 4.6 billion years ago from a collapsing cloud of gas. At its center is the Sun — containing 99.86% of the total mass.

Sun
Type G-type dwarf star Diameter 1,392,000 km Temperature (surface) 5,778 K Age 4.6 billion years
Mercury
Ø 4,879 km Distance 0.39 AU Orbit 88 days Moons 0
Venus
Ø 12,104 km Distance 0.72 AU Orbit 225 days Surface 464°C
Earth
Ø 12,742 km Distance 1.00 AU Orbit 365.25 days Moons 1
Mars
Ø 6,779 km Distance 1.52 AU Orbit 687 days Moons 2
Jupiter
Ø 139,820 km Distance 5.20 AU Orbit 11.9 years Moons 115
Saturn
Ø 116,460 km Distance 9.58 AU Orbit 29.5 years Moons 293 · ring system
Uranus
Ø 50,724 km Distance 19.2 AU Orbit 84 years Moons 29
Neptune
Ø 49,244 km Distance 30.1 AU Orbit 165 years Moons 16

Further bodies in the solar system:

Dwarf Planets
Pluto, Eris, Ceres, Haumea, Makemake. Too small to have cleared their orbital path, large enough to be round.
Asteroid Belt
Between Mars and Jupiter. Millions of rocky bodies, the largest: Ceres. Total mass < 4% of the Moon's.
Trojan Asteroids
Share Jupiter's orbit at the Lagrange points L4 and L5. Over 15,000 known Trojans.
Kuiper Belt
Beyond Neptune (30–50 AU). Contains Pluto and thousands of icy objects — source of short-period comets.
Comets
Chunks of ice and dust on elliptical orbits. As they approach the Sun, a glowing tail of gas and dust forms.
Oort Cloud
Hypothetical sphere of billions of icy bodies extending up to 100,000 AU — source of long-period comets.
→ The Solar System in Detail
→ Topics Overview Stellar Evolution Star Systems