Distance travelled by light in vacuum in one year, approximately10millionmillion kilometres = 10 to the power 13 km
Cosmological horizon
Border of the observable Universe, beyond which matter is not accessible to observations
In a static Universe, the cosmological horizon would be situated at the distance that light travels within the age of the Universe, which is about 46 billion light-years in our expanding Universe
Distribution of galaxies on the sky is both regular and non-regular
Cosmic web
Galaxies form cells similar to a honeycomb, with walls formed by galaxies and empty spaces (voids) inside the cells. Majority of galaxies are concentrated in the intersections of walls and in the knots where three walls intersect
Clusters of galaxies
Nearly spherical structures consisting of hundreds and thousands of galaxies
Groups of galaxies
Non-numerous clusters of galaxies, such as the Local Group consisting of the Milky Way and Andromeda Galaxy
Galaxies
Stellar systems consisting of billions of stars, gas and dust. 9/10 of the galactic material exists in a hidden, invisible form
Masses of galaxies vary from 10^9 to 10^12 solar masses, with the Milky Way being close to the upper limit
Sizes of galaxies (their visible parts) usually vary from 3 to 300 thousand light-years
Types of galaxies
Spiral
Spherical or elliptical
Irregular asymmetric
Quasars
Quasi-stellar objects, which are the mostdistantobjects in the observable Universe and are likely the nucleiofdistantgalaxies in a highlyactivestate
Types of stellar clusters
Globular clusters
Open clusters
Globular clusters
Spherical objects containing millions of stars, the oldest formations in the Milky Way with an age of 10-12 billion years
Open clusters
Contain tens, hundreds or thousands of stars, some are relatively young (e.g. Pleiades at 60 million years)
Stars are born in groups and clusters, not one at a time
Types of stars
Regular stars
Compact stars (white dwarfs, neutron stars, black holes)
Sizes of regular stars vary from slightly smaller than the Sun to enormous sizes of supergiants, while compact stars range from several kilometres to several thousand kilometres
Planetary systems
Apart from the Solar system, hundreds of multiplanetary systems with at least two confirmed exoplanets are known
The star with the most confirmed planets is the Sun with 8, while Kepler-90 and HD 10180 have the most exoplanets with 7 each
Cosmic bodies
Planets
Comets
Asteroids
Small planets
The diameter of the Earth is about 13,000 km
Macroscopic bodies are all the objects around us, consisting of a very large number of molecules
Microscopic bodies
Associations of molecules, such as cells and their components
Molecules
Vary in size from the simplest two-atomic molecules to complex polymeric macromolecules with molecular weights in the hundreds of thousands
Atom
Consists of a compact nucleus and an electron shell, with the size determined by the electron shell
The word "atom" comes from the Greek word "atomos" meaning indivisible, but atoms were later found to be composed of subatomic particles
Subatomic particles
Elementary particles (primary, indivisible)
Composite particles
Initially only protons, neutrons, electrons and photons were considered elementary particles, but more were discovered over time
Types of subatomic particles
Elementary particles
Composite particles
Elementary particles
Primary, indivisible particles, of which all the matter consists
Till the beginning of 1930s the only known representatives of elementary particles were proton, neutron, electron, and photon