The Universe 
Animated sequence of astronomical maps from different NASA and ESA missions, each showing a distinct scientific dataset using colour-coded data. Source.
Image credit: NASA, ESA, JPL-Caltech; Two Micron All-Sky Survey (2MASS); Planck Collaboration (HFI & LFI Consortia); NASA/WMAP Science Teamis Everything
Animated sequence of astronomical maps from different NASA and ESA missions, each showing a distinct scientific dataset using colour-coded data. Source.
It is vast, and we can only observe a portion of it (the observable universe). Beyond what we see, everything has been expanding since the Big Bang about 13.8 billion years ago. It includes everything that exists such as space, time, matter, and energy, from tiny particles to huge galaxies, and it remains in constant motion and expansion. The Milky Way is just one of billions of galaxies in the observable universe.
Large Scale Structures
Collections of galaxies bound together by gravity
Dark Matter & Dark Energy
The invisible substance and force that make up most of the universe.
Nebula
Nebulae are made of dust and gases—mostly hydrogen and helium
Galaxies
Galaxies consist of stars, planets, and vast clouds of gas and dust, all bound together by gravity.
Stars
Estimated stars in the universe: up to one septillion (10²⁴) stars
Exoplanets
An exoplanet is any planet beyond our solar system.
Supernova
A supernova is the biggest explosion that humans have ever seen
Neutron Stars
Neutron stars are formed when a massive star runs out of fuel and collapses
Black Holes
The most mysterious cosmic objects, much studied but not fully understood.
Quasars
Accreting supermassive black holes can be among the brightest objects in the universe
Our Milky Way
A spiral galaxy with a disk of stars over 100,000 light-years across, containing stars, planets, and vast clouds of gas and dust. Approximately 90% of its mass is invisible dark matter, while the visible 10% includes the luminous stars, interstellar gas (90% hydrogen, 10% helium), and dust, all held together by gravity.
The supermassive black hole in the center of the Milky Way is called Sagittarius A*. It is as massive as 4 million Suns. The Milky Way and Andromeda Galaxy are the two largest spiral galaxies in the Local Group and are moving toward each other under mutual gravity.
The Solar System
Our Solar System consists of the Sun and its orbiting celestial bodies, with the Sun located in a small section of the Milky Way called the Orion Arm, or Orion Spur. The Sun formed in a giant, spinning cloud of gas and dust called the solar nebula.
As the nebula collapsed under its own gravity, it spun faster and flattened into a disk. Most of the nebula's material was pulled toward the center to form our Sun, which accounts for 99.8% of our solar system's mass. Much of the remaining material formed the planets and other objects that now orbit the Sun. (The rest of the leftover gas and dust was blown away by the young Sun's early solar wind.)
- The first four planets from the Sun are Mercury, Venus, Earth, and Mars.
- These inner planets are called terrestrial planets because they have solid surfaces.
- The giant planets in the outer solar system do not have hard surfaces; they have swirling gases above a core.
- Jupiter and Saturn are gas giants, while Uranus and Neptune are ice giants.
- Dwarf planets include Pluto, Ceres, Makemake, Haumea, and Eris.
- Dwarf planets Pluto, Eris, Haumea, and Makemake are all Kuiper Belt objects that have moons.
- Ceres is the only dwarf planet in the inner solar system and is located in the main asteroid belt between Mars and Jupiter.
Sun
The overwhelming heart of the system. It contains 333,000 times the mass of Earth, exerting the gravity that keeps everything in orbit.
Planets
The eight major planets. Jupiter holds 71% of this mass; essentially, the solar system is the Sun, Jupiter, and some leftover debris.
Comets (Oort Cloud)
A vast, spherical shell of icy objects. While numerous, their combined mass is likely only 5 to 20 Earth masses, though this remains the hardest to measure.
Moons & Satellites
All natural satellites. Despite their numbers, their combined mass is small; Ganymede and Titan alone provide a significant portion.
Kuiper Belt
The icy ring beyond Neptune. It is surprisingly sparse, containing only about 1/10th to 1/30th the mass of the Earth.
Asteroids (Main Belt)
Rocky debris between Mars and Jupiter. The entire belt’s mass is less than 5% of our Moon; Ceres alone accounts for a third of it.
Meteoroids & Dust
Includes the Zodiacal dust cloud. While it covers a massive volume, the total mass is equivalent to just one large asteroid spread very thin.
Interplanetary Medium
The solar wind and plasma. Though it fills the 'empty' space, its density is so low it contributes virtually nothing to the system's total mass.
Merriam-Webster dictionary defines a planet as "any of the large bodies that revolve around the Sun in the solar system." But in 2006, the International Astronomical Union (IAU), the organisation responsible for defining and naming objects in our Solar System, adopted a new definition of the term “planet,” which led to Pluto's reclassification as a dwarf planet.
To be classified as a planet, a celestial body must meet three criteria:
- It must orbit a star (in our cosmic neighbourhood, the Sun).
- It must be large enough to have sufficient gravity to force it into a spherical shape.
- It must be large enough that its gravity has cleared away other objects of a similar size near its orbit around the Sun.
According to the 2006 IAU Resolution, "a dwarf planet is an object in orbit around the Sun that is large enough to pull itself into a nearly round shape but has not been able to clear its orbit of debris."
