People have interpreted patterns and images in the stars since ancient times. This 1690 depiction of the constellation of Leo, the lion, is by Johannes Hevelius.
Infrared image from NASA's Spitzer Space Telescope showing hundreds of thousands of stars in the Milky Way galaxy
Stellar evolution of low-mass (left cycle) and high-mass (right cycle) stars, with examples in italics
An example of a Hertzsprung–Russell diagram for a set of stars that includes the Sun (center) (see Classification)
Betelgeuse as seen by ALMA. This is the first time that ALMA has observed the surface of a star and resulted in the highest-resolution image of Betelgeuse available.
Onion-like layers at the core of a massive, evolved star just before core collapses
The Crab Nebula, remnants of a supernova that was first observed around 1050 AD
Artist's impression of the Sirius system, a white dwarf star in orbit around an A-type main-sequence star
This view of NGC 6397 includes stars known as blue stragglers for their location on the Hertzsprung–Russell diagram.
Some of the well-known stars with their apparent colors and relative sizes
The Pleiades, an open cluster of stars in the constellation of Taurus. These stars share a common motion through space.
Surface magnetic field of SU Aur (a young star of T Tauri type), reconstructed by means of Zeeman–Doppler imaging
The reflection nebula NGC 1999 is brilliantly illuminated by V380 Orionis. The black patch of sky is a vast hole of empty space and not a dark nebula as previously thought.
The asymmetrical appearance of Mira, an oscillating variable star
Internal structures of main sequence stars with masses indicated in solar masses, convection zones with arrowed cycles, and radiative zones with red flashes. Left to right, a red dwarf, a yellow dwarf, and a blue-white main sequence star
A cross-section of the Sun
Overview of consecutive fusion processes in massive stars

Astronomical object comprising a luminous spheroid of plasma held together by its gravity.

- Star

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Star formation

The W51 nebula in Aquila - one of the largest star factories in the Milky Way (August 25, 2020)
Hubble Space Telescope image known as Pillars of Creation, where stars are forming in the Eagle Nebula
Assembly of galaxy in early Universe.
ALMA observations of the Orion Nebula complex provide insights into explosions at star birth.
Dwarf galaxy ESO 553-46 has one of the highest rates of star formation of the 1000 or so galaxies nearest to the Milky Way.
LH 95 stellar nursery in Large Magellanic Cloud.
Composite image showing young stars in and around molecular cloud Cepheus B.
N11, part of a complex network of gas clouds and star clusters within our neighbouring galaxy, the Large Magellanic Cloud.
Star formation region Lupus 3.
The Orion Nebula is an archetypical example of star formation, from the massive, young stars that are shaping the nebula to the pillars of dense gas that may be the homes of budding stars.
Star-forming region S106.
Young stars (purple) revealed by X-ray inside the NGC 2024 star-forming region.
Star-forming region Westerhout 40 and the Serpens-Aquila Rift- cloud filaments containing new stars fill the region.

Star formation is the process by which dense regions within molecular clouds in interstellar space, sometimes referred to as "stellar nurseries" or "star-forming regions", collapse and form stars.

Fixed stars

The fixed stars (stellae fixae) compose the background of astronomical objects that appear not to move relative to one another in the night sky, unlike the foreground of Solar System objects, which appear to move.

Kepler, Johannes. Mysterium Cosmographicum, 1596. Kepler's heliocentric rendition of the cosmos, containing an outermost “sphaera stellar fixar,” or sphere of fixed stars.
Copernicus, Nicolaus. On the Revolutions of the Heavenly Spheres. Nuremberg. 1543. Print copy of Copernicus's work showing the model of the universe with the sun in the center and a sphere of “immobile stars” on the outside according to his theory of the cosmos.

Generally, the fixed stars are taken to include all stars other than the Sun.

Star catalogue

An illustration of the constellation Perseus (after Perseus from Greek mythology) from the star catalogue published by the German astronomer Johannes Hevelius in 1690

A star catalogue (Commonwealth English) or star catalog (American English) is an astronomical catalogue that lists stars.

Stellar evolution

Representative lifetimes of stars as a function of their masses
Artist's depiction of the life cycle of a Sun-like star, starting as a main-sequence star at lower left then expanding through the subgiant and giant phases, until its outer envelope is expelled to form a planetary nebula at upper right
Chart of stellar evolution
Simplistic representation of the stages of stellar evolution
Schematic of stellar evolution
Internal structures of main-sequence stars, convection zones with arrowed cycles and radiative zones with red flashes. To the left a low-mass red dwarf, in the center a mid-sized yellow dwarf and at the right a massive blue-white main-sequence star.
The evolutionary track of a solar mass, solar metallicity, star from main sequence to post-AGB
The Cat's Eye Nebula, a planetary nebula formed by the death of a star with about the same mass as the Sun
Reconstructed image of Antares, a red supergiant
The onion-like layers of a massive, evolved star just before core collapse (not to scale)
The Crab Nebula, the shattered remnants of a star which exploded as a supernova visible in 1054 AD
Stellar evolution of low-mass (left cycle) and high-mass (right cycle) stars, with examples in italics
Bubble-like shock wave still expanding from a supernova explosion 15,000 years ago

Stellar evolution is the process by which a star changes over the course of time.

Main sequence

A Hertzsprung–Russell diagram plots the luminosity (or absolute magnitude) of a star against its color index (represented as B−V). The main sequence is visible as a prominent diagonal band that runs from the upper left to the lower right. This plot shows 22,000 stars from the Hipparcos Catalogue together with 1,000 low-luminosity stars (red and white dwarfs) from the Gliese Catalogue of Nearby Stars.
Hot and brilliant O-type main-sequence stars in star-forming regions. These are all regions of star formation that contain many hot young stars including several bright stars of spectral type O.
Comparison of main sequence stars of each spectral class
Logarithm of the relative energy output (ε) of proton–proton (PP), CNO and triple-α fusion processes at different temperatures (T). The dashed line shows the combined energy generation of the PP and CNO processes within a star. At the Sun's core temperature, the PP process is more efficient.
This diagram shows a cross-section of a Sun-like star, showing the internal structure.
The Sun is the most familiar example of a main-sequence star
This plot gives an example of the mass-luminosity relationship for zero-age main-sequence stars. The mass and luminosity are relative to the present-day Sun.
Evolutionary track of a star like the sun
H–R diagram for two open clusters: NGC 188 (blue) is older and shows a lower turn off from the main sequence than M67 (yellow). The dots outside the two sequences are mostly foreground and background stars with no relation to the clusters.

In astronomy, the main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness.

Thermonuclear fusion

Process of atoms combining or “fusing” together with huge amounts of heat.

Edward Teller in 1958

There are two forms of thermonuclear fusion: uncontrolled, in which the resulting energy is released in an uncontrolled manner, as it is in thermonuclear weapons ("hydrogen bombs") and in most stars; and controlled, where the fusion reactions take place in an environment allowing some or all of the energy released to be harnessed for constructive purposes.


Chemical element with the symbol H and atomic number 1.

The Space Shuttle Main Engine burnt hydrogen with oxygen, producing a nearly invisible flame at full thrust.
Depiction of a hydrogen atom with size of central proton shown, and the atomic diameter shown as about twice the Bohr model radius (image not to scale)
Hydrogen gas is colorless and transparent, here contained in a glass ampoule.
Phase diagram of hydrogen. The temperature and pressure scales are logarithmic, so one unit corresponds to a 10x change. The left edge corresponds to 105 Pa, which is about atmospheric pressure.
A sample of sodium hydride
Hydrogen discharge (spectrum) tube
Deuterium discharge (spectrum) tube
Antoine-Laurent de Lavoisier
Hydrogen emission spectrum lines in the visible range. These are the four visible lines of the Balmer series
NGC 604, a giant region of ionized hydrogen in the Triangulum Galaxy

Stars such as the Sun are mainly composed of hydrogen in the plasma state.

Milky Way

Galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye.

The Origin of the Milky Way (c. undefined 1575–1580) by Tintoretto
A view of the Milky Way toward the constellation Sagittarius (including the Galactic Center), as seen from a dark site with little light pollution (the Black Rock Desert, Nevada), the bright object on the lower right is Jupiter, just above Antares
The shape of the Milky Way as deduced from star counts by William Herschel in 1785; the Solar System was assumed near center
Photograph of the "Great Andromeda Nebula" from 1899, later identified as the Andromeda Galaxy
Map of the Milky Way Galaxy with the constellations that cross the galactic plane in each direction and the known prominent components annotated including main arms, spurs, bar, nucleus/bulge, notable nebulae and globular clusters.
An all-sky view of stars in the Milky Way and neighbouring galaxies, based on the first year of observations from Gaia satellite, from July 2014 to September 2015.
The map shows the density of stars in each portion of the sky. Brighter regions indicate denser concentrations of stars. Darker regions across the Galactic Plane correspond to dense clouds of interstellar gas and dust that absorb starlight.
Artistic close-up of the Orion Arm with the main features of the Radcliffe Wave and Split linear structures, and with the Solar System surrounded by the closest large scale celestial features at the surface of the Local Bubble at a distance of 400-500 light years.
Diagram of the Sun's location in the Milky Way, the angles represent longitudes in the galactic coordinate system.
The structure of the Milky Way is thought to be similar to this galaxy (UGC 12158 imaged by Hubble)
A schematic profile of the Milky Way.
Abbreviations: GNP/GSP: Galactic North and South Poles
360-degree panorama view of the Milky Way (an assembled mosaic of photographs) by ESO, the galactic centre is in the middle of the view, with galactic north up
360-degree rendering of the Milky Way using Gaia EDR3 data showing interstellar gas, dust backlit by stars (main patches labeled in black; white labels are main bright patches of stars). Left hemisphere is facing the galactic center, right hemisphere is facing the galactic anticenter.
Overview of different elements of the overall structure of the Milky Way.
Illustration of the two gigantic X-ray/gamma-ray bubbles (blue-violet) of the Milky Way (center)
Observed (normal lines) and extrapolated (dotted lines) structure of the spiral arms of the Milky Way, viewed from north of the galaxy – the galaxy rotates clockwise in this view. The gray lines radiating from the Sun's position (upper center) list the three-letter abbreviations of the corresponding constellations
Clusters detected by WISE used to trace the Milky Way's spiral arms.
The long filamentary molecular cloud dubbed "Nessie" probably forms a dense "spine" of the Scutum–Centarus Arm
Galaxy rotation curve for the Milky Way – vertical axis is speed of rotation about the galactic center; horizontal axis is distance from the galactic center in kpcs; the sun is marked with a yellow ball; the observed curve of speed of rotation is blue; the predicted curve based upon stellar mass and gas in the Milky Way is red; scatter in observations roughly indicated by gray bars, the difference is due to dark matter
Comparison of the night sky with the night sky of a hypothetical planet within the Milky Way 10 billion years ago, at an age of about 3.6 billion years and 5 billion years before the Sun formed.
The Milky Way arching at a high inclination across the night sky, (this composited panorama was taken at Paranal Observatory in northern Chile), the bright object is Jupiter in the constellation Sagittarius, and the Magellanic Clouds can be seen on the left; galactic north is downward
The Milky Way viewed at different wavelengths

It is estimated to contain 100–400 billion stars and at least that number of planets.


A supernova ( or supernovas; abbr.

SN 1994D (bright spot on the lower left), a type Ia supernova within its host galaxy, NGC 4526
SN Antikythera in galaxy cluster RXC J0949.8+1707. SN Eleanor and SN Alexander were observed in the same galaxy in 2011.
Supernova remnant SNR E0519-69.0 in the Large Magellanic Cloud
"A star set to explode", the SBW1 nebula surrounds a massive blue supergiant in the Carina Nebula.
Multi-wavelength X-ray, infrared, and optical compilation image of Kepler's supernova remnant, SN 1604
Light curve for type Ia SN 2018gv
Light curves are used to classify type II-P and type II-L supernovae.
Artist's impression of supernova 1993J
In the galaxy NGC 1365 a supernova (the bright dot slightly above the galactic center) rapidly brightens, then fades more slowly.
Formation of a type Ia supernova
The layers of a massive, evolved star just before core collapse (not to scale)
Supernova types by initial mass-metallicity
Remnants of single massive stars
Within a massive, evolved star (a) the onion-layered shells of elements undergo fusion, forming an iron core (b) that reaches Chandrasekhar-mass and starts to collapse. The inner part of the core is compressed into neutrons (c), causing infalling material to bounce (d) and form an outward-propagating shock front (red). The shock starts to stall (e), but it is re-invigorated by a process that may include neutrino interaction. The surrounding material is blasted away (f), leaving only a degenerate remnant.
The atypical subluminous type II SN 1997D
SN 2008D, a type Ib supernova at the far upper end of the galaxy, shown in X-ray (left) and visible light (right)
Comparative supernova type light curves
Messier 61 with supernova SN2020jfo, taken by an amateur astronomer in 2020
The pulsar in the Crab Nebula is travelling at 375 km/s relative to the nebula.
The radioactive decays of nickel-56 and cobalt-56 that produce a supernova visible light curve
Isolated neutron star in the Small Magellanic Cloud
Periodic table showing the source of each element in the interstellar medium
Supernova remnant N 63A lies within a clumpy region of gas and dust in the Large Magellanic Cloud
The nebula around Wolf–Rayet star WR124, which is located at a distance of about 21,000 light-years

This transient astronomical event occurs during the last evolutionary stages of a massive star or when a white dwarf is triggered into runaway nuclear fusion.

Variable star

The Trifid Nebula contains Cepheid variable stars
The variable star Mira at two different times
A photogenic variable star, Eta Carinae, embedded in the Carina Nebula
Intrinsic variable types in the Hertzsprung–Russell diagram
Light curve of Mira variable χ Cygni
Herbig Ae/Be star V1025 Tauri
Images showing the expansion of the light echo of V838 Monocerotis
Light curves for FK Comae Berenices. The main plot shows the short term variability plotted from TESS data; the inset, adapted from Panov and Dimitrov (2007), shows the long term variability.
How eclipsing binaries vary in brightness

A variable star is a star whose brightness as seen from Earth (its apparent magnitude) fluctuates.