star is a huge luminous sphere of very hot plasma and gas. It is made up mainly of helium and hydrogen and maintains its shape thanks to the force of gravity. Our Sun is a yellowish star of average size, the youngest stars are bluish and the oldest are reddish.


What is a star?

Stars are huge spheres or celestial bodies composed of plasma , which are maintained by their force of gravity. They produce electromagnetic radiation in the form of visible light, thanks to hydrogen atoms that fuse in their nucleus.



It is a celestial body made up of plasma and gas , which is concentrated in one place by its force of gravity . They produce their own light thanks to the fusion of elements in their nucleus, they heat up and shine.


Characteristics of a star

  • Age: varies between one billion and eleven billion years. The most massive stars last a few million years, unlike the lowest mass stars , which burn their fuel slowly and can last thousands of years longer.
  • Chemical composition: in the Milky Way they  are composed of 71% hydrogen and 27% helium. Heavy elements are measured by the iron contained in the stellar atmosphere .
  • Diameter: it varies greatly in size, for example, our Sun is 1391016 km in diameter, but one of the superstars called Betelgeuse has a diameter of 1070 times that of the Sun.
  • Kinematics: its movement or stop is determined by astrometric measurements quantified in units of milli-seconds of arc per year. You can calculate the speed of movement through its distance and stop.
  • Magnetic field: it depends on its mass and composition. This is generated by the movement of electrical charges inside. The magnetic field decreases the surface activity of stars over time, thus slowing them down .
  • Mass: allows you to predict the evolution of a star, from its birth to its death. Mass can be measured using the Hertsprung-Russel (HR) diagram that relates luminosity, temperature, and mass. On the other hand, the mass of supermassive stars can be measured by the curvature of gravitational lenses, with the curvature the gravity of the star and with it its mass can be known. The combination of a star’s radius and mass determines its surface gravity.
  • Rotation: spectroscopic measurement and starspots allow us to know the rotation speed of stars.


Despite the large number of stars known today, the first billion years of the universe were quite dark. During that time only dark matter and gas clouds existed . However, after hundreds of millions of years, it was that the first stars were formed. As a result of the creation of the stars, the universe continued to generate significant growth for billions of years.

On the other hand, for the human race the stars have always had a historical importance. They have been related to religious practices of thousands of cultures and from ancient times to the present day they continue to be the object of study by astronomers.


Main sequence stars

  • Orange dwarfs
  • Yellow dwarfs
  • White
  • Blue
  • Subgiants

Giant and luminous stars

  • Red giant
  • Orange giant
  • Yellow giant
  • White giant
  • Blue giant

Supergiant stars

  • Blue supergiant
  • White supergiant
  • Yellow supergiant
  • Orange supergiant
  • Red supergiant

Faint stars, practically dead

  • Subdwarf
  • Brown dwarf
  • White dwarfs
  • Neutron star
  • Press
  • Magnets

Binary stars

  • Double
  • Binary
  • Eclipsing binary
  • X-ray binary


Stars are formed from large clouds of gas and dust from the galaxy and are known as nebulae . These nebulae form lumps of matter that gravitationally attract the surrounding dust and gas. The mass accumulation is compressed and fragmented, originating many bodies called protostars ; later these protostars capture more matter, are compressed and  heated , causing nuclear reactions due to high temperatures in their nucleus and forming the new star.

Stages of a star

  • Gigantic molecular clouds: the life of the star begins as a large gas cloud with a low temperature.
  • Protostars: the movement of the particles generates heat within the gas cloud.
  • T-Tauri:  begins its production of strong outward winds, turning it into a visible star without the help of x-rays or infrared light.
  • Main sequence star : the star reaches a hydrostatic equilibrium in which its gravity is compensated by the pressure from the outside. It is the longest stage.
  • Giant: At this stage, the core of the star turns to ice , causing it to collapse in on itself. It will continue to expand as it cools.
  • Core fusion: the previous stages will be repeated in cycle until iron begins to appear in the core of the star, absorbing energy and eventually causing a supernova .
  • Supernova – the explosion of the star’s core will become a visible object. Depending on the size of the star it can become a planetary nebula , a neutron star or even a black hole .

How much

From the Earth we see the stars as bright points at night, except for the Sun , which we can see large and yellowish due to its proximity to the Earth. The stars vary in size, we have the Sun measured 1,391,016 km in diameter , the star R Doradus measures 0.057 seconds of arc of angular diameter and Betelgeuse measured 1,070 times more than the sun.


The rate of energy production of a star allows us to determine the surface temperature and is usually calculated from the star’s color index.The temperature in the central region of a star is several million kelvin kelvins. The largest can have 50,000 K surface temperature , the sun has thousands of K.

What does the color of a star indicate

The color of stars depends on their surface temperature. The blue ones are the hottest and the red ones are the coldest. This is due to the electromagnetic spectrum , since ultraviolet light is more intense than infrared light. Thus, the blue color is related to more intense radiation than red.

On the other hand, color is related to the age of the stars. The youngest are more bluish and the oldest have reddish tones. When they are younger they generate more energy and reach higher temperatures, unlike older stars that generate less energy and their temperature decreases.

Featured stars

  • I Cephei
  • Sirius
  • UY Scuti
  • Arthur
  • Spica
  • R Doradus
  • Albireo
  • Bellatrix
  • Betelgeuse
  • Vega
  • Sun
  • LBV 1806-20
  • u Andromedae
  • Procyon
  • Canopus
  • Herculis
  • Wolf Rayet


The stars are of the utmost importance , as they have contributed to the creation of life as we know it. This is the case of the Sun, which has been of vital help for the photosynthesis of plants, it provides light and heat , it allows our bodies to absorb vitamin D, the wind exists and the water carries out its cycle. Without the sun our life on earth would not exist.

For astronomy , the stars have been important because they have helped to create an astral map, the best known are the constellations of the zodiac signs , although depending on the cultures and the place of observation these can change.

Also, astronomers used the movement of the sun against the stars to create the first calendars. The Egyptian astronomers, Babylonians, Greeks, Chinese and Islamic made great progress in terms of name and catalog stars.

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