Stars are bodies of enormous masses similars to the Sun, that rotate at low speeds, so they have almost spherical symmetry.

In the cover image you can see the ancient white dwarf explosión Credit: web “”

Stars are not rocky bodies, but are basically made up of hydrogen and helium (99%) as discovered British astronomer Cecilia Payne in the early 20th Century.

Much of what we now know about the stars is due to the painstaking and self-sacrificing work of some intelligent women. Some of them are very well known:

Cecilia Payne, Annie Cannon, Henrietta Leavitt, and Williamina Fleming

Stars are bodies of enormous masses.

Compared with the mass of the Sun, the smallest have about 1/12 of the solar mass, they are called brown dwarfs; the largest are between 120 and 200 times the mass of the Sun.

Many stars, including the Sun, rotate at low speeds, so they have almost spherical symmetry. Other fast-spinning stars have their equatorial radius significantly greater than their polar radius.

A high rotational speed generates surface temperature differences between the equator and the poles.

Sirius is the brightest star in the night sky. Credit: web “”

The star Vega, for example, has a rotation speed of 275 km / sec at the equator, which causes the poles to be at a temperature above 10,000 degrees Kelvin and the equator at a temperature of 7,900 degrees Kelvin.

How do stars form

Stars form in the densest regions of the molecular clouds present in nebulae.

Orion Nebula
Orion Nebula.   Crédito: web “”

Due to the gravitational force, the clouds of hydrogen molecules begin to concentrate, causing their density to increase progressively.

The increasingly intense gravitational collapse of hydrogen molecules causes mergers of these molecules and expansive nuclear reactions that balance the gravitational force.

Normally stars start their nuclear combustion with around 75% hydrogen and 25% helium along with small traces of other elements.

It is estimated that from when the mass of this nucleus is 1/12 of the mass of the Sun, the temperature is sufficient to ignite the nuclear furnace.

When a star collapses

If the collapsed mass were greater than 200 solar masses, the pressure of the core fusion would cause the star to explode violently.

The balance between both forces makes the stars that we see in the sky are as we know them.

Supernova  Credit: web “”

The star will die when the hydrogen in its core is depleted, which will make gravity no longer have anything to prevent the collapse of the star.

What are the parts of a Star

A typical star is divided into: core, radiation zone, convection zone, and atmosphere.

SUn parts
Various parts and areas of the sun   Credit: Microsoft illustration
  • In the core the nuclear reactions that generate its energy take place.
  • Radiation and convection zones transport this energy to the surface.
  • The atmosphere is the most superficial part of the stars and the only one that is visible.

The atmosphere is the coldest zone of the star and in it the phenomena of matter ejection take place.

There are distinguished in it: the chromosphere, the photosphere and the solar corona.

The solar corona is a very thin layer of the atmosphere, formed by ionized particles that, when accelerated by the star’s magnetic field, acquire high speeds that increase their temperature up to a million degrees.

In some non-massive stars, convection movements penetrate much inside, mixing processed material with the original.

Then you can see, even on the surface, part of that processed material. The star presents, in these cases, a superficial composition with more metals.

The composition of a star evolves throughout its cycle, increasing its content of heavy elements to the detriment of hydrogen.

Stars dissipate enormous amounts of energy in space, in the form of electromagnetic radiation, neutrinos, and stellar wind.

Sun Rays X
Sun X Ray. Credit: blog “”

Due to this incredible energy the stars shine and we can observe them in the night sky as points of light.

Why do stars produce such intense luminosity

The luminosity of stars has a very wide range, ranging from 0.001 to 3,000,000 times the luminosity of the Sun.

Sun   Credit: web “”

What is the source of the enormous energy that powers the stars and that produces this incredibly intense luminosity?

Gravitational contraction is a very large energy source, but it is not enough to explain the heat input over billions of years.

In the 1920s, Sir Arthur Eddington attributed the energy input to nuclear reactions.

In 1938, Hans Bethe delved into this theory, studying the detailed mechanism of nuclear fusion reactions, capable of maintaining the internal structure of a star.

His theory is valid for stars of intermediate or high mass and is called the Bethe cycle.

When were the stars formed?

Most stars are between 1 billion and 10 billion years old; some stars are older still. The oldest observed star, HE 1523-0901, has an estimated age of 13.2 billion years.

The Sun formed as a star 4.5 billion years ago.

The evolution of a star depends on its mass. Stars continually lose mass and in the last phases of their lives they lose it much more intensely and can end up with a final mass much lower than the original one.

When the mass decreases to a certain level and the star does not fuse material, it will go into a degenerative process that will cause it to collapse on itself due to gravity.

The NGC 2440 nebula, 4,000 light years from Earth.  Credit: web “”

The star NGC 2440, created by a star similar to the Sun in the last stages of its life.

It has expelled its outer layers which now form a cocoon around the stellar core. The material glows due to the ultraviolet light coming from the star.

The white point near the center is a white dwarf and was the core of the star.

Depending on its mass, the star can become a white dwarf, a neutron star, a black hole or explode and transform into a supernova.

Each year the Sun loses about 1020 grams of matter that are expelled by the solar wind.

Sun wind
Solar wind. Credit: Image obtained with a NASA optical telescope.

In the most massive stars this loss is greater from the beginning. Thus, a star with an initial 120 solar masses and metallicity equal to that of the Sun will end up expelling more than 90% of its mass in the form of a stellar wind and will end its life with less than 10 solar masses.

The death of a Star

When the star dies, in most cases, it transforms into a planetary nebula or a supernova, by which even more matter is expelled into interstellar space.

The ejected matter includes heavy elements, produced in the star, which will later form new stars and planets, thus increasing the metallicity of the Universe.

Star classes

Stars are classified into two large groups, according to their wealth in metals. Those with the greatest abundance of metals are called population I; while the metal-poor stars are part of population II.

Normally the metallicity is related to the age of the star. Younger stars have more heavy elements.

Evolution stars
Stars. Evolution of Stars. Credit: youtube.

Another stellar classification is the one that was already made by Hipparchus of Nicea and transmitted by Ptolemy, in a work called Almagest.

This system classified stars according to the intensity of their apparent brightness, seen from Earth.

Hipparchus established a scale of brightness of the stars. The brightest are classified as the first magnitude and the least bright, those that are almost invisible to the human eye, are those of the sixth magnitude.

Hipparchus of Nicaea (120-150 BC). Credit: Wikipedia

The modern classification is based on the spectrum of the detected light.

The classification called HD (by its author, Henry Draper, Harvard) distinguishes stars according to their light spectrum and their surface temperature.

A simple measure of this temperature is the star’s color index.

The classification is W, O, B, A, F, G, K, M, L and T going from higher to lower temperature.

  • Stars of type W, O, B, and A are very hot,
  • and those of type M, L, and T are considerably cooler.
  • Stars W and O are blue,
  • while stars with a lower surface temperature (classes K, M, L, or T) are reddish, such as Betelgeuse or Antares.

The classification based on the Yerkes Observatory catalog (made in 1943) is based on the luminosity class.

In the Yerkes classification the stars are distinguished: luminous supergiants, supergiants, luminous giants, giants, sub-giants, dwarfs (including the Sun), sub-dwarfs and white dwarfs.

Hubble Space Telescope image of NGC 6397. Credit: HST / NASA / ESA
  • About 10% of all stars are white dwarfs;
  • 70% are M-type stars,
  • 10% are K-type stars,
  • and 4% are G-type stars like the Sun.
  • Only 1% of the stars are of higher mass and types A and F.

Brown dwarfs, projects of stars that were left half because of their small mass, could be very abundant but their weak luminosity prevents a proper census.

Binary star systems

It is common for two, three or more nearby stars to be trapped together by their gravitational forces.

About 90% of the very massive stars in the Milky Way belong to binary systems.

And only 50% of low-mass stars form binary systems.

Evolution stars
Stars. Evolution of Stars. Credit: youtube.


Orion constellation
Orion constellation is easy to spot Credit: web “”

The brightest star in the constellation Orion is called Rigel and it is located on the supposed left foot of the Orion hunter figure.

It is located about 773 light years from Earth and its brightness is equivalent to 40,000 times that of the Sun.

Rigel, a bluish-white supergiant star, is actually a triple system in which the main star is orbited by two companions: Rigel ß and Rigel C that revolve around Rigel A.

The star Castor, of the constellation Gemini is actually a 6-star system, only discernible with powerful telescopes.

Star B in the constellation Unicorn is an impressive triple star system that forms a triangle discovered by William Herschell in 1781.

Sirius is the brightest star in the sky. Around it, a white dwarf star called Sirius ß orbits.

The study of binary stars is said to be key in understanding stellar evolution.

The Milky Way

The stars are not evenly distributed in the Universe, despite what it may seem to the naked eye, but are found grouped in galaxies that contain hundreds of billions of stars grouped, most of them, in the narrow galactic plane.

Milky Way
Actually astronomers are mapping the arms of the Milky Way. Credit: web “

More than 100,000 galaxies are known. One of them is the Milky Way, at one end of which our Sun is located.

Other times, stars are grouped into so-called stellar clusters, which are large concentrations ranging from tens to hundreds of thousands of stars.

In the Milky Way there are clusters that contain hundreds of thousands to millions of stars. Such is the case of the cluster called NGC 3603, of the Doradus cluster in the Large Magellanic Cloud.

Most of the characteristics of stars are usually measured using solar magnitudes as standards. The mass of the Sun is 1.9891 × 1030 kg. The masses of the other stars are measured in solar masses abbreviated as Msol.

Nebulae are cradles of stars

Star NGC 6357
Image of the nebula NGC 6357, cradle of Stars. Credit:  Observatory website, on November 18, 2012

These beautiful images were obtained with the Hubble Space Telescope.

Looking like a Gothic cathedral, the energetic stars in the center seem to explode and illuminate the entire nearby space.

The nebula NGC 6357 is the cradle of new stars and includes to the open cluster Pismis 24, which is home to several massive stars and is home to the star Pismis 24-1, which was listed as one of the most massive stars known (about 300 solar masses).

Recently, it was discovered that Pismis 24-1 is not a single star, but is at least a six-star system. This star is the brightest object seen in the image, just above the gas front.