Most stars in the sky, except the brightest, appear white or bluish to the naked eye because they are too dim for color vision to work.
Red giant stars are cooler and redder than red dwarf stars.
From the moment a star is born, until the hydrogen in its core is depleted, practically ninety percent of the life of that star passes.
During this phase of its life, the star shines less than the Sun.
The vast majority of the stars in the universe are dwarf stars and represent “normality” in stellar astrophysics.
First steps in star rating?
Until 1980, “astronomy” consisted of the study of “position and movements” of the celestial bodies.
Edward Pickering set out to go one step further and come to know the nature of the stars. He set out to discover the physical composition of the stars. That was already “astrophysics.“
This visionary professor began by placing a prism on the objective of the telescope, in order to obtain the light spectra of the stars.
This elemental technique had already been devised by William Hershel in 1798. Thus he made the first descriptions of the spectra of two well-known stars: Sirius and Arthur.
Soon after, in 1814, Joseph Fraunhofer began to study the lines that appeared in the spectrum of the Sun.
In 1861, Gustav Kirhhoff and Robert Bunsen used the lines discovered by Fraunhofer to identify the chemical elements in the solar atmosphere.
In 1862, Lewis Rutherford had obtained the first spectral plates of starlight.
In 1867, the Jesuit Angelo Secchi, made a classification of the stars, based on the chemical elements that their light spectra showed.
Star rating at Harvard
In 1881, at the Harvard Observatory, Edward Pickering had accumulated photographic plates with the most detailed stellar spectra captured up to that date.
Professor Pickering decided to offer temporary employment to his young housekeeper, Williamina Fleming, to work on sorting the material.
As the astute and clairvoyant professor expected, once her intelligence was at the service of an attractive cause, Williamina Fleming worked tirelessly and efficiently.
In this early stage, she identified and classified the spectra of more than 10,000 stars.
In 1886, the widow of Henry Draper, a pioneer in obtaining photographs of star spectra, decided to fund the work of the Harvard Observatory.
Edward Pickering did not waste a single moment. His first experience with a smart woman couldn’t have been better, so he hired nine other women.
He tasked them with performing routine calculations to analyze the photographs of the stars and classify the spectra recorded on the photographic plates.
This was undoubtedly a more challenging job for these young women than cleaning in a house or working in a factory.
Women were trained in the women’s universities in the area; and the team, led by Williamina Fleming, began to stand out for its efficiency and sagacity.
It was a wonderful team, and these young women became known as “Harvard computers.“
New star rating system
Williamina Fleming helped to develop a star assignment system, which basically consisted of assigning the star a letter, which depended on the amount of hydrogen observed in its spectrum.
The stars classified with the letter A were almost entirely made up of hydrogen, those classified with the letter B contained less hydrogen, and so on, 16 types of stars, from A to N.
Professor Pickering did not hesitate to make public recognition of her authorship and it is the basis of the spectral classification in use today: Harvard Classification.
Improvements to the Harvard ranking system
The system devised by Williamina Fleming served as the basis of work to develop a classification of stars based on the observed temperature.
In 1896, Annie Cannon joined the Harvard Computer team. She was commissioned to continue with the stellar classification of the Southern Hemisphere.
In an attempt to make improvements and streamline work on the cataloging system, Annie Cannon established classification rules based on the temperature of the stars.
These substantial advances in spectral classification are the basis of the system currently used.
In 1906, the Danish astronomer Ejnar Hertzprung suggested that the reddest stars, assigned as K and M in the Harvard Classification scheme, be divided into two groups:
- Giant stars, those that were much brighter than the Sun
- Dwarf stars, those that shone much fainter than the Sun.
Classification of dwarf stars.
The group of dwarf stars was later divided into seven subgroups:
- Red dwarfs: they are low-mass stars during their evolution.
- Yellow dwarfs: their masses are comparable to that of the Sun.
- Orange dwarfs: they are stars with a mass slightly greater than that of the Sun.
- Blue dwarf is a hypothetical class of very low mass stars that increase in temperature as soon as they reach the end of their life.
- White dwarfs: they are stars made up of electrons, which are in the final stage of their evolution. They don’t have enough mass to collapse into a neutron star or to explode as a supernova.
- Black dwarfs: these are white dwarfs that have cooled so much that they no longer emit any visible light.
- Brown dwarfs: they have little mass, less than 0.08 solar masses. This small mass is not enough to cause the fusion of hydrogen into helium.
Stars do not remain in their dwarf state for life, but instead become giants, although, in the course of their evolution, they may eventually revert to a white dwarf state.
The Sun, currently a dwarf star, will be a red giant in five billion years, and in another half a billion years it will be a dwarf again, this time a white dwarf.
The group of dwarf stars is technically called “luminosity class V” stars.
Red dwarf is a small and relatively cool star.
This type is formed by most of the stars, their mass and diameter values being less than half those of the Sun and a surface temperature of less than 4,000º K.
According to some estimates, red dwarfs represent three-quarters of the stars in the Milky Way; but, due to their low luminosity, they cannot be easily observed.
From Earth, none are visible to the naked eye. Proxima Centauri, the closest star to the Sun, is a red dwarf, as are twenty of the thirty closest stars.
Red dwarfs with less than 0.35 solar masses develop very slowly, harboring a constant luminosity and spectral type, so – in theory – their fuel will take a few billion years to run out.
White dwarf is a stellar remnant that is generated when a star of mass less than 10 solar masses has exhausted its nuclear fuel, and has expelled much of this mass in a planetary nebula.
White dwarfs are, along with red dwarfs, the most abundant stars in the universe.
97% of the stars that we know, including the Sun, go through this stage of stellar evolution.
Brown dwarfs are believed to be failed stars, as they contain the same materials as a star like the Sun, but with too little mass to shine.
Using Jupiter as a comparison, the brown dwarf is 10 times more massive, the low-mass star is 100 times more massive, and the Sun is approximately 1,000 times more massive.
The first verified brown dwarf was Teide-1, in 1995, at the Teide Observatory, in the Canary Islands.
The mass of this dwarf star is 25 times that of Jupiter. Canarian researchers referred to it as a superplanet.
María Teresa Ruiz, on March 15, 1997, was able to make a very important contribution to Astronomy.
Her gaze met an object she wasn’t looking for.
She at first she did not know what this object was. It didn’t look like a star; It could be a giant planet, a super Jupiter, or a brown dwarf.
Ultimately, it turned out to be a system of two brown dwarfs located in the southern constellation Hydra, approximately 61 light-years from Earth.
This object discovered by Dr. María Teresa Ruiz has been called Kelu-1 brown dwarf object.
The image that María Teresa obtained on March 15, 1997, she made through an infrared filter, with the 3.6-meter telescope, at the La Silla Observatory.
Brown dwarfs occupy the mass range between the heaviest gas giant planets and the lightest stars.
The mass of the largest brown stars is between 75 and 80 times the mass of Jupiter.
Nuclear fusion occurs in the youth of the star, but the atomic fuel disappears quickly, and its nuclear reaction cannot withstand the immense gravitational collapse.
Brown dwarfs continue to glow for a time due to residual heat from reactions and the slow contraction of the matter that forms them. But, they cool down until they reach an equilibrium.
Stars are classified by spectral class, with brown dwarfs being designated as M, L, T, and Y types.
Despite their name, brown dwarfs come in different colors: magenta, orange, or red.
Brown dwarfs are not very luminous at visible wavelengths.