Stellar classification . How to classify the stars according to their brightness
Summary
[ hide ]
- 1 History
- 2 Classical spectral types
- 1 Class O
- 2 Class B
- 3 Class A
- 4 Class F
- 5 Class G
- 6 Class K
- 7 M class
- 3 Other spectral types
- 1 Class W
- 2 Class L
- 3 Class T
- 4 Class C (R, N)
- 5 Class S
- 6 Classes P and Q
- 7 Class D
- 4 Brightness classes
- 5 Source
History
The oldest classification of stars is Hipparchus of Nicaea, which cataloged the stars around [[134 ane] according to their brightness, assigning magnitude 1 to the 20 brightest stars in the sky and magnitude 6 to those that were barely visible naked eye. With the invention of the telescope , the detection of stars invisible to the naked eye was made possible, which is why there are currently stars with a magnitude greater than 6. This classification scheme was adopted by Claudio Ptolemy and is still used even though it has been superseded by spectral types.
Classic spectral types
In the late 1880s, the widow of astronomer Henry Draper , Anna Palmer Draper , donated a significant sum of money to the Harvard College Observatory for the Henry Harper Memorial for the purpose of classifying the photographic spectrum of stars. The work was carried out by Edward Pickering and his assistant William Fleming , who developed a simple classification system by ordering the stars according to the intensity of the hydrogen spectral line and identifying 22 types from A to Q in decreasing order.
In later work to perfect the previous classification developed at the Harvard Observatory, the influence of other spectral lines was appreciated (H&K lines for calcium , D lines for sodium , etc.), revealing the duplication of some classes. Later, Annie Jump Cannonobserved that the temperature of the stars was the main differentiating characteristic of the different spectra, which led him to simplify the existing classes and reorganize them based on temperature, giving rise to the classic types O, B, A, F, G, K and M —a classification provisionally accepted by the IUA in 1922— to which later some special types have been added. The classification is completed by assigning within each class Arabic numerals (0 to 9) according to decreasing temperatures.
The classic types hottest to coldest are shown in Table comparing their masses, radii and luminosities with those of the sun . The colors in the table reflect those of the stars without the influence of the atmosphere.
Class O
They are very hot and luminous stars standing out in brilliant blue colors. Naos (in the constellation Puppis ) shines with a power close to a million times that of the Sun. These stars show very prominent lines of ionized and neutral helium and have faint Balmer lines (of hydrogen). They emit most of their radiation in the ultraviolet.
Class B
Extremely bright, like Rígel in Orion , a blue supergiant. The spectra of these stars show neutral helium lines and moderate hydrogen lines . Because stars O and B are so massive, they consume their energy much faster than other smaller stars, releasing huge amounts of energy and living for a short period of time of a few million years. At this time, they cannot stray too far from the star-forming regions in which they are born, so they usually appear in groups of several stars in what is known as OB1 associations, formed inside giant molecular clouds. The Orion OB1 association is the closest example.
A class
They are the most common stars that we observe with the naked eye. Deneb in the Swan is a very bright star while Sirius , the brightest star from Earth, is also a very close A-type star but not as large as Deneb. Class A stars show pronounced Balmer lines (of hydrogen) and also lines of ionized metals.
Class f
They are still very bright, high-mass stars, but they already belong to the main sequence. As an example you can consider Fomalhaut in Piscis Australis . Their spectra are characterized by weak Balmer lines and ionized metals. They are white with a slight yellow component.
Class g
They are the best known since our Sun belongs to this class, being a G2-type star. They show hydrogen lines even weaker than F’s and have ionized and neutral metal lines.
Class K
Orange stars somewhat cooler than the Sun. Some of them are giants and even supergiants like Antares, while other K stars like Alpha Centauri B belong to the main sequence. They show very faint hydrogen lines, if any, and sometimes some lines corresponding to neutral metals.
Class m
It is the most common of all by the number of stars. All red dwarfs belong to this class and more than 90% of all stars are of this type as Proxima Centauri. The M class also corresponds to most of the giants and some supergiants like Arcturus and Betelgeuse , as well as to the Mira variables. The spectrum of an M star has neutral metal and molecular lines but does not normally show hydrogen lines. The titania can be intense lines in the stars M.
Other spectral types
Class w
They are superluminous stars, with temperatures above 70,000 K, called Wolf-Rayet stars, whose peculiarity is to be formed mostly by helium instead of hydrogen. It is believed that these are supergiants at the end of their cycle in which the outer layer of hydrogen has been expelled by strong solar winds caused by high temperatures, leaving the hot helium nucleus bare. Depending on whether nitrogen or carbon predominates in its spectrum – and therefore in its outer layers – the WN and WC subtypes are distinguished respectively.
Class l
Stars with temperatures between 1500 and 2000 K. They receive their name from the lithium present in their nucleus. Since lithium is destroyed in nuclear fusion reactions in larger stars, its presence indicates the absence of such reactions, that is, they are very dark red brown dwarfs. In fact, they are so cold that they emit in the near infrared, allowing their low temperatures to clearly reveal metal hydrides and alkali metals in their spectrum.
Class T
They are stars with temperatures of the order of 1000 K, young and not very dense that are often found in the interstellar nebulae in which they formed. They are stars with just enough mass to complete their formation and if they do, they will be of the brown dwarf type. They are black, they do not emit in the visible spectrum but they do intensely in the infrared. At its temperature, complex molecules can form, showing intense methane lines in their spectra. Classes T and L could be the most frequent (even than the rest of the classes together) if recent hypotheses are confirmed. From the study of protoplanetary disks it follows that the number of stars that should be in the galaxy should be much higher than we know. It has been speculated that these discs compete with each other so that the first to form will become a proto-star that will interfere with the development of neighboring discs by attracting their gas, so that they will probably evolve according to the sequence main or remaining as virtually invisible brown dwarfs of classes L and T. Since these stars also live a long time (no stars below 0.8 [[M sol]] has died in the history of the galaxy) accumulate over time, increasing in number and proportion.
Class C (R, N)
Class C stars are carbon stars , dying red giants. Formerly, the R and N classes were considered, parallel to the G and M classes that have finally been reunited in the C class, the N0 class corresponding approximately to the C6.
Lessons
They show zirconium oxide (ZrO) lines in addition to – rarely instead of – titanium oxide (TiO). They are a class straddling the M and carbon stars. They present an excessive amount of zirconium and other chemical elements generated by the S process ( nuclear synthesis responsible for the formation of most of the elements heavier than iron) and an abundance of similar oxygen and carbon, resulting in both compounds being unavailable, forming carbon monoxide. carbon(CO). In M-class stars there is a glut of oxygen being available for the formation of TiO, while in class C there is a glut of carbon so carbon molecules can be formed. The relationship of these three types of stars and the main sequence suggests that there is actually a continuum in the stellar types of carbon abundances, which would add a new dimension to the stellar typology in addition to the temperature (the groups now described) and the brightness classes introduced in 1943 .
Classes P and Q
These classes are occasionally used to classify non-stellar objects, particularly planetary nebulae (P) and novae (Q).
Class D
Used for the classification of white dwarfs, the state in which most stars end their life. Based on the composition of the outermost layer of the dwarf’s atmosphere, five classes are considered:
- DA: Balmeri lines (hydrogen).
- DB: neutral helium .
- DO: ionized helium.
- DQ: molecular carbon in cold stars and atomic carbon in hot ones.
- DZ: rich in metals, especially calcium II.
- DC: no spectral line is strong enough to classify it in an earlier group.
- DX: spectral lines are not clear enough to fit into one of the above categories.
All classes are subdivided using Arabic numbers from 1 to 9 according to the temperature index n = 50400 / T (T: effective temperature of the star): 1 if it is above 37,500 K and 9 if it is less than 5500 K
Brightness classes
The Yerkes spectral classification or MKK system, sometimes Morgan-Keenan, came into being in 1943 with the publication of An Atlas of Stellar Spectra by astronomers William Wilson Morgan., Phillip C. Keenan and Edith Kellman of the Yerkes Observatory. The classification is based on the star’s surface gravity sensitive spectral lines that are related to its luminosity and is intended to complement the Harvard classification. Given that a giant is much larger than a dwarf being, however, their masses similar, the gravity and therefore the pressure and temperature on the surface of the giant star will be much lower than in the dwarf. These differences are also manifested in the luminosity of both bodies, affecting both the width and the intensity of the spectral lines, both of which can be measured.