Classification of the stars

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This article deals with the classification of stars in astronomy , various classifications are briefly listed below and, if necessary, dealt with in detail.

introduction

The classification of stars has long been an important part of astronomy. With improved technology, the stars could be divided into more and more precisely defined categories. In modern astronomy, two properties of stars play a decisive role: on the one hand, the absolute brightness (and closely related to it, the luminosity class ), on the other hand, the spectral class . These two properties are shown in the so-called Hertzsprung-Russell diagram , according to which the stars are further categorized according to their stage of development . Usually stars are classified according to the MK or Yerkes classification based on spectral class and luminosity class. The longest phase during which a star exists, the so-called main sequence phase, is also important . Our sun is currently lingering in the main row .

The spectral class can be clearly demonstrated experimentally on the basis of the spectral lines , while the absolute brightness is much more difficult to measure, which is mainly due to the difficult distance to be determined. The solution to this problem is to derive the absolute brightness or the luminosity also indirectly from the star spectrum . This is the so-called luminosity class of the MK system. From a physical point of view, the spectral class depends on the surface temperature of the star, while the luminosity class depends on the surface gravity of the star.

Some other properties, in combination, lead to a multitude of phenomena and star classes described. Many of these properties can be combined as desired and therefore usually do not lead directly to a statement about the star system being investigated, but, as already mentioned, only in the sum of the properties. Since not all phases of stellar evolution are fully understood, these are partly in an unclear relationship to one another, since many categories are either phenomenologically and / or theoretically founded. Some examples of these additional properties:

MK or Yerkes classification

The MK classification - after the first letters of the surnames of William Wilson Morgan and Philip C. Keenan , who first developed the system - also known as the Yerkes classification - after the Yerkes observatory they both worked on - and called the MKK system Introduced in 1943 by William Wilson Morgan, Phillip C. Keenan, and Edith Kellman.

This is a two-part classification scheme, which is composed of spectral types and the luminosity classes, the luminosity class being closely linked to the absolute brightness. The two parts, especially the spectral types, can also be used individually to classify stars.

Spectral types by spectra

The spectral classes represent different ranges of surface temperatures. The classification is based on spectral lines (absorption and emission lines ) in the spectra of the stars. The presence of spectral lines is directly related to the surface temperature of a star, as different elements can be ionized depending on the temperature . It has become common practice to designate the spectral classes O to A as early spectral classes , F to G as middle spectral classes and the others as late spectral classes . The designations early, medium and late come from the now outdated assumption that the spectral class says something about the stage of development of a star. Despite this erroneous classification, these terms are still in use today, and a star is considered sooner or later if its spectral class is closer to class O or class M compared to that of another.

In order to be able to classify the stars more precisely, the spectra are still graded in the individual classes from 0 to 9 (M0 is hotter than M9). With increasingly better instruments, it was possible to differentiate more finely over time, so that intermediate classes were defined, for example there are now even three additional classes between B0 and B1, called B0.2, B0.5, and B0.7. The spectral classes with their seven basic types (O, B, A, F, G, K, M) make up around 99% of all stars, which is why the other classes are often neglected.

class Characteristic colour Temperature in K typical mass
for main series ( M ☉ ) 		Example stars
O ionized helium (He II) blue 30000-50000 60 Mintaka (δ Ori), Naos (ζ Pup)
B. neutral helium (He I), Balmer series hydrogen blue White 10000-28000 18th Rigel , Spica , Achernar
A. Hydrogen, calcium (Ca II) white (slightly bluish) 07500 09750 03.2 Wega , Sirius , Altair
F. Calcium (Ca II), appearance of metals White yellow 06000- 07350 01.7 Prokyon , Canopus , Pole Star
G Calcium (Ca II), iron and other metals yellow 05000- 05900 01.1 Tau Ceti , Sun , Alpha Centauri A
K strong metal lines, later titanium (IV) oxide orange 03500- 04850 00.8 Arcturus , Aldebaran , Epsilon Eridani , Albireo A
M. Titanium oxide Red orange 02000- 03350 00.3 Betelgeuse , Antares , Kapteyns Stern , Proxima Centauri

Spectral classes outside the standard sequences

Some objects cannot be divided into the seven standard sequences and are still assigned a spectral class. These are the following:

Luminosity classes (state of development)

The luminosity class of a star is determined by properties that depend on its luminosity ; these are in particular the width and the strength (height) of the spectral lines . Giant stars, for example, have a lower gravitational acceleration in their photosphere than dwarf stars of the same temperature, which results in a lower pressure broadening of the lines, whereas the spectral class takes into account properties that primarily depend on its surface temperature.

Since the luminosity of a star in physical units depends on its mass, the size of its surface and its effective temperature, no statement about the luminosity class can be made with the value of the luminosity alone ; so z. For example, a star with about 100 times the luminosity of the sun could be a main sequence star, a sub-giant or a giant. To determine the luminosity class, you also need to specify the spectral class. Is this z. B.  M0 , a star with a hundredfold solar luminosity would be a red giant , the complete classification in the MK system (see below) would be M0III .

Luminosity class Star type
0 Hypergiant
I. Super giant
Ia-0, Ia, Iab, Ib Subdivision of the supergiants according to decreasing luminosity
II bright giant
III "Normal" giant
IV Sub giant
V Dwarf (main sequence star)
VI or sd (prefix) Subdwarf
VII or D (prefix) White dwarf

The luminosity class indicates the stage of development of a star, of which a star goes through several in its life.

When the “birth process” of a star is complete, it is usually a main sequence star (V). If its chemical composition deviates significantly from that of the other stars, in such a way that its atmosphere contains significantly fewer metals , this star can also be classified as a subdwarf (VI). In the case of the hot stars, with the spectral classes O and B, the main sequence is even more thick and also includes the luminosity classes IV and III. This is due to the fact that the massive stars there have a non-convective outer shell, so that the metallicity has a greater influence on the energy transport via the opacity.

Prefixes and suffixes

In the event of deviations from the defined standard, prefixes and suffixes help to make the classification more precise (see →  Spectral class # prefixes and suffixes ). Some of these prefixes and suffixes are obsolete due to the introduction of the luminosity class in the MK system.

Example stars classified according to MK and star class

star Spectral class Luminosity class Star class comment
Sun G2 V Yellow dwarf -
Sirius A A1 V Main sequence star of the spectral class A Overall classified as A1 Vm due to strong metal lines
Mintaka Aa1 O9.5 II Giant star of the spectral class O Brightest component in a multiple star system
Canopus F0 Ib Super giant -
Aldebaran K5 III Red giant -
Kapteyn's star M1 (sd) prefix Cool sub-dwarf -
HW Virginis B. (sd) prefix Hot sub-dwarf Double star system with a red or brown dwarf

UBV system

The UBV system is a different system than the MK system for classifying stars, with color indices taking the role of spectral class. It is a photometric system . Stand in it

  • U for the brightness in ultraviolet light with the main wavelength of 365  nm
  • B for the brightness at 440 nm (blue)
  • V for the brightness at 550 nm (yellow); V stands for visual , since the human eye perceives stars most strongly in the yellowish area.

Based on these reference values, three color indices are formed in the UBV system: UB, UV and BV, the latter being of greater importance for visual observers and e.g. B. is often given in star catalogs.

Examples

star (BV) color index Spectral class colour
Spica −0.23 B1 blue
Rigel ± 0.00 B8 bluish white
Deneb +0.09 A2 White
Sun +0.65 G2 yellowish
119 Tauri +2.06 M2 deep red

Star catalogs

Star catalogs are used to list the large number of stars according to various properties in book form or to save them in databases . The most important of these parameters are:

Star catalogs have different purposes. There are extensive catalogs with data from a survey and millions or more stars (such as the Tycho-2 catalog or Gaia DR2 ). But there are also specialized fundamental catalogs with the data of selected stars over extremely long periods of time. Another example of a special catalog would be the General Catalog of Variable Stars .

Star classes

In addition to the systematic classification of stars according to the MK system, there are also a large number of so-called star classes or star categories . These star classes follow different classification schemes and are usually defined in a star catalog or a database. A star can also belong to several star classes or be composed of several star classes in a binary star system, one of which describes the binary star system itself (e.g. AM Herculis star ).

Here are a few examples of the types of star classes:

  • Types that can already be easily described in the MK system such as yellow dwarf or red giant
  • Some star classes are based on the spectral class and a suffix , such as the Be stars or the Am stars
  • Others have a color in their name (e.g. blue stragglers ): the course is similar to that of the spectral class
    • blue star classes have a high surface temperature
    • yellow star classes comparable to the sun
    • red star classes a low surface temperature for a star
  • Many other star classes are based on a prototype that originally defined this star class. This is especially the case with variable stars . One does not have to assume, however, that a similar name describes a related star class - usually the prototype of several star classes is simply located in the same constellation . Due to the systematic name, the star classes based on it have the same name, although there is no further connection (e.g. W-Virginis-Star and HW-Virginis-Star , see also →  Naming variable stars ).

Populations (metal abundance)

With the help of the metal abundance, stars are also classified into populations, which enables conclusions to be drawn about their age. Populations roughly correspond to the time a star was formed, as the metals in galaxies continue to accumulate in the course of nucleosynthesis . In galaxies other than the Milky Way , such populations may be defined differently than in the Milky Way, for example, in the Magellanic Clouds, all stars are metal poor compared to the stars in the Milky Way. The stars are roughly sorted as shown in the following table.

class Assignment
Extreme population I Metal-rich newly formed stars.
Population I. Stars with solar metal abundance, typically a few billion years old.
Population II Stars with low metal abundance, from the time the Milky Way was formed.
Population III Postulated population of stars without metals, from the beginning of the universe. Although there must obviously have been Population III stars, no such stars are observed today. From this one concludes that Population III consisted only of relatively massive and therefore short-lived stars.

History (previous classifications)

Already in Babylonian astronomy - adopted by the Greek astronomer Hipparchus - stars were classified according to the so-called "size class" (also called "magnitudo") based on their apparent brightness as they can be observed from Earth . This free-eyed scale ( stars 1st to 6th magnitude ) was defined and expanded strictly logarithmically in 1850. Today it reaches as far as the faintest stars of 25th magnitude, which can just be resolved with the largest telescopes .

Since the apparent brightness no longer met the requirements of modern astronomy at the beginning of the 20th century, the absolute brightness was introduced as a new measure. According to it, each star is normalized to the size class that the star would appear to shine at a distance of 10 parsecs (32 light years). This energy radiation, also called luminosity , is one of the most important state variables in astrophysics and forms the basis for the classification of the stellar families in the Hertzsprung-Russell diagram (HRD).

19th and 20th centuries

The first attempts to bring order to the brightness and temperature of stars were made in 1865 by the Italian Father Angelo Secchi with a three-stage scale, and in 1874 by Hermann Carl Vogel with a system that also incorporated the theories of stellar evolution known up to that point constant changes. In 1868 Angelo Secchi developed the following four basic types:

  • Type I  : white and blue stars with a strong hydrogen line (A class)
  • Type II  : yellow stars with a weak hydrogen line, but numerous metal lines (G and K class)
  • Type III  : orange to red stars with complex bands (M class)
  • Type IV  : red stars with significant carbon lines and bands (carbon stars)

In 1878 he added another:

A new classification was developed based on extensive spectra by Henry Draper . Edward Charles Pickering began work on this in 1890, together with Williamina Fleming , Antonia Maury and Annie Jump Cannon . Pickering proceeded alphabetically and arranged the classes with capital letters from A to Z according to the Balmer series (transitions of the electron orbits in the hydrogen spectrum ). As a result of further research, this scheme was replaced by the so-called Harvard classification , which provided for a subdivision into the types AQ.

However, Annie Jump Cannon soon discovered that the order made no sense: according to the gradation, the blue and white glowing, hot O stars came after the red, relatively cool M and N stars. It also turned out that some of the classes were based only on exposure errors, or that they had no sense and could therefore be omitted. The gradation was no longer made dependent on the spectrum, but on the temperature of the stars. Based on these findings, the previous subdivision was rearranged around 1912, and the subdivision used today into the seven above-mentioned spectral classes followed.

Schematic comparison of the spectral classes O to M

Around 1950 a scale was defined from I ( supergiant ) to V (main sequence stars, formerly called "dwarfs") to classify according to luminosity. It was later supplemented by 0, Ia, Ib, VI (sub-dwarfs) and VII (white dwarfs), which ultimately resulted in the luminosity class of the MK system.

See also

Web links

literature

  • The Classification of Stars - Paperback by Jaschek & Jaschek , published by Cambridge University Press , July 1990; ISBN 0-521-38996-8 , bibcode : 1990clst.book ..... J
  • Joachim Krautter u. a .: Meyers Handbuch Weltall. 7th edition. Meyers Lexikonverlag, 1994, ISBN 3-411-07757-3 .
  • Arnold Hanslmeier : Introduction to Astronomy and Astrophysics. 2nd Edition. Spectrum Academic Publishing House, 2007, ISBN 978-3-8274-1846-3 .
  • RF Garrison: The MK Process and Stellar Classification. In: RF Garrison (Ed.): The MK Process and Stellar Classification. Proceedings of the Workshop in Honor of WW Morgan and PC Keenan , held at the University of Toronto, Canada, June 1983. David Dunlap Observatory - University of Toronto, Toronto 1984, ISBN 0-7727-5801-8 .
  • Carlos Jaschek , Mercedes Jaschek: The classification of stars. Cambridge University Press, Cambridge u. a. 1987, ISBN 0-521-26773-0 .
  • Theodor Schmidt-Kaler : Physical Parameters of Stars. In: K.-H. Hellwege (ed.): Landolt-Börnstein. Numerical values ​​and functions from natural sciences and technology. = Numerical data and functional relationships in science and technology. Group 6: Astronomy, Astrophysics and Space Research. = Astronomy, astrophysics and space research. Volume 2: Astronomy and Astrophysics, continuation and addition of Volume 1. Part volume b: K. Schaifers , HH Voigt (Ed.): Stars and star clusters. New Series. Springer-Verlag, Berlin a. a. 1982, ISBN 3-540-10976-5 .
  • (specifically on the "History" section) JB Hearnshaw: The Analysis of Starlight: One Hundred and Fifty Years of Astronomical Spectroscopy. Cambridge University Press, Cambridge (UK) 1990, ISBN 978-0-521-39916-6 .

Individual evidence

  1. ↑ The development of stars ( Memento of the original from March 5, 2016 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. ( PPT file, ~ 1.6 MB; HTML version ) - Seminar lecture at the HU-Berlin @1@ 2Template: Webachiv / IABot / ms2.physik.hu-berlin.de