A Herbig Ae/Be star (HAeBe) is a pre-main-sequence star – a young () star of

spectral type In astronomy, stellar classification is the classification of stars based on their spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting the ...

s A or B. These stars are still embedded in gas-dust envelopes and are sometimes accompanied by

circumstellar disks A circumstellar disc (or circumstellar disk) is a torus, pancake or ring-shaped accretion disk of matter composed of gas, dust, planetesimals, asteroids, or collision fragments in orbit around a star. Around the youngest stars, they are the reser ...

Hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-to ...

and

calcium Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar ...

emission lines are observed in their spectra. They are 2-8

Solar mass The solar mass () is a standard unit of mass in astronomy, equal to approximately . It is often used to indicate the masses of other stars, as well as stellar clusters, nebulae, galaxies and black holes. It is approximately equal to the mass ...

() objects, still existing in the

star formation Star formation is the process by which dense regions within molecular clouds in The "medium" is present further soon.-->interstellar space

(gravitational contraction) stage and approaching the

main sequence In astronomy, the main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar He ...

(i.e. they are not burning hydrogen in their center).

Description

In the

Hertzsprung–Russell diagram The Hertzsprung–Russell diagram, abbreviated as H–R diagram, HR diagram or HRD, is a scatter plot of stars showing the relationship between the stars' absolute magnitudes or luminosities versus their stellar classifications or effective te ...

, Herbig Ae/Be stars are located to the right of the

. They are named after the American astronomer George Herbig, who first distinguished them from other

star A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...

s in 1960. The original Herbig criteria were: *

Spectral type In astronomy, stellar classification is the classification of stars based on their spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting the ...

earlier than F0 (in order to exclude

T Tauri star T Tauri stars (TTS) are a class of variable stars that are less than about ten million years old. This class is named after the prototype, T Tauri, a young star in the Taurus star-forming region. They are found near molecular clouds and ide ...

s), * Balmer emission lines in the stellar spectrum (in order to be similar to

s), * Projected location within the boundaries of a dark

interstellar cloud An interstellar cloud is generally an accumulation of gas, plasma, and dust in our and other galaxies. Put differently, an interstellar cloud is a denser-than-average region of the interstellar medium, the matter and radiation that exists in the ...

(in order to select really young stars near their birthplaces), * Illumination of a nearby bright

reflection nebula Reflection or reflexion may refer to: Science and technology * Reflection (physics), a common wave phenomenon ** Specular reflection, reflection from a smooth surface *** Mirror image, a reflection in a mirror or in water ** Signal reflection, in ...

(in order to guarantee physical link with

star formation Star formation is the process by which dense regions within molecular clouds in The "medium" is present further soon.-->interstellar space

region). There are now several known isolated Herbig Ae/Be stars (i.e. not connected with dark clouds or nebulae). Thus the most reliable criteria now can be: *

earlier than F0, * Balmer emission lines in the stellar spectrum, *

Infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of Light, visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from ...

radiation excess (in comparison with normal stars) due to

circumstellar dust Circumstellar dust is cosmic dust around a star. It can be in the form of a spherical shell or a disc, e.g. an accretion disk. Circumstellar dust can be responsible for significant extinction and is usually the source of an infrared excess for s ...

(in order to distinguish from classical

Be star Be stars are a heterogeneous set of stars with B spectral types and emission lines. A narrower definition, sometimes referred to as ''classical Be stars'', is a non-supergiant B star whose spectrum has, or had at some time, one or more Balmer e ...

s, which have infrared excess due to free-free emission). Sometimes Herbig Ae/Be stars show significant brightness variability. They are believed to be due to clumps (

protoplanet A protoplanet is a large planetary embryo that originated within a protoplanetary disc and has undergone internal melting to produce a differentiated interior. Protoplanets are thought to form out of kilometer-sized planetesimals that gravitation ...

s and

planetesimal Planetesimals are solid objects thought to exist in protoplanetary disks and debris disks. Per the Chamberlin–Moulton planetesimal hypothesis, they are believed to form out of cosmic dust grains. Believed to have formed in the Solar System ...

s) in the circumstellar disk. In the lowest brightness stage the radiation from the star becomes bluer and linearly polarized (when the clump obscures direct star light, scattered from disk light relatively increases – it is the same effect as the blue color of our sky). Analogs of Herbig Ae/Be stars in the smaller mass range (<2 ) – F, G, K, M

pre-main-sequence stars – are called

s. More massive (>8 )

s in pre-main-sequence stage are not observed, because they evolve very quickly: when they become visible (i.e. disperses surrounding circumstellar gas and dust cloud), the hydrogen in the center is already burning and they are

main-sequence In astronomy, the main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar ...

objects.

Planets

Planets around Herbig Ae/Be stars include: * HD 95086 b around an A-type star * HD 100546 b around a B-type star

Gallery

A stellar fingerprint.jpg, IRAS 12196-6300 is located just under 2300 light-years from Earth. V1025 Tauri Taurus Molecular Nebula from the Mount Lemmon SkyCenter Schulman Telescope courtesy Adam Block.jpg, Herbig Ae/Be Star V1025 Tauri from the Mount Lemmon SkyCenter

References

Sources

* Thé P.S., de Winter D., Pérez M.R. (1994

0 * Pérez M.R., Grady C.A. (1997)
''Observational Overview of Young Intermediate-Mass Objects: Herbig Ae/Be Stars'', Space Science Reviews, Vol 82, p. 407-450
* Waters L. B. F. M., Waelkens, C. (1998)
''HERBIG Ae/Be STARS'', Annual Review of Astronomy and Astrophysics, Vol. 36, p. 233-266Herbig Ae/Be stars
* {{DEFAULTSORT:Herbig Ae Be star Star types Star formation 1960 in science