Blue giant

A Blue Giant is a star of spectral type O or B with the 10 - to 50-fold solar mass. The luminosity of Blue giant is higher than that of main-sequence stars.

Characteristics

While a red giant reach its expansion size only in the final stage of its stellar evolution and thereby expands to many times, achieved a Blue Giant this size already in a normal stage of development. The high mass leads to a high density, high pressure and high temperature of the matter inside the star. This results in comparison to lower-mass stars, high reaction rate. The resultant release of energy causes the surface temperature of the sun is up to 30,000 to 40,000 K significantly higher than the approximately 5500 K. Due to this high temperature, the emission maximum is ( according to Wien's law for a black body ) in the ultraviolet part of the light spectrum, which explains the blue color impression of these stars and hence its name.

The absolute visual magnitude MV reaches 9.5 and is of the same order of magnitude as the integral brightness of globular clusters and some dwarf galaxies. By a wind momentum -luminosity relation, the absolute luminosity can be determined with an accuracy of 25 percent. These stars are therefore lighter distance indicators than the classical Cepheids by the period-luminosity relationship.

In contrast to the abundant lower-mass stars, which have a lifetime of several billion years, such as our sun with about 10 billion years ago, through Blue Giants their hydrogen burning phase because of the high reaction rate in only a few tens of million years. After that, they are puffed up to the Red supergiants and end in a type II supernova.

The development of Blue giant of spectral type O is strongly influenced by the presence of a companion in a binary star system. At 70 percent of the O -star companion were with circulation periods of less than 1500 days ago. This exchange binaries from during or shortly after the main sequence phase of matter and torque. 20 to 30 percent of all massive stars in binary stars will merge within a few million years. 50 percent of all O-stars either lose their hydrogen-rich atmosphere and develop, for example, in Wolf -Rayet stars or gain from her companion substantial amounts of matter.

X-ray radiation and stellar wind

X-rays are often emitted from the blue giants and supergiants and in communication with the stellar winds of hot stars. The stellar winds are radiatively driven and are a consequence of the radiation pressure. The interaction cross-section is generally higher for heavy elements and therefore these elements are more accelerated. By shocks in the stellar wind, the kinetic energy is distributed evenly, which speeds of several thousand kilometers per second can be achieved. The wind density is dependent on the chemical composition of the atmosphere of the Blue Riesens and can reach up to 10-3 solar masses per year in Wolf -Rayet stars. The X-ray radiation is produced as bremsstrahlung in an interaction of the stellar wind with the interstellar medium, shock waves in the stellar wind near the stellar surface or in the collision of stellar winds in binary systems.

Blue giants and supergiants are components in high-mass X-ray binaries. The stellar wind of the blue giant is doing accreted by a black hole, a neutron star or a white dwarf of fairly rare. The matter is accelerated by the gravitational field of the compact star in the case and produced before the surface of a shock wave in which matter is braked abruptly. In contrast to the X-rays from pure wind interaction, which is soft, the X-rays from X-ray binaries clearly energetic ( harder ). In addition to the bremsstrahlung, it also leads to burst when the hydrogen - or helium- rich material on the surface of the compact star reaches a density in the used unrestrained thermonuclear reaction.

Not due to stellar winds are the X-ray binaries, which consist of a Be star and a compact companion. Due to the high rotational speed and pulsation may formed in the plane of rotation around the loading star in a disk from the surface abgeflossenem gas. If the compact star passes through the disk, it collects on the matter and the X-ray brightness varies with the orbital period of the binary system.

Variability

Blue Giant often show variable brightness as eruptive variables and / or pulsating variables. The case of pulsating variables, the atmosphere is unstable to oscillations due to the kappa mechanism. They include the

• Slowly pulsating B stars with periods longer than one day
• Alpha Cygni stars with their non-radial oscillations
• Beta Cephei stars
• PV TELESCOPI star; they are helium - and carbon-rich variables with the spectral type Bp

While all of these classes of stars within the instability strip, there seems to be a small group of early B supergiants, which can be found just outside of the known instability strip and the line profiles are variable with periods of less than 2 hours. This is usually interpreted as a non- radial vibration, as these periods are too short for rotation modulation.

The eruptive variables with irregular light change under the blue giants and supergiants are the

• Gamma Cassiopeiae and Be stars, whose atmosphere rapidly rotating detaches matter and forms equatorial slices
• Luminous blue variables with their pseudo photospheres due to strong fluctuations in the stellar wind
• And the Wolf -Rayet stars.

Supernova and Gamma Ray Burst

Contrary to the initial expectations explode Blue Giant directly as a core-collapse supernova. The best known example is the supernova 1987A, whose progenitor star was cataloged as B- supergiant Sanduleak -69 ° with the designation 202 and is no longer detectable since the explosion. In addition to part of the supernova of type II, the atmosphere at the time of the supernova explosion is rich in hydrogen also have the supernovae of type Ib and Ic blue supergiants as progenitor. These have already lost much of their atmosphere to the interstellar medium by strong stellar winds; therefore is no longer detectable hydrogen in the spectra of these supernovae.

Part of the gamma ray bursts produced in Blue supergiant in a supernova explosion. Gamma ray bursts are extremely luminous energy releases mainly in the area of ​​gamma radiation with a duration of a few seconds or minutes at cosmological distances. They are divided into short and long hard soft gamma -ray bursts, with some days at the site of gamma -ray bursts could be detected in some of the latter, a supernova outbursts of type Ic. These bursts probably arise in a supernova, in which a high-energy jet pierces through the atmosphere and shows exactly in the direction of the earth.

Upper mass limit

Blue giants are stars with the largest observed masses of up to 250 solar masses, such as in the supergiant R136a1. Mass upper limit should be achieved when the radiation pressure in equilibrium with the pressure due to the gravitational force. This Eddington limit is only valid for a value of 60 solar masses. Many Blue giants have significantly higher masses, because at its core outweighs the convective energy transport and thus a balance even up to masses of 150 solar masses is possible. This upper mass limit is dependent on the metallicity and is valid for protostars during star formation. The high radiation pressure leads to a fast stellar wind, resulting in a mass loss of about half the original mass within 10 million years ago. Even larger stellar masses of up to 250 solar masses may arise in a binary star system only through mergers of two massive stars. The star densities required for these mergers are only in young star clusters as in R136 and the Arches star cluster.

Observations

HE 0437-5439 is a " hypervelocity star" called Blue Giant, which is observed every three and a half years by the Hubble Space Telescope, and according to NASA, since already 100 million years back put Coming 200,000 light- years from the direction of the center of our Milky Way and with 715-723 km / s - thus double escape velocity - our galaxy is leaving at the time.

Warren Brown of the Harvard- Smithsonian Center for Astrophysics, published in 2010 as the first author of the study, the discovery in the journal The Astrophysical Journal Letters. Since the star but after 20 million years ago would have had to be burned, it is believed that it originated in a triple - star system. Before about 100 million years ago, the triple - star system came very close to the black hole at the center of our Milky Way. One of the three stars is there probably have been swallowed. The other two stars had been catapulted. " The larger of these two burned faster, ballooned into a red giant and verleibte thereby its partners. "

Examples

Blue giants are also relatively rare because of their short life span.