Yellow hypergiant

The Yellow Hyper giants are stars with absolute magnitudes of MV = -8 and effective temperatures 4000-8000 K. This corresponds to a spectral type of late A to early K and the luminosity class 0 or Ia. The Yellow hypergiants are very rare with only about a dozen well-known star in the Milky Way. The radii of the visible photosphere up to a few hundred solar radii. The Yellow hypergiants be known in English as hot hyper giants.

Properties

Yellow hypergiants show signs of strong mass loss by stellar winds, the mass losses have episodic character and can reach up to 0.05 solar masses per year. There are strong infrared excesses observed by cool zirkumstellares material which has flowed from the star and now absorbs radiation and emits again at low temperatures of 100 K. The strength of the stellar winds can often be determined from P- Cygni profiles, wind speeds were found by a few hundred kilometers per second. The stars show strong fluctuations in their surface temperatures due to the formation of pseudo- photosphere. The effluent gas in the vicinity of the atmosphere is so dense that the actual stellar surface is not accessible to the observer and can be analyzed only in the pseudo- photosphere re-emitted light.

Variability

Yellow hypergiants lie within the instability strip, and therefore belong to the pulsationsveränderlichern stars with semi- regular nature. The amplitude is usually low with amplitudes of up to 0.3 mag and pulsation periods of 300 to 1000 days. Besides enters irregular light change as a consequence of the temporary strong mass loss. The desquamated material reabsorbed radiation from the star and emits it again at low temperatures. Rho Cassiopeiae temporarily shows the spectrum of a red giant with a hyper spectral type M. The visual brightness falls during these periods from strong, because a large part of the radiation is emitted in the region of the infrared. This case episodes probably arise from the formation of negative density gradient in the outer atmosphere of the yellow supergiant. In combination with a very low gravitational acceleration in the photosphere oscillations can lead to the shedding of parts of the atmosphere. The pulsations in the star's atmosphere are the cause of the uniform mass loss by stellar winds, since they accelerate due to the large expansion of matter on the escape velocity addition.

Development

In the Hertzsprung -Russell diagram, the area of the Yellow hypergiants is almost empty and the area is therefore called yellow development gap. The state of development of the Yellow supergiants is interpreted differently.

• Either had the Yellow hypergiants during their time on the main sequence mass of about 25 to 40 solar masses. According to these sources, the Yellow hypergiants move toward lower temperatures. Probably the developing towards higher temperatures stars are unstable and are therefore not observed. The length of stay in the area of ​​the Yellow hypergiants is astronomically very short with a duration of about 10,000 years. After simulations, these stars are in the contraction of the helium nucleus state before a central helium burning is ignited.
• Others describe Yellow hypergiants as the successor of the red supergiant masses originating from more than 40 solar masses on the path to higher temperatures in the HR diagram. Accordingly, the yellow supergiant only in a short period of transition between the red supergiant on the way to the light Strengthening Blue Variables or to the Wolf -Rayet stars, which subsequently find their end in a supernova explosion.

Yellow hypergiants have been identified in type IIb supernovae SN 1993J and SN 2011dh on shots before the explosion at the site of the supernova and could no longer be detected several years after the eruption. Some yellow hypergiants seem to evolve very rapidly with an increase in the temperature of 1000 ° within 10 years in IRC 10420.

Examples

• Rho Cassiopeiae
• V509 Cassiopeiae
• V382 Carinae
• IRAS 17163-3907
• IRC 10420

Itemization

• Star class