Hypergiants, OF Stars and Wolf-Rayets
Example: Eta Carinae
a photograph of Eta Carinae, a blue hypergiant star. (Photo
credits: NASA/HST: J. Morse (Arizona
State U.), K. Davidson (U. Minnesota) et al., WFPC2, HST, NASA).
Eta Carinae (Tseen She, lit. ‘Heaven’s Altar’, or Foramen) is a blue hypergiant star and an LBV (large
blue variable) star some 7500-8000 light years from Sol situated in the Homunculus Nebula which was created from matter blasted off from the star. It is a variable (and probable) binary star and it is the most massive and most luminous star in the Milky Way Galaxy. In 1843 Eta Carinae brightened by a factor of at least ten (to magnitude -0.8), emitting almost as much light as a supernova and becoming the second brightest star in the sky, but it survived this eruption (called a pseudo-supernova). At its observed dimmest, between 1900 and 1940, it became magnitude 8. At the moment its magnitude is currently below 5 and so it can be seen with the naked eye. The star underwent an X-ray eclipse in 2003, most likely due to the dynamics of wind-wind collision with a companion star. These eclipses follow a 5.52 year cycle.
Eta Carinae is now thought to be a binary system, with the main larger blue hypergiant star possessing an initial mass of about 120 to 150 solar masses, before blasting off much of its material to reduce its mass to around 100 solar masses. The combined mass of the system is about 130 solar masses, with an optically invisible companion star of about 30 solar masses hidden by the cloud of material blasted off the primary star. This lighter companion star may be a Wolf-Rayet star.
Wolf-Rayet stars are very hot and luminous stars with a surface temperature of about 90 000 K with strong and broad emission lines of ionised helium, carbon, oxygen and nitrogen. These ionisation lines probably come from expanding atmospheres, expanding at 40 to 3000 km/s as the star rapidly loses mass. Wolf-Rayets are probably the centres of initially much more massive OF stars.
Eta Carinae. (Photo credits: NASA/HST: J. Morse (Arizona State
U.), K. Davidson (U. Minnesota) et al., WFPC2, HST, NASA).
remain: is there an upper mass limit to stars, and if so, then
what is it? How massive can a star be before it is too unstable
to survive even by rapid mass loss? There are only about a dozen
hypergiant stars in a typical large spiral galaxy. VY Canis
Majoris (VY CMa) holds the current record as the largest star known to terrestrial
observers and also one of the most luminous. It is about 2000
times the diameter of the Sun, 4900 light-years from Earth and
is red hypergiant (or perhaps a red supergiant or the supergiant
equivalent of a hypergiant star!)and a solitary star. It is
thought to have been an O star of about 30 - 40 solar masses
before it left the main sequence and is predicted to go
hypernova in about 100 000 years time. It's present spectral
class is M, with a surface temperature of about 3000 K.
The most massive star known (to terrestrial observers) is R136a1, another blue hypergiant ,near the Tarantula Nebula in the Large Magellanic Cloud, with an estimated mass of 265 - 320 solar masses (over the years the maximum size of stars keeps increasing with new discoveries!). It stole the title of most luminous star (at one time attributed to Eta Carinae) with a luminosity some 8.7 million times that of the Sun! Supermassive stars are hypothetical stars with a mass in excess of 1000 solar masses - no star has yet claimed this title!
Credits: NASA , ESA , N. Smith (University of Arizona), and J. Morse (BoldlyGo Institute) 2019.
Above: imaged in the ultra-violet, showing warm magnesium-rich gas (blue) and shock-heated nitrogen (red). https://hubblesite.org/contents/media/images/2019/18/4505-Image?keyword=Nebulas&news=true
Article updated: 8 April 2020