In the early 20th century two astronomers separately developed diagrams to identify classes of stars based on parameters such as luminosity and surface temperature. They got together and a unified diagram, called the Hertzsprung-Russell (H-R) diagram came into being. It makes a pretty wall poster too. The stars I’m about to describe are called Supergiant stars. They have their own niche on the H-R diagram.
Supergiant stars come in flavors (colors): Blue, White, Yellow, and Red… as do main sequence stars. But that’s where the similarity ends. Supergiant stars are stars living on the edge, on the edge of existence. Stars like our Sun live a nice long placid life, until near their end. But even their red giant phase looks placid by comparison.
Blue/white supergiant stars are born in molecular clouds of hydrogen, just how the Sun was born, but from the get-go they are way different. Their birth cloud is wracked by the baby blue supergiant’s extreme mass, like an 18-pound baby from a 120-pound mother! This massive accumulation of hydrogen becomes a star with attitude as it blows away any remaining stuff surrounding it. Not much can survive within a billion miles of this beast. They can have 300 times the mass and 100 times the diameter of the Sun. They are highly luminous and fusing hydrogen like there’s no tomorrow. Living about 30 to 200 million years. many will end in a type II supernova, producing a neutron star or black hole. Those managing to evolve without blowing up can become a yellow and eventually a red supergiant.
Yellow and red supergiant stars are mostly evolutionary. Some start out as a blue supergiant, but most start yellow, with 8-12 times the mass of our Sun. They can fuse helium in their core where stars like the Sun cannot. This makes their evolution into supergiants more rapid than the Sun’s evolution into a red giant. Helium fusion in the core makes these stars billow into bubbles many hundreds of times the size of the Sun. Their sheer size makes them extreme. The biggest, at least in our Milky Way galaxy, is UY Scuti at 1700 times the Sun’s diameter. If it were to replace our Sun, its photosphere would nearly reach Saturn.
Many red supergiant stars are also variable. They vary in luminosity due to the push-pull gyrations of helium fusion vs gravity. Betelgeuse and UY Scuti are examples.
Another variation occurs in transitional phases, such as Wolf Rayet (WR) stars. WR stars are a normal evolutionary stage of very massive stars. They fuse helium and heavier elements in the core and have shed off their hydrogen shell. They are some of the hottest and most luminous (ultraviolet) stars and many form the central star of planetary nebulae.
What’s in the Sky?
July 11; Before sunrise: southeast and east; See Mars and a gibbous Moon high up with Venus and Aldebaran near the eastern horizon.