Way back around 1910, Danish astronomer Ejnar Hertzsprung and American astronomer Henry Norris Russell independently found a curious pattern when they did a scatterplot correlation between stars' luminosities and temperatures.
The graph, now called the Hertzsprung-Russell Diagram in their honor, looks like this:
Most stars fall on the bright swatch running from the hot, bright stars in the upper left to the cool, dim stars in the lower right; the overall trend for these stars is that the lower the temperature, the lower the luminosity. Stars like this are called main-sequence stars. (If you're curious, the letter designations along the top -- O, B, A, F, G, K, and M -- refer to the spectral class the star belongs to. These classifications were the invention of the brilliant astronomer Antonia Maury, whose work in spectrography revolutionized our understanding of stellar evolution.)
There is also a sizable cluster of stars off to the upper right -- relatively low temperatures but very high luminosities. These are giants and supergiants. In the other corner are white dwarfs, the exposed cores of dead stars, with very high temperatures but low luminosity, which as they gradually cool slip downward to the left and finally go dark.
So there you have it; just about every star in the universe is either a main-sequence star, in the cluster with the giants and supergiants, or in the curved streak of dwarf stars at the bottom of the diagram.
Emphasis on the words "just about."
One star that challenges what we know about how stars evolve is the bizarre Stephenson 2-18, which is in the small, dim constellation Scutum ("the shield"), between Aquila and Sagittarius. At an apparent magnitude of +15, it is only visible through a powerful telescope; it was only discovered in 1990 by American astronomer Charles Bruce Stephenson, after whom it is named.
Its appearance, a dim red point of light, hides how weird this thing actually is.
When Stephenson first analyzed it, he initially thought what he was coming up with couldn't possibly be correct. For one thing, it is insanely bright, estimated at a hundred thousand times the Sun's luminosity. Only its distance (19,000 light years) and some intervening dust clouds make it look dim. Secondly, it's enormous. No, really, you have no idea how big it is. If you put Stephenson 2-18 where the Sun is, its outer edge would be somewhere near the orbit of Saturn. You, right now, would be inside the star. Ten billion Suns would fit inside Stephenson 2-18.
If a photon of light circumnavigated the surface of the Sun, it would take a bit less than fifteen seconds. To circle Stephenson 2-18 would take nine hours.
This puts Stephenson 2-18 almost off the Hertzsprung-Russell Diagram -- it's in the extreme upper right corner. In fact, it's larger than what what stellar evolution says should be possible; the current model predicts the largest stars to have radii of no more than 1,500 times that of the Sun, and this behemoth is over 2,000 times larger.
Astronomers admit that this could have a simple explanation -- it's possible that the measurements of Stephenson 2-18 are overestimates. But if not, there's something significant about stellar evolution we're not understanding.
Either way, this is one interesting object.
There's also a question about what Stephenson 2-18 will do next. Astrophysicists suspect it might be about to blow off its outer layers and turn either into a luminous blue variable or a Wolf-Rayet star (the latter are so weird and violent I wrote about them here a while back). So it may not be done astonishing us.
Puts me in mind of the quote from Richard Dawkins: "The feeling of awed wonder that science can give us is one of the highest experiences of which the human psyche is capable."
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