What's the densest object in the universe? The brightest? The loudest? In his new book Extreme Cosmos (Perigee, 2012), astronomer Bryan Gaensler reveals the cosmic record holders of these and many other titles. In an excerpt below, from the chapter "Extremes of Temperature," Gaensler explains the physics behind some of the hottest stars known:
We all know that if you heat something up, it glows. A poker in a fire shines a dull orange or red, while a conventional (incandescent) lightbulb works by heating up a tungsten filament to several thousand degrees so that it glows yellow or white. These are special cases of a universal process first properly explained by German physicist Max Planck: Virtually every object (whether on Earth or in space) radiates light, and the color of this light is tied to the object's temperature.
We can see this effect, known as "Planck’s law of black body radiation," in action whenever we look at the different colors of stars. Our Sun is a reasonably average star. Its surface temperature of 9,900 degrees F results in a yellowish light, just as Planck's equations predict.
Betelgeuse, a bright star in the constellation of Orion, is much cooler, about 6,900 degrees F, and so even to the naked eye has an easily identified red hue. The brightest star in the night sky, Sirius (also known as the "Dog Star"), has a surface temperature of about 18,000 degrees F, which gives it its bluish tinge.
But there are other stars, invisible to the naked eye, which are far hotter than Sirius. As we'll see a little later in this chapter, the real action is happening deep within a star's core, where the fury of nuclear fusion generates all a star's heat and light for up to billions of years. But when a typical star finally exhausts all its fuel, it puffs off most of its outer layers into a slowly expanding shell of gas, exposing the central core. This core, a small dense ball of helium, carbon, and heavier elements, is no longer burning any gas via nuclear fusion, but is still incredibly hot. This dying ember, known as a “white dwarf," is now among the hottest stars in the Universe, so hot that it lights up the surrounding shroud of expelled gas to form an exquisite glowing cloud known as a “planetary nebula.”
So just how hot is a newly formed white dwarf? The current record holder sits at the heart of a beautiful planetary nebula. This glowing gas cloud, referred to by astronomers as "NGC 6537" but more commonly known as the "Red Spider Nebula," is about 2,000 light-years away toward the constellation of Sagittarius. (One light-year is the distance you can travel in one year if you move at the speed of light, a total of just under 6 trillion miles. So 2,000 light-years is around 12,000 trillion miles!)
Throughout the 20th century, the central white dwarf in the Red Spider Nebula eluded detection, and its temperature remained unknown. There are two reasons why such stars are so hard to see. First, they are tiny objects buried at the very centers of glowing, luminous, surrounding clouds. Often the brightness and complexity of a planetary nebula hides its central star from view.
But the other reason is that, paradoxically, the star's extreme heat itself makes the star almost invisible. As we saw above, Planck's law of black body radiation dictates that an object's temperature determines its color. Sirius, with its surface at a temperature of 18,000 degrees F, is so hot that it glows blue.