How Hot Is the Hottest Star?

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.

What happens if a star is even hotter than blue Sirius? In such cases Planck's law still applies, but the resulting glow will be of a color beyond the range to which our eyes or ordinary telescopes are sensitive. In particular, objects much hotter than Sirius will glow in ultraviolet or X-ray light. Different temperatures, and their connection to color through the law of black body radiation, reveal that seemingly distinct phenomena such as ultraviolet light and X-rays are really just parts of the broad electromagnetic spectrum. The electromagnetic spectrum describes a whole range of different colors, well beyond the sliver of light that we can see with our eyes.

So white dwarfs are buried deep within their planetary nebulas, and are so hot that they don't emit much visible light, but instead radiate mainly in the ultraviolet and X-ray parts of the spectrum. It's thus not too surprising that the superheated star at the center of the Red Spider Nebula remained unseen for many decades. That situation finally ended in 2005, when Mikako Matsuura and colleagues used the powerful Hubble Space Telescope, located in orbit above the Earth's atmosphere, to identify a tiny speck of light corresponding to the white dwarf at the heart of the Red Spider. In this and subsequent studies, astronomers have been able to make a precision measurement of the star’s color, and then have used Planck's law of black body radiation to calculate its temperature.

The results are astonishing—the surface temperature of the star at the center of the Red Spider Nebula is an incredible 540,000 degrees F, more than 50 times hotter than the Sun, and 30 times hotter than mighty Sirius.

This amazing star, with its extreme temperature and the spectacular glowing nebula that surrounds it, is of more than mere academic interest. For in gazing at the Red Spider, we are seeing our future fate. Around 5 billion years from now, the Sun too will run out of fuel, and will similarly shed its outer layers. All that will remain of our star and its solar system will be a beautiful planetary nebula, illuminated by an intensely hot white dwarf at its center.

ScientificAmerican