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.

11 Comments

1. 1. jtdwyer 07:58 AM 7/27/12

   Also see
   http://www.eurekalert.org/pub_releases/2012-07/e-tbs072412.php

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2. 2. jobjobx 10:32 AM 7/28/12

   Planck's law is misleadingly explained.
   When an object heats up, the PERCENTAGE of light get higher for shorter wavelengths.
   However, the absolute amount of light increases at EVERY wavelength, ust more slowly at long wavelengths; so there's plenty of visible light coming from a white dwarf.
   
   Therefore, it's the other two reasons mentioned that make the star difficult to see: it's small, and it's obscured.

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3. 3. jtdwyer 11:30 AM 7/28/12

   This article states:
   "...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..."
   
   However, as stated by
   http://en.wikipedia.org/wiki/White_dwarf
   "A white dwarf is very hot when it is formed, but since it has no source of energy, it will gradually radiate away its energy and cool down. This means that its radiation, which initially has a high color temperature, will lessen and redden with time. Over a very long time, a white dwarf will cool to temperatures at which it will no longer emit significant heat or light, and it will become a cold black dwarf. However, since no white dwarf can be older than the age of the Universe (approximately 13.7 billion years), even the oldest white dwarfs still radiate at temperatures of a few thousand kelvins, and no black dwarfs are thought to exist yet."
   
   Doesn't this imply that most of the discussion about which is the hottest star referring to surface temperature? If I understand correctly, the cores of many stars producing fusion reactions within their cores may be hotter than the surface of a white dwarf, which has generally concluded its fusion processes, but since their heat and light must pass through their outer layers the surface of newly formed white dwarfs appear to radiate more heat.

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4. 4. QBitSci 06:18 PM 7/28/12

   This is an excerpt from a book, so I guess that the terminology used reflects that for it's target audience…? But it always seems weird to me to see temperatures quoted in degrees Fahrenheit for scientific items.
   – although degrees Kelvin are mentioned in the comments.
   
   540,000 degrees F is clearly exceptionally hot…
   But would be more informative expressed in degrees K or degrees C
   (After calculating, I got the answer 300,000 K - )
   
   Same issue with Pounds and Miles…
   - don't really belong even in 'popular science'…
   

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5. 5. Big Mama Roneck 01:43 AM 7/29/12

   QBitSci
   
   Yes, we live in a digital world but ship speed is still measured in 'knots' - ocean distances are measured in 'nautical miles' - tide charts are measured in 'fathoms' - Mt Everest is still described in 'feet' - eggs are sold by the 'dozen' - cattle are penned in 'yards' - auto tyres and rims are measured in 'inches' - tyre pressure is measured in 'PSI's' (pounds per sq. Inch.)the most expensive (and arguably best) watches in the world are 'analogue'...
   
   I can live with that. I get your point tho'.
   
   BMR

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6. 6. Kan Fukumin 02:36 AM 7/29/12

   Why don't you use more scientific units, such as the SI unit, or use Km instead of mile, use Celsius (C and Kelvin temperatures are nearly the same at high temperatures) instead of F, in scientific artical?
   Is this article a scientific article?

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7. 7. American Muse 12:37 PM 7/30/12

   I prefer the use of F rather than C (for temperatures only!) because:
   1. I grew up with it and am comfortable with the scale.
   2. It better intimates ambient temperatures for human consumption because the scale is broader (32-212 versus 0-100).
   3. There is no scientific reason to only count in tens.

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8. 8. Daniel35 02:37 PM 8/1/12

   "Its surface temperature of 9,900 degrees F results in a yellowish light, just as Planck's equations predict."
   
   Wait a minute! It isn't as though this prove Planck's equations. Do we have any other means to measure temperature of a star? Isn't it more that see the color, which tells us the temperature?

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9. 9. Daniel35 02:44 PM 8/1/12

   I agree that these figures should be in SI, each with English equivlent and maybe with a conversion factor. More of us might come see the lack of common sense of other systems.

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10. 10. bgaensler in reply to Daniel35 03:21 AM 8/2/12

    Hi,
    
    Yes, there are sophisticated ways to measure the temperature of a star, way beyond the scope of this short article. A detailed spectrum of the star's light allows one to assign a spectral and luminosity class to the star, based on the presence and absence of emission and absorption lines. These are compared to standard templates generated from sophisticated simulations to accurately determine the star's temperature.

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11. 11. bgaensler in reply to Daniel35 03:23 AM 8/2/12

    The article you're reading is an extract from my book, "Extreme Cosmos". The originally published version of the book was for Australia and New Zealand, and is full of appropriate SI units (kilometres, kilograms, etc.). However, when the book was republished in the US, the publisher made the decision to convert all the numbers to imperial units (miles, pounds, etc.). This extract is taken from the US edition, hence the US units used throughout.

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