Can Stars Be Cold?

If you’ve heard me say “oot and aboot”, you know I’m a Canadian. And we Canadians are accustomed to a little cold. Okay, a LOT of cold. It’s not so bad here on the West Coast, but folks from Winnepeg can endure temperatures colder than the surface of Mars.  Seriously, who lives like that?

And on one of those cold days, even on a clear sunny day, the Sun is pointless and worthless. As the bone chilling cold numbs your fingers and toes, it’s as if the Sun itself has gone cold, sapping away all the joy and happiness in the world. And don’t get me started about the rain. Clearly, I need to take more tropical vacations.

But we know the Sun isn’t cold at all, it’s just that the atmosphere around you feels cold. The surface of the Sun is always the same balmy 5,500 degrees Celsius. Just to give you perspective, that’s hot enough to melt iron, nickel. Even carbon melts at 2500 C. So, no question, the Sun is hot.

The Sun – It’s pretty hot. Credit: NASA/SDO.

And you know that the Sun is hot because it’s bright. There are actually photons streaming from the Sun at various wavelengths, from radio, infrared, through the visible spectrum, and into the ultraviolet. There are even X-ray photons blasting off the Sun.

If the Sun was cooler, it would look redder, just like a cooler red dwarf star, and if the Sun was hotter, it would appear more blue. But could you have a star that’s cooler, or even downright cold?

The answer is yes, you just have to be willing to expand your definition of what a star is.

Under the normal definition, a star is a collection of hydrogen, helium and other elements that came together by mutual gravity. The intense gravitational pressure of all that mass raised temperatures at the core of the star to the point that hydrogen could be fused into helium. This reaction releases more energy than it takes, which causes the Sun to emit energy.

The coolest possible red dwarf star, one with only 7.5% the mass of the Sun, will still have a temperature of about 2,300 C, a little less than the melting point of carbon.

But if a star doesn’t have enough mass to ignite fusion, it becomes a brown dwarf. It’s heated by the mechanical action of all that mass compressing inward, but it’s cooler. Average brown dwarfs will be about 1,700 C, which actually, is still really hot. Like, molten rock hot.

This artist’s conception illustrates the brown dwarf named 2MASSJ22282889-431026. Credit: NASA/JPL-Caltech

Brown dwarfs can actually get a lot cooler, a new class of these “stars” were discovered by the WISE Space Observatory that start at 300 degrees, and go all the way down to about 27 degrees, or room temperature. This means there are stars out there that you could touch.

Except you couldn’t, because they’d still have more than a dozen times the mass of Jupiter, and would tear your arm off with their intense gravity. And anyway, they don’t a solid surface. No, you can’t actually touch them.

That’s about as cold as stars get, today, in the Universe.

But if you’re willing to be very very patient, then it’s a different story. Our own Sun will eventually run out of fuel, die and become a white dwarf. It’ll start out hot, but over the eons, it’ll cool down, eventually becoming the same temperature as the background level of the Universe – just a few degrees above absolute zero. Astronomers call these black dwarfs.

We’re talking a long long time, though, in fact, in the 13.8 billion years that the Universe has been around, no white dwarfs have had enough time to cool down significantly. In fact, it would take about a quadrillion years to get within a few degrees of the cosmic microwave background radiation temperature.

Dwarf Star

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A dwarf star is a star that is not a giant or supergiant … in other words, a dwarf star is a normal star! Of course, some dwarf stars are much smaller (less massive, have a smaller radius, etc) than normal (or main sequence, not really massive) stars … and these have names, like white dwarf, red dwarf, brown dwarf, and black dwarf. Our very own Sol (the Sun) is a dwarf star … a yellow dwarf.

Looking more closely at this rather confusing class of objects: a dwarf star has a mass of up to about 20 sols, and a luminosity (a.k.a. intrinsic brightness) of up to about 20,000 sols (‘sol’ is a neat unit; it can mean ‘the mass of the Sun’, or ‘the luminosity of the Sun’, or …!). So just about every star is a dwarf star! Why? Because most stars are on the main sequence (which means almost all have luminosities below 20,000 sols), and only a tiny handful of main sequence stars are more massive than 20 sols. In addition, once a star has burned through all its fuel, it becomes a white dwarf (and, one day, a black dwarf), all of which are dwarf stars by this definition.

The most interesting class of dwarf star is, perhaps, the black dwarf star; it’s hardly a star at all (it doesn’t burn any fuel, except, perhaps, deuterium, for a few million years or so).

So why do astronomers have this classification at all? Hitting the history books gives us a clue … back when spectroscopy was getting started, among astronomers – and well before there was any kind of astronomy except that in the optical (or visual) waveband; think the second half of the 19th century – a curious fact about stars was discovered: the spectra of stars with the same colors could still be very different (and when their distances were estimated, these spectral differences were found to track luminosity). So while dwarf stars overwhelmingly dominate, in terms of numbers, the giants (and sub-giants, and supergiants) pretty much rule in terms of what you can see with your unaided vision.

Neatly linking one kind of dwarf (the Sun, as a yellow dwarf) to another (white dwarf) is Universe Today’s The Sun as a White Dwarf. Other Universe Today articles on dwarf stars (not only white dwarfs!) include Astronomers Discover Youngest and Lowest Mass Dwarfs, Brown Dwarfs Form Like Stars, and Observing an Evaporating Extrasolar Planet.

Astronomy Cast’s episode Dwarf Stars has more on this topic.

Black Dwarf

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A black dwarf is a white dwarf that has cooled down to the temperature of the cosmic microwave background, and so is invisible. Unlike red dwarfs, brown dwarfs, and white dwarfs, black dwarfs are entirely hypothetical.

Once a star has evolved to become a white dwarf, it no longer has an internal source of heat, and is shining only because it is still hot. Like something taken from the oven, left alone a white dwarf will cool down until it is the same temperature as its surroundings. Unlike tonight’s dinner, which cools by convection, conduction, and radiation, a white dwarf cools only by radiation.

Because it’s electron degeneracy pressure that stops it from collapsing to become a black hole, a white dwarf is a fantastic conductor of heat (in fact, the physics of Fermi gasses explains the conductivity of both white dwarfs and metals!). How fast a white dwarf cools is thus easy to work out … it depends on only its initial temperature, mass, and composition (most are carbon plus oxygen; some maybe predominantly oxygen, neon and magnesium; others helium). Oh, and as at least part of the core of a white dwarf may crystallize, the cooling curve will have a bit of a bump around then.

The universe is only 13.7 billion years old, so even a white dwarf formed 13 billion years ago (unlikely; the stars which become white dwarfs take a billion years, or so, to do so) it would still have a temperature of a few thousand degrees. The coolest white dwarf observed to date has a temperature of a little less than 3,000 K. A long way to go before it becomes a black dwarf.

Working out how long it would take for a white dwarf to cool to the temperature of the CMB is actually quite tricky. Why? Because there are lots of interesting effects that may be important, effects we cannot model yet. For example, a white dwarf will contain some dark matter, and at least some of that may decay, over timespans of quadrillions of years, generating heat. Perhaps diamonds are not forever (protons too may decay); more heat. And the CMB is getting cooler all the time too, as the universe continues to expand.

Anyway, if we say, arbitrarily, that at 5 K a white dwarf becomes a black dwarf, then it’ll take at least 10^15 years for one to form.

One more thing: no white dwarf is totally alone; some have binary companions, others may wander through a dust cloud … the infalling mass generates heat too, and if enough hydrogen builds up on the surface, it may go off like a hydrogen bomb (that’s what novae are!), warming the white dwarf quite a bit.

More from Universe Today: How Does a Star Die?, Why Do Stars Die?, and Hubble Discovers a Strange Collection of White Dwarf… Dwarfs.

The End of the Universe Part 1 and Part 2 are Astronomy Cast episodes worth listening to, as are The Life of the Sun and The Life of Other Stars.

References:
NASA
NASA: Age of the Universe
Wikipedia