Date: April 28, 2011
Title: A Thermal Tour of the Universe
Podcaster: L Cate Kendall
Link: http://ww.legofusker.net/astronomy.htm
Description: We all have a sense for what hot and cold mean in our lives, and in this podcast we will take a brief look at some of the places where temperatures are very cold or very hot, and what a huge range there is in the universe.
Bio: L Cate Kendal is a science writer, theoretical physicist and amateur astronomer living in Scotland. She has written for the magazine Astronomy Now and does science outreach volunteer work with the Royal Observatory in Edinburgh.
Sponsor: This episode of “365 Days of Astronomy” is sponsored by — NO ONE. We still need sponsors for many days in 2011, so please consider sponsoring a day or two. Just click on the “Donate” button on the lower left side of this webpage, or contact us at signup@365daysofastronomy.org.
Transcript:
A Thermal Tour Of The Universe
Hello. I’m L Cate Kendal and welcome to this podcast.
Temperature is something that we are all very used to; Whether it’s a lovely hot bath or having to go out in a snowstorm, we have a feeling for what hot and cold mean in our lives.
However, the changes of temperature we experience in our lives here on Earth are very small compared to what is going on beyond our planet. Today we will take a brief look at some of the places where temperatures are very cold or very hot, and what a huge range there is in the universe.
But what do we mean by temperature? An object with a high temperature has a lot of energy inside it. A good way of thinking about this is to imaging the constituent atoms or molecules of the object jiggling and bouncing around. The hotter the object’s temperature, the more energy it has inside it and the faster and more vigorously the atoms move. As the object cools and loses energy, this movement decreases.
So the colder the object gets the less energy it has and the less the atoms move. In theory, there is a point where all this motion should stop. This is defined as absolute zero, and you can’t get colder than that temperature, you can’t have less motion than none at all.
Absolute zero is defined as the start of the temperature scale which we will be using. The scientific world uses the Kelvin temperature scale, which is zero Kelvin at absolute zero, and goes up as high as we need. Other scales are in common use for the temperatures we encounter everyday, and I’ll mention one or two equivalents as we go. But as we climb higher in temperature, away from what we are used to, any differences between the scales become less important.
But in practical terms we can never get down to exactly absolute zero, it’s not possible to stop all motion completely, so it’s not the coldest temperature in the universe.
The coldest we humans have got to this point is 100 picoKelvin. That’s a within a hundred trillionths of absolute zero, and that was created by scientists in Helsinki at the University’s Low Temperature Lab in 1999. The coldest place we’ve observed in nature is the Boomerang Nebula in the constellation of Centaurus. This is a balmy 1 Kelvin. The gas in this nebula is expanding very quickly, and it’s this expansion that causes the drop in temperature.
I say drop, as this is the only place observed that has a temperature lower than the cosmic microwave background, which is considered as the average temperature of the universe.
You may have heard of this before, the cosmic microwave background (or CMB) covers the whole sky, and is shows virtually the same temperature everywhere (not counting stars or nebulae), 2.73 Kelvin. There’s more about what the CMB is and what it means elsewhere in these podcasts, but for our purposes it can be thought of as the temperature of normal “space”, unheated by stars or planets.
Although the CMB accounts for “space”, there are many objects in space that are considerably warmer and now we start meeting more familiar objects, such as those in our solar system.
It’s about 44K at the surface of the now defunct planet Pluto, 53K on Neptune and 63K on Uranus. And while these temperatures are warm compared to the CMB, these are still temperatures we humans should avoid direct contact with. Down at this temperature, even the gases that make up our breathable atmosphere are either liquid or solid. It’s too cold for us humans, who maintain our normal body temperature of about 37 degrees centigrade, or about 300 Kelvin, which is about 500 Kelvin warmer than on Pluto!
Here on Earth, we are comfortable at room temperature of 20 degrees centigrade. But our home can throw harsher temperatures at us, the coldest air temperature being 183 Kelvin (minus 89 degrees centigrade). At the other extreme there have been temperatures recorded of 326 Kelvin, that’s 54 degrees centigrade.
But in spite of these extremes, Earth has an acceptably warm surface. Venus is a different matter, although similar in size and mass to Earth, its average surface temperature is well over twice as big, about 730 Kelvin, hot enough to melt lead!
And under our feet is a different matter again. Down near the core of the Earth the temperature has risen to 5700 Kelvin, as matter is squashed together at high pressure. These are the sort of temperatures we encounter when we look at the surface of stars.
Inside stars, energy is released as the stars burn their nuclear fuel, so the fact that they are hot isn’t much of a surprise. But there is a huge range in their temperatures, wider than we have encountered so far in our climb up the Kelvin scale.
Small, cool stars that burn fuel very slowly can have a surface temperature as low as 500 Kelvin, but more usually they are two or three times that.
Average, middle-aged stars like our Sun have surface temperatures of about 6000 Kelvin. These stars will slowly evolve to become cool, red giants in their old age, their surface temperatures dipping down by a thousand degrees or so. The best-known red giant Betelgeuse, in Orion, has a surface temperature of roughly 3500K.
At the other extreme, are young, heavy stars. The bigger the star, the faster it consumes its fuel and the more energy will be released. So young heavy stars such as Rigel, also in Orion, have surface temperatures into the tens of thousands of Kelvin.
These have all been surface temperatures of stars, but now is a good time to note the temperature at the centre of stars is different from that of the surface. These are the temperatures needed to sustain the nuclear fusion reactions that power the star.
Our Sun, for example has a core temperature of about 13 million Kelvin, as hydrogen is burned to make helium. But at the core of a giant star it’s possible to get up to temperatures of a billion Kelvin, hot enough to burn heavier elements such as oxygen or silicon.
Beyond these temperatures we find ourselves in exotic and unfamiliar places. Supernovae explode with temperatures in the region of 10 billion Kelvin, ten times hotter than in the centre of the hottest stars. And further beyond this we enter the realm of high-energy particle physics. In particle collisions, the temperature starts to rise rapidly, reaching millions of times hotter than even a supernova, and this stretches our understand of what a temperature actually is.
In this podcast, we have spanned a huge scale of temperature, from within a fraction of absolute zero to the hottest and most violent events in the universe, and these have been very different from our experience of temperature here on Earth.
I hope that this has given you a sense of the amazing variety of temperatures in the universe, and that you have enjoyed this podcast. Thank you for listening.
End of podcast:
365 Days of Astronomy
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