When the universe began

How hot is it inside the sun? The science of heat involves millions and millions of degrees

Published July 22, 2012 12:00PM (EDT)

Excerpted from "Extreme Cosmos: A Guided Tour of the Fastest, Brightest, Hottest, Heaviest, Oldest and Most Amazing Aspects of Our Universe"

Let’s start with the Sun, a giant burning ball of gas, so hot and intense that it’s not safe to even look at. The Sun has a surface temperature of around 9900°F, which is hot, but not unimaginably so. The Sun’s surface is about five times hotter than a candle flame. 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 it 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.

The surface temperature of the star at the center of the Red Spider Nebula is an incredible 540,000°F, more than 50 times hotter than the Sun. 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.

Stars may have high temperatures at their surfaces, but the fiery hell in their interiors is unimaginably hotter. When the Sun began its life 4.6 billion years ago, the composition of its core is thought to have been very different from what we see today. This massive change in the core’s composition over the Sun’s lifetime, from 72% hydrogen at the beginning to 39% hydrogen now, provides a vital clue to the extremes going on in the Sun’s deep interior. It tells us that the heat and light of the Sun come from nuclear fusion, in which hydrogen is continuously converted into helium, releasing large amounts of energy in the process. This is the same phenomenon that provides the devastating power of a hydrogen bomb, but on a far larger scale.

As its name suggests, nuclear fusion involves the nuclei of two hydrogen atoms sticking together. But this is not something that can happen easily, because hydrogen nuclei have positive electrical charges, and two positive charges will furiously try to repel each other when brought close together. It is only when two hydrogen nuclei can be brought close enough to essentially touch that they will then bind to make helium.

The trick is to bring the two hydrogen nuclei together as quickly as possible. If they approach each other slowly, they will have time to exert their repulsive forces on each other and will stay apart. But aim them at each other at high speed, and their mutual electrical repulsion cannot slow them down enough to prevent a collision.

This process is achieved by heating the hydrogen to extraordinarily high temperatures. At this high temperature, individual hydrogen nuclei fly around randomly at enormous velocities, making fusion possible via high-speed collisions. Calculations show that the temperature needed in a star’s core to make this reaction proceed is about 9,000,000°F. At this temperature, all solids and liquids vaporize into gas, all molecules are broken apart into individual atoms, and electrons are then torn off these atoms to leave the central nuclei exposed.

The Sun is even hotter, with a core temperature of about 27,000,000°F! And, as mentioned above, the Sun is an unremarkable star. Heavier stars generate even hotter temperatures in their cores — up to 90,000,000°F.

The period of time during which a star converts hydrogen into helium in its core is known as the “main sequence,” and it occupies most of the star’s life. The Sun is at about the halfway mark of its main sequence phase, with another 5 billion years or so to go.

When a star has converted all the hydrogen in its core into helium, the fusion reactions at the center shut off, and the star begins to run into the health problems associated with old age.

With its heat source extinguished, the star’s core now begins to collapse under its own gravity, becoming smaller, denser, and even hotter. The star then becomes a “red giant.” Eventually the core of the star reaches around 180,000,000°F, a temperature at which helium nuclei now travel fast enough to collide and fuse to form carbon. The star now gets its “second wind,” and enters a period of relative stability known as the “horizontal branch” phase of stellar evolution.

But inevitably the helium, too, is used up. For a star like the Sun, this is now almost the end. The core will compress and heat up further, but there isn’t enough mass to trigger any more nuclear reactions. A series of complicated convulsions begins, eventually leading to a wind that blows off the star’s outer layers, producing a beautiful glowing planetary nebula like the Red Spider Nebula.

But for stars heavier than the Sun, the game is not yet over. Once all the helium has fused into carbon, the core heats up further, until at an unimaginable 1,000,000,000°F, carbon nuclei begin to fuse to form oxygen, neon, magnesium, and sodium. For the very heaviest stars, with masses more than eight to ten times that of the Sun, the primrose path has yet further to run. When the temperature of the star’s core rises above 2,700,000,000°F, oxygen nuclei fuse to form silicon, sulfur, and phosphorus. And then at around 5,400,000,000°F, silicon fuses to form iron.

At this point a star’s life is almost at an end, because iron is the most stable element in the Universe, and resists undergoing further fusion. With all nuclear reactions at an end, the largely iron core now collapses further, squeezed until it reaches a temperature of around 9,000,000,000°F. Yes, that’s 9 billion degrees. The core then catastrophically collapses to form a ball of almost pure neutrons, only about 15 miles across. The outer layers of the star fall onto this newly formed neutron star and rebound, causing a vast supernova explosion that rips the outer parts of the star apart and blasts them off into space at high speed.

The centers of stars are hot, but as you might expect, they’ve got nothing on the temperatures involved in the very earliest moments of the Universe. It’s now been about 13.7 billion years since the Universe itself began in a “Big Bang.” If we run the clock backward in time, the cosmos gets very toasty indeed. If we go back to the Universe’s very earliest moments, how hot do things get?

We’ll start with the situation just one second after the Universe began. We can’t make any measurements or observations of this era, but we can work backward from what we see now to make what we think are reasonably accurate calculations of the conditions at that time. When the Universe was one second old, the temperature everywhere was about 18,000,000,000°F, twice as hot as the core of a massive star at the last moment of its life. At this temperature, atoms could not yet exist: Their building blocks — protons, neutrons, and electrons — flew freely around in all directions, occasionally colliding, but with too much energy to ever stick together.

Let’s step back further, to one-millionth of a second after the Big Bang. The Universe’s temperature was now 18,000,000,000,000°F! The Universe was filled with small particles called “quarks,” which were beginning to stick together to form protons and neutrons.

Probably the earliest time we can meaningfully discuss is just 0.0000000000000000000000000000000000000000001 seconds after the Big Bang, when the temperature would have been 180,000,000,000,000,000,000,000,000,000,000°F. At earlier stages, the very concept of time becomes difficult to define, and we move into an era that goes beyond our current physical understanding. Furthermore, it becomes difficult to even say what we mean by temperature.

Excerpted from "Extreme Cosmos: A Guided Tour of the Fastest, Brightest, Hottest, Heaviest, Oldest and Most Amazing Aspects of Our Universe" by Bryan Gaensler. By arrangement with Perigee, a member of Penguin Group (USA) Inc., Copyright (c) 2011 by Bryan Gaensler.

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