Is A White Dwarf Hotter Than A Red Giant?

We as a species are curious. Our curiosity has taken us around the world and even beyond the borders of our planet to the endless expanse of space itself.

At this point in humanity’s history, we are still using that curiosity to discover more about the boundless universe, including information about its most dominant celestial bodies, the solar power stations known as stars.

Stars came into existence at the beginning of the universe and will continue to exist long after we are gone. Our species has long given respect and awe to these astronomical titans, with the closest one, our sun, being worshiped as a god in days long past.

Yet, it is only in the modern-day that we have examined stars in closer detail, working out the intricacies of their lives and the causes of their deaths.

An average star will go through several phases of its life, from a stellar nebula to a star to a red giant and down all the way to a white dwarf.

But these different phases will have different chemical makeups, temperatures, and heavy element compositions, which change their temperature and size over time.

With that in mind, a question people have been asking astronomers for some time is whether a white dwarf is hotter than a red giant?

And, if so, why? Today, we will take a closer look at star life cycles in order to gain some understanding of the situation.

What Is A Red Giant?

Before we answer the question at hand, we will discuss a little about what each of these stars is, starting with a red giant. A red giant is the beginning of the end of an average star’s life cycle.

You see, stars are constantly generating a thermonuclear reaction in their core, which involves fusing hydrogen into helium. Over time, helium will begin to be more dominant in the core, increasing in percent over time.

When the core of a star runs out of hydrogen to fuse, and it is all helium, the nuclear reaction will no longer produce enough energy to counteract the massive amount of gravity that exists within the sun.

This causes the core to contract slightly, making the core smaller, denser, and hotter. The increase in temperature will let the nuclear fusion take place in the shell that exists around the core, where there is still plenty of hydrogen.

The shell, now burning from the hydrogen-based reaction, will adhere to ‘the mirror principle’. This is an observation of a star’s structure where a physical action is reacted against in a similar manner.

As such, when the core contracts from the lack of a thermonuclear reaction, the shell will expand thanks to the presence of extensive thermal energy.

Of course, the causes and reactions to this are much more complex than what has been described, but the result is that the core contracts and heats up and the shell expands, absorbing energy as it goes, and begins cooling down.

Once sufficiently cool, the star will stop expanding, and as it cools it will become redder in color.

At this point, the star is now a red giant.

What happens to a red giant after this depends on its size.

We can use the sun as a reference point in a star’s life cycle, but it is still hard to gauge even with this comparison.

For one thing, in 5 billion years the sun itself will become a red giant, as will any other main-sequence star up to 10 times the sun’s size.

If the core of these stars is less than 2 times the sun’s mass, it will become a white dwarf. If the core of these stars is more than 2 times the sun’s mass, it will become a neutron star or a black hole.

What Is A White Dwarf?

Now that we’ve discussed the beginning of the death of a star, we can also discuss one of the possible ends for that star. Most stars in the universe will become white dwarfs, over 95% in fact.

A white dwarf is what happens when a star completely runs out of hydrogen to fuse in a thermonuclear reaction, in both the core and the shell.

During the red giant phase of a star’s life cycle, the hydrogen fusing will stop in the core and move to the shell and in the core, the helium, which now makes up almost 100% of the core, will begin fusing to oxygen and carbon.

Depending on the mass of the red giant, it may be unable to generate a high enough core temperature to fuse different substances. This will cause different varieties of the white dwarf at the end of the red giant’s life, from carbon-oxygen white dwarfs to helium white dwarfs.

In most cases, white dwarfs are carbon-oxygen white dwarfs and have a mass of carbon and oxygen in their core. While a part of the red giant, the core will continue to contract, becoming incredibly dense, unlike the shell of the star.

When the red giant is unable to continue the hydrogen-fusing thermonuclear reaction in its shell, the shell will shed giving birth to a planetary nebula and at the center of this nebula will be the remnant core of the former star.

This is what we call a white dwarf.

So, Is A White Dwarf Hotter Than A Red Giant?

You would think that a red giant must be hotter than a white dwarf, simply because it is earlier in the star’s life span, and it is still producing a thermonuclear reaction, right?

Well, not quite. You see, although a red giant’s core may be hot, the expansion of the shell where the hydrogen-fusing reaction is taking place is quite cool.

This is because the star has expanded up to 10 times its original size, and expansion makes the giant less compact and so temperature escapes more easily.

The redder color is a giveaway for its cooler temperature, as on the shell’s surface it sits at only around 3,000 degrees kelvin, and generally the brighter an object the hotter it is.

White dwarfs, on the other hand, no longer generate nuclear reactions, meaning that the dwarf has no source of energy. However, when they form, they are incredibly hot and are very, very dense, thanks to electron degeneracy pressure.

The heat from their initial formation when they were fusing in the red giant gives them a bright white color and the density of continually contracting means that they lose heat very slowly.

White dwarf’s surface temperature is estimated to be from 3000 degrees kelvin to up to a huge 30,000 degrees kelvin, over time though this decreases and with no thermonuclear reactions to replace the lost heat, the white dwarf will darken over time.

With this in mind, the surface temperature of a white dwarf is definitely hotter than the shell of a red giant.
However, if we take the core of a red giant, that is a different story.

During the fusing of helium, the red giant’s core can potentially reach 100,000,000 degrees kelvin, which is far, far hotter than a white dwarf, and it only starts to cool down once this fusion reaction has finished.

As such, a white dwarf is hotter than a red giant’s shell’s surface temperature, but it is far cooler than a red giant’s core.

Final Thoughts

The life cycles of stars are a strange and oddly familiar topic. They reach the extreme ends of temperature and explainable physics, yet we can recreate their life cycles in models or see similar things happen in our world, albeit at a much smaller scale.

Still, it is fascinating how the end of a star’s life still continues to generate so much energy and how they live so long. They live such a long time that we have never seen a white dwarf cease to be since the beginning of the universe, they continue to burn brightly even now.

We can only theorize about what happens after – with so-called ‘black dwarfs’ – but if it’s anything like comparing the heat of red giants and white dwarfs, I’m sure it will be incredibly fascinating to find out.