Learning more about 'white dwarf' stars

'White dwarf' stopwatches

It might be surprising that “white dwarf” stars, the relics of stars like our Sun that have run out of fuel, are useful cosmic stopwatches.

The resulting objects are about the size of the Earth, but still with almost the mass of the star. They are white hot, around 10,000 C, which is why these stars are known as white dwarfs.

With no source of energy, their fate is to radiate away all their heat, ending up as cold, dark cinders. However, this takes an extremely long time.

Assuming a white dwarf started off with a temperature of around 10,000 C, we can measure what that temperature has fallen to now and calculate how long ago that star became a white dwarf. That means we can estimate the ages of things containing white dwarf stars.

For example, the Hubble Space Telescope has detected many white dwarf stars in the central regions of our galaxy. These stars are around 13 billion years old, confirming other estimates of the age of the Milky Way—13.6 billion years.

A lot of astronomical attention is being paid to a pair of recently discovered white dwarf stars, poetically named WD J2147-4035 and WD J1922+0233, which have cooled to about 3,000 C and 3,300 C respectively.

From that, we estimate the first of those stars became a white dwarf around 10 billion years ago and the second roughly nine billion years ago.

If the stars that became these white dwarfs were like the Sun, then those stars would have formed a few billion years earlier, which brings them fairly close to the beginning of the universe.

This is particularly intriguing because it looks as though these stars had planet and because the light from those white dwarfs carries the signatures of elements such as potassium, calcium and lithium.

The only way they could have collected them is through the debris of destroyed planets falling onto their surfaces. The evidence is subtle, but detecting it was made easier because, unlike those white dwarfs the HST observed around the centre of our galaxy some 26,000 light years away, this pair of stars lie close by, around 100 light years away.

That means their light is strong enough for subtle indications of elements to be detected, with no confusion with the light from other stars nearby.

Galaxies form through the coalescence of smaller galaxies. During that process collisions between gas clouds stimulate the formation of lots of stars. Some of those stars would have had masses many times that of the Sun, and would have blown themselves up within a few million years.

The lower-mass stars became white dwarfs, which is why they can tell us the age of the Milky Way. However, it is really intriguing that at least two white dwarfs from stars that must have been among the first in our galaxy had planets.

To make planets you need elements like silicon, oxygen, iron, phosphorus and all the others. These did not exist when the universe began, just under 14 billion years ago.

The only elements available were hydrogen and helium. Luckily these are all you need to make stars. All the other elements are produced as waste products by the process of energy production in the stars, and distributed into space when these stars exploded at the ends of their lives.

We believe those early stars were supermassive, blue giants, which had very short lives—maybe a million years or two before exploding. That the two white dwarfs formed nine and 10 billion years ago, from sunlike stars with planets, suggests the raw materials needed for making planets existed soon after the first stars formed.

There is a downside to observing white dwarfs. Because they ration their energy radiation over many billions of years, they are incredibly dim.


After sunset, Jupiter lies in the south-east and Saturn in the south. Mars rises later. The Moon will be new on Nov. 23.

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.

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About the Author

Ken Tapping is an astronomer born in the U.K. He has been with the National Research Council since 1975 and moved to the Okanagan in 1990.  

He plays guitar with a couple of local jazz bands and has written weekly astronomy articles since 1992. 

Tapping has a doctorate from the University of Utrecht in The Netherlands.

[email protected]

The views expressed are strictly those of the author and not necessarily those of Castanet. Castanet does not warrant the contents.

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