January 31 – A White Dwarf Is Discovered!

Posted on January 31, 2016

When I say white dwarf, I am not talking about a Little Person. Instead, I am talking about a certain kind of star.

Or, rather, I am talking about a certain part in the life cycle of some stars.

Sirius is the brightest star in the Earth's night sky. As a matter of fact, it is almost twice as bright as the next brightest star, Canopus. Naturally, it was seen by the earliest humans (and their not-yet-human ancestors), and later by ancient humans, and later still by medieval and then Renaissance humans...all the way up til now – BUT on this date in 1862, an American astronomer named Alvan Graham Clark spotted something nobody else had ever seen before:

Sirius has a much dimmer companion star!

So now, the thing we called Sirius pre-1862 now has to be called the Sirius star system, and we call the two different stars that make up that system Sirius A and Sirius B.

The Sirius star system seems bright in Earth's sky partly because it is pretty close – and getting closer! For the next 60 thousand years, Sirius will seem to get slightly brighter and brighter and brighter (after that, the distance between star systems will start to widen again, but Sirius will still be the brightest star in our sky for another couple of hundred thousand years!).

But the other reason Sirius seems so bright is because Sirius A is seriously large and seriously bright. It is about twice as massive as the Sun, and it is 25 times more luminous than the Sun. (There are stars in the universe even more massive and even more luminous than Sirius A, but they are farther away. Likewise, there are some stars that are closer, but they aren't as big and bright.)

What about Sirius B? It is much smaller than Sirius A, which is why it is called a dwarf; it is only about the size—or volume—of Earth, although it has almost as much stuff—mass—as the Sun! You have probably already guessed that Sirius B is much less luminous than Sirius A, as well – so faint that it took scientists a long time to spot it next to its brighter companion.

According to Wikipedia, Sirius B is a whopping ten thousand times less luminous than Sirius A, in visible light (although it does emit more X-rays than its companion). 

It wasn't always so. Millions and millions of years ago, both Sirius A and Sirius B were huge blue stars, burning hot and bright. Sirius B happened to be the more massive one, so it burned up its hydrogen more quickly, fusing it into helium at a much quicker rate. Once the fuel ran out, the star sort of imploded, collapsing in on itself. Once that happened, things heated up so much that helium was able to start fusing into carbon and oxygen. At that point, about 120 million years ago, Sirius B ballooned out to become a red giant. The more the star expanded, the less gravitational attraction the outer layers of the star had for the inner core, and those layers drifted away. Eventually, all that was left of Sirius B was that inner core, which is what we see. It is a very dense, very hot body is not undergoing fusion anymore. It will slowly cool off more and more and more, until it becomes a black dwarf.

This is all part of the normal life cycle of average stars, including our sun and even Sirius B.


  • Here is a zoom to see Sirius B next to its much larger companion. 

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