The title of this post mentions Sirius A, which is not how we would typically talk about this star; usually, we say Sirius. Sirius A implies there must be a ‘B’ or more, which I will cover here. Sirius A is easy to find, being the brightest star in the night sky. If you imagine a line from the Southern Cross to Orion, you’ll find Sirius A and its companion white dwarf a little before you get to Orion, about a hand span before. What you won’t see easily is Sirius B.
First View of Sirius A’s Companion White Dwarf
Sirius B was first spotted by Alvan Clark back in 1862. He was an American telescope maker and astronomer, finishing an 18.5″ refractor on 31 January. While testing it, he discovered a tiny little companion to Sirius A named Pup. Unfortunately for Sirius A, it couldn’t solely claim the name “Sirius” from that moment anymore. It turns out it wasn’t the first time the Pup had stolen the limelight. It took another American astronomer, Walter Adams, to work out that the tiny companion to Sirius A was a bit unusual. He measured a spectral shift in the light coming from Sirius B and concluded it must be a very dense and compact object. His observations also helped to confirm Einstein’s General Relativity. Even though it was later found that Clark’s measurements were a bit off, they were close enough to verify the existence of a new object class.
Sirius A’s Compact Companion
The object he discovered is what we call a white dwarf. These objects are the remnant of stars with initial masses up to around eight times the mass of the Sun. More giant stars blow up in massive explosions called supernovae; smaller stars have a less dramatic ending. They shed a lot of mass through stellar winds and the loss of their outer layers; all that remains are weird little super-dense white dwarfs. Sirius B is about 98% of the mass of the Sun, so it must have started as a much larger star; astronomers think it may have been as big as five solar masses. What makes things a bit complex is that Sirius B is close to Sirius A at an average of 3 billion kilometres (about the distance from the Sun to Uranus). There was a time when Sirius B should have been called Sirius A, meaning there was a time when Sirius B was brighter than Sirius A (the brighter of the group gets to have the A suffix); we’ll still stick with calling it Sirius B to avoid confusion. The star would have evolved off the main sequence (where it fuses hydrogen into helium) and into the red giant branch, expanding to an enormous size.
Mass Transfer
As a red giant, Sirius B’s outer layers would have felt less of the influence of the star’s enormous gravity and a bit more influence from what is now Sirius A. There would have been some mass transfer from Sirius B to A. We don’t know how much, but this transfer may have affected both stars’ evolution. The mass of Sirius A is roughly two times the mass of the Sun, so it is still considerably smaller than what we think the mass of Sirius B was. After the red giant phase, Sirius B would have settled down into fusing helium into carbon. It probably had another big red giant phase before fusing carbon into oxygen in the core. That was probably it for Sirius B at five solar masses; to fuse oxygen, there would need to be more than eight solar masses.
White Dwarf Formation
Once the fusion stops in a star, the core contracts rapidly. In the case of Sirius B, it contracted until it was stopped by electron degeneracy pressure. If you squash some matter together, the electrons whizzing around the nucleus get forced closer and closer to other electrons. They eventually run out of room because the lower energy states get filled up; Pauli’s exclusion principle states that fermions must occupy unique quantum states; electrons are fermions. It’s like filling up a theatre where everyone must sit in a chair; you can’t have two people in one chair (even if they are friends), so once all the chairs are full, there’s no more room, so the doors get closed. This pressure caused by all of the electrons in their chairs (quantum states) holds back the gravitational collapse of the core. That’s what a white dwarf is: the original star’s core squished into a much smaller ball. For Sirius B, that equates to a little smaller size than the Earth!
The density of Sirius B is about 2 billion kg per cubic metre (a teaspoon of Sirius B would weigh about 10 tons!). The surface temperature is around 25,000 degrees. It’s a bit tricky to age a white dwarf, but based on its temperature and cooling models, astronomers think it’s probably about 150 million years old; that’s just the white dwarf phase. I managed to image Sirius A and its companion white dwarf, Sirius B on a night with excellent conditions at Star Safari.