January 31 – A White Dwarf Is Discovered!

nPostednon January 31, 2016

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

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nOr,nrather, I am talking about a certain part in the life cycle of somenstars.

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nSiriusnis the brightest star in the Earth’s night sky. As a matter of fact,nit is almost twice as bright as the next brightest star, Canopus.nNaturally, it was seen by the earliest humans (and theirnnot-yet-human ancestors), and later by ancient humans, and laternstill by medieval and then Renaissance humans…all the way up tilnnow – BUT on this date in 1862, an American astronomer named AlvannGraham Clark spotted something nobody else had ever seen before:

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nSiriusnhas a much dimmer companion star!

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

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nThenSirius star system seems bright in Earth’s sky partly because it isnpretty close – and getting closer! For the next 60 thousand years,nSirius will seem to get slightly brighter and brighter and brightern(after that, the distance between star systems will start to widennagain, but Sirius will still be the brightest star in our sky fornanother couple of hundred thousand years!).

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

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

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nAccordingnto Wikipedia, Sirius B is a whopping ten thousand times lessnluminous than Sirius A, in visible light (although it does emit morenX-rays than its companion).

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nItnwasn’t always so. Millions and millions of years ago, both Sirius Anand Sirius B were huge blue stars, burning hot and bright. Sirius Bnhappened to be the more massive one, so it burned up its hydrogennmore quickly, fusing it into helium at a much quicker rate. Once thenfuel ran out, the star sort of imploded, collapsing in on itself.nOnce that happened, things heated up so much that helium was able tonstart fusing into carbon and oxygen. At that point, about 120 millionnyears ago, Sirius B ballooned out to become a red giant. The morenthe star expanded, the less gravitational attraction the outer layersnof the star had for the inner core, and those layers drifted away.nEventually, all that was left of Sirius B was that inner core, whichnis what we see. It is a very dense, very hot body is not undergoingnfusion anymore. It will slowly cool off more and more and more, untilnit becomes a black dwarf.

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

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