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.
- Check out this short video to learn more about this life cycle.
- Here is a zoom to see Sirius B next to its much larger companion.
Also
on this date:
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