"What's the bright star to the south? Is that a planet?".
I've been asked that several times over the last month.
The star itself is Sirius. The brightest star in the sky, Sirius is also known as the "dog star". This is because it is in the constellation of Canis Major, or the Great Dog. It's name comes from Arabic, meaning "the scorched one". However, Sirius is not a lone star at all. It has a companion star, only about the size of the Earth. That companion star is known as Sirius B, meaning, by inference, that the brighter of the pair is properly denoted Sirius A. The tiny star is a white dwarf.
A few weeks ago, I said the planet Earth would
eventually be destroyed in about five billion years when the Sun turns into
a white dwarf. What we did not discuss, however, was the death of the Sun.
We had talked about how the Sun would turn into a red giant in the beginning
of it's demise, but not about what would happen later.
As you may recall, red giants are stars which have run out of their normal fuel of hydrogen, and are fusing helium instead. What happens when the helium runs out? When a star runs out of it's supply of helium fuel, gravity starts to compress the star back inwards, collapsing in on itself. When the atoms which had formerly comprised the outer shell of a star come together, the electrons around the atoms start to repel one another. Electrons repel each other because they all have a negative 1 charge. In subatomic physics, all like charges repel, just like similar poles of a magnet repel. The star only stops collapsing as the electrons in the star start to repel one another. When a star is being held up like this, we call it a white dwarf.
One tablespoon of a white dwarf would weigh about the same as mount Everest. After the Sun becomes a red giant, and devours the Earth, the Sun will come to it's final rest as a white dwarf, about the size of the Earth. Stars larger then the Sun can fuse other elements such as carbon and oxygen before they die. Stars over 40 percent larger then the Sun cannot be held up by electron repulsion, and collapse further in neutron stars, and black holes.
The Dogon people, in what is now Mali, may have held an interesting belief. In the early 1930's, it was recorded that these people believed that the star Sirius had an invisible companion, made of a metal which could not be found on Earth. What is interesting is that from the standpoint of chemistry, the matter which comprises white dwarfs is a metal, which does not occur naturally on Earth. The Dogon people, however, may have been informed of Sirius B by a team of anthropologists who had met with the Dogons a few years earlier.
Sirius B was, in fact, discovered by accident. An American telescope maker by the name of Alvan Clark was testing a new lens on Sirius in 1862 when he discovered Sirius B. The white dwarf is seen from Earth to have a magnitude (brightness) of about 8. If it were alone in space, you would not see it from Earth with the naked eye, but even a small pair of binoculars would reveal it. This makes it about as bright as the planet Neptune. Sirius B, however, is lost in the glare of it's brighter companion. The star Sirius A is magnitude -1, making it 9 magnitudes brighter than Sirius B. That means the brighter star is about 2.5 to the power of 9, or over 3800 times brighter than it's dimmer sister star. Also, they are always within 11 arc seconds, or about 1/350 of a degree away from one another. Look for Sirius (it shouldn't be too hard) in the south/south west skies early in the evening. It will be the brightest object in the area. It should be possible to see Sirius B with any telescope under good conditions, but I have never seen it with any telescope having a diameter of less than 8 inches. Try for yourself.
Clear skies, and good viewing.
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