Astronomy

Did you know what is the brightest star in the night sky? Yes, it is Sirius. We can easily find this star by looking south and to the east from the famous Orion constellation. The name “Sirius” comes from the Greek word for “searing” or “scorching,” certainly appropriate for this star that shines at the bright end of the “minus-first” (-1.47) magnitude.

Located in the constellation Canis Major, Sirius can be seen from almost every inhabited region of the Earth’s surface (those living north of 73.284 degrees cannot see it). In the Northern Hemisphere, it is known as a vertex of the Winter Triangle.

What appears as a single star to the naked eye is actually a binary star system, consisting of a white main sequence star of spectral type A1V, named Sirius A, and a faint white dwarf companion of spectral type DA2, named Sirius B. The reason for its brightness is not its intrinsic luminosity but its closeness to the Sun; at a distance of 2.6 pc or 8.6 light years, Sirius is one of our near neighbours. Sirius A is only about twice as massive as the Sun and, with an absolute magnitude of 1.42, has far less intrinsic luminosity than other bright stars such as Canopus or Rigel.

Though typically separated from each other by a few seconds of arc, Sirius B is terribly difficult to see in the glare of Sirius A. The only way the companion star can be both hot and dim is to be small, only 0.92 the size of Earth, the total luminosity (including its ultraviolet light) just 2.4 percent that of the Sun. The two orbit each other with a 50.1 year period at an average distance of 19.8 Astronomical Units, about Uranus’s distance from the Sun, a large orbital eccentricity carrying them from 31.5 AU apart to 8.1 AU and back again. They were closest in 1994 and will be again in 2044, while they will be farthest apart in 2019.

From the orbit (and spectroscopic data), astronomer find that Sirius A and B have respective masses of 2.12 and 1.03 times that of the Sun. Sirius B is the chief member of a trio of classic white dwarfs, the others Procyon B and 40 Eridani B. Its high mass and tiny radius lead to an amazing average density of 1.7 metric tons per cubic centimeter, roughly a sugar cube. White dwarfs are the end products of ordinary stars like the Sun, tiny remnants that were once nuclear-fusing cores that have run out of fuel. Most are balls of carbon and oxygen whose fates are merely to cool forever.

To have evolved first, Sirius B must once have been more massive and luminous than Sirius A. That its mass is now lower is proof that stars lose considerable mass as they die. Given the 250 million year age of the system, Sirius B may once have been a hot class B3 star that could have contained as much as 6 or 7 solar masses, the star losing over 80 percent of itself back into interstellar space through earlier winds.

The best time of year to view it is around January 1, when it reaches the meridian at midnight. Under the right conditions, Sirius can be observed in daylight with the naked eye. Ideally the sky must be very clear, with the observer at a high altitude, the star passing overhead, and the sun low down on the horizon.

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