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Sirius: Brightest Star in Earth's Night Sky

sirius
Follow the belt of Orion to find Sirius.
Credit: Starry Night Software

Sirius, also known as the Dog Star or Sirius A, is the brightest star in Earth's night sky. The name means "glowing" in Greek. With a visual magnitude of -1.46, the star is outshone only by several planets as well as the International Space Station.

Because Sirius is so bright, it was well-known to the ancients. What came as a surprise to astronomers was the discovery of a companion star, Sirius B, in 1862.

The companion is so much dimmer than Sirius — 10,000 times, in fact — that it wasn't until 2005 that astronomers were able to estimate its mass.

Locating Sirius

Sirius is highly visible in the Northern Hemisphere winter night sky because it has a high relative luminosity to other stars, and it's relatively close to Earth (8.6 light-years). If the star were placed next to Earth's sun, Sirius would outshine it more than 20 times over.

To find Sirius, use the belt of Orion as a pointer. The three stars point downward toward Sirius to the left. To be more precise, the position of Sirius is:  

  • Right Ascension: 06h 45m 08.9s
  • Declination: -16 degrees 42 minutes 58 seconds

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Nearby Star Smaller than Earth, Massive as Sun
Photograph of Sirius A, the brightest star in our nighttime sky, along with its faint, tiny stellar companion, Sirius B.
Credit: NASA, ESA, H. Bond (STScI), and M. Barstow (University of Leicester)

Sirius in history

Today, Sirius is nicknamed the "Dog Star" because it is part of the constellation Canis Major — Latin for "the greater dog." The expression "dog days" refers to the period from July 3 through Aug. 11 when Sirius rises in conjunction with the sun. The ancients felt that the combination of the sun during the day and the star at night was responsible for the extreme heat that is experienced during mid-summer.

The star is present in ancient astronomical records of the Greeks, Polynesians and several other cultures. Perhaps most notably, the Egyptians based their calendar on when Sirius was first visible in the eastern sky, shortly before sunrise. They called it the "Nile Star" because it always returned just before the river rose, and so announced the coming of floodwaters, which would renourish their lands.

In 1718, Edmond Halley — when studying Sirius and other stars — discovered that stars have "proper motion" relative to one another. Then in 1862, Alvan Graham Clark discovered Sirius had a faint companion, now known as Sirius B.

Recent observations

In 2005, astronomers using the Hubble Space Telescope precisely measured the mass of Sirius B for the first time — a mass that is 98 percent that of the sun, NASA stated.

The scientists used a spectrometer aboard the orbiting telescope to measure how its light is affected by gravity on the star. Previous attempts with ground-based observatories were foiled because Sirius itself is so bright, and along with the Earth's atmosphere spreading its light, it washed out its fainter companion (which is a white dwarf).

Astronomers have struggled to confirm a few older measurements of Sirius' system. One 1994 paper in Astronomy and Astrophysics suggested the binary system could actually have a third, tiny companion that rotates around Sirius every six years. Observations in 2008 with a European Southern Observatory 3.6 meter telescope, however, revealed nothing.

Some have suggested that Sirius was much redder in the past, based on ancient accounts that described the star as red. Among them was Greek astronomy Claudius Ptolemy in A.D. 150.

"There is no doubt about what Ptolemy means. He bluntly says "reddish, and other ancient sources corroborate him," wrote the University of Houston's R.C. Ceragioli in 1995.

However, since the matter first received serious attention at the Royal Society in 1760, astronomers have been unable to explain the discrepancy. The stars would not have evolved so quickly in a few thousand years, but there could be effects from the theoretical third companion.

Or, some say, observers could have been describing how Sirius appears when it is close to the horizon and the most affected by Earth's atmosphere.

— Elizabeth Howell, SPACE.com Contributor

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