Sirius star, also known as the Dog Star or Sirius A, is the brightest star in Earth’s night sky. The name means “glowing” in Greek — a fitting description, as only a few planets, the full moon and the International Space Station outshine this star.
Not counting the sun, the second-brightest star in all of Earth’s sky – next-brightest after Sirius star is Canopus. It can be seen from latitudes like those of the southern U.S. The third-brightest and, as it happens, the closest major star to our sun is Alpha Centauri. It’s too far south in the sky to see easily from mid-northern latitudes.
Because Sirius star is so bright, it was well-known to the ancients. But the discovery of a companion star, Sirius B, in 1862 surprised astronomers. The star that you can see with the naked eye is called Sirius A, or sometimes just Sirius.
January and February are perfect months for both Northern Hemisphere and Southern Hemisphere observers to view the brightest star in the sky: Sirius star. As part of the constellation Canis Major the Greater Dog, Sirius also earns the nickname of the Dog Star.
Sirius star is highly visible in the Northern Hemisphere’s winter night sky, because the star has a high luminosity, or intrinsic brightness, relative to other stars, and because it’s relatively close to Earth (8.6 light-years away). According to NASA, Sirius star has a mass that’s two times that of Earth’s sun. If the Sirius star were placed next to our sun, Sirius would outshine it more than 20 times over, according to NASA’s Astronomy Picture of the Day.
Sirius star is classified by astronomers as an “A” type star. That means it’s a much hotter star than our sun; its surface temperature is about 17,000 degrees Fahrenheit (9,400 Celsius) in contrast to our sun’s 10,000 degrees F (5,500 C). With slightly more than twice the mass of the sun and just less than twice its diameter, Sirius star still puts out 26 times as much energy. It’s a main-sequence star, meaning it produces most of its energy by converting hydrogen into helium through nuclear fusion.
Today, Sirius star 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, Space.com previously reported. The ancients felt that the combination of the sun during the day and the star at night was responsible for the extreme heat during mid-summer.
Astronomers express the brightness of stars in terms of stellar magnitude. The smaller the number, the brighter the star. The visual magnitude of Sirius star is -1.44, lower – brighter – than any other star. There are brighter stars than Sirius in terms of actual energy and light output, but they are farther away and hence appear dimmer.
Normally, the only objects that outshine Sirius in our heavens are the sun, moon, Venus, Jupiter, Mars and Mercury (and usually Sirius star outshines Mercury, too).
The star is present in ancient astronomical records of the Greeks, Polynesians and several other cultures. The Egyptians even went so far as to base their calendar on when Sirius star was first visible in the eastern sky, shortly before sunrise.
In 1718, English astronomer Edmond Halley discovered that stars have “proper motion” relative to one another, according to the Quarterly Journal of the Royal Astronomical Society. This means that stars, including Sirius star, move across our sky with a predictable angular motion with respect to more-distant stars.
From the Northern Hemisphere, Sirius star arcs across in the southern sky. From the Southern Hemisphere, it swings high overhead. It’s always easy to spot as the brightest point of light in its region of sky (unless a planet happens to be near it, which none are in early 2021).
More than 100 years after Halley’s finding, in 1844, German astronomer Friedrich Wilhelm Bessel published a scientific note in the Monthly Notices of the Royal Astronomical Society(opens in new tab) describing how Sirius star had been deviating from its predicted movement in the sky since 1755. Bessel hypothesized that an unseen companion star affected Sirius star’ motion.
Alvan Graham Clark, a U.S. astronomer and telescope maker, confirmed Bessel’s hypothesis in 1862, when the U.S. researchers spotted Sirius B through Clark’s newly developed great refractor telescope.
Sirius B is 10,000-times dimmer than Sirius star, according to NASA. It’s so dim, and therefore so difficult to see from Earth, that astronomers couldn’t estimate its mass until 2005, thanks to data from the Hubble Space Telescope.
The mass of a star is an important factor in the object’s stellar evolution, because it determines the star’s core temperature and how long and hot the star will burn. Astronomers can calculate the mass of a star based on its brightness, or luminosity, but this was challenging for Sirius B.
Sirius star has a small, faint companion star appropriately called the Pup. That name signifies youth, but in fact the companion to Sirius star is a dead star called a white dwarf. Once a mighty star, the Pup today is an Earth-sized ember, too faint to be seen without a telescope.
The luminosity of Sirius A overpowered ground-based observations, making it impossible to isolate the much dimmer luminosity coming from Sirius B, according to the Astronomical Society of the Pacific
In April, 2018, NASA launched the Transiting Exoplanet Survey Satellite (TESS), with the aim of its primary mission being to find exoplanets orbiting bright stars. Because Sirius star is a young star, it’s not likely to have planets orbiting it. TESS discovered 66 new exoplanets, according to NASA Exoplanet Exploration.