The sky — astronomy

Sirius: The Brightest Star in the Night Sky

Sirius (Alpha Canis Majoris) is the brightest star in the night sky because it is both much more luminous than the Sun and very close, about 8.6 light-years away. It seems to flash red, blue and green when low on the horizon because its light passes through a thick, turbulent layer of air that bends colors by slightly different amounts.

Why Sirius is so bright

Step outside on a winter evening in the Northern Hemisphere and look toward the south: one star outshines all the rest. That is Sirius, formally named Alpha Canis Majoris because it is the brightest star in the constellation Canis Major (the Greater Dog). Its apparent magnitude, the scale astronomers use for brightness where smaller numbers mean brighter objects, is about −1.46. Most stars you can see with the naked eye have positive values.

Two things explain this brilliance. First, Sirius is genuinely powerful. It is a white main-sequence star with roughly twice the mass of the Sun and about 25 times its luminosity. Its surface temperature is near 9,900 K, far hotter than the Sun's.

Second, it is close. Sirius lies only about 8.6 light-years away, making it one of the nearest star systems to our own. A light-year is the distance light travels in one year. The combination of “strong and near” makes it the leader of the night sky. Stars such as Rigel or Betelgeuse are far more luminous, but they are hundreds of light-years away. You can read about one of those giants in the article on Betelgeuse.

Sirius is easy to find: follow the line of the three belt stars of Orion down and to the left. Canis Major is one of the 88 constellations recognized by the International Astronomical Union (IAU). Because Sirius is the brightest star in the Greater Dog, it is nicknamed the Dog Star.

Distances to nearby stars are measured by parallax, the tiny apparent shift of a star against distant background objects as Earth orbits the Sun. Such measurements underpin modern catalogs, described in the article on the Gaia and Hipparcos star catalogs.

A double star: the white dwarf Sirius B

Sirius is not alone. The brilliant star we see, Sirius A, has a companion called Sirius B. It is a white dwarf, the dense leftover core of a star that has used up its nuclear fuel and shed its outer layers. Sirius B has about the mass of the Sun packed into a body roughly the size of Earth, so its matter is extraordinarily dense.

The discovery is a lovely example of scientific reasoning. In 1844 the German astronomer Friedrich Bessel noticed that Sirius did not travel in a straight line but wobbled slightly, and he concluded that an unseen companion must be orbiting it. In 1862 Alvan Graham Clark, while testing a new telescope, saw the companion itself: a faint point beside the dazzling primary.

The two stars circle a common center of mass about every 50 years. Their separation varies from roughly 8 to roughly 31 astronomical units (one astronomical unit is the average Earth–Sun distance). In visible light Sirius B is thousands of times fainter than Sirius A, so the glare of the primary makes it hard to spot in a small telescope. The Hubble Space Telescope and NASA's Chandra X-ray Observatory have both imaged the system.

For astronomers, Sirius B is a valuable natural laboratory for the physics of dense matter and for the ultimate fate of stars like our Sun.

Twinkling and the atmosphere

Many people notice that Sirius sometimes flashes red, blue and green, especially near the horizon. The star is not changing color by itself. The cause is Earth's atmosphere.

Starlight passes through layers of air with different temperatures and densities that are constantly moving. They bend the beam this way and that, an effect called scintillation, or twinkling. In addition, air refracts different colors by slightly different amounts, so the atmosphere acts like a weak prism. When Sirius is low, its light crosses a much thicker slice of air, the effect grows stronger, and pieces of the spectrum briefly show up separately as colored flashes.

Sirius is bright enough that the eye picks up these colors easily. Fainter stars twinkle too, but their color changes are hard to notice. Planets usually twinkle less because they appear as tiny discs rather than points, and the disturbances average out.

A practical tip: to enjoy the colorful show, watch Sirius when it is low in the sky on a clear, cold evening. Once it climbs higher, its light becomes steadier and whiter. For more advice on choosing a place and time, see how to watch the sky.

Sirius in ancient Egypt

To the ancient Egyptians Sirius was Sopdet, which the Greeks called Sothis. What mattered was its heliacal rising: the first morning of the year when the star briefly appears in the east before dawn, after a period when it is lost in the Sun's glare. In ancient Egypt this event fell roughly around the time the annual Nile flood began, so the star was linked with renewal and with the goddess Isis.

Observations of Sopdet also gave rise to the so-called Sothic cycle, the period it takes for the 365-day civil calendar, slowly drifting against the slightly longer true year, to line up again with the star's heliacal rising. Historians use it as a tool in dating.

The Romans connected the hottest part of summer, the “dog days,” with the heliacal rising of Sirius, although the star does not cause the heat; the seasons come from the tilt of Earth's axis. In this way Sirius left its mark on language, calendars and the cultures of many peoples, and today it remains a perfect starting point for anyone learning to read the night sky.

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