Sirius B is a white dwarf star and the faint companion to Sirius A, the brightest star in Earth's night sky (also called the "Dog Star"). It is part of the Sirius binary system, located in the constellation Canis Major, or the "Greater Dog."
Sirius B is a white dwarf, which means it is the leftover part of a star that used to have a moderate mass and ran out of fuel. It is the closest example of a white dwarf to Earth and is the secondary star in the Sirius system. Sirius A is the brightest star in the night sky, while Sirius B is too dim to be seen with the naked eye. Its brightness is only 2% of the Sun's. The Sirius system is the fifth-closest star system to the Sun, located 8.6 light-years away.
Like all white dwarfs, Sirius B is very dense. Its size is similar to Earth, but its mass equals that of the Sun. It is the leftover material from a star that was five times the mass of our Sun and existed about 100 million years ago. Sirius B has a current temperature of 25,000 K (24,700 °C; 44,500 °F), which is 2.5 times hotter than Sirius A and over four times hotter than the Sun. It no longer produces energy through nuclear fusion and will gradually cool as its heat escapes into space over time.
Because Sirius B is close to Earth, scientists have studied it for many years to search for planets orbiting it. So far, no planets have been found around Sirius B. However, current observations cannot completely rule out the possibility of some planetary structures existing.
Background
White dwarfs are the leftover parts of stars that are about the same size as our Sun after they use up their fuel. Stars create energy by combining hydrogen atoms into helium atoms. This energy keeps the star balanced between gravity pulling it in and the outward pressure from the energy. Over time, the core of these stars runs out of hydrogen, causing the outer layers to expand and the star to become a red giant, growing much larger than before. When the core no longer produces energy, it shrinks. Later, hydrogen fusion begins in a layer around the core, and if the core becomes hot enough, helium turns into carbon. In the final stage, the star releases its outer layers, leaving behind a dense core known as a white dwarf.
White dwarfs do not create energy through fusion anymore. Instead, they slowly release their leftover heat and cool over time. Eventually, they will become black dwarfs, but this process is expected to take over 10 trillion years—far longer than the current age of the universe, which is about 14 billion years. White dwarfs are extremely dense, with densities ranging from 100,000 to 100,000,000 grams per cubic centimeter. A small amount of white dwarf material, like a teaspoon, could weigh as much as 5.5 tons. One example is ZTF J1901+1458, which is more than 1.3 times the mass of the Sun but has a radius about the size of a distance between the Moon and Mercury. Other white dwarfs, such as Procyon B and van Maanen 2, have masses of 0.6 and 0.7 times the Sun’s mass, respectively, and sizes about 0.012 and 0.011 times the Sun’s radius.
Discovery and observations
The discovery of Sirius B began in the mid-1800s. In a letter dated August 10, 1844, the German astronomer Friedrich Wilhelm Bessel noticed that the seeming movement of Sirius A was not steady, a finding never seen before. This was the first clue that Sirius had an unseen companion. The dim companion, Sirius B, was first seen on January 31, 1862, by the American telescope-maker and astronomer Alvan Graham Clark. This happened during tests of an 18.5-inch (470 mm) aperture refracting telescope for the Dearborn Observatory. At the time, this was one of the largest refracting telescope lenses in the world and the largest telescope in the United States. Sirius B’s discovery was confirmed on March 8, 1862, using smaller telescopes.
In 1915, Walter Sydney Adams used a 60-inch (1.5 m) reflector telescope at Mount Wilson Observatory to study the light patterns of Sirius B. He found it was a faint, whitish star. This led scientists to conclude that Sirius B was a white dwarf, the second such star discovered after 40 Eridani B.
Because Sirius A is much brighter and closer to Sirius B, observing Sirius B was difficult in the 20th century. Measuring properties like size and temperature was especially hard due to the brightness of Sirius A. However, the mass of Sirius B could be measured more easily by studying the orbit of the binary star system. In ultraviolet light, Sirius B is brighter than Sirius A. With the development of ultraviolet satellites in the second half of the 20th century, such as the Extreme Ultraviolet Explorer (EUVE) and the International Ultraviolet Explorer (IUE), scientists made precise measurements of Sirius B’s properties. In 1998, astronomers combined data from EUVE and IUE to find that Sirius B had a temperature of 25,000 ± 35 K and a surface gravity of 10, more precise than earlier measurements. A year earlier, the Hipparcos mission measured the distance to the Sirius system using parallax, finding it to be 2.637 ± 0.011 parsecs (8.601 ± 0.036 light-years), a 20% improvement over previous estimates. Using this distance and ultraviolet data, scientists calculated Sirius B’s radius as 0.0084 ± 0.00025 times that of the Sun.
Characteristics
The mass of Sirius B can be found using Kepler's third law, which connects mass, the time it takes to orbit another star, and the average distance between the two stars. The first measurement in 1910 showed a mass of 0.94 times the Sun's mass. Later measurements, like one from 2017, found a mass of 1.018 times the Sun's mass with a small range of uncertainty. Sirius B is one of the most massive white dwarfs known, nearly twice the average mass of 0.6 times the Sun's mass.
The radius of Sirius B is 0.008098 times the Sun's radius, or about 5,635 kilometers (3,501 miles), which is 0.88 times Earth's radius (6,378 kilometers). This is much smaller than other white dwarfs, such as Procyon B or van Maanen 2. White dwarfs have a unique feature: the more massive they are, the smaller their size. Sirius B's mass and radius can also be measured using gravitational redshift, which gives values of 1.017 times the Sun's mass and 0.00803 times the Sun's radius.
White dwarfs do not create heat and cool over time. Scientists estimate their age as white dwarfs by calculating how long it took for them to cool to their current temperature, called the "cooling age." Sirius B has a mass of 1.02 times the Sun's mass and a temperature of 25,000 K (24,727 °C). This is much hotter than the Sun (5,772 K or 5,499 °C) or Sirius A (9,845 K or 9,572 °C). Using this method, Sirius B is estimated to be 126 million years old as a white dwarf. This is about half the total age of its system, which is 230 million years. The other half is the time it took for the star to burn its fuel and become a white dwarf.
Theories suggest Sirius B's original star had a mass of 5.0 times the Sun's mass and was a B-type main-sequence star of class B5V. Its higher mass means it was once brighter than Sirius A and burned its core hydrogen faster. The system's orbit was also smaller in the past, with the closest point between the stars predicted to have been between 1.5 and 1.6 AU. During its red giant phase, Sirius B expanded to hundreds of times the Sun's size, possibly transferring some mass to Sirius A without engulfing it.
Sirius B is mainly made of carbon and oxygen, created by helium fusion in its original star. A layer of lighter elements covers this, with materials separated by mass due to strong gravity. The outer atmosphere of Sirius B is now almost entirely hydrogen, the lightest element, and no other elements are seen in its spectrum.
Search for planets
Sirius B has been studied many times to search for planets that might orbit it. Scientists have used methods such as measuring changes in the star's speed, taking direct images, and tracking changes in the star's position. However, no planets have been found around Sirius B. Using special telescopes like VLT/SPHERE and the Hubble Space Telescope, researchers have determined that giant planets with masses about 10–35 times that of Jupiter cannot exist near Sirius B at distances of a few astronomical units (AU). Data from Gaia DR3 shows changes in Sirius B's movement that may or may not be fully explained by its companion star, Sirius A. A small planet with a mass less than 1–2 times that of Jupiter, located between 0.5 and 1.3 AU from Sirius B, cannot be ruled out. Other white dwarf stars, such as PSR B1620−26, have been found to have planets orbiting them.