Because Sirius 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.
The Moon And Star On Earth
Sirius 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 (opens in new tab), Sirius has a mass that's two times that of Earth's sun. If the 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 (opens in new tab).
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, 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.
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 was first visible in the eastern sky, shortly before sunrise. According to Joe Rao of Space.com, the Egyptians called Sirius the "Nile Star," because it always returned just before the river rose and so announced the coming of the floodwaters that would nourish their lands.
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 (opens in new tab). This means that stars, including Sirius, move across our sky with a predictable angular motion with respect to more-distant stars.
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 had been deviating from its predicted movement in the sky since 1755. Bessel hypothesized that an unseen companion star affected Sirius' 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.
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. 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 (opens in new tab).
It wasn't until 2005, when a team of astronomers assembled data collected by the Hubble Space Telescope, that scientists were able to measure the mass of Sirius B for the first time. They found that the star has a mass that is 98 percent that of Earth's sun.
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 is a young star, it's not likely to have planets orbiting it. TESS discovered 66 new exoplanets, according to NASA Exoplanet Exploration (opens in new tab), but none have been discovered orbiting Sirius.
The two main stars are Alpha Centauri A and Alpha Centauri B, which form a binary pair. They are about 4.35 light-years from Earth, according to NASA (opens in new tab). The third star is called Proxima Centauri or Alpha Centauri C, and it is about 4.25 light-years from Earth, making it the closest star other than the sun.
Proxima Centauri, meanwhile, is about one-fifth of a light-year or 13,000 AUs from the two other stars, a distance that makes some astronomers question whether it should even be considered part of the same system.
Proxima Centauri may be passing through the system and will leave the vicinity in several million years, or it may be gravitationally bound to the binary pair. If it's bound, it has an orbital period around the other two stars of about 500,000 years.
To the naked eye, the Alpha Centauri A and B shine as one, making them the third brightest "star" in our night sky. The two separate stars can be seen through a small telescope, making the system one of the finest binary stars that can be observed. Proxima Centauri is too faint to see unaided, and through a telescope it appears about four diameters of the full moon away from the other two.
Alpha Centauri A, also known as Rigel Kentaurus, is a yellow star of the same type (G2) as the sun, although slightly larger, according to NASA (opens in new tab). It is three times closer to Earth than the next nearest star like our sun, according to NASA (opens in new tab).
The planet is also in the star's habitable zone, that just-right range of distances from a star where liquid water can exist on the surface of a body. Proxima b lies just 4.7 million miles (7.5 million km) from its host star and completes one orbit every 11.2 Earth-days.
As a start, scientists need to be able to look for signs of an atmosphere. From there, the investigators can extrapolate whether that atmosphere (if present) allows liquid water to flow on the surface. Even the surface temperature of the planet, which would also affect habitability characteristics, depends on the atmosphere.
The planet may also be so close to its star that it is tidally locked, meaning it always shows the same face to its host star, just as the moon shows only one face (the near side) to Earth. This arrangement would make one side of the planet very warm and the other very cold unless winds could distribute the heat around the planet. If that stark temperature difference does exist, it would be a severe challenge to any life.
Scientists are continuing to study red dwarf stars to better understand the habitability of worlds like Proxima b. (NASA's Transiting Exoplanet Survey Satellite, or TESS, is an exoplanet-hunter particularly adept at spotting planets around this category of star.)
In November 2017, scientists discovered Ross 128b, another planet in the habitable zone of a red dwarf that is nearly as close to Earth as Proxima Centauri is, but that appears to be a much quieter star.
However, finding out more about its atmosphere will require a next-generation ground-based telescope. (The James Webb Space Telescope, set to launch in late 2021, can't gather the necessary observations because the planet does not transit across the face of its star.)
The former Soviet Union logged the first successes with its Luna program, starting with Luna 1 in 1959. NASA followed with a series of robotic Ranger and Surveyor spacecraft that performed increasingly complex tasks that made it possible for the first human beings to walk on the Moon in 1969.
American astronauts have planted six American flags on the Moon. But that doesn't mean the United States has claimed it; in fact, an international law written in 1967 prevents any single nation from owning planets, stars, or any other natural objects in space.
The moon, meanwhile, is a relatively small, solid mass. As you probably know, people have even landed on the moon and walked on its surface. That type of activity would never be possible with a star due to the heat and non-solid surface. Despite its luminous appearance, the moon has remarkably little in common with the sun and other stars.
There are over 3,000 satellites currently orbiting Earth, but only one of them is natural. That natural satellite, of course, is the moon. Made of rock and metal, the moon has existed alongside Earth for over 4.5 billion years.
Little Pluto is so small and remote it was not discovered until 1930, orbits 40 times farther from the Sun than we do. Light from the Sun takes about 5 and 1/2 hours to reach it and roughly the same time to return to Earth. By the time the light reaches us, it has spread out so much that the planet looks very dim, and requires a good telescope to spot. PhotoPhoto of Pluto and its moon Charon, as seen with the Hubble Space Telescope in 1994.
The brightest star in our skies is the "dog star", Sirius (pronounced Sea' ree us). It's the primary star in the constellation of the big dog, Canis Major. Sirius is roughly 9 light years away. Think of what you were doing 9 years ago. That's when the light we see from Sirius tonight first began its journey to us. Not far from Sirius in the sky is the bright star Betelgeuse (pronounced Beetle' juice). It is so far that its light takes 430 years to reach us. Light that we see tonight from Betelgeuse left it in the late 1500's.
In the same part constellation, Orion, as Betelgeuse but even farther away is the Orion Nebula, a place where we see new stars forming. Its distance is 1500 light years, meaning that the light we see from it left more than a thousand years before the invention of the telescope.
The farther away an object in space lies, the longer it takes its light to get to us and the older that light is when it reaches Earth. As we look deeper and deeper into the Milky Way Galaxy (the island of stars in which we live), we are looking deeper into the past. Light can take tens of thousands of years or more to reach us from distant parts of our galaxy, which is roughly 100,000 light years wide.
Earth is intimately tied to the star around which it revolves, the Sun, and the satellite that revolves around Earth itself, the Moon. Without the Sun, of course, life on Earth simply could not exist, not just because of the need for light but to an even greater degree because of the energy it supplies. For that matter, Earth itself would not exist: our planet appears to have developed from the same cosmic cloud that formed the Sun, and without the Sun to hold it in place with its gravitational pull, Earth would go spinning off into space. For all its influence on human life, however, the Sun has less impact on the tides than the Moon, which is smaller but much closer. Together, these two bodies literally define time in human experience, which has been marked by the movements of the Sun and Moon from a time before civilization began.
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