If you look up the details of W Ursae Majoris, you’ll find that its two stars are about a million miles apart. The way astronomers figure that distance, though, is from the centers of the two stars. When you measure the distance between their surfaces, the stars are a whole lot closer. In fact, they’re touching. That makes them a contact binary – one of thousands discovered so far.
Many stars move through the galaxy with one or more companion stars. Their distances from each other vary greatly. Some can be light-years apart. But if they’re born close to one another, they might eventually spiral together. That might be caused by magnetic fields, the exchange of gas between the stars, or some other process.
W Ursae Majoris shows how that plays out. One of its stars is a little bigger, heaver, and brighter than the Sun. The other is about half the Sun’s mass. They’re in such close contact that they share their outer layers of gas. That makes them about the same temperature and color.
But they’re not the same brightness. As the stars orbit, once every eight hours, they cross in front of one other as seen from Earth. So the system’s brightness varies – the result of sibling stars in a tight embrace.
W Ursae Majoris is in the great bear, which includes the Big Dipper. The dipper is high in the sky at nightfall. W Ursae Majoris is well to the upper left of its upside-down bowl, and is visible through binoculars.
Script by Damond Benningfield
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2:15
Moon and Antares
The most massive stars are seldom alone. Most of them have one or more companions – stars that are bound to each other by their mutual gravitational pull. Such stars were born together, from the same giant cloud of dust and gas. Dense clumps in the cloud collapsed and split apart, giving birth to heavy stars.
One possible example is Antares, the bright star that represents the heart of Scorpius. It stands close to the left of the Moon at dawn tomorrow.
The star we see as Antares is a supergiant. It’s more than a dozen times as massive as the Sun, and hundreds of times wider. If it took the Sun’s place in our own solar system, it would swallow the four innermost planets, including Earth.
Another big star accompanies the supergiant. It’s about seven times the Sun’s mass, and five times its diameter.
But it’s unclear whether the two stars actually form a binary. They move through space in the same direction and at the same speed. But they’re separated by more than 500 times the distance from Earth to the Sun – an especially wide gap. At that range, it would take up to a couple of thousand years for the stars to orbit each other. But we’ve only known about the smaller star for a few hundred years. That’s not long enough to trace any possible orbital motion.
So while the two stars probably form a binary, the case isn’t quite conclusive.
We’ll talk about some stars that are definite binaries tomorrow.
Script by Damond Benningfield
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2:15
Lost Twin?
The Sun is an unusual star in several ways. It’s the only star known to support life. It’s in the top 10 percent of the most-massive stars in the galaxy. And it’s in the minority in another way: it moves through the galaxy alone.
Like all stars, the Sun probably was born in a cluster – a group of hundreds or thousands of stars. But the Sun’s birth cluster fell apart, with the stars going their own ways.
Most stars hang on to one or more of their siblings. Most of them form binaries – two stars bound by their gravity. Others form systems of three stars or more – all the way up to seven. The heavier a star, the more likely it is to have companions. For stars like the Sun, the odds are about 50-50.
The Sun has no companions today. But it could have in the distant past. In its crowded nursery, close encounters with other stars could have pulled the Sun and any possible companions away from each other.
Astronomers haven’t found any long-lost binary “twin” of the Sun. But they have found some stars that could have been born in the same cluster. A great example is HD 162826. Astronomers at McDonald Observatory found that its composition and motion are very close matches to the Sun’s.
HD 162826 rises above Vega, the brightest star of Lyra. They’re in the northeast in late evening. But you need binoculars to see this likely sibling to the Sun.
More about binaries tomorrow.
Script by Damond Benningfield
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2:15
Bellatrix
Orion is packed with stars that are much bigger, brighter, and heavier than the Sun. But only one of them is likely to draw hisses from Harry Potter fans: Bellatrix. Its name comes from Latin, and means “female warrior.” The name was applied to one of Harry Potter’s nastiest foes – Bellatrix Lestrange.
Regardless of its name, the star itself is impressive. It’s almost eight times the mass of the Sun, so it “burns” its nuclear fuel in a hurry. As a result, the surface of Bellatrix is tens of thousands of degrees hotter than the Sun. Combined with its great size, that makes the star thousands of times brighter than the Sun.
Because of its high-speed reactions, Bellatrix won’t live very long. The star is about 25 million years old – just half of one percent the age of the Sun. Yet it will expire in a few million years. It’s probably not massive enough to explode as a supernova. Instead, it will expel its outer layers in a more gentle process. That will leave a small, hot, dense corpse – a feeble ending for a powerful star.
Orion is in the west and southwest at nightfall. Look for its three-star belt, parallel to the horizon. The hunter’s brightest stars, Betelgeuse and Rigel, stand above and below the belt. Bellatrix is the next-brightest, to the upper right of the belt. It’s among the brightest stars in the night sky.
Script by Damond Benningfield
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2:15
Moon and Spica
The full Moon has an especially close companion tonight: Spica, the brightest star of Virgo. From the eastern half of the country, they’ll appear to almost touch each other as night falls. They’ll be a bit farther apart for those in the west, but still close.
A dozen American astronauts walked on the Moon, with the final steps coming half a century ago. Their missions are among those commemorated today – the International Day of Human Spaceflight. The day was established by the United Nations, in 2011, to “celebrate the beginning of the space era for all mankind.”
That era began on this date in 1961, when the Soviet Union launched the first human into space. Yuri Gagarin made one orbit around Earth aboard Vostok 1, then parachuted to the ground after re-entering the atmosphere. Exactly 20 years later, the United States launched the first space shuttle. The U.N. General Assembly named those missions, along with the first Moon landing, as motivation for the Day of Human Spaceflight.
The first celebration of the date came in 2001. A private group created “Yuri’s Night,” which continues today. It combines science and engineering with music, art, and other activities.
Space centers, museums, planetariums, and other venues host events to celebrate the date. If you can’t make it, though, celebrate on your own by watching the Moon – the site of the first human steps beyond our own planet.
Script by Damond Benningfield
USA