StarDate Billy Henry

When it comes to understanding a star, it’s all a matter of perspective. The angle at which you view the star makes a big difference in what you know about it.
Consider Vega, the leading light of the constellation Lyra and one of the brighter stars in the northern sky. It’s in the northeast at nightfall, and climbs high overhead later on.
For a long time, astronomers thought Vega was about three times as massive as the Sun, and no more than a hundred million years old. So when they discovered a cloud of dust grains around Vega, they thought it might be raw material for planets.
But it turns out they were seeing Vega from a different angle than thought. We’re looking almost directly down on one of the star’s poles. That perspective makes it more difficult to measure Vega’s details.
Once they knew the correct angle, astronomers determined that the star spins so fast that it’s almost ripping itself apart. They also found that Vega’s a bit smaller than thought, and hundreds of millions of years older.
Today, perhaps the best estimate puts Vega’s age at about 455 million years. That’s too old for the dust around Vega to be making new planets. In fact, Vega appears to already have at least one planet — a giant that’s much bigger than Earth, and much closer to its star. So the dust probably is debris from collisions between asteroids or other bodies — maybe even fully grown planets.
Script by Damond Benningfield

Like a mixed litter of puppies, sibling stars don’t necessarily look alike. That can make it hard to figure out which stars are related to each other.
One example is Regulus, the heart of the lion. It’s close to the left or lower left of the Moon as darkness falls.
To the eye alone, Regulus looks like a single bright star. Instead, it’s a system of four stars — two pairs of stars that are separated by a third of a light-year.
What we see as Regulus is the brightest of the four stars. Known as Regulus A, it’s much bigger, heavier, and brighter than the Sun. It’s paired with a star that’s so faint, and so close to Regulus A, that we can’t actually see it, even with the largest telescopes. It reveals its presence only to special instruments. It’s probably a stellar corpse — a white dwarf.
The other pair is known as Regulus BC. Both of its stars are smaller, less massive, and fainter than the Sun. One of them, in fact, is less than one percent of the Sun’s brightness.
Despite the differences in the four stars, there’s evidence that they’re all siblings. They’re all about the same distance away, for example — about 79 light-years. The pairs are close enough to each other for their gravity to hold them together. And the stars move through the galaxy in the same direction, and at the same speed — indications that they’re members of a stellar family.
Script by Damond Benningfield

Our home galaxy is a cosmic melting pot. While many of its stars were born in the Milky Way, many others came from outside. They were born in smaller galaxies that were captured by the Milky Way. Over time, the smaller galaxies were ripped apart, and their stars were scattered throughout the Milky Way. And one of the Sun’s close neighbors may be an example.
Kapteyn’s Star is about 13 light-years away. Only about 20 other star systems are closer. The star is about a third the size and mass of the Sun. Don’t try looking for it, though — it’s only one percent of the Sun’s brightness, so you need a telescope to see it.
The star’s composition and motion don’t match that of the stars that are native to the Milky Way — or at least to the Milky Way’s bright disk. Kapteyn’s Star has a lower proportion of heavier elements — an indication that it’s extremely old. And it orbits the center of the galaxy in the opposite direction from most of the stars around it.
The combination suggests that Kapteyn’s Star came from the Milky Way’s halo, a vast region that surrounds the disk. In fact, there’s evidence that Kapteyn’s and a few other stars came from a giant cluster known as Omega Centauri.
It’s likely that the cluster is the core of a small galaxy captured by the Milky Way long ago. The stars in the galaxy’s outer precincts were pulled away — making them new residents of our home galaxy.
Script by Damond Benningfield

The biggest globular star cluster in the Milky Way Galaxy may not be a child of the Milky Way. Instead, it may be a sort of orphan — the surviving core of a smaller galaxy that was captured by the Milky Way.
Omega Centauri contains perhaps 10 million stars, all packed into a dense ball about 150 light-years across. In the cluster’s middle, the stars are packed so tightly that they’re only about a tenth of a light-year apart. Compare that to our part of the galaxy, where the nearest neighbor star is more than four light-years away.
The cluster appears to be about 12 billion years old — one of the older clusters in the entire galaxy. But the composition of its stars, and the way it orbits the center of the Milky Way, suggest that Omega Centauri wasn’t born here. Instead, it appears to be one of many dwarf galaxies captured by the Milky Way. Over time, the stars in the smaller galaxy’s outer regions were pulled away. So over the eons, that left only Omega Centauri’s core — a possible “orphan” adopted by the Milky Way.
Omega Centauri is about 16,000 light-years away, in the constellation Centaurus, which is quite low in the south at nightfall. If you live south of about Dallas, and you have dark skies, you might just make out the cluster as a hazy patch of light about as wide as the Moon.
We’ll talk about one of the possible escapees from Omega Centauri tomorrow.
Script by Damond Benningfield

As you look out into the night sky, you might think that you’re seeing a true sampling of the stars — in other words, a little of everything. That’s not the case, though. Most of what you see is the Milky Way’s equivalent of the one percenters — stars that are among the galaxy’s biggest and brightest.
An example is in the constellation Centaurus, which wheels quite low across the south on June nights. It’s so low, in fact, that much of it stays below the horizon for those of us in the United States. That includes its brightest star, Alpha Centauri.
The brightest star most of us can see in Centaurus is Menkent, a name that means “the shoulder of the centaur.” It’s due south at nightfall.
Like many of the other stars that are visible to the unaided eye, Menkent looks so bright because it’s nearing the end of its life. It’s converted the original hydrogen fuel in its core to helium. That’s made the core get smaller and hotter, which in turn made the star’s outer layers puff up. Today, Menkent is more than 10 times wider than the Sun, and about 60 times brighter.
Dying stars like Menkent are rare. Most of the stars in the galaxy are still in the prime of life, so they’re not as bright as they’ll become later on. But because stars like Menkent are so big and bright, they’re greatly over-represented in the stars we see — stellar giants glowing brightly in the night sky.
Script by Damond Benningfield

Earth lies in the middle of the Sun’s habitable zone. That’s the distance from the Sun where conditions are most comfortable for life. And astronomers are concentrating their hunt for life in other star systems in their habitable zones.
There’s an idea that galaxies have habitable zones as well. The zones would have a good mix of chemical elements, not too much radiation, and a low risk of exploding stars.
Stars consist mainly of hydrogen and helium. But to make planets like Earth, you need a good supply of heavier elements — things like oxygen, silicon, and iron. Those elements are forged in the hearts of stars, then blown out into space when the stars die. So to have good conditions for life, you need a region where lots of dead stars have “seeded” the galaxy with heavy elements.
But you don’t want to be too close to stars that die in massive explosions. They can damage a planet’s atmosphere, exposing life to high levels of radiation. So you want to be away from a galaxy’s core, where stars are packed close together — including ones that might explode.
The cores of most galaxies also harbor supermassive black holes. As they pull in stars and gas they produce lots of radiation, saturating the space around them.
So in our home galaxy, the Milky Way, you want to be away from the core, but not too far away. And that’s just where Earth is — about half way from the center to the edge — in the galactic habitable zone.
Script by Damond Benningfield