A star in the constellation Cetus brightens and fades dramatically every 11 months. At its brightest, it’s fairly easy to see. At its faintest, it’s visible only through a telescope. Because of that change, a 17th-century astronomer called the star Mira – from the Latin word for “wonderful.”
The star changes because it pulses in and out like a beating heart. Mira’s in the final stages of its red-giant phase of life. Its core is no longer producing nuclear reactions. Instead, it’s fusing hydrogen and helium in thin shells around the core.
Mira’s outer layers are puffed up by radiation from the shells. At the maximum, that inflates the star to about 400 times the diameter of the Sun. That’s also when its surface is coolest and faintest. As the outer layers cool, they fall inward, making the surface hotter and brighter. At minimum, the star is about 330 times the Sun’s diameter.
Each time it puffs up, Mira loses a little of the gas at its surface. Within the next million years or so, it’s likely to expel all the gas in its outer layers. That will leave only its hot but dead core – a white dwarf.
Astronomers have discovered thousands of stars like Mira. And many others will undergo the same phase, including the Sun – in about six billion years.
Mira climbs into view in the east by 8:30 or 9. But it’s in the “fading” part of its cycle, so you need a telescope to see it.
Script by Damond Benningfield
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Eclipsing Binaries
Algol does something amazing. Every 2.9 days, the star fades to just one-third of its usual brightness. In centuries past, the stars were thought to be unchanging. A star that changed so blatantly was a bit scary. So it was given a name to match: “Algol” comes from an Arabic phrase that means “head of the demon.”
But the star’s odd behavior isn’t scary it all – Algol fades as the result of eclipses.
The system consists of three stars. Two of them form a tight binary. The members of the binary orbit each other once every 2.9 days. We see the system edge-on, so the two stars eclipse each other. One star is much brighter than the other. When the fainter star crosses in front of it, the system fades dramatically. When the bright star covers up the faint one, though, the difference is tiny – much too subtle to see with the eye alone.
Astronomers have cataloged hundreds of eclipsing binaries. And the eclipses are important. They reveal the relative sizes and masses of the two stars, details about their orbit, and more. So there’s nothing to fear from these up-and-down star systems.
Algol is low in the northeast at nightfall, in Perseus. It should be at its brightest tonight. The faint part of its cycle will happen during daylight for the next few cycles. It’ll be visible during nighttime later in the month.
Sometimes, a star can change brightness all on its own, and we’ll have more about that tomorrow.
Script by Damond Benningfield
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Orionid Meteors
The Orionid meteor shower should be at its most active the next few nights. And there’s no Moon to get in the way, so it should be a pretty good show.
The shower is named for Orion because its meteors appear to “rain” into the sky from Orion the hunter. The constellation climbs into good view after midnight, so that’s when the shower is at its best – between midnight and dawn. You don’t have to look at Orion to see the meteors, though – they can blaze across any part of the sky.
The meteors are bits of debris from Comet Halley. The comet sheds grains of dust as it orbits the Sun. When Earth crosses the comet’s path, some of those grains plunge into the atmosphere. They instantly vaporize, creating the streaks of light known as meteors.
Most of the dust grains are no bigger than pebbles. But a few are larger. They form brilliant streaks that are visible even in a somewhat light-polluted sky. And some of them can leave glowing trails that remain visible for a couple of minutes.
The shower has been declining in recent years. Halley’s Comet is near its greatest distance from Earth, so there aren’t as many bits of comet dust in this part of its orbital path. Even so, the shower could produce 20 or more meteors per hour at its peak.
To watch the Orionids, find a dark but safe site away from city lights. Bundle up against the autumn chill, then sit back and watch the sparks from Halley’s Comet.
Script by Damond Benningfield
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Moon and Venus
Venus doesn’t have any moons. But it does share its orbit around the Sun. Astronomers have discovered 20 asteroids known as “co-orbitals,” but there could be many more. These big space rocks follow roughly the same path as Venus. But they won’t stay in that lane forever. And when they leave it, they could threaten Earth.
These objects are nudged along by the gravity of Venus and the Sun. They generally stay well ahead of or behind Venus. Only one follows exactly the same orbit as the planet. The others move in and out a bit, getting closer to the Sun, then moving farther away.
Over the long term, though, their orbits aren’t stable, so they can break free and head elsewhere. A recent study found that of the 20 known objects, six could threaten Earth within the next 12,000 years. And three of them are especially dangerous. All three are at least a thousand feet in diameter, so they could cause major damage if they hit our planet.
A study also found that there could be many more of these Venus groupies. They stay so close to the Sun in our sky that they’re hard to see through the solar glare. And they move quickly, making them even harder to find. But a new telescope in Chile might pick out some of them – helping us find potential threats far in advance.
Look for Venus near the Moon in the dawn sky tomorrow. It’s the brilliant “morning star,” so you can’t miss it.
Tomorrow: an autumn meteor shower.
Script by Damond Benningfield
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California Nebula
California is the land of the stars. It’s also in the stars as the California Nebula – a cloud of gas and dust that looks like the outline of the state. It’s more than a thousand light-years away, in Perseus.
The nebula belongs to a giant star-forming complex – the Perseus O-B-2 association. The region has given birth to many class O and B stars – the biggest and brightest of all stars.
The California Nebula probably is energized by one of those stars, known as Xi Persei. The star is more than 30 times the mass of the Sun, and tens of thousands of degrees hotter. At that temperature, it produces huge amounts of ultraviolet energy.
When that radiation zaps hydrogen atoms, it splits them apart. When they link back up, the atoms produce red light – the main color of the nebula. Oxygen and other elements produce their own colors, but they’re not nearly as common as hydrogen.
The California Nebula probably is about a hundred light-years long. It’s likely to split into smaller clumps that will collapse to form even more stars. But radiation and winds from Xi Persei and other big stars will blow away much of the nebula’s material – limiting the number of new stars for this cosmic California.
Perseus climbs into good view, in the northeast, in early evening. Xi Persei is visible to the naked eye, near the bottom of the constellation. But you need a telescope to see the faint outline of the California Nebula.
Script by Damond Benningfield
USA