StarDate Billy Henry

When astronomers look at Spica, they see double. Or at least their instruments do. The system consists of two stars. But they’re so close together that even the biggest telescopes see them as a single point of light. So it took a technique called spectroscopy to “see” the system as a binary.



The technique splits the light from a star into its individual wavelengths. Each chemical element imprints its own “barcode” in that spectrum of light.



In the case of Spica, there are two sets of those barcodes. And they shift back and forth a tiny bit — the result of the orbital motion of the stars around each other.



Careful study of the two spectra has revealed many details about the system.



For example, the main star is much bigger and more massive than the Sun. It’s destined to explode as a supernova. The other star isn’t quite as impressive, but still far more impressive than the Sun. It probably won’t explode, but instead will leave a small but heavy corpse.



The stars orbit each other once every four days. Their surfaces are only a few million miles apart — so close that the gravity of each star distorts the shape of the other — one more amazing finding about this impressive double star.



To the eye alone, Spica looks like a single bright star. Tonight, it’s close to the lower left of the Moon at nightfall. The Moon slides toward it during the night, so they’re especially close as they set, in the wee hours of the morning.



Script by Damond Benningfield

A horse and rider gallop across the north and northwest on May evenings. They’re in the handle of the Big Dipper, which is high in the sky at nightfall and low in the northwest at dawn.



They’re the stars Mizar and Alcor. Mizar is the brighter of the two, with fainter Alcor just a whisker away. They’re so close together that the skywatchers of bygone centuries thought of them as a horse and rider.



Mizar is a system of four stars, all of which are a little hotter and brighter than the Sun. Alcor consists of two stars — one of them more impressive than the Sun, the other less impressive. Both systems lie about 80 light-years from Earth.



One question that astronomers have asked for centuries is whether the two systems are bound to each other, or if they just happen to line up in the same direction in the sky.



They’re both members of a wide-spread cluster. That makes Mizar and Alcor stellar siblings — they formed at the same time, from the same cloud of gas and dust.



Just how close their relationship is has remained a mystery. Early observations said the systems were perhaps two or three light-years apart. At that range, they probably would not be gravitationally bound to each other.



But observations over the past few years by the Gaia space telescope put the separation at roughly one light-year. That may be close enough for them to be saddled together as a single unit — just like a horse and rider.



Script by Damond Benningfield

Millions of black holes inhabit our home galaxy, the Milky Way. Most of them probably roam the galaxy alone, so we never see them. Luckily for science, though, many of them have companion stars. That makes it possible for astronomers to “see” and learn about the black holes.



An example is a system called Swift J1357. It was discovered in 2011 by Swift, a space telescope that studies the X-ray sky. The system is thousands of light-years away, and appears to be outside the galaxy’s disk of stars.



J1357 consists of two objects: a black hole and a small, faint companion star. The black hole’s gravity pulls in hot gas from the companion. The gas spirals around the black hole, heating up and forming a faint disk.



The black hole appears to be at least nine times the mass of the Sun. It and the companion orbit each other once every two and a half hours — one of the tighter orbits of any known black-hole binary system.



J1357 produces outbursts of X-rays every few years. They may occur when too much gas piles up in the disk. It gets so hot that it causes an eruption of particles and energy — an outburst that reveals more about this intriguing system.



Swift J1357 is much too faint to see with the eye alone, but we can spot its location. It’s close to the left of Spica, the brightest star of Virgo. It’s about a third of the way up the southeastern sky at nightfall.



Script by Damond Benningfield

Jupiter is missing. The solar system’s biggest and heaviest planet is nowhere to be seen. And it won’t return to view for a few weeks.



Jupiter is passing behind the Sun as seen from Earth, so it’s hidden in the Sun’s brilliant glare. It’ll be closest to the Sun on Saturday afternoon. That point is known as conjunction, and it happens every 12 and a half months.



Jupiter is known as a “superior” planet. That means its orbit is outside Earth’s orbit around the Sun. So Jupiter sometimes lines up directly opposite the Sun in our sky, shining brightly all night long. About half a year later, though, it passes behind the Sun and out of sight.



The planet doesn’t necessarily pass directly behind the Sun. This time, for example, it slides about half a degree from the Sun — less than the width of your finger held at arm’s length. Even so, it’s so close to the Sun that astronomers can’t point their telescopes toward it. And radio “static” from the Sun means that flight controllers lose contact with spacecraft at Jupiter for a while. The craft operate on their own, and save their observations to beam to Earth when the Sun is out of the way.



For casual skywatchers, Jupiter should return to view in a month or so. It’ll be quite low in the eastern sky during the dawn twilight. It’ll be in better view as we move into summer — climbing farther from the Sun, and shining like a brilliant beacon in the early morning sky.



Script by Damond Benningfield

The Moon and the heart of the lion stand close together this evening. The lion’s heart is the star Regulus, to the lower right of the Moon at nightfall.



We see Regulus and the Moon for different reasons. Regulus is a star — a brilliant ball of hot gas. It generates its own light — so much light that it’s visible across almost 80 light-years. That means we see Regulus as it looked almost 80 years ago. So if anything big were to happen to the star tonight, Earth wouldn’t know it until early in the 22nd century.



The Moon doesn’t produce any light of its own. Instead, it reflects sunlight. The Moon looks big and bright only because it’s far closer to us than any other celestial object — an average of about a quarter of a million miles. That’s a bit more than one light-second, so we see the Moon as it looked a bit more than one second earlier.



Tonight, the Sun lights up a little more than half of the lunar hemisphere that faces our way. Many people think of that phase as a “half” Moon. And based on its appearance alone, that’s perfectly correct.



But a “half” Moon is also known as a quarter Moon. And at the current angle, it’s the first quarter Moon. That may sound confusing, but there’s a simple explanation: At first quarter, the Moon is one-fourth of the way through its month-long cycle of phases. It’s half of the way through the cycle when it’s full. And the cycle starts over at new Moon — when it’s not visible at all.



Script by Damond Benningfield

A binary star system about 30 million light-years away is an impostor. When astronomers first saw it, they thought it was a supernova — the titanic explosion of a massive star. They even called it one: Supernova 2000ch. But the system is still there.



The system is in the spiral galaxy NGC 3432, in a clump of young, bright, heavy stars.



By the end of last year, 2000ch had produced 23 outbursts. They’ve been about six and a half months apart. Based on that interval, the way the system brightens and fades, and other details, astronomers have developed a model of what’s going on.



The system’s main star is dozens of times the mass of the Sun, and a million times brighter. It’s also unstable — it puffs in and out.



A smaller companion star follows a lopsided orbit. During its close approaches, it stirs up the brighter star, triggering an outburst. And if the main star is in its “puffier” phase, then the outburst is especially bright.



The system may be building up to a much bigger outburst in the fairly near future. And after that, the massive star could explode as a supernova — no longer an “impostor,” but the real thing.



NGC 3432 is in Leo Minor, the little lion. It’s too faint to see without a telescope. But it’s high above the Moon as darkness falls tonight. The bright star Regulus — the heart of the big lion — is to the left of the Moon. More about that meeting tomorrow.



Script by Damond Benningfield