StarDate

In Shakespeare’s play “Julius Caesar,” Caesar declares himself to be “as constant as the north star.” Unfortunately for Caesar, though, the current North Star, Polaris, turns out to be not very steady at all — its brightness can vary by quite a bit.

No star is completely steady. Even the Sun’s overall brightness changes — by about a tenth of a percent over a period of more than a decade. But some stars vary by a lot.

Variable stars fit into a couple of main categories.

Stars like Polaris actually pulse in and out like a beating heart. In the case of Polaris, each “beat” lasts several days. As the star’s diameter changes, so does its brightness. For some stars in the same class as Polaris, the change in brightness is even more dramatic.

The other category is eclipsing stars. The two stars in such a system orbit in such a way that they regularly pass in front of each other as seen from Earth. When one star covers the other, the system’s brightness can drop by quite a bit.

Perhaps the most famous example is Algol — the “demon” star. About every three days, the system’s fainter star passes in front of the brighter one. That drops the overall brightness by more than two-thirds — a change that gave the star a “spooky” reputation.

Algol is high in the west at nightfall, in the constellation Perseus. Polaris is right where it always is — marking due north every night of the year.

We’ll talk about some stars that get especially close tomorrow.

Script by Damond Benningfield

We haven’t seen life anywhere in the solar system besides Earth. Yet several worlds seem to have the potential for life. The list includes Mars, Jupiter’s moon Europa, and even Pluto.

Another one is Enceladus, a moon of Saturn. It has most of the ingredients for life — including some of the chemistry of life.

Enceladus is coated with almost pure ice, so it looks almost pure white. Below the surface, though, it’s quite complicated. An ocean of liquid water lies below its crust. And volcanic vents at the bottom of the ocean belch hot water that contains a lot of minerals. That gives Enceladus the main ingredients for life: water, heat, and chemistry.

And the chemistry keeps getting more favorable. Scientists are still analyzing observations from the Cassini spacecraft, which orbited Saturn for more than a decade. It flew through plumes of water from Enceladus that shoot out through cracks in its crust.

The analysis has revealed organic compounds attached to grains of ice. And a few months ago, the list was expanded to include key ingredients in amino acids — some of the basic building blocks of life.

None of that means that anything is living in the ocean. But the discoveries do show that we can’t rule out the possibility of life on this bright, icy moon.

Its parent planet, Saturn, is quite low in the southeast at first light. Tomorrow, it stands near the crescent Moon. The planets Jupiter and Mars line up to their upper right.

Script by Damond Benningfield

A day after covering up Mars, the crescent Moon takes aim at another planet: Jupiter, the giant of the solar system. They’ll be low in the southeast at first light tomorrow. Jupiter looks like a brilliant star. This time, though, the Moon won’t quite catch up to it — they’ll stay a few degrees apart.

The Moon is actually on a sort of tour of the bright outer planets. Early today it passed by Mars, the fourth planet from the Sun. Tomorrow it’s Jupiter, the fifth planet. And on Thursday it’ll stand near Saturn, the sixth planet.

The fact that the three of them line up in order, and fairly equally spaced, is a result of their clock-like procession around the Sun.

Each planet moves at its own pace. Mars, for example, loops through the background of stars in about two years. Jupiter takes about 12 years to make that same circuit, while Saturn takes almost 30 years.

The difference is a result of their distances from the Sun. A planet that’s farther from the Sun has to cover a greater distance than one that’s closer — like being in the outer lane of a race track. And thanks to the laws of orbital motion, a more-distant planet moves more slowly than a closer one.

Over the coming weeks, the grouping of Mars, Jupiter, and Saturn will get even tighter. Mars will pass by both worlds in March. And Jupiter and Saturn will draw tighter through the spring — before they begin to pull apart in summer.

We’ll have more about the Moon and Saturn tomorrow.

Script by Damond Benningfield

Mars and the Moon team up for a bit of stage magic tomorrow morning. As seen from parts of the United States, Mars will disappear — hidden behind the crescent Moon.

Such an event is called an occultation. It happens because the Moon and planets all stay close to the ecliptic — the Sun’s path across the sky. Their exact paths diverge from the ecliptic by a few degrees, though. So the geometry has to be just right for the Moon to pass in front of a planet and block its light. As a result, such encounters are rare.

In the past, occultations were important ways to learn about Mars. Precise timing of the event helped astronomers measure the planet’s diameter, for example, and refine models of its orbit around the Sun. Today, spacecraft provide much better measurements, so occultations are less important.

They’re still interesting to watch, though. This one will be visible across much of the western half of the United States. The view will be best from the Mountain Time Zone, where the Moon will cover Mars by around 4:45 a.m. By then, the two worlds will be high enough in the sky for you to watch as the Moon passes between Earth and Mars.

Parts of the Central and Pacific zones will see at least part of the occultation. But the east will miss out because the Sun will be rising by the time Mars vanishes.

Those who miss the occultation, though, will still see a beautiful conjunction, with orange Mars quite close to the crescent Moon.

Script by Damond Benningfield

A beautiful feature of the constellation Puppis is giving astronomers a rare treat — a chance to watch the final years of a dying star.

The star is at the center of a cloud of gas and dust. The cloud contains a lot of sulfur, so it’s known as the Rotten Egg Nebula.

In pictures from Hubble Space Telescope, it looks like a squid that’s trying to escape from danger. Its arms and tentacles are folded behind it, and a cloud of “ink” squirts from its base.

The nebula is produced by a red giant — a star that’s old and bloated. The nuclear reactions in its core are beginning to shut down. As that happens, the star is blowing its outer layers into space. They race outward in opposite directions at close to a million miles per hour. That forms “jets” that span more than a light-year — the body and arms of the squid.

The jets ram into gas and dust around the star. They slow down and spread out, forming the squid’s ink.

This phase of a star’s life doesn’t last long, so astronomers don’t get much chance to study it. In a thousand years or so, in fact, the star will have shed all of its outer layers. That’ll create an even more impressive cloud — a planetary nebula. It’ll glow for thousands of years, illuminated by the star’s hot but dead core — a white dwarf.

The Rotten Egg Nebula is near the northern border of Puppis. On February evenings, the nebula stands to the left of Sirius, the brightest star in the night sky.

Script by Damond Benningfield

Breakups are never quite even.

Consider the ancient constellation Argo Navis. It represented the ship that carried Jason and the Argonauts on their quest to capture the golden fleece. But it was so big that, in 1752, astronomer Nicolas Louis de Lacaille split it into three constellations: Vela, the sails; Carina, the keel; and Puppis, the poop deck — the high deck at the back of the boat.

The breakup wasn’t quite equal, though. Puppis got 40 percent of the ship’s original area. That makes it the 20th largest of the 88 modern constellations.

Puppis is a southern constellation — it’s far south of the celestial equator. That makes it tough to see from northern latitudes. From the northern half of the United States, in fact, only part of it is in view — it’s cut off by the southern horizon.

From the southern half of the country, though, Puppis rotates low across the southern sky on February nights. It nudges into view in the southeast by a couple of hours after sunset, and stands almost south by 11 p.m. or so.

A couple of moderately bright stars stand at Puppis’s upper edge. But the poop deck is best spotted by looking for Sirius, the brightest star in the night sky. Puppis is below Sirius as the constellation rises, and to the lower left of the brilliant star later on.

Puppis is home to several star clusters. Two of them are to the left of Sirius. And one of them is home to a dying star that’s blowing itself apart. More about that tomorrow.

Script by Damond Benningfield

One of the reddest stars in the galaxy bounds across the south on February evenings. It’s in the “fur” of Lepus, the rabbit. The star’s main name is R Leporis. But it’s better known as Hind’s Crimson Star, after the astronomer who discovered it, in the 19th century.

The star looks so red for a couple of reasons. And both of them are related to its age.

Hind’s Crimson Star is in the final stages of life, so its outer layers have puffed up to giant proportions — about 75 times the diameter of the Sun. That’s made the surface of the star cooler, so it shines an orangey red.

At the same time, a lot of carbon has been dredged from inside the star and pulled to the surface. The carbon absorbs blue light, enhancing the red.

Some of the carbon, along with other elements, is being blown into space by a strong “wind” from the star. The wind is much thicker than the wind from the Sun. And it’s a first step in the star’s demise. Eventually, the star’s outer layers will all blow out into space, forming a beautiful bubble. Over tens of thousands of years, the bubble will spread out and vanish from sight. After that, only the star’s hot core will remain — a tiny stellar corpse known as a white dwarf.

Lepus is in the south as night falls, below Rigel, the bright blue-white star that marks the foot of Orion the hunter. Hind’s Crimson Star is too faint to see with the eye alone. But a small telescope reveals its rich color — the color of a dying star.

Script by Damond Benningfield

The first spacecraft to leave the solar system carry roadmaps. Plaques on Pioneer 10 and 11 include a diagram that points the way to the Sun. The landmarks for that roadmap are 14 pulsars — the tiny, rapidly spinning corpses of once-mighty stars. If another civilization finds either probe, it should be able to use the map to navigate to the solar system.

Scientists and engineers here on Earth are looking at ways to use pulsars to navigate through the solar system. In particular, they’re using millisecond pulsars, which spin from dozens to hundreds of times per second. Like a lighthouse, such a pulsar sends out a beam of radio waves, X-rays, or other forms of energy. If Earth happens to line up with such a beam, we see “pulses” of light.

Pulsars are small but heavy. Their great density and high-speed rotation make them extremely stable. So their pulses of energy serve as excellent clocks. Comparing the timing from several pulsars — such as the four depicted here — should allow a spacecraft to triangulate its position anywhere in the solar system.

A test in 2018 used an X-ray telescope aboard the International Space Station. It was able to plot its position with an accuracy of three miles. And today, the European Space Agency is using pulsars to double check its GPS satellites.

In the future, probes to the planets and beyond might be guided by pulsars — landmarks on a galactic roadmap.

Script by Damond Benningfield

[SFX: Pulsar sounds] Those are the voices of pulsars — the corpses of exploded stars. More than 2,000 have been discovered so far. But several projects are looking for more.

A pulsar forms when a massive star dies. The core collapses into a ball no bigger than a city, but up to twice as massive as the Sun. As the pulsar spins, a powerful magnetic field generates beams of energy. They can sweep across Earth like a lighthouse, producing “pulses” of radio waves and other forms of energy.

The timing of a pulsar’s “beat” can reveal the location, mass, and size of a pulsar. That tells astronomers how such stars form and evolve. It also can reveal the presence of a companion star or planet, and details about the companion. And changes in timing might even reveal gravitational waves rippling through the universe.

One major search is being conducted with the giant Arecibo radio telescope in Puerto Rico. The search has revealed more than 200 pulsars. Many of those were discovered by students at universities in Texas and Wisconsin. The students have sifted through observations compiled over many years.

Another major search uses the Green Bank Telescope in West Virginia. It involves students from high schools and colleges across the country. The students learn about how science works, while astronomers get new pulsars to ponder — rotating beacons throughout the galaxy.

Script by Damond Benningfield

Almost a millennium ago, a star announced its death in dramatic fashion — as a brilliant new star in the constellation Taurus. It was bright enough to see in daylight for weeks.

The star left behind an expanding cloud of debris that resembles the outline of a crab. It’s near the tip of one of the bull’s horns, which is high in the sky this evening. But you need a good telescope to see it.

The remnant is known as M1, the Crab Nebula. It was born when a heavy star exploded as a supernova. The star’s outer layers have spread out to form a cloud that’s several light-years across.

At the center of the cloud is the star’s crushed core — a neutron star. It’s several times the mass of the Sun, but only as wide as a small city.

The explosion that created the nebula also caused the neutron star to spin more than 30 times a second — a rate of about four million miles per hour at its equator.

The explosion also created a powerful magnetic field. As the star spins, the field causes it to beam energy into space. Radio telescopes detect the beam as on-off “pulses” of radio waves, so the neutron star is also known as a pulsar.

The pulsar produces a “wind” of charged particles that sculpts the interior of the surrounding cloud. And it causes the pulsar to slow down — but only by about ten millionths of a second per year. So the Crab Pulsar will keep spinning for a long time to come.

More about pulsars tomorrow.

Script by Damond Benningfield