ASTRONOMY
April 2006, Vol. 34, No. 4, pp. 62-67

Copyright © by Kalmbach Publishing Company. April 2006. All rights reserved. Reprinted with permission.

All About the North Star

Polaris, navigators' guiding light, is evolving right before astronomers' eyes.

By Ken Croswell

The North Star The North Star outshines all others in the constellation Ursa Minor the Little Bear. Earth's spin axis now points less than 1 degree from Polaris, which is why you never see the star rise or set. (Credit: Bill and Sally Fletcher)

     Most of us find comfort in the Sun's stability. But this stability means astronomers can't observe its changes to study how stars evolve over time. Fortunately, one bright star has been changing right before their eyes. Ironically, it's an object long revered as a symbol of stability and fidelity, one that, for ages, has aided mariners and explorers: Polaris, the North Star. Controversial new research suggests Polaris is brightening. It's now more than 15-percent brighter than it was 100 years ago--and more than twice as bright as it was 2,000 years ago.

The Remarkable North Star

     Polaris now lies less than 1° from the North Celestial Pole, an extension of Earth's spin axis and the point around which the northern sky appears to rotate. But the North Star wasn't always located so conveniently.

     Earth's rotational axis wobbles, causing it to point in different directions. Normally, the axis points at nothing in particular; but for the past several centuries, it's been aimed toward Polaris. In fact, this wobble is bringing Polaris ever closer to the North Celestial Pole, and it will continue to do so until the year 2102. Rarely has Earth been blessed with such a good North Star.

     Many nonastronomers mistakenly think the North Star is the brightest in the sky. It does, however, rank a respectable 2nd magnitude, which makes it bright enough to see even from light-polluted locations.

     While much of its fame rests on its utility for finding true north, Polaris also reveals an observer's latitude on Earth. If Polaris stands 30° above the horizon, you're at 30° north latitude. If it's 60° high, you're at 60° north. And if Polaris appears directly overhead, you're at Earth's North Pole.

A Star with a Pulse

     The North Star's remarkable traits don't end there. Polaris belongs to a class of stars whose story began in September 1784, when English astronomer Edward Pigott discovered the fluctuating light of a yellow star named Eta Aquilae. A month later, his friend John Goodricke found similar variations in another yellow star, Delta Cephei, and the stars came to be called Cepheids.

     Cepheids are yellow supergiants of spectral class F or G that pulsate like a human heart--they expand and contract, brighten and fade. Cepheids are such luminous stars that astronomers can see them in galaxies beyond our own.

     A pulsating supergiant is remarkable enough. But in 1907, Harvard astronomer Henrietta Leavitt uncovered the most astonishing Cepheid property: The longer a Cepheid takes to pulsate, the more light it emits into space. Thus, Cepheids make excellent yardsticks for measuring distances to other galaxies. Simply measure a Cepheid's period of pulsation--which is easy to do--and you know its true brightness. Compare the true brightness with the Cepheid's apparent brightness, and you learn something much more precious: how far the star is from Earth.

     In the 1920s, Edwin Hubble used Cepheids in so-called spiral nebulae to show these objects are really galaxies beyond our own. Cepheids also helped Hubble discover the universe's expansion. Today, using the space telescope named for Hubble, astronomers observe Cepheids to measure the universe's expansion rate.

     Polaris is a Cepheid, too. In fact, it's the closest and brightest member of this important stellar class. But unlike most stars of this type, Polaris' light barely fluctuates, so astronomers didn't discover its Cepheid nature until the 1910s.

     Most Cepheids pulsate with the precision of a Swiss watch. Eta Aquilae, the first Cepheid discovered, completes one pulsation cycle every 7 days, 4 hours, 14 minutes, and 34 seconds--exactly.

     Not Polaris. Its 3.97-day period gains 3 or 4 seconds a year. Furthermore, in 1983, Armando Arellano Ferro, a Mexican astronomer working at the University of Toronto, reported the star's already feeble flickerings were fading further. During the early 1900s, its light varied by 0.10 to 0.15 magnitude. In the 1980s, however, it fluctuated by just 0.05 magnitude.

     The rise and fall of Polaris' light continued to decline. A team led by Canadian astronomer Don Fernie predicted that, in 1994, the star would be as constant as, well, the North Star. Polaris seemed destined to make history as the first ex-Cepheid ever seen.

The Beat Goes On

The Polaris System Two Stellar companions orbit the North Star. The closer Ab component orbits about as far from its primary as Uranus does our Sun. Using the Hubble Space Telescope, astronomers recently imaged this component for the first time. (Credit: Astronomy: Roen Kelly)

     That didn't happen. Not only did Polaris keep pulsating--recently its pulsations have strengthened. After fluctuating by a mere 0.02 magnitude in the early 1990s, the light now varies by 0.03 to 0.05 magnitude.

     What's going on? In 1997, the European satellite Hipparcos provided fresh insight. It measured the star's parallax and revealed Polaris is 430 light-years from Earth. For Polaris to be seen as a 2nd-magnitude star at this distance, it must emit 2,400 times more light than the Sun. Put another way: In an hour, Polaris casts more light into space than the Sun does in 3 months.

     In fact, it's too bright. Polaris is too luminous to be a normal Cepheid with a 3.97-day period. A Cepheid as big and bright as Polaris should pulsate more slowly, just as a large musical instrument like a tuba produces a lower pitch than a small one like a trumpet. Polaris' period should be around 6 days. This longer period is called the star's fundamental pulsation mode. But just as musical instruments have overtones, or harmonics, so do Cepheids. Polaris seems to be pulsating in its first overtone.

     That explains a lot, astronomers say. "Overtone pulsators tend to have periods which change unusually rapidly--for reasons that we don't understand," notes Nancy Evans of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts. Many Cepheids pulsate in their first overtone, and this pattern is especially common among short-period Cepheids like Polaris. (It takes months for the largest, brightest Cepheids to complete a single pulsation.) In addition, overtone pulsators tend to show modest light fluctuations, just as Polaris does.

     A normal Cepheid, such as Eta Aquilae, brightens fast and fades slowly, whereas Polaris takes just as long to brighten as it does to fade. In 2004, Evans and her colleagues used NASA's Wide-Field Infrared Explorer (WIRE) satellite to measure the star's light to the incredible precision of ¹/10,000 of a magnitude. Their analysis confirmed that the rise and fall of the North Star's light shares traits of stars known to be overtone pulsators.

A Bright Future?

     Meanwhile, in 2002, a team led by Edward Guinan of Pennsylvania's Villanova University began observing Polaris--and tracing its history. The group's conclusion: Polaris is getting brighter.

     "That was the most startling thing," says Guinan. "It looks as though over the past 50 years--from 1955 to the present--its average magnitude changed from about 2.02 to 1.97." He decided to go back and look at even earlier data. "This gets you into trouble rather fast, because you have to worry about nonstandard systems for measuring brightness. So you have to recalibrate everybody's observations, putting in modern values for their comparison stars." When Guinan and undergraduate Scott Engle did so, they found Polaris was even fainter in the 1800s--between magnitudes 2.2 and 2.3.

     "We decided to do the full thing," says Guinan. "We went back and we looked in Ptolemy's catalog--and Polaris is listed as 3rd magnitude. So that was curious, but you know, he did make mistakes."

     While Claudius Ptolemy, who lived nearly 2,000 years ago, didn't have the precision instruments of modern astronomers, it's pretty easy to see the difference between 2nd- and 3rd-magnitude stars. Even someone who's never seen the Big Dipper's seven stars before can pick out the one shining at 3rd magnitude; the rest are 2nd magnitude.

     A book Guinan found during a visit to Iran convinced him Polaris had been fainter in the past. "I gave a little talk on different types of variable stars, and I threw in Polaris. An astronomer there, Yousef Sobouti, mentioned a catalog by Al Sufi [903-986], a Persian astronomer....His measures of the stars in the Little Dipper were much better than Ptolemy's measurements--they were nearer to the modern values--and Polaris was listed as 3rd magnitude again."

     Guinan doesn't know what's causing Polaris to change. "The changes in period, the changes in light are much faster than you predict by standard stellar-evolution theory," he says. "But my guess is that it's undergoing some kind of internal change."

     Not everyone agrees Polaris is brightening. David Turner, a Canadian astronomer at Saint Mary's University in Halifax, isn't convinced. He says German astronomer Julius Schmidt measured the star at its current brightness in the 1850s.

     Furthermore, Turner thinks the Hipparcos distance--430 light-years--is wrong. Based on the apparent and absolute brightnesses of stars near Polaris, which form a small group to which the North Star may belong, Turner thinks Polaris is only 300 light-years from Earth. As Turner notes, Hipparcos has flubbed other distances--most famously, to the Pleiades star cluster. If Polaris is only 300 light-years away, it must be less luminous, so much so that Polaris would be pulsating in its fundamental, rather than its overtone, mode.

     Turner thinks Polaris has just evolved from being a blue main sequence star, like Regulus in Leo, into a yellow supergiant going through a pulsation phase on its way to becoming a red supergiant. Such stars are rarer than other Cepheids because they evolve faster, but Turner says this explains recent changes in the star's period and amplitude. In his view, Polaris' period is lengthening because the star is getting bigger and redder--and will someday stop pulsating altogether. So far, though, no one has been able to detect a steady color or temperature change, so no one knows whether Polaris is getting redder or bluer.

     Fortunately, there are easy ways to resolve many of the disputes. Using the Hubble Space Telescope's (HST) Fine Guidance Sensors, Howard Bond of the Space Telescope Science Institute in Baltimore and his colleagues plan to measure a precise parallax for Polaris. This will either confirm or refute the Hipparcos distance. An accurate HST distance will determine whether Polaris is a fundamental-mode pulsator, as Turner believes, or an overtone pulsator, as Evans and Guinan think. And if the star is really brightening, as Guinan and Engle maintain, it will take just a few more years of observations to show it.

     There's one thing everyone agrees on. Polaris is young--about 100 million years old--so any planets it may harbor probably haven't yet developed life. As Guinan notes, the star moves through space in the same direction as the Pleiades, a cluster with stars of similar age, so Polaris probably formed at the same time as the cluster. And if the Hipparcos distance to Polaris turns out to be right, the North Star is nearly as far as the Pleiades cluster, about 435 light-years away.

     Says Turner: "It's a star that will prove interesting to follow for years to come--whatever it happens to do." If, against all odds, the star does have planets with intelligent life, Polarians have no idea how special their star is to us--just as we have no idea whether the Sun is some other planet's North Star, guiding sailors across alien seas, and perhaps even inspiring extraterrestrial playwrights to wax poetic.

Ken Croswell a Harvard-trained astronomer, is a frequent contributor to Astronomy and author of several books, including Magnificent Universe (Simon & Schuster, 1999).

Polaris and Friends

Stars in the Bowl Stars in the Bowl of the Big Dipper, an asterism formed by Ursa Major's brightest stars, guide observers to Polaris. The "pointer stars" are Merak and Dubhe. (Credit: Bill and Sally Fletcher)

     Polaris is actually a triple star. The brightest member of the system is the Cepheid, an F7 supergiant about 5 times more massive than the Sun. Both stars have similar surface temperatures--about 6,000 K--so their color is similar, too. Polaris pulsates every 3.97 days.

     Polaris B, an F-type main sequence star, orbits 18" away and shines at 9th magnitude--easy to see in a small telescope. If both Polaris and this companion are exactly 430 light-years from Earth, then they're separated by 2,400 astronomical units, or about 60 times Pluto's distance from the Sun. (One astronomical unit, or AU, is Earth's average distance from the Sun, about 93 million miles [150 million kilometers].) Polaris B must take tens of thousands of years to complete an orbit.

     The system's third member orbits Polaris A every 29.6 years, tugging on it and altering its velocity with this same period. But this star went unseen until 2005, when the Hubble Space Telescope finally spotted it. It's an F main sequence star. On average, this companion lies as far from the supergiant as Uranus lies from the Sun.--K.C.

Anachronism or Irony?

     I am constant as the northern star,
     Of whose true-fix'd and resting quality
     There is no fellow in the firmament.
     The skies are painted with unnumb'red sparks,
     They are all fire, and every one doth shine;
     But there's but one in all doth hold his place.

     In 1599, when William Shakespeare put these words into the mouth of Julius Caesar, Polaris was about 2° farther from the North Celestial Pole than it is today. The trouble is, the North Star lay even farther away in Caesar's time.

     Shakespeare's plays include anachronisms, and his "constant" North Star may be an example of one. But the playwright may have intended these words as ironic. Conspiring senators murder Caesar just a few lines after he utters them.--K.C.

Two Stars Compared

Constellation
     Polaris: Ursa Minor
     Sun: --

Distance (light-years)
     Polaris: 430
     Sun: --

Apparent magnitude
     Polaris: 1.97
     Sun: -26.74

Absolute magnitude
     Polaris: -3.6
     Sun: 4.83

Relative brightness
     Polaris: 2,400
     Sun: 1

Spectral type
     Polaris: F7
     Sun: G2

Color
     Polaris: Yellow-white
     Sun: Yellow

Surface temperature
     Polaris: 6,000 K
     Sun: 5,770 K

Relative mass
     Polaris: 4.3 +/- 1.1
     Sun: 1

Relative radius
     Polaris: 46
     Sun: 1

Age (years)
     Polaris: About 100 million
     Sun: 4.6 billion

Relative iron abundance
     Polaris: 1
     Sun: 1

Relative carbon abundance
     Polaris: 0.4
     Sun: 1

Relative nitrogen abundance
     Polaris: 2.5
     Sun: 1