T Tauri helps tell important story


By Tom Burns - Stargazing



Buried in the bright stars of Taurus’ V-shaped head is a star that, at its brightest, is visible only in a medium-sized amateur telescope. At its dimmest, only a large observatory telescope will ferret it out.

T Tauri, as it is called, hardly seems worth the effort. However, this pulsating variable star has garnered the attention of astronomers since its discovery in 1857. It turns out to be a critical link in the long history of a star’s birth, life and death.

T Tauri’s long history began in 1781. William Herschel had discovered the first planet since prehistoric times when he accidentally spied the planet Uranus.

Nobody questioned Uranus’ planetary status. It was clearly large enough to be a planet and had an orbit that matched the other planets. Also, Uranus’ position in the solar system verified a very odd theory about the planets.

In those days, many astronomers demanded mathematical perfection in all things.

Bode’s Law, as it was called, seemed to predict the distances of the planets from the sun in exact numerical ratios.

Most astronomers considered Bode’s Law a coincidental curiosity until Herschel discovered Uranus, which turned out to be right where Bode said it would be.

But there was a fly in the mathematical unguent. The fly landed in the gap between Mars and Jupiter. A planet ought to orbit there if Bode’s calculations were correct.

Astronomers began a frenzied search for the missing planet. However, they were trumped by a purely accidental discovery by a humble Sicilian monk.

On Jan. 1, 1801, Giuseppe Piazzi was making routine observations of stars to plot their exact positions. Imagine his surprise when one of the stars appeared to move slowly against the starry background.

Piazzi’s observations placed the new object, called Ceres, precisely in the Bodean gap between Mars and Jupiter.

Piazzi had discovered a planet, maybe. The trouble was that the fly wouldn’t leave the ointment. Ceres was obviously a tiny thing — one thousand times fainter than Jupiter or Mars, the planets that bracket it in the solar system. In fact, modern measurements indicate that it is a mere 579 miles wide, just over one-fourth the diameter of Earth’s moon.

Also, in 1802, just a year after Piazzi’s discovery, Wilhelm Olbers discovered another “planet,” which he called Pallas, in the Bodean gap.

With the discovery of Juno in 1804 and Vesta in 1807, Ceres’ planetary status began slowly to fade away.

Still, a few astronomers were obsessed with the difficult and time-consuming task of discovering more asteroids, as they came to be called.

The main difficulty is buried in its name. The Latin word “aster” means star, and “oid,” also of Latin derivation, means “looks like,” loosely translated. The darned things look for all the world like stars.

The only way to tell them apart was to observe them on several occasions over days or weeks. The ones that moved against the starry background were probably asteroids. Astronomers used a given asteroid’s motion to determine its orbital path around the sun, and the asteroid was officially discovered.

In 1840, English astronomer John Russell Hind entered the asteroidal fray. Born in 1823, he discovered an interest in astronomy at the early age of 12. His family, who apparently disapproved of his astronomical inclinations, sent him at the tender age of 17 years to London as an apprentice to a civil engineer. He soon abandoned civil engineering when he was offered a post at the Royal Observatory at Greenwich, a prestigious job for a 17 years old.

In 1840, he moved on to the directorship of the George Bishop’s Observatory and its beautiful refracting telescope with a 7-inch diameter lens. By today’s standards, the telescope was small, but Hind certainly made the most of it.

His early interest appears to have been in variable stars, stars that vary regularly in their brightness. In 1845, he discovered one of the most magnificent in the heavens, a deep red variable in the constellation Lepus, the Hare. It is still called Hind’s Crimson Star.

By 1847, he had caught the asteroid bug and quickly discovered two of them, Iris and Flora, the seventh and eighth asteroids identified as such. He went on to discover 10 of them, the last of which was Urania in 1857.

Along the way, he discovered and observed many stars that weren’t asteroids. As he carefully plotted the objects in the head stars of Taurus, he observed a very faint star we now call T Tauri. Subsequent observations confirmed that the object did not move against the starry background. It was only a star — and not a very interesting one at that. So he moved on.

Years later, astronomers began to notice the slight variations in T Tauri’s brightness. Such variations indicate that it is indeed one of Hind’s coveted variable stars. However, another fly was buzzing around the ointment.

Astronomers now suspect that T Tauri’s main period of pulsation from dim to bright and back to dim again extends to a whopping 4,000 years. Within that main pulsation cycle, T Tauri varies unpredictably and erratically in its brightness.

T Tauri’s unpredictable behavior would have been difficult to record except for another of Hind’s discoveries.

At the same time Hind first observed T Tauri, he also discovered an adjacent cloud of gas and dust that has come to be called Hind’s Variable Nebula. The nebula appears to be illuminated by T Tauri so that the nebula’s variations in brightness mirror the changes in the star.

Observers, Hind included, could see the nebula fairly easily between 1852 and 1861. After 1861, the nebula slowly began to fade from view.

During the mid-1860s, only fragments of the nebula were visible, and it took the largest telescopes of the time to see them. By 1868, it had disappeared completely.

It reappeared in 1890 but then only in the long-exposure photographs made by E.E. Barnard and S.W. Burnham. Then it vanished again until 1899 when a few traces of it were again captured photographically.

In 1920, the nebula began to brighten and fade again. The “phantom nebula,” as it is sometimes called, has brightened steadily since the 1930s. I had no trouble seeing it in my large amateur telescope a few years ago.

That’s strange behavior for a nebula, and it’s even stranger for the star illuminating it. Variable stars pulsate with clockwork-like regularity. Nearby Algol in the constellation Perseus completes a cycle of brightening and dimming in two days, 20 hours, and 49 minutes. You really can set your watch by it.

In the constellation Cetus, Mira, the miracle star pulsates over a 332-day period. You can set your calendar by it.

T Tauri is, well, weird.

It wasn’t until 1945 that American astronomer Alfred Harrison Joy noticed that T Tauri was but one of a class of stars bearing similar characteristics. They are fittingly called T Tauri stars since T Tauri was the first one discovered.

T Tauri stars turn out to be stars at a pivotal point in their evolution.

A typical star begins its life as a protostar, a clump of gas and dust in a much larger cloud of mostly hydrogen gas called a nebula. Its clumpiness means that it has a bit more gravity than the rest of the cloud.

As a result, the clump begins to contract into a spinning ball of gas that is still surrounded by a cocoon of gas from the nebula.

As the clump contracts and begins to spin, it flattens out into what is called an accretion disk. Gases in the disk swirl to its center. As a result, a denser ball of gas forms at the center of the accretion disk.

In a critical middle phase between a protostar and a true star, the central ball’s continuing contraction generates a considerable amount of energy. That outflow of energy blows away the outer cocoon of gas and dust. The ball of contracting gas thus emerges from its shroud and becomes visible to our eyes.

The contraction continues until, deep within the core of the ball, the density and temperature gets high enough to force its hydrogen atoms to fuse into helium. A fully fledged star, a hydrogen bomb of enormous proportions, is born in that cataclysmic moment.

At less than a million years old, T Tauri turns out to be the prototype of that middle, transitional phase. It is the fetus out of which a fully formed star will emerge in 100 million years or so.

Its wild fluctuations in brightness are the result of the complexity of its contraction, the fetal kicks that indicate new stellar life.

Over hundreds of millions of years, the tiny remnants of the accretion disk of T Tauri may clump together into planets and asteroids.

Little did Hind know it, but in some far distant future that he could have never imagined, his accidental discovery of a star at that middle stage of evolution will then have come full circle.

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By Tom Burns

Stargazing

Tom Burns is the former director of the Perkins Observatory in Delaware.

Tom Burns is the former director of the Perkins Observatory in Delaware.