Star baby: T Tauri shows that a stellar nursery can be a rough-and-tumble place to live

Natural History, Dec, 2003 by Charles Liu

In 1852 the English astronomer John Russell Hind, exploring the constellation Taurus through his telescope, round a dim star that wasn't noted on his charts. The new star, named T Tauri, has since become something of a minor celebrity among astronomers. It owes its fame primarily to its status as a stellar ingenue: it's just a million or so years old, which, for a star about as massive as the Sun, is very young indeed. Nowadays all stars of similar age and mass are known as T Tauri stars: they're immensely important because they afford astronomers a chance to study, by inference, the early history of our own solar system and Sun, born more than four and a half billion years ago.

Astronomers have learned a lot about this stellar baby, the original T Tauri, in the past 150 years. Among the most significant discoveries has been that T Tauri was part of a multiple birth: astronomers identified a second star in the system in 1981 and a third star in 1097. Another study, made early in 2003, suggested that a close encounter between the second and third star, acting as a kind of gravitational slingshot, had hurled the third star out of the system. Now a research team led by Elise Furlan, an astronomer at Cornell University in Ithaca, New York, has discovered that, if the slingshot hypothesis is true, the T Tauri system probably includes a fourth object, too.

T Tauri is about two one-hundredths of a percent the of our Sun. That's approximately the difference between a baby only a few days old and a middle-aged adult. Although people of both ages are alike in having head, limbs, and torso, their behavior and physical development bear hardly any resemblance at all. Much the same is true for a novice star like T Tauri and a middle-aged star like our Sun.

Like a human baby, a stellar baby is unpredictable. When Hind discovered T Tauri, it was shining at about magnitude ten--roughly one-fiftieth the brightness visible with the unaided eye. In the following forty years, though, the star gradually dimmed by 98 percent, then, inexplicably it started brightening again. Today it's about as bright as it was when Hind first saw it. The star is still inconstant, however; modern measurements show that, even from one day to the next, T Tauri's brightness can change by as much as half its typical output. Although the causes of the variation remain unclear, the star's interaction with the gassy, dusty environment in which it was born certainly plays a big role.

An infant star, like a baby, also is hungry T Tauri is so young that the nuclear fusion of hydrogen into helium, which makes mature stars shine, hasn't even begun. Without nuclear fusion, the star's luminosity depends on gravity: matter falling onto the stellar surface from the surrounding gas cloud glows as it acccelerates; and the protostellar gas ball, as it collapses inward from its own weight, also becomes hot enough to glow. Plenty of power is generated as a consequence, enough to make T Tauri shine, but the energy isn't as steady and predictable as nuclear fusion. The thick clouds still swirling around the star further accentuate the swings in luminosity. As T Tauri consumes the matter around it, it grows in mass and in energy output, often spitting up swirling streams of energetic particles called T Tauri winds.

One big difference, though, between human and stellar child-birth is the frequency of twins and triplets. A woman's chances of bearing twins are typically about one in a hundred. But astronomers think that as many as two-thirds of all new stars are born as binaries or multiples. T Tauri was no exception. As I noted earlier, at least three components have been identified, known as North, South-A (the 1981 discovery), and South-B (1997).

Astronomers have long suspected that such systems, with several young stars in close orbit about one another, might occasionally fling one of their components out or-the system in a gigantic gravitational game of crack-the-whip. High-precision radio astronomy observations made in early 2001 suggested that the stars in T Tauri were doing just that: South-B seemed to be exiting the system, after having swung around South-A in a curlicue path during the past two decades.

Furlan and her collaborators aimed the 200-inch Hale Telescope, at the Palomar observatory in California, at the system not to investigate the gravitational slingshot, but to study T Tauri's circumstellar environment. (The radio data had not yet been published when the Furlan team began their work.) Using an infrared camera fitted with adaptive optics designed to cancel out the distorting effects of Earth's atmosphere, the astronomers were able to plot the apparent positions of South-A and South-B to accuracies of better than ten milliarcseconds. That's about 1/400,000 of a degree of arc, or the apparent diameter of a penny at a distance of 200 miles. After the radio observations were published, and they compared that data with their own, however, Furlan's group realized that South-B isn't the star moving along the crack-the-whip trajectory after all. It is apparently still orbiting South-A. Nevertheless, the Furlan team doesn't suspect that the earlier data were faulty. On the contrary, they think those measurements actually detected a fourth body in the T Tauri system. That body tentatively named South-C, is probably being ejected after a close encounter with South-B.