Temperamental monsters: massive stars may slim down in eruptive bursts

Science News,  Sept 23, 2006  by Ron Cowen

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"[I]t is now again on the increase. It is, and has been for a month, brighter than [the star] Canopus. Half-way indeed between him and Sirius, and very red."

--Astronomer Charles P. Smyth, from letter dated Jan. 1, 1845

Late in 1837, a dim Milky Way star called Eta Carinae suddenly blossomed and soon became the second-brightest star, next to Sirius, in the night sky. During the next 19 years, a period now known as the Great Eruption, the star ejected two billowing, mushroom-shaped gas clouds, each 100 times as wide as our solar system and containing enough mass to make 10 suns.

Eta Carinae suffered another substantial, but less dramatic, eruption in 1890, and evidence is accumulating that it had undergone several other severe outbursts during the past 10,000 years. Even today, the star, which tips the scale at a whopping 100 solar masses, hurls the equivalent of two Earths of gas and dust into space each day.

Although the outbursts have made Eta Carinae a striking spectacle, astronomers have long regarded the star as a freak of nature. Astronomical models indicate that most heavyweights expel large amounts of matter before they die, but that they eject this material relatively slowly, over their entire 3-to-4-million-year lifetimes. But new evidence--a combination of theory and observations--suggests that Eta Carinae's temperamental behavior may be the norm, not an anomaly, among extremely massive stars.

Simulations indicate that the first stars to form were all extremely massive. Because these stars were the main sources of every element heavier than helium in the early universe, the evidence of widespread temperamental behavior may prompt a new look at how the cosmos acquired its assortment of chemical elements, including those necessary for life. The findings may in particular shed new light on the fate of the first stars in the universe.

"There's a paradigm shift in our understanding of massive stars" that may affect views of how these stars live and die, the remnants they leave behind, and their contribution to the chemical makeup of the early universe, says Nathan Smith of the University of the California, Berkeley.

WINDS OF CHANGE All massive stars lead short lives. The heaviest ones have a life span only about a thousandth of that of a star such as the sun. They can weigh 50 to 150 times the mass of the sun at birth, but during their life spans, they lose much of that mass via a steady, outgoing wind. Eventually, they die a fiery death in an explosion called a supernova. Before exploding, these stars, then known as Wolf-Rayet stars, have lost their outer atmosphere and slimmed down to a mere 10 to 20 times the mass of the sun.

Until the explosion, the stars burn hydrogen at their cores, transforming it into helium. The nuclear reaction creates ultraviolet (UV) radiation, which ionizes an array of elements in the stars' outer layers. These ionized atoms absorb the radiation, and the kick imparted by the process blows off gas, creating a continuous, outward flow of matter. But just how much mass those winds carry away is now an open question, Smith and others say.

Astronomers had assumed that the UV-absorbing ions are distributed smoothly and uniformly throughout a star's outer layers. Calculations using that assumption show that the winds are indeed intense enough to put heavy stars on a slow but steady diet, reducing their mass by tens of solar masses over several million years.

Recent studies indicate that observers may have overestimated the strength of stellar winds. The new data show out that the material that absorbs radiation is unevenly distributed in the atmospheres of stars.

Researchers measure the brightness of a star to deduce the amount of mass carried off by a wind. The greater the emission, the larger the mass loss. But an atmosphere that consists of dense clumps of ions will radiate more strongly than an atmosphere containing the same amount of material distributed more uniformly. If astronomers don't account for the higher intensity of light emitted by a dumpier atmosphere, they can be fooled into thinking that the wind carries away more mass than it really does.

"Currently accepted mass-loss rates may need to be revised downward as a consequence of previously neglected clumping, note Joachim Puls of the University of Sternwarte in Munich and his colleagues in a review article recently posted on the Internet (http:/xxx.lanl.gov/astro-ph/0607290).

According to Smith, Stanley P. Owocki of the University of Delaware in Newark, and other researchers, the total wind may be only one-tenth as strong as models had indicated. That's too gentle to blow away all the matter that astronomers know must be expelled by massive stars before they explode.

"Steady winds are simply inadequate for the envelope shedding needed to form a Wolf-Rayet star" Smith and Owocki note in the July 1 Astrophysical Journal Letters. Smith also reviewed the evidence for episodic outbursts among heavyweights in May at a meeting on massive stars at the Space Telescope Science Institute in Baltimore.