Shadowy partner: astronomers may have detected what lurks in the shadow of the giant star Eta Carinae

Natural History, Oct, 2004 by Charles Liu

In the southern constellation Carina, literally the "keel" of a larger group of stars called Argo Navis--the celestial ship of Jason and the Argonauts--a giant star lies shrouded in mystery: Eta Carinae is a titanic object--at least fifty and perhaps as much as 120 times the mass of the Sun--enveloped in a thick cocoon of glowing, dusty gas. The star is pumping out energy millions of times faster than the Sun is--probably faster, in fact, than just about any other star in the Milky Way. For that reason alone, understanding the workings of Eta Carinae would go a long way toward unlocking the details of the birth, growth, and death of stars.

But there's more: Eta Carinae also "hiccups." Actually, that's putting it mildly--the star unabashedly belches energy, varying so wildly in brightness over the years that its behavior has fascinated and baffled astronomers since the early nineteenth century. From 1837 until 1858, the star's mood swings were so visually amazing that the period was labeled the Great Eruption. In April 1843, Eta Carinae, despite its distance (nearly 8,000 light-years from Earth) briefly became the second-brightest star in the night sky. Throughout most of the three succeeding decades, though, its energy output plunged to less than a thousandth of that value. Since then, its luminosity has steadily risen again, though on average it is still less than 1 percent of what it was in the glory days of the Great Eruption.

Although the cause of the huge brightness variations remains unclear, astronomers have noted since 1984 that some of the variations are periodic. In 1996 the Brazilian astronomer Augusto Damineli confirmed that, since the 1940s, Eta Carinae's output has pulsed briefly but substantially every five and a half years. To an astronomer, periodic behavior in a star usually signals one of two things. First, the pulses could be caused by interior processes--the best-known examples are Cepheid variables, whose periodicity helps astronomers measure distances to nearby galaxies. Second, the pulses could be the consequence of a rotational effect: the star could be spinning at a regular rate, or a second, companion star could be orbiting it with a regular period.

For several years most Eta Carinae experts have supported the companion-star hypothesis. There's just one big problem: despite years of searching, no such star has ever been found. But recently a group of astronomers led by Nathan Smith of the University of Colorado in Boulder announced they detected ultraviolet shadows that were moving in accord with the 5.5-year cycle of Eta Carinae, suggesting the star may have a hidden partner.

The Great Eruption left a souvenir for us to see today: two globular blobs of hot, glowing gas, the equivalent of roughly one and a half million Earth-masses, bulging outward from the star in opposite directions. Dubbed the Homunculus--I guess it looked like a little man to the astronomer who named it in 1950--the two blobs are each expanding at more than a million miles an hour. The Homunculus is a fascinating observational target in its own right--it's the star-stuff of Eta Carinae, violently ejected by the Great Eruption. But our stunning view of materials blasted off the star has its downside: the Homunculus obscures the star itself, making it impossible to separate Eta Carinae from its surrounding gas with any Earthbound observatory.

Enter the Hubble Space Telescope. Because Hubble orbits some 400 miles above the Earth's surface, it is immune to the obscuring effects of our atmosphere. The telescope can thereby resolve Eta Carinae itself, distinguishing the star from its surrounding nebulosity. As an added bonus, Hubble's instruments can detect astronomical emissions of ultraviolet radiation that would otherwise be absorbed by Earth's atmosphere. Thus Hubble opens a major window in the electromagnetic spectrum for the study of the universe.

That is particularly important for the study of Eta Carinae. The star itself emits copious quantities of UV radiation, whereas the gas and dust surrounding it scatter and absorb UV. So patterns of light and shadow--not just the light and shadow of visible light, but of UV as well--provide important clues about the shape and clumpiness of the Homunculus and the other dusty clouds surrounding the star.

Smith and his collaborators take full advantage of those facts in their search for Eta Carinae's partner. Imagine, they suggest, that a companion star about thirty times the mass of the Sun is orbiting Eta Carinae. The companion would be an impressive star in its own right, but still dwarfed by Eta Carinae. Next, imagine that the companion's 5.5-year orbit is highly elliptical, shaped more like a kayak than a circle, with Eta Carinae near one end.

The companion star would emit plenty of its own UV radiation, but this radiation would be blocked in the direction of Eta Carinae by the thick nebulosity of the giant star's surrounding gas, dust, and stellar wind. For more than five years of its orbit, while it is far from Eta Carinae, the companion would shine its UV light toward us largely unobscured. But during the several-month period when the companion star is next to or behind Eta Carinae, the companion's emission would be dramatically altered. The ultraviolet shadow cast by Eta Carinae, coupled with our alignment with the two stars, would illuminate different parts of the nebula around Eta Carinae.