Mankind’s Explanation: Eta Carinae

  Original Version for Printing

   {What Astronomers do know about Eta Carinae is that it belongs to a rare class of stars called Luminous Blue Variables, or LBVs, objects whose temperature and mass approach the absolute maximum believed possible for a star. Eta Carinae appears to tip the scales at a mass 100 to 120 times larger than the sun while its surface temperature broils at a temperature that ranges between about 22,000 and 50,000 degrees Fahrenheit. Compare this to the surface temperature of the sun, which comes in around 10,000 degrees Fahrenheit.

   The most obvious features in the Hubble images are the two large and grayish bipolar lobes, shaped somewhat like an hourglass. With a total mass somewhere around three times that of the sun, their glow comes mainly from starlight radiated by Eta Carinae that reflects off the ubiquitous dust in the lobes. Ejected from the star during the 1843 eruption, each lobe is expanding outward at the rate of about 1.5 million miles per hour. At this great speed - fast enough for a spaceship to travel to the moon and back three times in an hour - the lobes have expanded in 150 years to span about four trillion miles, or some 0.7 light year.

   In all likelihood, the lobes are mostly hollow, though astronomers have detected evidence of some dust within them. No one knows whether the lobes are shaped like spheres or cones. Either way, they apparently formed when matter was ejected from the star’s polar regions. Perhaps this happened because Eta Carinae spins rapidly, possesses a powerful magnetic field, belongs to a binary star system, or some combination of these factors. Regardless, the ejecta from the 1843 eruption had a harder time escaping from near the equator and was forced to seek the path of least resistance, traveling outward from the poles.

   Less obvious but also seen first in Hubble images was the strange equatorial disk tilted between 52 and 60 degrees to our line of sight and about 90 degrees to the two lobes. Faintly resembling a ceiling fan with many blades, the disk consists of many curious objects moving at a wide range of speeds. Three mysterious blobs appear embedded within the disk only a few light-days (about fifty billion miles) from Eta. Flying outward from the star at about 100,000 miles an hour, all three arranged around the outside edge of the equatorial disk’s largest fan (called the Paddle and visible as the triangular-shaped bright area above and to the right of the Homunculus’s center). Astronomers don’t yet know what caused these blobs to erupt from the star so asymmetrically, though their speed and distance from the star suggest they were ejected in 1889.

   Within the Paddle itself, however, several small regions move at much slower speeds, as low as 30,000 miles per hour. These relatively sluggish speeds imply that the features were ejected from Eta several hundred years ago, long before the eruptions of the 19th century.

   The equatorial disk also contains several mysterious and fast- moving jets, or “bullets” as some scientists have labeled them. The northern jet is shooting away from the star at a tremendous velocity, estimated as high as 3.4 million miles per hour. As it rockets outs it appears to be pushing its way through the interstellar medium of nitrogen gas that surrounds Eta Carinae and was ejected in a much earlier, unrecorded eruption. Although some scientists believe that the jet’s origin is linked to the 1889 eruption, others contend that it - along with most of the equatorial disk - formed during the Great Eruption of 1843.

   In fact, all the data gathered about the equatorial disk so far presents scientists with an exceedingly confusing picture of its origin. Depending on when and where they look, different astronomers get different results. Perhaps Ted Gull of NASA’s Goddard Space Flight Center puts it best when he says “the disk appears to be the accumulation of many outbursts.”

   Even more baffling is the disk’s odd radial appearance, with its fans, spokes, and jets all pointing toward the star. As Davidson and Roberta Humphreys, also at the University of Minnesota, have written, “We regard the radial streaks as warning arrows pointing inward toward some extraordinary phenomenon near the central star.”

   Such intensely large, hot objects must periodically shed additional large amounts of mass to remain stable. What causes these larger eruptions remains a mystery, though astronomers suspect the incredibly high mass and temperature play key roles. The most popular hypothesis says that the star’s luminosity is so great that it occasionally overpowers the gravity that holds the star together. The star becomes unstable; its outer layers pulse in and out as if unsure whether they wish to remain in place or gush into space. Eventually an eruption occurs and the outer layers are flung away.

   With the loss of this shell of hot gas, the star cools and its surface temperature drops to a relatively low 13,000 degrees Fahrenheit. At the same time, its electromagnetic output shifts from the high-energy ultraviolet to less energetic optical radiation, so though the star is now cooler, it radiates more brightly at wavelengths our eyes can see. Hence, while the star seems brighter, its overall output undergoes no intrinsic change.

   After an eruption, these strange stars become stable for long periods, with their visible luminosity generally holding steady (though small, irregular fluctuations are not unusual). Whether the huge outbursts happen more than once is simply not known. If so, they are separated by thousands of years.

   It is also far from clear whether the war between gravity pulling inward and the pressure of radiation pushing outward is the sole cause of these giant eruptions. Some scientists think that turbulence and convection on the star’s surface either contributes to or causes the outbursts. Others believe that no current theory can be correct because none seems to explain adequately what instigates the eruption as well as what makes them stop.}


All information contained within the brackets was acquired from: Astronomy, February 2000 issue, Scoping out the Monster Star, pages 40-42, by Robert Zimmerman, published by Kalmbach Publishing Co. copyright 1999


Return to Main Document


Return to Title Page 2