|
By Bob Calverley
The USC News Service
When USC astronomer Gerrie Peters looks up at
the sky on a clear night, she does not wonder what stars she sees. She is looking at the one in 30 that are “Be stars,”
and after almost 30 years she is beginning to understand their very complex twinkle.
Be stars are the supermodels of the universe. Young and very hot, they have fascinated
celestial observers with their dynamic beauty and explosive outbursts. And Be stars come accessorized with a flashy,
gaseous disc that waxes and wanes.
At least, that is how astronomers like Peters picture Be stars, for there are no telescopes powerful enough to reveal
one with its disc. Astronomers have painstakingly created a portrait of Be stars with data gathered from observatories
on Earth and from spacecraft.
Most of the data is spectroscopic. When astronomers use a spectroscope to splay the starlight into a spectrum, the
result is creased with fine absorption lines representing light that has not successfully traveled to Earth. Since
every atom, molecule and ion absorbs light in a characteristic wavelength, the absorption lines provide clues about the
star.
“Interpreting a star’s absorption lines tells you about the temperature, gravity, chemical composition of the
atmosphere, turbulence and rotation speed,” Peters said. “Be stars also have an emission spectrum, which provides
more information.”
The term “Be stars” is a contraction of “B-emission” stars. Of the roughly 9,000 stars visible to the unaided eye from
Earth, about 20 percent, or 1,800, are simply “B stars,” and 20 percent of these, or about 360 stars, are “Be (B-emission)
stars.”
B stars are three to 14 times as massive as the sun and a toasty 11,000 to 30,000 degrees Kelvin (19,000 to 53,000
Fahrenheit). In comparison, the “surface” of the sun is a mere 6,000 degrees Kelvin (10,000 degrees Fahrenheit).
“Look at any bright, bluish star in the sky and you are probably looking at a B star,“ Peters said. Because B stars are
often 10,000 times brighter than the sun, they can be seen at a great distance, and so we can see a lot of them. But
they are still relatively uncommon stars. For every hot, bright star like them, there are 100 stars the size of our sun,
and for every sun there are 10 smaller stars.
“Physics favors forming smaller, cooler stars,” Peters said. “Most of the stars around us are small but we can’t see
them.”
The more massive a star, the hotter it is, and the faster it travels down the stellar evolutionary path. By standards
of the universe, Be stars don’t grow very old. “Most of them are less than 100 million years old, so they were born
and formed after the age of the dinosaurs,” Peters said.
The first known Be star, gamma Cassiopeia, was identified in 1867 by Angelo Secchi, an Italian priest peering through
a newly invented spectroscope. Like many Be stars, gamma Cassiopeia has large swings in brightness and color changes
that are more easily seen with a spectroscope.
Peters has her own special star, mu Centauri in the constellation Centaurus. It hangs low in the southern sky in the
spring and summer and was classified as a Be star in 1895. She began watching it while she was in graduate school in the
early 1970s.
Peters, a research scientist with the Space Sciences Center, has her own special star, mu Centauri in the constellation
Centaurus. It hangs low in the southern sky in the spring and summer and was classified as a Be star in 1895. She began
watching it while she was in graduate school in the early 1970s.
“It has a history of going through stages of having a disc and not having one, and while I was in grad school the disc
was fading away,” she said. “It was gone by 1977.”
But it came back with a vengeance when Peters had the good fortune to be watching. She was at the Kitt Peak National
Observatory 50 miles southwest of Tucson in 1985 looking at mu Centauri through a spectrograph when she saw it explosively
rebuilding its disc. She discovered that the star underwent emission episodes that lasted a day or two each, in which
the star blasted stellar material into space.
“The material goes into a temporary disc that lasts from a week to a month,” she said. Tipped off by Peters, astronomers
around the world began watching mu Centauri. “While I was watching I saw it eject a tremendous amount of material in a
period of just 12 minutes.”
Stellar material is about 90 percent hydrogen, 9 percent helium and 1 percent everything else. The discs around Be
stars are not stable structures because the disc’s viscosity drains its energy, Peters said. As it loses energy, over
a period of years, months, weeks, days or even minutes, the material in the disc cascades back into the star.
Peters’ 1985 observation of mu Centauri spurred a German group and an American researcher to launch separate studies of
short-term activity in Be stars that will be the focal point of an International Astronomical Union Colloquium next
June in Alicante, Spain. She has been invited to present a review of Be star research, and the conference will explore
hypotheses on what makes Be stars tick.
In 1930 astronomer Otto Struve had theorized that Be stars were massive, hot stars rotating so fast they were on the
verge of breaking up. Big, hot stars rotate faster than smaller stars: The sun completes a rotation in a month, although
some of its material completes the rotation in 26 days. (A star is a globe of gas and so doesn’t behave like solid rock.)
A typical Be star rotates about once a day, Peters said, which is about 80 percent of breakup velocity. While that is
fast, it isn’t as fast as Struve had hypothesized. With the advent of space data, Struve’s model had bitten the astronomical
dust by 1980 to be replaced by two competing explanations for a Be star behavior.
THEORY ONE, Peters said, is called non-radial pulsation. In this model, the surface of a Be star is going in and out
like a pulsing balloon, creating waves that precess around the star’s equator in the opposite direction of rotation.
The wave crests are hotter than the troughs, and astronomers can “see” the hot and cool spots as they parade across the
line of sight of their telescopes or other instruments. What observers actually see are changes in the spectra caused
by the changing temperature.
Some of the stars have two hot spots and two cooler spots. More complex Be stars have four of each. Each hot-cold spot
pair is not necessarily traveling around the star at the same speed, so at certain times, Peters said, the waves complement
each other, producing an even hotter spot with enough energy to eject material into space.
But some astronomers doubt that Be stars would have enough energy under the non-radial pulsation theory to eject material.
So a second theory holds that magnetic fields are responsible for some of the Be star activity. This second model has not
had much support, Peters said, because while magnetic fields are commonly found in smaller, cooler stars such as the sun,
“there is no obvious physical reason why a hot star should have a surface magnetic field.”
Nevertheless, Peters’ most recent paper on mu Centauri, analyzing 1994 observations of the star when it was flaring
material into a disc, concedes that the data could be interpreted to support the existence of a magnetic field.
STARS, INCLUDING Be stars, also have “winds.” Wind on a star is not the same as wind on the Earth. The sun’s wind,
called solar wind, is the outflow of charged particles, mostly protons and electrons. It is weak by standards of the
universe, but if not for the protection of Earth’s magnetic field, the ionizing radiation of the solar wind would kill
us all. Be stars, which are emissive stars, have wind up to 10 million times as strong as the solar wind.
“The spectral signature for wind is in the far ultraviolet, which does not penetrate the Earth’s ozone layer,” Peters
said. “We didn’t know anything about winds on stars until we had spectrographic data from space.”
In a 1997 paper, Peters analyzed data from a long series of observations of 15 Be stars. In some of the stars there was a
correlation between the cycles of the surface wind and the pulses of light the star emits. The rate at which the light
pulsed was faster than the star’s rotation, corresponding to the hot crests passing the observer’s line of sight as
explained by the non-radial pulsation theory.
“The bulk of the observations seem to imply that NRP (non-radial pulsation) is not only responsible for the light
variability but also for the modulation of the wind,” she said.
It is unlikely that astronomers will ever be able to completely understand Be stars in our lifetime. As Peters says,
“We’ve known about them for less than 130 years. We’ve had good equipment for less than 30, and we are limited to
observations in a single line of sight.”
So, to even the most powerful telescope, a Be star is just a pinpoint of light, a little twinkling star.
|
|
