University of Wisconsin

Space Astronomy Laboratory




Objects: 2203, 2205, 2209, 2215, 2217, 2221, 2222, 2223, 2230, 2231, 2234, 2235, 2236, 2237, 2239, 2271, and 2273
UW Astronomer: Karen Bjorkman

The astrophysical zoo is a wonderful place to find strange and sometimes maddening objects to study and try to understand. There are many different types of interesting and unusual stars in our Milky Way galaxy. Among the hottest are the O and B stars, which are also some of the youngest and brightest stars found. Although they make up only about 11% of all stars, the massive O and B stars have a disproportionate importance in affecting the life cycle of the universe. They process hydrogen and helium into heavier elements, and recycle material into the interstellar medium (ISM) through supernova explosions and stellar winds. Supernova explosions often trigger the formation of new stars by compressing and heating the ISM.

Elements heavier than iron are formed almost exclusively in supernova explosions, and supernovae only occur among stars of about 6 times the mass of the sun or more. O stars range in mass from about 25 times the mass of the sun up to 60-100 times the mass of the sun, while B stars range from about 10 times the mass of the sun up to 25 times the mass of the sun. Since the lifetimes of such massive stars are short compared to most stars, their effect on the evolution of the galaxy is large.

The other important role of massive stars in affecting galactic evolution comes about via stellar winds. These outflows, which cause O and B stars to lose significant portions of their outer layers during their lifetimes, can modify the evolution of massive stars. The winds also affect the surrounding ISM.

The formation of new stars in the regions of groups of O and B stars (called OB associations) is affected because the winds sweep out much of the gas and dust remaining in the area after the massive stars form. In the case of O stars the winds may actually "blow bubbles" of relatively empty space around the star. This usually prevents lower mass stars from forming in the region, and may eventually lead to the cessation of star formation in the immediate vicinity. The material blown off by stellar winds enriches and replenishes the ISM.

Not all O and B stars are alike - for example, some of them show bright lines, called emission lines, in their spectra. These stars are referred to as Oe and Be stars, where the "e" indicates that emission lines are present. Most O and B stars show only dark, or absorption, lines, so the presence of emission lines is a sign that something unusual is occurring in the Oe and Be stars. Astronomers are interested in understanding what it is about these stars that causes them to be different from "normal" O and B stars. Emission lines present in the spectra of early-type stars almost always suggest that a large amount of gas or dust surrounds the star.

The first Be star was discovered by Angelo Secchi in 1866, when he noted that there was a bright emission line of hydrogen in the spectrum of the star Gamma Cassiopeiae, the third brightest star in the constellation of Cassiopeia. Since then, several thousand stars have been classified as Be based on emission lines in their optical spectra. Usually, the emission lines are those of hydrogen, but occasionally lines of iron are also seen in emission.

While the initial discovery and classification of Be stars was based on optical spectra, the absence or presence of emission lines in any one spectrum may not necessarily pinpoint whether a star is a Be star. Often the spectrum of a single star may vary between that of a normal B star, a Be star, and a Be-shell star (one which shows very sharp, narrow absorption lines) over a period of time. The star may change its spectral characteristics in times that vary widely from star to star, ranging from as short as several weeks to as long as tens of years, and it is generally not periodic. Consequently, a star is considered to be a Be star if it has ever shown evidence of emission in its optical spectrum. Be stars make up about 15-20% of all B stars.

Although the distinctive nature of Be stars was first recognized because of the simple observation of bright lines, over 100 years of study have shown that Be stars are unusual in many other respects and in many other wavelength regions. Each spectral region provides information about a different part of the material around the star.

The number of known Be stars now stands at over 3000, and it continues to increase as new Be stars are discovered among supposedly normal B stars. The changing nature of Be star spectra suggests that the B and Be stars may be simply different phases of behavior of the same type of star rather than distinctly different classes of objects. Be stars also are known to show substantial changes in brightness in times as short as a few hours to as long as several years.

Optical studies of the polarization of the light from Be stars have provided evidence that the material around Be stars may be concentrated toward the equator of the star, rather than evenly distributed in all directions around the star. The polarization of light from Be stars is also known to change with time. Stars that are losing material (for example, due to a stellar wind) show evidence of this mass-loss by the shape of the lines in their spectra.

The first star discovered to be losing mass was the star P Cygni, so the particular shape of lines in mass-losing stars is known as a P Cygni line profile. These lines are often seen in the ultraviolet portion of the spectrum, which can only be observed by telescopes in space. Ultraviolet observations show that fast, hot winds are a common characteristic of Be stars. Another important characteristic of Be stars in the ultraviolet is the presence of "superionization"; that is, an indication that the material in Be star winds is much hotter than is present in the winds of normal B stars.

Even though Be stars make up a large fraction of all B stars, they are not well understood. The problems inherent in understanding Be stars are problems which are applicable to other hot stars as well. Since Be stars are known to be rapidly spinning, the question of how this affects the star and the material around it is an important one.

An understanding of how rapid rotation may combine with the stellar wind to cause the stars to lose their outer layers faster applies to O and B stars alike. Therefore, an understanding of the nature of Be stars, which are the most prominent examples of rapidly rotating hot stars, is a key to a complete understanding of the nature of hot stars in general. Increased mass-loss rates affect the ISM as well, so there is also a tie with star formation and the evolution of galaxies.

The Wisconsin Ultraviolet Photo-Polarimeter Experiment (WUPPE), one of the telescopes aboard the Astro-1 Space Shuttle Mission, will be observing Be stars. WUPPE will obtain polarimetry of these stars in the ultraviolet - something which has never before been done. The data from WUPPE will help in learning more about how the material around Be stars is distributed, where the hottest gas is located around the stars, and what might be responsible for making a B star into a Be star.