What are the Be Stars?

The “Be X-ray Binaries” have been mentioned on this site a number of times.

A Be X-ray Binary consists of a Be Star plus a neutron star, where as a result of close encounters with the Be Star, the neutron star accretes matter from the Be Star and in the process emits x-rays.

But, what is a “Be Star” and what makes them so special?

We need a little context, so let’s start with the 200 billion stars in the Milky Way Galaxy. For hundreds of years, people have looked at the stars in the sky and wondered: “What’s going on?”.

Throughout history some of these wondering people have spent much of their lives looking at the stars in detail and recording what they have observed. Two properties of stars, brightness and color, have provided a framework for distinguishing one kind of star from another. Two stars that have nearly the same color and brightness are considered to be a “kind” of star, and when all the stars are plotted on a 2 dimensional graph of brightness on the y-axis, and color on the x-axis, it has been found that instead of being spread all over the graph, the stars actually are clumped together. The graph is called a “Hertzsprung-Russell Diagram“, or “H-R Diagram” for short.

The diagram shows a diagonal clump of stars from the upper left of the diagram, stretching down to the lower right of the diagram.

  • The “brightest stars” are near the top of the diagram, and the “dimmest stars” are near the bottom.
  • The “hottest stars” are on the left side of the diagram, and the “coolest stars” are on the right side of the diagram.
  • The hottest stars are blue and the coolest stars are red, which corresponds to the colors of the rainbow, where
    • the blue (ultra-violet) side of the rainbow contains the shortest wavelengths of visible light (hottest) and
    • the red side of the rainbow contains the longest wavelengths of visible light (coolest).

In any event, the brightest, hottest stars are in the upper left of the diagram, and they are classified as O-type supergiants. The second brightest, hottest stars are just a bit down and to the right of the supergiants and they are classified as B-type stars. In general, the B-type stars are giant stars with mass in the range of 3-17 times the mass of the Sun, and radius 3-10 times the radius of the Sun.

Within the class of B-type hot giant stars, there is a distinct subclass, called the “Be Stars“. These are the stars that are of interest to us in the context of binary x-ray pulsars, because it turns out that a large number of binary x-ray pulsars have a Be star as a companion in Be X-ray Binary systems.

Before looking at the binary systems, let’s first focus on the Be Stars themselves.

About 1 out of 6 B stars is a Be star. The main distinguishing feature is that a Be star is a rapidly rotating giant, such that its shape is that of an oblate spheroid, which means there is a large circumstellar bulge or disc wrapped around the equator of the Be star. Normal B stars do not have a circumstellar disc.

The disc of the Be star, itself, emits its own specific radiation, known as a Balmer emission line, which is a normal hydrogen emission line that is typically simply part of a star’s emission spectrum. However, the fact that this emission line appears as an enhanced addition to the normal continuous emission spectrum of the star, means something is going on above the photosphere of the star, where most of the star’s normal continuous emission spectrum comes from. What is different about the Be stars is that they have this circumstellar disk bulging out around the equator of the Be star, so it is reasonably assumed that this extra Balmer emission line is coming from the disk and not the main body of the Be star.

It also turns out that the Balmer line is spread around its normal value indicating that the emission is coming from different parts of the disk that have different velocities with respect to the observer. The velocity has been calculated to indicate that it is as if the material in the disc is rotating around the star at a Kepler velocity meaning that the disk is orbiting around the main body of the star like a satellite in a stable orbit. As the disc rotates around the main body the part of the disc that is coming toward the observer shifts the emission line to a higher frequency (shorter wavelength) and the part of the disc that is moving away from the observer shifts the emission line to a slower frequency (longer wavelength). These frequency shifts are as a result of the well-understood physical Doppler shift mechanism. The net effect is that the line does not appear as sharp as it would if it was being emitted from a stationary source, because the Doppler effect shifts the emissions from the disc based on the velocity of the material in the disc as seen by an observer. i.e. the material in the part of the disc approaching the observer, has the frequency of its emission line shifted to a higher value, and the material in the part of the disc moving away from the observer, has the frequency of its emission line shifted to a lower value. The net effect is that the total picture of the data is spread wider in frequency, in fact to the point that there is often a double peak on the line where one of the  peaks is associated with the material moving toward the observer, and the other peak is associated with material moving away from the observer.

The velocity of the Kepler orbit is on the order of 200 km/sec. A “Kepler orbit” is a stable orbit where a mass has a distance and speed relative to the star where the orbit would be nearly a stable circle. In general, given a distance from the central star, there is a velocity associated with that distance that results in a stable circular orbit.

An interesting thought to consider is what if a neutron star was embedded in this disc and orbiting the star right along with all the gaseous material that comprises the disc.

We are not going to say that this is actually what is going on with the Be stars, but it is a concept worth keeping in mind when we start to consider the overall aspects of the Be X-ray binary systems.

The Be X-ray binary systems generally consist of a neutron star in an eccentric elliptical orbit going around the Be star, and there are bursts of x-rays when the neutron star encounters the circumstellar disc at the periastron of the NS orbit around the Be star.

These are just concepts to keep in mind, when we discuss how the NS-Capture theory explains the Be X-ray Binary systems (BeXRB’s).