What Is A B-Type Star And Why Are They So Important To Astronomers?

How we categorize stars

Astronomers trying to make sense of the differences between stars realized that the wavelength of light at which its output peaks is among its most important features. Combined with the presence or absence of certain spectral lines, this wavelength reveals the temperature of the star’s outer layers. For very young or very old stars, that temperature changes relatively quickly, but for most of their lives, stars’ temperatures are a very good indication of their mass, which in turn determines their lifespan. 

During the long years on what is called the “main sequence,” temperature is also a good guide to how much energy a star is releasing, from which we can calculate its distance.

It’s arbitrary where we draw the lines between one category of stars and another, but having a categorization system is certainly useful. You’d expect such a system to have A-type stars as the brightest through to G, but for historical reasons, things got scrambled.

Illogically, B-type stars are hotter and more massive than A-type, with temperatures between 10,000 K (9,727 °C or 17,540 °F) and 30,000 K (29,727 °C or 53,540 °F), but B is still the one letter that’s in its expected place, being the second hottest category, behind O-type stars.

How big stars shape the universe

While in their main sequence phase, all stars turn hydrogen to helium, but for most B-type stars, their greatest impact comes from the end of their lives.

Stars with masses more than eight times that of the Sun explode as supernovae, and in the process, they not only produce a lot of other elements, but distribute them far and wide. Most of the Earth’s most common elements are made in supernovae. Although small amounts of these elements are formed through other processes, along with the majority of some rarer elements, there’s little chance there would be enough elements beyond oxygen on the periodic table to form Earth-like planets without supernovae.

Big stars have far too short lifespans for life to develop around them, but through their deaths are crucial to life’s existence around smaller stars, even though having them too close can be fatal. 

But why B-type?

Not all supernovae come from B-type stars, however, and not all B-type stars will end up as supernovae. For one thing, Type Ia supernovae are the product of white dwarfs, left behind by smaller stars.

Even if we ignore that, the least massive B-type stars have masses around twice that of the Sun, too light to become a supernova.

On the other hand, all O-type stars probably end up as supernovae, although there’s a controversial theory that some skip that stage and go straight to black holes.

So, you might wonder why we care about B-type stars. Aren’t O-type where it’s at? Aside from acknowledging that all stars matter, it’s important to realize O-type stars are now very, very rare, representing about one in 3 million stars.

There were probably a lot more O-type stars in the early universe, so many of the elements that make us up may be left over from then. Today, however, and for quite a long time before now, most supernovae have come from B-type stars, with O-type a rare curiosity, sometimes in the form of exotic hypernovae. 

By the time stars become supernovae, they have moved off the main sequence, but it’s likely that most famous supernovae like 1987a and SN 1054 were once B-type stars.

Our guides to the universe

We’ve learned a great deal of what we know about the universe from supernovae. Type Ia supernovae have revealed the existence of dark energy and accelerating expansion, but core-collapse supernovae have taught us about other important features, such as neutrinos and, perhaps one day, dark matter. 

If we had to wait around for an O-type supernovae, we’d have learned only a small fraction of this.

Even before they explode, B-type stars are very important for research. Recently, we covered a study indicating that fast-moving stars in the galactic halo were fired at us by the Large Magellanic Cloud. It needed to use very massive stars because smaller stars are too faint to detect at such distances, other than in their brief red giant phase. On the other hand, the study relied on getting a statistically significant sample, and there just aren’t enough O-type stars that fit the bill for that. Consequently, the researchers looked for B-type stars.

That’s a common pattern for astronomers looking to explore more distant parts of our own galaxy, or to study the behavior of individual stars in nearby galaxies. O-type stars are often too rare, and the other five categories are too faint, so B-types light the way.

Famous B-type stars

The most familiar B-type stars are Rigel, part of Orion, and the more massive of the binary Regulus, the brightest star in Leo.

Regulus aside, there is a surprising lack of familiar B-type stars, mainly because there are none within 80 light-years of Earth. Algol A, also known as the “demon star” and the second brightest in Perseus, is the next most well-known example.

Sirius’s main star and Vega both narrowly fail to qualify, being right at the top of the A-type stars.

Paikauhale is one of the two naked eye stars that sit either side of Antares, the beating heart of Scorpio.

Southern Hemisphere residents would be familiar with Acrux, the brightest star in the Southern Cross.

There are also some very famous stars that had the right mass to have been B-type once, but have since shifted off the main spectrum, including Antares itself and Betelgeuse.