Herbig Ae/Be stars


When a dust-cloud in space is heavier than a certain mass, called the Jeans mass, it will collapse to form a star. In the very beginning, before it becomes a main sequence star like our sun, it will be called a protostar. These objects are very hot because of the energy coming from the collapsing cloud. There are three different types of protostars:

  • The least heavy, a T-Tauri star.
  • The medium weight one, a Herbig Ae/Be star
  • Very heavy protostars.

A Herbig Ae/Be star is a protostar that is so heavy it starts burning hydrogen quickly. What you see when you look at these kinds of stars is a normal star, but itís surrounded by dust, because the dust hasnít had time to form planets or planetoids, fall on the star or be blown away by stellar winds yet.
Because of the dust clouds you can detect Herbig Ae/Be stars quite well using spectroscopy. You will see the spectrum of a normal young star, but a large amount of infrared (from the dust surrounding the star) is added. You will find different extra lines for the gas and dust.
On the differences between a T-Tauri star and a Herbig Ae/be the most important one is that a T-Tauri star hasnít started fusion yet because of its lower mass. By the time a T-Tauri star will start fusing it is older and the dust will not be so visible. A herbig Ae/Be star develops quicker than a T-Tauri star.
Because it is fusing already, a Herbig Ae/Be star is much brighter than a T-Tauri star. It isnít hidden inside its nebula like a T-Tauri star is, it shines through.
What is also an unexplained fact is that the heavier the protostar, the more chance it has that it has close stellar companions. You have a much larger chance of seeing other objects near a herbig Ae/be star than near a T-Tauri star.

Currently, much research is being done on magnetic fields. You can detect a magnetic field, because the particles in the dust cloud will align themselves along the magnetic field lines and that will cause light coming from the star to be polarized. You can estimate the shape and strength of the magnetic field from the polarization of the light. These magnetic fields are especially interesting, because they are believed to be one of the reasons that planets will form. In these early type stars, planet formation is a very interesting and as of yet relatively little understood area of research.
A second type of research is being done on the spectroscopy of the dust cloud surrounding the herbig Ae/Be star. This is not only used to find and classify Herbig Ae/Be stars, but also to detect different elements and their availability in the disk. Again this is interesting for research on planet formation, a gas planet like Jupiter requires quite large amounts of hydrogen to be formed, while a planet like ours requires more heavy elements like iron to form.
The third type of research that is being done is done on Herbig Ae/e stars that will shine in X-rays also. Uncertain is which reason is the exact cause of this radiation, but there are several explanations.
The first one is called the dynamo effect. What happens is that magnetic particles are spinning around the star creating an electric field, like a bicycle dynamo. This builds up charge which discharges sending out X-rays.
The second explanation is that fast moving stellar winds from the star hit gas and heat it up so much it will then send out X-Rays.

Herbig AeBe stars

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