Star Formation

Where did the heavy elements in the Universe come from? What happens to molecular gas when it is contracted in the presence of the magnetic field? In which conditions does star formation become possible? These and other questions are answered by Berkeley Professor Emeritus of Physics and Astronomy, Christopher McKee.

Star formation is one of the most important problems in astrophysics. I see it as being at the nexus of astrophysics. If you want to understand how galaxies evolve – galaxies consist of stars, so it’s really come down to then how the stars evolve. If you want to understand where did the heavy elements in the Universe come from, those heavy elements are made in stars. So we have to understand how the stars form.

One of the most active fields in astronomy today is the study of exoplanets, planets around other stars. Planets form around stars. You cannot understand how planetary systems form, unless you understand how stars form. So it’s really one of the very basic questions. It’s important to realize that the Sun was formed about 4,5 billion years ago, but the star formation in the galaxy continues today, it’s been gradually decreasing over the last ten billion years, but we still make about one or two solar masses of stars per year in the galaxy.

Evidence for fractal structure in the pulsations of stars Scientists have found a star whose pulsation frequencies' ratio is close to the golden mean

The stars are formed in molecular clouds. The molecular clouds are large regions of gas. In fact, much of the molecular gas is in so-called giant molecular clouds, which can have sizes up to a hundred parsecs worth (that’s three hundred light years in size). And this gas is dense by interstellar standards – it can have densities of thousands of particles per cubic centimeter. They’re very dusty, that is, they have tiny particles that absorb light, and that’s very important, because first of all, the molecular hydrogen in these molecular clouds is actually formed on the surface of these dust grains. And secondly, the dust grains block the ultraviolet radiation which could destroy the molecular hydrogen. So it’s very important for the gas to be able to be shielded like this so that it can be molecular.

So, let me now talk briefly about how massive stars form. Massive stars are very important, because they’re the ones that are responsible for formation of the heavy elements. In particular, stars more than about 8 solar masses undergo what’s called a ‘core collapse supernova explosion’, in which, after exhausting its nuclear fuel, the core of the star contracts and then explodes in a titanic explosion ejecting heavy elements into the interstellar medium. How do stars like that form?