Unbelievable Power: The Physics of Nuclear Blast Waves

Why the ArXiv of the future will look like Authorea

and 1 collaborator

Open Peer Review with Authorea

and 3 collaborators

The spin rate of pre-collapse stellar cores: wave driven angular momentum transport in massive stars

and 4 collaborators

The Value of Ignorance in Science

and 4 collaborators

Asteroseismology Can Reveal Strong Internal Magnetic Fields in Red Giant Stars

and 4 collaborators

*This is the author’s version of the work. It is posted here by permission of the AAAS for personal use, not for redistribution. The definitive version was published in Science on Vol. 350 no. 6259 pp. 423-426 , DOI: 10.1126/science.aac6933*

Internal stellar magnetic fields are inaccessible to direct observations and little is known about their amplitude, geometry and evolution. We demonstrate that strong magnetic fields in the cores of red giant stars can be identified with asteroseismology. The fields can manifest themselves via depressed dipole stellar oscillation modes, which arises from a magnetic greenhouse effect that scatters and traps oscillation mode energy within the core of the star. The *Kepler* satellite has observed a few dozen red giants with depressed dipole modes which we interpret as stars with strongly magnetized cores. We find field strengths larger than $\sim\! 10^5 \,{\rm G}$ may produce the observed depression, and in one case we infer a minimum core field strength of $\approx \! \! 10^7 \,{\rm G}$.

Are we alone in the Universe? The Drake Equation

**Previous “Habitable Planets” – Next “Astrobiology”**

There is on average one planet orbiting every star in the Universe \citep{2013ApJ...764..105S, 2012Natur.481..167C}. If this sounds exciting, you might wanna read the previous post in this series. Our Galaxy (the Milky Way) is an immense disk of gas and stars with a diameter of about 100 000 light years, hosting about 100 billion stars and, therefore, also about 100 billion planets. Take a deep breath. Now, it turns out the Milky Way is just one of 100 billion galaxies that populate our Universe, a colossal expanding stretch of spacetime with an age of 13.7 billion years. The math is trivial: There are about 10 000 000 000 000 000 000 000 = 10^{22} planets out there. This number is extremely large. Apparently larger than the number of grains of sand found in every beach and every desert on Earth.

But how many of these planets host life? And in particular, **how many planets host intelligent life we might be able to communicate with**?

In order to estimate the number of technological civilizations that might exist among the stars, in 1961 Frank Drake proposed the following simple equation:

Notes on Mixing Length Theory

and 1 collaborator

Are we alone in the Universe? The Fermi Paradox

**Previous “Astrobiology” – Next “Interactive Drake Equation”**

With an estimated diameter of 93 billion light years and age of 13.7 billion years, our Universe is an astonishingly big place that’s been around for a very long time. When you look up, you only get a short glimpse at a fraction of the hundreds of billions of stars that populate our Galaxy (which in turn is one of hundreds of billions in the cosmos), but it’s enough to make you wonder: “Are we alone?” In the previous post we discussed the likelihood of the emergence of (intelligent) extraterrestrial life. Starting from the famous Drake Equation and using recent findings in astrophysics and some astrobiology arguments, we obtained a simple way to estimate *N*, the number of communicative civilizations in our Galaxy. This reduces to the product of the chance of emergence of intelligent life *f*_{i} and the longevity *L* (in years) of a civilization’s communicative phase:

\begin{equation}\label{eq:Drake_simplified} N \approx \, \frac{1}{4}\, f_i \, L \,. \end{equation}