We present some new ideas about the possibility of life developing around sub-giant and red giant stars. Our study concerns the temporal evolution of the habitable zone. The distance between the star and the habitable zone, as well as its width, increases with time as a consequence of stellar evolution. The habitable zone moves outward after the star leaves the main sequence, sweeping a wider r...

Many binary star systems are not wide enough to contain the progenitor stars from which they were made. One explanation for this is that when one star becomes a red giant a common envelope forms around both stars in the binary system. The core of the giant and its companion star continue to orbit one another inside the envelope. Frictional energy deposited into the common envelope may lead to i...

Binary population synthesis shows that mass transfer from a giant star to a mainsequence (MS) companion may account for some observed long-orbital period blue stragglers. However, little attention is paid to this blue straggler formation scenario as dynamical instability often happens when the mass donor is a giant star. In this paper, we have studied the critical mass ratio, qc, for dynamicall...

We find that the infrared excess around HD 233517, a first-ascent red giant, can be explained naturally if the star possesses an orbiting, flared, dusty disk. We estimate that the outer radius of this disk is 45 AU and the total mass within the disk is 0.01M . We speculate that this disk is the result of the engulfment of a lowmass companion star that occurred when HD 233517 became a red giant....

Repeated radial velocities have been measured for a sample of 10 subgiant CH (sgCH) stars over a period of ∼15 years. Long-term velocity variations are exhibited by all but one star, and spectroscopic orbits have been calculated for six of them. The periods are long, ranging from 876 to 4144 days. The distribution of eccentricities and mass functions for sgCH star orbits are similar to those fo...

We revisit the problem of why stars become red giants. We modify the physics of a standard stellar evolution code in order to determine what does and what does not contribute to a star becoming a red giant. In particular, we have run tests to try to separate the effects of changes in the mean molecular weight and in the energy generation. The implications for why stars become red giants are dis...

We revisit the problem of why stars become red giants. We modify the physics of a standard stellar evolution code in order to determine what does and what does not contribute to a star becoming a red giant. In particular, we have run tests to try to separate the effects of changes in the mean molecular weight and in the energy generation. The implications for why stars become red giants are dis...