SpaceTime

Image: protoplanetary disc surrounding the young star HL Tauri, a very young T Tauri star in the constellation Taurus, approximately 450 light-years (140 pc) from Earth in the Taurus Molecular Cloud. These new ALMA observations reveal substructures within the disc that have never been seen before and even show the possible positions of planets forming in the dark patches within the system. Credit: ALMA (ESO/NAOJ/NRAO) T Tauri stars are pre-main-sequence stars in the process of contracting to the main sequence. Their central temperatures are too low for hydrogen fusion. Instead, they are powered by gravitational energy released as the stars contract, while moving towards the main sequence, which they reach after about 100 million years. Roughly half of T Tauri stars have circumstellar disks, which in this case are called protoplanetary discs because they are probably the progenitors of planetary systems like the Solar System.

Image: Virgo Supercluster. Credit: Andrew Z. Colvin The Virgo Supercluster is a region with a diameter of 33 megaparsecs (~1000 times larger the Milky Way's diameter) containing at least 100 galaxy groups and clusters.

Image: The Milky Way. Credit: Serge Brunier Using γ-ray data from the Fermi Large Area Telescope, various groups have identified a clear excess emission in the inner Galaxy, at energies around a few GeV. This excess attracted great attention, because it has properties typical for a dark matter annihilation signal.

Image: Spiral galaxy NGC 253  Credit: Robert Gendler/Jim Mistin The modern paradigm of cold dark matter with a cosmological constant (ΛCDM) predicts that galaxies form hierarchically - growing through the gradual merging of many smaller galaxies.

Image: Artistic representation of Doulbe Neutron Stars. Credit: NASA/Goddard Space Flight Center Double Neutron Stars (DNS) have to survive two supernovae and still remain bound. For this reason these systems are a unique and rare population of neutron stars and sets strong limits on the nature of the second collapse. Moreover, DNS emit gravitational radiation and consequently their orbit decays and they merge. This make DNS systems prime candidates for detection of gravitational radiation.

For orbital period generally we refer to the sidereal period, that is the temporal cycle that it takes an object to make a full orbit, relative to the stars. This is the orbital period in an inertial (non-rotating) frame of reference (365,25 days for the earth).