Thaisa

Artemis II is not just going "to the Moon," but rather traveling to meet it at an exact point in space. It’s all about orbital mechanics.

🆕 A return to the Crab Nebula 🦀 Nearly 1000 years ago, astronomers saw something blazing in the sky – a supernova so bright it was visible in daylight. Today, astronomers continue to watch its remnant (the Crab Nebula) evolve. 1/3

What we usually recognize as the Andromeda Galaxy is only its bright stellar disk, but in reality M31 extends far beyond that familiar spiral shape. Surrounding it is an enormous and extremely faint environment dominated by hydrogen gas, part of what we call the circumgalactic medium. This extended halo reaches hundreds of thousands of light-years into space, making the true physical size of the galaxy much larger than what is visible in ordinary photographs. The reddish structures seen in very deep images trace ionized hydrogen, commonly detected through H-alpha emission. This light is produced when hydrogen atoms are energized by ultraviolet radiation, coming from massive stars, past episodes of star formation, shock heating, or even the diffuse extragalactic radiation field, and then recombine, releasing a characteristic red glow. The emission is extraordinarily faint, close to the detection limits of modern imaging, which explains why these clouds were only clearly identified in the last decade despite Andromeda being one of the most photographed objects in the sky. These hydrogen clouds are not passive material. They are part of the galaxy’s baryonic cycle. Some of the gas has been expelled from the disk by supernova explosions and stellar winds, while other portions likely originate from intergalactic gas slowly accreting onto Andromeda or from smaller satellite galaxies that have been gravitationally stripped during past interactions. Over cosmic time, this reservoir can cool and fall back toward the disk, helping to sustain future star formation. The structures also preserve dynamical history. Filaments and diffuse streams of gas are thought to be linked to gravitational encounters with companion galaxies such as M32 and M110, leaving large-scale gaseous signatures similar to the stellar streams observed in galactic halos. In this sense, the hydrogen surrounding Andromeda acts as a record of past mergers and interactions. Perhaps the most striking implication is scale. Observations with ultraviolet spectroscopy have shown that Andromeda’s gaseous halo is so extended that it nearly overlaps with the halo of the Milky Way. The space between our galaxies is therefore not empty but filled with extremely tenuous gas belonging to the Local Group environment. Seen this way, the faint hydrogen clouds reveal that a galaxy is not just a luminous island of stars. It is a dynamic system continuously exchanging matter with its surroundings, growing, recycling gas, and evolving through interactions that unfold over billions of years.

Cold Dark Matter (CDM) is a central component of the standard cosmological model used to describe the composition, evolution, and large-scale structure of the Universe. The term cold refers to the fact that the particles constituting this form of matter possessed very low thermal velocities compared with the speed of light at the time when cosmic structures began to form, meaning they were non-relativistic. The term dark indicates that these particles do not interact appreciably with electromagnetic radiation: they neither emit, absorb, nor scatter light, and therefore cannot be observed directly. Their presence is inferred solely through their gravitational effects. Within the ΛCDM framework, cold dark matter accounts for approximately 27% of the total mass–energy content of the Universe, while ordinary baryonic matter contributes about 5% and dark energy roughly 68%. This partition is tightly constrained by observations of the cosmic microwave background, large-scale galaxy clustering, gravitational lensing, and the expansion history of the Universe. CDM provides the dominant gravitational backbone upon which all visible cosmic structures are built. A defining role of cold dark matter is its control over structure formation. Because CDM particles move slowly and interact weakly, they preserve small primordial density fluctuations rather than erasing them through free streaming. The free-streaming length of CDM is therefore very short, a direct consequence of either a relatively large particle mass or a production mechanism that yields low velocities in the early Universe. This allows gravitational collapse to occur efficiently on small scales. Structure formation proceeds hierarchically, in a bottom-up manner: small dark matter halos form first and subsequently merge to produce larger systems such as galaxies, galaxy groups, and clusters. On the largest scales, the gravitational evolution of cold dark matter naturally produces the filamentary network known as the cosmic web. Dark matter forms interconnected filaments, sheets, and dense nodes, acting as a gravitational scaffold. Ordinary matter flows into these potential wells, where it cools, condenses, and forms stars and galaxies. The observed distribution of galaxies closely traces this underlying dark matter structure, providing strong empirical support for the CDM paradigm. From a particle-physics standpoint, cold dark matter is assumed to be non-baryonic and effectively collisionless, interacting with ordinary matter primarily through gravity. Proposed particle candidates include weakly interacting massive particles (WIMPs), axions, and other beyond-Standard-Model entities, although no candidate has yet been confirmed experimentally. A broad experimental program continues, encompassing direct detection, indirect searches, and collider experiments. Despite its success on large and intermediate scales, cold dark matter has historically faced challenges on sub-galactic scales. Two of the most discussed are the cusp–core problem and the missing satellites problem. Numerical simulations of collisionless CDM predict steep central density cusps in dark matter halos, while observations of dwarf and low-surface-brightness galaxies often suggest flatter, core-like density profiles. Similarly, simulations predict many more small satellite halos around galaxies like the Milky Way than the number of observed luminous satellites. Crucially, many cosmologists now regard these tensions as likely consequences of baryonic physics rather than evidence against cold dark matter itself. Energetic feedback from baryonic processes, such as repeated supernova explosions or activity from central black holes, can redistribute gas within galaxies and dynamically heat the surrounding dark matter. This process can flatten initially cuspy density profiles into cores, potentially resolving the cusp–core discrepancy. Likewise, many low-mass dark matter halos may simply be unable to retain gas after reionization or strong stellar feedback, preventing star formation altogether. These halos would exist but remain effectively invisible, naturally addressing the missing satellites problem without altering the fundamental properties of dark matter. Cold dark matter remains the most robust and successful framework for explaining the formation and evolution of cosmic structure across a vast range of scales. Its gravitational influence is firmly established, and many apparent small-scale discrepancies may be reconciled through improved modeling of baryonic processes. The fundamental nature of the dark matter particle itself, however, remains one of the most important open questions in modern cosmology and astroparticle physics.

How the International Space Station was built [🎞️ Jared Owen]