John Baez's post explains that the common notion of black holes shrinking due to Hawking radiation is a simplification. While Hawking radiation exists, it's emitted by the hot "quantum atmosphere" surrounding the black hole, not the singularity itself. This atmosphere is formed from infalling matter interacting with intense spacetime curvature. The black hole's mass, contained within its event horizon, only decreases because this infalling matter effectively loses some energy as Hawking radiation before crossing the horizon. Therefore, it's more accurate to say the black hole's atmosphere radiates and shrinks as it loses energy, indirectly causing the enclosed black hole to shrink over vast timescales.
The post explores the surprising discrepancy between the estimated and observed rates of supernovae. While theoretical models predict hundreds of billions of supernovae across the observable universe annually, current surveys only detect a small fraction of that. This vast difference isn't due to faulty models, but rather the difficulty in observing these explosions. Dust, intervening galaxies, and the sheer expanse of the universe obscure the majority of supernovae from our view, making their detection a challenging endeavor despite their immense power. This explains why, even with sophisticated telescopes, we only observe a relatively tiny number compared to the predicted cosmic abundance.
HN commenters generally expressed awe at the sheer scale of supernovae occurring in the observable universe, with some emphasizing the vastness of space this implies. Several pointed out that the article's title was misleading as it conflated observable universe numbers with those in our own galaxy, where supernovae are much rarer. One commenter highlighted the counterintuitive fact that distant supernovae, though individually fainter, are collectively brighter than those nearby due to the sheer number at those distances. There was also discussion about the accuracy of the estimates, the methodology used, and the different types of supernovae. Some users shared links to further resources and tools like a supernova simulator. A few commenters jokingly lamented the lack of easily visible supernovae from Earth.
The article discusses the challenges in defining "Sun-like" stars, crucial for identifying potentially habitable exoplanets. While basic parameters like mass and temperature are a starting point, truly Sun-like characteristics also encompass age, metallicity, and magnetic activity cycles. The Sun's unusually low activity compared to similar stars is highlighted, raising questions about whether this quiescence is typical for G-type dwarfs and its implications for habitability. Ultimately, finding a truly "Sun-like" star requires a nuanced approach beyond simple categorization, emphasizing the need for ongoing research to understand the full complexity of stellar evolution and its influence on surrounding planetary systems.
HN users discussed the challenges of defining "Sun-like," noting that even small variations in a star's properties can significantly impact planetary habitability. Some pointed out the difficulty in observing other stars with the same level of detail as our Sun, making comparisons inherently limited. The potential for long-term stellar variability was also highlighted, along with the fact that our understanding of stellar evolution continues to evolve, making any definition of "Sun-like" subject to revision. A few commenters mentioned the Kepler mission's contribution to identifying potentially habitable exoplanets and the ongoing search for biosignatures. Finally, there was a brief discussion of the challenges in characterizing planetary atmospheres and the possibility of non-water-based life.
After over a decade, ESA's Gaia space telescope has completed its primary mission of scanning the sky. Gaia has now mapped nearly two billion stars in the Milky Way and beyond, providing unprecedented details on their positions, motions, brightness, and other properties. This immense dataset will be crucial for understanding the formation, evolution, and structure of our galaxy. While Gaia continues observations on an extended mission, the core sky survey that forms the foundation for future astronomical research is now finished.
HN commenters generally expressed awe and appreciation for the Gaia mission and the sheer amount of data it has collected. Some discussed the technical challenges of the project, particularly regarding data processing and the complexity of star movements. Others highlighted the scientific implications, including improving our understanding of the Milky Way's structure, dark matter distribution, and stellar evolution. A few commenters speculated about potential discoveries hidden within the dataset, such as undiscovered stellar objects or insights into galactic dynamics. Several linked to resources like Gaia Sky, a 3D visualization software, allowing users to explore the data themselves. There was also discussion about the future of Gaia and the potential for even more precise measurements in future missions.
Summary of Comments ( 79 )
https://news.ycombinator.com/item?id=44015872
Hacker News users discuss the surprising assertion that dead stars don't radiate, focusing on the definition of "dead" in this context. Several point out that even black dwarfs, the theoretical endpoint of stellar evolution, would still emit some radiation due to Hawking radiation, though at an incredibly low temperature and over vast timescales. The discussion also touches on the complexities of heat death, the challenges of simulating such long-term processes, and the limitations of current scientific understanding regarding proton decay and other long-term phenomena. Some users highlight the immense timescales involved, emphasizing the difference between theoretical predictions and observable reality. Others express fascination with the concept and appreciate the thought-provoking nature of the article.
The Hacker News post titled "Dead Stars Don’t Radiate" generated a moderate amount of discussion, with several commenters engaging with the premise and its implications.
One of the most compelling threads revolved around the definition of "dead" in the context of stars. A commenter pointed out that even black holes, often considered the ultimate dead stars, emit Hawking radiation, albeit at incredibly low levels. This sparked a discussion about the timescale over which this radiation would lead to complete evaporation, highlighting the immense durations involved in cosmological processes. Another user added to this by explaining that while a true black dwarf (a hypothetical, fully cooled white dwarf) wouldn't radiate, the universe isn't old enough for any to have formed yet. This clarification helped contextualize the article's claim within the current understanding of stellar evolution.
Several comments focused on the idea of heat death, the theoretical state where the universe reaches maximum entropy. Users discussed the implications of a universe where no temperature differences exist, and whether this truly represents a "dead" state. One comment explored the possibility of quantum fluctuations allowing for localized pockets of energy even in a heat-dead universe, leaving a glimmer of potential for activity even in such an extreme scenario.
A more technical discussion branched off, delving into the thermodynamics of black holes. Users debated the nature of the information paradox and the role of entropy in black hole evaporation. This conversation touched on complex theoretical concepts, showcasing the depth of understanding some commenters brought to the discussion.
A few commenters expressed their appreciation for the article's clarity and its ability to explain complex physics concepts in an accessible manner. This sentiment highlighted the value of clear scientific communication and the article's success in achieving this goal.
Finally, a couple of comments offered additional resources, such as links to relevant Wikipedia articles and other scientific papers, further enriching the conversation and providing avenues for deeper exploration of the topic.