When we think of mysteries, the usual suspects are the Bermuda Triangle, ancient lost cities, or unsolved crimes. But the universe has its enigmas that make Earth's riddles resemble child's play. Dive into this cosmic conundrum where stars are disappearing without a trace!
Let's hit on some freaky cosmic disappearances:
The Fading Stars: Imagine observing a star for years and then, one night, it's just... gone. Astronomers have documented instances of stars that dramatically dimmed and then seemingly vanished from the night sky1. No supernova, no black hole – just a cosmic vanishing act. Some stars show periodic or erratic dimming, which isn't always easily explained by known astrophysical phenomena. One of the most famous recent cases is the star KIC 8462852, also known as Tabby's Star or Boyajian's Star. Its irregular dimming patterns led to various theories, from a swarm of comets blocking its light to large alien megastructures. Typically, massive stars end their lives in a spectacular explosion known as a supernova. However, some stars have been observed to dim and then disappear without any detectable supernova event. A study conducted using the European Southern Observatory's Very Large Telescope (VLT) reported an example of this phenomenon. Researchers identified a massive star in the Kinman Dwarf galaxy that seemed to have vanished without undergoing a supernova explosion [5]. Some suggest these fading stars might have transformed directly into a black hole without a supernova, or it's obscured by dust or other material, blocking their light temporarily. Some stars are inherently variable, with brightness changing over time.
Galaxy Ghosts: Entire galaxies seem to be missing the matter they should contain. Dark matter was proposed as the solution, but recent studies suggest that even with dark matter included, some galaxies are inexplicably lighter than they should be 2.
Most galaxies, including our own Milky Way, are believed to be surrounded by vast halos of dark matter, which doesn't emit, absorb, or reflect light. These halos exert gravitational forces and play a key role in galaxy formation and dynamics. Even though they can't be observed directly, the presence of dark matter is inferred from its gravitational effects on visible matter and light.
Low Surface Brightness (LSB) Galaxies are as large as our Milky Way but have so few stars that they can barely be seen against the night sky. Because they're so dim, LSB galaxies can easily be missed in sky surveys, making them "ghostly" in appearance. They pose questions about galaxy formation and evolution because their low density challenges our understanding of these processes.
Ultra-diffuse galaxies (UDGs) are as large as typical galaxies but have far fewer stars, making them extremely diffuse. Discovered in clusters like the Coma Cluster, these galaxies are so sparse that you could see right through them. Their existence raises questions about how such sparsely populated galaxies could form and survive in dense galaxy clusters.
The Galactic Missing Baryon Problem refers to the discrepancy between the predicted amount of baryonic (ordinary) matter in the universe from Big Bang nucleosynthesis and the observed amount of baryonic matter from the Cosmic Microwave Background. A significant portion of the baryons in the universe was "missing" or not accounted for. Recent observations have suggested that these missing baryons might be found in the warm-hot intergalactic medium, which is a diffuse, warm-hot gas between galaxies.
In all these cases, the "ghostly" aspect of these galaxies or galaxy-related phenomena arises from the challenges associated with observing, detecting, or understanding them using our current observational tools and theoretical frameworks. They're like the phantoms or shadows of the cosmos – entities we know or believe exist but remain elusive or intangible in various ways.
Signal Silence: Scientists have been detecting 'Fast Radio Bursts' (FRBs) - mysterious and powerful signals from deep space for years. While some sources of FRBs have been identified, others remain a perplexing puzzle, their origins shrouded in cosmic secrecy 3. The first FRB was identified in 2007 in archived data originally recorded in 2001 by the Parkes Radio Telescope in Australia. Initially, FRBs were thought to be one-off events. However, some FRBs have been observed to repeat, albeit irregularly. The repeaters challenge the idea that FRBs come from cataclysmic, one-time events, such as a star explosion. There could be other causes, such as the collision of two dense neutron stars, highly magnetized neutron stars that can emit bursts of energy, the collapse of a neutron star into a black hole, and possibly alien civilizations.
The Great Void: In our universe filled with galaxies, stars, and matter, there exists an enormous region, roughly 1 billion light-years across, with... nothing. This colossal emptiness, known as the Eridanus Supervoid, baffles scientists as to why and how such a vast space is devoid of typical cosmic content 4.
Conclusion: While we've made leaps and bounds in our understanding of the cosmos, it's humbling (and a tad terrifying) to realize how much we don't know. From disappearing stars to mysterious radio signals, the universe dazzles and baffles us with its grand magic show.
References:
Villarroel, B., Nyholm, A., Karlsson, T., & Comerón, S. (2016). The search for failed supernovae with the Large Binocular Telescope: confirmation of a disappearing star. ↩
de Blok, W. J. G., & McGaugh, S. S. (1997). The dark and visible matter content of low surface brightness disc galaxies. ↩
Petroff, E., Barr, E. D., Jameson, A., Keane, E. F., Bailes, M., Kramer, M., ... & Of, C. (2016). A real-time fast radio burst: polarization detection and multiwavelength follow-up. ↩
Szapudi, I., Kovács, A., Granett, B. R., Frei, Z., Silk, J., Burgett, W., ... & Flewelling, H. (2015). Detection of a supervoid aligned with the cold spot of the cosmic microwave background. ↩
Allan, T. R., Groh, J. H., Mehner, A., & Smith, N. (2020). The possible disappearance of a massive star in the low-metallicity galaxy PHL 293B. Monthly Notices of the Royal Astronomical Society, 495(1), 55-68.↩