A quick follow-up to yesterday’s post. The idea of a stream of debris or even large objects like comets or asteroids from another star continues to resonate with me. The odds on identifying such a stream in terms of origin seem stupendous, but the benefits of doing so would be obvious. I notice that another kind of stellar stream is in the news, one involving not debris but entire stars. The Icarus stream is a grouping of stars that seem to have been tidally disrupted by the Milky Way, probably from an earlier encounter between the parent galaxy and a dwarf galaxy. Digging a bit, I learned that we can carry the idea of stellar streams back to the work of Donald Lynden-Bell, who in 1995 proposed the stream concept to explain the long structure or filament of stars evidently tidally stripped from the Sagittarius Dwarf Spheroidal Galaxy, the latter being a satellite galaxy of the Milky Way. The Sgr dSph, as it is known, actually contains four globular clusters within it. It travels a...

In a tantalizing article in The Conversation, Robert Nichol (University of Surrey) offers a look at where new physics might just be emerging in conjunction with the study of dark energy. Nichol is an astronomer and cosmologist deeply experienced in the kind of huge astronomical surveys that help us study mind-boggling questions like how much of the universe is made up of matter, dark matter or dark energy. We’ve assumed we had a pretty good idea of their proportions but a few issues do arise. One of them seems particularly intriguing. Nichol’s article asks whether dark energy, regarded as a constant, may not actually vary over time. That’s quite a thought. The consensus over a universe made up of normal matter (5 percent), dark matter (25 percent) and dark energy (70 percent) came together early in our century, with dark energy taking the role of the cosmological constant Einstein once considered. Although he came to reject the idea, Einstein would doubtless take great interest in...

While I’m immersed in the mechanics of exoplanet detection and speculation about the worlds uncovered by Kepler, TESS and soon, the Roman Space Telescope (not to mention what’s coming with Extremely Large Telescopes), I’m daunted by a single fact. We keep producing great art showing what exoplanets in their multitudes look like, but we can’t actually see them. Or I should say that the few visual images we have captured thus far are less than satisfying blobs of light marking hot young worlds. Please don’t interpret this as in any way downplaying the heroic work of scientists like Anne-Marie Lagrange (LESIA, Observatoire de Paris) on Beta Pictoris b and all the effort that has gone into producing the 70 or so images of exoplanets thus far found. I’m actually just pointing out how difficult seeing an exoplanet close up would be, for the goal of interstellar flight that animates our discussions remains hugely elusive. The work continues, and who knows, maybe in a century we’ll get a...

I’ve reminisced before about crossing Lake George in the Adirondacks in a small boat late one night some years back, when I saw the Milky with the greatest clarity I had ever experienced. Talk about dark skies! That view was not only breathtaking on its own, but it also raised the point about what we can see where. Ponder the cosmic optical background (COB), which sums up everything that has produced light over the history of the universe. The sum of light can be observed with even a small telescope, but the problem is to screen out local sources. No telescope is better placed to do just this than the Long Range Reconnaissance Imager (LORRI) aboard the New Horizons spacecraft. Deep in the Kuiper Belt almost 60 AU from the Sun, the craft has a one-way light time of over eight hours (Voyager 1, by comparison, shows a one-way light time of almost 23 hours at 165 AU). It’s heartening that we’re continuing to keep the Voyagers alive even as the options slowly diminish, but New Horizons is...

Something interesting is going on in the galaxy NGC 6240, some 400 million light years from the Sun in Ophiuchus. Rather than sporting a single supermassive black hole at its center, this galaxy appears to have two, located about 3000 light years from each other. A merger seems likely, or is it? Centauri Dreams regular Don Wilkins returns to his astronomical passion with a look at why multiple supermassive black holes are puzzling scientists and raising questions that may even involve new physics. By Don Wilkins Super massive black holes (SMBH), black holes with a mass exceeding 100,000 solar masses, don’t behave as expected. When these galaxies collide, gas and dust smash into each other forming new stars. Existing stars are too far apart to collide. The two SMBH of the galaxies converge. Intuition foresees the two massive bodies coalescing into a single giant, Figure 1. The Universe, as frequently happens, ignores our intuition. The relevant force is dynamical friction. [1-4] As a...

I hardly need to run through the math to point out how utterly absurd it would be to have two civilizations develop within a few light years of each other at roughly the same time. The notion that we might pick up a SETI signal from a culture more or less like our own fails on almost every level, but especially on the idea of time. A glance at how briefly we have had a technological society makes the point eloquently. We can contrast it to how many aeons Earth has seen since its formation 4.6 billion years ago. Brian Lacki (UC-Berkeley) looked into the matter in detail at a Breakthrough Discuss meeting in 2021. Lacki points out that our use of radio takes up 100,000,000th of the lifespan of the Sun. We must think, he believes, in terms of temporal coincidence, as the graph he presented at the meeting shows. Note the arbitrary placement of a civilization at Centauri B, and others at Centauri A and C, along with our own timeline. The thin line representing our civilization actually...

Put two massive galaxy clusters into collision and you have an astronomical laboratory for the study of dark matter, that much discussed and controversial form of matter that does not interact with light or a magnetic field. We learn about it through its gravitational effects on normal matter. In new work out of Caltech, two such clusters, each of them containing thousands of galaxies, are analyzed as they move through each other. Using data from observations going back decades, the analysis reveals dark and normal matter velocities decoupling as a result of the collision. Collisions on galactic terms have profound effects on the vast stores of gas that lie between individual galaxies, causing the gas to become roiled by the ongoing passage. Counter-intuitively, though, the galaxies themselves are scarcely affected simply because of the distances between them, and for that matter between the individual stars that make up each. We need to keep an eye on work like this because...

Our recent discussions of X-ray beaming to propel interstellar lightsails seem a good segue into Don Wilkins’ thoughts on the Chandra mission. Chandra, of course, is not a deep space probe but an observatory, and a revolutionary one at that, with the capability of working at the X-ray wavelengths that allow us to explore supernovae remnants, pulsars and black holes, as well as making observations that advance our investigation of dark matter and dark energy. This great instrument swims into focus today because it faces a funding challenge that may result in its shutdown. It’s a good time, then, to take a look at what Chandra has given us since launch, and to consider its significance as efforts to save the mission continue. We should get behind this effort. Let's save Chandra. by Don Wilkins On July 23, 1999, the Chandra X-ray Observatory deployed from Space Shuttle Columbia. Chandra along with the Hubble Space Telescope, Spitzer Space Telescope (decommissioned when its liquid helium...

Don’t you love the way the cosmos keeps us from getting too comfortable with our ideas? The Hubble Constant (H0), which tells us about the rate of expansion of the universe, is still a hot issue because observations from both the Hubble Space Telescope and JWST don’t tally with what the European Space Agency’s Planck mission concluded from its data on the Cosmic Microwave Background (CMB). How exactly do we fine tune the standard model of cosmology to make sense of this? The so-called Hubble Tension is hardly the only issue raised by the acquisition of new and better data, although it may be the biggest. All kinds of questions linger about what dark energy is, not to mention dark matter. Of course, challenging observations are hardly limited to cosmology. Dialing down to the stellar level, new work has emerged challenging the way white dwarf stars evolve. Contrary to all expectation, some white dwarfs seem to stop cooling, and can indeed live to a satisfying old age. A white dwarf is...

If individual star systems show us a wide variety of formation scenarios – and we just examined recent ESO work on circumstellar disks in different star-forming regions – the variety in galaxy evolution is even more spectacular. I’m reminded here of an unusual find when my uncle Roland died unexpectedly and I became his executor. Clearing out his house preparatory to sale, I discovered a series of astronomy photographs that he had blown up to huge scale. An image of M31, the great spiral of Andromeda, was fully six feet long and gorgeously mounted. I remembered nights as a child when he would visit from Florida and point out celestial objects for me to observe with my 3-inch reflector. M31, he told me then, was considerably wider than the Moon in the sky. When I checked, I found that Andromeda had an angular size of 3 degrees, as opposed to about half a degree for the Moon. Even so, our spectacular sister galaxy is actually a difficult catch, with only its brighter central region...

A kilonova at the wrong place and time would spell trouble for any lifeforms emerging on a planetary surface. Just how we found out about kilonovae and the conditions that create them, not to mention their hypothesized effects, is the subject of Don Wilkins’ latest, a look at Cold War era surveillance that wound up pushing astronomy’s frontiers. That work now causes us to ponder the formation of an ‘island of stability’ in which exists a set of superheavy element isotopes with unique properties. It also raises interesting questions about our Solar System's history and possible exposure to a nearby event. Based at Washington University in St. Louis, Don’s interest in deep space exploration here probes the formation and structure of matter in processes we’re only beginning to unlock. by Don Wilkins Setting out to discover something on Earth can sometimes reveal an unexpected result from a far more interesting source. As a case in point, consider what happened in August of 1963, when...

I hadn’t intended to return so quickly to the issue of high-redshift galaxies, but SPT0418-47 jibes nicely with last week's piece on 13.5 billion year old galaxies as studied by Penn State’s Joel Leja and colleagues. In that case, the issue was the apparent maturity of these objects at such an early age in the universe. Today’s work, reported in a paper in The Astrophysical Journal Letters, comes from a team led by Bo Peng at Cornell University. It too uses JWST data, in this case targeting a previously unseen galaxy the instrument picked out of the foreground light of galaxy SPT0418-47. In both cases, we’re seeing data that challenge conventional understanding of conditions in this remote era. This is evidence, but of what? Are we wrong about the basics of galaxy formation? Do we need to recalibrate the models we use to understand astrophysics at high-redshift? SPT0418-47 is the galaxy JWST was being used to study, an intriguing subject in its own right. This is an infant galaxy...

When something turns up in astronomical data that contradicts long accepted theory, the way forward is to proceed with caution, keep taking data and try to resolve the tension with older models. That would of course include considering the possibilities of error somewhere in the observations. All that is obvious enough, but a new paper on JWST data on high-redshift galaxies is striking in its implications. Researchers examining this primordial era have found six galaxies, from no more than 500 to 700 million years after the Big Bang, that give the appearance of being massive. We’re looking at light from objects 13.5 billion years old that should be anything but mature, if compact, galaxies. That’s a surprise, and it’s fascinating to see the scrutiny to which these findings have been exposed. The editors of Nature have helpfully made available a peer review file containing back and forth comments between the authors and reviewers that give a jeweler’s eye look at how intricate the...

Some years back, I reminisced in these pages about reading Poul Anderson’s World Without Stars, an intriguing tale first published in 1966 about a starship in intergalactic space that was studying a civilization for whom the word ‘isolation’ must have taken on utterly new meaning. Imagine a star system tens of thousands of light years away from the Milky Way, a place where an entire galaxy is but a rather dim feature in the night sky. Poul Anderson discussed this with Analog editor John Campbell: One point came up which may interest you. Though the galaxy would be a huge object in the sky, covering some 20? of arc, it would not be bright. In fact, I make its luminosity, as far as this planet is concerned, somewhere between 1% and 0.1% of the total sky-glow (stars, zodiacal light, and permanent aurora) on a clear moonless Earth night. Sure, there are a lot of stars there — but they’re an awfully long ways off! For more on galactic brightness, see The Milky Way from a Distance. The...

Interesting things happen at the edge of the Solar System. Or perhaps I should say, at the boundary of the heliosphere, since the Solar System itself conceivably extends (in terms of possible planets) further out than the 100 or so AU that marks the heliosphere's boundary at its closest. The fact that the heliosphere is pliable and reacts among other things to the solar cycle in turns means that the boundary is a moving target. It would be useful if we could get something like JHU/APL's Interstellar Probe mission out well beyond the heliosphere to help us understand this morphology better. But let's think about the heliosphere's boundaries from the standpoint of incoming spacecraft. Because deceleration at the destination system is a huge problem for starship mission planning. A future crew, human or robotic, could deploy a solar sail to slow down, but a magsail seems better, as its effects kick in earlier on the approach. Looking at the image below, however, suggests another...

Interstellar objects are much in the news these days, as witness the flurry of research on ‘Oumuamua and 2I/Borisov. But we have to be cautious as we look at objects on hyperbolic orbits, avoiding the assumption that any of these are necessarily from another star. Spanish astronomers Carlos and Raúl de la Fuente Marcos dug several years ago into the question of objects on hyperbolic orbits, noting that some of these may well have origins much closer to home. Let me quote their 2018 paper on this: There are mechanisms capable of generating hyperbolic objects other than interstellar interlopers. They include close encounters with the known planets or the Sun, for objects already traversing the Solar system inside the trans-Neptunian belt; but also secular perturbations induced by the Galactic disc or impulsive interactions with passing stars, for more distant bodies (see e.g. Fouchard et al. 2011, 2017; Królikowska & Dybczy?ski 2017). These last two processes have their sources beyond...

Every time we look in a new place, which in astrophysics often means bringing new tools online, we find something unexpected. The news that an object has been detected that, for one minute in every 18, becomes one of the brightest radio sources in the sky, continues the series of surprises we've been racking up ever since first Galileo put eye to telescope. So what is this object, and why is it cause for such interest? Here's astronomer Natasha Hurley-Walker (Curtin University/International Centre for Radio Astronomy Research), who is lead author of the paper on the discovery: "This object was appearing and disappearing over a few hours during our observations. That was completely unexpected. It was kind of spooky for an astronomer because there's nothing known in the sky that does that. And it's really quite close to us—about 4000 lightyears away. It's in our galactic backyard." Image: A new view of the Milky Way from the Murchison Widefield Array in Western Australia, with...

Looking into Astro2020's recommendations for ground-based astronomy, I was heartened with the emphasis on ELTs (Extremely Large Telescopes), as found within the US-ELT project to develop the Thirty Meter Telescope and the Giant Magellan Telescope, both now under construction. Such instruments represent our best chance for studying exoplanets from the ground, even rocky worlds that could hold life. An Astro2020 with different priorities could have spelled the end of both these ELT efforts in the US even as the European Extremely Large Telescope, with its 40-meter mirror, moves ahead, with first light at Cerro Armazones (Chile) projected for 2027. So the ELTs persist in both US and European plans for the future, a context within which to consider how planet detection continues to evolve. So much of what we know about exoplanets has come from radial velocity methods. These in turn rely critically on spectrographs like HARPS (High Accuracy Radial Velocity Planet Searcher), which is...

Only time will tell whether humanity has a future beyond the Solar System, but if we do have prospects among the stars -- and I fervently hope that we do -- it's interesting to speculate on what future historians will consider the beginning of the interstellar era. Teasing out origins is tricky. You could label the first crossing of the heliopause by a functioning probe (Voyager 1) as a beginning, but neither the Voyagers nor the Pioneers (nor, for that matter, New Horizons) were built as interstellar missions. I'm going to play the 'future history' game by offering my own candidate. I think the image of the black hole in the galaxy M87 marks the beginning of an era, one in which our culture begins to look more and more at the universe beyond the Solar System. I say that not because of what we found at M87, remarkable as it was, but because of the instrument used. The creation of a telescope that, through interferometry, can create an aperture the size of our planet speaks volumes...

The 36 dish antennae at ASKAP -- the Australian Square Kilometre Array Pathfinder in outback Western Australia -- comprise an interferometer with a total collecting area of about 4,000 square meters. ASKAP has commanded attention as a technology demonstrator for the planned Square Kilometer Array, but today we're looking at the discovery of a highly polarized, highly variable radio source labeled ASKAP J173608.2?321635, about 4 degrees from galactic center in the galactic plane. According to Ziteng Wang, who is lead author of the study on this signal and a University of Sydney PhD student, the observations are strikingly different from other variable radio sources: "The strangest property of this new signal is that it has a very high polarisation. This means its light oscillates in only one direction, but that direction rotates with time. The brightness of the object also varies dramatically, by a factor of 100, and the signal switches on and off apparently at random. We've never...