Ask Ethan: Could The Energy Loss From Radiating Stars Explain Dark Energy?
“What happens to the gravity produced by the mass that is lost, when it’s converted by nuclear reactions in stars and goes out as light and neutrinos, or when mass accretes into a black hole, or when it’s converted into gravitational waves? […] In other words, are the gravitational waves and EM waves and neutrinos now a source of gravitation that exactly matches the prior mass that was converted, or not?”
For the first time in the history of Ask Ethan, I have a question from a Nobel Prize-winning scientist! John Mather, whose work on the Cosmic Microwave Background co-won him a Nobel Prize with George Smoot, sent me a theory claiming that when matter gets converted into radiation, it can generate an anti-gravitational force that might be responsible for what we presently call dark energy. It’s an interesting idea, but there are some compelling reasons why this shouldn’t work. We know how matter and radiation and dark energy all behave in the Universe, and converting one into another should have very straightforward consequences. When we take a close look at what they did, we can even figure out how the theory’s proponents fooled themselves.
Radiating stars and merging black holes do change how the Universe evolves, but not in a way that can mimic dark energy! Come find out how on this week’s Ask Ethan.
Hubble Catches New Stars, Individually, Forming In Galaxies Beyond The Milky Way
“There are a massive variety of star-forming regions nearby, and Hubble’s new Legacy ExtraGalactic UV Survey (LEGUS) is now the sharpest, most comprehensive one ever.
By imaging 50 nearby, star-forming spiral and dwarf galaxies, astronomers can see how the galactic environment affects star-formation.”
Within galaxies, new stars are going to be formed from the existing population of gas. But how that gas collapses and forms stars, as well as the types, numbers, and locations of the stars that will arise, is highly dependent on the galactic environment into which they are born. Dwarf galaxies, for example, tend to form stars when a nearby gravitational interaction triggers them. These bursts occur periodically, leading to multiple populations of stars of different ages. Spirals, on the other hand, form their new stars mostly along the lines traced by their arms, where the dust and gas is densest. Thanks to the Hubble Space Telescope, we’re capable of finding these stars and resolving them individually, using a combination of optical and ultraviolet data.
The best part? These are individually resolved stars from well outside our own galaxy: in 50 independent ones. Here’s what Hubble’s new LEGUS survey is revealing.
Astronomers Confirm Second Most-Distant Galaxy Ever, And Its Stars Are Already Old
“Scientists have just confirmed the second most distant galaxy of all: MACS1149-JD1, whose light comes from when the Universe was 530 million years old: less than 4% of its present age. But what’s remarkable is that we’ve been able to detect oxygen in there, marking the first time we’ve seen this heavy element so far back. From the observations we’ve made, we can conclude this galaxy is at least 250 million years old, pushing the direct evidence for the first stars back further than ever.”
When it comes to the most distant galaxies of all, our current set of cutting-edge telescopes simply won’t get us there. The end of the cosmic dark ages and the dawn of the first cosmic starlight is a mystery that will remain until at least 2020: when the James Webb Space Telescope launches. Using the power of a multitude of observatories, we’ve managed to find a gravitationally lensed galaxy whose light comes to us from over 13 billion years ago. But unlike previous galaxies discovered near that distance, we’ve detected oxygen in this one, allowing us to get a precise measurement and to estimate its age.
For the first time, we have evidence from galaxies, directly, that the Universe’s first stars formed no later than 250 million years after the Big Bang. Here’s how we know.
How Do The Most Massive Stars Die: Supernova, Hypernova, Or Direct Collapse?
“When we see a very massive star, it’s tempting to assume it will go supernova, and a black hole or neutron star will remain. But in reality, there are two other possible outcomes that have been observed, and happen quite often on a cosmic scale. Scientists are still working to understand when each of these events occurs and under what conditions, but they all happen. The next time you look at a star that’s many times the size and mass of our Sun, don’t think “supernova” as a foregone conclusion. There’s a lot of life left in these objects, and a lot of possibilities for their demise, too. We know our observable Universe started with a bang. For the most massive stars, we still aren’t certain whether they end with the ultimate bang, destroying themselves entirely, or the ultimate whimper, collapsing entirely into a gravitational abyss of nothingness.”
How do stars die? If you’re low in mass, you’ll burn through all your fuel and just contract down. If you’re mid-ranged, like our Sun, you’ll become a giant, blow off your outer layers, and then the remaining core will contract to a white dwarf. And the high-mass stars can take an even more spectacular path: going supernova to produce either a neutron star or a black hole at their core. But that’s not all a high-mass star can do. We’ve seen supernova impostors, hypernovae that are even more luminous than the brightest supernova, and direct collapse black holes, where no explosion or even ejecta exists from a star that used to be present and massive. The science behind them in incredible, and while there are still uncertainties in predicting a star’s fate, we’re learning more all the time.
Come get the fascinating physics behind how the most massive stars die. You might think “supernova” every time, but the Universe is far more intricate and complex than that!
Hubble’s Greatest Discoveries Weren’t Planned; They Were Surprises
“And if we head out beyond our own galaxy, that’s where Hubble truly shines, having taught us more about the Universe than we ever imagined was out there. One of the greatest, most ambitious projects ever undertaken came in the mid-1990s, when astronomers in charge of Hubble redefined staring into the unknown. It was possibly the bravest thing ever done with the Hubble Space Telescope: to find a patch of sky with absolutely nothing in it — no bright stars, no nebulae, and no known galaxies — and observe it. Not just for a few minutes, or an hour, or even for a day. But orbit-after-orbit, for a huge amount of time, staring off into the nothingness of empty space, recording image after image of pure darkness.
What came back was amazing. Beyond what we could see, there were thousands upon thousand of galaxies out there in the abyss of space, in a tiny region of sky.”
28 years ago today, the Hubble Space Telescope was deployed. Since that time, it’s changed our view of the Solar System, the stars, nebulae, galaxies, and the entire Universe. But here’s the kicker: almost all of what it discovered wasn’t what it was designed to look for. We were able to learn so much from Hubble because it broke through the next frontier, looking at the Universe in a way we’ve never looked at it before. Astronomers and astrophysicists found clever ways to exploit its capabilities, and the observatory itself was overbuilt to the point where, 28 years later, it’s still one of the most sought-after telescopes as far as observing time goes.
Hubble’s greatest discoveries weren’t planned, but the planning we did enabled them to become real. Here are some great reasons to celebrate its anniversary.
6 Facts You Never Imagined About The Nearest Stars To Earth
“4.) There are no neutron stars or black holes within 10 parsecs. And, to be honest, you have to go out way further than 10 parsecs to find either of these! In 2007, scientists discovered the X-ray object 1RXS J141256.0+792204, nicknamed “Calvera,” and identified it as a neutron star. This object is a magnificent 617 light years away, making it the closest neutron star known. To arrive at the closest known black hole, you have to go all the way out to V616 Monocerotis, which is over 3,000 light years away. Of all the 316 star systems identified within 10 parsecs, we can definitively state that there are none of them with black hole or neutron star companions. At least where we are in the galaxy, these objects are rare.”
In the mid-1990s, astronomy was a very different place. We had not yet discovered brown dwarfs; exoplanet science was in its infancy; and we had discovered 191 star systems within 10 parsecs (32.6 light years) of Earth. Of course, low-mass stars have been discovered in great abundance now, exoplanet science has thousands of identified planets, and owing to projects like the RECONS collaboration, we’ve now discovered a total of 316 star systems within 10 parsecs of Earth. This has huge implications for what the Universe is actually made of, which we can learn just by looking in our own backyard. From how common faint stars are to planets, lifetimes, multi-star systems and more, there’s a huge amount of information to be gained, and the RECONS collaboration just put out their latest, most comprehensive results ever.
We’ve now confidently identified over 90% of the stars that are closest to us, and here’s what we’ve learned so far. Come get some incredible facts today!
The Milky Way Is Still Growing, Surprising Scientists
“It’s no big secret that galaxies grow over time. The force of gravity is powerful enough to pull smaller galaxies, gas clouds, and star clusters into larger ones, even over distances of millions of light years. Our own Milky Way has likely devoured hundreds of smaller galaxies over its lifetime, and continues to absorb the dwarf satellites which surround us. But there’s a steadier, more subtle way that galaxies grow: by continuing to form stars from the gas already inside. While most of the stars that form will do so in the plane or central bulge of a spiral galaxy like our own, a new study has shown that galaxies also grow outward over time, meaning that their physical extent increases in space. The implication is that our own galaxy is increasing in size by 500 meters per second: growing by a light year every 600,000 years.”
Imagine a galaxy all by its lonesome out there in the Universe. It’s full of stars, with gas, dust, plasma, and dark matter permeating all throughout it. What’s going to happen to the galaxy over time? You might think that it will continue to form new stars in its spiral arms, while older stars burn out and eventually die. All of that is true, but there’s a subtle but important effect that really adds up over cosmic time: the physical extent of where stars can be found grows as even isolated galaxies age. The Milky Way itself is growing at a rate of 500 m/s, typical of spiral galaxies around this size. It means that by time the Universe is three times as old as it presently is, Milky Way-like galaxies will have grown to be twice as large as they presently are.
While our galaxy itself won’t ever make it to that stage, due to our upcoming merger with Andromeda, many will. Come get the full story here.
Ask Ethan: What Happens When Stars Pass Through Our Solar System?
“How bad would it be if a star passed near the Sun? How close/large would it have to be to pose serious danger? How likely would such an event be?”
Space is a pretty empty place; it’s more than four light years to the nearest star. But despite this, we’re moving through the galaxy at around 220 km/s, passing and being passed by other stars at about 10% of that speed. Over long periods of time, stars occasionally make close passes by our own, meaning that they could pertub the Oort cloud, the Kuiper belt, or (if they got close enough) even the orbits of the planets themselves. Which of these is a realistic concern, and how often do these events actually occur? Moreover, when they do occur, what are the implications for what we’ll experience here on Earth? Will there be a pretty light show? A series of cometary impacts? Or a complete disruption of our orbit?
There’s only one way to find out, and that’s to calculate it so we know! Let’s walk you through exactly that, and what it means, for this week’s Ask Ethan!
Why Isn’t Our Universe Perfectly Smooth?
“This seems, at first glance, to pose a tremendous problem. If inflation stretches space to be flat, uniform, and smooth, indistinguishably so from perfection, then how did we arrive at a clumpy Universe today? Both Newton’s and Einstein’s theories of gravity are unstable against imperfections, meaning that if you start with an almost-but-not-quite perfectly smooth Universe, over time, the imperfections will grow and you’ll wind up with structure. But if you start with perfect smoothness, with literally no imperfections, you’re going to remain smooth forever. Yet this doesn’t jibe with the Universe we observe at all; it had to have been born with imperfections in its matter density.”
One of the great successes of cosmic inflation is to set up the initial conditions for the Big Bang that we knew we needed, including giving us a Universe that had the same temperature and density everywhere. But this couldn’t have been a perfect smoothness, otherwise we’d never have formed stars, galaxies, and the cosmic large-scale structure we observe today in the space we inhabit. So how did we come to be clumpy? The Universe must have been born with initial imperfections in them. If you treat inflation as a classical field, you’ll never get them that way, but if you recognize that it’s a quantum field, with the associated quantum fluctuations that we know must be there, the whole story changes. Not only does inflation give you these cosmic imperfections, but it gives you the full spectrum of them that you can then go check against observations.
These predictions were made in the early 1980s, and were verified decades later by COBE, WMAP, and Planck. It’s a huge victory for a great scientific theory!
Space Is Full Of Planets, And Most Of Them Don’t Even Have Stars
“When we look at our Universe, where our own galaxy contains some 400 billion stars and there are some two trillion galaxies in the Universe, the realization that there are around ten planets for every star is mind-boggling. But if we look outside of solar systems, there are between 100 and 100,000 planets wandering through space for every single star that we can see. While a small percentage of them were ejected from solar systems of their own, the overwhelming majority have never known the warmth of a star at all. Many are gas giants, but still more are likely to be rocky and icy, with many of them containing all the ingredients needed for life. Perhaps, someday, they’ll get their chance. Until then, they’ll continue to travel, throughout the galaxy and throughout the Universe, vastly outnumbering the dizzying array of lights illuminating the cosmos.”
According to the International Astronomical Union, planets need to have enough mass to pull themselves into hydrostatic equilibrium, they need to orbit a star and not any other object, and they need to clear their orbits in a certain amount of cosmic time. But what do you call an object that would have been a planet, if only it were in orbit around a star, but instead wanders through the heavens alone, unbound to any larger masses? These rogue planets are surprisingly ubiquitous in our galaxy and beyond, and we expect that they’ll far outnumber not only the stars, but even the planets that are found orbiting stars. Where do these rogue worlds come from? A percentage of them are orphans, having been ejected from the solar system that they formed in, but the overwhelming majority ought to have never been part of a star system at all.
Come learn how even though space is full of planets, many containing the ingredients for life, most of them don’t even have stars to orbit to give them a chance.