ASK ASTRO Astronomy’s experts from around the globe answer your cosmic questions. cosmologists call “dark energy,” burn not only hydrogen, but also helium. But after about produces repulsive gravitational 1 to 2 billion years, it will exhaust its supply of nuclear effects that cause the average fuel entirely and its core will contract into a white dwarf distance between galaxies to made of carbon and oxygen, while the outer layers of its increase faster and faster over atmosphere drift away as a planetary nebula. time. Determining dark energy’s White dwarfs are roughly the size of Earth, but the true nature remains one of the Sun as a white dwarf will be about 200,000 times denser greatest mysteries in theoretical than our planet. These objects no longer burn fuel to physics today. generate light or heat, but because they start out hot So, does the existence of dark — 10,000 kelvins or more — and have immensely high energy in our accelerating density, they continue shining with residual heat and universe mean that space is cool slowly. It takes a white dwarf roughly 10 trillion expanding everywhere, even on years (nearly 730 times the current age of the universe, small scales such as those inside which is 13.7 billion years) to cool off enough that it no of an atom, where most of the longer gives off visible light and becomes what astrono- volume is effectively “empty” mers term a black dwarf. On March 7, 2004, space? The short answer is no! So, the Sun won’t become a black dwarf for trillions We do know that meteorites exist on the surface of NASA’s Mars rover Spirit captured a And we should all count ourselves of years — and, in fact, no black dwarfs exist yet, simply Mars. As of July 2018, at least six meteorites had been streak in the martian lucky that we live in such a because the universe has not been around long enough confirmed and formally named, all found by robotic sky (enhanced in the inset at upper right). It universe. Thankfully, the local to allow even the earliest stars to reach this stage. rovers. But could they have produced light on the way is likely the first image Galaxy clusters like electromagnetic forces that hold Alison Klesman down? The answer is yes — despite the atmosphere’s of a meteor seen on MACS J1206.2-0847 the atom’s positively charged nucleus and negatively Associate Editor low density, meteors should be visible on Mars. another world. NASA/JPL/ stay together because CORNELL/TEXAS A&M the force of gravity charged electrons together, as well as the strong nuclear In 2008, researchers announced they’d seen, in data among neighboring Expanding forces confining the nucleus, are significantly stronger taken by the Mars Global Surveyor satellite, signatures galaxies overpowers the force of dark than the potentially disruptive forces of dark energy. of a meteor shower in the martian atmosphere. The energy — and the On larger scales, electromagnetic and gravitational shower had taken place in April 2003, as material from expansion of the space forces prevent planets and moons from expanding as Comet 79P/du Toit-Hartley streaked through the air, universe — on the scale of the cluster. the universe expands. On still larger scales, the force of leaving a fleeting layer of plasma in the atmosphere ESA/HUBBLE & NASA gravity that binds together systems like stars, solar about 50 to 59 miles (80 to 95 kilometers) above Mars’ IF THE UNIVERSE IS EXPANDING, systems, galaxies, and galaxy clusters is similarly stron- surface. The satellite hadn’t seen the meteors them- QI DOES THE SPACE INSIDE AN ATOM ger than the local effects of dark energy, which needs selves, but it did detect signs of the resulting plasma. EXPAND, TOO? SINCE THE SPACE INSIDE vast swaths of space to cause cosmic-scale accelerated From the ground, Mars rovers have also looked to AN ATOM IS MOST OF ITS VOLUME, expansion. Still, cosmologists can easily imagine “big the skies in search of meteors. A June 2, 2005, paper in THAT MEANS THAT MATTER WOULD BE rip” universes where dark energy was stronger than Nature reported that a streak in the martian sky, imaged EXPANDING AT THE SAME RATE. these local forces, either early on or at late times in March 7, 2004, by the Mars Exploration Rover Spirit, Hugh Cedric cosmic history. In such universes, dark energy could was likely a meteor associated with debris from Comet Beijing, China have prevented stars or galaxies from forming or even 114P/Wiseman-Skiff. If so, it is the first image of a A white dwarf such as IK Pegasi B (lower left) is much smaller eventually ripped apart atoms themselves as the than the Sun (also shown, for reference), but hundreds of meteor seen on another world. However, based on its We’ve known since the early 20th century that expanding universe accelerated itself into oblivion. thousands times denser than Earth. When a white dwarf cools location and motion, researchers could not rule out the AI the universe is expanding — after observations Needless to say, if this was such a universe, we wouldn’t enough that it no longer gives off visible light, it becomes a possibility that the streak may have been the defunct black dwarf. The Sun won’t become a black dwarf for trillions by Edwin Hubble and others showed that other galaxies be here to discuss it! of years, and even the oldest white dwarfs have not had time to Viking 2 orbiter, which still orbits the Red Planet. Andrew Friedman SEND US YOUR are almost all moving away from us — and the greater cool enough to become black dwarfs yet. RJHALL, CHRKL (WIKIMEDIA COMMONS) Additionally, the panoramic cameras on Spirit and QUESTIONS their distance, the faster they’re doing so. Astronomers Assistant Research Scientist, University of California, San Diego its sister Mars rover, Opportunity, were periodically Send your long expected that this cosmic expansion should be pointed upward to observe the martian sky at night, IS IT POSSIBLE TO SEE METEORS astronomy questions slowing down due to the combined gravitational pull WHEN WILL THE SUN BECOME A looking for telltale streaks left by meteors. In composite via email to askastro@ ON MARS, OR IS THE ATMOSPHERE of the universe’s seen and unseen matter. However, in BLACK DWARF? QI images taken by Spirit on November 18, 2005 — a time astronomy.com, or QI write to Ask Astro, the late 1990s, two teams of astronomers — one led by Isaiah Charnow TOO THIN? when Mars was passing through the debris trail left by Brian Schmidt and Adam Riess and the other by Saul Irvington, New York Allan Burger Comet 1P/Halley — three streaks that may be meteor P.O. Box 1612, Perlmutter — discovered evidence, using exploding Passaic, New Jersey trails through the martian atmosphere appear. Waukesha, WI 53187. stars in other galaxies, that cosmic expansion was not, in Black dwarfs are the very last stage of Sun-like We use the term “meteor” to refer to the light However, streaks can also be caused by cosmic rays Be sure to tell us your full name and fact, slowing down, but actually accelerating. The three AI stars. Currently in its main sequence hydrogen- AI produced when material from space enters a hitting the camera’s detector — so, again, it’s hard to where you live. team leaders later shared the 2011 Nobel Prize in Physics burning phase, our Sun has about 4.5 billion years left planet’s atmosphere. Friction between the air and the tell for sure whether the images captured meteors. Unfortunately, we for this accomplishment. The mysterious culprit, origi- before it enters the next stage of its life, puffing up to fast-moving object creates light. If any part of the object Alison Klesman cannot answer all nally conceived of by Albert Einstein and which modern become a red giant. As a red giant, the Sun will eventually survives to impact the ground, it is called a meteorite. Associate Editor questions submitted. 70 ASTRONOMY • APRIL 2020 WWW.ASTRONOMY.COM 71.