THE SOUTHWORTH PLANETARIUM 70 Falmouth Street Portland, Maine 04103 (207) 780-4249 usm.maine.edu/planet 43.6667° N 70.2667° W Founded January 1970 2022-2023: LXXI Sunrise: 6:57 a.m. Sunset: 6:45 p.m. Civil twilight begins: 6:29 a.m. Civil twilight ends: 7:13 p.m. Sun's host constellation: Pisces Moon phase: Waning gibbous moon (66% illuminated) Moonrise: 1:46 a.m. (03/14/2023) Moonset:10:23 a.m. (03/14/23) Julian date: 2460017.29 'To the mind that is still, the whole Universe surrenders." -Lao Tzu THE DAILY ASTRONOMER Monday, March 13, 2023 If a Black Hole Entered the Solar System and it didn't make a sound, would we know it was here? Or, more sensibly, what would happen to the bodies within the solar system? Most importantly, of course, is the matter of what would happen to Earth, named in a recent poll as Sol's most significant planet. Though the eventuality is as remote as a dust mote in a fern patch on an alien world in Andromeda, discussing it softens our neurons with some rough kneading. We'll pretend that a stray black hole ventures into the solar system. According to stats issued by the cosmic cogniscenti, our galaxy alone might harbor 100 million of these space ripping monsters. So, having one of this stampede wander onto our pasture doesn't seem too far fetched. It is far-fetched, of course, because our galaxy is so unfathomably huge that you can comfortably store 100 million around without tripping. However, like every other single piece of paper ever scribbled on, this is a contained Universe and we can do what we wish within it. So, if we wish to imagine that a black hole happens by, well, a black hole will happen by..and there it is: roaring Viking-like onto our defenseless shores. Since this is our own little cosmos, we can even pretend that black holes behave like frothing Visigoths that wander where their moods take them, as opposed to moving in response to the galaxy's matter distribution: the actual governing force. First issue is the shore itself. What does it mean to "enter" the solar system? Well, fortunately for those who prefer aggravation to simplicity, the solar system's boundary is not a sharp demarcation line, such as the surreal border separating July flowers and the Magna Carta. We'll state that the Oort Cloud defines our outer perimeter: this "cloud," for lack of a sensible term, is the spherical distribution of cometary nuclei enclosing the solar system. Even though we picture it as a gossamer shell of negligible thickness, it is a rarefied region between 2,000 to possibly 100,000 AU* from the Sun. This range encompasses billions of miles of space! In fact, the Oort Cloud's outer edge might extend more than a light year from the Sun, or, more than a quarter of the distance to the nearest star system, Alpha Centauri. Within this vast volume one could encounter billions of comet nuclei spread far apart. A black hole could pass through the Oort cloud easily. This is a lovely juncture to dispel a popular misconception pertaining to black holes. They are not wandering storm drains. A black hole would not "swallow" everything in the solar system as though it were drawn down a tube. Instead, the black hole gravitational effects would disrupt bodies within its vicinity. Remember that a stellar sized black hole contains a star's mass within a comparatively minuscule volume. Consequently, the more powerful effects would be tidal.** Any objects close to it would be ripped apart. Also, the black hole could induce such perturbations into the Oort Cloud comets that it would likely send a swarm of them toward the inner solar system. Now, after raging through the Oort Cloud, our menacing interloper would pass through another region called the heliosphere, the boundary where the interstellar gases repel the solar wind. The heliosphere "edge," called the heliopause, is believed to be approximately 100 AU* from the Sun, or thereabouts. Some might consider this region as the solar system's circumference. Once sliding through the heliosphere, the visitor would encounter the comparatively more dense Kuiper Belt: a disc of nuclei from which short period comets (those with periods of less than 125 Earth years) originate. The resultant disruption will send another comet armada toward the inner solar system. Some close bodies would likely be captured, but these would be few and far between. At this point, one wonders if we'd know about the black hole. The answer is that we could, but not by direct observation. Instead, astronomers could observe the motions of planets the black hole would perturb. We recall that every massive object attracts every other massive object. And, the outer planets and other bodies would react to the black hole's proximity, by either being pushed or pulled depending on their positions relative to the black hole. The outer planets would act like sentries, alerting us to the black hole's presence. Let's now have some real fun and assume the black hole voyages toward the inner solar system en route to Earth. What would we see? Well,if the black hole is not surrounded by an accretion disk,*** we'd have a difficult time observing it. A black hole is a region where gravity is so strong not even light can escape from it. However, if it were close enough, it would literally distort the starlight around it. We might not see the object, but we'd see peculiarities in the night sky. Of course, we might not pay much attention to the sky, because a black hole close enough to cause such visual distortions would soon make short work of our planet. Earth would NOT be sucked into it, but would literally be ripped apart by it. The black hole's tidal forces would be so strong that not only would it draw away all the gases enveloping the planet, thereby snuffing out all life within a few minutes, but it would also tear Earth's solid structure into pieces. These pieces would become pieces until all the material was reduced to its component subatomic particles. Our home world would become so much flotsam enclosing the voracious black hole. In other words, we'd die. This is a basic idea of what would transpire if a wayward black hole frolicked through our neighborhood. We would know about it before it wandered too close, but, of course, if it did, we couldn't do much to stop it. On that ticklish note, enjoy your Monday! *AU: astronomical unit. Defined as the Sun and Earth's mean separation distance (approximately 149,597,871 kilometers, or 92,955,807 miles.) The innermost bodies comprising the Oort Cloud would be 2000 times farther from the Sun than Earth. Pluto, for comparison, is about 40 times more distant from the Sun than Earth. (quite a bit of variation in this distance, of course.) **A teeny tidal trouble tutorial: the physical term for tidal effects are "differential gravitational effects." If our aim was to elucidate the concept in the first sentence, we failed. In a physically neat world, a body's gravitational influence on another object would be constant. For instance, a massive object would pull on your head with the same force it exerts on your feet. For Earth, these forces are essentially equal. It's true that your head's distance from the planet is greater than your foot's distance, but this difference is so slight as to make the gravity differential negligible. However, if Earth's volume were crushed down into a little ball, the difference would be significant: enough to rip you asunder. ***an accertion disk consists of matter around a black hole. This matter spirals in toward the black hole. Since material close to the black hole moves faster than the stuff farther away, it develops a differential rotation. The closer particles move faster than the more distant one. They rub against each other so energetically, they generate high energy x-rays. To subscribe or unsubscribe from the "Daily Astronomer" http://lists.maine.edu/cgi/wa?A0=DAILY-ASTRONOMER