The University of Texas at Arlington

The University of Texas at Arlington

UTA Planetarium

UTA Planetarium

Ask the Astronomer Q&A

Tag: "gravity"

  • Can we experience weightlessness near a black hole?
  • I don’t think that you would feel weightless around a black hole, because the difference in gravity between your head and feet is so great that you will be stretched into a long thin string. Even if you can get into a fast orbit around the black hole that would attempt to balance the gravitational forces, creating a free fall situation (exactly the same situation the astronauts have around Earth) you would still feel this stretching on your body. Some astronomers like to call this stretching effect, spaghettification, because it stretches you into a thin piece of spaghetti. You will eventually become stretched so much, that the bonds holding your atoms together will break, so it won’t be you that enters the black hole, but the protons, neutrons and electrons that make you up.

    (Tags:  black hole  gravity)
  • Can we make Jupiter, Saturn, Uranus or Neptune into mini-Suns using lasers to ignite the fusion process, like they do at the National Ignition Facility? If so, will their moons begin to rotate instead of being tidally locked?
  • Your question is an interesting one. However, I must say that there is no way to turn these planets into mini-Suns by sending laser beams. Here are several major problems with your suggestion. First, laser beams would be damped in atmospheres of these planets and the beams would never reach central parts of the planets. Second, lasers beams are highly collimated, which means that we could only trigger a fusion process very locally in the upper parts of the planetary atmospheres, where gas density and temperature are relatively low. Third, the fusion process (if ever triggered) would neither be sustained nor spread throughout the atmosphere. Fourth, neither of these planets would be able to sustain nuclear reactions in their interiors (even if we found a way to ignite them there) because the interior temperatures and densities in these planets are not high enough to initiate any fusion process. Finally, I do not think that our current lasers are strong enough to even trigger a very localized fusion process in the most upper parts of atmospheres of these planets. Now, the fact that the moons are tidally locked has to do with the planet’s gravity, which would not change whether nuclear reactions would occur or not.

    (Tags:  gravity  jupiter  laser  light  neptune  planet  saturn  sun  uranus)
  • Could phenomena occur where planets align in a way that impacts Earth?
  • There are no planetary alignments in the next few decades, Earth will not cross the galactic plane in 2012, and even if these alignments were to occur, their effects on the Earth would be negligible. Each December the Earth and Sun align with the approximate center of the Milky Way Galaxy but that is an annual event of no consequence.

    (Tags:  2012  apocalypse  black hole  earth  gravity  milkyway galaxy)
  • General relativity proved that light can bend based on a lot of gravity being forced onto the light. What are the two proofs of this?
  • The fact that light is bent by gravity was predicted by Einstein's General Theory of Relativity. The first observational evidence was given by Eddington and his team, who measured the effect during a solar eclipse in 1919. The measured value was very close to the one predicted by Einstein's theory.

    (Tags:  albert einstein  eclipse  gravity  light  theory of general relativity)
  • Gliese 710, an Orange dwarf star, will be headed our way in 1.5 million years. Gliese 710 is expected to come within 1.1 light years of our Solar System, perturbing the Oort Clouds and its comets. Can it grab some of the moons of the giant gas planets (Jupiter, etc) and make them satellites for itself, re-arranging our Solar System?
  • It is not likely that Gliese 710 will have any effect on the moons of the gas giant planets because 1.1 light years is still a very long way away. For reference, the Sun is about 8 light minutes away from Earth. Pluto is about 5.6 light hours away from the Sun. At 1.1 light years away, Gliese will be over 5 trillion miles from the solar system. This may be close enough for the star’s gravity to have an effect on the edge of the Oort Cloud, as you mentioned, but this will likely mean an increase in meteor showers and comets. It would not completely rearrange our solar system.

    (Tags:  brown dwarf  earth  gravity  jupiter  meteor  planet  pluto  saturn  solar system)
  • How can an astronomer test to see if gas and dust from a star are being drawn into a black hole?
  • There are two ways of knowing this: 1. When dust and gas approaches a black hole, strong gravitational forces spin the dust and gas nearly at the speed of light. Accelerating particles produce X-ray radiation which radiates perpendicular to the orbital plane of the gas and dust. We can detect this radiation if the radiation direction is just right (towards Earth). The first discovered black hole, Cygnus X-1, was detected this way. 2. Computer simulations suggest if a star is nearby a black hole, star atmosphere (dust and gas around stars) would be pulled by the gravity of the black hole.

    (Tags:  black hole  gravity  light  physics  star  universe)
  • How can black holes be so small but have such an incredibly large mass?
  • The mass of a black hole is determined by the mass of the star. When a star explodes as a supernova, much of the matter is sent into space, but the core of the star remains and collapses in on itself until a black hole is formed. The mass gets compressed so much that the electrons combine with protons in the nucleus of an atom - creating neutrons. If the compression stops the core becomes a neutron star instead of a black hole. Astrophysicists are not sure how matter can condense further than this to create a black hole because this process is unexplained by our current understanding of the universe. We can determine the radius of a black hole however, using an equation called the Schwarzschild radius. For a black hole of mass m, the Schwarzschild radius RS is given by RS = 2Gm/c2, where G is the universal gravitational constant and c is the speed of light. The Schwarzschild radius for a black hole of solar mass is about 3 km/1.9 mi.

    (Tags:  black hole  gravity  neutron star  physics  supernova)
  • How does the Sun's gravity keep Pluto in orbit, since it is so far away?
  • The Sun’s gravitational effect extends infinitely away from an object. The size and distance of the objects does not matter. In order to stay in orbit around the Sun, an object just has to follow Kepler’s Law of Motion: a^3=p^2. Where a is the semi-major axis and p is the period of rotation. In Pluto’s case a=39.264 AU (1 AU is equal to the distance between the Earth and the Sun – so Pluto is 39 times farther from the Sun than Earth is) and it’s period, p, is just over 246 years.

    (Tags:  gravity  pluto  sun)
  • How would gravity effect a planet if it was cube? How would it be like on the corners?
  • Gravity is inversely related to the square distance of an object from the center. That means the gravity force would decrease at the corners, and increase towards the center of each face of the cube.

    (Tags:  gravity)
  • I am led to understand that the Earth has an atmosphere due to the fact we have a magnetic field that deflects the solar wind. It is further my understanding that Mars lost most of its atmosphere (and subsequently most of its water) because the core cooled and Mars lost its magnetic field. Why then does Venus have such a thick atmosphere given that it has no magnetic field combined with the fact that it is much closer to the sun and therefore is exposed to solar wind to a much higher degree than the Earth is?
  • Actually, Earth has an atmosphere because of the size of the planet. Since gravity is directly related to the size (mass) of an object, the larger it is, the more gravity it possesses. Earth’s gravity is able to hold on to the molecules that make up our atmosphere. Mars, being about ½ the size of Earth, has much less gravity. This allowed the atmosphere on Mars to “evaporate” away into space. Venus still has an atmosphere because it is very close in size to Earth, about 90% Earth’s mass.

    (Tags:  earth  gravity  mars  venus)
  • If a black hole is a tunnel in a new universe, why haven't humans tried to explore it yet?
  • There are a few reasons why humans haven't yet explored a black hole to see if there is a tunnel to another universe. Right now, astronomers and mathematicians have proven that it is mathematically possible that a black hole is a gateway to another universe -- sometimes called a white hole, or worm hole. But, we have no way to test if this is actually true. The closest black hole is many thousands of light years away, which means that if we were traveling at the speed of light - which is currently impossible — it would take a few thousand years to reach the black hole. Then, if we can imagine that we have made it to the black hole, once a person or satellite enters the black hole, we will no longer receive any transmissions from them because they will be caught in the black hole’s extreme gravity. This means that even if we could safely send someone through the black hole, we would never know what happened to them, regardless if they made it through safely or not.

    (Tags:  astronaut  black hole  gravity  physics  theory of general relativity  universe)
  • If Astronauts are traveling in zero gravity, why can't we travel at the speed of light? Doesn't that mean they are weightless?
  • Actually, no. The effects of gravity are felt everywhere, even in space. When an astronaut experiences weightlessness or zero-G they aren’t actually feeling a lack of a gravitational field, but instead they are experiencing free-fall. The shuttle, in Earth’s orbit, is actually free-falling around Earth, so we don’t feel a pull directly towards the surface. That is why an astronaut - and all objects – float in space. Also, gravity is what causes us to have weight, not mass. Our weight can fluctuate depending on where we are in the universe. On Earth an adult may weigh 150 pounds, but on the moon, which has a smaller gravitational field, the same person would way only 50 pounds. But his mass, the amount of material that makes the adult, does not change wherever he is. You can determine your mass by dividing out 9.8m/s^2 (which is the pull we feel from the Earth – its gravitational effect) from your weight as measured on Earth.

    (Tags:  astronaut  earth  gravity  moon  physics)
  • If the Sun's gravity holds the planets in orbit, why is it that we can not duplicate this?
  • Gravity is something we can replicate. It is a force that is proportional to the mass of two (or more) objects and the distance between those objects. Gravity is what holds you to the Earth. It’s what keeps the Moon in orbit around us, and us in orbit around the Sun. If we had sufficient enough mass we would be able to create a “second sun” that would change the orbits of all the planets. The only problem is creating that much mass – the sun makes up over 90% of all the mass in our solar system. It weighs 1,980,000,000,000,000,000,000,000,000,000 kg! That’s 333,000 times more than the Earth weighs.

    (Tags:  gravity  planet  sun)
  • If we suppose Jupiter has moved from the place that it was originally created to the spot it is now in our solar system, what affect would it's gravitational movement have on Earth?
  • The current model that astronomers use to explain how the solar system formed, does not include any changes in orbit after the planet forms. There may be slight adjustments to a planet’s orbit due to mass changes over time, but nothing that will explain back and forth movements of the planets, because that would require an external force. For Jupiter to move closer or farther away from the Sun, there would have to be an external force tugging it - allowing the orbital period to change. But there is no known source in the solar system that would have the power to change the orbits by that much. Even though the planets have a small gravitational effect on each other, a change in planet configuration wouldn’t cause a dramatic change to the orbits of the other planets. Most of the gravity in the solar system comes from the Sun, so as long as the Sun remains in the system, the orbits won’t change much (if at all). That’s why Earth and Mars don’t orbit around Jupiter when they get in conjunction with Jupiter.

    (Tags:  earth  gravity  jupiter  mars  solar system  sun)
  • Is it possible for a satellite to maintain an orbit on the far side of the Moon (always hidden from Earth), given the complicating effects of Earth's gravity? If it is possible, what would the semi-major axis be?
  • No, it would not be possible for a satellite to orbit completely hidden from the Earth. To keep a satellite on the far side of the moon, it would have to orbit the Earth, not the Moon, every 28 days. Using Kepler’s law of planetary orbits, we can find the semi-major axis of the object with this equation (p^2/(a^3). Where a is the semi-major axis and p is the period. We know we want the period to be the same as the moon’s, 28 days. Which means that the semi-major axis would be equal to the moon’s semi-major axis. So the only way for the satellite to remain on the far side of the moon, is to place it on the moon itself. It is possible for the object to share the moon’s orbit (by trailing behind or ahead of the moon), but then we would be able to see it.

    (Tags:  earth  gravity  moon  satellite  telescope)
  • Is it possible that the gas giant planets are just planets that haven't fully formed yet? Like gravity hasn't fully brought them together and compressed them to a solid yet?
  • The gas giant planets are fully formed planets. They just don’t have solid ground. But they definitely have gravity. Everything that has mass, has gravity. The more massive an object, the more gravity it has. And the gas giants have A LOT of mass. Jupiter, for example, is 318 times more massive than Earth and has 2.5 times more gravity than Earth. This incredible amount of gravity holds the planet together and compresses the gas into a liquid closer to the center.

    (Tags:  gravity  jupiter  neptune  planet  saturn  uranus)
  • Isn't it possible that other planets rotate in and out of our solar system on longer cycles in elliptical orbits. E.G. Nibiru at 3,600 year cycles that our current civilization has not observed?
  • Although it is theoretically possible that more planets exist in our solar system that we have not detected, it is very unlikely. Everything in the universe has gravity, and this gravity effects everything in the universe. The closer 2 objects are to each other, the more their gravity effects surrounding objects. The same is true for large objects. The larger the object, the more gravity will effect surround objects. These effects are measurable by astronomers, even outside our own solar system. Astronomers have used this technique to find hundreds of planets in orbit around other stars. Since we have never seen any gravitational interaction on our Sun or the planets in our solar system, astronomers believe there are not any very large planets beyond the orbit of Pluto.

    (Tags:  2012  gravity  nibiru  physics  planet  pluto  solar system  sun)
  • Isn't there strong evidence to indicate that planets are formed as a collection of 'cold stars?' Planets are so variable in their composition, isn't it unlikely that they were formed by the 'snowball' effect that most scientists seem to prefer as an explanation? We see stars forming and cooling. We don't observe dust particles conglomerating in space into massive planets by cumulative means. Can scientists explain the diversity of composition of planets if they are indeed formed from rings of orbiting dust from a single central star?
  • Astronomers are still working on models that can accurately predict what type of planet will form around various stars. It is currently unknown why some planets can form with solid surfaces and some as pure gas (like Jupiter). Astronomers have created models that explain how outer planets (like those of our solar system) could form as gas giants. But the models don’t work if the gas giants are close to their host star. But, we have detected many examples of gas giant planets that orbit their host star in just a few days. We do know planets form from gas & dust (and NOT from cold stars), which accumulates - after some dynamic processes – in the form of a disc around a star (or stars in case of close binary systems). Additional processes occur within the disc such as gravitational bonding and cooling, which eventually results in the build-up of planets. The idea of "cool stars" is probably a misunderstanding in the sense that when stars age (and typically, attain a cooler surface temperature), they lose mass to free space, which is the decisive step for the existence of gas & dust necessary for planets to form. This gas and dust is thought to be “recycled” when it becomes a large enough cloud for gravity to collapse it again into a new star. This process takes many thousands (even millions) of years.

    (Tags:  exoplanet  gravity  planet  star)
  • What causes planets to rotate?
  • Planets are formed inside giant clouds of gas and dust that begin to rotate and collapse in on itself until it forms a star and planets. The rotation of the planets, is the left over rotation from the gas cloud that formed the planet.

    (Tags:  gravity  physics  planet  solar system  star  sun)
  • What happens if you get too close to a black hole?
  • Near the visible edge of the black hole (called the "event horizon") gravitational forces are so great that you would be stretched out like a long piece of spaghetti. Eventually your atoms and molecules would be ripped apart, and your mass would add to the mass of the black hole.

    (Tags:  black hole  gravity)
  • What holds the planets in place?
  • The planets are held in place by the Sun's gravity. Because the Sun is by far the largest object in the solar system, it extends its gravitational force far out into the solar system. Anything that comes inside our solar system will be sent into orbit around our Sun.

    (Tags:  gravity  physics  planet  solar system  sun)
  • What is a black hole?
  • A black hole is what happens to large stars when they run out of energy. During the main part of a its life, a star exists in a balance. Gravity tries to pull the star in closer to the middle and energy pressure pushes the star out from the core. These forces cancel out for most of the star's life. When the energy production quits in large stars, gravity pulls the star in rapidly and violently, creating a shockwave which blasts apart the outer part of the star. The inner part continues to collapse from the gravity, to the point where the gravity becomes "infinite." Nothing can get out of this corpse star, not even light, which is pulled down by the gravity, as well.

    (Tags:  black hole  gravity  light  sun)
  • When is the Sun predicted to become a black hole?
  • The Sun will never become a black hole. A black hole is what remains after a star, many times larger than the Sun, dies. The star will explode in a supernova, and then collapse in on itself, until it disappears completely, leaving only its gravity behind. The Sun, which is too small to become a black hole, will die. In about 5 billion years, the Sun will use up all its Hydrogen, and expand. Then it will shoot off the outer layers of the atmosphere, leaving just the core behind. We see many examples of stars that have died this way, just search the internet for pictures of Planetary Nebulae. One of the most famous of these planetary nebulae is the Ring Nebula, in the constellation of Lyra, the Harp.

    (Tags:  black hole  gravity  physics  star  sun)
  • When will we get actual pictures of the class m planets recently discovered by Kepler? Artist conceptions are cool and optimistic, however the truth is always more amazing.
  • Unfortunately, astronomers do not have the technology to actually see, or take pictures of, exoplanets. Most exoplanets are discovered by indirect methods, like looking for a dimming in a star’s light, or for the wobble in a star’s orbit – which is caused by a planet pulling on the star. In order to directly see the planet, astronomers would need much larger telescopes than have currently been built, in order to see the incredibly small bodies at great distances. One of the closest exoplanets discovered is about 22 light years away – that’s approximately 135 trillion miles away – and is only 4 times the size of Earth (at approximately 32,000 miles in diameter)! This is way too small to be seen by our telescopes. So for now, the best we can do is imagine what the planets look like, with artist conceptions.

    (Tags:  exoplanet  gravity  light  observatory  planet  star  telescope)
  • Why are planets round?
  • Simply put, planets are round because of gravity. Gravity pulls all the matter of a planet equally towards the center, creating a ball. Objects that are smaller, like asteroids, are not round, because there isn’t enough gravity to compress the object into a sphere. Instead they look like giant potatoes floating in space. In fact, planets themselves are not perfectly round either. We call them oblate spheres. Because they rotate on their axis as they go around the sun, the motion flattens the top and bottom of the planet a little bit, making the diameter at the equator greater than the diameter measured from the poles. This same effect is seen on the Sun and stars.

    (Tags:  asteroid  earth  gravity  planet  star  sun)
  • Why do planets rotate around the sun?
  • Planets and stars are formed in immense clouds of gas and dust. This gas and dust collapses in on itself and begins to spin as it collapses. The densest parts of the cloud become stars and planets. The planets get their motion around the Sun because of the rotation of the cloud that formed them. And Newton tells us that they will continue to orbit the Sun forever (an object in motion will stay in motion unless acted upon by another force).

    (Tags:  gravity  physics  planet  solar system  sun)
  • Why doesn't Mercury have moons?
  • Most moons are captured asteroids or rocks left over from the formation of the solar system. Since Mercury is so close to the Sun, asteroids and comets that come close by are captured by the Sun’s gravity. They are either pulled in to the Sun and burned up or they go around the Sun as comets. Mercury doesn’t have as strong a pull of gravity as the Sun, so it is unlikely that it would be able to redirect the path of any incoming objects to turn them into moons.

    (Tags:  asteroid  comet  gravity  mercury  moon  sun)