The University of Texas at Arlington

The University of Texas at Arlington

UTA Planetarium

UTA Planetarium

Ask the Astronomer Q&A

Tag: "physics"

  • Are planetary distances measured to the Sun’s surface or its center?
  • The answer is actually neither. Distances in the solar system are measured from an object's center of which can be deduced from Kepler’s laws. If an object is perfectly spherical, the center of mass would be the center of the object. However, the Sun and the planets are not perfectly spherical, so the center of mass is somewhere off center. For planets in the Solar System, either the semi-major axis is given as “distance”, or the average distance is given. Average distance is simply half of (perihelion + aphelion), with other words, half of (largest distance plus smallest distance).

    (Tags:  physics  planet  solar system  sun)
  • 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)
  • I know that the Earth goes around the Sun, the Sun goes around the Milky Way, and the Milky Way is moving somewhere too. I was wondering, if there is a totally static point in space, how fast is the Earth moving from that static point of view?
  • Earth's average distance to the Sun is 150,000,000 km (93 million miles), therefore the distance it travels as it circles the Sun in one year is that radius x 2 x pi, or 942,000,000 million kilometers in a year of 24 hours/day x 365 1/4 = 8,766 hours so you divide to get 107,000 km/h or about 67,000 mph. You could also say the Earth moves around the Sun at 30 km/s. The Sun circles the center of our Galaxy at about 250 km/s. Our Galaxy is moving relative to the 'average velocity of the Universe' at 600 km/second (

    (Tags:  earth  physics  solar system  sun  universe)
  • 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 I were to fast-forward into the future, and I was setting up a base on Callisto, what might be the best place to do that? Would somewhere like in the Lofn crater be a good place to put it? If so, what would the area look like? Would I be able to see Jupiter from there? Would the ground feel and look like snow? Would the thin atmosphere carry any sort of sound?
  • Callisto is a tidally locked moon which means that the same side is always facing Jupiter (like our Moon). Unfortunately, the Lofn crater is located on the far side the moon (56˚ S and 23˚W) so your base would never see Jupiter. The surface of Callisto is covered by ice (water ice), carbon-dioxide and silicates; so it wouldn’t feel like snow, it would feel like hard ice. The thin atmosphere is so tiny; you can almost consider there is no atmosphere. It is surprising that this tiny moon has an atmosphere; however, it is not stable. The atmosphere is actually running away and is probably being replenished by the surface carbon-dioxide. Such an atmosphere wouldn’t carry any sound.

    (Tags:  jupiter  moon  physics  solar system)
  • If the weight of our planet does not fluctuate... Does this include matter and mass ON the planet as well?
  • It depends on how the weight is calculated. If we estimate the density as a function of the radius of Earth, we can calculate the total mass up to its surface, not including the matter on the planet (air, trees, animals, etc.). If we calculate the mass from Newton’s 3rd law - using the Sun’s or the Moon’s mass (assuming that their mass are well known) – then we will figure the mass of Earth with everything on the planet. However, there will not be a significant difference as the air mass and the matter on the surface is negligible. Moreover, Earth’s weight is not constant. Every year, 100,000 pounds of dust and debris falls on Earth from the sky. Of course, this increase is negligible and will not affect the planet.

    (Tags:  earth  moon  physics  sun)
  • 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)
  • 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 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 light year?
  • A light year is a unit of distance. The distances to the stars are so great, it is inconvenient to use terms like miles or kilometers. So we use "light year." A light year is the distance a beam of light travels in a year. Light goes around 186,000 miles per second (in a vacuum) and in a year travels almost six trillion miles. Now, instead of saying Alpha Centauri is 25,000,000,000,000 miles away, it is much easier to say "around 4.3 light years" away. As a consequence, things we see in space are also removed from our own time. The light reaching Earth now from Alpha Centauri left 4.3 years ago. So we are actually seeing what the star looked like 4.3 years ago, even if we use a telescope. Other visible stars are dozens to hundreds of light years away, and the galaxies are millions and billions of light years away.

    (Tags:  light  physics)
  • What is the SOFIA?
  • SOFIA is an acronym for the Stratospheric Observatory For Infrared Astronomy. It is a telescope mounted to a Boeing 747 airplane. The airplane flies high above Earth's atmosphere to study objects in the infrared part of the electro-magnetic spectrum.

    (Tags:  astronomy  electromagnetic spectrum  light  observatory  physics)
  • What's the difference between 3rd dimensional and 4th dimensional?
  • The main physical difference between 3-dimensions (3D) and 4-dimensions (4D) is that 3D refers to space and 4D refers to space-time, which means that the extra fourth dimension describes time. It is important to point out that time and space must be combined together and considered on the same footing. We do live in 4D space-time, however, in every day life we experience 3D space and 1D time as two separate entities.

    (Tags:  albert einstein  physics)
  • 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 the Milky Way collide with Andromeda? Are those estimates accurate?
  • The Andromeda Galaxy will collide with the Milkyway in 3 billion years as they are separated by 2.2 million light years and approaching to each other at approximately 500,000 km/hour speed. Are those numbers accurate? Yes and No. We measure distances and velocities accurately. That would allow us to give a specific date and time for the collision, if the galaxies were like solid objects. However, galaxy collisions are not similar to collision of 2 vehicles. Galaxies don’t have sharp boundaries that you can tell when the collision starts. Moreover, the collision will last approximately 1 billion years. During the collision, it is unlikely that any star will run into another! If there is life on a planet at that time, probably the life will not be affected. After the collision, galaxies change their shape, they either merge or form 2 “new” galaxies orbiting each other. Based on scientific data, numerous simulations have been computed and galaxy collision animations created. More specific information and animations can be found at the following link:

    (Tags:  andromeda galaxy  milkyway galaxy  physics  star  universe)
  • 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 is the earth not at 0° solar longitude on March 21 (the vernal equinox, 1st day of spring), but at 180°? Confusingly, the vernal equinox supposedly marks the 0°
  • In the diagram (, Earth is about at 90 degrees, if you consider the vernal equinox at +X-axis. On March 21, Earth would be at 180 degrees, just like you said. Now, why Earth is not at zero degrees since it is vernal equinox? Because we determine equinoxes according to the Sun’s position, not Earth’s. Put Earth on the Vernal Equinox point (180 degrees). Which direction you see the Sun?

    (Tags:  earth  physics  sun)
  • Why is the speed of light referred to as the letter "c"?
  • The letter 'c' used for speed of light stands for 'constant', which means that the speed is the same in all inertial frames of reference. It has become common to use 'c' after the formulation of the Special Theory of Relativity by Einstein in 1905.

    (Tags:  albert einstein  light  physics)
  • Will we ever be able to travel at the speed of light or faster?
  • That’s a great question, but it is difficult to answer. Theory tells us that as we increase our speed to the speed of light, we will get more massive, and if we get more massive, we will need more fuel to propel us at that speed. So as we continue to get closer to the speed of light, we will get infinitely massive. The only thing that can actually reach the speed of light today, is light, which has no mass. So in order to go the speed of light, we will either need to learn how to propel an infinite mass, or discover how to make ourselves mass-less. As far as going faster than the speed of light, that may never be possible, since light itself cannot go faster than the speed of light. But who really knows, our technology gets better every day, and perhaps sometime in the future we will discover a way to travel fast enough to leave our solar system and travel among the stars.

    (Tags:  light  physics  speed of light  theory of general relativity)
  • Will we ever have the ability to teleport ourselves from one place to another?
  • No. Teleportation involves moving an object at the speed of light (or faster). But this is not possible since General Relativity tells us our mass increases indefinitely as we approach the speed of light. The speed of light is also much faster than any speed we have ever achieved on Earth. The speed of light is 186,000 miles per second!

    (Tags:  albert einstein  light  physics  speed of light  theory of general relativity)