Astrostem

What is the temperature in space?

Temperatures in space depend on whether the thermometer is in sunlight or darkness. Near the Earth and the Moon, objects in direct sunlight can heat up to temperatures of about 250 degrees F (121 degrees C). In the shade, objects can cool down to around -250 degrees F (-156 degrees C). This extreme range is the reason why the thermal designs of spacecraft and spacesuits are so important.

If space ends, what's on the other side?

If space is infinite, there is nothing on the other side. If space is finite because it has been bent around upon itself because of gravity, then again there is nothing on the other side of it because there is no seam. It looks like the surface of a smooth ball which represents a piece of flat paper bent upon itself.

Exactly what is gravity?

We do not know exactly. We can define what it is as a field of influence, and with general relativity we can define a language which states that it is a property of our real world that is mathematically equivalent to not just the geometry of space-time, but equivalent to space-time itself. Some think that it is made up of particles called gravitons, which flit about at the speed of light just as photons do. In any true fundamental sense, we do not know what gravity is, we only know how it operates in various corners of our universe. Without gravity, there would be no space and time.

How do we know that the Milky Way is a spiral galaxy and not some other kind of flat system of stars?

We know it is a kind of flat system of stars because when we look at the night sky, the Milky Way`s stars are arrayed along a rather thin band across the sky to form the familiar Milky Way. Telescopically, we can study other galaxies and we see that they come in three basic types: Spirals, Ellipticals and Irregulars. The Milky Way cannot be of the last two categories because no amount of juggling of position of the Sun in these types of galaxies would give us the kind of night sky that we see. So by the process of elimination, the Milky Way must be some kind of spiral galaxy.

If the Milky Way was the size of a coffee cup, how big would the rest of the universe be?

The Milky Way has a radius of about 50,000 light years. The visible universe has a radius of about 15 billion light years or 300,000 times the size of the Milky Way. So for an 8-centimeter-wide coffee cup as the Milky Way, the VISIBLE universe would be a sphere about 48 kilometers (almost 30 miles) in radius...give or take.

Is it some kind of conspiracy that astronomers only talk about other galaxies and not the Milky Way?

No. Galactic research is one of the most active areas in astronomy. It is just that out of the hundreds of research papers written about this subject every year, few attract much media attention. So the average person on the street doesn`t hear much about it. In large terms, astronomers study the interstellar medium and stellar evolution in our galaxy, but discussions of the large-scale shape of the Milky Way are hampered by the fact that we can never get outside of the Milky Way to really see what it looks like. As for conspiracies, you will not find them in science, except that lack of interest and resources can sometimes be interpreted as a conspiracy when in fact there simply are not enough astronomers and resources to advance a subject very fast at a given time. But over a few decades, most subjects in astronomy get their proper due and make progress. You just have to be patient; after all, we have to leave something for our children to do!

Why can the Hubble Space Telescope see distant galaxies, but not objects beyond Pluto?

The Hubble Space Telescope cannot see objects beyond Pluto because the distant things are so luminous that even at their great distances they outshine the feeble sunlight that would be reflected from small bodies beyond the orbit of Pluto.

Can the oldest galaxies in the universe be found by looking in any direction?

Actually, the oldest galaxies we can see are right next door to us in the present universe. When we look out into space using the Hubble Space Telescope for example, we see images of galaxies that become younger and younger the further out we look. We actually see images of the youngest galaxies in the universe the farther out we look. Because there are probably over 100 billion galaxies in our visible universe, and because there are only about 42,000 square degrees in the surface of the sky, that means that in every square degree we will see nearly 3 million galaxies. Most of these will be the most distant and hence `youngest` ones. So it doesn`t seem to matter what direction you look.

What happens when two galaxies collide?

On the largest scales, the changing gravitational fields cause the galaxies to distort their shapes tremendously to produce great streams of stars and gas that are often ripped from each of the galaxies and hurled into intergalactic space. Most of the matter, however, settles back into a new system which often looks nothing like either of the galaxies before the event. When the interstellar clouds in each of the galaxies collide, they can trigger bursts of star formation resulting in very massive, short lived, stars being formed. These stars form in large numbers and over small enough regions their light can combine to turn the galactic collision into a so-called star-burst system. If the cores have massive black holes, the systems can flare-up into quasar, or near-quasar brilliance for millions of years. For individual stars, they are so small compared to their average distances that you might get only a handful of close encounters or impacts out of literally hundreds of billions of stars. Planetary orbits could be upset, however, either by the changing galactic gravitational fields or by close encounters with passing stars.

Why send people into space when robotic spacecraft usually cost less?

Humans and robots each have their own special roles in space. Robots are best qualified for missions that require precise, repetitive measurements or maneuvers, and for missions that last for a long time, such as trips to the outer planets. Humans remain better equipped than robots for tasks that involve analytical decision-making or constant adjustments. By launching humans into space, we also gain unique insights into the workings of the human body, many of which are masked or changed by gravity when a person is on the Earth.

When was the last solar eclipse seen from North America, and when will the next one happen?

The last solar eclipse was on May 10, 1994. It was annular and could be seen in the United States. The next U.S. eclipse will be on May 20, 2012, and will also be annular. Similar eclipses occur 18 years apart in the Saros Cycle.

Why don`t solar eclipses occur exactly at noon?

Because the geometry required for a total solar eclipse has nothing to do with local noon. It has to do with when the lunar shadow sweeps across your location during the time when the Sun is above the horizon. Even so, it is possible for the Sun to be in full eclipse before it rises at your particular location!

Exactly how tall is Olympus Mons on Mars?

Olympus Mons is a shield volcano 374 miles (624 km) in diameter (approximately the same size as the state of Arizona), 16 miles (25 km) high. To compare, the largest volcano on Earth is Mauna Lo Mauna Loa is a shield volcano 6.3 miles (10 km) high and 75 miles (120 km) across. The volume of Olympus Mons is about 100 times larger than that of Mauna Lo In fact, the entire chain of Hawaiian Islands (from Kauai to Hawaii) would fit inside Olympus Mons!

How can I name or buy a star?

We do not provide a star-naming service.

How are stars born?

We haven`t been able to observe in detail all of the stages in this process, but from many specific young objects we have studied, it all seems to start with a dense, interstellar cloud many light years across. For reasons that we do not fully understand, small regions within such a cloud perhaps a fraction of a light year across begin to collapse under their own gravity. As the collapse continues, the center of this core region becomes denser and denser, climbing from only 100 atoms per cubic centimeter to millions of atoms per cubic centimeter and higher. As it collapses, whatever very slight rotation it originally had gets amplified so that, like a figure skater on ice, it spins faster and faster. Although the gas falling along the axis of the collapsing cloud feels nothing more than the gravitational force of the central core, along the equator of the object, centrifugal forces due to its spinning become so strong that they impede the collapse along this direction. The cloud collapses into a flattened disk with a central bulge containing most of the mass, and it is in this central object that the star will be born. We see many rotating cloud cores like this, and many with a disk-like shape, so we are pretty certain we understand this phase of the evolution of stars, but what we don`t fully understand is why the core collapses in the first place. There seem to be many things that can cause this physically; perhaps all of them occur in one cloud or another.

What is on the other side of a black hole?

We don`t know, other than theoretically, what might be inside of a black hole (if that is what you mean by "the other side"). Inside, we encounter the surface of the object that collapsed to make the black hole in the first place, and because we cannot tunnel inside this body, all we see is a solid surface. Mathematicians and physicists have theorized a hollow black hole that doesn`t require the collapse of a star to form it. This type of black hole is imagined to have an exotic geometry and be a place where time travel is possible.

Can a star clog a black hole that is swallowing it?

No. A black hole is not really like a drain pipe! In fact, if it consumes matter at too ferocious a rate, the radiation pressure generated by the in-falling matter provides tremendous resistance to the flow of matter. The rate at which matter can fall into a black hole is regulated to what is called the Eddington Accretion Rate, which depends on the mass of the black hole. Also, it all depends on whether the star has been captured into orbit or is just passing by. Orbital capture means that the black hole, over the course of millions of years, can leisurely nibble away at the star "without choking."

What happens when black holes touch?

Their event horizons distort into a dumbbell shape so that the midpoint between them initially lies on the surface of both event horizons. Then in the next instant, from the standpoint of a local observer who is falling onto them, it is inside the merged horizon.

For a black hole the size of our Solar System, could you avoid its singularity?

Not for a non-rotating black hole. The internal geometry prevents avoiding the central singularity for a time much longer than the light travel time from the event horizon to its center...a few hours. For rotating black holes, in principle you could avoid the singularity by simply choosing not to enter the equatorial plane where the ring singularity lives.

What is the importance of black holes to cosmology?

Indirectly, they tell us that our relativistic theory of gravity and space-time provided by Einstein`s general relativity is fundamentally correct, so that when we use these same equations to study cosmology we have some confidence that they may be correct. Directly, black holes tell us that the universe can hide much of its matter in a way that still contributes to the total mass of the universe, but may not contribute to the abundances of certain primordial elements such as hydrogen and helium. If enough black holes were produced soon after the Big Bang but before the first few minutes, this could have an impact on the relationship between how rapidly the universe is expanding and the origin of the primordial element abundances. But it is expected that most black holes formed long after the Big Bang by stellar evolution, and these black holes may contribute to the missing mass in the universe up to the maximum limit set by the primordial element abundances themselves. It is a bit complex, but black holes are cosmologically important from many different standpoints.

Do black holes die if they are not fed?

No, they just stay at the same mass until they slowly evaporate over trillions of years.

How much younger are astronauts after they return from orbit?

When an astronaut returns home they will be 0.000023 seconds younger for each day they spend in orbit. For a one-year stay on the International Space Station, the returning astronauts will be about 0.0085 seconds younger. Not much to get excited about, but it is measurable on a good stopwatch if the experiment is set up correctly.

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