Questions | Universe Today

Question: How Does a Star Die?

Answer: So a star has reached middle age by fusing hydrogen into helium. Then what happens? Once a star has run out of usable hydrogen that it can convert into helium, a star then takes one of several paths.

If the star is 0.5 solar masses (half the mass of our sun), electron degeneracy pressure will prevent the star from collapsing in upon itself. Due to the age of the universe, scientists can only use computer modeling to predict what will happen to such a star. Once it has finished its active phase (hydrogen to helium), it becomes a white dwarf.

A white dwarf can come about in one of two ways; first, if the star is very small, electron degeneracy pressure simply stops the collapse of the star, it is out of hydrogen, and it becomes a white dwarf. Secondly, and more commonly, the core of the star can still be surrounded by some layers of hydrogen, which continue to fuse and cause the star to expand, becoming a red giant.

A red giant is a star in the process of fusing helium to form carbon and oxygen. If there is insufficient energy to make this happen, the outer shell of the star will shed leaving behind an inert core or oxygen and carbon - a remnant white dwarf. If enough energy is involved in the casting off of stellar casings, a nebula can form. If said white dwarf is in a binary system, it could become a type 1A supernova, but this is very rare. Instead, it is thought that a white dwarf will eventually cool to become a black dwarf - in theory because there are no white dwarfs older than the universe, black dwarfs are theoretical only because there hasn't been enough time for one to form.

If a star that has reached the end of its productive phase is below the Chandrasekhar Limit - 1.4 times the mass of our Sun - it will become a white dwarf; over this limit, it will become a neutron star. If a star is larger than about 5 times the mass of the sun, when the hydrogen fusing stops, a supernova will take place and the rest of the material will condense into a black hole.

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Question: How does a star form?

Answer: A star is formed out of cloud of cool, dense molecular gas. In order for it to become a potential star, the cloud needs to collapse and increase in density.

There are two common ways this can happen: it can either collide with another dense molecular cloud or it can be near enough to encounter the pressure caused by a giant supernova. Several stars can be born at once with the collision of two galaxies. In both cases, heat is needed to fuel this reaction, which comes from the mutual gravity pulling all the material inward.

What happens next is dependent upon the size of the newborn star; called a protostar. Small protostars will never have enough energy to become anything but a brown dwarf (think of a really massive Jupiter). A brown dwarf is sub-stellar object that cannot maintain high enough temperatures to perpetuate hydrogen fusion to helium. Some brown dwarfs can technically be called stars depending upon their chemical composition, but the end result is the same; it will cool slowly over billions of years to become the background temperature of the universe.

Medium to large protostars can take one of two paths depending upon their size: if they are smaller than the sun, they undergo a proton-proton chain reaction to convert hydrogen to helium. If they are larger than the sun, they undergo a carbon-nitrogen-oxygen cycle to convert hydrogen to helium. The difference is the amount of heat involved. The CNO cycle happens at a much, much higher temperature than the p-p chain cycle.

Whatever the route… a new star has formed.

The life cycle of a star is dependent upon how quickly it consumes hydrogen. For example, small, red dwarf stars can last hundreds of billions of years, while large supergiants can consume most of their hydrogen with a comparably short few million years. Once the star has consumed most of its hydrogen, it has reached its mature state. This is how a star forms.

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Question: What are the rings of Saturn made of?

Answer: Saturn's rings are made of ice chunks of various sizes surrounded by Saturn's 60 moons and moonlets!

Some of the chunks of ice, ranging in size from mere microns to several meters have been in existence since the formation of Saturn itself; while others are continuously broken down and reformed by repeated collisions with each other.

Scientists have categorized several distinct rings based on size and composition and each ring is separated by a visible gap.

The way you learned your ABC's in Kindergarten does not apply when describing Saturn's rings. Beginning from the innermost ring and moving progressively outward, they are not in alphabetical order.

The faint innermost ring of Saturn is the D ring followed by the C ring. Both are faint and contain no moons (that have been discovered to date). The B ring is the largest and brightest of Saturn's rings and contains several visible gaps. The Cassini Division is the large gap that separates the Saturn's B ring from its A ring.

The Encke Gap is located within the A ring, which contains the moon Pan. Wobbles in the A ring are caused by resonance disruptions from other orbiting moons. Recently, scientists have discovered that new ring material may be generated from Saturn's moons. Saturn's A ring also contains the small moon Daphnis. Several other small moonlets have been discovered in the A ring, detectable only by the visible, v-shaped disruptions they cause.

Saturn's F ring lies outside the A ring and contains the moons Prometheus and Pandora. Janus and Epimetheus are moons special enough to have their own ring designation. The particles in this ring are generated by meteoroids hitting the surface of these two moons.

The G ring is located between the F ring and the E ring. It is very faint and is acted upon by the orbit of Mimas (contained in the E ring)

Saturn's E ring is its wide, outermost ring filled with very fine particles that are thought to be generated by the moon Enceladus. Mimas, Tethys and Dione are the other moons contained within the E ring.

Rhea, Titan, Hyperion and Iapetus are Saturn's largest moons and orbit the planet outside the E ring.

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Question: Why is Mars red?

Answer: Well, the short answer is that it isn't. For most of the planet, the red layer only covers a couple of millimeters and at its deepest, two meters.

The red color comes from various oxides of iron (hematite mostly) in very, very fine particles, and trace amounts of other elements including titanium, chlorine and sulfur.

One possible way the dust was created was by harder basalt rocks, which contain more feldspar, grinding against the softer basalt to create fine dust particles.

All of that iron had to come from somewhere: volcanoes. The best information that we have is that the surface of mars below the red layer is made up of hardened, low viscose lava: basalt. The concentration of iron in Mars' basalt is higher than that of Earth, which is why Earth is much less red.
And that is why Mars is red.

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Question: What is the North Star?

Answer: Aside from knowing that it was an ancient tool that mariners used to find their way across the unknown seas and is well, in the North, I am left curious.

Firstly, you might expect one of the most famous stars in the night sky to be one of the brightest, but it isn’t; not by a long shot. That honor belongs to Sirius and many less bright stars besides. The North Star shines with a humble brightness that belies its navigational importance.

Polaris, or the North Star, sits almost directly above the North Pole; therefore, it is a reliable gauge of North if you find yourself lost on a clear night without a compass. Stars that sit directly above the Earth’s North or South Pole are called Pole Stars. Interestingly, the North Star hasn’t always been, nor will it always be the Pole Star because the Earth’s axis changes slightly over time, and stars move in relation to each other over time.

You can also approximate your latitude by measuring the angle of elevation between the horizon and the North Star. There is no equivalent star in the South Pole, but Sigma Octantis comes close. It isn’t very useful for navigational purposes as it isn’t very bright to the naked eye. Instead, navigators use two of the stars in the Southern Cross, Alpha and Gamma to determine due South.

The North Star is easy to find if you can first locate the Little Dipper. The North Star lies at the end of the handle in the Little Dipper (Ursa Minor). For a point of reference, The Big Dipper (Ursa Major) lies below the little dipper and their handles point in opposite directions. The two stars in the end of the ladle of The Big Dipper point to Polaris. Also, both The Big Dipper and The Little Dipper remain in the sky all night long, rotating in relation to the Earth’s axis.

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Question: What is a Binary star?

Answer: The term binary star is a misnomer because it is actually a star system made up of usually two stars that orbit around one center of mass – where the mass is most concentrated. A binary star is not to be confused with two stars that appear close together to the naked eye from Earth, but in reality are very far apart – Carl Sagan far!

Astrophysicists find binary systems to be quite useful in determining the mass of the individual stars involved. When two objects orbit one another, their mass can be calculated very precisely by using Newton's calculations for gravity. The data collected from binary stars allows astrophysicists to extrapolate the relative mass of similar single stars.

There are several subcategories of binary stars, classified by their visual properties including eclipsing binaries, visual binaries, spectroscopic binaries and astrometric binaries.

Eclipsing binary stars are those whose orbits form a horizontal line from the point of observation; essentially, what the viewer sees is a double eclipse along a single plane; Algol for example.
Visual binary stars are systems in which two separate stars are visible through a microscope that has an appropriate resolving power. These can be difficult to detect if one of the stars’ brightness is much greater, in effect blotting out the second star.

Spectroscopic binary stars are those systems in which the stars are very close and orbiting very quickly. These systems are determined by the presence of spectral lines – lines of color that are anomalies in an otherwise continuous spectrum and are one of the only ways of determining whether a second star is present. It is possible for a binary system to be both a visual and a spectroscopic binary if the stars are far enough apart and the telescope being used is of a high enough resolution.

Astrometric binary stars are systems in which only one star can be observed, and the other’s presence is inferred by the noticeable wobble of the first star. This wobble happens as a result of the smaller star's slight gravitational influence on the larger star.

So now you can answer the question, "what is a binary star?"

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Question: What is a shooting star?

Answer: A shooting star is another name for a meteoroid that burns up as it passes through the Earth's atmosphere.

Most of the shooting stars that we can see are known as meteoroids. These are objects as small as a piece of sand, and as large as a boulder. Smaller than a piece of sand, and astronomers call them interplanetary dust. If they're larger than a boulder, astronomers call them asteroids.

A meteoroid becomes a meteor when it strikes the atmosphere and leaves a bright tail behind it. The bright line that we see in the sky is caused by the ram pressure of the meteoroid. It's not actually caused by friction, as most people think.

When a meteoroid is larger, the streak in the sky is called a fireball or bolide. These can be bright, and leave a streak in the sky that can last for more than a minute. Some are so large they even make crackling noises as they pass through the atmosphere.

If any portion of the meteoroid actually survives its passage through the atmosphere, astronomers call them meteorites.

Some of the brightest and most popular meteor showers are the Leonids, the Geminids, and the Perseids. With some of these showers, you can see more than one meteor (or shooting star) each minute.

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Question: What is a neutron star?

Answer: Neutron stars are formed when large stars run out of fuel and collapse. To get a neutron star, you need to have star that's larger than about 1.5 solar masses and less than 5 times the mass of the Sun.

If you have less than 1.5 solar masses, you don't have enough material and gravity to compress the object down enough. You only get a white dwarf. This is what will happen to our own Sun one day.

If you have more than 5 times the mass of the Sun, your star will end up as a black hole.

But if your star is right in between those masses, you get a neutron star.

The neutron star is formed when the star runs out of fuel and collapses inward on itself. The protons and electrons of atoms are forced together into neutrons. Since the star still has a lot of gravity, any additional material falling into the neutron star is super-accelerated by the gravity and turned into identical neutron material.

Just one teaspoon of a neutron star would have the mass of over 5 x 10¹² kilograms.

A neutron star actually has different layers. Astronomers think there's an outer shell of atomic nuclei with electrons about 1 meter thick. Below this crust, you get nuclei with increasing numbers of neutrons. These would decay quickly on Earth, but the intense pressure of the gravity keeps them stable.

When neutron stars form, they maintain the momentum of the entire star, but now they're just a few kilometers across. This causes them to spin at tremendous rates, sometimes as fast as hundreds of times a second.

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Question: Can You See the Great Wall of China from Space?

Answer: This is a popular myth: the Great Wall of China is the only human structure that can be seen from space. But it's not true. The reality is that you can't easily see the wall from space with the unaided eye.

In fact, when China's first astronaut, Yang Liwei, went into space, he said that he couldn't see the structure of the wall from out his capsule window.

NASA astronaut Leroy Chiao attempted to photograph the Great Wall of China from out his window on the International Space Station. He photographed a region of Inner Mongolia, about 200 miles north of Beijing. Chiao and NASA believe they can identify parts of the Great Wall in his photograph, but it's not easy to see.

The ancient pyramids at Giza are easy to see out the window of the International Space Station, however.

This myth goes even further, though. People somehow think that the Great Wall of China can be seen from the Moon. The Apollo astronauts confirmed that you can't see the Great Wall of China from the Moon. The best you can see is the white and blue marble of our home planet.

With all of the human construction, many buildings and other structures can be seen from space. But you can't see the Great Wall of China from space.

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Question: What was the first animal in space?

Answer: The first rocket ever sent to space probably carried bacteria or some other accidental passenger. But the first animals ever intentionally sent into space were fruit flies launched aboard a V2 rocket in 1947. US scientists were studying the effects of radiation at high altitude.

A rhesus monkey called Albert 1 became the first monkey launched into space on June 11, 1948; also on board a US-launched V2 rocket.

These were just suborbital flights, though. The first animal to actually go into orbit was the dog Laika, launched on board the Soviet Sputnik 2 spacecraft on November 3, 1957. Unfortunately, Laika died during the flight.

At least 10 more dogs were launched into space and on sub-orbital flights by the Soviets until April 12, 1961, when Yuri Gagarin became the first human in space.

Since those first historic launches, many monkeys, chimpanzees, rats, mice, frogs, spiders, cats and even a tortoise were launched into space.

Read more about Laika's mission in this article.

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Question: How Far is Space?

Answer: The distance to space is surprisingly small - space isn't very far away. You would have to get pretty close to Earth to be able to see the thin atmosphere surrounding our planet.

The reality is that there is no clear boundary between the Earth and outer space. As you climb in altitude, the atmosphere gets less dense as you go. You need to get about 350 km (220 miles) above the Earth's surface to get to the point where a spacecraft can orbit the planet. Any lower than that, and the spacecraft will bump into too much atmosphere to remain in orbit very long.

But various agencies have designated lower altitudes for the beginning of space. The Federation Aeronautique Internationale has established the Karman line at an altitude of 100 km (62 miles). Above this altitude, a vehicle would need to be going more than orbital velocity to get enough lift from the atmosphere to support itself.

NASA considers people to be astronauts once they've traveled higher than 80 km (50 miles).

And the Ansari X-Prize competition required entrants to travel above an altitude of 100 km twice in two weeks to win the $10 million prize.

The Earth's atmosphere extends out 10,000 km above the surface of the planet; although, it is extremely tenuous at this altitude. Only the lightest gases, like hydrogen and helium can be found there. It is from this layer of the atmosphere, called the exosphere, that molecules from Earth can escape into space.

So, if you're wondering how far space is, I would go with the 100 km (62 mile) answer. It's a pretty safe one.

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Question: What is on the other side of a black hole?

Answer: There is no other side.

Science fiction has populated the idea that a black hole serves a portal to another world. If you could pass through, where does a black hole go? Perhaps you'll come to some other dimension, or re-emerge from some other part of the Universe?

No, a black hole only leads to death, for you, your spaceship, and another else that's unlucky enough to fall in.

Imagine you fell into a star like our Sun, there would be no question what would happen to you. The intense heat, gravity and pressure would kill you. If you compress more than 5x the mass of the Sun into a tight little area, you get a black hole. But the gravity, heat and pressure are all still there, just much more intense.

If you actually fell into a black hole, the tidal forces pulling at you are so extreme that the force on your feet is dramatically stronger than the force at your head. You would be stretched out and torn into pieces, and then those pieces would be torn into pieces. You would eventually be pulled into a stream of atoms, winding their way down to the surface of the black hole. For this process, scientists have a technical term: spaghettification.

Let's say you could survive this journey. Where does the black hole lead? No where. All of the mass of the star that came before the black hole is still there, pulling at you with all its gravity. This intense gravity would tear every molecule apart, and all the atoms. Protons and electrons would be crushed together to create neutrons, and then these would be crushed together even further into some kind of exotic form of superdense matter.

It's even possible that the heart of a black hole is single point of infinitely small size, containing the mass of many stars. This black hole is not a portal to anywhere, it's just a final destination.

Here's an article I did about how to maximize your time while falling into a black hole.

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Question: How far away are the planets?

Answer: To answer this question, we'll consider the distance from each of the planets to the Sun. Before we start though, there's a term in astronomy that you'll want to remember: astronomical unit. One astronomical unit (or AU) is the distance from the Earth to the Sun.

You should also note that all of the planets orbit the Sun in ellipses. This means that their distance to the Sun varies depending on what point they are in their orbits. Astronomers call the closest point perihelion, and the most distant point aphelion.

As a special bonus, I'll also let you know the closest point that each of the planets gets to Earth.

Mercury
Closest: 46 million km (.307 AU)
Furthest: 70 million km (.466 AU)
Average: 57 million km (.387 AU)

Closest to Mercury from Earth: 77.3 million km

Here are some Mercury interesting facts.

Venus
Closest: 107 million km (.718 AU)
Furthest: 109 million km (.728 AU)
Average: 108 million km (.722 AU)

Closest to Venus from Earth: 40 million km

Mars
Closest: 205 million km (1.38 AU)
Furthest: 249 million km (1.66 AU)
Average: 228 million km (1.52 AU)

Closest to Mars from Earth: 65 million km

Jupiter
Closest: 741 million km (4.95 AU)
Furthest: 817 million km (5.46 AU)
Average: 779 million km (5.20 AU)

Closest to Jupiter from Earth: 588 million km

Saturn
Closest: 1.35 billion km (9.05 AU)
Furthest: 1.51 billion km (10.12 AU)
Average: 1.43 billion km (9.58 AU)

Closest to Saturn from Earth: 2.57 billion km

Uranus
Closest: 2.75 billion km (18.4 AU)
Furthest: 3.00 billion km (20.1 AU)
Average: 2.88 billion km (19.2 AU)

Closest to Uranus from Earth: 2.57 billion km

Neptune
Closest: 4.45 billion km (29.8 AU)
Furthest: 4.55 billion km (30.4 AU)
Average: 4.50 billion km (30.1 AU)

Closest to Neptune from Earth: 4.3 billion km

As a special bonus, we'll include Pluto too, even though Pluto is not a planet anymore.

Pluto
Closest: 4.44 billion km (29.7 AU)
Furthest: 7.38 billion km (49.3 AU)
Average: 5.91 billion km (39.5 AU)

Closest to Pluto from Earth: 4.28 billion km

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Question: Why Does Jupiter Have a Red Spot?

Answer: Jupiter is beautiful to look at, both in a backyard telescope and with the Hubble Space Telescope. Even in the smallest telescope you can see atmospheric bands across the planet, and its 4 largest moons orbiting around it. With larger telescopes you can see the famous Great Red Spot - a swirling storm larger than the Earth.

Astronomers have been observing the Great Red Spot for more than 300 years, so it's incredibly long lasting. But it is changing. In 2004, the Great Red Spot was only half the size it was a century earlier, reaching a total size of 40,000 km. At this current rate, astronomers think it will become circular by 2040.

Thanks to recent missions to Jupiter, like Galileo, Cassini and New Horizons, scientists are learning a tremendous amount about how the atmosphere on Jupiter works.

Hot gases in Jupiter's atmosphere are constantly swirling around, rising and falling. As cooler gas falls down through the atmosphere, the Coriolis force causes the region to start swirling. These eddies can last for a long time because there is no solid ground on Jupiter to create friction.

These eddies can move around and merge into one another, creating larger and larger storms, which last for longer periods. According to theories, several of these larger storms came together, combining their energy to create the long lived Great Red Spot.

We can see this process happening again with another spot on Jupiter called Oval BA. It was first discovered a few years ago when three white storms merged together. This new "Red Spot Jr." has been stable in the atmosphere of Jupiter for more than 6 years now. It and the larger Red Spot have even passed each other several times now, and not caused much damage.

Scientists think the Great Red Spot has its characteristic red color because the strength of the storm is digging down into Jupiter's atmosphere, bringing complex organic molecules to the high atmosphere.

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Question: Why Can't We Launch our Trash Into Space?

Answer: Now wouldn't that be a tidy solution to a big problem? Gather together all the garbage, bundle it up and fire it off into space. Maybe just dump it into the Sun. We could live in a world without trash.

There are just two problems: humans produce an enormous amount of garbage; and rocket launches are extremely expensive.

It's been estimated that launching material on the space shuttle costs about $10,000/pound ($22,000/kg). Even if engineers could bring down prices by a factor of 10, it would still be thousands of dollars to launch the garbage into space. Let's imagine a wonderful dream world, where launch costs could be brought down to $1,000/kg - a factor of 1/20th the cost to launch on the space shuttle.

It has also been estimated that the United States alone produces 208 million metric tonnes of garbage per day… per day! So, to launch all that trash into space would cost the United States $208 trillion per day… per day!

The gross domestic product of the United States was $13.13 trillion in 2006, which works out to be about $36 billion a day. In other words, the United States would need to spend 5,800 times its daily gross domestic budget, just to launch its trash into space.

What about nuclear waste? A nuclear reactor releases about 25-30 tonnes of spent fuel every year. With our dream budget of $1,000/kg, that would cost about $25 million to launch a single reactor's waste into orbit. According to Wikipedia, there are 63 operating reactors in the US, so it would cost about $1.6 billion/year to dispose of the nuclear waste generated.

It's been estimated that Yucca Mountain - the United State's current plan to store nuclear waste - will cost about $58 billion to store waste over the course of 100 years. So storing waste in Yucca Mountain will cost about 1/3rd the price of launching that material into space. Not to mention the terrible risk of launching rockets full of nuclear waste into space - imagine what might happen if a rocket exploded in mid-flight…

I'm sure I've made some math errors here somewhere…

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Question: How Long Will Life Survive on Earth?

Answer: It feels like the Earth is forever. But we know it formed around 4.5 billion years ago, and it will last another 7.5 billion years or so, when the Sun becomes a red giant, and probably destroying the Earth.

But our climate will become unlivable long before that. According to Peter Ward and Robert Brownlee, in their book, The Life and Death of Planet Earth, things are going to heat up much, much earlier.

That's because the energy output coming the Sun is gradually increasing. Not enough to change the climate in our lifetimes, or even millions of years. But in the span of hundreds of millions of years, things are going to heat up.

A model developed by researchers at Pennsylvania State University calculated that the energy coming from the Sun will heat up the planet so much that the oceans will evaporate within a billion years or so.

But this is just the end of a series of terrible things that will happen to the planet as the Sun's energy output increases.

As the climate becomes warmer, the cycle of silicate rock weather speeds up, removing carbon dioxide from the atmosphere and sequestering it as calcium carbonate in the oceans. Without carbon dioxide, plants won't be able to survive, and everything relying on them dies too.

According to their calculations, Earth has been habitable for 4.5 billion years, and only has half a billion years left.

Here's an article I did about the end of the Universe, just in case you really want to look ahead.

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Question: How Does the Earth Protect Us From Space?

Answer: Our Earth keeps us very safe from a dangerous Universe that's always trying to kill us in new and interesting ways.

Risk: Cosmic rays are high energy particles fired at nearly the speed of light by the Sun, supermassive black holes and supernovae. They have the ability to blast right through your body, damaging DNA as they go. Long term exposure to cosmic rays increases your chances of getting cancer. Fortunately, we have our atmosphere to protect us. As cosmic rays crash into the atmosphere, they collide with the oxygen and nitrogen molecules in the air.

Risk: Gamma rays and X-rays. As you know, radiation can damage the body. Just a single high-energy photon of gamma rays can cause significant damage to a living cell. Once again, though, the Earth's atmosphere is there to protect us. The molecules in the atmosphere absorb the high-energy photons preventing any from reaching us on the ground. In fact, X-ray and gamma ray observatories need to be built in space because there's no way we can see them from the ground.

Risk: Ultraviolet radiation. The Sun is bathing the Earth in ultraviolet radiation; that's why you get a sunburn. But the ozone layer is a special region of the atmosphere that absorbs much of this radiation. Without the ozone layer we would be much more exposed here on the surface of the Earth to UV rays, leading to eye damage and greater incidence of skin cancer.

Risk: Solar flares. Violent explosions on the surface of the Sun release a huge amount of energy as flares. In addition to a blast of radiation, it often sends out a burst of plasma traveling at nearly the speed of light. The Earth's magnetosphere protects us here on Earth from the effects of the plasma, keeping it safely way from the surface of the planet. And our atmosphere keeps the X-ray/gamma ray radiation out.

Risk: Cold temperatures. Space itself is just a few degrees above absolute zero, but our atmosphere acts like a blanket, keeping warm temperatures in. Without the atmosphere, we'd freeze almost instantly.

Risk: Vacuum. Space is airless. Without the Earth, there'd be no air to breath, and the lack of pressure damages cells and lets water evaporate out into space. Vacuum would be very, very bad.

If you'd like to hear more about cosmic rays, listen to this episode of Astronomy Cast.

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Question: Will the Large Hadron Collider Destroy the Earth?

Answer: No.

As you might have heard in the news recently, several people are suing to try and get the Large Hadron Collider project canceled. When it finally comes online, the LHC will be the largest, most powerful particle accelerator ever constructed.

If there's something wrong with it, the LHC might have the power to damage itself, but it can't do anything to the Earth, or the Universe in general.

There are two worries that people have: black holes and strange matter.

One of the goals of the Large Hadron Collider is to simulate microscopic black holes that might have been generated in the first few moments of the Big Bang. Some people are worried that these artificial black holes might get loose, and then consume the Earth from within, eventually moving on to destroy the Solar System.

The physicists are confident that any black holes they create will evaporate almost instantaneously into a shower of particles. In fact, the theories that predict that black holes can be created also predicts that black holes will evaporate. The two concepts go hand in hand.

The other worry is that the Large Hadron Collider will create a theorized material called strangelets. This "strange matter" would then be able to infect other matter, turning the entire planet into a blog of strange matter.

This strange matter is completely theoretical, and once again, the same theories that say it might be produced in the Large Hadron Collider also rule out any risks from it.

One of the most important considerations is the fact that the Moon is struck by high energy cosmic rays that dwarf the power of the Large Hadron Collider. They were likely blasted out of the environment around a supermassive black hole.

These have been raining down on the Moon for billions of years, and so far, it hasn't turned into a black hole or strange matter.

You can read more about the Large Hadron Collider lawsuit here. Or how it might create wormholes, a view into other dimensions, or unparticles.

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Question: Why are more distant galaxies moving away faster?

Answer: As you know, the Universe is expanding after the Big Bang. That means that every part of the Universe was once crammed into a tiny spot smaller than a grain of sand. Then it began expanding, and here we are, 13.7 billion years later with a growing Universe.

The expansive force of dark energy is actually accelerating the expansion even faster. But we won't bring that in to make things even more complex.

As we look out into the Universe, we see galaxies moving away from us faster and faster. The more distant a galaxy is, the more quickly it's moving away.

To understand why this is happening, go and get a balloon (or blow one up in your mind). Once you've got it blown up a little, draw a bunch of dots on the surface of the balloon; some close and others much further away. Then blow up the balloon more and watch how the dots expand away from each other.

From the perspective of any one dot on the surface of the balloon, the nearby dots aren't expanding away too quickly, maybe just a few centimeters. But the dots on the other side of the balloon are quite far away. It took the same amount of time for all the dots to change their positions, so the more distant dots appeared to be moving faster.

That's how it works with the Universe. Because space itself is expanding, the more further a galaxy is, the faster it seems to be receding.

Thanks to Cassandra for the question.

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Question: How Can Galaxies Move Away Faster Than Speed of Light?

Answer: Einstein's Theory of Relativity says that the speed of light - 300,000 km/s - is the maximum speed that anything can travel in the Universe. It requires more and more energy to approach the speed of light. You could use up all the energy in the Universe and still not be traveling at light speed.

As you know, most of the galaxies in the Universe are expanding away from us because of the Big Bang, and the subsequent effects of dark energy, which is providing an additional accelerating force on the expansion of the Universe.

Galaxies, like our own Milky Way are carried along by the expansion of the Universe, and will move apart from every other galaxy, unless they're close enough to hold together with gravity.

As you look at galaxies further and further away, they appear to be moving faster and faster away from us. And it is possible that they could eventually appear to be moving away from us faster than the speed of light. At that point, light leaving the distant galaxy would never reach us.

When that happens, the distant galaxy would just fade away as the last of the photons reached Earth, and then we would never know it was ever there.

This sounds like it breaks Einstein's theories, but it doesn't. The galaxies themselves aren't actually moving very quickly through space, it's the space itself which is expanding away, and the galaxy is being carried along with it. As long as the galaxy doesn't try to move quickly through space, no physical laws are broken.

One sad side effect of this expansion is that most of the galaxies will have receded over this horizon in about 3 trillion years, and future cosmologists will never know there's a great big Universe out there.

You can read more about this in an article I did called the End of Everything.

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Question: How Big Can Planets Get?

Answer: Here in the Solar System, we have three kinds of planets: the inner terrestrial planets, the gas giants, and the ice planets. Sadly, Pluto is no longer a planet, so we won't deal with that here. We know how big our planets are, but how big can planets actually get in other Solar Systems. What are the biggest possible planets?

Let's start with terrestrial planets, like our Earth. We'll set the size of the Earth and 1 Earth radius, and the mass as 1 Earth mass. We've seen that terrestrial planets can get smaller, with Mars and Mercury, and astronomers have detected larger terrestrial planets orbiting other stars.

The largest known rocky planet is thought to be Gliese 436 c. This is probably a rocky world with about 5 Earth masses and 1.5 times our planet's radius. Amazingly, this planet is thought to be within its star's habitable zone.

What's the largest possible rocky planet? For this I put in an email to Dr. Sean Raymond, a post doctoral researcher at the Center for Astrophysics and Space Astronomy (CASA) at the University of Colorado. Here's what he had to say:

"The largest "terrestrial" planet is generally considered the one before you get too thick of an atmosphere, which happens at about 5-10 Earth masses (something like 2 Earth radii). Those planets are more Earth-like than Neptune-like."

Gas giants, of course, can come much larger. Jupiter is 317 times more massive than Earth, and 11 times larger. You could fit 1,400 Earths inside Jupiter.

The largest known extrasolar planet (at the time of this writing) is TrES-4, which is located 1,400 light years away in the constellation Hercules. The planet has been measured to be 1.4 times the size of Jupiter, but it only has 0.84 times Jupiter's mass. With such a low density, the media was calling TrES-4 the puffy planet.

And once again, how large can they get? Again, here's Dr. Raymond:

"In terms of gaseous planets, once they reach 15 Jupiter masses or so there is enough pressure in the core to ignite deuterium fusion, so those are considered "brown dwarfs" rather than planets."

What is the biggest planet in the Solar System?

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Question: What is the distance to the Moon?

Answer: Here's a quick answer. The average distance from the center of the Earth to the center of the Moon is 384,403 km (238,857 miles).

Wait!

Before you write that into your homework assignment, you've got to realize that the Moon travels around the Earth in an ellipse. That means it gets closer and further, depending on where it is in its orbit. At its closest point, the Moon gets to 363,104 km (225,622 miles), and at its furthest point, it's 405,696 km (252,088 miles).

This variation can make the size of the Moon noticeably different from full Moon to full Moon. When the Moon is at its closest point, this is called perigee. Here's a picture that illustrates the difference.

How have scientists been able to measure this distance so accurately? When the Apollo astronauts visited the Moon, they left behind some special mirrors that reflect light back towards the Earth. Scientists here on Earth can fire lasers at the Moon, and measure the time it takes to make the trip within the scale of picoseconds. Four of these lunar retro-reflectors are still functional.

In fact, their experiments are so accurate they can measure the distance to the Moon within millimeters. For more information on this project, check out the APOLLO experiment page.

Now that you know the distance to the Moon, find out how long it takes to get to the Moon.

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Question: What is the far side of the Moon?

Answer: Did you ever notice that the Moon always looks the same? Sure, it waxes and wanes from a new moon to a full moon, but the bright and dark patches on the Moon always look the same. In fact, these features are so familiar that people call it the Man in the Moon.

This is because the Moon always points the same face towards the Earth. The Moon does actually rotate on its axis, it's just that the amount of time it takes to make a complete orbit around the Earth matches the amount of time it takes to complete one rotation. In both cases, this is 27.3 days.

So, when you hear people refer to the far side of the Moon, they're talking about the part of the Moon that always faces away from the Earth. Until we sent spacecraft into orbit around the Moon to take pictures, nobody on Earth had ever seen what the far side of the Moon looks like.

But why does this happen? Over the few billions years since its formation, the Moon has become tidally locked with the Earth. In the distant past, the Moon had different rotation and orbital speeds, and it showed all of its sides to our planet. But the gravity of the Earth tugged at the irregular shapes on the Moon, causing it to slow its rotation down until it was exactly the same length as its orbit.

The Earth, on the other hand, has so much mass that the force of gravity from the Moon pulling on Earth can't overcome its rotational speed. The Moon does create the tides, though, and causes the ground to rise and fall - it's just such a small amount that you can't feel it.

Sometimes people mistakenly call this the dark side of the Moon. But there is no dark side of the Moon. Think about it, when we're seeing a new moon, that's because the familiar part that we can always see is in shadow. But at that point, the far side will be bathed in sunlight.

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Question: What kind of star is our Sun?

Answer: As you probably know, our Sun is just a star. It's our closest, most familiar star, but it's still just a star. With a great big Universe out there, populated with countless stars, astronomers have been able to see examples of stars in all shapes, sizes, metal content and ages.

According to their system of classification, the Sun is known as a yellow dwarf star. This group of stars are relatively small, containing between 80% and 100% the mass of the Sun. So the Sun is at the higher end of this group. The official designation is as a G V star.

Stars in the this classification have a surface temperature between 5,300 and 6,000 K, and fuse hydrogen into helium to generate their light. They generally last for 10 billion years.

But there's more to this question, because G V Stars can experience several different stages. Some are newly forming, others are in their middle ages, and others are nearing the end of their lives.

Our Sun is right in the middle ages, in a time known as the main sequence. It has already lived for 4.3 billion years, and will likely last another 7 billion years or so. At that point, it will balloon into a red giant star, and eventually collapse down into a white dwarf.

The Sun also belongs to the Population I group of stars, which contain relatively large amounts of heavier elements. The first ever stars, made from pure hydrogen and helium are Population III. These exploded as supernovae, producing fusing the lighter elements into heavier and heavier elements. Our Sun, then, contains the metal from previous generations of stars that went supernova.

Some other examples of the yellow dwarf star group include Alpha Centauri, Tau Ceti and 51 Pegasi.

For the quick answer, the Sun is a Population I yellow dwarf star, in the main sequence.

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Question: How Big is Apophis?

Answer: In case you haven't heard, Asteroid 99942 Apophis is a near Earth asteroid that astronomers think will make a close flyby to the Earth in 2029. When its trajectory was first calculated back in 2004, it had one of the closest visits to Earth astronomers had seen, and had a 2.7% chance of hitting the Earth.

But follow-up observations brought that risk down to 1 in 45,000. Right now, astronomers think that Apophis is essentially no risk to the Earth. In April, 2008 media reported that a 13-year old German student had caught a math mistake made by NASA, and the risk of an Earth strike was actually 1-45. This later turned out to be a hoax.

Because of its close approach to Earth, space advocacy societies, including the Planetary Society think that Apophis would make an ideal target for a human mission, and allow engineers to test out strategies for moving asteroids away from dangerous Earth-crossing orbits.

So back to the original question, how big is Apophis? The best estimate puts it at 270 meters (885 feet across), and it has a mass of 2.1 x 10¹⁰ kg. To give you a sense of scale, the Eiffel Tower in Paris is 324 meters tall.

But now you know its mass and size, you're probably wondering: what would happen to the Earth if it struck? NASA estimated that a strike by Apophis would release the equivalent of 880 megatons of energy. Just as a comparison, the object that carved out Meteor Crater in Arizona probably released 3-10 megatons of energy.

If Apophis struck land, it would flatten thousands of square km of land, killing millions of people if it hit a densely populated area. But it wouldn't cause the kinds of long term climate destruction that 1 km and larger asteroids can do. If it hit an ocean, it would create devastating tsunamis in all directions.

Here's an article explaining techniques that might be used to move an asteroid. And here's NASA's official page on Apophis.

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