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This question comes from William.

He writes "Hi Fraser, I was wondering if you could do a video on pulsars like you did on quasars."

You got it.

Stars are held in perfect balance between the pressure of gravity pulling them inward, and the outward force of radiation.

Once stars runs out of fuel, they collapses in on themselves - it's the amount of mass decides what happens next.

The most massive stars detonate as supernovae, and can explode or collapse into black holes.

If they're less massive, like our Sun, they blast away their outer layers and then slowly cool down as white dwarfs.

But for stars between 1.4 and 3.2 times the mass of the Sun, they may still become supernovae, but they just don't have enough mass to make a black hole.

These medium mass objects end their lives as neutron stars, and some of these can become pulsars or magnetars.

Gravity overwhelms the atomic bonds of the matter in a neutron star, crushing protons and electrons together into neutrons. This is how they get their name.

A star that used to be more than a million kilometers wide is now less than 20 kilometers across.

This material is so dense, that a single sugar cube's worth weigh about 100 million tonnes on Earth.

And you would need to be traveling 100,000 km/s to escape a neutron star's pull - about 1/3rd the speed of light.

So that's how you get a neutron star. But what about these pulsars?

When these stars collapse, they maintain their angular momentum. But with a much smaller size, their rotational speed increases dramatically, spinning many times a second.

This relatively tiny, super dense object, emits a powerful blast of radiation along its magnetic field lines.

Although this beam of radiation doesn't necessarily line up with it's axis of rotation.

And so, from here on Earth, astronomers detect an intense beam of radio emissions several times a second, as it rotates around like a lighthouse beam.

This is a pulsar.

The first one was detected in 1967 by Jocelyn Bell Burnell and Antony Hewis.

They detected a mysterious radio emission coming from a fixed point in the sky that peaked every 1.33 seconds.

Although they were certain it had a natural origin, they named it LGM-1, which stands for "little green men", and subsequent discoveries have helped astronomers discover the true nature of these strange objects.

Pulsars have been discovered emitting many different wavelengths of light, from radio to visible and even X and gamma rays.

There have been a total of 1600 found so far, and the fastest discovered emits 716 pulses a second.

When a pulsar first forms, it has the most energy and fastest rotational speed. As it releases electromagnetic power through its beams, it gradually slows down.

Within 10 to 100 million years, it slows to the point that its beams shut off and the pulsar becomes quiet.

When they are active, they spin with such uncanny regularity that they're used as timers by astronomers.

Pulsars help us search for gravitational waves, probe the interstellar medium, and even find extrasolar planets in orbit.

It has even been proposed that spacecraft could use them as beacons to help navigate around the Solar System.

On NASA's Voyager spacecraft, there are maps that show the direction of the Sun to 14 pulsars in our region. If aliens wanted to find our home planet, they couldn't ask for a more accurate map.

I hope that helps, William.

This question comes from Sheldon Grimshaw.

"I've heard that there are more stars in our Universe than there are grains of sand on all the beaches on Earth. Is this possible?"

Awesome question, and a great excuse to do some math.

As we learned in a previous video, there are 100 to 400 billion stars in the Milky Way and more than 100 billion galaxies in the Universe - maybe as many as 500 billion.

If you multiply stars by galaxies, at the low end, you get 10 billion billion stars, or 10 sextillion stars in the Universe - a 1 followed by 22 zeros.

At the high end, it's 200 sextillion.

These are mind bogglingly huge numbers.

How do they compare to the number of grains of sand on the collective beaches of an entire planet?

This type of sand measures about a half millimeter across.

You could put 20 grains of sand packed in side-by-side to make a centimeter. 8000 grains in one cubic centimeter.

If you took 10 sextillion grains of sand, put them into a ball, it would have a radius of 10.6 kilometers.

And for the high end of our estimate, 200 sextillion, it would be 72 kilometers across.

If we had a sphere bigger than the Earth, it would be an easy answer, but no such luck. This might be close.

So, is there that much sand on all the beaches, everywhere, on this planet?

You'd need to estimate the average volume of a sandy beach and the average amount of the world's coastlines which are beaches.

I'm going to follow the estimates and calculations made by Dr. Jason Marshall, aka, the Math Dude.

According to Jason, there about 700 trillion cubic meters of beach of Earth, and that works out to around 5 sextillion grains of sand.

Jason reminds us that his math is a rough estimate, and he could be off by a factor of 2 either way. So it could be 2.5 sextillion or there could be 10 sextillion grains of sand on all the world's beaches.

So, if the low end estimate for the number of stars matches the high end estimate for the number of grains of sand, it's the same.

But more likely, there are 5 to 10 times more stars than there are grains of sand on all the world's beaches.

So, there's your answer, Sheldon. For some "back of the napkin" math we can guess that there are more stars in our Universe than there are grains of sand on all the beaches of Earth.

Oh, one more thing.

Instead of grains of sand, what about atoms?

How big is 10 sextillion atoms?

How huge would something with that massive quantity of anything be?

Pretty gigantic. Well, relatively at least. 10 sextillion of anything does sound like a whole lot.

If you were to make a pile of that many atoms... guess how big it would be.

It'd be about.... (gesture big then gesture small) 4 times smaller than a dust mite.

Which means, a single grain of sand has more atoms than there are stars in the Universe.

Previously, I've talked about how quickly the Earth is rotating, and I've talked about why it's spinning.

But what would happen if the Earth suddenly stopped rotating?

Keep in mind, this is totally impossible.

Still, I find topics like this super interesting to think about.

If the Earth actually stopped rotating instantly, it would be very *very* bad for everything on the surface of the planet.

Consider this. If you're standing on the equator, you're traveling at 465 meters/second from West to East.

If the Earth put on the brakes, your momentum would keep you flying at the same speed until *friction* slowed you down.

Imagine the jumbled forests, oceans, buildings and unfortunate animals after that event - it would be carnage.

The atmosphere is also turning at 465 meters/second with us. If everything suddenly stopped, it would be like a simultaneous hurricane of epic proportions on almost every part of the planet.

The closer you get to the poles, however, the less damage it would cause.

If you were standing on the North Pole, you'd hardly feel it.

Let's imagine the Earth slowed down more gradually, taking a few days to come to a halt that didn't tear the landscape into fine mist of dirt, seawater and a variety of organic material.

The stars would hang fixed in the sky; only the planets would move around, continuing their paths around the Sun.

A day on Earth would now last 365 days. In other words, it would take a full year for Earth to fully orbit the Sun, bringing it back to the same location in the sky. Each year would include one long day, and then one long night. You can imagine how that would be devastating to plants and animals.

We get a hint of this at the poles, where you can experience weeks of permanent night and then weeks of permanent day. But imagine 6 months of night, followed by 6 months of day.

The planet's tilt gives us our seasons. As we define the Earth's tilt as its rotation in relation to the Sun. A stationary planet would effectively have no tilt at all.

Without that tilt, there'd be no axis, and no seasons. Although we would still have North and South poles.

Because of its rotation, turning once every day, the Earth is actually a flattened ball, or if you prefer, an oblate spheroid.

Without that rotation the Earth would become a perfect sphere. This sounds interesting and probably harmless, but it's actually a *big* problem.

Because of the Earth's bulge in the middle, the oceans are held out at the equator by 8 km. On perfect sphere Earth, the world's oceans would redistribute, flooding many regions of the planet with an immense volume of water.

We'd end up with a single continent around the middle of the planet, with oceans surrounding the north and south poles.

Let's assume that if the Earth stopped turning, the Earth's core would stop rotating within it as well.

This natural dynamo generates the magnetic field that surrounds our planet and protects life from radiation from space.

Whatever life still alive would be irradiated from the Sun and cosmic rays if it didn't live underground.

So, if the Earth suddenly stopped turning, it would be very bad, even catastrophically bad.

Fortunately, it's completely impossible for this to happen.

Imagine a really bad day. Perhaps you're imagining a day where the Sun crashes into another star, destroying most of the Solar System.

No? Well then, even in your imagination things aren't so bad... It's all just matter of perspective. Fortunately for us, we live in out the boring suburbs of the Milky Way. Out here, distances between stars are so vast that collisions are incredibly rare.

There are places in the Milky Way where stars are crowded more densely, like globular clusters, and we get to see the aftermath of these collisions. These clusters are ancient spherical structures that can contain hundreds of thousands of stars, all of which formed together, shortly after the Big Bang.

Within one of these clusters, stars average about a light year apart, and at their core, they can get as close to one another as the radius of our Solar System.

With all these stars buzzing around for billions of years, you can imagine they've gotten up to some serious mischief. Within globular clusters there are these mysterious blue straggler stars. They're large hot stars, and if they had formed with the rest of the cluster, they would have detonated as supernovae billions of years ago.

So scientists figure that they must have formed recently. How?

"Hi, I'm Professor Mark Morris of the University of California at Los Angeles in the Department of Physics and Astronomy."

Astronomers think they're the result of a stellar collision. Perhaps a binary pair of stars merged, or maybe two stars smashed into one another.

"When you see two stars colliding with each other, it depends on how fast they're moving. If they're moving at speeds like we see at the center of our galaxy, then the collision is extremely violent. If it's a head-on collision, the stars get completely splashed to the far corners of the galaxy. If they're merging at slower velocities than we see at our neck of the woods in our galaxy, then stars are more happy to merge with us and coalesce into one single, more massive object."

There's another place in the Milky Way where you've got a dense collection of stars, racing around at breakneck speeds... near the supermassive black hole at the center of the galaxy. This monster black hole contains the mass of 4 million times the Sun, and dominates the region around the center of the Milky Way.
Stars whip around it, like comets dart around our Sun, and interactions are commonplace.
There's another scenario that can crash stars together. The Milky Way mostly has multiple star systems. Several stars can be orbiting a common center of gravity. Many are great distances, but some can have orbits tighter than the planets around our Sun.

When one star reaches the end of its life, expanding into a red giant, It can consume its binary partner. The consumed star then strips away 90% of the mass of the red giant, leaving behind a rapidly pulsating remnant.

What about when galaxies collide? That sounds like a recipe for mayhem.
Surprisingly, not so much.

Read more at http://www.universetoday.com/107244/c...
Thanks to Professor Mark Morris at UCLA - visit the Physics and Astronomy program homepage at http://www.pa.ucla.edu/.