Archive for the ‘Introductory Astronomy’ Category

What is a Black Hole?

August 9, 2009

Black holes is one of the most popularized exotic space objects. Even if you are far from astronomy, chances are you still heard about black holes. What black holes really are? How do they form and how do we know they exist?

Let’s start with a gravity explanation. Go outside and throw a stone as far as you can (just please don’t hurt anyone :-) Your stone will fall down on Earth, because there is a gravity force between any two objects which have masses. Stone has a mass (and you can feel it like stone’s weight), Earth has a mass (and very big one!), so these two masses create a force which we call gravity.

If you climb somewhere high in the mountains and throw the same stone exactly the same way it will fly further, because gravity force weakens with the distance. In university we even measured different gravity force on different building floors, as you climb up the stairs you can see how gravity force weakens (this is one of the reasons why most of the modern athletic world records were created in high altitude stadiums). So, the point is — the further you are from the object the lower gravity force is, the closer you are to the object — the stronger gravity force is.

Gravity is different in different parts on Earth. Thats right, nothing is very simple in the real world. Image from: sciencefriday.com

Gravity is different in different parts on Earth. That's right, nothing is very simple in the real world. Image from: sciencefriday.com

Now let’s talk about launching satellites. You saw how your stone is falling back to Earth after you threw it. This is not what we want when we launch satellites. What is the real difference between your stone throw and satellite launch? The main difference is in speed. Use your imagination a bit: imagine if you are stronger than you are, then you obviously will throw stone further. Now imagine that you are a superman, if a superman throws a stone it can fly so far, that it will turn around the Earth and hit him in the head from behind. Minimum speed required for the stone to fly around the Earth (very close to the surface) is called first cosmic speed and equals about 7.8 km/s (4.8 mi/s). This speed is different for different altitudes, that’s why high flying satellites have lower speeds than low flying satellites. If you want to launch a spacecraft to some other planet, then you need a higher velocity, about 11.2 km/s (6.9 mi/s) for Earth. This is called second cosmic velocity or escape velocity and it means how fast you should fly to be able to completely escape gravitational field.

We use rockets to give our satellites enough speed for them not to fall back down on Eaarth. Image from: ilslaunch.com

We use rockets to give our satellites enough speed for them not to fall back down on Earth. Click for bigger picture. Image from: ilslaunch.com

I wrote about what is happening in the Sun’s core in my other introductory astronomy article about the Sun. Basically it is a huge hydrogen bomb non stop blast. Why the Sun is still in one piece? Why it didn’t brake down because of all those blasts happening there 24/7? It’s a gravity force which keeps it together. Pieces of Sun are trying to fly away, but Sun is so heavy, that it’s gravity keeps it in one piece. Every star is in a balance between these two forces. Gravity is trying to collapse the star, while wild blasts of energy is trying to expand the star. Star lives while it’s balanced.

What happens when star runs out of fuel? Those thermonuclear processes inside the star consume hydrogen, when star runs our of hydrogen nothing stops it from collapsing anymore. Star begins to shrink and it’s density grows. Imagine that you are standing right there on the star surface. As star shrinks your weight will increase: star mass stays the same, your mass stays the same, but distance between you and the star center decreases, so gravity force grows. As gravity force grows, cosmic velocities also grow, so you need higher and higher speeds to leave the star as it shrinks.

Star lives while there is a balance between two forces. Image from: astro.psu.edu

Star lives while there is a balance between two forces. Image from: astro.psu.edu

If star is heavy enough nothing can possibly stop it from total shrinking (at least nothing we currently know of), so it will be collapsing and in one point the second cosmic velocity for this star will reach the speed of light. Currently we know that nothing can travel faster than light, so at that point nothing can possibly leave this star. This kind of collapsed star is called a black hole.

It took me quite a while to explain what black holes really are, but it’s much better now when you know that there is nothing mysterious there — just the object with gravity field is so intense that even light can not escape from it. If you will stand on the “surface” of black hole and turn on your flashlight then light beam from it will bend, fly around a black hole and hit you in the head from behind :-). You can make black hole out of anything if you shrink it enough, you don’t need collapsing stars for it. If you will collapse our Earth into the ball with this radius, it will become a black hole too.

All things described above is a pure theory, we could come up with it (and we did) just on paper, without any observations. Why are we so sure that black holes exist? They don’t shine (they are black holes, remember?), they don’t reflect the light, can we possibly detect them? Yes we can! When charged particles are captured by black hole they don’t just fall on it right away — they are spinning around it as they fall, imagine some kind of spiral with every spin they come closer. Every accelerated charged particle emits electromagnetic energy and we are able to capture that energy. While we can’t capture black hole itself we can capture the things it is doing to it’s neighborhood. For example many stars are in fact double stars, our single Sun actually belongs to a minority of star population. Let’s say we have a double star and one of them collapses into a black hole. How will it affect the neighbor? Black hole will suck it’s material, spin it very fast around and ultimately consume. But again, while star material spins around the black hole we can detect radiation it emits.

Artist impression of black hole consuming a star. This kind of events we can detect with our telescopes. Image from: nrao.edu

Artist impression of black hole consuming a star. This kind of events we can detect with our telescopes. Image from: nrao.edu

OK, enough fancy talk for today, if you want to learn more about black holes you can search about how do they evaporate, this is an introductory article, so I try not to cover much of advanced stuff.

Introductory Astronomy: Sun

August 1, 2009

You also may be interested in my post about hydrogen bomb with pictures and videos.

Sun is the Center

Some time ago I wrote about the difference between stars and planets, read that post first if you haven’t yet. Sun is the closest star to Earth. Almost all the energy we use is a transformed Sun energy. Food we eat is the Sun energy, gas we burn is the Sun energy. Our star is a pure energy. It is huge comparing to our planet and it is the center of our Solar system.

Sun is really huge and heavy comparing to planets in solar system. Thats why it is the center. Image from: www.vjkrishna.com

Sun is really huge and heavy comparing to planets in solar system. That's why it is the center. Image from: http://www.vjkrishna.com

Where Does Sun Get It’s Energy From?

Sun mass creates a tremendous pressure in its core pressure is so high that hydrogen atoms are squeezed and transformed to helium. During this transformation a lot of energy is released and this is the energy which we see as a light, which warms us and which feeds all the plants on our planet. Hydrogen -> helium transformation is called a thermonuclear reaction. This is the kind of reaction we want to be able to reproduce on Earth, but failed so far. Well we did reproduce it in the form of hydrogen bomb, but that’s not what we really would like to have.

Hydrogen bomd destruction comes from the same reaction which powers our Sun. Image from: ovalscream.wordpress.com

Hydrogen bomb destruction comes from the same reaction which powers our Sun. Click for bigger version. Image from: ovalscream.wordpress.com

How Ideal Is The Sun?

In human history Sun was always associated with some God. It was always some kind of ideal object, that’s why it was so difficult in the beginning to accept that it has black spots on it’s surface. Just think about it: black spots on a god’s surface. Our mind passed a long way in understanding the world, something you take for granted today some time ago was a big deal.

Ideal Sun appeared to have dark spots. Image credit NOAA.

"Ideal" Sun appeared to have dark spots. Image credit NOAA.

In telescope Sun is not even smooth like it may look for a naked eye, it is granulated. They say granules come from the convection flows (hot masses rise to the top, cold ones sink to bottom).

Sun surface is not even smooth, its granular. Image credit: NASA.

Sun "surface" is not even smooth, it's granular. Image credit: NASA.

And one more thing defeats the “ideal & smooth round” Sun image — solar prominence. Our star has a very strong magnetic field and consists of plasma (it’s not the sci fi gun, it’s a state of material, for example the flame in your campfire is plasma too, basically plasma is a very hot gas).   Sometimes plasma is ejected from the Sun (it’s a constant super hydrogen bomb explosion inside of it, remember?) ejected mass often “hangs” on a magnetic string, because plasma consists of charged particles which interact with Sun’s magnetic field.

Sun plasma ejections are very often hang on magnetic strings, thats where their tricky shape comes from. Image credit: NASA

Sun plasma ejections (top right on this photo) are very often "hang" on magnetic strings, that's where their tricky shape comes from. Image credit: NASA

Sun Cycle

Many processes astronomy deals with happen in cycles. Earth has seasons, Moon has phases and Sun has a cycle too. It lasts about 11.2 years and on the peak of activity Sun shines about 1% brighter comparing to the low activity period. This cycle may be much more important than we currently think, because some studies found connection between solar cycle and global temperature, human health or even prices of wheat :-)

How Do We Study Sun?

Little update: I came across nice video from NASA with a lot of information about solar monitoring satellites. The cool thing is that they use real 3D model with real satellites and real time scale to demonstrate everything. You can see how satellites use planets to alter their orbits and stuff like that. Shortly speaking — video worth watching.

Read my previous article about Moon if you haven’t yet.

Introductory Astronomy: Moon

July 31, 2009

Where did Moon Come from?

Moon is the brightest object on the night sky. It has a great influence on our planet and living creatures (werewolves for example). Where did Moon come from? There are several hypothesis about it.

  1. Melted Earth was rotating fast and piece of it flew out and became a Moon. In this case though Moon should consist from the same stuff as Earth, but as we know it has much lower density. Apparently it lacks iron a lot.
  2. Moon formed somewhere else in the Solar system (some place without iron ;-) and was captured by Earth (Mars for example captured it’s satellites from asteroid belt). When we got samples from Moon they showed a lot of similarities with our Earth materials (for those who wants details: isotope composition was the same), so that’s where the third theory came from:
  3. Earth was hit by a big space body when it was young. A lot of debtis from Earth’s crust (crust has a small amount of iron comaring to core) flew into space and formed the Moon. This third one is the most accepted currently.
Five Hours After Impact, based on computer modeling by A. Cameron, W. Benz, J. Melosh, and others. Copyright William K. Hartmann

Five Hours After Impact, based on computer modeling by A. Cameron, W. Benz, J. Melosh, and others. Copyright William K. Hartmann

Moon Flies Away

Every year distance between Earth and Moon increases by about 1 cm (0.5 inch). Reflectors mounted by Apollo crews helped us to precisely measure distance to Moon and discover that it flies away.

Alien Bases on the Moon

Japanese lunar satellite Kaguya created gravity anomalies Moon map. What’s anomaly? We know how Moon’s mass and radius, so we know how strong gravitational force should be on it’s surface. In some places this force is higher than calculated value (this called positive anomaly and represented by red color on the picture below) in some places force is lower than calculated value (this called negative anomaly and represented by blue color on the picture below):

Moon gravity anomalies map by Kaguya. Red -- high gravity, blue -- low gravity. Image credit: JAXA

Moon gravity anomalies map by Kaguya. Red -- high gravity, blue -- low gravity. Left picture is Moon's side we see from Earth, right picture is the far side of the Moon. Image credit: JAXA

We all know that those anomalies mean alien bases under Moon’s surface, seriously, what else it could be :-)?

–> Read previous chapter about Earth.
–> Read next chapter about Sun.