The Death of a Small Star

A few weeks ago we talked about the birth of a star, how tiny pieces of matter start clumping together in very cold and dark regions of space, becoming a protostar, and blowing its shell out with the massive amount of energy from fusion.

A star is born in the darkness. It lives anywhere between millions of hundreds of billions of years, then the star dies just as fantastically. We’re new to this party, we’ve only been around for a tiny piece of history, because we are the sons and daughters of a star that died a long time ago.

Like in birth, the way a star dies has everything to do with how massive it is. We start with stars with a low mass.

Structure of an Asymptotic Giant Branch (AGB) star

Low Mass Stars (less than about 4 solar masses)

As a star burns, all the hydrogen in the core is being converted into helium. As the hydrogen is depleted, in a complicated sequence of events, helium will start to fuse together in shells and will start giving the star enough energy to expand. The star eventually becomes a red giant  - stars with very large sizes with relatively cool surface temperatures. The star’s core could be the size of the Earth, and its envelope be as big as the Earth’s orbit!

The Dredge-Up

Energy in a star is conducted in two ways: convection and radiation. Convection is what happens in a stove when you heat up water, the water at the bottom gets warm, loses density, and will “float”, and will be replaced with cooler water which will eventually heat up and rise. Water on top of the pot will cool off, get heavy, and sink to heat up again. This causes a “convective loop”.

Radiation is what makes you warm when a fireplace is running, the infrared radiation will excite the molecules on your skin and warm you up. It’s how the sun makes you hot in a hot sunny 4th of July beach day. Good thing you’re sitting on the water, because it’s really freaking hot. Great, now I want to go paddleboarding, but my body will never forgive me.

When a star starts fusing helium in the core, three elements are created: carbon, nitrogen, and oxygen. These elements then rise to the surface of the star through convection. A star’s life may have up to three dredge-ups, the third is very rich in carbon, and when a star undergoes this it’s often called a carbon star.

Mass Loss and Stellar Wind

So the star is now huge, and luminious, and gravity at the surface is very weak – the core is very low in density and spread out over a very large area. Any kind of disturbance would send star-stuff out into empty space. Remember that the outer layer has carbon, nitrogen, and oxygen? These stars actually blow soot into space.

It is this carbon soot that is the building block for our carbon-based life forms, and it’s this nitrogen and oxygen that you’re breathing right now.

Taste that air, feel your skin. You are experiencing what once was the core of a dying star, blown into space during a star’s death.

Planetary Nebulae

We’re going to use the previous picture to guide us for this one.

The helium burning shell is burning, pushing the hydrogen shell away, and creating carbon and oxygen that fall into the middle of the star. As the helium in that shell burns out, the hydrogen burning shell can crush into itself, and creating more helium, which falls into the temporarily dormant helium shell. After a certain point in temperature, a helium-shell flash will occur, which blows star stuff out of the star, increasing the luminosity of the star, and then starting the cycle again. These flashes happen every 100,000 to 300,000 years, and they give rise to the planetary nebulae. We discussed these some time ago, they are very beautiful objects that only live for a few ten or hundred thousand years or so.

The Ring Nebula M57, located 2,300 light years from Earth, is an example of a planetary nebula.

The Eskimo Nebula (or the Clownface Nebula) - notice the small, hot nucleus of the star in the middle. Super hot and about the size of our Earth, it is densely packed with carbon and oxygen in the center, and helium, and hydrogen in its outer shell.

As these stars blow their shells in a stacatto, their hot, bright centers are discovered further and further. These, in turn, become what are known as white dwarfs – hot, bright stars that used to be the heart of a very large red star. They are not very large, sometimes they’re similar to the size of the Earth. The odd thing about them is that the heavier they are, the smaller they are. This is backwards from what we’re used to, it’s due to something called electron degeneracy. To put it simply, in a white dwarf the size is supported by degenerate electrons. The more massive the star, the bigger its density, which creates an increase in pressure, and it overcomes the repulsive force of these electrons, so it shrinks.

It takes a loooong time for a white dwarf to twinkle out, they are very hot, but they are very dim.

This is the end of a star that’s less than four solar masses. It changes fuel sources, starts blowing its shell, increases in luminosity, and the core will start pulsating to release what’s left of it: a carbon-oxygen rich core that will cease to burn fuel and will become dimmer and dimmer over billions of years until it becomes a black dwarfs,  stars that have stopped emitting significant heat or light, objects of theory since the universe is not old enough to have produced any.

Soon: a large star dies very differently – you may have heard of it – a supernova. The most cataclysmic explosion since the Big Bang.

Categories: Astrophysics | 1 Comment

What did I get myself into? I was supposed to save money, but after a string of expenses I ended up paying the same as a cheaper paddleboard, and about \$200 more of what I could have gotten at Costco. This board is mine now, and when I come up with a design from the front, it’ll be mine forever.

Here is a brief journey from day zero to day, what, 50??

The Doppler Effect

Today we’ll talk briefly about a popular phenomena that few people understand: the Doppler Effect.

Sound and light are waves. The pitch of the sound and the color of light are a result of the frequency of these waves, that is to say, how many times the wave “peaks” in one second. This is otherwise known as “Hertz” – the musical note “A” is 440Hz, therefore it peaks 440 times in one second.

Visible light corresponds to electromagnetic waves between 400 and 790 THz, that’s Terahertz, or trillions of times per second. But there’s another way to look at it, the higher the frequency, the shorter the distance between the wave peaks. That’s what is referred to as wavelength.

The wavelength for visible light is 390nm to 750nm (that’s nanometers, or a billionth of a meter). The small wavelength corresponds to the highest frequency, because the shorter the distance between peaks, the more times it “peaks per second”.

Visible light colors by wavelength, in billionths of a meter.

Distilling the above diagram to break down the waves into what they individually (sort of) look like,

Lower frequency waves (red) have longer wavelengths. High frequency waves (magenta/purple) have shorter wavelengths.

So there’s an inverse relationship between frequency and wavelength, and it is expressed by a very simple equation:

$f = \frac{c}{\lambda}.$

Where f = frequency, c = speed of light (300 million meters per second), and the funny squiggly line (lambda) is wavelength. For example, if lambda was 1 meter, frequency would be large (300000000/1, which equals 300000000 Hertz, the frequency VHF TV signals). If we increase the wavelength, to say, 100 million meters, then the frequency would be very small (300000000/100000000, which equals 3 Hertz, the frequency of ELF waves). So when one goes up, the other goes down. This is why algebra is important, people, a lot of the theory is accessible to you with only a basic understanding in mathematics.

We are assuming, however, that the source of these waves is static. But everything is moving. What happens if something is approaching you, or if something’s getting further away? What happens is the Doppler Effect.

If you're in front of the red particle, waves seem to be of a higher frequency and shorter wavelength. If you're behind it, it looks like the waves are of a lower frequency and higher wavelength.

This is readily apparent when a police siren zooms by you, as it approaches, the sound has a higher pitch, but when it passes you, the pitch seems to drop to a “lower note”. The wavefront has passed you and now the waves are “stretched out”.

Well, it just so happens, light behaves the exact same way!! Since red is a lower frequency than blue, when objects race away from us they turn just a tinier bit red (because the frequency is lower on the back end), and objects coming close to us turn a tiny bit more blue!

You may be asking yourself: “to know that something is a tiny bit more red,  you’d have to know its original color. This stuff is remarkably far away from us. who do you think you are, you loon??”

I actually have an answer for you. Every element gives off a signature color band. Just like gold is always yellow, Hydrogen and other common space elements have a signature “light spectrum”, and if this signature shifts all the way to the red a little bit, we know how fast this Hydrogen (or whatever it is) molecule is getting away from us. Here’s a great graphic I found on a certain open-source collaboration wiki that’s very popular in the internet and teachers absolutely hate:

Redshift of spectral lines (the little black bands) in the optical spectrum of a bunch of galaxies (right) as opposed to our own sun, which we know ain't going nowhere (left). Notice how the galaxies on the right have spectral bands that have moved up towards the red.

ALAS we come to the point of the blog post: everything in the observable universe IS SHIFTED TOWARDS RED. Only ONE THINGS could explain this: THE UNIVERSE IS EXPANDING EVERYWHERE.

I’m  going to blow your mind one  more time: the bigger the “redshift”, the faster the object is getting away from us. It just so happens that the further away something is, the bigger the redshift, which means that farther objects are going faster than closer ones. THE UNIVERSE SEEMS TO BE EXPANDING EVERYWHERE AND ACCELERATING TOWARDS THE EDGES. This is described by Hubble’s Law using Hubble’s Constant. We can look at an object, know how fast it’s going, and therefore know how far away it is.

We have just taken a look at the sun, then we look at a distant star, and just from their colors we can discern whether it’s getting away from us or coming towards us. When we look pretty much anywhere, the same thing is happening. We even realized that the universe is ballooning at an exponential rate, and we can use this to infer distances in the range of millions of freaking light years.

Humans may be tiny and meaningless specks of dust living on a slightly bigger speck of dust in the middle of a bright speck of dust amongst a cluster of beautifully colored specks of dust. Yet we have managed to unlock some of the biggest mysteries just by looking at old light from minutes after the Big Bang occurred. Avoiding an anthropocentric ego-trip, there’s something special about us, but it’s not necessarily in the way we handle our business and treat ourselves, but in how sometimes we achieve what seemed to be once impossible.

Categories: Astrophysics | | 2 Comments

What is Anti-Matter?

Anti-Matter! Stuff from legend, science fiction, both good and terrible books in the strange section at Barnes and Noble, is it real? What’s it like? Why is it so hard to find? And why is it so uncommon?

These questions, and more, summarily answered by science’s hive mind. I’m only but one small remora sucking from the side of the massive whale of knowledge.

Anti-matter, strictly speaking, is made of anti-particles. There’s an anti-version of every existing particle out there, leading to atoms of anti-particles. Hydrogen has anti-Hydrogen, Lithium has anti-Lithium, and so on.

Anti-matter (and anti-particles, but from now on I’ll just refer to them as the former) behave in exactly the same way as their opposite particles. If a baseball was made of anti-leather, anti-hide, and anti-rubber (or whatever baseballs are made out of), it would fly just the same as a regular baseball, it would look the same, it would weigh the same. It would be near impossible to detect what kind of particle it’s made of. In fact, we could even say we’re made of anti-matter, and that regular matter is very hard to detect.

How Do We Even Know it Exists?

We’ve observed it, and very recently (at the CERN particle accelerator in Switzerland) we were able to create many anti-Hydrogen atoms and held them in place for about 17 minutes (nature.com). This is a lot longer than previously achieved in other experiments, and now we can do some really good science on these observations.

The anti-particles eventually escape their lock-down and annihilate with particles around them, giving us otherwise-unexplainable flashes of light. So here we are: we have Hydrogen locked in a chamber, and it looks like regular Hydrogen, and behaves the same way, weighs the same, but then it disappears in a flash of light. It ain’t magic.

So How Do We Know it’s Anti-Matter?

Matter and anti-matter annihilate when they come in contact. Annihilation is the conversion of these particles into gamma (high energy light) and other particle-antiparticle pairs. To know that a anti-matter makes up a baseball, we’d have to throw it into something made of matter and see if it blows up in a fantastic flash of light. Chances are your hand is made of matter too, so even holding it would bring some unfortunate (but awesome) consequences.

How Do We Know the Earth is Made out of Regular Matter?

If there was anti-matter in the Earth, it’d be annihilating with the surrounding regular stuff, and we’d be able to detect it. But so far nothing has really been observed, with one exception: it was just recently discovered that anti-matter thunderstorms create anti-matter above them (bbc.co.uk).

So If it’s So Similar, Then Equal Amounts Were Created in the Big Bang!

Not quite so, it seems. Anti-matter is a lot less prevalent in the observable Universe as matter is. Why? No one knows for sure (actually, do we *really* ever know *anything* for sure?)

Baryogenesis is the process by which there’s a population difference between the two kinds of particles. Baryons are the names of the elementary particles that created everything we know after the Big Bang, so the question is, why were there more Baryons than anti-Baryons? Hint: the most popular Baryons are protons and neutrons.

You HAVE to Have Some Practicality to This.

And there is – anti-matter is used in the medical field for PET imaging, and recently it’s been found effective in the treatment of some cancers (engr.psu.edu).

No, it’s not currently feasible to make anti-matter weapons, Dick Cheney. Nice try.

The Universe continues to marvel us, the deeper we look, the more we realize that we have no idea about where we come from and where we could end up, and the things that we could achieve. When you hold something in your hand, you’re not holding just a coffee mug full of hot liquid, but a microcosm of particles that were created and survived many billions of years ago. It’s a bit much to think about, at times overwhelming, but it’s important to have the presence of mind to realize that we’re just a thread in a massive weave of existence.

Resonance and Harmonics

Acoustic resonance is the increased tendency for an object to vibrate at a particular frequency that it’s configured for. For example, a guitar string tuned to E will vibrate on its own when it “hears” another E – you don’t have to strike the first string in order for it to move.

Harmonics are multiples of a base, or fundamental, frequency. The “A” note is known to be 440Hz. Another note that plays with A that is a multiple of 440Hz is a harmonic – 220Hz, 110Hz, 880Hz.

In music, resonance and harmonics are used to create complicated orchestras and symphonies that defeat our understanding of nature. We can always describe a musical note as a wave, but no book in science truly understands the effect that music has on us.

My point is – we don’t have to be in similar frequencies to understand each other, nor do we have to wait for someone else to take action – we play our string so others’ start moving on its own.

Categories: music | Tags: , , | 5 Comments

The new LHC particle accelerator in Geneva is operating at 480 bunches per beam. A bunch is about a hundred billion protons, each one carrying the energy of about a mosquito in flight. When put together, 480 of these bunches have the energy of a 20,000 ton aircraft carrier going at 3 knots. When the LHC is at full capacity, because it’s actually designed to run at 2808 bunches per beam, the beam will have the energy of the same carrier going at 13 knots. Imagine a carrier going at 13 knots and hitting just about any object. I can’t imagine any man-made object that could resist such an impact.

To imagine how many protons are in a bunch, if these protons were each the size of a marble, a bunch would be as long as the distance between Earth and Uranus, and about as wide as the distance between the Earth and the Moon. They are so far apart that the distance between the marbles would be from Miami, FL to Charleston, SC. These bunches are made to collide against each other to cause proton-proton interactions, but they’re so far apart that we only get a few events per beam.

Particle physics is a statistical game, so we have to get a bunch of events and samples in order to get results. The LHC has already exceeded its expectations, and it’s not even at a fifth of its capacity.

For more pictures of the LHC during construction, see the following: http://www.boston.com/bigpicture/2009/11/large_hadron_collider_ready_to.html

Categories: particle physics | | 2 Comments

The Moon’s Locked On

Ever notice that we always see the same side of the moon? If all planets rotate, and all moons rotate, and even the sun rotates, why is it that our moon doesn’t seem to?

In fact, it does, but it’s our fault we only see one side. The moon is in Tidal Lock with the Earth.

Ride the Tide

We all know the tide to be something very specific: the water in the ocean goes up and down throughout the day. It is caused by the gravitational tug of the Sun and the Moon, when the Sun is directly overhead, there is a tendency for water to rise a few feet in some locations. If both Sun and Moon are overhead, then we get the Spring Tide when levels are particularly high.

What’s that Got to do With the Moon?

Interestingly enough, the Moon suffers the same ailment, and to the Moon, we’re its Sun.

Many many (MANY) years ago, when the moon was originally created, it was rotating just like any other satellite. Over time, however, the surface of the moon was deformed ever so slightly, made oblong, with the pointy end facing the Earth:

As the moon rotates, it keeps this bulgy shape, but now the pointy end isn’t pointing towards Earth anymore. Earth’s gravity will try to reshape the moon so the pointy end points towards the Earth, but since the Moon is made out of rock, that’s not so easy.

The result is that the Earth produces a force that tugs on that bulge, into something called Bulge Dragging. (editor’s note: searching for “bulge dragging” images on Google can be a disturbing experience, I ended up settling for a Wikipedia image.)

The Moon spins, but it has a bulge the Earth can tug on

In the top green Moon, the Earth’s gravitation re-shapes its surface. In the bottom one, the Moon has rotated to its red configuration, but Earth still tugs on the bulge (creating torque), and tries to keep the bulge facing towards itself. Since the tug is stronger than the Earth’s ability to re-shape the Moon, the Moon slows down more and more until one day it stops giving us a different face. If the moon were made out of water, it would be so easy to re-shape the bulge that the Earth would never have a sideways bulge to tug on!

So in a nutshell, our intense gravitational field made the Moon into a bulgy shape, giving us a bulge to tug on. Since the Moon is hard, it’s not easy for that bulge to flow across its surface, we exerted torque on the Moon until we eventually left it permanently oblong in the same direction, and we shall never see the other side of the Moon from Earth.

Libration

The moon is not perfectly still to our eyes, however, because all of this we just described is true to the center of the Earth, and we’re really just on its surface. Also, the Moon has an eccentric orbit, and our spin direction and the Moon’s are different. This effect has us see slightly more of the Moon’s surface over the course of time, but over a long period of time, we can see almost 60% of the Moon (so that’s an extra 10%). Here’s a simulated image showing Lunar Libration:

So, What’s the Other Side Look Like?

The other side is interesting: it’s seen too many space collisions. This makes sense, since the part that faces us is a lot less likely to be hit by asteroids or comets.

It’s nothing like what we’re used to. It was only recently photographed. We call it the Far Side of the Moon. (Note to Pink Floyd fans: there’s no “dark side” of the moon, all sides are equally lit as the Moon travels around its orbit.)

Categories: Astrophysics | | 1 Comment

A Map of the Night Sky

Have you ever wanted to be able to look at the sky and know right away what it is you’re looking at? If you’ve spent 5 minutes with me outdoors at nighttime you may have seen me point out over two dozen bright stars in the sky. You’d be surprised to know how easy this is to accomplish. I don’t memorize a map of the stars, instead, I’ve learned what the relative positions of stars are. During the winter, I use Orion, and in the summer, I use something else. That is what we’re here for today.

If you wait ’til nighttime, you can look into the sky and see one of the most recognizable constellations in the sky: Ursa Major, otherwise known as the Big Dipper.

The Big Dipper is special, everyone knows what it is, and it is a roadmap for the rest of the sky. If you know a few points on the map, you can point out several other constellations as you become more familiar with the spring-summer night sky.

A simplified roadmap to the late spring/early summer sky

The Big Dipper has a long handle, a top and a bottom to the pot, and two sides: one towards the handle and one away. Let’s learn some of these rules and then use the two images below to help you find these stars, that way you don’t have to go outside and get used to night vision or stand in the bitter cold. Use the drawing on top to get a quick familiarization, then find Ursa Major in the next three drawings for an accurate depiction of the night sky. Now let’s try some quick and simple exercises. (hint: the first star in the Big Dipper is named Alkhaid. This will assist you in finding it. If the black screenshots are too small, there’s one with a green border at the bottom that should assist you).

• If you follow the arc handle starting from the pot and going away, you end up in Arcturus. It is commonly known as “Follow the arc to Arcturus”. It is the major star in the Bootes constellation.
• If you keep going along that Arc, you’ll come against another bright star: Spica, the brightest star in the Virgo constellation.
• Follow the star connecting the handle to the pot, and go down from there to the bottom star in the handle-side of the pot, and you eventually make it to Regulus, the brightest star in the Leo constellation.
• With the handle-to-pot star, and going to the opposite star in the pot, you go straight to Pollux: one of the two (and the brighter of) the Gemini Twins. The other is Castor.
• A very important one: follow the two pot stars, opposite the handle, going “up” the pot: this goes straight to Polaris, the famous North Star. It’s not super bright, but it’s bright enough, and it’s also the first star in the Little Dipper. I like to remember this as “The Pot Always Spills North” (I made that up, which is why it’s kind of a lame mnemonic)
• If the first star in the Big Dipper handle is a certain distance away from the Gemini Twins, at the exact “opposite” side of the Twins is Betelgeuse: Orion’s Shoulder. This is a red supergiant that is ready to supernova in the “near” future – you and I may have been dead for hundreds of thousands of years before this happens, but when it does, it’ll outshine the moon for a few days.

See what other patterns you can discern, and make your own rules. Go out one night and see if you can spot just one of these. If you do this occasionally, before long, you’ll know all of the brightest stars in the sky.

A screenshot of the Stellarium software, free for download from http://stellarium.org! My apologies if it's all too small, it was a challenge to fit all of this in here :)

Same Stellarium screenshot, this time with constellation lines

Again the same screenshot, but this time it's got art.

A slightly more technical shot of the night sky around these times

Categories: Astronomy | Tags: , , | 2 Comments

Nebulae – Clouds Alight in a Deep Ocean of Darkness

We took a brief look at nebulae, but some of you fed back to me that we didn’t take far enough of a look. A nebula, as explained a few days ago, is a big mass of gas and dust that either reflects or fluoresces light. They are a showcase for the stars that inhabit and surround them, much like a beautiful personality makes a stunning woman even more brilliant.

In this post, we shall look at some of the most beautiful nebulae ever witnessed, both from the ground and from our in-orbit superstar, the Hubble Space Telescope.

There’s many other amazing things to witness through the telescope eyepiece, like the Magellanic cloud and the star cluster, but those shall have to wait.

The Eagle Nebula

The Eagle Nebula is part of a diffuse emission nebula, if you recall from a few days ago, emission nebula shine because a nearby star ionizes the gas and excites its electrons, and when they de-excite they give off light via fluorescence. The most famous portion of this nebula is called “The Pillars of Creation” because gas-forming dust is part of the tips of these pillars, and if you also recall, these are small chunks of dark nebula where the protostars form.

The Pillars of Creation

The Triangulum Emission Garren Nebula

Another emission nebula where the gas is ionized by a cluster of stars in its midst. This nebula is a region inside the Traingulum Galaxy, and it exists 2.7 million light years away. That is, by any measure, extremely far away. It is so bright that if it were equally far away from us as the Orion Nebula it would outshine Venus, the brightest object in the sky after the sun and moon. What a sight to behold:

The Triangulum Emission Garren Nebula NGC 604

The Crab Nebula

The Crab Nebula is a supernova remnant that has a strong pulsar in the center – a rapidly spinning neutron star that acts very much like an x-ray lighthouse (specifically, this one spins about 30 times per second, about as much as an old phono record, and measures about 30km across, about as much as a a small city). After a star is destroyed in a cataclysmic explosion, the new elements that are manufactured from the explosion are blown away further in pulsar winds and are ionized by this high energy flux.

The Crab Nebula

The spinning star in the center requires some special attention, because it is so freaking cool. How can something so large and so dense spin so fast? Imagine a mass of several suns, in the area of a small city, spinning 30 times per second. Its magnetic pole is putting out the x-ray and gamma radiation, which is the source of its lighthouse beam. Remember Emily Noether and her conservation of angular momentum? Surely you’ve sat in a chair, with arms and legs stretched out, and started spinning slowly. As soon as you contract your extremities your spin becomes much faster. This is conservation of angular momentum – this star went from being very large to very small, and has essentially sped itself up by shrinking millions upon millions of times in size. Here is a picture of this monstrous beauty:

The Crab Neutron Pulsar

You can see the direction it sweeps in, the gas ionized in a disc by the super intense x-ray and radio radiation being emitted from the pulsar. It’s a good thing we can look at it from this far away: the radiation created from this pulsar would wipe all life from our planet in the  blink of an eye.

This is the sort of thing that’s left over after all the atoms in our body were created.

The Flame Nebula

Another emission nebula, this time in everyone’s favorite constellation Orion. It is lit by the easternmost star in the belt, that is to say, if you’re looking at the belt and Betelgeuse (the red supergiant) is on top, and Rigel (the superhot blue) is on the bottom, it’s the leftmost of the belt.

The dark strands in the middle are dark nebula, that tenuous place where the building blocks of life are put together for the first time.

The Flame Nebula NGC 2024

Planetary Nebula

The Cat’s Eye Nebula, pictured first, is actually a planetary nebula: a star lost its envelope via a process called mass loss. The star runs out of hydrogen to burn, so it stars fusing other materials, then it switches back, and these thermal pulses shoot out in intervals. The star essentially ejects its outer shells and atmosphere into some of the most complex celestial constructs ever witnessed by our species.

In the second picture, the concentric circles from this thermal shock are processed to make them more obvious. It’s important to remember that not all of these pictures are what you would see if you peered at the nebula, sometimes exposures are hours long to gather as much light as possible, and sometimes invisible colors are recolored to show structure.

Discovered in the 18th century, these cosmic butterflies were named for their resemblance to gas-giant planets. Planetary nebulae are actually the remains of stars that once looked a lot like our sun. When sun-like stars die, they puff out their outer gaseous layers. These layers are heated by the hot core of the dead star, called a white dwarf, and shine with infrared and visible-light colors. Our own sun will blossom into a planetary nebula when it dies in about five billion years.

The Cat's Eye Nebula NGC 6543

The Cat's Eye Nebula, processed to show its concentric rings from thermal shock, or so we think, sort of.

The Ring Nebula M57, located 2,300 light years from Earth, is another example of a planetary nebula.

This infrared image from NASA's Spitzer Space Telescope shows the Helix planetary nebula, located about 700 light-years away in the constellation Aquarius. Spitzer's infrared view of the Helix nebula, the eye looks more like that of a green monster's. Infrared light from the outer gaseous layers is represented in blues and greens. The white dwarf is visible as a tiny white dot in the center of the picture. The red color in the middle of the eye denotes the final layers of gas blown out when the star died. The brighter red circle in the very center is the glow of a dusty disk circling the white dwarf (the disk itself is too small to be resolved).

Protoplanetary Nebula

Protoplanetary nebula are the period during the asymptotic giant branch event when the initial mass loss starts the planetary nebula formation. What I’m saying is, a protoplanetary nebula is the precursos to the planetary nebula we just saw.

The Boomerang Nebula and the Egg Nebula are excellent and beautiful examples of this: stars are currently ejecting themselves as they form into things of the sort of the Cat’s Eye Nebula. These objects are dazzling and many times defy logic.

Boomerang Protoplanetary Nebula

The Egg Protoplanetary Nebula

See? Even stars suffer from flatulence.

False Color Imagery

Earlier I mentioned that most astrophotography has to be enhanced in order to bring out the detail, structure, and mystery from many of these celestial bodies to light (pun intended). Here’s an example of an emission nebula, the Pelican Nebula (also known as the North America Nebula due to its shape). This is a very active nebula in both formation and star generation, so in a few million years it’ll look like something completely different.

Depending on what portion of the light spectrum you look at, you see different features of the Pelican Nebula. Only the "visible" shows "true" color.Of particular notice is how visible light can't see many features infrared can - especially protostars.

And That’s Just a Tiny Fraction Of It

There are so many trillions upon trillions of galaxies, and each of them holding million of nebula, all unique, a true sight to behold. The telescope in my balcony can only see a few of these, for example, the Crab Nebula looks like a little swath of white light. I’d have to tether my camera to the eyepiece for an entire hour, while my telescope tracks the sky with specially built drive motors, in order to get an exposure that would bring out the detail in its structure.

Now that we have some of these supertelescopes in orbit we are able to witness the universe in a new light, free from atmospheric turbulence and interference from light pollution and our own sun and moon. The next generation of orbital telescopes are on their way to space right now, and we shall peer even deeper into the incredibly rich 15 billion year history that precedes us.

Classifying Stars – Luminosity, Brightness, Temperature, and Color

Stars come in many different colors and sizes!

A few days ago, we talked briefly about how stars are made. But no two stars are identical: they can vary in size, internal pressure, temperature, composition, age, mass, mass loss, density, spectra, brightness, and luminosity. Today we’ll talk about some of their visual characteristics. Tomorrow, we’ll talk about their types, and how these two are related.

Luminosity and Brightness

Luminosity is the amount of energy a star gives off each second. It is measured in watts (just like the light bulb). Brightness is the apparent energy that reaches us, and it depends on distance. The farther away a star, the less bright it looks. It is measured in watts per meter squared.

brightness = luminosity / 4*pi*distance2

If we measure the brightness of a star via equipment on Earth, and we can figure out the distance using methods such as parallax (which we also talked about a few days ago), we can figure out the luminosity of a star! This is the first step towards identifying what kind of star we’re looking at.

luminosity = brightness*4*pi*distance2

Color

The color of a star is determined by its surface temperature. Kinda like the flame you see on occasion, we know that the red flame is the lamest, followed by the yellow flame, so on until the blue flame which is the hottest. Same with stars, except many times hotter. Different than a flame, however, is that colder stars can emit in the infrared (and therefore invisible to the human eye), and really hot stars emit mostly in the ultraviolet (which is equally invisible). 70% of the stars in our galaxy are in the infrared, which is why we have infrared telescopes, and they must doctor their images to false color so you and I can marvel at their artificially natural beauty.

A star actually emits at many wavelengths, for example, our sun emits across the visible spectrum but we all know that it also emits UV (which is why we wear lotion in the beach) and the infrared. The hotter the star, the more it emits in general, with a peak at a certain wavelength. Observe the following charts,the colored bars represent visible spectra.

Stars emit their peak energy at different wavelengths depending on their surface temperature (shown in the Kelvin scale). The hotter the star is, the bigger the area under the curve (therefore putting out more energy in its photons) and the peak energy shifts to the left towards the ultraviolet. Lower energy stars emit less energy, and peak towards the right.

Our sun is a bit like the chart in the middle, it puts out light at all frequencies in the visible range at about the same rate, with a slight peak in yellow, which is why it’s kind of yellow.

Some stars are SO HOT at the surface that they put out nothing more than x-ray radiation. They are so dense that all they’re made out of are neutrons, with the mass of several suns compressed into the area of a small city. If you held a chunk of this in your hand (assuming you could) and you let it drop, it would crash through the ground, shattering rock and iron until it cut its way into the center of the Earth, overshoot it, come back, overshoot again, until being assimilated into our iron core. How will I ever explain this to the insurance agency?

Wien’s law states that the peak wavelength is inversely proportional to temperature, and it gives us a number in nanometers.

wavelengthpeak = 2900000nm/Temperature

There You Have It

So this is why stars look different colors: they are different temperatures. Using some back-of-the-envelope math, we can figure out the surface temperature and emission spectra of stars. It’s very reliable too – so when things don’t match up, we can ask followup questions to the nature of these fusion behemoths. Tomorrow we’ll put more of this together and dive deeper into the mystery of our stellar ancestors.