Astro-Lesson: Supernova

This week is going to be about one of the most massive explosions in the Universe: Supernova. Last week, when discussion the star life cycle, I had mentioned supernovae* briefly as an outcome of the death of super massive stars. But that is only one kind of supernova, there happens to be two types which are further subdivided.

 *The plural of supernova is supernovae.

So, get ready to learn a little bit about supernovae and some see some awesome pictures.

The Crab Nebula, the remnant of a supernova recorded by Chinese astronomers in 1054. (Credit: NASA)

A supernova is an extremely luminous explosion of a star with a burst of radiation that often briefly outshines the entire galaxy in which the star resides. It can take several weeks or months for a supernova to fade, over this time it can emit as much energy as the Sun over its whole life span. The explosion expels most, if not all, of a star's matter into space, creating a shock wave. The shock wave sweeps up dust and gas from the star an the interstellar medium, creating at supernova remnant. These remnants are usually what you see in images from the Hubble Space Telescope and other telescopes.

Supernova 1994D (the bright 'star' on the bottom left) in Galaxy NGC 4526. (Credit: HST/NASA/ESA)

The word nova means "new" in Latin, referring to what appears to be a bright new star in the night sky. Occasionally these explosions cause what appears to be a new star in the sky. The prefix 'super-' separates a supernova from an ordinary nova, which also involve a star increasing in brightness, though to a lesser extent and through a different mechanism.
 
Types of Supernova: there are two basic types of supernova, and a couple of other distinctions:

Illustration of different ways a supernova is formed. (Source)

Type Ia: These result from some binary star systems in which a white dwarf absorbs matter from a companion. (What kind of companion star is best suited to produce Type Ia supernovae is hotly debated.) The idea is that so much mass piles up on the white dwarf that its core reaches a critical density that results in an uncontrolled fusion of carbon and oxygen, thus detonating the star.

Type Ib and Ic: These ones look similar to Type Ia when looking at their spectrum, but are distinguished because they lack certain different lines of spectra. The lack of spectra means that elements in the core may have been lost due to other means, and in general Type Ib/c may be referred to as stripped core-collapse supernovae. These types of supernova are also incredibly rare, and their sources might also be the progenitors of gamma ray bursts.

The onion like layers of a star's core before going supernova.

Type II: A common supernova type, usually found in the spiral arms of galaxies and not in elliptical galaxies, they are distinguished by the presence of hydrogen in their spectrum. These are known to be caused by the rapid collapse and violent explosion of a massive star. There exists several subcategories of Type II supernova, including II-L, which has a steady, linear decline in light over time; II-P which has a slower decline (a plateau) followed by normal decay; IIn, the "n" denotes narrow, which have narrow hydrogen emission lines in their spectrum (probably caused by blue variable stars); and IIb, which initially resembles a Type II supernova but later has a spectrum resembling Type Ib.

A different image of the Crab Nebula, a composite image of the radiation in the nebula's core region. A pulsar (a type of neutron star) can be seen near the center, pushing out jets of particles. (Credit: NASA/HST/CXC/ASU/J. Hester et al.)

Those are the general types of supernova known to astronomers. After the explosion the cores are left behind and usually create a neutron star or a black hole. Supernova are pretty rare events in a galaxies, the Milky Way experiences one about every 50 years, though the last one seen from Earth was in 1604. This last one was known as Kepler's Star and was easily visible in the night sky, brighter then all the planets except Venus. It was visible during the day for over 3 weeks.

False-color of the remnant of SN 1604 (Kepler's Star). (Credit: HST/NASA/ESA)

There are several large candidates in the Milky Way that might go supernova in the next million years, these stars include VY Canis Majoris, Betelgeuse, and Eta Carinae. Once these stars explode, they will provide a vital part in stellar evolution. Supernova explosions are the source of many heavy elements including uranium and plutonium. All of these elements get shot out into space and form clouds of dust that eventually condense and form new stars, or the shock wave can trigger star formation in an already present cloud. This is likely the cause of formation for our own star. This includes the Earth and us. We owe our existence to these violent explosions, the death of a star. As Carl Sagan would say, "We're made of star stuff."

Thanks for reading! This was my 100th post on this blog. It hardly seems like it, I only started back in February. I really have enjoyed writing these posts for my readers, and I enjoy the feedback. I hope you have learned some new things about astronomy from what I've written, or at least enjoy the pictures. I think supernovae are a good way to celebrate. Thank again for reading and I plan to continue this for as long as I can.

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Posted in Astro-Lesson,Astronomy,Neutron Stars,Stars,Supernova 18 comments

Changing Things Up.

So, my schedule has changed around and I don't think I will be posting the Astro-Lessons on Tuesday afternoon anymore. Mostly this is because I play Frisbee with a bunch of friends.

We had been playing on Thursdays, but Tuesdays seem to be working out better. Unfortunately, I used to spend most of my Tuesday putting together a good Astro-Lesson, but this week I just didn't have the time and no idea what to write really pulled at me.

So, I am thinking of moving the Astro-Lesson topics to Wednesday afternoon. And I have a good idea what I want to write for the next one. I'll also be bringing the poll back, it was just easier to do the last few ones without it. I felt that the star life cycle was a natural progression after the Sun. And tomorrow, to continue on that, I am going to do supernovae. It will also be my 100th post, so I figure I'll celebrate with a BANG!

In addition to that, for the rest of the week, I might only be posting every other day or so. It's going to be pretty busy with our party this weekend. Once again, the real world takes precedence over blogging, but I am totally excited.

No new astronomy or science-y stuff from me today! But if you really want to learn something, I suggest reading this NASA article on the Alpha Magnetic Spectrometer that Space Shuttle Endeavour is delivering to the International Space Station. I know some people have asked questions about it, what it is, and what it does: http://www.nasa.gov/mission_pages/shuttle/main/amsprocessing.html

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Posted in Blogger,internet,NASA,personal,Supernova 31 comments

Astro-Lesson: The Star Life Cycle

This week's topic kinda picks up from last weeks topic, the Sun. From our short and limited view of the cosmos, we have been able to discern something amazing, the life cycles of stars. It turns out stars have a birth, then long lives of constant work converting hydrogen to helium and other elements, and then they die, either by withering away or massive violent explosion.

Art of the life of a Sun-like star.

 There is a significant reason why knowing this is important. A star can last for BILLIONS -- or even TRILLIONS!-- of years, and yet humans have only been around for about 200,000 years, but a brief moment of any star's life. But if it isn't possible for us to observe the entire life span of a star, how have we figured it out? Well, there are lots of stars out there, of all different ages that show features that relate to other stars, this allows us to work with something to make a model out of.

The best analogy I can provide is this: Imagine being in the woods, deep in a seemingly infinite forest that stretches to the horizon. You are surrounded by trees, while you are stuck to a tree yourself, the only one that you can see up close. Your tree appears to be a sturdy adult tree, analogous to our Sun. Looking out into the forest you see a variety of trees; without knowing the life cycle of the tree, they may not seem otherwise connected. You have seeds on the ground, saplings sprouting up, small trees, big trees, really big trees, and rotting logs on the ground.

Sagittarius Star Field - Our Forest.

This analogy can go further; those rotting logs provide subsistence for the next generations of trees, much like supernovae provide matter for new stars. And occasionally you might hear the crash in the distance of a tree falling, much like a supernovae or gamma ray burst going off. And what lies beyond the horizon remains a mystery since you are tied to one tree, just like massive size of the Universe.

Now the trick is, you have only a minute to gaze out into this forest to try discern what is what. And that brings the analogy home with what humanity has been able to accomplish. In our brief glimpse, we have been able to identify saplings, healthy full-grown trees, various species of trees, and dying or dead trees, of course in stellar terms. So lets now look at a few of these stages of the life of a star.

Concept art of the birth of a star in a molecular cloud. (NASA)

Protostar: This is where a star begins to form. A molecular cloud or nebula of dust and gas that starts "clumping" together. As the atoms gather, their gravitational attraction pulls in more atoms, making a larger "clump". This process is known as accretion. Lots of reactions happen inside an unstable protostar, which can have influences on a possible planetary system. Once the protostar achieves and maintains equilibrium, balance between gravity pulling atoms toward the center and gas pressure pushing heat and light away from the center, it becomes a star.

Not all stars are born equal! Stars are born into a variety of sizes and colors, depending on their composition, mass of formation nebula, and temperature. These stars lead vary different lives, of varying length, and die in dramatically different ways.

Small-Medium Stars: The overwhelming majority of stars, at least in the Milky Way, fall into this category, including the Sun, up to 1.5 times the mass of the Sun. These are amongst the longest living stars, the Sun is predicted to last 14 billion years, and it is roughly 4.6 Billion years old now. Smaller stars than the Sun can last billions of years longer. The smaller a star is, the longer it will live.

Hertzsprung–Russell diagram, commonly used to classify stars.

The majority of the life of a star will take place during a phase called the main sequence. This is when the star fuses hydrogen to produce helium in high-temperature and high-pressure reactions near the core.

After a small star like the Sun burns up all of its hydrogen, it leaves the main sequence. It begins to burn helium instead, converting helium atoms into carbon. The star loses its previous equilibrium, so in order to maintain it and keep cool, the star 'puffs out.' The star is now called a red giant, and is the first step in old age. The Sun may expand out to roughly the distance of Earth, 250 times its current size!

A red giant is very unstable, and may expand and contract, these stars are known as variable stars. This period if very short, only lasting a couple million years over the billions of years in a star's life. Soon the helium will burn up and and the star will again change as it switches to the last phase of fusion -- carbon burning.

The Cat's Eye Nebula is a planetary nebula. (NASA)

When a star switches to fusing carbon, the core contracts inward, down to about the size of the Earth. The outer layers of a star are blown off into space into a planetary nebula. Often beautiful to see from Earth, these are the dying throws of a star. The star becomes a white dwarf, densely packed, but not massive enough to become an neutron star or black hole. A white dwarf burns slowly and will gradually fade into a black dwarf, a cold dark mass. However, the Universe is not old enough for any black dwarf stars to exist yet.

A VERY Giant Star (the biggest known)

Huge and Giant Stars: These are stars much more massive then the Sun, with drastically shorter lifetimes. There are two classifications here based on the predicted end results of the star. Here we will call a star that is between 1.5 to 3 times the mass of the Sun a huge star; and a star greater than 3 times the mass of the Sun will be a giant star.

These stars spend a relatively brief period on the main sequence, since they are larger and burn hotter, they burn their hydrogen quickly. When they begin helium burning, they enter a special sort of phase, they become red supergiants, the largest known stars in the Universe by volume.

The onion-like layers of a massive star just before core collapse. (Not to scale.)

These stars are so large, they begin to fuse elements in an onion like structure near the end of their lives. Fusion of elements all the way up to iron on the periodic table is possible before a core collapse. The core of the star essentially is unable to support its weight against gravity. A massive explosion then occurs, a supernova, which violently blasts the upper layers of the star into space, leaving behind the core.

One of two things happens to the core: If it was a huge star, the core becomes a neutron star. If it was a giant star, it becomes a black hole. You can read more about black holes and neutron stars in one of my older articles.

The Crab Nebula, the remains of a supernova first observed around 1050 AD.

Tiny Stars: A special case in stellar physics, these are stars smaller then 0.5 times the mass of the Sun. These stars never fuse helium into higher elements, they do not have the mass to exert the pressure on the core. They are known as red dwarfs, like Proxima Centuri. These stars can exist on the main sequence for up to trillions of years and have life spans longer then when the Universe is predicted to end. They may eventually fade to white dwarfs, but there is no real way of knowing.

That accounts for an overview of star life cycles. I hope this helps you to understand some of the differences between stars and their end results. Astronomers were able to piece this together looking at the clues that the Universe has given us, from our brief glimpse. We have been able to determine the amazing course of existence for the very things that gave us life.

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Posted in Astro-Lesson,Black Holes,Main Sequence,Neutron Stars,Planetary nebula,Red Giant,Star Life Cycle,Stars,Sun,Supernova,White Dwarf 34 comments

Astro-Lesson: Gamma Ray Bursts.

First: Good to see that I got some votes out of you all and looks like Gamma-Ray Bursts just barely won. I'll have a new poll up tomorrow I suppose for next weeks Astro-Lesson.

Second: I decided to start calling these things Astro-Lessons because they are little lessons in astronomy that are just supposed to help build a pretty basic understanding of the Universe we live in.

Now onto the lesson:

NASA concept art of a gamma-ray burst

Gamma-Ray Bursts, also known as GRBs, are short-lived bursts of gamma ray photons associated with immense explosions that have been observed in distant galaxies. These are the most luminous electromagnetic events known. They typically last only seconds, but can be milliseconds quick, or even as slow as several minutes. After the burst, there is typically an "afterglow" in longer wavelength (X-ray, ultraviolet, optical, infrared, and radio).

Gamma rays, if you're unfamiliar with them, are a form of electromagnetic radiation at high frequency (very short wavelength) produced by subatomic interactions, such as radioactive decay, fusion, and fission. Gamma rays are a health hazard since they are a form of ionizing radiation, which is what makes GRBs scary.

NOT LIKE THAT...

Imagine, if you will, a microwave so powerful that it would cleanse the entire galaxy of life. It would literally fry everything. That would be the power of a gamma-ray burst in our galaxy.

GRBs are caused by a special type of supernovae, hundred of times brighter then typical ones (astronomers can pick out supernovae in distant galaxies) and a million trillion times brighter then the Sun. There aren't a very common event though, GRBs are detected roughly ONCE PER DAY in any random direction of the sky.

Until recently, astronomers knew very little about GRBs. In fact, their discovery was a bit accidental. During the Cold War in the 1960s, U.S. military satellites were watching for Soviet nuclear weapons testing in violation of the test ban treaty. The satellites carried gamma ray detectors since a nuclear explosion produces gamma rays. But they began noticing these huge gamma ray bursts coming from deep space. These bursts remained a mystery up into the early 90s. There were no indications as to how far away the GRBs were, if they originated at the edge of our solar system, or in the Milky Way, or further away.

Hubble catches a GRB in action.

A combination of satellite observations with ground-based follow-up observations and theoretical work began to unveil one of the biggest mysteries in modern astronomy. It turns out that GRBs occur incredibly far away, near the edge of the observable Universe in distant galaxies. (Though one of my favorite explanations was that GRBs were massive nuclear weapons being used in a galactic war.)

As astronomers began to learn more about GRBs, they began to notice differences in individual events. These differences were in the length of the event, in which there were two classifications: long-duration (longer than 2 seconds) and short-duration (less than 2 seconds). The short-duration ones can last for a few milliseconds though, and average about 0.3 seconds (300 milliseconds). Long-duration bursts can last up to several minutes and average around 30 seconds.

Infographic describing theories on both long and short GRBs.

It is believed that entirely different physical properties cause long and short duration GRBs. The long ones are the ones that astronomers feel confident in their knowledge of. The short GRBs are only theoretically described and remain a mystery. So there isn't much else to say about short-duration GRBs, they exist and their are missions to study them, but the data is so fleeting, it is hard to pinpoint how far away they are or what causes them.

For the long-duration GRBs there is a good amount known. In the 1990s is when astronomer's discovered the "afterglow" which allowed the origin of the GRB to be pinpointed. This "afterglow" pointed to galaxies at immense distances, Billions of light years away. Some of these GRBs first occurred before the Earth, being only 4 billion years old, even existed! The most recent likely happened when the Earth was young, perhaps before the first microbes formed.

The faint smudge of a galaxy in the center of this Hubble image is where a GRB exploded in 1997.

In tracing GRBs back to their origin, astronomers began to study the galaxies that were the source of these explosions. Unfortunately there is no definite answer for what causes a GRB, but they are associated with a special sort of supernova seen in these distant galaxies, dubbed hypernova for their immense size and magnitude. The "smoking gun" that linked the two happened in March 2003 when the afterglow from a GRB perfectly matched the optical spectrum of a supernova in that galaxy.

What causes the supernova and the GRB is the mystery though. It would require an incredibly massive amount of energy, either from a incredibly massive star or a black hole or neutron star. Some of the theories suggest neutron star collisions while others suggest the collapse of massive stars.

But even though gamma-ray bursts seem like scary unpredictable events, we got two things going for us. One, these things have all happened very, very far away where they cannot hurt us, although the closest ones have been known to cause satellite interference. And two, there is no indication that these burst are happening in closer, more modern galaxies.

If GRB's were still common in modern galaxies as we know them, they would occur a lot closer and more often. So there is some sort of point where these events must die off. The idea is kind of that star formation hit a certain limit, perhaps the right mixture of elements and the right temperatures are just not available anymore to create the stars responsible for GRBs. So, we are likely safe from any threat of annihilation by these incredibly violent events.

Further Reading: 

Wikipedia entry on gamma-ray bursts

NASA's website on gamma-ray bursts

UC Berkeley's website on gamma-ray bursts 

Addendum: I did some more research (actually I watched the 3rd episode of Phil Plait's Bad Universe on Discovery Channel, which coincidentally happened to premiere yesterday [I watched it after I wrote this post]) and it turns out the safe distance things is a bit iffy. We seem pretty safe from gamma-ray bursts, but there is a very extremely small chance that we might get hit, there is no real good way to know. But there is evidence that an extinction event that happened in the past may have been caused by a GRB. I don't know that it's a definitive theory, but it is a plausible one with reactions of the atmosphere and massive extinction.

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Posted in Astro-Lesson,Astronomy,Galaxies,Gamma Ray Bursts,Supernova,Universe 22 comments