Star Clusters

This is a lovely post for us to write because we get to all come together, but also slightly sad because this is the last of our posts on stellar evolution. Yes, we finally get to star clusters.  First question to ask any astronomer investigating clusters would be, “open or globular?”  Yes, just like any other astronomical object these have the same old tools we’ve used for many years, some types, and some very interesting problems.  Let’s see what clusters have to offer us astronomers.

Clusters in general are fantastic tools for any astronomer, with photometric, spectroscopic, and other studies done to find out what they are and why they’re important.  They are groups of stars that all formed from the same nebula.  This means they are all at the same distance, and there’s a higher than normal density of stars in that location that have the same age, the same chemical composition, and different masses.  The difference in mass is important, and it reflects how different stars can form in a region.  For all these reasons, clusters are perfect for studying stellar evolution, finding distances to phenomena, or better understanding properties of galaxies or certain stars.  Guess these clusters really show how “united we stand” can be quite the useful strategy (at least for us astronomy nerds anyway).

So let’s open up the first mystery present, open clusters.  These are famously filled with all sorts of massive, higher metallicity stars.  Yep, that means they can produce all sorts of fancy objects as per our understanding of stellar evolution.  Since they are massive, we also consider these clusters to be younger (millions of years old).  This logically follows into another observation.  Open clusters are defined by being in a galaxy’s disk.  This is the area of most star formation, and it makes sense younger star clusters would be forming there.  Last for these clusters is that they are usually sparser in member stars, having hundreds or thousands if a lot, because of the high mass stars.

See Explanation.  Clicking on the picture will download the highest resolution version available.

The Pleiades, or M45 because it is easier to pronounce, spell, and it gives us an excuse to bring up the Messier catalog again.

If we still have your attention then great.  If not, then perhaps we can look into the celestial sphere for a far more well-rounded cluster.  Alright, bad introductions to the next type aside, we have globular clusters.  These are older clusters up to gigayears old, in a galaxy’s halo (outside the disk that is normally seen) and typically having millions of stars.  Since they are older, they also have lower metallicity, and cluster members are not as massive stars.  As shown, clusters link together mass, evolution, age, and density of stars in different locations.  But don’t let these general properties fool you, globular clusters can still have blue stragglers.  Blue stragglers are unusually massive stars in globular clusters, which may be caused by some sort of binary star.

See Explanation.  Clicking on the picture will download the highest resolution version available.

This would be the brightest globular cluster we can see, Omega Centauri.  There are all sorts of fancy random-looking blue lights, and just look at that bright core…hopefully it doesn’t somehow cluster together against us.

So far we’ve defined what clusters are, and how they’re useful.  Let’s exemplify how useful they are.  The Color-Magnitude Diagram is essentially an H-R Diagram that plots up the color and magnitude of stars (determined using photometry, or amount of light, and spectroscopy, or the type of light we receive).  This may not sound useful, but by taking different clusters at different ages, astronomers can actually see how a whole entire cluster of stars evolves over time.  If you’re wondering how, we’ve mentioned it previously in this post because by seeing the different ages for clusters, then we can see the numbers of stars, the location of the stars, and qualities of those stars.  They in fact show exactly how population I (like stars in open clusters) and population II stars (like stars in globular clusters) are distinguished .

One last aspect of clusters to explore is how we classify them.  People have actually spent time trying to systematically distinguish certain clusters from one another.  One example is the Shapley-Sawyer morphology classification, which classifies different globular clusters by how dense they appear.  There is also a Trumpler classification of open clusters, which shows how nebulous and rich (concentrated with stars) the open cluster is.  Well, enough classes, that’s all for this lesson.

So yes, this is how they classify open clusters. As you can see, they all look fairly similar, so basically Trumpler developed really good skills at distinguishing clusters.

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Sources and links for further reading (links to images are below):

Color-Mag Diagrams

Blue Stragglers, Stellar Evo with clusters

Open

Globular

Both types of clusters

Cluster classification

Images

Pleiades: NASA/APOD, http://apod.nasa.gov/apod/ap120903.html

Omega Centauri: NASA/APOD, http://apod.nasa.gov/apod/ap130501.html

Trumpler Classification: http://www.astrophoton.com/trumpler_class.htm

Exotic/theoretical objects

As both of your authors are away doing research over the summer (yes, of course, we’re both doing astro research), we haven’t had the time to post new and exciting blog posts for a while (not to mention the constant pile of schoolwork). Once we return, we’ll explain the research that we each did (or maybe astronomical requests if we get some!), but in the meantime, here’s a new post to satisfy your needs for astronomy knowledge.

Well, I guess we can’t say that rarity rocks since this is not a geology blog (unless the readers out there like that).  But let’s get going about some rare stuff that’s out of this world!  Yes, in the universe there are exotic, weird, and sometimes mind blowing objects that we shall explain in this post.

Exotic stars are technically defined as compact objects that are not made of electrons, protons, and neutrons and are in a degenerate state (those degenerates!).  But to change things up we’ll also introduce some other weird objects.  After all this is astronomy, when can we ever leave it simple?  Mind you much of this is theoretical, so most people would be as confused as you or we are, but it’s interesting anyway.

To start, there are exotic stars.  Please don’t scratch your head too much yet.  But let’s make a list to explain:

  • Quark and strange stars: If a neutron star manages to compress further, then just like with white dwarfs to neutron stars, they can compress into quark matter.  This increases density, and a specific type is known as a strange star (strange, we know).  They are made up of strange quarks (a specific type of quark…hopefully that wasn’t too quarky).

    If you look closely enough there’s a difference. Theoretically. Maybe…just trust us here.

  • Electroweak stars: Apparently in this case quarks can be turned into leptons by the electroweak force , which is what keeps electrons in orbit around a nucleus and allows for nuclear fission.  They again arise when a quark star becomes denser.
  • Preon stars: The next step in our evolution of this post.  A bit more compact and a possible dark matter candidate.  But there is some argument against their existence, though that applies to all of these objects  in general, considering they are all theoretical.
  • Boson stars: A boson is essentially a force.  So as to not force you to take forever to read this, this star is somehow made of pure force (yes, the force is indeed strong with this one).  They are also a possible dark matter candidate.  Let’s not forget that they are theoretically transparent and in general would be difficult to detect.

In general, the stuff here isn’t necessarily normal.  But what that’s not to say that black holes, pulsars, and anything we talk about in the future or past are not weird.  We just wanted a post for these objects.  Pretty much out of all of these so far the quark star is most likely (there are possible quark novae, most famously SN 2006gy).  Now to name a few more astronomically odd objects.

That is one big bang (oh wait, it’s not a big bang!). But it is SN 2006gy in all its luminous glory.

This may also be relatively confusing, but there is also the dark star (it is made of dark matter, there is annoyingly another dark star).  Dark matter interactions produces heat instead of normal fusion with matter.  This is thought to form in the earlier universe and normal fusion wouldn’t occur because the dark matter collisions holding the star would prevent the normal matter in it from fusing.

Next up to confuse us all is the quasi-star.  Another early universe star.  These are thought to be black hole stars that are held by matter falling into a black hole.  This could come from massive protostars to an extent collapsing into a black hole, but without outer layers being blown away. (The matter normally gets blown away and leaves a black hole with supernovae).  These are associated with the theoretical population III stars that were the earliest stars in the universe.

Hopefully this can explain somewhat how quasi-stars are made.

There are also theories about how stars or degenerate objects can tidally capture each other.  One possibility is the Thorne-Zytkow object (TZOs, we needed at least one new acronym, we know how you all missed them).  This oddball consists of a giant star that manages to capture a neutron star that sinks to its core.  So yes, a neutron star within a giant star (we’re hoping you have that nerdy astro moment where you go woah as well).  Apparently the outer layers of the giant star can be shed such that a white dwarf core is left over, forming a binary system with the neutron star.

So how does any of this have to do with stellar evolution?  To start, the exotic stars themselves serve as logical in-between phases from neutron stars to black holes.  After all, with some room between the Chandrasekhar limit and the TOV limit then perhaps there could be some more room between when a star is pure neutron matter and when a star condenses far enough to become a black hole.  Also, theoretical physics likes to make things more complex (because why not).  In addition, there are many objects which astronomers just plainly have trouble explaining, like binary compact objects or the earliest stars.

This friends is how black holes are made (next up, even more in between steps!).

So, as a final note from rvtau and astroisstellar, may your summer be quite…stellar.  Also, thank you to anyone who keeps reading, we really appreciate it!  Perhaps when we get back we’ll make some sort of anniversary type of thing.

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Sources and links for further reading (links to images are below):

Carroll & Ostlie, An Introduction to Modern Astrophysics, 2nd edition, p. 691

Images

Binary Stars (Part II)

Well, we rotate again within this topic.  Now in your view is on how we classify these binary stars, after those lovely properties described.  Also, remember, some past topics in the general/history sections still apply, like Kepler’s laws.  But now for types, we will even include binary stars that only appear to be binary stars.  Each type effectively uses a major astronomical technique to better understand the motion and properties of a binary system.  We will explain all the concepts that go behind it here, and then the math to apply it after this.

To start, let’s see what we have to work with.  What I mean by that is an optical double of course!  As stated, these are stars that just appear to move as binaries, but aren’t really.  They are produced when two stars fall into one’s line of sight, and they generally can pop up if you’re looking through a telescope at stars that aren’t even  within hundreds of parsecs of each other.  Some stars that appear close, like with a constellation, can do this at times.  Simply looking at a light curve (like we did with the variable star posts) reveals great distinction.

Seeing double again? From: UNL Astronomy.

Next we get physical with our binaries (no violence intended, disclaimer: this site is for all ages).  Physical binaries are bound by gravity; basically they are real binaries.  Therefore, we have various types based on the many ways we determine the properties of  systems.  This goes into finding the mass of the whole system or each object, rotational speed, recessional or radial velocity (speed towards or away from us), and various qualities of each star.

Yes, with this we can finally see the truth! From Discovery Space.

The first we shall talk about are the visual binaries and astrometric binaries.  We can spot the orbits of the binary stars in both these cases, and therefore the motions can tell us information about the period and separation of orbit for the two stars.  This is important since with that we can apply Kepler’s Laws (this is the perfect time to give a blast from the past with our Kepler post).  But the orbit can be at an angle, which is known as an inclination from our line of sight.  This has to be resolved when making calculations.  Afterwards, the total mass of a system and the center of the mass can be determined, which can lead to figuring out the mass of each star or object, and the types of stars or objects in the system.  Observing two stars can reveal a TON of data (which is a true theme of astronomy: making observations and calculations can lead to major discovery).  But you cannot do all this without knowing the system’s distance from us.  Without that, you cannot know the angular separation or inclination, linear distances, etc.

Also, you may be wondering whats the difference between visual and astrometric?  The visual part you can see, the astrometric part you measure.  Binary systems in a nutshell (yum, nuts…don’t call us astro people crazy now!).  The only real things to add on about that is much of this is because one part of a system can be brighter or too close to distinguish, and therefore basic laws of physics like Newton’s laws can be applied to find out much about the system.  This has extended to discovering exoplanets around stars, since these laws apply to any two orbiting bodies in the UNIVERSE.

Moving on, we have yet again those eclipsing binaries.  Okay, we’ve mentioned them a few times…they come up.  We already sort of discussed their set up, but let’s talk more explicitly about measuring them.  As stated, one object can block the other.  An interesting result is that we can find both the time of orbit or eclipsing and the brightness of an orbiting object.  Since we can find the brightness of each object, we can find distance, and all the other stuff mentioned with astrometric binaries.  So, a light curve plotting the brightness and eclipsing is extremely important here.  Looking at the shape and amount of dip after each eclipse can sometimes tell about relative size (whether one star much larger/more massive than another).  Very regular, very neat.  Eclipsing binaries can also be something called spectroscopic, another type of binary star.  But again, here eclipsing binaries and timed blocking can result in important exoplanet discovery.

Let’s talk quickly about spectrum binaries next.  This is the case where two stars cannot be resolved (sort of like the opposite of an optical double).  But the spectra produced by the stars immediately show the binary system.  Certain stars show more absorption lines of helium or calcium because stars have different masses, temperatures and different amounts of elements.  One star cannot be simultaneously hot and cold, so the presence of spectral lines associated with both hotter and colder stars would lead astronomers to consider a binary system.  The Doppler effect, where the lines of a star can be shifted, also takes place here so radial velocities of the stars can be measured by spectra.

If a binary pair orbits along our line of sight, a shift in spectra can be seen, known as a spectroscopic binary.  This is if the luminosities of the star can be compared, a double-line spectroscopic binary.  But there is also the case where one star can be brighter than the other, meaning only one set of spectra can be seen, a single-line spectroscopic binary.  In addition to looking at the spectra lines themselves, the velocity of a spectroscopic binary can be plotted to see their blueshift, redshift, or periods and radial velocities.  Therefore, an eclipsing binary’s analysis or photometry can show the brightness or period, and the spectroscopic aspect can show the Doppler effect, which can link to the speed of the system.  The double-line is better for analysis since you can actually see the whole system and analyze the speed and mass of the objects.

A double-line spectroscopic binary in all its beauty. First (double-line spectroscopy showing movement) and third (velocity curve) from Dept. of Phys and Astro at University of Tennessee. Second (showing Doppler effect in spectral lines and motion of a binary system) from Australia Telescope Outreach and Education

So you’re a normal piece of matter, say an electron, going along, and you suddenly get hit by a proton!  What happens when you get this high speed collision?  Well, you get X-ray radiation of course!  When you have two stars having a ton of this occur because of matter accretion (or accumulating matter from one star due to gravity pulling off a layer from another star) it’s called an X-ray binary.  This means that a compact object must have enough gravity to pull off material from its binary partner.  This can be created by a normal mass star turning into a white dwarf through stellar evolution while a star in the system is still a red giant, meaning the white dwarf will have enough gravity to pull off the loose Hydrogen layer from the red giant.  Alternatively, more massive stars can become neutron stars or black holes and still retain the system.  We mention them because they are quite important, and they too can be measured using a variety of physics between the rate of accretion and the angular motions and energy transfer that occurs with accretion.

This system sure is hot stuff! From Northern Arizona Meteorite Laboratory Glossary.

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TL;DR

The post summarized the different classifications of binary stars.  They are characterized by various methods of identification and analysis of data, all of which is very important.  In addition, many binary systems can have compact components and can exist in a variety of ways.  They can be a pair of any objects, be it brown dwarfs, white dwarfs, black holes, neutron stars, any type of star at any point of its stellar evolution except perhaps protostars, and planets and moons even.  Therefore, it is important to not only classify the systems by the type of objects, but by the way we see and understand our surroundings.

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Sources and links for further reading (links to images are below, some topics covered above that aren’t found are found in the General):

General

Eclipsing binary

Spectroscopic

Visual

Optical

Astrometric

Photometric

XRBs

Carroll & Ostlie, An Introduction to Modern Astrophysics, 2nd edition, p. 180-198

Images

UNL Astronomy: http://astro.unl.edu/classaction/images/binaryvariablestars/opticaldoublesandvisualbinaries.html

Discovery Space: http://www.discoveryspace.net/index.asp?Cat_id=631

Dept. of Physics and Astronomy at University of Tennessee: http://csep10.phys.utk.edu/astr162/lect/binaries/spectroscopic.html

Australia Telescope Outreach and Education: http://outreach.atnf.csiro.au/education/senior/astrophysics/binary_types.html

Northern Arizona Meteorite Laboratory Glossary: http://www4.nau.edu/meteorite/Meteorite/Book-GlossaryX.html

Extrinsic Variable Stars

We don’t have quite the humongous post for you, sorry, we were both busy and unsure how to present these types of stars (but no worries, we are back in your view for this new post!).  Interestingly, it’s not always what’s on the inside that counts.  So, this post considers extrinsic variables.  Following weeks will probably cover either math, more about certain variable stars, or other general information.  But now, shall we move onto all our lovely extrinsic variables?  These things don’t have the same large number of types as their intrinsic cousins in terms of causes, but they are quite important in our search for life outside the solar system, making them quite close to home…

The power of the sun (varies over time). Yep, one of the closest variable stars. From: NASA, Sun picture

The principal topic here will involve binary stars, but some can be non-binary stars.  They may not seem common because they require all sorts of optical conditions or non-uniformity in a star, but they are definitely out there.  Events on the outside of the star are the essential cause.  For this, the two chief classes include eclipsing binaries and rotating types.

Let’s start by exploring the Rotating variables.  Circle around these  folks, things are about to get pivotal!  Rotating variables pretty much are the extrinsic variables that aren’t always in a multiple star system.  Interestingly, differences in luminosity can be created by magnetic fields, stellar spots (like the sunspots we talked about in the what are stars post, it makes a lot of sense considering what they are!), and non-spherical shapes for stars.  These stars really aren’t spotty at all in fact!  After all, sunspots are characteristic of this class since they are cooler regions on the outside of a star, related to magnetic fields created from rotation.  This makes the pulsations and discovery fairly consistent.

Yep, a real deal picture of starspots. Hopefully the magnetism here attracts you (hopefully not fatal attraction?)! From: University of St. Andrews

Here are the major types (sadly, we have to introduce…long names):

Alpha2 Canum Venaticorum (ACV): Main sequence stars of type B8p-A8p (the p means having peculiar spectral lines) with strong magnetic fields.

Aph10111814

Light curve. From: Planet Hunters, ACV light curve

Rapidly osccilating Alpha2 CVn (ACVO): These non-radially pulsate, and they are of type A spectral class (meaning they are massive) with relatively fast periods.

BY Draconis (BY): White dwarfs with occasional light changes and even flares.  This is from the rotation making the spots on the star, creating non-uniform surface brightness.  Our Sun could even be considered of this type, and sometimes they can be considered eruptive variables too.

Light Curve. From: BY Draconis light curve

Rotating ellipsoidal (ELL): Close binaries with ellipsoidal components (ones where the stars of a system can actually be stretched from gravity of each star in the system pulling eachother).  Spica is a famous example.

Light curves…again…it’s ellipsoidal? Sorry if this is getting repetitive. From: Ellipsoidal variable light curve

FK Comae Berenices (FKCOM): Rotating giants with non-uniform surface brightness (types G to K).  They can be spectroscopic binary systems, where the variability is caused by sunspots from rotation (in case you haven’t noticed, yes the pattern is as we said, you definitely want to read the stars post about sunspots for these rotating variables or perhaps we’ll make a post more on binary stars).

But yep, we’re back with another light curve. From: FK Comae Berenices light curve

Optically variable pulsars (CM Tau, PSR): These rotate very quickly and are known for strong magnetic fields (basically, pulsars are rotating, extrinsic variables, so yes even post-supernova objects can be classified as variable stars…classify all the things!).  Refer to the neutron star post for more on this (heh, not going to bother with another light curve).

SX Arietis-type (SXARI): Main sequence type B stars that are high temperature forms of ACV types.  Helium is prevalent in spectra, and they have those good ol’ sunspots and magnetic fields.

Now be ready, we don’t want to block your view from learning at all.  But we have eclipsing binary stars up next!  To start, the general definition is that the orbit has to be oriented so when we see it one object can pass in front of the other (this is termed lining up with the ecliptic, basically the stars line up with our line of sight).  They have VERY characteristic light curves and dips.  These stars are really nice in their periodic motions and observations, which makes O-C diagrams extremely useful here (in fact, their predicted variability can be theoretically easier than intrinsic variables).

I hope we are all inclined to see this. The system can be tilted to our view, sometimes one star can block another. From: University of Tennessee eclipsing binaries

Okay fine, that wasn’t the end of light curves. But hey…it’s what’s on the inside that counts, right? Oh wait…From: David Darling

Looking at the graph above we can derive two facts if values were given.  Pretty much, we could figure out the apparent magnitude of the stars depending on the dips (as seen, the larger dip is from the larger star, but you would have to assess that from data given) and the period of the system if not given (which as said can be useful for O-C diagrams).

We can name three major examples (this also could includes the not as stellar planetary transits used to detect exoplanets…those can be eclipsing, but we’ll leave those for another day):

Algol: Yes, a ghostly star, sometimes considered a demon.  A spectroscopic binary observed since Egpytian times.  They are ellipsoidal, with one B-type star and a K-type star (so it can be blue and red).  Aside from the usual, it presents what’s called the Algol paradox.  Normally a star with more mass evolves and reaches end stages more quickly.  Somehow, the lower mass star is a giant in this case, while the higher mass star is on the main sequence.  This is most likely due to accretion of matter.

The Algol system, notice the various dips. Hopefully this uneclipses all our views as to what this could look like. From: University of Tennessee the Algol system

Beta Lyrae: Yet another secretive star known since ancient times.  It follows traditional definition, but it has a particularly large rate of period change, making it most likely an active system (this could be similar to the Algol paradox).

Visual Light Curve

Beta Lyrae…not much more to say on this…light curve again? From AAVSO Beta Lyrae light curve.

W Ursae Majoris: Out of the examples, a relatively more recent discovery.  The peaks for this system seem almost equal.  But it most likely is also an active system like with Beta Lyrae.  This has probably created some period variation that has been detected over time, and the system also seems to have some star spots.

Hah, yes, another light curve! You really aren’t escaping these, sorry. From: AAVSO W Ursae Majoris light curve

Epsilon Aurigae:  Just to throw it in here because it is probably one of the most mysterious eclipsing binaries ever.  It has a REALLY long period, even the eclipse itself is long.  In fact, there was a year of astronomy around 2009 where people in part made lots of observations because of its rare period.

It may actually be caused by a gap in a relatively large accretion disk that occasionally doesn’t block the light. Yes, this is probably the definition of weird. It’s like having pizza dough in space spin, get really large, and normally it doesn’t happen, but it gets so thin you can sort of see through it. From: AAVSO Epsilon Aurigae

Another model of the system. Again, extremely unlikely. Hopefully you didn’t think we wouldn’t end with one last light curve! From: Hopkins Phoenix Observatory Epsilon Aurigae

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TL;DR

Oh, the beauty of stars.  They prove that it’s BOTH what’s on the inside AND the outside that matters!  Extrinsics, though, show what’s on the outside.  Whether it be pushing and pulling, magnetism, and rotation for rotating variables, or just straight on eclipsing lined up with our sight sadly blocking out stars on occasion, these stars are yet again far out.  While they may not seem exciting, some can be…quite weird.

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Sources and links for further reading (links to images are below):

General:

Rotating (ellipsoidal)

Eclipsing

Images:

Supernova Remnants

Well, this post will hopefully be smaller, but just as explosive (or implosive?) as the last one!  Today, we are dealing with the actually large topic of supernova remnants, or SNRs.  These things are technically nebula, but not the kind that we start a star off with. No, these are actually for the end of a star!  There are many types, stages, effects, and definitely much significance from yet another one of our magnificent acronyms.

As for types, there are effectively two ways to categorize them.  One is by the original SN that created them.  Let’s start off with that (but technically this isn’t an exact type like the actual SNR types).  Essentially, a SN happens and we are left with the star expanding and releasing heat at extremely high rates.  For all SNs, the SNR that is produced essentially follows the properties of that SN.  For example, Type Ia SNRs would all generally be around the same mass, and the most massive element in them would be iron.  SNRs are also used to find the progenitor event or SN.  Some Type Ia SNRs have been used to create models of white dwarf explosions, leading to the double degenerate progenitor model.  Type Ib, Ic, and II SNRs, on the other hand, give us even higher mass elements, such as gold and even up to uranium. They can also have interesting objects (neutron stars and black holes) left over, and produce neutrinos – which sadly cannot travel faster than light.  I guess you could say they just make quite the big bang.

You’d better thank SNRs for this! Credit: Thomas Jefferson National Accelerator Facility

Next we have the actual way they are classified.  For these remnants of the past, there are many types based on the shape of the SNR and the radiation detected.  Shell types are characterized by being…a shell.  That is, the outer edges are brighter compared to the inner edges.  Next, there are crab remnants, plerions, or pulsar wind nebulae (PWN).  They have less exact shapes than their shelled-up cousins.  They also notably have a pulsar in the center and emit X-rays.  But yes, it basically is based after the Crab Nebula in case you haven’t guessed it.  Then we also have composite remnants, mixtures of both depending on what wavelength is being viewed.  This creates two subtypes of composites.  Thermal composites stay cooped up in shells in the radio, but in X-rays they are crab-like (though, not exactly…), while plerionic composites somehow only show spectral lines in certain parts of the x-ray spectrum, and also have shells.

Cas A, a shell SNR. Credit: University of Minnesota and HST

The Crab Nebula, a crab-type SNR (we figured we might as well go with the obvious example). Credit: European Southern Observatory

W44, a composite SNR (radio = orange, infrared = red, x-ray = blue, gamma-ray = magenta). Credit: NASA/DOE/Fermi LAT Collaboration, ROSAT, JPL-Caltech, and NRAO/AUI

As for stages, there are a few different opinions.  Since NASA uses three, we will too (just for fun).  You may be asking, how are there stages for an SNR?  All the stages of stellar evolution are already over!  But wait, these stages generally just explain what happens after the SN goes bang.  So, we start off with free expansion, where the a shock wave is formed at constant temperature and expansion velocity.  That lasts a few hundred years.  Then we have the Sedov or adiabatic phase (adiabatic means no heat flow occurs).  During this phase the SNR starts to slow down and cool, and parts of the SNR start to mix.  This develops a magnetic field inside, and it lasts from 10,000 to 20,000 years.  The reason we say no heat transfer occurs is because energy is not actually released by radiation from the gas.  Pretty much, so far the SNR has gotten bigger, and it’s starting to have the original insane amounts of heat dissipate throughout it a little bit.  But then we come to our third phase, the snow-plow phase (sadly, no snow will be used in this explanation).  After the SNR has cooled down, electrons actually recombine with elements that were ionized, radiating energy to the point where the SNR actually starts to cool down Remember however, that this is astronomy, so our “cooling down” means it is still pretty hot.  But the snowball effect of recombining causes the expansion speed continues to slow down, and eventually, the material goes into the interstellar medium (ISM), forever…until it is used for another star! Got to love the circle of (stellar) life.

There are a few confusing things that maybe should be cleared up.  So let’s switch wavelengths (or topics)!  For an SNR, there is certainly a forward shock wave or bow shock.  What this means is that the magnetic fields that are being created meet space, and end up forming a curved shape, like a bow.  The word “shock” itself mainly refers to the transfer of energy carried by the expansion.  Sometimes a reverse shock also drives inward, slowing down the expansion.  The next topic of discussion are cosmic rays.  Pretty much, they are high energy radiation of charged particles, but they are puzzling because their energies are so high that we don’t know what they could have come from.  Theoretically, they may come from SNRs, which have a mechanism that accelerates charged particles like electrons to extremely high speeds.  Another aspect that we haven’t mentioned yet is Rayleigh-Taylor instabilities, which are basically differences in density that create mixing and loss of energy within an SNR.  For example, if you’ve ever seen two fluids with different densities (such as oil and water) in a cup, you’d know the more dense substance would sink.  But space isn’t precisely a cup…meaning we get a sort of twisted finger-like structure.

Rayleigh-Taylor instabilities in a glass of water. Credit: American Physical Society

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TL;DR — After all this talking, if you haven’t noticed, SNRs are actually important to astronomy.  They are classified based on the type of SN that produced them, or by their shape and radiation.  We aren’t always able to see SNs, but we can find their SNRs to see their properties.  They show fantastic examples of how elements spread around the universe, which means you better thank astronomy for your gold watch or jewelry!  SNRs can show interesting properties such as bow shocks and Rayleigh-Taylor instabilities. To add more fuel to the fire (or star going supernova), they may even produce the mysterious cosmic rays.  Also, they just look pretty awesome.

Sources and links for further reading:

Carroll and Ostlie, An Introduction to Modern Astrophysics 2nd edition, p. 633 to 646

Supernovae

We apologize for the absence, as we have been extremely and unexpectedly busy.

It’s Christmas time folks!  And what do we have at the top of the tree (if you celebrate)?  A star, right?  The perfect reminder to bring us back to our lovely discussion on stars!  So, we have discussed the life of a star.  Now things get interesting.  We have reached the end of a star’s lonely existence.  Yet another difference between a star’s end and a human’s end is that in some cases a star can have one of the most awesome events in the universe: a supernova.  Summarized, it is an explosion of a star.  But in detail there are multiple types, all fascinating.

Take that and put it on your Christmas tree! Okay, maybe you don’t celebrate or it doesn’t seem that festive, but it’s still awesome. From: http://hubblesite.org/newscenter/archive/releases/2007/10/image/a/

Each type has a progenitor, which makes spectra important as they show the amount of elements present and rate of fusion.  For example, during an SN enough energy is released such that higher than average fusion can take place.  So, if you ever wondered where iron or even heavier than iron elements came from, thank SNe.  Other ways to determine the type follow similar reasoning, including light curves, energy, and progenitor events.  The main summary of the types of SNe is the popular Minkowski-Zwicky scheme or classification. We will now begin splitting up these wonderful things, despite the fact that SNe involve fusion.  There are two types: I and II.  A major way to characterize them is by spectra.  Type I SNe lack Balmer hydrogen emission lines, while type II SNe show Balmer hydrogen emission lines in their spectra.

From: http://www.ubthenews.com/tychosnova.htm

The Type Ia (thermonuclear) SN is the most different.  It consists of a binary pair of two stars, one being a white dwarf.  As stated previously in the WD, PN, Type Ia post , they go through an uncontrolled thermonuclear explosion as the white dwarf becomes unstable upon reaching the Chandrasekhar limit.  The mechanism of explosion is important to note as the other three main types explode from gravitational collapse.

The other three main types are known as core-collapse SNe, which we explained at the end of our post on high-mass stellar evolution.  As the name indicates they collapse from the sheer amount of matter in one space, creating the appropriately named implosion-explosion event.  Type Ib and Ic are thought to mostly come from WR stars, so they appeared to be very similar.  But they were differentiated by spectra, where the Type Ib has shed its hydrogen shell before collapsing, while the Type Ic has shed its helium shell too. (Despite being caused by totally different mechanisms, both Type Ia and Type Ic lack hydrogen and helium lines; they are differentiated by the strong silicon lines in Type Ia.) As we said last time, the shedding of layers before explosion makes them sometimes named stripped-core collapse SNe.  Type II SNe are massive stars, of generally about 8 to 12 solar masses that have enough mass to collapse and then either continue collapsing or explode.

They also differ by location.  Type Ia are located in most areas as they only require a white dwarf and a binary system, and this is a more likely outcome for most stars.  Type Ib, Ic, and II all are generally in the galactic spiral arms in areas of relatively recent star formation since they require more massive, population I stars, which have to be massive enough to explode.

Another difference involves energy and light curves.  Type Ia SNe have the highest and most consistent peak; the rest depend on mass.  Also, out of the SNe, the only one with the possibility for a plateau or non-linear falling luminosity is the type II SN.  Visually the light curves are like so (click to enlarge):

From Wikipedia Supernova Light Curves

Next is the Type II SN and its core-collapse.  As the core collapses, protons and electrons are pushed together, forming neutrons and neutrinos.  This accounts for the high neutrino signatures that come from these types.  In fact, the relatively recent issue of faster than light neutrinos involved this very type of SN!  The neutrinos have such high densities that they develop radiation pressure, but the star essentially collapses in on itself, rebounds, and creates a shock wave.  These effects are named neutrino outburst and rebound shock.  Type II SNe can also be split up into types P, L, n, and b.  Type II-P and II-L stand for plateau and linear, which are the shapes of the light curve. The plateau type forms from hydrogen ionizing after the explosion, increasing luminosity, while the linear type fully expels its hydrogen layer.  Type IIn SNe are detected by narrow hydrogen emission lines in spectra, sometimes thought to be caused by LBVs.  Type IIb SNe show weak hydrogen lines initially, but the lines later become untraceable and it starts to resemble the spectra of a Type Ib SN.  The famous example of this is Cassiopeia A.  Another very high mass explosion is the hypernova, or a pair instability SN.  The pair instability occurs when atoms collide with gamma rays to form electrons and positrons, which creates a pressure to slightly reduce the core’s overall pressure.  This leads to a large-scale collapse.

For all the supernovae remnants (SNRs) can form.  For type Ia SNe they will always be large nebulae, but the other types can leave massive objects too.  These include a neutron star or a black hole.  The cause for the neutron star is that neutronization or neutrino outburst from neutron degeneracy pressure, analogous to the electron degeneracy pressure holding up a white dwarf, stops full collapse.  Black holes, on the other hand, collapse to a point.  Both objects have very odd and interesting properties.

To finish off we aren’t fooling that there are also supernova impostors, originally thought to be a type V SN.  These are also possibly LBV eruptions, but they don’t seem to be full supernovae.  As stated LBVs are unstable, so they can throw off large amounts of mass and heat that up.  They are recent but important events that are still being researched.

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TL;DR:

The easiest summary of supernovae is that they are characterized by many ways.  But they have great importance.  They explain how we could have any jewelry (in case you wanted to get some for a holiday present).  Even greater, they allow for a better understanding of the universe.  This is by giving ways to calibrate distance measures and  understand properties of stars.  Just to the point, they’re awesome.  It’s not Thanksgiving, but we should certainly thank SNe anyway.

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References and further reading:

SNe in general:

Type Ib+Ic SNe:

Type II SNe:

SN imposter:

White Dwarfs, Planetary Nebulae and Type Ia Supernovae

In our last post about low-mass stellar evolution, we left off at the AGB.  So, what comes next?  We said of white dwarfs (WDs) and planetary nebulae (PNe), but these can get somewhat detailed, so they deserve a post of their own.  Especially since WDs have a very interesting relationship to a certain event we call a supernova (SN; we adore our acronyms).

After a star gets to the pulsating red giant phase, it eventually gets to the point where it doesn’t have the energy to continue fusion, usually by the C/N/O elements.  When this happens there is insufficient radiation pressure outwards to repel the gravity pushing the star inwards, making it condense (sadly we can’t condense all this astronomy information).  With that, the outer layers of the star shed off to form a PN.  Some may ask why they are called planetary nebulae, since they in fact do not have any planets at all.  It comes from the fact that early astronomers couldn’t see them clearly, and with PNe being generally green-blue, they looked similar to the planet Uranus.

Most PNe are indicators of the later stages of a low-mass star.  Therefore, they are generally found with Population I stars, younger stars closer to the galactic plane or center.  Like other nebulae, they also look awesome and are among the most popular targets for astrophotography.  An interesting fact is that they don’t only form as a spherical shell, but they can come in many shapes, such as a bipolar shape, and also rings and ellipses.  In general, PNe are distinguished by shape, so another major question is how these shapes form.  Many of the different shapes are theorized to be from high-speed stellar winds, but the rings most likely come from an ejection of material that occurred at a different time from the main ejection.

As for their colorful nature, PNe have red emission lines from hydrogen and have green lines from doubly ionized oxygen, or OIII.  They are created from the star’s light ionizing atoms, ripping electrons away and then recombining.  The lines from a PN are also known as forbidden lines, not because they shouldn’t happen, but because in a lab we could normally only produce these spectra under high density.  Space, on the other hand, is an odd, low-density environment.

The many shapes, colors, and sizes of these PNs. They may not be planetary, but they are certainly stellar! Credit: Bruce Balick and HST from http://www.astro.washington.edu/users/balick/WFPC2/

There are also Fast-Low Ionizing Emission Regions (FLIERs).  These are strange red-colored regions of a PN that are exactly what their name says.  But sadly their origin is not fully understood.  Some say they seem like they should be moving outward, but they could be stationary or even moving inward.  So yes, this lovely universe of ours likes to seem to make little sense.

Saturn Nebula (NGC 7009)

You can see the FLIERs at the sides. So…perhaps you can see what’s going on here. Credit: David Darling

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They can have multiple features.  They may seem like degenerates for being aged as they are, but really they are fascinating.  They can be categorized as many types of stars. They can go ahead and nova or even go SN (for fun, let’s just say that they are little things with quite the Napoleon complex).  Yes, my fellow astronomy-lovers, we have gotten to the white dwarf (WD).

WDs are the result of  a star collapsing in on itself due to lack of radiation pressure, disrupting hydrostatic equilibrium, and allowing gravity to push inwards.  They come from stars that have run out of fuel, and they’re very small, being around the mass of the Sun shrunk down to the size of the Earth.  They are made mostly of the C, N, O elements because by that point a medium-mass star can’t fuse much more.  The color of the WD comes from the fact that it’s quite hot. It may not seem like it, but WDs can actually be detected in X-ray better than in visible light.

So then, the next question would be why doesn’t the star just continue to collapse?  It doesn’t even have fusion occurring to oppose the collapse.  Well, WDs actually have stability. They fulfill hydrostatic equilibrium, due to the Pauli exclusion principle, which works independent of temperature.  The compaction of the WD produces such a high density that all the atoms are smashed together to the point where electrons start to collide.  The Pauli exclusion principle says that two particles cannot occupy the same space with the same energy and spin, since they are effectively identical.  So, what happens when you have a lot of electrons in a little space?  Some that are in the same state collide.  Blasphemy!  In actuality a pressure keeps these two electrons apart, which creates pressure that stops total collapse.  This state of matter, as mentioned in the light and matter post, is known as “degenerate matter.”

Similar picture with electrons

And to summarize this. There’s not much space, so we are under high pressure here! This can oppose even the strong force of gravity. Credit: Chandra X-ray

Next, how do we detect them?  Well, Friedrich Bessel did manage to spot the white dwarf companion of Sirius in the visible range through careful observation (we’ll call this one Sirius business).  WDs can (and often do) form in binary star systems, which is an especially important way to detect any sort of object that’s normally tough to see, because if we cannot see the target object, the companion is still affected by that object.

And yet another interesting part to the WD is the famous discovery of one Subrahmanyan Chandrasekhar.  To get to the point, he found that WDs actually have a mass limit.  His work was highly debated at the time, but now it is heavily accepted.  So just how large can a star be before its mass overcomes even the pressure trying to oppose gravity?  The answer is about 1.4 solar masses.

What’s interesting about this is that there are actually multiple types of WDs, because the Chandrasekhar Limit is an upper mass limit.  This means smaller stars (even red dwarfs) can still form WDs, which have differing amounts of the elements C, N, O, and Ne.  For an analogy of how massive a WD is imagine one teaspoon of WD matter.  This is about 16 tons.  That is, if we had a beach ball of WD matter, that would still be heavier than an ocean liner.

white dwarf types

And now the summary of WD types. Credit: David Darling

Ultimately these stars would form into black dwarfs, fading away into another boring death.  But does this always happen?  Well, since it’s the universe, of course not!  In fact, WDs not only have multiple types, but they can even pulsate.  This will be discussed in more detail later (see, we like to mimic stars and evolve this blog as we go along).  Aside from spectral differences from different elements they also can differ by the presence of magnetic fields.

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But the mass limit has even more interesting implications.  See, WDs can be in binary systems with a red giant star with unstably held outer layers.  Those layers can be ripped off the companion star and accumulate around the WD in a process known as accretion.  Aside from our fancy names, accretion allows for material to get close enough to the WD such that fusion can occur.  This flash of very high amounts of fusion all at once is what we call a nova.  The name comes from the fact that when astronomers first saw these bright flashes, they thought new stars were being made.  But the even greater thing is that when a WD gains more than 1.4 solar masses, it produces one of the many magnificent events in the universe, an supernova.

Now, to be specific SNe can be split into two types, and they are further split up from there.  The type of SN produced by a WD is known as a Type Ia SN.  There is hot debate over what causes it, perhaps at a thermonuclear level!  Okay, not that hot, but one theory is the above mentioned accretion.  There is also what’s known as the double degenerate progenitor (a progenitor is a fancy word meaning “the thing that created it”), which states that a binary system of two WDs with a total mass greater than 1.4 solar masses collided and, exceeding the limit, exploded. But what is the exact cause of an SN?  Well, so much mass is gained in such a little space that runaway thermonuclear fusion occurs when it burns up all at once, releasing a massive amount of energy.

A nice summary of the SN’s creation. Credit: hyperphysics

Type Ia SNe are well-known for multiple reasons.  They are located almost everywhere, since WDs are just older low-mass main sequence stars.  They are thought to be able to fuse carbon/oxygen all the way up to iron/nickel, creating the heavy elements of the universe. The spectra is noted for a lack of Balmer hydrogen lines, and further distinguished by the presence of silicon in their spectra.  Another major note is that since there is a mass limit on WDs, the light curves of a Type Ia SN should always be the same. This means they are what’s called a standard candle; that is, they can be used to consistently measure what distance the SN was from earth.  The absolute magnitude for a Type Ia is approximately -19.5.  This is essential for understanding distances across the cosmos and can be used to calibrate Hubble’s Law.

Lastly, we will show how this is really, really powerful.  An initial comparison is that some note that this is enough to equal the entire Sun’s energy emitted over its whole lifetime.  But let’s continue to show another way.  We said earlier in the apparent/absolute magnitude post that we can relate brightness or luminosity to magnitudes.  Let’s apply this now.  Given the absolute magnitude of the Sun is +4.83 and a solar luminosity is about 4×1026 W we can now find it:

B2/B1=100(m1-m2)/5

L of the SN/1 solar luminosity =100(4.83+19.5)/5

=5395106225.15 solar luminosities = 2×1036 W.

Now to put this relative.  The global energy consumption per year is about 15 terrawatts.  That is, we could run the entire world’s energy supply and then some  for oh about 1023 years.  I believe it is undebatable to say this is insane.  Now, others say that the energy output is about 2×1044 J.  So, take this all as you will.

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TL;DR:

Now to condense (perhaps into a star, or even a WD!) all this information.  After we have a red giant star it sheds its loosely held layers to form a PN.  This PN can have many different colors and shapes.  At the center the remaining core of mass condenses into a white dwarf, composed of degenerate matter.  This means it is so condensed electrons repel eachother, creating an electron degeneracy pressure opposing gravity.  Beyond that, WDs have a mass limit known as the Chandrasekhar limit, being 1.4 solar masses. WDs can also form in binary systems.  If the partner accretes, or accumulates, matter to the WD then the WD can quickly fuse it and form an extremely bright nova.  If either this goes past the Chandrasekhar limit OR if a WD combines with another WD and exceeds the limit then the result is a beautiful Type Ia SN.  Type Ia SNe are valuable for creating heavier than normal elements, being common, and being standard in peak luminosity.  This is useful for calculation of large distances which leads to a better understanding of what we like to call the universe.

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Sources and further reading:

Planetary Nebulae:

FLIERs:

White Dwarfs:

Type Ia SNe:

Carroll and Ostlie, An Introduction to Modern Astrophysics, 2nd edition (pg 446-474,  524-529, 557-578)