Blog Moving

(by galacticinteractions) Feb 13 2014

I'm on the road for four days, so I may not manage to complete this in time. However, my blog is moving. I don't know exactly where it will land, but I will certainly link to it from Hopefully I will still have access to this blog here in order to be able to post a link to the new site, but that's not assured.

There is some nasty politics in the back end of There is one person (the one responsible for the last couple of days of downtime we had) who maintains control of the domain name despite the fact that this has repeatedly caused demonstrable problems for the site, and a couple of other flat disrespectful folks who make this a community I no longer want to be a part of. So, off I go.

Update: I've just registered There's nothing there yet, but that's where this site will land.

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Thermonuclear Supernova in M82

(by galacticinteractions) Jan 23 2014

I discovered (via random folks posting to my twitter feed) this morning that we (as in "humanity") discovered a new supernova in the galaxy M82 in the last day or so. This is very cool for a lot of reasons.

The Galaxy

So what is M82? M82 is a nearby galaxy, as galaxies go. It's not quite a spiral or an elliptical; it's usually categorized as an irregular galaxy. It's very close to the galaxy M81; the two galaxies are less than a degree apart on the sky, and are found not far from the tip of the bowl of the Big Dipper.

M81 and M82. Image from SDSS via SkyView, stacked by me to make a color image.

In the image above, M81 is the face-on spiral galaxy towards the bottom, and M82 is the thin line of a galaxy above it. If you look at this system with a radio telescope, you see that the two galaxies actually share a stream of gas; it's clear that the two of them are interacting with each other.

M82 is also what we call a starburst galaxy. This is a galaxy that's undergoing a burst of star formation. Stars are being formed in it much faster than they are in a typical everyday star-forming galaxy like our own. When stars are being formed rapidly, this will include some high-mass, short-lived stars. Higher mass stars may only live a few million or a few ten million years, which is a very short lifetime for a star. (For comparison, our star will live nearly ten billion years.) These high-mass stars also die with a bang: they explode in acore-collapse supernovae. So, you'd expect core-collapse supernovae to be more common in starburst galaxies than in regular everyday galaxies, simply because the high-mass stars that are the progenitors of core-collapse supernovae are being made at a faster rate there than they are elsewhere.

Unfortunately, often these starburst galaxies are very dusty, and the sites of most active current star formation are buried deep inside dust. As such, some of those supernovae may explode but never be caught because the light of the explosion is shrouded by dust.

M81 and M82 are very nearby galaxies, on the cosmic scale. According to NED, M82 is between 3 and 5 Mpc away; I'll use NED's "average" value of 3.8 Mpc. That means that M82 is 12 million light-years away. Whereas the supernova was just seen here on Earth in the last couple of days, it actually exploded 12 million years ago.

The Supernova

The new supernova was reportedly discovered by amateur astronomers in Russia (please suggest a better link or citation to said amateurs in the comments if you have one!), and has been confirmed by numerous people. As the image in the buzzfeed "discovery" link I just gave you shows, this supernova is actually on the outskirts of M82. What's more, this supernova has been shown by its spectrum to be a thermonuclear supernova rather than a core-collapse supernova.

Very massive stars (more than eight times the mass of the sun) end their lives in a core-collapse supernova, blowing themselves all to hell and leaving behind a neutron star. Stars of lower mass end their lives in a much more sedate (but still pretty awesome) planetary nebula and leave behind a white dwarf. A white dwarf is a star that's about the mass of the Sun, but only the size of the Earth. It is hot just because of left over heat from its formation; it's no longer actively generating energy inside it. It just sits there and cools off. A typical white dwarf is made up of carbon and oxygen (although in a rather exotic state that I won't go into right now). It's not as mind-bogglingly dense as a neutron star, but it's still amazingly dense. There is an upper limit to the possible mass of a white dwarf star (at 1.4 times the mass of the Sun); one over the size of that would collapse under gravity. If a real white dwarf does grow to this size (either by colliding with another white dwarf, or by pulling matter off of a companion star over time), it starts to collapse under gravity, but doesn't complete that collapse. It gets dense enough to trigger runaway nuclear fusion of the carbon that composes it. Basically all of the carbon in the white dwarf undergoes fusion, and a chain reaction of fusion continues until all the atoms have combined to make iron (or something right about the same size as iron). In other words, the white dwarf becomes... a thermonuclear bomb that is one and a half times the mass of the sun.


That's what happened in M82.

Because these supernovae are so amazingly bright, they can be seen out to extreme distances. I'm not talking M82; I'm talking many, many, many times farther away. Whereas M82's supernova exploded just 12 million years ago, we've seen supernova out to distances that means they exploded 10 billion years ago. This property, together with other properties, of this type of supernova makes them lighthouses useful for measuring the history of the Universe. Because we can see them so far away, we can see supernova from when the Universe was much younger. Indeed, it was measurements of these supernovae that allowed us to measure the history of the expansion rate of the Universe and discover that the Universe's expansion is actually accelerating.

It'll be fun to watch over the next weeks and months as this new nearby supernova gets brighter and then fades away.

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Grumpy Cat Plays Scrabble

(by galacticinteractions) Jul 14 2013


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Grumpy Cat reacts to ubiquitous surveillance

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Updated Obama Poster

(by galacticinteractions) Jun 12 2013


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My reaction (as student & teacher) to "let's have class outside!"

(by galacticinteractions) Apr 26 2013


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The Higgs Boson: a talk in Second Life tomorrow morning (April 6)

(by galacticinteractions) Apr 05 2013

It's been a year since I've given a public outreach physics and astronomy talk in Second Life. I used to do these things fairly regularly as a part of MICA (the Meta-Institute of Computational Astronomy). However, the MICA project has completed, its island in Second Life has gone online, its Second Life groups have been disbanded, and MICA no longer really exists. (Its website is still up, and should stay up for at least a little while. If I were smart, I'd probably make sure to download and archive elsewhere all of the audio recordings of my own talks....) A write-up of what MICA did and was all about is available at, and was published in the conference proceedings of a SLActions conference on virtual worlds

I've always meant to find other venues for continuing to do popular talks in virtual worlds. Someday, I'd like to escape from Second Life's walled garden and start doing these talks in an OpenSim grid, and even did the first steps for trying to get set up to do them in my own region on OSGrid. However, of course, the audience in Second Life for now is still far bigger.

Fortunately, the Exploratorium, the excellent science museum in San Francisco, has a presence in Second Life. This Saturday (tomorrow, 2013 April 6) at 10AM pacific time (17:00 UT) I'll be giving a talk about the Higgs boson in the Exploratorium region in Second Life. Remember, basic Second Life accounts are free. Drop by if you're interested.

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Dark Matter found? Don't break out the champagne just yet.

(by galacticinteractions) Apr 03 2013

You may have seen announcements that Dark Matter has been "found". I don't believe there's a publicly available scientific paper on this yet, so the original source for this is two press releases from CERN: One from four days ago and one from today.

First, I want to say what is meant by dark matter being "found" here. It's not evidence that previously-uncertain Dark Matter exists. We already know that Dark Matter exists; the Bullet Cluster observations several years ago was unambiguous confirmation that non-baryonic dark matter exists. We don't know what it is from that, but we know it exists. (Here is a podcast I did three years ago about the evidence for the existence of Dark Matter, including the Bullet Cluster.) So what does it mean to say that these new CERN results may have "found" Dark Matter?

Although we know Dark Matter exists, there remain a huge number of mysteries about it. Many of these can be summarized under: what is it? All we really know is that it's not made out of baryons, that is, protons and neutrons. So, it can't be an excess of dim stars or rogue planets (a model that was once considered a real possibility for our Galaxy's dark matter). Thus far, we've observed it because of the effects of its gravity. We've seen it in comparisons of the structure in the Universe to models of structure growth from early-Universe conditions; in the dynamics of galaxy clusters and galaxies; and through gravitational lensing. It would be nice to observe it in other ways.

To "find" Dark Matter, we'd like to do one (or more) of two or three things. Either, we'd like to see the results of decay products in our atmosphere or in space because of interactions of Dark Matter particles out in space. Or, we'd like to have an actual Dark Matter particle interact with a particle detector we have on Earth (analogous to how we see neutrinos from the Sun). Or, we'd like to actually make some of the stuff in a collider like the LHC at CERN in Switzerland, and see its decay products or signature there.

The current announcement from CERN is potentially of the first type. There is a detector, the "Alpha Magnetic Spectrometer" or AMS, on the International Space Station. This spectrometer is measuring electrons and positrons (the antiparticles of electrons) coming from space— that is, cosmic rays that are electrons and positrons. They see too many positrons for what we'd expect. One possible reason for the excess of positrons is that they are the result of very rare Dark Matter annihilations in our Galactic halo. (Although such annihilations, if they are happening, would be rare, there is so much bloody Dark Matter out there that if it's doing this, it would produce enough excess positrons for us to observe.)

What's really been detected is a positron excess, which is interesting all by itself. Whatever it turns out that this positron excess is coming from, it's going to be at least new astronomy, and potentially also new physics. It may not be as sexy and headline-worthy as "WE FOUND TEH DARK MATTER!!!1!!one!", but it will still be interesting, and will tell us something about nature. What's been seen is consistent with it coming from the Dark Matter halo of our galaxy, but other sources can't be ruled out yet. As more data is collected, the investigators running this experiment will be able to test whether the details of what is seen remain consistent with what would be expected from Dark Matter, versus other possible sources.

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Image of a Thermomnuclear Supernova Progentior

(by galacticinteractions) Feb 14 2013

Holy cow, it's been a long time since I blogged.

The class I'm teaching right now is 3d Computer Modelling and Animation. Perhaps the hardest thing about it is figuring out if the word Modelling has one or two l's in it... it depends on whether you're in the USA or Canada, I think.

For this class, I'm making all of the students do a major project. Some of them are doing some pretty interesting things, and already several of them have figured out things about Blender (the 3D software we're using, a quite powerful free package that you should check out yourself) that I don't know myself. A couple are playing around with motion tracking, in order to add 3D rendered elements into a live action video scene. One is building a game using the Blender game engine. Others are doing various other animations.

I've decided to take on a project myself. For this project, I am going to model a white dwarf in a mutual orbit with a main sequence or red giant star, pulling matter off of it into an accretion disk. During the animation, the white dwarf will go critical, and explode in a supernova, blowing itself way, and blowing off some of the outer layers of the companion star.

So far, I've managed to create the basic progenitor model, and do a little bit of animation of the textures so that the disk is spinning, the star's surface is roiling, and the gas bridge between the star and the disk looks a little like it's streaming. Here's a rendered frame from what I've done so far:

Click to embiggen (CC-BY-3.0)

I'll certainly post the full animation once I've completed it. Next, I'm going to have to start worrying about how to deal with the supernova. Eventually, I'll set the whole thing to music.

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Does your vote count?

(by galacticinteractions) Nov 06 2012

This being election day, you're seeing a lot of people telling you to get out and vote, saying that if you think your vote doesn't count you're abdicating democracy, reminding you how many people who died so that you could vote, etc.

I have to admit I find these exhortations both facile and manipulative. If we're talking about the presidential election, sure, you can make an argument that "every vote counts". That argument is not really practical, however, because of the electoral college. Every vote for president counts only in a small number of battleground states. I used to live in California; everybody knows that California is going to go to Obama. The Obama and Romney campaigns certainly know it; how much time and effort did they spend trying to sway California voters? Now, strictly speaking, if everybody who would vote for Obama figured it didn't matter and as a result didn't vote, then, yes, Romney could pull out a surprise win. But, while that's a theoretical possibility, let's be realistic here about how likely that is. It's not going to happen. As a result, if you tell somebody in California that their vote for the president really matters, you come across looking either naive, or manipulative.

So should you vote anyway? Yes. Two reasons.

First, to stay in practice. The USA's current system of elections is horribly corrupt. Jimmy Carter has spent a lot of time overseeing elections in other countries, and he says that about our elections. Also, check out, a website related to Lawrence Lessig's book Republic Lost. It's easy to become cynical, to realize that everybody running for any office is dancing to the tune of large campaign donors, and to give up and not bother voting. However, you must vote, both because there are differences between candidates, and because you need to stay in practice, and we need to keep voting as "a thing" that we do in the USA, in hopes that we do manage to fix the corrupt system.

The second reason is: there are elections other than president. If you live in California, no, it doesn't matter who you vote for for president; Obama's going to get your electoral college votes, whether you like it or not. However, there are congressional districts whose representatives are not a foregone conclusion. And, in many states, the Senate seats may well not be a foregone conclusion. Congress matters. You need to vote there. Additionally, there are going to be state and local elections that matter. You need to vote there. With all of these other things, there isn't an electoral college making your votes irrelevant; in these other races, every vote does matter.

So, yes, get out and vote. But, please, let's stop pretending that every individual vote for president matters, because that's simply not the reality of the situation.

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