In my Top 14 Astronomy Pictures of 2010, I started off with a galaxy I called the Milky Way’s fraternal twin; it looks a lot like ours, but has some differences that were worth pointing out.

In one of those coincidences that makes me smile, only a few days later the folks at Hubble Space Telescope released another spiral galaxy image, and this one… well, it’s a beauty:

That’s really something! It’s so pretty I made it my desktop image. Click it to see it in all its 2800 x 2400 pixel galactaliciousness.

The name of this galaxy is UGC 12158. It’s a face-on barred spiral; the bar refers to that rectangular block of stars in the center. Some spirals have a spheroidal central bulge, like Andromeda does, but quite a few have a bar-shaped hub. The Milky Way does, in fact, and observations using radio and infrared telescopes (able to pierce the dust obscuring our view) show that our bar is actually pretty hefty. The small picture here shows an illustration of the Milky Way based on these observations, and we think it’s a pretty accurate representation. The resemblance to UGC 12158 is obvious.

When I first saw this Hubble picture, I was impressed with the beauty of this galaxy I had never seen before. But then I realized something… Y’know, I have a lot of experience looking at Hubble images. I spent years working on them, and after a while you get a feel for them. It’s just practice, and you get what almost feels like instinct about some things. So when I saw this picture and I got that odd (but familiar) feeling in my head, I knew to pursue it. It didn’t take me more than a few seconds to nail it down: this galaxy is big. The size of the star images, the smoothness of the galaxy itself, the way the image feels… I just knew that this was no tiny galaxy.

So I went to the release page for it, and when I saw the distance, I was shocked: that galaxy’s not big, it’s freaking huge. (more…)

Type Ia supernovae are very important exploding stars. It’s thought that this particular type of supernova has a very special property: they all explode with about the same energy. This makes them very valuable, because it means that if you can simply measure how bright they appear to be, you can figure out how far away they are. It’s like seeing headlights on the highway; dim ones are far away, and bright ones are close.

Of course, in reality, it’s not that easy. But after a Herculean effort, astronomers in the late 1990s figured they had been able to account for any small differences in brightness and could use these stars as "standard candles", benchmarks to calculate cosmic distances. Because they’re so bright, they make great milestones because they can be seen pretty much all the way to the edge of the observable Universe.

The thing is, it’s not clear how a type Ia actually forms. There are two models, both involving white dwarfs. These are the ultradense remnants of dead stars, the exposed cores of stars after they shed their outer layers. The Sun will one day be a white dwarf (in about 6 – 7 billion years, so don’t hold your breath). Because of complicated quantum physics, it turns out that white dwarfs can only have so much mass; if they exceed about 1.4 times the mass of the Sun they can collapse, either forming an even denser neutron star, or exploding as a supernova.

The first model of a Type Ia is a white dwarf orbiting a star like the Sun. The intense gravity of the dwarf draws material off the normal star, a process called accretion. The matter piles up, the mass limit is exceeded, and BANG! Supernova.

Well, it’s a lot more complicated than that, but close enough.

The second idea is that you have two white dwarfs orbiting each other. Over time they spiral in (this time due to relativistic effects called gravitational waves), get too close together, merge, and BANG! Supernova.

Astronomers have always assumed that the accretion scenario is the far more common of the two, because it takes a long time for two dwarfs to merge, whereas accretion can happen easily if a dwarf happens to be paired up with a normal star (which should be pretty common). But how do you tell which is which?

It turns out that the two different scenarios leading up to the explosion have two very different effects: accretion makes a lot of X-rays, while a merger does not. So astronomers did what you’d expect: they pointed the Chandra X-Ray Observatory at a bunch of galaxies and observed supernovae. What they found was pretty surprising: the amount of X-rays from Type Ia supernovae in nearby galaxies was 30 – 50 times lower than what would be expected from accretion. In other words, their observations strongly favor the idea that it’s the merger of white dwarfs that cause Type Ia supernovae.

Well! I was pretty surprised to hear that. Like other astronomers, I figured it was accretion that was the culprit. Now mind you, there are some caveats here. They observed elliptical galaxies, which tend to have an older population than spirals, so you might see more mergers than accretions. Also, it’s possible things were different in the past, and when we observe very distant galaxies were seeing them as they were billions of years ago.

But still, you just don’t expect to see what the astronomers saw, so it seems to me like they’re on to something here.

This has some interesting ramifications. It certainly affects a lot of fields of astronomy, like how binary stars form and change over time. But it may also affect cosmology, the study of the birth, evolution, and eventual fate of the Universe itself. If Type Ias are caused by a different scenario than previously thought, could it mean that our measurements of the distant Universe are wrong?

I asked this question specifically at the Chandra press conference, and was told that the two different scenarios produce explosions with pretty much the same energy, so this may only affect cosmological measurements a small amount. However, right now our theoretical models of the merger scenario are still pretty rough, so it’s unclear if the peak brightnesses of the two models are the same.

This may affect our measurements of dark energy, the mysterious pressure that seems to be accelerating the expansion of the Universe. My gut reaction is that this won’t matter a huge amount, since we have lots of independent ways of measuring dark energy, and they all appear to be in rough agreement. But this means we have one more thing to take into account in those measurements. And it may prove to be useful; if we can distinguish between the two supernova generators, our measurements will get that much more accurate.

I have to say I’m pleased with this; I studied supernovae in college and grad school, eventually studying one for my PhD (though it was of an entirely different flavor from this kind). I remember reading a long technical paper about the different Type Ia scenarios back then: it’s been a mystery for a long, long time. But with perseverance, the right equipment, and more than a touch of cleverness, we’re a big step closer to figuring this all out!

Related posts:
Fireworks and Pinwheels (an overview of Type Ia supernovae)
Dark Energy site open for business (explaining dark energy)
The Universe is 13.73 +/- .12 billion years old
What astronomers do (about the discovery of dark energy)
The cosmological not-so-constant

Image credits: NASA, ESA, The Hubble Key Project Team, and The High-Z Supernova Search Team, and NASA/CXC/M.Weiss (adapted a bit by The Bad Astronomer)

As I mentioned in an earlier post, I attended the first few days of the American Astronomical Society meeting this week. I went as a member of the press, as I have for the past few years. The press room is a fun place; lots of old friends, banter across the table, and, of course, the press releases.

I had a stack in my mailbox, so I poked through them. One in particular caught my eye. And how could it not? In oversized, bold print the headline ran: "THE LONG OVERDUE RECURRENT NOVA T PYXIDIS: SOON TO BE A TYPE Ia SUPERNOVA?"

Hmmm. Recurrent novae are binary systems, where a dense white dwarf is stealing matter from its companion. The matter piles up, and eventually detonates, causing a huge flash of light (that’s the nova part). After time, the system settles down, the matter starts piling up, and the cycle starts again (that’s the recurrent part). Lots of recurrent novae are known, and are fairly well understood.

T Pyxidis is a fairly regular nova, blowing its lid every 20 years or so. It’s currently overdue, since the last event was in 1967. Using ultraviolet observations and new models of the system, astronomer Edward Sion and his team concluded it may actually explode soon as a supernova, an event far more energetic than a mere nova. Worse, their models indicate the system is "much closer" than previously thought: about 3300 light years away. In the last paragraph of their press release, it says:

An interesting, if a bit scary, speculative sidelight is that if a Type Ia supernova explosion occurs within [that distance] of Earth, then the gamma radiation emitted by the supernova would fry the Earth, dumping as much gamma radiation (~100,000 erg/square centimeter) into our planet [sic], which is equivalent to the gamma ray input of 1000 solar flares simultaneously.

AIIIIEEEEE!!! We’re all gonna die!

Hubble’s view of T Pyxidis from 1997, showing a shell of expanding matter from an earlier eruption.

Ahem. Except, really, no. I rolled my eyes when I read that bit. A Type Ia does put out more high-energy radiation than a Type II supernova, which is caused when a massive star’s core collapses and the outer layers are ejected. That’s what most people think of when they hear about a supernova. Those have to be really close to hurt us, certainly closer than 25 light years. But even with their added power, a Type Ia just doesn’t have the oomph needed to destroy our ozone layer (as the press release indicates) from 3300 light years away. It would have to be far closer than that.

I missed that press conference, but oh, how I wish I had been there! My friend Ian O’Neill was able to track down some details, and found out that astronomers (including another friend, Alex Filippenko, who is an expert’s expert on supernovae) at the meeting took Sion to task for this claim. It looks like Sion used the wrong numbers for the gamma ray emission for a Type Ia event, instead using the emission from a gamma-ray burst… a far, far, far more energetic event, and dangerous from several thousand light years away.

I don’t generally have too big an issue with a scientist getting a number wrong, but it depends on the circumstance. Issuing a press release saying, essentially, we’re all gonna die means they should do some due diligence. And in this specific case — they used the phrase "fry the Earth" for Pete’s sake! — means I am less willing to cut them slack. People get scared from stuff like this, and it’s simply wrong to feed that fire without making really sure you have your numbers straight first.

I’ll note that scientists tend not to write press releases, and it can be hard to rein in the PR author if they are not that familiar with the science (which I’ve seen many times). But even if the numbers in the PR were correct, the phrasing of that last paragraph is unacceptable. Whoever wrote the release should have known the media would zero in on that phrase.

Ian O’Neil, in his post at Discovery News, points out The Daily Telegraph did just that, printing an article with the headline, "Earth ‘to be wiped out’ by supernova explosion". The UK paper The Sun — which is so awful fish complain when you wrap them in it — had a similar article with the tagline, "A star primed to explode in a blast that could wipe out the Earth was revealed by astronomers yesterday."

Sheesh.

It’s too bad. There was no need to disaster-porn this release up the way it was done. Recurrent novae and Type Ia supernovae are fascinating, well worth our attention for any number of reasons including of course their potential danger. But it’s a not-too-fine line between piquing interest and tarting up the science.

Artwork credits: Casey Reed, Dana Berry.

What does a star on the edge of death look like? Perhaps not what you think:

This series of images [as usual, click to embiggen], from the European Southern Observatory’s Very Large Telescope, will take some ‘splainin. Hang on.

A supernova — an exploding star — is among the brightest single objects in the known Universe. A supernova can release as much energy in a single second as the Sun will in a thousand years.

Most people think of supernovae as massive stars exploding at the end of their lives, but there is another kind. When the Sun finally dies in a few billion more years, it will shed most of the material making up its outer layers, revealing the white-hot, dense core. This superhot ball will have half the mass of the Sun in it, but only be the size of the Earth. We call such a thing a white dwarf.

If a white dwarf orbits a normal star like the Sun, it can draw material off. This matter piles up on the surface and can eventually detonate like a stellar thermonuclear bomb. We call these Type Ia supernovae.

The thing is, massive stars are bright, so we can see them a long way off. We know of many stars in our galaxy that can blow that way (though all too far away to hurt us). But a Type Ia progenitor is faint, and hard to spot. Usually, the first notice we get of one is when it explodes, and we see the sudden and vast increase in light in a distant galaxy.

But astronomers have spotted a potential Type Ia supernova in our own galaxy, a ticking time bomb about 25,000 light years away. Called V445 Puppis, in November 2000 it underwent an explosive event: not a supernova, but a regular nova, the detonation of small (in cosmic terms) amount of material. Still, it ejected a lot of matter — several times the mass of the entire Earth — at very high speed, about 24 million kilometers per hour (14 million mph). That would reach from the Earth to the Moon in one minute flat. Over the course of several years, astronomers have taken images of the expanding debris, and the change — seen in the picture above — is dramatic, lovely, and terrifying.

The debris did not expand spherically because the two stars are in a tight orbit, circling each other rapidly. The matter drawn off the normal star forms a thick disk around the white dwarf. When the material on the surface exploded, it couldn’t go through the disk, so it went up and down, above and below the disk. Over time it forms what’s called a bipolar structure, because it comes out of the poles of the star. We see lots of similar bipolar objects, but not usually in a system that’s about to go bye-bye.

Tellingly, there is no detectable hydrogen in the system. The surface of the white dwarf appears to be mostly helium, and the normal star looks to be dumping only helium on the white dwarf. Type Ia supernovae are hydrogen poor, even lacking it completely, so that fits.

Also, the mass of the white dwarf in V445 Puppis is on the thin hairy edge of the maximum it can be before it blows. When a white dwarf reaches 1.4 times the mass of the Sun, it goes kablooie (I had to calculate this as a homework problem in grad school). V445′s mass? 1.35 times that of the Sun.

Yikes.

So when will the system go off? Hard to say. It may not be for thousands of years, or even longer. At that distance, it will be very bright in the sky, brighter than Venus. It won’t hurt us; it’s way too far away to to do that. But a nearby supernova of this type would be a huge boon to astronomy! It’s this flavor of supernova we use to measure the expansion of the Universe (since they are so bright they can be seen very far away, and tend to blow up with the same brightness every time).

It’s a little funny to think that the death of a star so many quadrillions of kilometers away can actually be a benefit to us. But remember, the calcium in our bones and iron in our blood came from supernovae like the one V445 Puppis will eventually become, so not only do we learn more about the Universe from them, we owe our very existence to them as well.