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SYFY WIRE Bad Astronomy

Ethereal echoes from a dying star

By Phil Plait
The starburst galaxy M82, with red tendrils of gas and dust being expelled by stars in its center. Credit: NASA, ESA and the Hubble Heritage Team (STScI/AURA). Acknowledgment: J. Gallagher (University of Wisconsin), M. Mountain (STScI) and P. Puxley (NSF)

Given the level of wizardry astronomers now wield with observations and images, it takes something pretty special to get me to whisper "oh, wow" quietly as I sigh in wonder.

But this animation made me do exactly that. It shows a flash of light expanding into the dark, an echo of the last scream from a dying star:

The expanding light echo from supernova 2014J in the galaxy M82. Credit: NASA, ESA, G. Bacon, J. DePasquale, and Z. Levay (STScI) Acknowledgment: Y. Yang (Texas A&M/Weizmann Institute of Science)

Oh, wow.

I wasn't just being poetic above. This is quite literally an echo: a light echo.

There exists material between the stars, dust and gas strewn thinly throughout space. It's usually dark, unless it happens to lie near a star and can either reflect that light or be energized by it. But the vast majority of this interstellar medium, as astronomers call it, is quite dark.

A supernova, though, is a powerful event. When a star explodes, the flash of light is crushingly bright. It can light up this material even when it's very far away. And that's what the video depicts, the wave of light from the star as it expands and lights up this dust. No material you see in it is actually moving; only the light is.

The geometry for this is a bit complicated; I did some work on light echoes for my Ph.D. and found a lovely paper by Paul Couderc showing the math; it's from 1939 and if you read French that'll help. But in the end this all has to do with how light moves.

Light travels extremely rapidly (300,000 kilometers per second), but not infinitely fast. It takes time to move across space, and space is vast. When a star explodes, the flash of light expands outward in a sphere. A year after a supernova, the pulse of light will have moved one light-year, which is 10 trillion kilometers. Material at that distance from the supernova will then get lit up by the light and reflect it. A year later, material 20 trillion km out gets lit, and so on. From Earth, we see this as an expanding circle around the supernova*.

Science aside, it's mesmerizing, isn't it?

But come on. You know I won't put science aside.

The starburst galaxy M82, with red tendrils of gas and dust being expelled by stars in its center. Credit: NASA, ESA and the Hubble Heritage Team (STScI/AURA). Acknowledgment: J. Gallagher (University of Wisconsin), M. Mountain (STScI) and P. Puxley (NSF)

The supernova in the animation is called SN 2014J, the 10th named supernova that year. It occurred in the nearby galaxy M82, what is known as a starburst galaxy: one that has an unusually high rate of star birth. Near the core of M82 lie about 200 young massive clusters of stars, containing tens of millions of stars total. A lot of these stars are very massive and luminous, and they blow fierce winds of material away. Combined, this is a mighty wind, so vast it can be seen in images of the galaxy, tendrils of gas and dust literally blowing out of the galaxy from its heart.

The star that blew up to form SN 2014J was not a massive star like these, though. It was a white dwarf, the old, dead remnant of a star that was probably once like the Sun but ran out of fuel in its core and blew off its outer layers. It was in a binary system, orbiting another star. There are a couple of ways stars like this can explode. If the other star were a normal star like the Sun the white dwarf could draw material off its companion, gaining enough mass to ignite a wave of thermonuclear fusion that blew it up. If the other star was also a white dwarf, they could have collided, again triggering what is in effect a nuclear fusion bomb.

The galaxy M82 with the exploding star SN 2014J highlighted. Credit: NASA, ESA, A. Goobar (Stockholm University), and the Hubble Heritage Team (STScI/AURA)

Because M82 is only 12 million light-years away — very close as galaxies go — this supernova was visible even to small telescopes. In fact, it's one of the very few supernovae I've seen with my own eyes! I was in Tucson, Arizona, for a Science Getaways vacation (plug plug) about a month after the light from the explosion first reached Earth. With clear skies, we set up my 20 cm. ‘scope in a parking lot, and with a little help from another astronomer who knew that part of the sky, I pointed it toward M 82 (near the bowl of the Big Dipper and relatively easy to find). The supernova was just barely visible in the eyepiece. But it was there. It was amazing to see it, knowing what it was: The cataclysmic death of a star, emitting a billion times as much light as the Sun, dimmed by its terrible distance.

What I didn't know at the time is that this expanding flash of light would so beautifully light up the material in space around it. That echo was far fainter than the explosion itself, and it took Hubble to see it clearly. But we see light echoes from other objects as well (most famously V838 Monocerotis), and we can learn a lot about not only the star that exploded but the environment around them by studying the echoes.

By looking, for example, at how smooth the circle is you can learn about how clumpy the material is. The color (for example, using a blue filter versus red) can reveal the sizes of the dust grains (usually made up of long carbon molecules or tiny clumps of silicate (rocky) material) doing the reflecting. This material is extremely difficult to study because it's so dark, so the supernova acts like a flashbulb, illuminating it.

While M82 is the nearest starburst galaxy to us, and thus heavily studied, any extra tool in our toolkit we can use is welcome. And if it also provides a stunning display of how math, science, and art combine, then all the better.

* After one year, all the material 1 light-year from the supernova will get lit up, which defines the surface of a sphere. However, from Earth we don't see all that material lit up at the same time! Think of it this way: Imagine a blob of gas one light-year away from the event in the Earth's direction, and another blob one light-year away on the opposite, far side of the explosion. They both get lit at the same time, but the light from the blob on the far side will take an extra two years to get to us! That's because it has to cross the distance back to the supernova and then to the near-side blob, catching up to it: an extra trek of two light-years in distance and two years in time. When you do the math (again, as shown in Couderc's paper), what we actually see at any one time is the surface of a paraboloid (a 3D parabola) with the apex on the far side of the supernova and opening toward us. Like I said, it's complicated, but the math is fun to play with. In any event, no matter where the blobs are, we see an expanding circle of light because that's what a paraboloid looks like when you look down it.