Tag: U Cam

A dying star weaves a spiral in the night

By Phil Plait | October 10, 2012 11:01 am

The number of ways stars can find to die bizarre deaths will never cease to amaze me.

Some explode, supernovae which blast radiation across the Universe. Others fade away slowly over hundreds of billions of years, longer than the cosmos has been around. Some blow off winds of gas and dust, taking on strange shapes from perfect spherical shells to elongated structures that look like two jellyfish kissing.

And then some – a very few – are like R Sculptoris, a red giant on the thin hairy edge of death. And its death is both spectacular as well as just plain old damned weird.

Check THIS out:

This is not a drawing! It’s actual data, observations of R Sculptoris made using the Atacama Large Millimeter/submillimeter Array (ALMA). ALMA looks at light far too low energy for our eyes to see; it’s actually out past infrared in the spectrum. Cold dust and gas emits light at this wavelength, including carbon monoxide. That molecule is created copiously in red giants and shines brightly in the submillimeter, making it easy to see with ‘scopes like ALMA. That’s nice, because CO can be used as a tracer for other, harder to detect molecules like hydrogen. Looking at CO really tells you a lot about what’s going on in the gas and dust.

And what’s going on? Ah, this is the really cool part.

When a star like the Sun (either a bit less massive, or up to about 8 times as massive) ages, the core heats up, which causes the outer part of the star to expand (like a hot air balloon), turning it into a red giant. The details are complicated – read this post on a similar star where I explain it in more detail (and you want to because the details are awesome) – but the bottom line is that helium builds up in a thin shell outside the star’s core, where it fuses into carbon. The fusion rate is insanely sensitive to temperature, and periodic imbalances in temperature cause vast and very sudden increases in the fusion rate – and by sudden I mean over a timescale of just a handful of years, the blink of an eye to a star. Called a thermal pulse, this huge fireball of energy is dumped into the star’s interior, blows upward like a tsunami, and then blasts material clear off the star’s surface.

The result is an epic paroxysm which blows out a massive wave of material, expanding in a sphere around the star. We’ve seen this before, like in the star U Cam. After a few years, you get an eerie detached shell of expanding material, like a smoke ring trillions of kilometers across.

OK, so that’s the thin shell thing on the outside. So what’s the deal with R Sculptoris that makes that freaky inner spiral pattern?

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CATEGORIZED UNDER: Astronomy, Cool stuff, Pretty pictures

Similar by eye, not by nature

By Phil Plait | July 16, 2012 6:50 am

In a funny coincidence, my friend Travis Rector, an astronomer at the University of Alaska who takes amazing and incredibly beautiful astrophotographs, posted an image that may look familiar. It’s of NGC 6751, a planetary nebula about 5000 – 7000 light years away:

[Click to embiggen, and note I rotated the image 90° to make it fit better on the blog.]

It’s very pretty, but also very complex. Planetary nebulae are actually gaseous structures created when stars a bit more massive than the Sun die. When they turn into red giants they blow a dense, slow wind of gas into space — that can be seen in NGC 6751 as the big blue halo surrounding the whole thing. The edge is a bit brighter as it slams into gas floating in between the stars and piles up a bit. The halo is probably something like 1.5 light years across.

The interior part is actually the most interesting, You can see the star in the center. About halfway out to the big halo is a ring of material that looks like it’s broken up into individual knots like pearls on a necklace. Inside that ring is something that looks like a disk seen face-on, with faint filaments stretching from the central star out to the ring.

Studies of this object indicate this is in fact the case. Although it’s not clear, I suspect the central star may have enveloped and swallowed a companion star or giant planet, spinning it up to a much faster rotation speed. Otherwise, it’s hard to explain why it has a ring and disk of material; a solitary star could never spin fast enough to eject such a structure. This would also explain some of the more complex features of the nebula as well. If this is true, we’re nearly roughly down on the pole of the star (or maybe off by 20° or so), since the disk is very close to being a circle; if the system were tilted by much more the ring would be more elliptical, like looking at the rim of a glass from an angle.

Anyway, I said this nebula may look familiar; that’s because I recently posted a picture of the star U Cam, which is much like the Sun but a lot older. Like the central star of NGC 6751, it too is dying, but in the case of U Cam it’s blasting out shells of material on a very short timescale. The ring we see around it is only about 700 years old, and was created in an event that lasted mere decades. The shells and rings in NGC 6751 took thousands of years to make, and the halo is probably more than 50,000 years old!

And while both huge structures are the results of dying stars, and both have an overall similar appearance, they’re very different in actual shape and origin. The halo around U Cam is like a soap bubble: a thin shell that has a sharp edge but is probably mostly empty on the inside. Astronomers call this a "detached shell". NGC 6751, on the other hand, is more like a balloon filled with gas. And down near the star NGC 6751 has that complex disk/ring affair going on, while U Cam is probably mostly empty space all the way down to the surface of the star. The gas making up the shell of U Cam has about the same mass as the Earth, while the nebula in NGC 6751 is tens of thousands times more massive!

There are lots of other differences between them as well, but I find it remarkable (literally, since I’ve been sitting here remarking on it!) that two objects with such similar appearance – and both caused by the efforts of dying stars – can actually be so different. Nature is filled with such things, of course. Spiral galaxies look similar to swirls of cream in a coffee cup, though the physics is entirely different. Stars and drops of water are spherical, but for two different reasons (gravity for the stars, surface tension for the droplets).

Sometimes moons look like Death Stars, but that may be pushing this a bit too far.

I think that’s cool. Shapes of objects can be revealing of their nature, but you have to be careful when making judgement based on shape alone. You have to look deeper to reveal the true nature.

If there’s a life lesson in that, feel free to find it on your own.

Image credit: D. Tran (PAL College), T.A. Rector (University of Alaska Anchorage), T. Bridges (Queen’s U.) and the Australian Gemini Office

Related Posts:

The Ghost in the Shell
Tears of a dying star
The green ghost of a distant dead star
The knotty halo of the Cat’s Eye

CATEGORIZED UNDER: Astronomy, Cool stuff, Pretty pictures

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