Planetary Nebulae, and the White-Hot Dwarfs that Light Them

Monday night I was able to get some images with the imager, and conditions were great nearly the entire evening. One of the objects I imaged was Messier 46 (or, rather, I walked a volunteer through imaging it, as I was doing some training). M46 is an open cluster in the constellation Puppis the Poop Deck not too far away from Sirius, the brightest star in the night sky.

Messier 46 in Puppis, with the planetary nebula NGC 2438 visible near the top.

M46 by itself is a fairly unremarkable (if fairly rich) open cluster of perhaps up to 500 stars about 30 light-years across, located about 4,500 light-years from us and receding at a speed of 41.4 km/s. What's interesting is that at first glance it appears to have a planetary nebula (cataloged as NGC 2438, visible above the middle of the cluster) embedded in it. However, the nebula does not appear to share the motion of the cluster, and is just a chance alignment along our line of sight.

The nebula itself is turns out to be pretty interesting. It is located closer to us than M46 at about 2,900 light-years, and has the amazing property that its central white dwarf star is one of the hottest stars known, measuring in at an astounding 75,000 K. (That's about 74,700 °C, or 134,672 °F.)

To put those number in perspective, our Sun's surface is about 5,778 K (or about 9,940 °F), while the very hottest O-type main-sequence stars (ones that are in their hydrogen-fusing phase, about 90% of the lifetime of all stars) might get up to 30,000 K (53,540 °F). Note that all these temperatures refer to the star's surface temperature – even in our fairly low-on-the-temperature-scale Sun the core temperature is over 15 million K (about 27 million °F). That still means that the Earth-sized ultra-dense sphere of electron-degenerate carbon and oxygen at the center of NGC 2438 is over two-and-a-half times hotter than the hottest stars operating under fusion power alone, and nearly thirteen times hotter than our Sun.

Planetary nebulae are so-named not because they have anything to do with planets, but because in the early telescopes through which they were discovered they looked somewhat like them (sort of a faint, fuzzy disc-shape). They come about when a star less than ten times the mass of the Sun runs out of hydrogen in its core to fuse, at which point it begins to work its way up the periodic table fusing heavier and heavier elements in its core while its outer layers puff up and the star becomes a red giant. The outer layers slowly puff off into space to form the nebula, while the core eventually stalls at carbon and oxygen (or maybe neon, if the star has between eight and ten times the Sun's mass) and forms a white dwarf, which is basically just the exposed core of a former star.

At this point no new energy is being released, so the white dwarf enters a long period of cool down. Due to their composition of electron-degenerate matter, temperatures inside the white dwarf are a fairly steady 1,000,000 K or so (about 18 million °F), while the outside cools off as it radiates energy into space, lighting up the outer layers it puffed off earlier and making the planetary nebula visible. Eventually the nebula disperses into the interstellar medium, and the white dwarf is left to gradually get cooler, and cooler, and cooler...

To end this post, I thought I'd mention that the hottest white dwarf found to date had a surface temperature of an incredible 200,000 K (360,000 °F) – almost three times hotter than the white dwarf in NGC 2438, nearly seven times hotter than the hottest O-type stars, and a whopping thirty-five times hotter than the surface of our own star. Pretty amazing, huh? On the flip side, the coolest white dwarfs found have surface temperatures of just about 4,000 K (6,740 °F) – they've already cooled off to less than the Sun's surface temperature.

Check back tomorrow, when I'll show you a picture of what happens when a star with more than ten times the mass of the Sun runs out hydrogen in its core. Hint: it's a lot more explosive!

Edit (3/24/12): As you can probably tell, it's way past tomorrow and I still haven't gotten that post up. That's because it turned out to be much longer than I anticipated when I started it, and then I went to work for two days. I will have it up in the next few days, though.