Dead Stars Tell No Tales… Or Could They?

The galaxy is full of stars that are dead or dying. You’d think that their stories would be over and we could move on to the next star, right?

Au contraire… as it turns out, some dead stars are surrounded by the dust of asteroids they chewed up and spat out. And, astronomers are getting a chance to “bite” that dust again by studying the infrared emissions from that material with the spectroscopic “eye” of Spitzer Space Telescope.

The data are intriguing, even if you’re not used to looking at it in X-Y plots like this one.  It shows that the asteroid dust surrounding a dead “white dwarf” star contains silicates. That’s a very common mineral here on Earth — many of our rocks are made of silicates. So are rocks on the Moon and the other rocky planets such as Mars and Venus. And, as it turns out, silicates are found in clouds of debris around other kinds of stars and in the the ghostly shrouds of planetary nebulae.  So, how do asteroids get chewed up and and ground to dust around a white dwarf star?

We all know the story of planetary formation around young stars. Dusty material left over from star birth swirls around the star in a kind of shroud. The dust and particles stick to each other over time, forming larger and large protoplanetary lumps. Eventually, if you stick enough of this stuff together and wait a while (say a few million or so years), you get planets. What’s left over circulates around as chunks of rocks. We call those chunks “asteroids.”

Now, fast-forward through the life of the star and its associated planetary system — say, one similar to our Sun. When such a star gets old and cranky, it puffs itself up into a red giant. That action eats up the inner planets and jostles whatever asteroids and outer planets that survive the angry red giant phase. As the star continues to die, it does a most amazing thing:  it blows off its outer layers and then shrinks down into a skeleton of its former self. The end result is a white dwarf, an  object with an intense gravitational field. If anything wanders too close — say an asteroid — it gets shredded to pieces by the gravitational pull of the white dwarf. That scatters around a lot of dust.  The chemical fingerprints of the elements in that dust can be picked out by the infrared spectrograph on Spitzer Space Telescope (for example).

Spitzer looked at eight white dwarf systems and found the dust to be very rich in a glassy silicate material like olivine (which is commonly found here on Earth). The scientists who did the looking, led by Mike Jura at University of California at Los Angeles, plan to search out more of these “dust-bitten” regions around white dwarfs. What they find from the tales these dead stars tell will give us some pretty unique insights into how other star systems treat their planets and asteroids as they form, grow, evolve, and then die.

Note: this story was first released during the AAS meeting a couple of weeks ago. I headlined it at the time, but have been wanting to give it more detailed treatment. For more information, check out the Spitzer press release.