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The Question

(Submitted January 25, 2001)

From what I've read, the elements heavier than Iron in atomic weight can only be created by Supernovae. Similarly, elements beyond Hydrogen, Helium and Lithium can only be created in the core of stars during the process of their lives.

The constituent elements of stars including our Sun can be determined by spectrographic analysis. I believe that our Sun, and many other stars already have heavy elements within them which indicates that they were, at least in part, formed by material from ancient Supernovae. I believe spectrographic analysis can also be used to determine the overall distribution of matter in whole galaxies just like it is used on stars (I'm speculating this, since I've never heard it before, but it seems logical).

Since Supernovae, and stellar evolution occur over time, there should be more heavy elements now (at our present point in cosmic history), than there were near the beginning. So what do we see when we look back in time at distant galaxies.

My questions are:

If we do spectrographic analysis on galaxies from oldest to youngest, does a change in the distribution of heavy elements show up?

If so, what does it tell us about the rate at which Supernovae occur on average?

How many Supernovae per galaxy per century are required to account for the current distribution of heavy elements found in stars?

The Answer

First, it does not require a supernova to create elements heavier than iron. Heavy elements can also form in the cores of massive stars before they go supernova (s-process isotopes). Secondly, some elements beyond helium are formed in planetary nebulae. Some can also be formed through cosmic ray collisions. So the picture is a bit more complicated.

Now for your questions. Galaxy metallicity (the fraction of heavy elements) can be derived from emission line spectroscopy of planetary nebulae and H-II regions in nearby spirals and irregulars, and from absorption line spectroscopy of large ensembles of stars in elliptical galaxies. The metallicities of galaxies depends on the star formation history (how many generations of supernova producing stars) and whether the newly synthesized metals can be retained. The latter depends mostly on mass (i.e., gravitational binding energy) --- low mass galaxies lose a lot of metals in galactic winds.

On getting metallicity of distant galaxies from optical spectroscopy: it's a much tougher thing than getting the metallicity of a star. For a star, you solve for temperature, gravity, and chemical abundances. But a galaxy has a population of stars of different temperatures, gravities, and chemical abundances (from old M dwarfs that have been around for billions of years to O stars that were borne yesterday). You need a pretty good idea of the stellar populations before you can make any inferences on the metallicity.

Another problem: you need high resolution, high signal-to-noise spectra. Cosmologically distant galaxies (the ones which may show a significant difference) are all too faint (at least they were, before the days of 10m class telescopes).

For a typical stellar population one supernova is produced for every 100 solar masses of stars formed --- this can give a rough idea of the rate in a galaxy of a given age and mass (in a galaxy like the Milky Way, 1-10 per century or so). The rate of Supernovae is probably not constant over the age of the galaxy, and also is not uniform across the width of the galaxy. Metallicity increases the closer you get to galactic center, due to the higher density of stars.

Cheers,

Hans Krimm, Bram Boroson, Eric Christian, Kazunori Ishibash, Mike Loewenstein, and Koji Mukai for "Ask an Astrophysicist"

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