Does the Sun have an iron surface? (Intermediate)

I recently found an website named and they contend that the surface of the sun is solid iron, did something change? My understanding was that iron was the last element to degrade before stars blew up. I always understood that all of the other elements would be gone leaving only iron at the end of a star.

You are not misinformed. The commonly accepted paradigm for the the make-up of the Sun has stayed the same, and there is a lot of evidence to back it up. There are a few people who believe that the Sun is made of iron, and believe that they have found strong evidence to support this. As we have stated in other places, we do not feel that the "Curious" site is not the appropriate place to debate the indepth details of such models, although I should note that it takes only basic Physics to find flaws in the idea of a solid iron surface to the Sun (for example the fact that iron is vapourized at the temperature of the Sun's surface). If you do want more details I would like to refer you to the Bad Astronomy and Universe Today Forum which hosted this thread in June-July 2005 in which the author of debates his model with various other members of the forum.

The gaseous sun model, rather than being outdated as the website you refer to suggests, is backed up by several lines of very recent evidence. If you study stellar structure at an undergraduate major level you will see how the standard model can predict the radius, surface temperature and other observable properties of stars, and how it does this very well. In fact the solar neutrino problem (the fact that the standard solar model overpredicted neutrino emissions of the Sun relative to what was observed), instead of pointing to a change in solar physics instead points to new properties for neutrinos! Helioseismology (observing sound wave propagating through the solar interior by their effect on the surface), also backs up the model. So there is very good (and recent) evidence to support the idea that the Sun is made mostly of gaseous hydrogen. In fact Astronomers think most of the universe is made of gaseous hydrogen, so it would be rather strange if the Sun were vastly different.

There is a small amount of iron in the Sun, because the Sun was made from a gas cloud which must have been enriched by the iron (and other elements) made in several previous stars and supernova. When the Sun runs out of hydrogen it will start fusing heavier and heavier elements together. Stars heavier than the Sun will continue this process until they make iron. Iron is the last element which when fused releases energy, heavier elements take an input of energy to fuse (but release energy by fission), so once a star has made iron there is no more energy available from nuclear fusion and it will die. The Sun is not massive enough even to ignite Carbon burning (heavier elements require hotter temperatures - and therefore bigger stars to ignite). Once the Sun has made Carbon in it's fusion in it's core will cease, leaving behind a slowly cooling and dim white dwarf star.

So nothing has changed, and your previous ideas about how the Sun works are believed to be correct by the majority of people.

Actually there is an interesting history behind the idea that the Sun was made of iron. It's a fairly recent question to ask what the Sun is made of and what the source of its energy is. Less than 200 years ago this wasn't a question that had even been thought of. Mid 19th century models suggested that maybe the Sun was just a ball of hot (molten) iron, but even simple calculations show that without an internal energy source such a Sun would cool down much too quickly for this to be possible. Around 150 years ago it was realised that the radiation from the Sun can be used to measure it's temperature, giving a surface temperature of almost 6000 C (~10,000 F), much too hot for solid iron (or solid anything for that matter). Undergraduate physics calculations of hydrostatic equilibrium for the Sun (taking into account the balance between gravity and the outward pressure of the material it is made of), also quickly show that the Sun must be gaseous. Early models for the source of energy for the Sun thought that it might be due to a slow contraction of the Sun (energy released from gravity), but this also led to a age for the Sun which was much too young. Spectra of the Sun can identify the most common elements on the surface of the Sun as hydrogen and helium - matching well with models of the most common elements in the universe as a whole. All of this work paved the way for the start of nuclear physics and our current understanding of the Sun as a giant gaseous hydrogen fusion reactor.

About the Author

Karen Masters

Karen Masters

Karen was a graduate student at Cornell from 2000-2005. She went on to work as a researcher in galaxy redshift surveys at Harvard University, and is now on the Faculty at the University of Portsmouth back in her home country of the UK. Her research lately has focused on using the morphology of galaxies to give clues to their formation and evolution. She is the Project Scientist for the Galaxy Zoo project.

Twitter:  @KarenLMasters

Search Our Q&A Archive

Most Popular

Our Reddit AMAs

AMA = Ask Me (Us) Anything