A very cool picture of the very first stars

Thursday, November 3rd, 2005 at 12:36 PM by Juan

I teach a cosmology course every once in a while, and it is a testament to modern astronomy that cosmological observations are becoming commonplace enough that I have to revise my class notes extensively every time I teach the course. A few years ago it was the Wilkinson Microwave Anisotropy Probe and its outstanding picture of the Cosmic Microwave Background that showed that the universe is 13.7 Billion years old and probably spatially flat (that is, on large scales the angles in a triangle still at up to 180° and the universe is infinite in extent with no bounds).

(Click on image for full-size version.) This Spitzer Space Telescope image shows both the original infrared image (top) and the image once all the foreground stars and galaxies have been removed, revealing the Cosmic Infrared Background.

Now, in a press release from yesterday, astronomers have announced that they used the Spitzer Space Telescope to shoot an extremely deep infrared image of the night sky in a small section of the sky in the constellation Draco. This is very similar to the famous Hubble Deep Field image shot by the Hubble Space Telescope a few years back, except that this image was shot at infrared wavelengths. The hope was that after removing the light from all the foreground stars and galaxies (seen in the top image), they could enhance the contrast and see the faint cosmic infrared background due to the “first stars” (also called “Population III” stars by astronomers).^[1]

The first stars formed within 200 million years of the Big Bang (or 13.5 Billion years ago) and formed from almost pure Hydrogen and Helium. Because they form from almost pure hydrogen and helium, these “first stars” are quite different from any of the subsequent stars that form in the universe. It turns out that hydrogen and helium in space do a poor job of cooling off. In space, since you can’t loose heat through conduction, the only way a gas can lose heat is by having the atoms in the gas collide and convert some of their energy into light. How efficient this process can be depends on the number of spectral lines seen in the gases spectrum. Since hydrogen and helium atoms have relatively few spectral lines, they can’t easily convert heat energy into light and thus cool off.

Stars exist due to a property called hydrostatic equilibrium which is the balance between two forces, gas pressure outward versus the pull of gravity inward. As a star is forming, if the gas can cool off efficiently by converting heat into light, the gas pressure drops, allowing gravity to pull the star together to the point where it can start fusion in its core and “light up.” However, the gas forming the first stars couldn’t cool off easily, therefore you needed a lot more gas to assemble a star with enough gravity to hold itself together. Thus the first stars were believed to be much more massive than the most massive stars existing today, maybe up to a million times the mass of the Sun! A star with that kind of mass burns extremely quickly since it would burn much brighter than our Sun and thus it would have a lifetime as short as a few hundred years (for comparison, the Sun’s lifetime is 10 billion years)! It would then supernova (or hypernova) and contribute the heavier elements it had formed through nuclear fusion to the interstellar gases. These heavier elements have many spectral lines and so subsequent generations of stars can form from a gas which cools off much more easily, so they can be a lot smaller. This means there will never again be stars as large or as short lived as the “first stars.”

What the Spitzer folks think they are seeing is the light from these first stars, emitted 13.5 billion years ago, redshifted by the expansion of the universe into the far infrared. If confirmed, I think this stands as one of the crowning achievements of humanity (or at least astronomers): We can now see the most distant stars in the universe, objects that will never again exist in our universe, by looking at light emitted when the universe was only 1.5% its current age.

Posted in Astronomy News

One Response to “A very cool picture of the very first stars”

1. Centauri Dreams » Blog Archive » The Light of Ancient Stars wrote on 11/3/05 at 2:14 pm :

   [...] Centauri Dreams note: It is important to emphasize that the new Spitzer observations reveal not individual stars but variations in background radiation that seem to point to star formation. Nonetheless, the data seem clean, as discussed in this nature.com article from which the above quote from Alexander Kashlinsky comes. In fact, the team found clusters in the background radiation in four different wavelengths in different parts of the sky and at different times of year, which seems to rule out local interference. The paper itself is Kashlinsky, A., Arendt, R.G. et al., “Tracing the first stars with fluctuations of the cosmic infrared background,” Nature 438, 45-50 (3 November 2005). Finally, Juan Cabanela at Saint Cloud State University (MN) offers a fine discussion of the first stars in terms of hydrostatic equilibrium. [...]