SMA Research: Dying Stars

Although both the human eye and the SMA can see thousands of stars, there are very few stars which can be seen by the human eye and the SMA. That is because most of the stars we see at night are in their middle age, while the SMA sees stars most clearly when they are very young or very old.

Old stars become visible to the SMA when they begin to expel their outer envelopes back into space. Such stars swell to hundreds of times the diameter of our Sun, and they are cool enough for dust and molecules to form in their upper atmospheres. The dust is particularly important because the radiation from the star can push upon the dust and drive it away from the star, back into interstellar space. The dust particles collide with gas molecules, which are also driven into space. Because some of this dust and gas was formed from the products of nuclear "burning" deep within the star, these stellar winds enrich the interstellar gas with heavy elements such as those which are the building blocks of life.

The SMA can make images of both the dust and the gas expelled from old stars. Because molecules emit and absorb radiation at specific frequencies, the SMA can determine the chemical composition of the gas by measuring the frequencies at which the radiation is most intense. Such spectroscopic observations have shown that the gas expelled by old stars has a rich chemistry, with many ions and free radicals which are unstable in earth-bound laboratories. By measuring a variety of frequencies the temperature and pressure of the gas can be estimated. Also, the SMA can very accurately measure the doppler shift of gaseous material around stars, which tells us how the gas is moving. Because the geometry of the emitting material is often well approximated by a simple expanding shell, knowing how the gas is moving allows a three-dimensional model of the shell to be produced.

SMA Research on Evolved Stars


Ken Young, Nimesh Patel

Chin-Fei Lee, Naomi Hirano, Shigehisa Takakuwa, Hsien Shang, Nagayoshi Ohashi, Ram Rao

On-going collaborators:
Hiroko Shinnaga


The concentric shells of dust surrounding the evolved star IRC+10216 are shown in grey in the image above (Mauron & Higgens, 1999). Also shown are false color maps of the distribution of several simple molecular species within the star's envelope. Stars of this type are the universe's main source of carbon, nitrogen and oxygen (which, along with hydrogen, constitute most of the material in our bodies). Since IRC+10216 is enshrouded in dust it cannot be seen in visible light. However, the cold envelope of expanding gas, as well as the star itself, are easily seen by the SMA. The SMA can separate the light emitted by different chemical components and watch the complex sequence of chemical reactions which occur as the envelope expands into space (Patel et al. 2011).