The CHIPSat logo. Credit: NASA

The CHIPS mission is studying the very hot, very low-density gas in the vast spaces between the stars in our local astronomical neighborhood. The majority of the power radiated by this hot gas occurs in the short-wavelength, extreme ultraviolet region of the electromagnetic spectrum, centered around 170 Angstroms. This is a relatively unsurveyed band and the emissions at this wavelength may contain important clues about the process of cooling that takes place in the Local Bubble of the Interstellar Medium.

In our galaxy alone, there are several hundred billion stars. Nearby stars are easy to see, although most of the stars in the Milky Way are so distant that their combined light appears as a fuzzy band stretching across the sky on a clear night. Equally easy to see is the space between the stars - but how often do you wonder about this space? Most of us have an idea that these vast spaces are empty, a perfect vacuum. The material between the stars is known as the Interstellar Medium (ISM) and contains important clues about the formation and evolution of galaxies.

The ISM literally contains the seeds of future stars, and all the stars we see were once formed out of the same kind of diffuse gas and dust. When the gas in the ISM cools and collapses, the gas forms clumps that can evolve into stars and planets. Stars, in turn, expel enriched material back into the ISM as they age. One of the biggest puzzles in astrophysics is understanding this cyclical process.

Processes that heat the ISM are fairly well understood. Stellar winds blow through galaxies, transferring huge amounts of energy. Massive stars explode as supernovae shortly after their formation, stirring and heating the gas out of which they formed. These injections of energy from supernovae and stellar winds profoundly affect the ISM and determine the rate at which new stars form.

The Interstellar Medium -- What is it?
About 99% of the ISM is gas (hydrogen and helium); the remaining one percent consists of heavier elements and dust. The gas is extremely dilute, with an average density of about 1 atom per cubic centimeter. The air we breathe is approximately 30 quintillion (30,000,000,000,000,000,000) times more dense than the ISM. Picture this: an "empty" coffee mug in the ISM would contain about 500 hydrogen molecules. The same "empty" coffee mug sitting on your desk contains about 1500 quintillion gas molecules - mostly nitrogen, hydrogen and oxygen.

The dust in the ISM is made of tiny, irregularly shaped particles of silicates, carbon, ice and iron. In areas where the dust is thick, the light from nearby stars can be completely blocked - similar to the way dark clouds block light from the Sun.

Thinner clouds of interstellar dust may dim the light passing through, without completely blocking it. This is known as extinction. The interstellar dust scatters blue light more effectively than red light - which means that most of the light that reaches us through the interstellar dust is reddish. This is known as interstellar reddening. A similar process happens on Earth at sunset - which is why sunsets often appear red. Light from nearby stars also can be reflected from the interstellar dust, similar to the way light from a car's headlights can reflect off fog.

Unlike the dust in the interstellar medium, which can only reflect or block light, the gas in the interstellar medium glows in visible and many other wavelengths. In the region of hot, newly formed stars, clouds of hydrogen gas are ionized by the ultraviolet radiation emitted from the stars. When free electrons recombine with the ionized hydrogen, visible red light is emitted from the hydrogen gas. This accounts for the red colors in photographs of emission nebulae, such as the Trifid and Orion Nebulae.

In the hottest regions of the interstellar medium, hydrogen and helium are fully ionized, or stripped of their electrons. Spectral features in the light emitted by this gas, therefore, originate from heavier elements. At one million degrees Kelvin, the brightest spectral features are predicted to arise from partially ionized iron atoms in the interstellar medium. CHIPS will be the first mission to search for these spectral emission features with sufficient sensitivity to detect them and sufficient resolution to distinguish them from one another.

The Local Bubble: our astronomical neighborhood
Our solar system is located in an unusual region of space called the Local Bubble. The Local Bubble is about 300 light years in diameter and is filled with extremely low-density gas (about 0.001 gas molecules per cubic centimeter) - this is much less dense than the average ISM surrounding it. The coffee mug that would contain about 500 hydrogen molecules in the ISM would only contain 1 hydrogen molecule (or maybe none at all!), if it were in the Local Bubble. This gas also is extremely hot - about one million Kelvin, or almost 200 times as hot as the surface of the sun! Astronomers believe that a supernova explosion may have created this bubble - that is, the explosion "blew" most of the gas and dust from the interstellar medium outward. It is this extremely diffuse gas, inside the Local Bubble, that the CHIPS mission is studying.

Within the Local Bubble are smaller, denser clouds of interstellar gas. Our sun and solar system, along with some other nearby stars, are within but near the edge of one such cloud that is roughly 20 light-years in diameter.

CHIPS science objectives
The key questions about the Interstellar Medium that the CHIPS mission will seek to answer:

• At what wavelengths does the majority of the power radiated by local hot gas emerge?
  
  
• What are the physical processes by which the hot interstellar gas of the Local Bubble cools?
  
  
• What is the thermal pressure of hot gas in the Local Bubble?
  
  
• What is the structure and distribution of hot interstellar gas within 300 light years of the Sun (the "Local Bubble")?
  
  
• What is the history and evolution of the Local Bubble?
  
  
• How can this knowledge be applied to other diffuse hot plasmas in the Universe?