Chapter 20 Between the Stars: Gas and Dust in Space
20.6 Interstellar Matter around the Sun
Learning Objectives
By the end of this section, you will be able to:
- Describe how interstellar matter is arranged around our solar system
- Explain why scientists think that the Sun is located in a hot bubble
We want to conclude our discussion of interstellar matter by asking how this material is organized in our immediate neighborhood. As we discussed above, orbiting X-ray observatories have shown that the Galaxy is full of bubbles of hot, X-ray-emitting gas. They also revealed a diffuse background of X-rays that appears to fill the entire sky from our perspective ([link]). While some of this emission comes from the interaction of the solar wind with the interstellar medium, a majority of it comes from beyond the solar system. The natural explanation for why there is X-ray-emitting gas all around us is that the Sun is itself inside one of the bubbles. We therefore call our “neighborhood” the Local Hot Bubble, or Local Bubble for short. The Local Bubble is much less dense—an average of approximately 0.01 atoms per cm3—than the average interstellar density of about 1 atom per cm3. This local gas has a temperature of about a million degrees, just like the gas in the other superbubbles that spread throughout our Galaxy, but because there is so little hot material, this high temperature does not affect the stars or planets in the area in any way.
What caused the Local Bubble to form? Scientists are not entirely sure, but the leading candidate is winds from stars and supernova explosions. In a nearby region in the direction of the constellations Scorpius and Centaurus, a lot of star formation took place about 15 million years ago. The most massive of these stars evolved very quickly until they produced strong winds, and some ended their lives by exploding. These processes filled the region around the Sun with hot gas, driving away cooler, denser gas. The rim of this expanding superbubble reached the Sun about 7.6 million years ago and now lies more than 200 light-years past the Sun in the general direction of the constellations of Orion, Perseus, and Auriga.
A few clouds of interstellar matter do exist within the Local Bubble. The Sun itself seems to have entered a cloud about 10,000 years ago. This cloud is warm (with a temperature of about 7000 K) and has a density of 0.3 hydrogen atom per cm3—higher than most of the Local Bubble but still so tenuous that it is also referred to as Local Fluff ([link]). (Aren’t these astronomical names fun sometimes?)
While this is a pretty thin cloud, we estimate that it contributes 50 to 100 times more particles than the solar wind to the diffuse material between the planets in our solar system. These interstellar particles have been detected and their numbers counted by the spacecraft traveling between the planets. Perhaps someday, scientists will devise a way to collect them without destroying them and to return them to Earth, so that we can touch—or at least study in our laboratories—these messengers from distant stars.
Key Concepts and Summary
The Sun is located at the edge of a low-density cloud called the Local Fluff. The Sun and this cloud are located within the Local Bubble, a region extending to at least 300 light-years from the Sun, within which the density of interstellar material is extremely low. Astronomers think this bubble was blown by some nearby stars that experienced a strong wind and some supernova explosions.
For Further Exploration
Articles
Goodman, A. “Recycling the Universe.” Sky & Telescope November (2000): 44. Review of how stellar evolution, the interstellar medium, and supernovae all work together to recycle cosmic material.
Greenberg, J. “The Secrets of Stardust.” Scientific American December (2000): 70. The makeup and evolutionary role of solid particles between the stars.
Knapp, G. “The Stuff between the Stars.” Sky & Telescope May (1995): 20. An introduction to the interstellar medium.
Nadis, S. “Searching for the Molecules of Life in Space.” Sky & Telescope January (2002): 32. Recent observations of water in the interstellar medium by satellite telescopes.
Olinto, A. “Solving the Mystery of Cosmic Rays.” Astronomy April (2014): 30. What accelerates them to such high energies.
Reynolds, R. “The Gas between the Stars.” Scientific American January (2002): 34. On the interstellar medium.
Websites and Apps
Barnard, E. E., Biographical Memoir: http://www.nasonline.org/publications/biographical-memoirs/memoir-pdfs/barnard-edward.pdf.
Cosmicopia: http://helios.gsfc.nasa.gov/cosmic.html. NASA’s learning site explains about the history and modern understanding of cosmic rays.
DECO: https://wipac.wisc.edu/deco. A smart-phone app for turning your phone into a cosmic-ray detector.
Hubble Space Telescope Images of Nebulae: http://hubblesite.org/gallery/album/nebula/. Click on any of the beautiful images in this collection, and you are taken to a page with more information; while looking at these images, you may also want to browse through the slide sequence on the meaning of colors in the Hubble pictures (http://hubblesite.org/gallery/behind_the_pictures/meaning_of_color/).
Interstellar Medium Online Tutorial: http://www-ssg.sr.unh.edu/ism/intro.htm. Nontechnical introduction to the interstellar medium (ISM) and how we study it; by the University of New Hampshire astronomy department.
Messier Catalog of Nebulae, Clusters, and Galaxies: http://astropixels.com/messier/messiercat.html. Astronomer Fred Espenak provides the full catalog, with information and images. (The Wikipedia list does something similar: https://en.wikipedia.org/wiki/List_of_Messier_objects.)
Nebulae: What Are They?: http://www.universetoday.com/61103/what-is-a-nebula/. Concise introduction by Matt Williams.
Videos
Barnard 68: The Hole in the Sky: https://www.youtube.com/watch?v=8No6I0Uc3No. About this dark cloud and dark clouds in interstellar space in general (02:08).
Horsehead Nebula in New Light: http://www.esa.int/spaceinvideos/Videos/2013/04/The_Horsehead_Nebula_in_new_light. Tour of the dark nebula in different wavelengths; no audio narration, just music, but explanatory material appears on the screen (03:03).
Hubblecast 65: A Whole New View of the Horsehead Nebula: http://www.spacetelescope.org/videos/heic1307a/. Report on nebulae in general and about the Horsehead specifically, with ESO astronomer Joe Liske (06:03).
Interstellar Reddening: https://www.youtube.com/watch?v=H2M80RAQB6k. Video demonstrating how reddening works, with Scott Miller of Penn State; a bit nerdy but useful (03:45).
Glossary
- Local Bubble
- (or Local Hot Bubble) a region of low-density, million degree gas in which the Sun and solar system are currently located
- Local Fluff
- a slightly denser cloud inside the Local Bubble, inside which the Sun also lies