- published: 19 Aug 2015
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In astronomy, the interstellar medium (or ISM) is the matter that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, dust, and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space. The energy that occupies the same volume, in the form of electromagnetic radiation, is the interstellar radiation field.
The interstellar medium is composed of multiple phases, distinguished by whether matter is ionic, atomic, or molecular, and the temperature and density of the matter. The thermal pressures of these phases are in rough equilibrium with one another. Magnetic fields and turbulent motions also provide pressure in the ISM, and are typically more important dynamically than the thermal pressure is.
In all phases, the interstellar medium is extremely dilute by terrestrial standards. In cool, dense regions of the ISM, matter is primarily in molecular form, and reaches number densities of 106 molecules cm−3. In hot, diffuse regions of the ISM, matter is primarily ionized, and the density may be as low as 10−4 ions cm−3. Compare this with a number density of roughly 1022 cm−3 for liquid water. By mass, 99% of the ISM is gas in any form, and 1% is dust. Of the gas in the ISM, 89% of atoms are hydrogen and 9% are helium, with 2% of atoms being elements heavier than hydrogen or helium, which are called "metals" in astronomical parlance. The hydrogen and helium are a result of primordial nucleosynthesis, while the heavier elements in the ISM are a result of enrichment in the process of stellar evolution.
Neil deGrasse Tyson ( /ˈniːəl dəˈɡræs ˈtaɪsən/ born October 5, 1958) is an American astrophysicist and science communicator. He is currently the Frederick P. Rose Director of the Hayden Planetarium at the Rose Center for Earth and Space, and a research associate in the department of astrophysics at the American Museum of Natural History. Since 2006 he has hosted the educational science television show NOVA scienceNOW on PBS, and has been a frequent guest on The Daily Show, The Colbert Report, Real Time with Bill Maher, and Jeopardy!. It was announced on August 5, 2011, that Tyson will be hosting a new sequel to Carl Sagan's Cosmos: A Personal Voyage television series.
Tyson was born as the second of three children in the borough of Manhattan in New York City, but was raised in the Bronx. His mother, Sunchita Feliciano Tyson, was a gerontologist and his father, Cyril deGrasse Tyson, was a sociologist, human resource commissioner for the New York City mayor, John Lindsay, and was the first Director of HARYOU. Tyson attended the Bronx High School of Science (1972–1976, astrophysics emphasis) where he was captain of the wrestling team and was editor-in-chief of the school's Physical Science Journal. Tyson had an abiding interest in astronomy from the age of eleven, following his visit to the Hayden Planetarium at age nine. Tyson recalls that "so strong was that imprint [of the night sky] that I'm certain that I had no choice in the matter, that in fact, the universe called me." He obsessively studied astronomy in his teens, and eventually even gained some fame in the astronomy community by giving lectures on the subject at the age of fifteen.
Christopher Jonathan James Nolan (/ˈnoʊlən/; born 30 July 1970), also known as Chris Nolan is a British/Americanfilm director, screenwriter and producer.
He received serious notice after his second feature Memento (2000), which he wrote and directed based on a story idea by his brother, Jonathan Nolan. Jonathan went to co-write later scripts with him, including the Batman series and The Prestige. He also first collaborated with Wally Pfister, who would photograph all his subsequent films. After directing Insomnia (2002), Nolan pitched an idea for a reboot of the Batman film franchise to Warner Bros., eventually making a successful trilogy consisting of Batman Begins (2005), The Dark Knight (2008) and The Dark Knight Rises (2012).
Inception (2010) was an original screenplay by Nolan, a heist film set in the world of "shared dreaming." Nolan co-founded Syncopy Films with his wife, Emma Thomas, and they have produced all his films since The Prestige (2006).
Nolan has also worked with screenwriter David S. Goyer, film editor Lee Smith, composers David Julyan and Hans Zimmer, special effects coordinator Chris Corbould, and actors Christian Bale, Cillian Murphy, Jeremy Theobald, Tom Hardy, Ken Watanabe, and Michael Caine.