Interstellar medium

• 1. Introduction

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  The interstellar medium is the matter and radiation filling the vast space between star systems in a galaxy. Imagine the emptiness between stars not as a perfect void, but as a faint, complex sea of gas, dust, and energy threading through the galaxy at densities far below anything achievable in a laboratory on Earth. The question that gripped astronomers for centuries was simple: what, if anything, is actually out there? And the answers, when they came, reshaped the entire picture of how galaxies live and die.

  By mass, 99% of the interstellar medium is gas and just 1% is dust. Yet that dust scatters and absorbs starlight so effectively that dark rifts appear in the band of the Milky Way visible to the naked eye on a clear night. The medium is not uniform. It exists in multiple distinct phases, ranging from icy molecular clouds just ten to twenty degrees above absolute zero, to scorching coronal gas at temperatures between one million and ten million degrees. How can such extremes coexist? And what keeps the whole system from collapsing into stars, or dispersing into nothing?

  On the 25th of August 2012, a human-built machine crossed into this territory for the first time. Voyager 1, launched decades earlier, slipped past the boundary of the solar system and began returning direct measurements from interstellar space. Its twin, Voyager 2, followed on the 5th of November 2018. What both probes found there, and what ground-based observers had been piecing together for more than a century, is the subject of this documentary.

• 2. A Sea of Phases

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  Molecular clouds sit at the coldest end of the spectrum, with temperatures between ten and twenty degrees kelvin and densities reaching a trillion molecules per cubic metre. At the opposite extreme, coronal gas the hot ionized medium fills between thirty and seventy percent of the volume of the Milky Way's disk, at temperatures of one to ten million degrees, yet its density can fall as low as one ten-thousandth of a particle per cubic centimetre.

  Between these extremes lie the cold neutral medium, the warm neutral medium, and the warm ionized medium. Each phase has a characteristic temperature, density, and state of hydrogen, the atom that makes up 91% of all atoms in the gas by number. The warm ionized medium, for instance, occupies between twenty and fifty percent of the disk's volume at around 8,000 degrees kelvin, sustained largely by radiation from massive OB stars.

  OB stars are so hot that some of their photons carry energy above the Lyman limit of 13.6 electron volts, enough to strip electrons from neutral hydrogen. Those photons are absorbed almost immediately by any neutral hydrogen they encounter, sustaining a dynamic equilibrium between ionization and recombination that keeps gas near such stars in the warm ionized phase. The boundary where the ionizing photons are finally used up marks a sharp transition called an ionization front, where the warm ionized medium gives way to the warm neutral medium.

  A different transition governs the cold neutral medium. Between the warm and cold neutral phases, a temperature and density range exists where cooling runs away spontaneously. Gas that tips into this range sheds heat faster than it can absorb it, cascading down to the cold phase. This runaway behavior means the gas tends to avoid the intermediate temperatures, producing the distinct phases rather than a smooth continuum.

• 3. Stars, Supernovae, and the Cycle

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  Stars are born deep inside large complexes of molecular clouds, typically a few parsecs across. But their birth is also their first act of disruption. When OB stars ignite inside a molecular cloud, their ionizing radiation converts the dense surrounding gas into a classical H II region at around 8,000 degrees kelvin. Because the gas was still at molecular cloud densities, it is suddenly at vastly higher pressure than the surrounding interstellar medium. The overpressure drives the ionized gas outward in what physicists call a Champagne flow, eroding the cloud from within.

  This expansion continues until either the molecular cloud is fully consumed or the OB stars exhaust their fuel, a process taking a few million years. Then the OB stars explode as supernovae, sending blast waves through the warm gas and heating it to coronal temperatures. Supernova remnants expand and cool over several additional million years before the gas returns to average interstellar pressure. The cycle is self-reinforcing: supernovae heat gas, heated gas expands, expanding gas eventually cools into the raw material for new molecular clouds and new stars.

  Stellar winds from young star clusters inject additional energy, creating superbubbles of hot gas that are visible to X-ray satellite telescopes. The Sun currently travels through the Local Interstellar Cloud, an irregular clump of the warm neutral medium a few parsecs across, which itself lies within the Local Bubble, a roughly 100-parsec radius region of coronal gas. This bubble is almost certainly the remnant of ancient supernovae that cleared the surrounding medium long before the Sun passed through.

• 4. Composition and Chemistry

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  Hydrogen and helium together account for nearly all the mass in the interstellar medium. By mass, the gas breaks down to roughly 70% hydrogen, 28% helium, and 1.5% heavier elements. Those heavier elements, collectively called metals by astronomers regardless of their chemical identity, include carbon, oxygen, and nitrogen. Hydrogen and helium trace back to primordial nucleosynthesis in the early universe, while the heavier elements were forged inside stars and scattered into the interstellar medium through stellar winds and supernova explosions.

  Polycyclic aromatic hydrocarbons, or PAHs, are one of the most intriguing chemical species found in the medium. In February 2014, NASA announced an upgraded database for tracking PAHs across the universe, noting that more than 20% of all carbon in the universe may be associated with them. PAHs appear to have formed shortly after the Big Bang and are found near new stars and exoplanets. In September 2012, NASA scientists reported that PAHs subjected to interstellar conditions are transformed through hydrogenation, oxygenation, and hydroxylation into more complex organic molecules, representing a step toward amino acids and nucleotides, the building blocks of proteins and DNA.

  In April 2019, the Hubble Space Telescope confirmed the detection of buckminsterfullerene, C60, also called buckyballs, in the interstellar medium. These large, cage-shaped carbon molecules survive in interstellar space, adding to the picture of a chemically active environment rather than a sterile void. In September 2020, evidence emerged of solid-state water in the medium, specifically water ice mixed with silicate grains in cosmic dust.

• 5. Dust, Light, and the Dark Rifts

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  Dust grains account for 1% of the interstellar medium by mass, but their effect on starlight is profound. When light from a distant star passes through a dusty region, the grains scatter and absorb photons, reducing the total brightness in a process called extinction. Because blue light is absorbed more efficiently than red, distant stars appear redder than they actually are, a phenomenon called reddening. Both effects allow astronomers to detect and map the dust by observing millions of stars.

  The dark rifts visible in the band of the Milky Way on a clear night are caused by absorption of background starlight by dust in molecular clouds within a few thousand light years of Earth. The effect decreases rapidly with increasing wavelength and becomes almost negligible at mid-infrared wavelengths beyond five micrometres. This wavelength dependence means that infrared and radio telescopes can see through dust that blocks optical light entirely.

  Extinction mapping has become one of the most powerful tools for charting the three-dimensional structure of the interstellar medium. Since the Gaia mission provided accurate distances to millions of stars, astronomers could determine how much dust lies in front of each star from its reddening, and then locate the dust along the line of sight by comparing stars projected close together on the sky but at different distances. By 2022, this technique made it possible to generate a map of interstellar structures within three kiloparsecs, or about ten thousand light years, of the Sun.

• 6. Observing the Invisible

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  The 21-centimetre spectral line of neutral hydrogen is one of the most productive observational tools in all of astronomy. Radio telescopes pick it up across the entire disk of the Galaxy, giving a detailed view of the warm neutral medium's distribution and motion. In the cold neutral medium, the same line appears in absorption rather than emission, as cold foreground gas absorbs photons from warmer background gas behind it.

  Molecular clouds are traced primarily through carbon monoxide, CO, which emits at 115 gigahertz corresponding to a change from one to zero quanta of angular momentum. Hundreds of other molecules have been detected in molecular clouds, each with many lines spanning millimetre and sub-millimetre wavelengths. The most common molecule in clouds, molecular hydrogen H2, is usually undetectable directly because it remains in its ground state except when excited by rare events such as interstellar shock waves.

  In October 2020, Voyager 1 and Voyager 2 detected a significant unexpected increase in density in the space beyond the solar system. Researchers concluded this implies a large-scale density gradient in the very local interstellar medium in the direction of the heliospheric nose. Radio waves traveling through the interstellar medium are affected by the plasma in ways that also reveal structure: variations in electron density cause interstellar scintillation, which broadens the apparent size of distant radio sources, while the delay of radio pulses from pulsars at lower frequencies, proportional to the column density of free electrons, provides a tool for mapping ionized gas and estimating pulsar distances.

• 7. A Century of Discovery

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  The word interstellar was coined by Francis Bacon in the context of the ancient theory of a literal sphere of fixed stars. For much of the following two centuries, the space between stars was debated as either a true vacuum or filled with a hypothetical fluid. In 1862 a journalist still wrote of the ether filling interstellar spaces.

  The modern era of interstellar science opened in 1864, when William Huggins used spectroscopy to determine that a nebula is made of gas. Huggins worked from a private observatory equipped with an 8-inch telescope with a lens by Alvan Clark. From around 1889, Edward Barnard pioneered deep sky photography, finding what he called holes in the Milky Way. By 1899 he was writing that one could scarcely conceive a vacancy with holes in it unless there was nebulous matter covering the apparently vacant places. Those holes are now recognized as dark nebulae, dusty molecular clouds recorded in his Barnard Catalogue.

  The first direct detection of cold diffuse matter in interstellar space came in 1904. Johannes Hartmann observed the binary star Mintaka, also known as Delta Orionis, with the Potsdam Great Refractor and found that absorption from the K line of calcium did not share in the periodic displacements caused by the orbital motion of the binary. The stationary nature of the absorption line proved the gas was not in the star's atmosphere but in a separate cloud along the line of sight. This discovery launched the systematic study of the interstellar medium.

  Interstellar sodium was detected by Mary Lea Heger in 1919 through stationary absorption of the D lines at 589.0 and 589.6 nanometres toward Delta Orionis and Beta Scorpii. Viktor Ambartsumian later introduced the concept that interstellar matter occurs in the form of discrete clouds, a notion now central to the field. Victor Hess's discovery of cosmic rays in the same era prompted Kristian Birkeland, described as a Norwegian explorer and physicist, to write in 1913 that the whole of space might be filled with electrons and flying electric ions, and that the greater part of the material masses in the universe might be found not in stellar systems or nebulae but in empty space.