What is the interstellar medium?

The vacuum of space between planetary systems within our Solar System The space between galaxies in the Local Group The gas and dust that fills the space between stars throughout a galaxy The region of space between the Sun and the nearest star, Alpha Centauri

What fraction of the ISM by mass is gas versus dust?

About 50% gas and 50% dust — they are evenly mixed About 99% gas and 1% dust by mass About 10% gas and 90% dust — most ISM mass is in solid form About 99% dark matter and 1% ordinary gas and dust

What is the coldest phase of the ISM, and what makes it special?

Hot ionized gas at millions of Kelvin — produced by supernova explosions Warm neutral hydrogen at 6,000-10,000 K — detected by visible light emission Molecular clouds at 10-30 K — the only phase cold and dense enough for gravity to form new stars Cold neutral hydrogen at 50-100 K — detected by radio emission at 21 cm

How do astronomers detect cold neutral hydrogen gas in the ISM?

By the visible light it absorbs from background stars, creating dark hydrogen absorption lines By its infrared thermal emission, similar to detecting the heat from a warm object By its radio emission at a wavelength of 21 centimeters, produced when hydrogen atoms flip their electron spin By the X-rays it emits when struck by cosmic rays from supernova remnants

What creates the hottest phase of the interstellar medium?

The intense radiation from the most massive blue stars heats surrounding gas directly Supernova explosions blast hot gas outward, creating bubbles of million-Kelvin plasma Dark matter particles colliding in the disk release energy as heat Friction from gas orbiting the galactic center heats the ISM uniformly

What are interstellar dust grains made of?

Tiny particles of gold, iron, and other heavy metals ejected from old stars Tiny particles of silicates and carbon compounds, typically 0.1 to 1 micrometer in size Microscopic ice crystals similar to those found in Earth's atmosphere Compressed packets of dark matter that have cooled to solid form

What is interstellar reddening?

Dust particles emit red light of their own when heated by nearby stars Stars near the galactic center have cooler surfaces and naturally emit more red light Dust scatters blue light more than red, making distant stars appear redder than they actually are The Doppler effect redshifts the light of stars moving away from us in the galaxy

What is the cosmic recycling cycle of the ISM?

Gas falls into the galactic center, powers Sgr A*, and is ejected back into the disk Stars form from ISM clouds, process elements through fusion, and return enriched gas to the ISM when they die, seeding future generations The ISM is completely static — it does not change or interact with stars Gas from the ISM falls into planets and is permanently locked away, slowly depleting the galaxy

Where do most of the heavy elements in your body (like carbon, oxygen, and iron) ultimately come from?

They were created in the Big Bang alongside hydrogen and helium They were created by cosmic rays hitting hydrogen gas in the ISM Previous generations of stars that fused lighter elements into heavier ones and then returned them to the ISM when they died They were imported from other galaxies via intergalactic gas streams

Which tool has been most important for mapping the large-scale distribution of the ISM across the Milky Way?

Visible-light photography of emission nebulae, which outlines the star-forming regions Gamma-ray detectors that sense cosmic rays produced in supernova remnants Radio telescopes detecting 21-cm hydrogen emission, which passes through dust and reveals gas across the entire galaxy Gravitational lensing surveys that detect the mass of gas clouds between us and distant quasars