How is the 21-cm line of hydrogen formed and why is this line such an important tool for understanding the interstellar medium? O The 21-cm line is formed when hydrogen atoms in which electrons have escaped the atom through ionization and the 21-cm line appears when the electron recombines with the nucleus of hydrogen. When a hydrogen nucleus recombines with the electron through collisions, it will subsequently emit a photon with a wavelength of 21 cm, giving rise to the line. The line is important because it is produced by neutral hydrogen everywhere. Since cold hydrogen atoms make up the largest part of the interstellar medium, the 21-cm line allows us to study the most common component of interstellar gas. The 21-cm line is formed when hydrogen atoms in which the proton and electron are aligned "flip" so that the proton and electron are anti-aligned. Hydrogen atoms that are in the anti-aligned state will be excited into the aligned state by collisions, and will subsequently emit a photon with a wavelength of 21 cm, giving rise to the line. The line is important because it is produced by neutral hydrogen everywhere. Since cold hydrogen atoms make up the largest part of the interstellar medium, the 21-cm line allows us to study the most common component of interstellar gas. The 21-cm line is formed when hydrogen atoms in which the proton and electron are aligned "flip" so that the proton and electron are anti-aligned. Hydrogen atoms that are in the anti-aligned state will be excited into the aligned state by collisions, and will subsequently emit a photon with a wavelength of 21 cm, giving rise to the line. The line is important because it is produced by ionized hydrogen everywhere. Since hot hydrogen atoms make up the largest part of the interstellar medium, the 21-cm line allows us to study the most common component of interstellar gas. O The 21-cm line is formed when hydrogen atoms in which the proton and electron are aligned "flip" so that the proton and electron are anti-aligned. Hydrogen atoms that are in the anti-aligned state will be excited into the aligned state by collisions, and will subsequently emit a photon with a wavelength of 21 cm, giving rise to the line. The line is important because it is produced by molecular hydrogen everywhere. Since hot hydrogen atoms make up the largest part of the interstellar medium, the 21-cm line allows us to study the most common component of interstellar gas. The 21-cm line is formed when hydrogen atoms in which electrons have escaped the atom through ionization and the 21-cm line appears when the electron recombines with the nucleus of hydrogen. When a hydrogen nucleus recombines with the electron through collisions, it will s ubsequently emit a photon with a wavelength of 21 cm, giving rise to the line. The line is important because it is prod uced by ionized hydrogen everywhere. Since hot hydrogen atoms make up the largest part of the interstellar medium, the 21-cm line allows us to study the most common component of interstellar gas. O The 21-cm line is formed when hydrogen atoms in which electrons have escaped the atom through ionization and the 21-cm line appears when the electron recombines with the nucleus of hydrogen. When a hydrogen nucleus recombines with the electron through collisions, it will subsequently emit a photon with a wavelength of 21 cm, giving rise to the line. The line is important because it is produced by molecular hydrogen everywhere. Since hot hydrogen atoms make up the largest part of the interstellar medium, the 21-cm line allows us to study the most common component of interstellar gas