interstellar medium (ISM)
Definition: This interstellar medium is both the leftover material from earlier generations of stars and the raw material for the next generation — understanding it explains where stars come from and why star formation continues billions of years after the Galaxy formed.
Why it matters here: Star formation begins where gravity locally overwhelms pressure support in the coldest, densest interstellar gas, and ends once nuclear fusion switches on and hydrostatic equilibrium takes over.
Relationship: Describe the major phases of the interstellar medium and their physical conditions.
Boundary or common confusion: Picturing star formation as instantaneous or as occurring uniformly throughout interstellar space. Star formation is localized to specific, rare, dense cloud cores and unfolds over roughly 10⁵-10⁶ years from core collapse to zero-age main sequence, a blink of an eye compared to a star’s subsequent main-sequence lifetime but still far longer than any human timescale.
molecular cloud
Definition: Molecular clouds are cold (about 10-20 K) and dense (hundreds to thousands of molecules per cm³ compared to about 1 atom per cm³ for the ISM average) — conditions found nowhere else in interstellar space.
Why it matters here: Star formation begins where gravity locally overwhelms pressure support in the coldest, densest interstellar gas, and ends once nuclear fusion switches on and hydrostatic equilibrium takes over.
Relationship: Young stellar clusters are consistently found still embedded in or immediately adjacent to molecular clouds, and never found isolated far from any cloud material, exactly as expected if clouds are the birthplace of stars.
Boundary or common confusion: Picturing star formation as instantaneous or as occurring uniformly throughout interstellar space. Star formation is localized to specific, rare, dense cloud cores and unfolds over roughly 10⁵-10⁶ years from core collapse to zero-age main sequence, a blink of an eye compared to a star’s subsequent main-sequence lifetime but still far longer than any human timescale.
H II region
Definition: A cloud can only begin gravitational collapse if its self-gravity exceeds internal pressure support, a competition captured qualitatively by the Jeans criterion: collapse is favored by high density and low temperature, and disfavored by high temperature and low density.
Why it matters here: Star formation begins where gravity locally overwhelms pressure support in the coldest, densest interstellar gas, and ends once nuclear fusion switches on and hydrostatic equilibrium takes over.
Relationship: As a collapsing core’s central density and temperature rise, it becomes a protostar, radiating primarily in the infrared because it is still enshrouded in the dusty remnant of its parent cloud.
Boundary or common confusion: Picturing star formation as instantaneous or as occurring uniformly throughout interstellar space. Star formation is localized to specific, rare, dense cloud cores and unfolds over roughly 10⁵-10⁶ years from core collapse to zero-age main sequence, a blink of an eye compared to a star’s subsequent main-sequence lifetime but still far longer than any human timescale.
Jeans mass
Definition: Because the Jeans mass falls with increasing density and decreasing temperature, only the coldest, densest cores within a molecular cloud (not the diffuse ISM as a whole, and not warmer parts of the same cloud) are unstable to collapse — explaining why star formation is localized to specific cloud cores rather than occurring everywhere at once.
Why it matters here: Star formation begins where gravity locally overwhelms pressure support in the coldest, densest interstellar gas, and ends once nuclear fusion switches on and hydrostatic equilibrium takes over.
Relationship: A cloud can only begin gravitational collapse if its self-gravity exceeds internal pressure support, a competition captured qualitatively by the Jeans criterion: collapse is favored by high density and low temperature, and disfavored by high temperature and low density.
Boundary or common confusion: Picturing star formation as instantaneous or as occurring uniformly throughout interstellar space. Star formation is localized to specific, rare, dense cloud cores and unfolds over roughly 10⁵-10⁶ years from core collapse to zero-age main sequence, a blink of an eye compared to a star’s subsequent main-sequence lifetime but still far longer than any human timescale.
Equation connection: \( \text{collapse favored when: gravitational energy} > \text{thermal (pressure) energy} \iff M > M_{\rm Jeans} \propto T^{3/2}\rho^{-1/2} \)
protostar
Definition: As a collapsing core’s central density and temperature rise, it becomes a protostar, radiating primarily in the infrared because it is still enshrouded in the dusty remnant of its parent cloud.
Why it matters here: Star formation begins where gravity locally overwhelms pressure support in the coldest, densest interstellar gas, and ends once nuclear fusion switches on and hydrostatic equilibrium takes over.
Relationship: A protostar reaches the main sequence (becomes a genuine, hydrogen-fusing star) once its core temperature is high enough to sustain steady nuclear fusion, at which point outward radiation and gas pressure balance gravity (hydrostatic equilibrium, Lecture 04) and the star stops contracting.
Boundary or common confusion: Picturing star formation as instantaneous or as occurring uniformly throughout interstellar space. Star formation is localized to specific, rare, dense cloud cores and unfolds over roughly 10⁵-10⁶ years from core collapse to zero-age main sequence, a blink of an eye compared to a star’s subsequent main-sequence lifetime but still far longer than any human timescale.
T Tauri star
Definition: As a collapsing core’s central density and temperature rise, it becomes a protostar, radiating primarily in the infrared because it is still enshrouded in the dusty remnant of its parent cloud.
Why it matters here: Star formation begins where gravity locally overwhelms pressure support in the coldest, densest interstellar gas, and ends once nuclear fusion switches on and hydrostatic equilibrium takes over.
Relationship: A protostar reaches the main sequence (becomes a genuine, hydrogen-fusing star) once its core temperature is high enough to sustain steady nuclear fusion, at which point outward radiation and gas pressure balance gravity (hydrostatic equilibrium, Lecture 04) and the star stops contracting.
Boundary or common confusion: Picturing star formation as instantaneous or as occurring uniformly throughout interstellar space. Star formation is localized to specific, rare, dense cloud cores and unfolds over roughly 10⁵-10⁶ years from core collapse to zero-age main sequence, a blink of an eye compared to a star’s subsequent main-sequence lifetime but still far longer than any human timescale.
zero-age main sequence
Definition: Star formation is localized to specific, rare, dense cloud cores and unfolds over roughly 10⁵-10⁶ years from core collapse to zero-age main sequence, a blink of an eye compared to a star’s subsequent main-sequence lifetime but still far longer than any human timescale.
Why it matters here: Star formation begins where gravity locally overwhelms pressure support in the coldest, densest interstellar gas, and ends once nuclear fusion switches on and hydrostatic equilibrium takes over.
Relationship: Trace the observable stages from molecular cloud core to protostar to zero-age main-sequence star.
Boundary or common confusion: Picturing star formation as instantaneous or as occurring uniformly throughout interstellar space. Star formation is localized to specific, rare, dense cloud cores and unfolds over roughly 10⁵-10⁶ years from core collapse to zero-age main sequence, a blink of an eye compared to a star’s subsequent main-sequence lifetime but still far longer than any human timescale.