Star Formation
Protostar Formation
From molecular cloud to baby star — gravitational collapse over millions of years
A protostar is a stellar embryo — collapsing molecular cloud core that has not yet ignited hydrogen fusion. Process: dense regions of giant molecular clouds (GMCs) gravitationally collapse → form Class 0 → Class I → Class II → Class III protostar. Initially hidden by dust; revealed as evolution progresses. Protostar accretes mass from envelope and disk. T Tauri phase: pre-main-sequence stage; visible. Becomes main sequence star when core T reaches 10⁷ K. Total formation: 10⁵-10⁷ years depending on mass.
- BirthplaceGiant molecular clouds (GMCs), 10-100 light-years across
- Formation time~10⁵-10⁷ years (mass-dependent)
- Class 0Deeply embedded; mostly envelope
- Class IDisk + envelope; jets
- Class IIT Tauri phase; visible disk
- Class IIIPre-main-sequence; dust dispersed
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Why protostar formation matters
- Star birth. Where stars come from.
- Planet formation. Disks form planets.
- Galactic evolution. Star formation shapes galaxies.
- Initial mass function. Why stars have specific masses.
- Astrobiology. Solar nebula precursor of life.
- Stellar physics. Earliest stages.
- Observation frontier. ALMA, JWST advance field.
Common misconceptions
- Stars form quickly. 10⁵-10⁷ years.
- Star formation is simple. Multiple phases, complex physics.
- Each cloud forms one star. Multiple stars; binary systems common.
- Sun formed from supernova. Solar nebula formed from molecular cloud (some seeded by past SN).
- Protostars look like stars. Embedded; not visible early on.
- Star formation is inefficient. Only ~1% of cloud mass forms stars.
Frequently asked questions
How does star formation begin?
Giant molecular cloud (10-100 ly across, 10³-10⁶ M_sun) becomes unstable. Gravity overcomes thermal pressure (Jeans criterion). Cloud fragments. Dense cores collapse. Core T initially low (~10 K). As it collapses, T rises. Eventually inner core becomes "first hydrostatic core" (~50 AU, 100 K). Further collapse → second hydrostatic core (~0.001 AU, ~2000 K). This is the protostar.
What's the Jeans criterion?
Mass M_J above which a gas cloud collapses gravitationally. M_J = (5kT/Gμm_H)^(3/2) × (3/4πρ)^(1/2). For typical molecular cloud: M_J ~ 10-100 M_sun. Above this: cloud fragments and collapses into stars. Below: pressure prevents collapse. Sets typical mass scale for stars.
What happens in Class 0?
Initial collapse phase. Object deeply embedded in dust envelope; not visible in optical. Detected via infrared/submillimeter. Most mass in envelope, less in protostar. Strong jets/outflows characteristic. Lasts ~10⁵ years. Smallest, most embedded. Hard to study.
What's a T Tauri star?
Pre-main-sequence star ~10⁶ years old. M < 2 M_sun. Cool (3000-4000 K) but luminous due to large radius. Surrounded by accretion disk. Outflows and jets common. Variable brightness. Surface activity (spots, flares) high. Eventually becomes main sequence by 10⁷ years for sun-like stars.
How long does formation take?
Depends on mass. Massive (~10 M_sun): fast — 10⁵-10⁶ years. Sun-like (1 M_sun): 30-50 Myr from molecular cloud to main sequence. Low-mass (M dwarfs): up to ~100 Myr. Most time spent in early phases. Rapid main sequence approach for massive stars.
What's a protoplanetary disk?
Disk of gas and dust around protostar. Forms during collapse due to angular momentum. Material spirals inward through disk → onto star. Some material remains and forms planets. Disk lifetime: ~10⁵-10⁷ years before dispersal. Visible in many young stellar objects.
How is star formation observed?
Multi-wavelength approach. Infrared/submillimeter: penetrates dust. Radio: molecular line observations. Optical: T Tauri stars visible. Recent: ALMA telescope reveals spectacular disk structures. Spitzer, Herschel, JWST all contribute. Star-forming regions (Orion, Taurus, etc.) actively studied.